Final Report Summary - GRASP (Green advanced space propulsion)
Executive summary:
Over the last 30 years, chemical propulsion for space application relied nearly fully on propellants which are highly toxic and in most cases carcinogenic. This included fuels such as hydrazine, monomethylhydrazine (MMH), unsymmetrical dimethylhydrazine (UDMH) and oxidisers such as mixed oxides of nitrogen (e.g. MON-3) and nitrogen tetroxide (NTO). Although those propellants have very good performance and in particular have an extensive flight history, their handling requires complex and costly precautions to protect human operators and the environment.
In 2007/8 a consortium consisting of 12 entities was established and the project GRASP was initiated. The GRASP consortium set out to identify possible alternatives to those toxic propellants. However, better protection of the human operators and environment as well as the increasing costs due to the toxicity of those propellants were not the only reasons for this quest. In view of the fact that over the last three decades the limitations in handling e.g. of hydrazine has become more and more restrictive, the GRASP team pointed out the possibility that the use of such propellants might become impossible or exceedingly expensive in the near future. Unfortunately for the relevant industry, this hypothesis of GRASP became reality with the recent action (June, 2011) of the European Chemical Agency (ECHA) in the framework of Reach, to put hydrazine on a watch list, potentially, outlawing the use of hydrazine in the near future.
Although worldwide different research groups have been working on the development and investigation of green propellants, the relevant industry and also space agencies were rather reluctant to participate in such efforts in spite of the above mentioned thread. Contributing to this hesitation is the lack of comprehensive and unbiased information about green propellants. The GRASP effort was designed to provide such information so that decision makers from industry or space agencies can re-evaluate their future strategy in respect to green propellants.
Starting with more than 100 green propellant candidates, the GRASP team down-selected a small number of propellants, which are very likely to be realistic candidates for a replacement of hydrazine and other toxic propellants. This was achieved by an intensive research and development (R&D) effort combining numerical/analytical as well as experimental expertise. Following the three years of GRASP, the relevant industry is now in a much better position to judge the suitability of the green propellant concept. This again puts it in a much better position to utilize the new knowledge and implement green propellants in their near term strategy. In view of the above mentioned action from REACH the input from GRASP could not have come to a better point in time.
Project context and objectives:
The GRASP project aimed to provide the European industry with alternative propellants to replace the currently used highly toxic and carcinogenic propellants (e.g. hydrazine, MMH, UDMH). These alternative, so-called, green propellants will reduce the potential harm to human operators and the environment and thereby significantly reduce the associated handling costs. Even more important, in view of the recent action of ECHA which might outlaw the use of hydrazine, the availability of green propellant technology might ensure the competitiveness of the European space industry.
The search for such a propellant is driven by four essential industrial needs:
1. Ensuring the competitiveness of the European industry in a challenging and continuously changing environment
2. Reduction of costs
3. Reduction in exposure to toxic and/or carcinogenic substances
4. Performance improvements.
Laboratory-scale evaluations in the past, conducted worldwide have indicated that several green propellant candidates can indeed provide a performance comparable or even better than those of presently used propellants. In addition, various studies have shown that the use of green propellants can provide long-term financial benefits. However, industry as well as space agencies have shown in public little interest in green propellant technology and a broad implementation of low toxicity propellants seemed still far from reality. There are many reasons for this reluctance but the main ones are:
1. The extensive experience the space community/industry has collected with the presently used toxic propellants
2. The satisfactory performance of the presently used propellants
3. The costs associated with the development and qualification of new hardware (propulsion, valves, tanks etc.)
4. The costs associated with the replacement of existing infrastructure such as test facilities, filling station at launch pads and many others
These are very valid reasons for entities which operating in a very conservative environment. On the other hand, the GRASP consortium and fellow researchers pointed out the historically proven tendency in the last 40 years towards more and more restrictive national and international laws with respect to the use of such dangerous substances. GRASP also pointed out that in the long term, an implementation of propellants with lower toxicity might reduce the total costs (in spite of the initial, probably substantial, investment) and ensure the competitiveness in a more stringent legal environment. In order to convince the space community/industry to switch to a particular green propellant, the following issues, amongst others, must be demonstrated:
1. Similar or better performance than those propellants used currently
2. System advantages
3. Detailed investigation of the storage and handling properties
4. Compatibility with existing hardware
5. Likelihood of reduced costs in the long-term.
The goal of the GRASP project is not to provide incremental progress but to conduct a systematic down-selection to identify the most promising green propellant candidate(s) and implement those candidates in demonstrator propulsion systems.
Besides the development of propulsion systems, another more general objective of GRASP was the development of a standard for green propellant technologies, to be achieved by collecting and generating sufficient data and information about a large number of green propellant candidates. This will include a comprehensive database of the physical characteristics of the candidate propellants, as well as theoretical performance, availability, stability, material compatibility and any other pertinent property.
The above was to be achieved by means of a down-selection of green propellant candidates. Several major assessments will function as filters during this process. An initial general assessment, including a comprehensive survey of green propellant technologies, will provide a first crude filter to identify the candidate propellants (and catalysts) that will be experimentally investigated in the following stage of the project. Besides being a general survey, this assessment will also address questions with regard to where green propellants can compete with current propellants, as well as definitions of minimum propellant requirements (specific impulse, toxicity, physical properties, etc.) and system requirements. Depending on the propellants selected, the data will be collected either by making a literature survey (for well-known propellants) or by measuring their properties (for new or scarcely characterised propellants).
Following this initial assessment, an extensive experimental study of the selected green propellant technologies was to be initiated. The nature of this investigation depends on the propellant and will have two main parts. For those propellants for which past experience exists (e.g. hydrogen peroxide), an investigation of catalysis will be conducted with the goal of optimising the performance (decomposition rates and temperatures, etc.). In parallel with this effort, new propellants will be manufactured and their characteristics will be evaluated. This experimental effort will provide a unique and comprehensive database allowing a further selection process for the next stage. Such a database will establish a standard for green propellants, which is urgently needed as a reference not only for this particular project but for all green propellant related research in the future.
The GRASP effort has three primary fields to be investigated: propellants, catalysis and propulsion systems. For a successful implementation of green propellants substantial advancements in all three fields are needed as described in the following.
Propellants:
The physical properties of a propellant must be known in detail in order to design a propulsion system. For instance, the density and viscosity of the propellant will have a major impact on the feed system, whereas the heat of formation and the chemical composition will determine the specific impulse and the combustion temperature and thus influence the design of the combustion chamber, cooling cycle and the nozzle. Toxic and/or explosive properties will influence how the propellant can be handled and how the thruster testing can be performed. Depending on the propellant, the data are either already available from the literature (e.g. for commercially available propellants) or need to be evaluated (for new or scarcely characterised propellants). The GRASP team will identify the most urgent need of the various propellants to be investigated.
For some propellants already a solid experience base has been established in literature and propulsion systems may already exist (on laboratory or even space proven level). However, performance improvements need to be realised and system issues need to be investigated further. Other possible green propellants (e.g. ionic liquids such as HAN and ADN derivatives) have been investigated in the past but still require significant research to identify their characteristics and performance. Propulsion systems for such propellants exist in general only at laboratory level. In some cases a propellant manufacturing procedure will be developed, taking safety issues into account. Some objectives for those propellants are:
1. Formulation and preparation of ionic liquid propellants (ADN and derivatives, HAN etc.)
2. Characterisation of propellants (Hf, Hs, Hm, Cp, Mp, burn rate, rheology, freesing, etc.)
3. Explosive properties (card gap, detonation, fire, heat)
4. Experimental evaluation of the thermal and catalytic decomposition
5. Determination of reaction balance
6. Perform transportation hazard classification.
A third category of propellants could be labelled 'future' green propellants such as new derivatives of ADN based or other ionic liquids and silanes. These propellants offer theoretically significant performance improvements but have not been investigated to an extent where such performance improvements have been experimentally proven. The general tasks with regard to such propellants are:
1. development of a propellant formulation
2. definition of manufacturing procedures
3. propellant characterisation (properties)
4. performance evaluation (theoretical and experimental).
Before designing a propulsion system, the compatibility of all construction materials that will be in contact with the propellant must be tested. This includes materials used in the tanks, valves, seals, piping, etc. This is to prevent possible violent reactions that might lead to major failure of the propulsion system, or degradation that might lead to reduced lifetime of the propellant, or to prevent corrosion that might lead to valve failure or blocking of small orifices. A significant amount of data for well-known materials and propellants can be found in the literature. However in the case of new propellants, for example HAN or ADN-based monopropellants, GRASP will conduct compatibility tests if such data is not yet available.
Catalysis:
As in the case of hydrazine, most of the green propellants (mainly the oxidisers) require some means to initiate the chemical reaction to release the energy contained in the chemical bonds. This process is in general called catalysis. Numerous methods exist to decompose the propellants, including several types of structural support for the catalyst (foams, pellets and advanced monolithic structures), as well as an extensive number of catalyst materials.
One of the most important goals of this project is to identify the most appropriate catalyst for the investigated propellants, including performance as well as system issues. The investigation with regard to the catalyst has two levels. The first is the development/adaptation of existing catalyst technologies (both heterogeneous and homogeneous) for hydrogen peroxide according to the requirements identified in the preliminary assessment (such as mass flow rate, decomposition temperature etc.). The second level is the development of new catalysts for ionic liquids (HAN, ADN etc.). General tasks for both components with regard to the catalyst investigation include:
1. Definition of catalyst type
2. Definition of the type and shape of the catalyst carrier (foam, pellets, ceramic monoliths)
3. Characterisation of the manufactured catalyst by X-ray diffraction, scanned electron microscopy (SEM), surface area evaluation and other appropriate methods before and after testing
4. Evaluation of the catalytic reaction in batch reactors, etc.
5. Numerical treatment of the decomposition processes and support for catalyst design
Propulsion systems:
Although the above are essential elements in the GRASP effort, the final goal of GRASP is implementation of the green propellants into a propulsion system. In general two kinds of propulsion systems with different mission applicability will be investigated; a low thrust system (1 to 10 N) and a high thrust system (higher than 100 N).
General replacement of 1-10 N class thruster (e.g. Hydrazine systems) for attitude control.
This thruster class is interesting because is constitutes one of the largest market share in the propulsion business. Due to the relative large number of manufactured thruster/per year this thruster class promises the highest potential for cost savings for the industry due to the replacement of toxic propellants.
Thruster in the higher than 100 N class
This thruster class has a similar interesting market share (although smaller than the 1 to 10 N class thruster) for roll and pitch control of larger satellites. Furthermore, this thrust range is interesting for descending stages for planetary missions on Mars etc. This again is a classical mission applicability for green propellants, e.g. hydrogen peroxide, since hydrazine thrusters could contaminate the environment with nitrogen (mainly in the form of ammonia) and therefore endanger the capability to detect traces of life.
The propulsion system development will occur in a two-step fashion. Initial testing will be conducted in so-called elegant bread board (EBB) propulsion systems The tested EBB units will include low thrust and high thrust units. The EBB units will investigate the suitability of the chosen propellants/catalyst on a propulsion system level and shall provide first indication of thrust range, throttling capability, specific impulse, lifetime assessment etc.
In a second step and following a final down-selection, advanced versions of the EBB so-called Demonstration units (DM) will be developed. Other than in the case of the EBBs, the DMs might exhibit a reduced diagnostic access or modularity compared to the EBB. However, those systems which have been assessed as suitable for further development to a DM should focus less on diagnostic and modularity but aspire to provide a performance assessment with a higher confidence level than possible with the EBB version. For example, an extended investigation of lifetime issues is expected as well as an improved thruster performance assessment.
The consortium was assembled such that all the necessary competence in the three fields is available. Each participant in a particular field has competences which others may lack and therefore the consortium as a whole is complementary. This present configuration not only ensures a very extensive coverage of the green propellant field of activity, but also provides competence in every step from basic research to a product development.
Project results:
GRASP aspired to investigate alternative propellants with regard to their suitability to replace presently used highly toxic propellants. By investigating a large number of possible candidates through theoretical as well as experimental efforts, GRASP aspired to identify the most suitable ones for a near term replacement of e.g. hydrazine etc. In order to achieve those goals GRASP has to:
1. develop a process to collect, manage and analysis an extensive amount of data such that conclusive decision for down-selection of more than 100 green propellant candidates can be made (see assessment methodology in the following)
2. develop new and advanced propellants and appropriate catalysts
3. develop propulsion systems suitable for the selected propellants.
The following paragraphs will address the achievements of each of those three fields.
Assessment methodology
The investigation of green propellants and their suitability for replacing presently used, highly toxic propellants is a complex matter due to the extensive decisive parameters deciding its suitability. To comprehensively investigate the suitability of only one substance/propellant is a major task; to conduct such a comprehensive study for a large number of potential green propellants candidates is unrealistic. Therefore one has to conceive an assessment methodology which allows an effective and manageable way to deal with a large number of data and a system of criteria hierarchised in a proper way.
The challenge in this work is the definition of the assessment criteria and their prioritisation respectively. If unwisely chosen, a criterion in an early assessment might exclude a very promising propellant. On the other hand, another propellant might be allowed to proceed further which, in the end, proves to be a poor propellant. While the former is difficult to look at in hindsight, the quality of the assessments will be judged by the propellants finally chosen.
The GRASP team therefore established a system of assessments, each with a range of criteria, which were designed such that unrealistic candidates are identified very early in the process and only more promising candidates were allowed to proceed further in the project.
In effect the GRASP project was guided by this series of assessments conducted at frequent intervals. Each assessment was based on different criteria or properties which were subject of the investigation efforts prior to the establishment of the assessment.
The first assessment, called the general assessment, had two major features:
1. For all substances which were considered green propellant candidates, all available data and background information relevant to its use as propellant have been collected (literature review). As a result, GRASP has provided the community with a unique data collection about green propellants (those information can be extracted from the GRASP project webpage).
2. A first down-selection was conducted based on toxicity, performance, storability and TRL. Based on the parameters from above and also considering availability issues (ITAR etc.), the GRASP consortium identified as a result of the first assessment 28 propellants (roughly one fourth of the original candidates) as promising green propellant candidates to be included in the upcoming assessments.
Following the first assessment which was mostly based on theoretical calculations and literature reviews, the subsequent assessment utilised the full potential of the versatile GRASP consortium. In total the GRASP consortium has roughly 30 different facilities available, allowing to measure a large range of properties of propellants and catalysts. At this stage, application relevant properties such as decomposition efficiencies, ignitability, ignition delays and such became the driving selection criteria.
Again several propellants were found more advantages than others. However, it has to be stated that some of the propellant candidates were not rejected but rather put aside for the time being. This was done not because they might be poor propellants but because of issues such as poor or non-existing availability of the propellant itself (e.g. HNF, higher silanes) or because the GRASP consortium did not have the facility to investigate them further. In total, the second assessment identified seven propellants as subjects for the Selection (third) and Final (fourth) assessment.
The last two assessments (Selection and Final assessment) were largely focused on the propulsive performance (combustion efficiencies, specific impulse and, if possible, minimum impulse bits) of the propellants and the developed propulsion systems. As this stage of the work the assessments were less a down selection of propellants but rather an investigation and verification of their propulsive performances.
In the end, i.e. the Final assessment, two monopropellants (FLP-106, H2O2), one oxidiser (H2O2), three fuels for bipropellant systems (kerosene, turpentine, d-limone) and one fuel for hybrid systems (HDPE) have been selected.
All of those propellants have a much lower toxicity, are non-carcinogenic, have acceptable or, as in the case of FLP-106, better performance than the toxic alternatives, are available and can be considered to have obtained a TRL of three to four during GRASP. This and their successful testing show the quality of the assessment methodology chosen by GRASP (see also comment above) and therefore this methodology might be considered as a standard for similar future efforts.
Propellant and catalyst development and analysis
The first down-selection of propellant candidates was based purely on performance calculation and on relevant data (toxicity etc.). This resulted in 28 substances which:
1. have significantly reduced toxicity
2. are storable liquids at standard conditions
3. have performances comparable with the one of the toxic alternatives.
In the second assessment additional candidates were put aside due to issues of availability (e.g. the only manufacture of HNF in Europe ceased its operation) or safety issues (recent information about HAN showed rather serious safety issues).
All propellants have been investigated experimentally during the GRASP project. The nature of the assessments differs very much depending on the propellants itself (e.g. background, history) and their possible applications (mono-, bipropellant, hybrid). In the following some of the major outcomes and achievements are summarised according to their envisioned application, i.e. mono-, bipropellant or hybrid systems.
Efforts related to monopropellants
Several green monopropellants we identified by GRASP with the potential to replace the toxic standard monopropellant hydrazine. However, as mentioned above, only two have managed to be included in the experimental efforts of GRASP, namely hydrogen peroxide and the ADN based propellant FLP-106. Most of the alternatives, although very promising candidates, have open questions with regard to safety issues and availability.
Hydrogen peroxide was also identified as oxidiser for bipropellant systems. The majority of the efforts related to hydrogen peroxide will be reported in the bipropellant section. Only the thruster development work will be summarised in this chapter.
FLP-106 is an ammonium dinitramide based propellant, similar to the LPP-103S which is was used on the recent (2010) maiden flight of a green propellant based thruster system on-board of the PRISMA mission. However, FLP-106 offers a slightly better specific impulse and impulse density. Albeit some information about FLP-106 was already published by FOI prior to GRASP, many open questions remained. GRASP has tackled some of them including. The efforts with regard to FLP-106 and other ionic liquids based on FLP-106 were spearheaded by FOI. Significant effort was contributed by LACCO, DLR and Fotec.
Propellant characterisation:
Motivation: FLP-106 is a rather new propellant and not all properties relevant to its potential use as a propellant are known yet.
Achievements: Various, afore GRASP unknown properties of FLP-106 have been evaluated and measured. This includes heat of melting and heat of solution of ADN and heat of solution of the fuel. The knowledge of those properties allows in the future a better assessment of the propellant.
Safety and handling characterisation:
Motivation: establishment of a comprehensive manual in order to allow a safe handling of FLP-106
Achievements: All relevant information for safe storage and handling were compiled. Extensive card gap tests conducted during GRASP indicated that FLP-106 should not be considered as a hazard class 1.1 material. The manufacturing manual allowed two more beneficiaries to initiate manufacturing of FLP-106 on their premises.
Development of catalyst with cold-start capability:
Motivation: While present catalytic systems for ADN based propellants are mainly based on thermal decomposition a purely catalytic one limits the need for internal heaters and reduces the complexity and power consumption of the thruster system.
Achievements: GRASP developed catalysts which showed in laboratory test the capability to reduce the on-set temperature for ADN decomposition from around 155 °C to 130 °C. Albeit this is not yet a cold start capability it showed the general feasibility of this notion and offered guidance for future improvements. The tests were also repeated for FPL-106, i.e. ADN+ H2O+Fuel. Again, in the presence of a catalyst the decomposition onset was significantly reduced (60 °C).
Material compatibility tests:
Motivation: To avoid pre-mature decomposition of the propellant (e.g. in the tank) or contamination of the propellant originating from the wetted materials itself it is mandatory to investigate relevant materials upon their compatibility to FLP-106.
Achievements: GRASP accomplished to test and verify the material compatibility of a significant number of metals and polymers which are standard construction materials in aerospace industry.
COTS testing:
Motivation: Utilisation of so-called commercial of-the-shelf components, i.e. flight qualified components or subsystems in order to show compatibility with FLP-106.
Achievements: A semi-automatic test stand for valve endurance testing was designed and built. Initially testing of a flight representative valve was successful and confirmed the facility design.
Impact of fuel variations and additives:
Motivation: Assessment of various methods to further improve FLP-106
Achievements: radical promoters as additive to the fuel and variation of the standard FLP-106 fuel were tested with regard to their impact on decomposition onset temperature and performance. While the radical promoters did not succeed in lowering the decomposition onset temperature several of the tested fuels show clear indication of further performance improvements.
Dynamic flow reactor test:
Motivation: batch reactor tests provide essential information about decomposition behaviour such as onset temperature etc., but essentially they provide only a snap-shot with regard to the situation one would find in a thruster like situation. A more realistic set-up such as a dynamic flow reactor is required to assess processes occurring in a thruster type environment.
Achievements: a dynamic flow reactor simulating a thruster was designed and tested. While the test facility failed to provide any data from stationary testing due to a structural failure of the flow bed itself, the tests itself provided essential knowledge about decomposition onset and required injector properties. As such, those tests and the results will guide any future testing.
Hydrogen peroxide was the subject of intense research within GRASP. Two 20 N monopropellant systems based on hydrogen peroxide were developed and tested within GRASP by two different entities using two different approaches for the decomposition of the propellant. In addition, a 300 N bipropellant thruster investigated a dual mode insofar that the thruster was operated with only hydrogen peroxide prior to injection of the fuel. In this monopropellant mode, thrust in the range of 80 N was generated.
The afore mentioned two 20 N monopropellant systems were operated successfully and allowed a comprehensive study of the catalytic activity and its impact on the thruster performances. Decomposition efficiencies were found to be on average around 95 % and c* efficiencies of the thruster systems as high as 95 % were reported.
As mentioned above, with the exception of the above, the hydrogen peroxide related research will be discussed in following bipropellant chapter.
Efforts related to bipropellants
Seven fuels and one oxidiser for bipropellant systems were included in the experimental efforts following the second assessment. Hydrogen peroxide was identified as the most promising oxidiser. The alternatives, namely oxygen and nitrous oxide were discarded due to the fact that they are not storable liquids (at standard conditions). Oxygen is a cryogenic and nitrous oxide is gaseous at standard conditions and liquefies only at about 52 bar. Although such a property might be favourable (self-pressurising), from the beginning of the project it was decided to focus only on storable liquid propellants within GRASP. Furthermore, the option of using oxidisers based on ionic liquids (HAN, ADN, etc.) for bipropellant systems was put aside due to the very low TRL of this technology.
During the project several fuels were dropped. It has to be pointed out that none of them has been discarded because they are not promising green propellants with regard to their predicted performance or toxicity. In some cases they were put aside due to some open questions with regard to ITAR or patent issues (e.g. tetramethylethylenediamine). Other reasons were potential storage problems such as the formation of organic peroxides during storage (Isopropyl alcohol (or 2-propanol), dibutyl ether, dimethyl ether, propyl ether) or just simply the fact that they were not available or extremely expensive such as in the case of silanes.
The remaining fuels have a very diverse background with regard to their general and space related use. For example, while kerosene and ethanol have been investigated as propellants and e.g. kerosene derivatives are presently used as fuels for bipropellant systems, for others such as dipentene or turpentine, only marginal and outdated information, if any at all, about their propulsive use are available.
In the following some of the efforts with regard to the fuels are summarised:
Ignition tests:
In spite of the diverse background of the fuels, the GRASP consortium decided that with regard to the suitability as a fuel in a bipropellant system the ignition behaviour is at this stage of the project one of the most relevant property. Two different experimental studies using different methods and facilities were therefore initiated. It is important to note that both entities (Deltacat, Fotec) investigate the ignitability of fuels with regard to the use of hydrogen peroxide as oxidiser - in other words, the ignitability as a function of temperature was to be investigated. Both entities established new facilities to investigate the ignitability of various fuels. Deltacat investigated the ignition delay of all seven different fuels in the same facility. This allowed a very good relative comparison of the ignitability of those seven fuels. As to be expected all fuels show decreasing ignition delay for increasing temperatures. Most interesting outcome is the one related to the two more uncommon fuels, dipentene and d-limone. Those two propellants have one of the lowest ignition delays over a large part of the investigated range of temperatures.
Fotec investigated the ignition properties of kerosene and ethanol. With regard to those two propellants, Fotec's result confirm the one from DELTACT, namely that ethanol is more difficult to ignite than kerosene (JetA-1). As a matter of fact, auto-ignition tests with Kerosene as fuel indicated reliable and repeatable ignition at pre-ignition pressures with lowest values in the range from four to five bar. The corresponding necessary auto-ignition temperatures were in the range 400 - 450 °C, which provides sufficient margin to theoretical decomposition temperatures (690°C). Ethanol on the other hand required much higher pre-ignition chamber pressures and temperatures. The lowest pre-ignition pressure, where auto-ignition was obtained was 15bar. The corresponding pre-ignition temperature was 600 °C.
Based on the result from the ignition tests, kerosene, d-limonene, dipentene and turpentine were chosen as the fuel baselines for subsequent implementation into propulsion modules.
Oxidiser
As mentioned before, the baseline oxidiser for GRASP is hydrogen peroxide in a concentration of 87.5% (by wt%). Hydrogen peroxide is manufactured by Evonik Industry AG under the brand name Propulse. Propulse has a standardised composition of stabilisers. Although substantial research with hydrogen peroxide was conducted in the past, many open questions remained. Most important, considering its planned utilisation as propellant, is its effective decomposition. In other words, the identification of the most suitable catalyst for the individual application (i.e. thrust level) is one of the most important tasks. Due to the importance of this, GRASP dedicated a significant part of its effort to this topic. Development of new catalyst types and their experimental assessment were conducted in parallel to a substantial effort in developing new numerical tools for an improved understanding of the catalytic processes and a better prediction of catalyst performances.
Other efforts with regard to hydrogen peroxide included storage testing and stabiliser variation investigation.
Catalyst development
Fifty five catalyst variations in terms of nature, shape and material of the catalyst carrier, active catalyst, manufacturing method variations and others were manufactured and tested. The test matrix consisted of variations of the catalyst carrier material (various ceramics and metal), as well as the nature of the catalyst carrier (monoliths, pellets, gauzes, foams). Within those variations, further variations were with regard to their geometry, the nature of active catalysts and last but not least the manufacturing processes itself were varied. In total more than 250 catalysts were experimentally assessed. The amount of variations allowed what can be considered the most comprehensive assessment of catalyst properties available up to now in the research community.
Other than Deltacat and Fotec, SOTON and UN developed their own in-house capability to manufacture catalysts. UN investigated two different manganese-based systems for the decomposition of highly concentrated solutions of hydrogen peroxide:
1. manganese-yttrium-zirconium mixed oxide nano-composites, obtained through a sol-gel route and deposited on a yttria-stabilised zirconia monolithic support
2. manganese-zirconia catalysts, obtained by direct deposition of a proper precursor of the active phase on the monolithic carrier.
It has to be noted that in particular the second method is of high interest since it circumvents the need for applying a washcoating and therefore would reduce the complexity of the manufacturing process. In the experimental assessment the direct deposition was found to be superior to the sol-gel catalyst. Although even the direct deposition ones did not reach their design performance they were successfully used for the subsequent thruster tests. In conclusion, both technologies are supposed to be very promising but will require more time to reach maturity.
In terms of performances of the manufactured catalysts it is to be pointed out that a significant number of catalysts developed during GRASP have shown major improvements compared to existing ones. Based on the knowledge gained up to now, they are fully suitable for the respective tasks in propulsion systems. Decomposition efficiencies above 95% were obtained by several catalyst types. Prior to GRASP, most existing catalysts suffered from insufficient cold-start capabilities and long transition times (tens and longer). Such a situation reduces their attractiveness for application in propulsion systems. During GRASP, catalysts have been developed with transition times lower than one sec and very reliable cold-start capabilities.
Equally important as the cold start capability is the catalyst lifetime, i.e. maximum total hydrogen peroxide through put. For the medium thrust range (100-300 N), Deltacat and SOTON succeeded in developing a catalyst with a proven lifetime of 38 kg of total through put. At the end of their test campaign, the performance of the catalyst was nearly unchanged. For the low thrust range (1 N), Fotec succeeded in showing catalyst lifetimes of roughly 18 kg total through put which corresponds to roughly 1/3 of the total lifetime of a standard 1 N hydrazine based propulsion system (e.g. EADS CHT1). For future efforts it is important to repeat such tests in order to obtain a statistical relevant number of data points and in order to investigate manufacturing variations, if any and their impact on the lifetime assessment. During GRASP the number of catalysts tested with regard to their lifetime was limited mainly due to limited time.
In this context it is important to note the development of a rapid catalyst lifetime assessment facility by SOTON and Deltacat. With this facility it is possible to obtain a preliminary assessment of the catalyst lifetime in a comparable very short time and using only a fraction of the propellant required in a standard lifetime assessment. Those results have yet to be verified by comparison with lifetime assessments conducted in standard fashion.
Analytical/numerical decomposition model development:
As mentioned above, catalysis of hydrogen peroxide is highly complex and not yet fully understood. The development of analytical and numerical models was therefore an essential element in the development of advanced catalysts.
The models developed during GRASP incorporate a much higher level of detail than any of the models suggested in the past. This allowed investigation of the decomposition and the parameters which influence the decomposition in much greater detail and allows more accurate prediction.
In a first step several models were established, each focusing on different aspects such as the pressure conditions and pressure drops across the catalyst bed or thermal conditions within the catalyst bed and decomposition chamber. In a second step the models were verified by means of the data obtained in the framework of the experimental catalyst assessment. Several significant improvements of the previous understanding of the decomposition process are the result of this effort. For example, the very strong impact of the thermal properties of the decomposition chamber has been predicted by the models and verified by experimental testing. As an example in the context of small mass flow rates, the thermal mass of the catalyst itself but also the thermal mass of the surrounding structure can significantly impact the decomposition efficiency in the initial phase. The model shows such an accuracy that the influence on the decomposition caused by relatively small effects such as variations of the thermal mass of the catalyst can be predicted. The accuracy of the prediction has been verified by tests. The design of some of the catalysts manufactured in the 2nd and 3rd period of GRASP is based on predictions made by the models. The fact that those catalysts have shown significant improvements is a clear sign for the validity of the models. Furthermore, the models succeeded in shedding some light into unexplained observations during past decomposition testing, namely, decomposition efficiencies above 100% or melting of the catalysts core even though maximum decomposition temperatures should have been far below the melting temperature of the catalyst material. The explanation of both effects is linked to a pre-heating of the hydrogen peroxide due to thermal conduction from the catalyst hot region to its colder upstream region. This effect was predicted by the numerical tools and subsequently confirmed in experiments.
Similar to the above, the models predicted a radial temperature distribution across a single channel in the catalyst and across the catalyst itself. Measurement of the former was not possible due to limited diagnostic access into the small channels of a catalysts but the latter was experimentally verified. This has significant impact since indicates that published results have to be re-visited and future test set-ups have to be adapted in order to provide reliable results.
In conclusion, the developed models:
1. have succeeded in increasing our understanding of the hydrogen peroxide decomposition processes in general
2. successfully guided the catalyst design process
3. explained afore unexplainable observations
4. succeeded in making predictions which have been confirmed later in experiments.
Storage investigation
The high potential of hydrogen peroxide as a monopropellant as well as an oxidiser is undoubted. In the past, test conducted by various research groups have indicated also suitable storage properties. It further has to be noted that hydrogen peroxide is one of the not so many green propellants with flight heritage (SynCom II, Early Bird). However, there is a need to verify the storability with technologies available nowadays in Europe (both in terms of materials and also of hydrogen peroxide qualities).
Consequently, SOTON and Fotec kept several samples of hydrogen peroxide in storage for a large part of the project. In addition to the standard propulse, Evonik also manufactured samples with variations of the standard stabiliser cocktail including increased phosphate content, increased acid content and increased stannate content. All samples show in time small weight variations (indicative for changes in concentration) which are within the measurement accuracy. This would allow the conclusion that within the test duration (up to 28 months) and storage conditions (Al transport container from Evonik and storage temperatures of 3-5°C) hydrogen peroxide is fully storable. However, some of the samples show a weight increase instead of a decrease which one would see if any decomposition took place. The reasons for that are unclear at this time. The samples are kept and will be observed even after the GRASP project is finalised.
Two unwanted potential effects of increasing the stability of hydrogen peroxide are a decrease of decomposition efficiencies and, in the worst case, a poisoning of the catalyst. Consequently, the hydrogen peroxide samples with the above mentioned stabiliser variations were tested with various catalysts. An indeed, while the increased phosphate content lead to a significant catalyst performance decrease for all tested catalysts, the increased acid content as well as increased stannate content did neither lower the decomposition efficiency nor, after 1 kg of total through put, harm the catalyst in any detectable way.
In conclusion, the storage tests show promising results, insofar that all four samples (standard and three stabiliser cocktail variations) show only concentration changes within the measurement accuracy (around 0.2 %). However, the results are somewhat tainted due to the fact that some samples showed an unexpected weight increase instead of the expected weight decrease. Pending the impact those stabilisers have on the stability of the hydrogen peroxide, in terms of compatibility with various catalyst, the tests clearly show that phosphates can have detrimental impact on the catalytic activity while increased amount of acids and stannates did not show any harmful effect.
With regard to storage capability and general handling safety, it has to be pointed out, that over a period of 2 ½ years in the GRASP period not a single accident occurred with hydrogen peroxide such as blow outs or run-away decompositions. This in spite of the fact that the work was conducted in numerous different facilities and working environments. On the one hand this is to be credited to all the beneficiaries who worked with hydrogen peroxide but also largely due to hydrogen peroxide itself which in spite of being a high energy content oxidant is relative simple to work with.
Efforts related to hybrids
Based on the first Assessment HDPE and hydrogen peroxide were selected for the hybrid system. No dedicated efforts with regard to HDPE have been conducted within GRASP.
Design of a methane based system
In a deviation of the general requirement for storable propellants but reasoned with the extreme potential and interest from propulsion system developer, GRASP also included methane and LOX as propellants. Initial numerical efforts focused on simulation of injection system and the thermal conditions within the combustion chamber. Results indicated that the combustion efficiency is strongly influenced and improved by introducing a swirled flow in the injector inner channel. Higher combustion efficiency can be also obtained for longer combustion chambers, but this influence is weak and does not balance the benefits of a shorter combustion chamber such as lower weight and better cooling efficiency.
According to numerical simulations of the combustion analyses of all cases it was shown that the best configuration of methane/oxygen gaseous thruster is for pintle injector type and for coaxial with inner swirl.
In a second step, a more detailed design study of the engine cycle was initiated. A comparison of regenerative cooling, radiative cooling and film cooling was conducted and its benefits and draw backs were identified.
It has to be noted that all GRASP activities with regard to methane and LOX have been of only theoretically nature both with regard to the propellants itself and the propulsion system based on this propellant combination.
Propulsion System Development
Within the GRASP project the final step in the propellant selection process was the test in a propulsion thruster test bed. As a matter of fact, this step itself was subdivided into two steps.
In a first step the eight propellants (identified in the second assessment, 'Preliminary assessment') were tested in the following 10 so-called Elegant Bread Board (EBB) units (note that in the following a similar or even identical thruster hardware which is used to test two different propellant (or propellant combination) is considered to be two units):
1. 1 N bipropellant propulsion system (hydrogen peroxide/ethanol)
2. 1 N bipropellant propulsion system (hydrogen peroxide/kerosene)
3. 20 N monopropellant propulsion system (FLP-106 (ADN))
4. 20 N monopropellant propulsion system (hydrogen peroxide)
5. 300 N bipropellant propulsion system (hydrogen peroxide/Kerosene)
6. 300 N bipropellant propulsion system (hydrogen peroxide/dipentene)
7. 300 N bipropellant propulsion system (hydrogen peroxide/D-limone)
8. 300 N bipropellant propulsion system (hydrogen peroxide/turpentine)
9. 20 N monopropellant propulsion system (hydrogen peroxide)
10. 200 N hybrid propulsion system (hydrogen peroxide/HDPE).
Testing of those EBBs had two major objectives:
1. a first assessment of their feasability (e.g. can auto ignition of various fuels in combination with decomposed hydrogen peroxide be achieved) and performance and
2. provide data for a further propellant selection to be implemented in advanced EBBs so called Demonstration units (DM).
Testing of the EBB units provided in all cases first a large amount of performance data and information to assess the propellants and the associated propulsion units. The only exception of this was the unit for FLP-106 testing. A premature and uncontrolled decomposition of the propellant due resulted in a damage of the test unit not permitting further testing.
Based on those EBB test results and according to the workplan, a further down-selection of propellants and associated propulsion units was conducted. Three units have been chosen for a further design and final testing, namely 300 N bipropellant (hydrogen peroxide/dipentene/turpentine); 200 N hybrid (hydrogen peroxide/HDPE); 1 N bipropellant (hydrogen peroxide/kerosene).
Using the benefit of the experience gained in the EBB testing campaign the units were re-designed and upgraded respectively. Subsequent testing of those three systems can be claimed to be a full success. To name only some improvements: DC and SOTON developed and implemented a new type of injector and nozzle, both improved significantly the thruster performance. UN improved their EBB design by using an advanced catalyst bed and nozzle design. FT implemented a new engine cycle (radiation cooling) and new materials for the combustion chamber (Pt-Rd).
Testing of those units allowed a confirmation of the selection process. All tested DM systems allowed evaluation of propellant characteristics (e.g. ignition behaviour, combustion stability) and thruster performance (c* efficiencies and specific impulse). Equally important the testing provided detailed information about the way forward for those systems and the selected propellants respectively.
Potential impact:
The GRASP project was from the beginning conceived to tackle a large range of issues. Not only that it included initially more than 100 propellants but it also covered a large number of topics ranging from the investigation of basic propellant properties to propulsion system tests.
A significant number of conference papers and some peer-reviewed publications is a clear sign of the scale and quantity of the scientific achievements of GRASP.
Presently no technologies which were developed during GRASP have been patented yet. It is in the belief of the project coordinator that some GRASP technologies however are very close to a maturity level at which patenting them is a real option.
GRASP has achieved more than submitting a number of publications and obtaining patents. It has shifted the topic of green propellants into the focus of the community and industry. It has established an awareness of essential topics such as propellant toxicity, hazardous levels and, most important, has shown the alternatives to the presently toxic propellants. But GRASP has not only pointed out the alternatives but has also, step by step, developed the necessary tools and technologies to use some of those alternatives and finally verified the feasibility and performance with dedicated propulsion hardware.
The GRASP project coincided with the decision of REACH in 2011 to put hydrazine on a list of substance of high concern which might lead to a phasing out of this essential propellant in Europe as early as 2017 - exactly the situation GRASP has from the beginning pointed out as a real possibility. Considering the fact that hydrazine is one of the pillars of space flights, this decision is very critical for the European space community. GRASP could not have come to a better point in time. Knowledge and access to it is critical at this time. It is needless to say that the interest in green propellants has sky rocketed since the announcement of REACH. And GRASP is helping by accepting invitations from companies, space agencies and universities for seminars and general information dissemination. Also as a direct result of the efforts of GRASP, several projects including GRASP beneficiaries related to green propellants are in preparation or have already been submitted to the European Commission or the European Space Agency.
At this point in time, GRASP and the results from it might very well be the incubator for a very rapid development of real alternatives to the presently utilised toxic propellants.
List of websites:
http://www.grasp-fp7.eu
Over the last 30 years, chemical propulsion for space application relied nearly fully on propellants which are highly toxic and in most cases carcinogenic. This included fuels such as hydrazine, monomethylhydrazine (MMH), unsymmetrical dimethylhydrazine (UDMH) and oxidisers such as mixed oxides of nitrogen (e.g. MON-3) and nitrogen tetroxide (NTO). Although those propellants have very good performance and in particular have an extensive flight history, their handling requires complex and costly precautions to protect human operators and the environment.
In 2007/8 a consortium consisting of 12 entities was established and the project GRASP was initiated. The GRASP consortium set out to identify possible alternatives to those toxic propellants. However, better protection of the human operators and environment as well as the increasing costs due to the toxicity of those propellants were not the only reasons for this quest. In view of the fact that over the last three decades the limitations in handling e.g. of hydrazine has become more and more restrictive, the GRASP team pointed out the possibility that the use of such propellants might become impossible or exceedingly expensive in the near future. Unfortunately for the relevant industry, this hypothesis of GRASP became reality with the recent action (June, 2011) of the European Chemical Agency (ECHA) in the framework of Reach, to put hydrazine on a watch list, potentially, outlawing the use of hydrazine in the near future.
Although worldwide different research groups have been working on the development and investigation of green propellants, the relevant industry and also space agencies were rather reluctant to participate in such efforts in spite of the above mentioned thread. Contributing to this hesitation is the lack of comprehensive and unbiased information about green propellants. The GRASP effort was designed to provide such information so that decision makers from industry or space agencies can re-evaluate their future strategy in respect to green propellants.
Starting with more than 100 green propellant candidates, the GRASP team down-selected a small number of propellants, which are very likely to be realistic candidates for a replacement of hydrazine and other toxic propellants. This was achieved by an intensive research and development (R&D) effort combining numerical/analytical as well as experimental expertise. Following the three years of GRASP, the relevant industry is now in a much better position to judge the suitability of the green propellant concept. This again puts it in a much better position to utilize the new knowledge and implement green propellants in their near term strategy. In view of the above mentioned action from REACH the input from GRASP could not have come to a better point in time.
Project context and objectives:
The GRASP project aimed to provide the European industry with alternative propellants to replace the currently used highly toxic and carcinogenic propellants (e.g. hydrazine, MMH, UDMH). These alternative, so-called, green propellants will reduce the potential harm to human operators and the environment and thereby significantly reduce the associated handling costs. Even more important, in view of the recent action of ECHA which might outlaw the use of hydrazine, the availability of green propellant technology might ensure the competitiveness of the European space industry.
The search for such a propellant is driven by four essential industrial needs:
1. Ensuring the competitiveness of the European industry in a challenging and continuously changing environment
2. Reduction of costs
3. Reduction in exposure to toxic and/or carcinogenic substances
4. Performance improvements.
Laboratory-scale evaluations in the past, conducted worldwide have indicated that several green propellant candidates can indeed provide a performance comparable or even better than those of presently used propellants. In addition, various studies have shown that the use of green propellants can provide long-term financial benefits. However, industry as well as space agencies have shown in public little interest in green propellant technology and a broad implementation of low toxicity propellants seemed still far from reality. There are many reasons for this reluctance but the main ones are:
1. The extensive experience the space community/industry has collected with the presently used toxic propellants
2. The satisfactory performance of the presently used propellants
3. The costs associated with the development and qualification of new hardware (propulsion, valves, tanks etc.)
4. The costs associated with the replacement of existing infrastructure such as test facilities, filling station at launch pads and many others
These are very valid reasons for entities which operating in a very conservative environment. On the other hand, the GRASP consortium and fellow researchers pointed out the historically proven tendency in the last 40 years towards more and more restrictive national and international laws with respect to the use of such dangerous substances. GRASP also pointed out that in the long term, an implementation of propellants with lower toxicity might reduce the total costs (in spite of the initial, probably substantial, investment) and ensure the competitiveness in a more stringent legal environment. In order to convince the space community/industry to switch to a particular green propellant, the following issues, amongst others, must be demonstrated:
1. Similar or better performance than those propellants used currently
2. System advantages
3. Detailed investigation of the storage and handling properties
4. Compatibility with existing hardware
5. Likelihood of reduced costs in the long-term.
The goal of the GRASP project is not to provide incremental progress but to conduct a systematic down-selection to identify the most promising green propellant candidate(s) and implement those candidates in demonstrator propulsion systems.
Besides the development of propulsion systems, another more general objective of GRASP was the development of a standard for green propellant technologies, to be achieved by collecting and generating sufficient data and information about a large number of green propellant candidates. This will include a comprehensive database of the physical characteristics of the candidate propellants, as well as theoretical performance, availability, stability, material compatibility and any other pertinent property.
The above was to be achieved by means of a down-selection of green propellant candidates. Several major assessments will function as filters during this process. An initial general assessment, including a comprehensive survey of green propellant technologies, will provide a first crude filter to identify the candidate propellants (and catalysts) that will be experimentally investigated in the following stage of the project. Besides being a general survey, this assessment will also address questions with regard to where green propellants can compete with current propellants, as well as definitions of minimum propellant requirements (specific impulse, toxicity, physical properties, etc.) and system requirements. Depending on the propellants selected, the data will be collected either by making a literature survey (for well-known propellants) or by measuring their properties (for new or scarcely characterised propellants).
Following this initial assessment, an extensive experimental study of the selected green propellant technologies was to be initiated. The nature of this investigation depends on the propellant and will have two main parts. For those propellants for which past experience exists (e.g. hydrogen peroxide), an investigation of catalysis will be conducted with the goal of optimising the performance (decomposition rates and temperatures, etc.). In parallel with this effort, new propellants will be manufactured and their characteristics will be evaluated. This experimental effort will provide a unique and comprehensive database allowing a further selection process for the next stage. Such a database will establish a standard for green propellants, which is urgently needed as a reference not only for this particular project but for all green propellant related research in the future.
The GRASP effort has three primary fields to be investigated: propellants, catalysis and propulsion systems. For a successful implementation of green propellants substantial advancements in all three fields are needed as described in the following.
Propellants:
The physical properties of a propellant must be known in detail in order to design a propulsion system. For instance, the density and viscosity of the propellant will have a major impact on the feed system, whereas the heat of formation and the chemical composition will determine the specific impulse and the combustion temperature and thus influence the design of the combustion chamber, cooling cycle and the nozzle. Toxic and/or explosive properties will influence how the propellant can be handled and how the thruster testing can be performed. Depending on the propellant, the data are either already available from the literature (e.g. for commercially available propellants) or need to be evaluated (for new or scarcely characterised propellants). The GRASP team will identify the most urgent need of the various propellants to be investigated.
For some propellants already a solid experience base has been established in literature and propulsion systems may already exist (on laboratory or even space proven level). However, performance improvements need to be realised and system issues need to be investigated further. Other possible green propellants (e.g. ionic liquids such as HAN and ADN derivatives) have been investigated in the past but still require significant research to identify their characteristics and performance. Propulsion systems for such propellants exist in general only at laboratory level. In some cases a propellant manufacturing procedure will be developed, taking safety issues into account. Some objectives for those propellants are:
1. Formulation and preparation of ionic liquid propellants (ADN and derivatives, HAN etc.)
2. Characterisation of propellants (Hf, Hs, Hm, Cp, Mp, burn rate, rheology, freesing, etc.)
3. Explosive properties (card gap, detonation, fire, heat)
4. Experimental evaluation of the thermal and catalytic decomposition
5. Determination of reaction balance
6. Perform transportation hazard classification.
A third category of propellants could be labelled 'future' green propellants such as new derivatives of ADN based or other ionic liquids and silanes. These propellants offer theoretically significant performance improvements but have not been investigated to an extent where such performance improvements have been experimentally proven. The general tasks with regard to such propellants are:
1. development of a propellant formulation
2. definition of manufacturing procedures
3. propellant characterisation (properties)
4. performance evaluation (theoretical and experimental).
Before designing a propulsion system, the compatibility of all construction materials that will be in contact with the propellant must be tested. This includes materials used in the tanks, valves, seals, piping, etc. This is to prevent possible violent reactions that might lead to major failure of the propulsion system, or degradation that might lead to reduced lifetime of the propellant, or to prevent corrosion that might lead to valve failure or blocking of small orifices. A significant amount of data for well-known materials and propellants can be found in the literature. However in the case of new propellants, for example HAN or ADN-based monopropellants, GRASP will conduct compatibility tests if such data is not yet available.
Catalysis:
As in the case of hydrazine, most of the green propellants (mainly the oxidisers) require some means to initiate the chemical reaction to release the energy contained in the chemical bonds. This process is in general called catalysis. Numerous methods exist to decompose the propellants, including several types of structural support for the catalyst (foams, pellets and advanced monolithic structures), as well as an extensive number of catalyst materials.
One of the most important goals of this project is to identify the most appropriate catalyst for the investigated propellants, including performance as well as system issues. The investigation with regard to the catalyst has two levels. The first is the development/adaptation of existing catalyst technologies (both heterogeneous and homogeneous) for hydrogen peroxide according to the requirements identified in the preliminary assessment (such as mass flow rate, decomposition temperature etc.). The second level is the development of new catalysts for ionic liquids (HAN, ADN etc.). General tasks for both components with regard to the catalyst investigation include:
1. Definition of catalyst type
2. Definition of the type and shape of the catalyst carrier (foam, pellets, ceramic monoliths)
3. Characterisation of the manufactured catalyst by X-ray diffraction, scanned electron microscopy (SEM), surface area evaluation and other appropriate methods before and after testing
4. Evaluation of the catalytic reaction in batch reactors, etc.
5. Numerical treatment of the decomposition processes and support for catalyst design
Propulsion systems:
Although the above are essential elements in the GRASP effort, the final goal of GRASP is implementation of the green propellants into a propulsion system. In general two kinds of propulsion systems with different mission applicability will be investigated; a low thrust system (1 to 10 N) and a high thrust system (higher than 100 N).
General replacement of 1-10 N class thruster (e.g. Hydrazine systems) for attitude control.
This thruster class is interesting because is constitutes one of the largest market share in the propulsion business. Due to the relative large number of manufactured thruster/per year this thruster class promises the highest potential for cost savings for the industry due to the replacement of toxic propellants.
Thruster in the higher than 100 N class
This thruster class has a similar interesting market share (although smaller than the 1 to 10 N class thruster) for roll and pitch control of larger satellites. Furthermore, this thrust range is interesting for descending stages for planetary missions on Mars etc. This again is a classical mission applicability for green propellants, e.g. hydrogen peroxide, since hydrazine thrusters could contaminate the environment with nitrogen (mainly in the form of ammonia) and therefore endanger the capability to detect traces of life.
The propulsion system development will occur in a two-step fashion. Initial testing will be conducted in so-called elegant bread board (EBB) propulsion systems The tested EBB units will include low thrust and high thrust units. The EBB units will investigate the suitability of the chosen propellants/catalyst on a propulsion system level and shall provide first indication of thrust range, throttling capability, specific impulse, lifetime assessment etc.
In a second step and following a final down-selection, advanced versions of the EBB so-called Demonstration units (DM) will be developed. Other than in the case of the EBBs, the DMs might exhibit a reduced diagnostic access or modularity compared to the EBB. However, those systems which have been assessed as suitable for further development to a DM should focus less on diagnostic and modularity but aspire to provide a performance assessment with a higher confidence level than possible with the EBB version. For example, an extended investigation of lifetime issues is expected as well as an improved thruster performance assessment.
The consortium was assembled such that all the necessary competence in the three fields is available. Each participant in a particular field has competences which others may lack and therefore the consortium as a whole is complementary. This present configuration not only ensures a very extensive coverage of the green propellant field of activity, but also provides competence in every step from basic research to a product development.
Project results:
GRASP aspired to investigate alternative propellants with regard to their suitability to replace presently used highly toxic propellants. By investigating a large number of possible candidates through theoretical as well as experimental efforts, GRASP aspired to identify the most suitable ones for a near term replacement of e.g. hydrazine etc. In order to achieve those goals GRASP has to:
1. develop a process to collect, manage and analysis an extensive amount of data such that conclusive decision for down-selection of more than 100 green propellant candidates can be made (see assessment methodology in the following)
2. develop new and advanced propellants and appropriate catalysts
3. develop propulsion systems suitable for the selected propellants.
The following paragraphs will address the achievements of each of those three fields.
Assessment methodology
The investigation of green propellants and their suitability for replacing presently used, highly toxic propellants is a complex matter due to the extensive decisive parameters deciding its suitability. To comprehensively investigate the suitability of only one substance/propellant is a major task; to conduct such a comprehensive study for a large number of potential green propellants candidates is unrealistic. Therefore one has to conceive an assessment methodology which allows an effective and manageable way to deal with a large number of data and a system of criteria hierarchised in a proper way.
The challenge in this work is the definition of the assessment criteria and their prioritisation respectively. If unwisely chosen, a criterion in an early assessment might exclude a very promising propellant. On the other hand, another propellant might be allowed to proceed further which, in the end, proves to be a poor propellant. While the former is difficult to look at in hindsight, the quality of the assessments will be judged by the propellants finally chosen.
The GRASP team therefore established a system of assessments, each with a range of criteria, which were designed such that unrealistic candidates are identified very early in the process and only more promising candidates were allowed to proceed further in the project.
In effect the GRASP project was guided by this series of assessments conducted at frequent intervals. Each assessment was based on different criteria or properties which were subject of the investigation efforts prior to the establishment of the assessment.
The first assessment, called the general assessment, had two major features:
1. For all substances which were considered green propellant candidates, all available data and background information relevant to its use as propellant have been collected (literature review). As a result, GRASP has provided the community with a unique data collection about green propellants (those information can be extracted from the GRASP project webpage).
2. A first down-selection was conducted based on toxicity, performance, storability and TRL. Based on the parameters from above and also considering availability issues (ITAR etc.), the GRASP consortium identified as a result of the first assessment 28 propellants (roughly one fourth of the original candidates) as promising green propellant candidates to be included in the upcoming assessments.
Following the first assessment which was mostly based on theoretical calculations and literature reviews, the subsequent assessment utilised the full potential of the versatile GRASP consortium. In total the GRASP consortium has roughly 30 different facilities available, allowing to measure a large range of properties of propellants and catalysts. At this stage, application relevant properties such as decomposition efficiencies, ignitability, ignition delays and such became the driving selection criteria.
Again several propellants were found more advantages than others. However, it has to be stated that some of the propellant candidates were not rejected but rather put aside for the time being. This was done not because they might be poor propellants but because of issues such as poor or non-existing availability of the propellant itself (e.g. HNF, higher silanes) or because the GRASP consortium did not have the facility to investigate them further. In total, the second assessment identified seven propellants as subjects for the Selection (third) and Final (fourth) assessment.
The last two assessments (Selection and Final assessment) were largely focused on the propulsive performance (combustion efficiencies, specific impulse and, if possible, minimum impulse bits) of the propellants and the developed propulsion systems. As this stage of the work the assessments were less a down selection of propellants but rather an investigation and verification of their propulsive performances.
In the end, i.e. the Final assessment, two monopropellants (FLP-106, H2O2), one oxidiser (H2O2), three fuels for bipropellant systems (kerosene, turpentine, d-limone) and one fuel for hybrid systems (HDPE) have been selected.
All of those propellants have a much lower toxicity, are non-carcinogenic, have acceptable or, as in the case of FLP-106, better performance than the toxic alternatives, are available and can be considered to have obtained a TRL of three to four during GRASP. This and their successful testing show the quality of the assessment methodology chosen by GRASP (see also comment above) and therefore this methodology might be considered as a standard for similar future efforts.
Propellant and catalyst development and analysis
The first down-selection of propellant candidates was based purely on performance calculation and on relevant data (toxicity etc.). This resulted in 28 substances which:
1. have significantly reduced toxicity
2. are storable liquids at standard conditions
3. have performances comparable with the one of the toxic alternatives.
In the second assessment additional candidates were put aside due to issues of availability (e.g. the only manufacture of HNF in Europe ceased its operation) or safety issues (recent information about HAN showed rather serious safety issues).
All propellants have been investigated experimentally during the GRASP project. The nature of the assessments differs very much depending on the propellants itself (e.g. background, history) and their possible applications (mono-, bipropellant, hybrid). In the following some of the major outcomes and achievements are summarised according to their envisioned application, i.e. mono-, bipropellant or hybrid systems.
Efforts related to monopropellants
Several green monopropellants we identified by GRASP with the potential to replace the toxic standard monopropellant hydrazine. However, as mentioned above, only two have managed to be included in the experimental efforts of GRASP, namely hydrogen peroxide and the ADN based propellant FLP-106. Most of the alternatives, although very promising candidates, have open questions with regard to safety issues and availability.
Hydrogen peroxide was also identified as oxidiser for bipropellant systems. The majority of the efforts related to hydrogen peroxide will be reported in the bipropellant section. Only the thruster development work will be summarised in this chapter.
FLP-106 is an ammonium dinitramide based propellant, similar to the LPP-103S which is was used on the recent (2010) maiden flight of a green propellant based thruster system on-board of the PRISMA mission. However, FLP-106 offers a slightly better specific impulse and impulse density. Albeit some information about FLP-106 was already published by FOI prior to GRASP, many open questions remained. GRASP has tackled some of them including. The efforts with regard to FLP-106 and other ionic liquids based on FLP-106 were spearheaded by FOI. Significant effort was contributed by LACCO, DLR and Fotec.
Propellant characterisation:
Motivation: FLP-106 is a rather new propellant and not all properties relevant to its potential use as a propellant are known yet.
Achievements: Various, afore GRASP unknown properties of FLP-106 have been evaluated and measured. This includes heat of melting and heat of solution of ADN and heat of solution of the fuel. The knowledge of those properties allows in the future a better assessment of the propellant.
Safety and handling characterisation:
Motivation: establishment of a comprehensive manual in order to allow a safe handling of FLP-106
Achievements: All relevant information for safe storage and handling were compiled. Extensive card gap tests conducted during GRASP indicated that FLP-106 should not be considered as a hazard class 1.1 material. The manufacturing manual allowed two more beneficiaries to initiate manufacturing of FLP-106 on their premises.
Development of catalyst with cold-start capability:
Motivation: While present catalytic systems for ADN based propellants are mainly based on thermal decomposition a purely catalytic one limits the need for internal heaters and reduces the complexity and power consumption of the thruster system.
Achievements: GRASP developed catalysts which showed in laboratory test the capability to reduce the on-set temperature for ADN decomposition from around 155 °C to 130 °C. Albeit this is not yet a cold start capability it showed the general feasibility of this notion and offered guidance for future improvements. The tests were also repeated for FPL-106, i.e. ADN+ H2O+Fuel. Again, in the presence of a catalyst the decomposition onset was significantly reduced (60 °C).
Material compatibility tests:
Motivation: To avoid pre-mature decomposition of the propellant (e.g. in the tank) or contamination of the propellant originating from the wetted materials itself it is mandatory to investigate relevant materials upon their compatibility to FLP-106.
Achievements: GRASP accomplished to test and verify the material compatibility of a significant number of metals and polymers which are standard construction materials in aerospace industry.
COTS testing:
Motivation: Utilisation of so-called commercial of-the-shelf components, i.e. flight qualified components or subsystems in order to show compatibility with FLP-106.
Achievements: A semi-automatic test stand for valve endurance testing was designed and built. Initially testing of a flight representative valve was successful and confirmed the facility design.
Impact of fuel variations and additives:
Motivation: Assessment of various methods to further improve FLP-106
Achievements: radical promoters as additive to the fuel and variation of the standard FLP-106 fuel were tested with regard to their impact on decomposition onset temperature and performance. While the radical promoters did not succeed in lowering the decomposition onset temperature several of the tested fuels show clear indication of further performance improvements.
Dynamic flow reactor test:
Motivation: batch reactor tests provide essential information about decomposition behaviour such as onset temperature etc., but essentially they provide only a snap-shot with regard to the situation one would find in a thruster like situation. A more realistic set-up such as a dynamic flow reactor is required to assess processes occurring in a thruster type environment.
Achievements: a dynamic flow reactor simulating a thruster was designed and tested. While the test facility failed to provide any data from stationary testing due to a structural failure of the flow bed itself, the tests itself provided essential knowledge about decomposition onset and required injector properties. As such, those tests and the results will guide any future testing.
Hydrogen peroxide was the subject of intense research within GRASP. Two 20 N monopropellant systems based on hydrogen peroxide were developed and tested within GRASP by two different entities using two different approaches for the decomposition of the propellant. In addition, a 300 N bipropellant thruster investigated a dual mode insofar that the thruster was operated with only hydrogen peroxide prior to injection of the fuel. In this monopropellant mode, thrust in the range of 80 N was generated.
The afore mentioned two 20 N monopropellant systems were operated successfully and allowed a comprehensive study of the catalytic activity and its impact on the thruster performances. Decomposition efficiencies were found to be on average around 95 % and c* efficiencies of the thruster systems as high as 95 % were reported.
As mentioned above, with the exception of the above, the hydrogen peroxide related research will be discussed in following bipropellant chapter.
Efforts related to bipropellants
Seven fuels and one oxidiser for bipropellant systems were included in the experimental efforts following the second assessment. Hydrogen peroxide was identified as the most promising oxidiser. The alternatives, namely oxygen and nitrous oxide were discarded due to the fact that they are not storable liquids (at standard conditions). Oxygen is a cryogenic and nitrous oxide is gaseous at standard conditions and liquefies only at about 52 bar. Although such a property might be favourable (self-pressurising), from the beginning of the project it was decided to focus only on storable liquid propellants within GRASP. Furthermore, the option of using oxidisers based on ionic liquids (HAN, ADN, etc.) for bipropellant systems was put aside due to the very low TRL of this technology.
During the project several fuels were dropped. It has to be pointed out that none of them has been discarded because they are not promising green propellants with regard to their predicted performance or toxicity. In some cases they were put aside due to some open questions with regard to ITAR or patent issues (e.g. tetramethylethylenediamine). Other reasons were potential storage problems such as the formation of organic peroxides during storage (Isopropyl alcohol (or 2-propanol), dibutyl ether, dimethyl ether, propyl ether) or just simply the fact that they were not available or extremely expensive such as in the case of silanes.
The remaining fuels have a very diverse background with regard to their general and space related use. For example, while kerosene and ethanol have been investigated as propellants and e.g. kerosene derivatives are presently used as fuels for bipropellant systems, for others such as dipentene or turpentine, only marginal and outdated information, if any at all, about their propulsive use are available.
In the following some of the efforts with regard to the fuels are summarised:
Ignition tests:
In spite of the diverse background of the fuels, the GRASP consortium decided that with regard to the suitability as a fuel in a bipropellant system the ignition behaviour is at this stage of the project one of the most relevant property. Two different experimental studies using different methods and facilities were therefore initiated. It is important to note that both entities (Deltacat, Fotec) investigate the ignitability of fuels with regard to the use of hydrogen peroxide as oxidiser - in other words, the ignitability as a function of temperature was to be investigated. Both entities established new facilities to investigate the ignitability of various fuels. Deltacat investigated the ignition delay of all seven different fuels in the same facility. This allowed a very good relative comparison of the ignitability of those seven fuels. As to be expected all fuels show decreasing ignition delay for increasing temperatures. Most interesting outcome is the one related to the two more uncommon fuels, dipentene and d-limone. Those two propellants have one of the lowest ignition delays over a large part of the investigated range of temperatures.
Fotec investigated the ignition properties of kerosene and ethanol. With regard to those two propellants, Fotec's result confirm the one from DELTACT, namely that ethanol is more difficult to ignite than kerosene (JetA-1). As a matter of fact, auto-ignition tests with Kerosene as fuel indicated reliable and repeatable ignition at pre-ignition pressures with lowest values in the range from four to five bar. The corresponding necessary auto-ignition temperatures were in the range 400 - 450 °C, which provides sufficient margin to theoretical decomposition temperatures (690°C). Ethanol on the other hand required much higher pre-ignition chamber pressures and temperatures. The lowest pre-ignition pressure, where auto-ignition was obtained was 15bar. The corresponding pre-ignition temperature was 600 °C.
Based on the result from the ignition tests, kerosene, d-limonene, dipentene and turpentine were chosen as the fuel baselines for subsequent implementation into propulsion modules.
Oxidiser
As mentioned before, the baseline oxidiser for GRASP is hydrogen peroxide in a concentration of 87.5% (by wt%). Hydrogen peroxide is manufactured by Evonik Industry AG under the brand name Propulse. Propulse has a standardised composition of stabilisers. Although substantial research with hydrogen peroxide was conducted in the past, many open questions remained. Most important, considering its planned utilisation as propellant, is its effective decomposition. In other words, the identification of the most suitable catalyst for the individual application (i.e. thrust level) is one of the most important tasks. Due to the importance of this, GRASP dedicated a significant part of its effort to this topic. Development of new catalyst types and their experimental assessment were conducted in parallel to a substantial effort in developing new numerical tools for an improved understanding of the catalytic processes and a better prediction of catalyst performances.
Other efforts with regard to hydrogen peroxide included storage testing and stabiliser variation investigation.
Catalyst development
Fifty five catalyst variations in terms of nature, shape and material of the catalyst carrier, active catalyst, manufacturing method variations and others were manufactured and tested. The test matrix consisted of variations of the catalyst carrier material (various ceramics and metal), as well as the nature of the catalyst carrier (monoliths, pellets, gauzes, foams). Within those variations, further variations were with regard to their geometry, the nature of active catalysts and last but not least the manufacturing processes itself were varied. In total more than 250 catalysts were experimentally assessed. The amount of variations allowed what can be considered the most comprehensive assessment of catalyst properties available up to now in the research community.
Other than Deltacat and Fotec, SOTON and UN developed their own in-house capability to manufacture catalysts. UN investigated two different manganese-based systems for the decomposition of highly concentrated solutions of hydrogen peroxide:
1. manganese-yttrium-zirconium mixed oxide nano-composites, obtained through a sol-gel route and deposited on a yttria-stabilised zirconia monolithic support
2. manganese-zirconia catalysts, obtained by direct deposition of a proper precursor of the active phase on the monolithic carrier.
It has to be noted that in particular the second method is of high interest since it circumvents the need for applying a washcoating and therefore would reduce the complexity of the manufacturing process. In the experimental assessment the direct deposition was found to be superior to the sol-gel catalyst. Although even the direct deposition ones did not reach their design performance they were successfully used for the subsequent thruster tests. In conclusion, both technologies are supposed to be very promising but will require more time to reach maturity.
In terms of performances of the manufactured catalysts it is to be pointed out that a significant number of catalysts developed during GRASP have shown major improvements compared to existing ones. Based on the knowledge gained up to now, they are fully suitable for the respective tasks in propulsion systems. Decomposition efficiencies above 95% were obtained by several catalyst types. Prior to GRASP, most existing catalysts suffered from insufficient cold-start capabilities and long transition times (tens and longer). Such a situation reduces their attractiveness for application in propulsion systems. During GRASP, catalysts have been developed with transition times lower than one sec and very reliable cold-start capabilities.
Equally important as the cold start capability is the catalyst lifetime, i.e. maximum total hydrogen peroxide through put. For the medium thrust range (100-300 N), Deltacat and SOTON succeeded in developing a catalyst with a proven lifetime of 38 kg of total through put. At the end of their test campaign, the performance of the catalyst was nearly unchanged. For the low thrust range (1 N), Fotec succeeded in showing catalyst lifetimes of roughly 18 kg total through put which corresponds to roughly 1/3 of the total lifetime of a standard 1 N hydrazine based propulsion system (e.g. EADS CHT1). For future efforts it is important to repeat such tests in order to obtain a statistical relevant number of data points and in order to investigate manufacturing variations, if any and their impact on the lifetime assessment. During GRASP the number of catalysts tested with regard to their lifetime was limited mainly due to limited time.
In this context it is important to note the development of a rapid catalyst lifetime assessment facility by SOTON and Deltacat. With this facility it is possible to obtain a preliminary assessment of the catalyst lifetime in a comparable very short time and using only a fraction of the propellant required in a standard lifetime assessment. Those results have yet to be verified by comparison with lifetime assessments conducted in standard fashion.
Analytical/numerical decomposition model development:
As mentioned above, catalysis of hydrogen peroxide is highly complex and not yet fully understood. The development of analytical and numerical models was therefore an essential element in the development of advanced catalysts.
The models developed during GRASP incorporate a much higher level of detail than any of the models suggested in the past. This allowed investigation of the decomposition and the parameters which influence the decomposition in much greater detail and allows more accurate prediction.
In a first step several models were established, each focusing on different aspects such as the pressure conditions and pressure drops across the catalyst bed or thermal conditions within the catalyst bed and decomposition chamber. In a second step the models were verified by means of the data obtained in the framework of the experimental catalyst assessment. Several significant improvements of the previous understanding of the decomposition process are the result of this effort. For example, the very strong impact of the thermal properties of the decomposition chamber has been predicted by the models and verified by experimental testing. As an example in the context of small mass flow rates, the thermal mass of the catalyst itself but also the thermal mass of the surrounding structure can significantly impact the decomposition efficiency in the initial phase. The model shows such an accuracy that the influence on the decomposition caused by relatively small effects such as variations of the thermal mass of the catalyst can be predicted. The accuracy of the prediction has been verified by tests. The design of some of the catalysts manufactured in the 2nd and 3rd period of GRASP is based on predictions made by the models. The fact that those catalysts have shown significant improvements is a clear sign for the validity of the models. Furthermore, the models succeeded in shedding some light into unexplained observations during past decomposition testing, namely, decomposition efficiencies above 100% or melting of the catalysts core even though maximum decomposition temperatures should have been far below the melting temperature of the catalyst material. The explanation of both effects is linked to a pre-heating of the hydrogen peroxide due to thermal conduction from the catalyst hot region to its colder upstream region. This effect was predicted by the numerical tools and subsequently confirmed in experiments.
Similar to the above, the models predicted a radial temperature distribution across a single channel in the catalyst and across the catalyst itself. Measurement of the former was not possible due to limited diagnostic access into the small channels of a catalysts but the latter was experimentally verified. This has significant impact since indicates that published results have to be re-visited and future test set-ups have to be adapted in order to provide reliable results.
In conclusion, the developed models:
1. have succeeded in increasing our understanding of the hydrogen peroxide decomposition processes in general
2. successfully guided the catalyst design process
3. explained afore unexplainable observations
4. succeeded in making predictions which have been confirmed later in experiments.
Storage investigation
The high potential of hydrogen peroxide as a monopropellant as well as an oxidiser is undoubted. In the past, test conducted by various research groups have indicated also suitable storage properties. It further has to be noted that hydrogen peroxide is one of the not so many green propellants with flight heritage (SynCom II, Early Bird). However, there is a need to verify the storability with technologies available nowadays in Europe (both in terms of materials and also of hydrogen peroxide qualities).
Consequently, SOTON and Fotec kept several samples of hydrogen peroxide in storage for a large part of the project. In addition to the standard propulse, Evonik also manufactured samples with variations of the standard stabiliser cocktail including increased phosphate content, increased acid content and increased stannate content. All samples show in time small weight variations (indicative for changes in concentration) which are within the measurement accuracy. This would allow the conclusion that within the test duration (up to 28 months) and storage conditions (Al transport container from Evonik and storage temperatures of 3-5°C) hydrogen peroxide is fully storable. However, some of the samples show a weight increase instead of a decrease which one would see if any decomposition took place. The reasons for that are unclear at this time. The samples are kept and will be observed even after the GRASP project is finalised.
Two unwanted potential effects of increasing the stability of hydrogen peroxide are a decrease of decomposition efficiencies and, in the worst case, a poisoning of the catalyst. Consequently, the hydrogen peroxide samples with the above mentioned stabiliser variations were tested with various catalysts. An indeed, while the increased phosphate content lead to a significant catalyst performance decrease for all tested catalysts, the increased acid content as well as increased stannate content did neither lower the decomposition efficiency nor, after 1 kg of total through put, harm the catalyst in any detectable way.
In conclusion, the storage tests show promising results, insofar that all four samples (standard and three stabiliser cocktail variations) show only concentration changes within the measurement accuracy (around 0.2 %). However, the results are somewhat tainted due to the fact that some samples showed an unexpected weight increase instead of the expected weight decrease. Pending the impact those stabilisers have on the stability of the hydrogen peroxide, in terms of compatibility with various catalyst, the tests clearly show that phosphates can have detrimental impact on the catalytic activity while increased amount of acids and stannates did not show any harmful effect.
With regard to storage capability and general handling safety, it has to be pointed out, that over a period of 2 ½ years in the GRASP period not a single accident occurred with hydrogen peroxide such as blow outs or run-away decompositions. This in spite of the fact that the work was conducted in numerous different facilities and working environments. On the one hand this is to be credited to all the beneficiaries who worked with hydrogen peroxide but also largely due to hydrogen peroxide itself which in spite of being a high energy content oxidant is relative simple to work with.
Efforts related to hybrids
Based on the first Assessment HDPE and hydrogen peroxide were selected for the hybrid system. No dedicated efforts with regard to HDPE have been conducted within GRASP.
Design of a methane based system
In a deviation of the general requirement for storable propellants but reasoned with the extreme potential and interest from propulsion system developer, GRASP also included methane and LOX as propellants. Initial numerical efforts focused on simulation of injection system and the thermal conditions within the combustion chamber. Results indicated that the combustion efficiency is strongly influenced and improved by introducing a swirled flow in the injector inner channel. Higher combustion efficiency can be also obtained for longer combustion chambers, but this influence is weak and does not balance the benefits of a shorter combustion chamber such as lower weight and better cooling efficiency.
According to numerical simulations of the combustion analyses of all cases it was shown that the best configuration of methane/oxygen gaseous thruster is for pintle injector type and for coaxial with inner swirl.
In a second step, a more detailed design study of the engine cycle was initiated. A comparison of regenerative cooling, radiative cooling and film cooling was conducted and its benefits and draw backs were identified.
It has to be noted that all GRASP activities with regard to methane and LOX have been of only theoretically nature both with regard to the propellants itself and the propulsion system based on this propellant combination.
Propulsion System Development
Within the GRASP project the final step in the propellant selection process was the test in a propulsion thruster test bed. As a matter of fact, this step itself was subdivided into two steps.
In a first step the eight propellants (identified in the second assessment, 'Preliminary assessment') were tested in the following 10 so-called Elegant Bread Board (EBB) units (note that in the following a similar or even identical thruster hardware which is used to test two different propellant (or propellant combination) is considered to be two units):
1. 1 N bipropellant propulsion system (hydrogen peroxide/ethanol)
2. 1 N bipropellant propulsion system (hydrogen peroxide/kerosene)
3. 20 N monopropellant propulsion system (FLP-106 (ADN))
4. 20 N monopropellant propulsion system (hydrogen peroxide)
5. 300 N bipropellant propulsion system (hydrogen peroxide/Kerosene)
6. 300 N bipropellant propulsion system (hydrogen peroxide/dipentene)
7. 300 N bipropellant propulsion system (hydrogen peroxide/D-limone)
8. 300 N bipropellant propulsion system (hydrogen peroxide/turpentine)
9. 20 N monopropellant propulsion system (hydrogen peroxide)
10. 200 N hybrid propulsion system (hydrogen peroxide/HDPE).
Testing of those EBBs had two major objectives:
1. a first assessment of their feasability (e.g. can auto ignition of various fuels in combination with decomposed hydrogen peroxide be achieved) and performance and
2. provide data for a further propellant selection to be implemented in advanced EBBs so called Demonstration units (DM).
Testing of the EBB units provided in all cases first a large amount of performance data and information to assess the propellants and the associated propulsion units. The only exception of this was the unit for FLP-106 testing. A premature and uncontrolled decomposition of the propellant due resulted in a damage of the test unit not permitting further testing.
Based on those EBB test results and according to the workplan, a further down-selection of propellants and associated propulsion units was conducted. Three units have been chosen for a further design and final testing, namely 300 N bipropellant (hydrogen peroxide/dipentene/turpentine); 200 N hybrid (hydrogen peroxide/HDPE); 1 N bipropellant (hydrogen peroxide/kerosene).
Using the benefit of the experience gained in the EBB testing campaign the units were re-designed and upgraded respectively. Subsequent testing of those three systems can be claimed to be a full success. To name only some improvements: DC and SOTON developed and implemented a new type of injector and nozzle, both improved significantly the thruster performance. UN improved their EBB design by using an advanced catalyst bed and nozzle design. FT implemented a new engine cycle (radiation cooling) and new materials for the combustion chamber (Pt-Rd).
Testing of those units allowed a confirmation of the selection process. All tested DM systems allowed evaluation of propellant characteristics (e.g. ignition behaviour, combustion stability) and thruster performance (c* efficiencies and specific impulse). Equally important the testing provided detailed information about the way forward for those systems and the selected propellants respectively.
Potential impact:
The GRASP project was from the beginning conceived to tackle a large range of issues. Not only that it included initially more than 100 propellants but it also covered a large number of topics ranging from the investigation of basic propellant properties to propulsion system tests.
A significant number of conference papers and some peer-reviewed publications is a clear sign of the scale and quantity of the scientific achievements of GRASP.
Presently no technologies which were developed during GRASP have been patented yet. It is in the belief of the project coordinator that some GRASP technologies however are very close to a maturity level at which patenting them is a real option.
GRASP has achieved more than submitting a number of publications and obtaining patents. It has shifted the topic of green propellants into the focus of the community and industry. It has established an awareness of essential topics such as propellant toxicity, hazardous levels and, most important, has shown the alternatives to the presently toxic propellants. But GRASP has not only pointed out the alternatives but has also, step by step, developed the necessary tools and technologies to use some of those alternatives and finally verified the feasibility and performance with dedicated propulsion hardware.
The GRASP project coincided with the decision of REACH in 2011 to put hydrazine on a list of substance of high concern which might lead to a phasing out of this essential propellant in Europe as early as 2017 - exactly the situation GRASP has from the beginning pointed out as a real possibility. Considering the fact that hydrazine is one of the pillars of space flights, this decision is very critical for the European space community. GRASP could not have come to a better point in time. Knowledge and access to it is critical at this time. It is needless to say that the interest in green propellants has sky rocketed since the announcement of REACH. And GRASP is helping by accepting invitations from companies, space agencies and universities for seminars and general information dissemination. Also as a direct result of the efforts of GRASP, several projects including GRASP beneficiaries related to green propellants are in preparation or have already been submitted to the European Commission or the European Space Agency.
At this point in time, GRASP and the results from it might very well be the incubator for a very rapid development of real alternatives to the presently utilised toxic propellants.
List of websites:
http://www.grasp-fp7.eu