Final Report Summary - HISP (High Performance Solid Propellants for In-Space Propulsion)
Executive Summary:
Spacecrafts are propelled using chemical or electrical propulsion systems. Electrical propulsion provides superior specific impulse but very low thrust. If high thrust is required, chemical propulsion is the only possible alternative. Today the majority of spacecrafts use chemical propulsion and this is also expected in the foreseeable future. The only way to drastically increase the performance of chemical propulsion systems is to improve the propellants. Depending on the propellants used, chemical propulsion systems are divided in two major categories:
• Liquid propellant rockets (monopropellant and bi-propellant systems)
• Solid propellant rockets
Liquid rockets provide high performance and adjustable thrust but are complex, costly and use toxic propellants. The benefits of solid rockets are their storability, compactness and simplicity. No propellant delivery system is required which enables a huge improvement in reliability and cost. One disadvantage with solid propellants is however their relatively low performance.
The aim of the HISP project was to improve the performance of solid propellants by using the new high energy density oxidizer ammonium dinitramide, ADN, an energetic binder based on glycidyl azide polymer, GAP, and high energy density fuels such as aluminium hydride, nano-aluminium and activated aluminium. These materials have all been known for many years but using them has shown to be a challenging effort due to problems concerning manufacturing, chemical stability and compatibility.
The project has substantially increased the technology readiness level of these materials and shown that solid propellants with substantially higher performance, compared to state of the art solid propellants, can be developed. Unfortunately, the burning rate of propellants based on the energetic polymer GAP seems high for many space applications. However, propellant based on ADN and non-energetic polymers is as a very interesting green alternative to current perchlorate based solid propellants.
Project Context and Objectives:
Solid propellant rocket motors have been used to propel spacecrafts in numerous missions since first used in the upper stage of the first U.S. Satellite Explorer I in 1958. They have been utilized in many different in-space applications such as:
• Large kick motors for orbital insertions
• Retrorockets for de-orbital manoeuvres
• Retrorockets, in combination with airbags, for soft landings
• Small spin rockets for spin stabilized spacecrafts
One well known spacecraft is the Magellan Venus probe launched from the Space Shuttle in 1978, using a solid rocket motor. On arrival at Venus, after a 15 month journey, a second solid propellant motor was fired to place the spacecraft in its orbit around the planet. The same type of motor has been used in several other missions until 1997, among them the Jupiter spacecraft Galileo in 1989. Another example is the Mars Observer, launched in 1992, which used a solid rocket motor to boost its way to Mars. More recently solid propellant rocket motors are considered for deorbiting in the ESA Clean Space Initiative and by NASA for the ascend module of the Mars sample return mission.
There is an ever increasing objective to significantly reduce the time, cost and mass of spacecrafts and to increase the scientific return of space exploration missions. The time and mass required for a spacecraft to reach its destination is directly linked to its propulsion system. The only way to significantly improve the performance of a propulsion system is to develop propellants with higher specific impulse and higher density. Using propellants with higher performance enables increased payload. A typical interplanetary transfer stage may contain 10 tons of solid propellant. If the specific impulse is increased by 10%, the mass of the spacecraft can be reduced significantly, or its scientific payload can be increased by several hundred of kilograms.
State of the art solid propellants are based on the oxidizer ammonium perchlorate (AP), NH4ClO4, and aluminium powder embedded in a hydroxyl terminated polybutadiene (HTPB) polymer binder matrix. AP is in many ways an excellent oxidizer due to its relative low explosive hazard and the possibility to tailor its ballistic properties. However, propellants based on HTPB/AP/Al have low specific impulse compared to current storable bi-propellants.
HISP stands for High performance solid propellants for In-Space Propulsion, or solid propellant with High Isp (specific impulse). As the name indicates, the objective of the project was to develop solid propellants with substantially higher performance compared to solid HTPB/AP/Al based propellants.
To accomplish this, the new high energy density oxidizer ammonium dinitramide, ADN, an energetic binder based on glycidyl azide polymer, GAP, and high energy density fuels such as aluminium hydride (AlH3), nano-aluminium and activated aluminium has been used in the project. Using these materials has in the past shown to be a challenging task due to problems concerning manufacturing, chemical stability and compatibility. In order to develop high performance solid propellants the technology level of these materials needed to be improved substantially.
Project Results:
Improving the technology readiness level of solid propellants containing new high energy density ingredients require not only propellant development as such, but also to improve the production methods of these materials. In the HISP project substantial improvements have been made concerning the following topics:
• ADN production improvement
• GAP production improvements
• GAP binder development
• High energy density fuels
• Propellant formulation and characterization
• Rocket motor testing
The results for each of these topics are presented below.
ADN production improvement
Ammonium dinitramide, ADN, NH4N(NO2)2, is an environmentally benign high energy inorganic oxidizer salt, first synthesized in 1971 at the Zelinsky Institute of Organic Chemistry in Moscow. It is today produced by EURENCO Bofors in Sweden. The material is of high purity but the cost is high. In order to implement ADN in future propulsion applications, the production method needs to be improved to reduce the cost. To some extent the cost can be decreased by improving the current production method. However, to drastically reduce the cost new production methods are required and production in larger scale is needed.
The current used ADN production method is a three step synthesis starting from ammonium sulfamate. The production improvement on ADN has concentrated on; recirculation of chemicals used in the process, finding a new solvent for ADN crystallization for higher production rate, a study of ADN properties relevant for drying, handling and packaging operations, and a new synthesis route.
Studies of the current ADN production process have shown it is possible to recover chemicals from the process. Guanylurea salt recovered from the second process step was recycled and tested for production of GuDN. The yield was as expected, after losses accounted for by the solubility of GuDN in the acid/water. Ammonium sulfate from the final process step was recovered in good yield and ADN produced from this recovered ammonium sulfate fulfilled the specification.
ADN is crystallized by removing water from the reaction mixture by azeotropic evaporation with a solvent, propanol. Propanol occurs as 1-propanol and 2-propanol. Today 2-propanol is used, but this work shows that 1-propanol is to prefer. ADN of good quality has been produced using 1-propanol and a method suitable for large scale production has been established. 1-Propanol is technically possible to recover with a purity of at least 99 mole-percent from an azeotropic water mixture, by extractive distillation. The optimum results are achieved with a distillation column operating at low pressure (500 mbar). The yield of this process is estimated to be 80% but the designed process will recover 94% of the initially entered propanol. In this work benzene was used for the extractive distillation but if the process is to be scaled up to industrial scale another solvent should be used.
The drying and handling operations could be improved if the ADN crystals are washed with a non-solvent prior to drying, which diminish the crystal lumping tendency. The dried product should be stored and handled at a humidity no higher than 45 % RH, but not at super-dry condition.
A new synthesis route, a two-step nitration of urea with preliminary separation of nitramide has been investigated. Using this process, the problem of ammonium ions in the spent acid would be avoided. A total yield of 40% of dinitramide has been achieved. This is rather low but since urea is very cheap this is not considered a problem. The process is still not ready for scale up, especially the hydrolysis of dinitrourea into nitramide requires more efforts.
The ADN production method was evaluated to make a prospect for industrialization, including estimation of future prices. A conceptual process design (CPD) was made for the improved ADN production process.
To assess the economic feasibility of large scale production of ADN, the Aspen program was used to simulate the production process and the results of the simulation were used as inputs for an economic feasibility analysis. Separation, crystallization and acid recovery sections were not included in the study; neither were storage facilities including piping for raw materials, intermediates and final product. This was related to the lack of precise kinetic data and thermodynamic properties of several substances that made the modelling of the reaction section of the plant not straightforward. The design of the chemical process and its specifications were based on an annual production rate of ammonium dinitramide of 2500 tonnes/year.
Investing in a facility for production of 2500 tonnes of ADN per year is a long term project. To give a view of future prices in medium term perspective, a process for a medium scale ADN production was defined, based on current production know-how and possible improvements established within this project. Then prices for ADN produced using the improved process in plant with capacities of 40-400 ton ADN/year were estimated.
According to the CPD an estimated future cost for large scale production of ADN could be in the order of 15 €/kg. This price does not include profit or return on investments. Neither are costs for separation, recrystallization, acid recovery, waste treatment etc. included. Some of these items will increase and some decrease the cost of ADN, but all together will have a small effect on the price of ADN.
The estimated price for ADN produced according to the EUB defined process described in the deliverable D2.3 is approximately 62-65 € for 40-400 tonnes per year, when all investments are paid off and with costs for media, personnel, chemicals etc. at the 2013 cost level. The bottle-necks in the described process are the crystallization and the nitration operations.
In general, the performance of a solid propellant increases with increased solid loading (oxidizer particles and aluminium powder). If too much is added, the viscosity of the uncured propellant slurry becomes too high. To obtain a castable propellant formulation with sufficient low viscosity and high solid loading, particles with minimum spatial extension are required. For this reason spherical particles are preferred. The particle shape of ADN as received from the synthesis is needle shaped and thus not suitable for formulation. Methods to produce ADN particles with suitable morphology have thus been studied.
Several different particle production methods were evaluated in order to select the method to be scaled up. The selection was based on the maturity of respective particle production method and the approximate particle size produced. Usually bimodal particle sizes are used in propellants. The desired sizes of respective fraction are in the range of 200 – 250 µm and 20 – 50 µm respectively. The results from the study show that:
• Spray prilling can produce spherical particles of desired size and shape. The method is mature for
up scaling
• Recrystallization
Cool crystallization gives particles with a high aspect ratio and broad particle size distribution.
Currently this method is not mature for up scaling
Solvent/non-solvent technique gives a small particle size distribution and small particles with
a favourable aspect ratio
• Jet milling seems as a promising method but the particles produced are too small to be used in the
project
• Emulsion prilling is currently not mature for up scaling since the process- or material-parameters
need to be modified
The two most suitable methods were spray prilling and solvent/non-solvent crystallization. In a propellant more large particles are used than small. Due to this and the reason that solvent/non-solvent crystallization of ADN in larger quantities has not been performed it was decided to select spray prilling as the method to be scaled up to a capacity of producing at least 10 kg ADN particles per day. The apparatus is further described in deliverable D3.3 “Scaling up of selected production method”.
The capacity of the spray prilling apparatus built is reported in HISP deliverable D3.4 “Production of ADN particles for WP7 using new method”. The capacity of the up scaled spray prilling apparatus has been verified. During four days a total amount of 45 kg prilled ADN was produced and the maximum amount of prills produced during one day was 16 kg. It was also found that jet milling was a convenient method to obtain small ADN particles.
A theoretical study has been performed on future industrial scale production to estimate the future cost of ADN particles. With an annual production of 40-400 ton ADN/year, the cost for spray prilled ADN is estimated to be in the range of 65 to 85 €/kg and the cost for jet milled ADN is estimated to be approximately 60 €/kg, at 2013 cost level. Costs for investments are not included in the cost estimates and thus the actual costs will be higher during the first years of production.
GAP production improvements
Glycidyl azide polymer, GAP, is an energetic pre-polymer manufactured by EURENCO France. Production of GAP is made using a two-step process. First an intermediate (PECH) is produced from polymerization of epichlorhydrinein dichloroethane, then GAP is formed by azidation. The azidation reaction is quite time consuming.
A theoretical study of possible process improvements and validating lab studies have been carried out. Dichloroethane (DCE) used as a solvent in the GAP production process most probably will have to be replaced in the future due to its environmental impact. The studies of the GAP production process has shown that replacement of DCE by dichloromethane as a solvent for the polymerization reaction can be considered as an intermediate solution but a long-term solution is needed.
A new analytical method to follow the azidation reaction has been developed and applied to follow GAP production in industrial scale. The azidation reaction time could not be decreased, despite more efficient stirrer or larger quantity of catalyst. Washing/decantation is possible improve at lab scale by adding a solvent. This improvement will be validated at industrial scale production.
GAP binder development
GAP in combination with ADN provides high specific impulse and good ballistic properties. However, the mechanical properties of GAP needed to be improved in order to obtain a propellant with sufficient elasticity and strength. In HISP delivery D4.2 “Development of curing systems” the mechanical properties of GAP were studied by using isocyanate free curing systems (ICT) and more conventional curing systems (FOI) and in delivery D4.3 “Improvement of mechanical properties” it was shown that the mechanical properties of GAP could be improved by optimizing the curing systems used.
The work performed at ICT showed that all binder formulations and ingredients were compatible with ADN. The mechanical properties obtained were however not in the range as desired but it still seemed possible to obtain a propellant formulation with acceptable mechanical properties without or with a low content of plasticizer. The most promising binder formulations was selected and characterized.
At FOI several types of GAP based binders were investigated with respect to mechanical properties. The binder with best mechanical properties was based on GAP, cured with mixture of Desmodur N3300 and Desmodur W, using 1,4-butanediol as chain extender. This material had an excellent strength (2.56 MPa) and elasticity (730%). The second best material was GAP, cured with Desmodur N3400. It had a strain of 630% and a strength of 0.59 MPa. Unfortunately, none of these materials cured properly in the presence of ADN. Successful curing and reasonable mechanical properties were however obtained by omitting the chain extender. The influence of four different bonding agents was also studied. The preferred bonding agent resulted in increased strength and elasticity of the selected binder, but adding more than 0.5 phr resulted in gassing problems during mixing.
High energy density fuels
In current solid rocket propellants aluminium is used as fuel. It has a relatively high specific density, abundantly available, burns with a high temperature (which is good for generating high pressures and hence high thrust levels) and is compatible with the other ingredients in the formulation.
The standard Al used is of micrometric size, typically spanning over the range 10 to 50 μm. It is a well-known fact that combustion of micro-sized Al implies two-phase flow losses in supersonic gasdynamic nozzles and a series of additional possible problems (throat erosion, slag accumulation, incomplete combustion, etc.) depending on the specific operating conditions and motor configuration. For example, incomplete combustion is mainly associated with insufficient residence time in the combustion chamber (i.e. small motors). At any rate, two-phase flow losses are the main reason for the poor combustion efficiency generally observed in metalized formulations. The elusive agglomeration phenomena, leading to burning particles much larger than the initial Al powder, are the key factor for this effect. By decreasing the Al ignition temperature using activated Al or nano-Al, the agglomeration effects can be mitigated, although not totally eliminated.
Another way to increase the specific impulse is by replacing aluminium with aluminium hydride. Although the density of aluminium hydride is lower than that of aluminium (meaning that one can take less aluminium hydride per unit volume compared to aluminium and hence less energy is available), the combustion energy released by aluminium hydride more than compensates for the difference in specific density. However, its use has been limited due to its incompatibility with several components and its apparent instability in general. Research was performed in the past on this latter issue. The literature mentions that research was conducted on the effect of air, impurities, heat, radiation and water on aluminium hydride and no decisive conclusions were given for all cases.
Hence, in order to obtain the desired performance two main roadmaps were followed; the enhancement of ideal performance on one side, and the reduction of specific impulse losses (mainly derived from two-phase flow) on the other. An initial basket of high-energy density fuels comprising three types of nanoaluminum powders, three chemically activated batches of micrometric aluminium particles and a sample of aluminium hydride was available at the beginning of the project. In addition other standard metal powders were tested for comparison.
The ingredients under examination were shortlisted pursuing a compromise between high active metal content and powder reactivity. The project addressed also the properties of powder blends aiming at a compromise among the properties of the individual ingredients. After a preliminary characterization of the fuel powders alone and as ingredients of standard propellants, a relative grading among the aluminium powders was obtained.
During the first two years of the project, the production of aluminium hydride was attempted in batches of 20 – 30 grams which turned out to be impure, necessitating more work on smaller scale methodology. Research activity carried out on small scale batch production identified the cause for the failure. Before continuing with larger scale production, some vital tests with the nearly pure AlH3 were performed, being the long term stability of the alane and its compatibility with the other propellant ingredients classified as major issues for the final application. It turned out that AlH3 was compatible with all ingredients, including ADN, but that the long term stability was insufficient for rocket propulsion applications.
Alternative routes to produce alane were investigated and the route using silicium tetrachloride was reproduced from literature. After some fine tuning of the heating program also needed in this route yielded the same purity of AlH3 compared to the conventional synthesis (i.e. using lithium aluminium hydride with aluminium chloride) route. From a simplicity point of view, there is not that much difference between the two routes. However, safety issues associated with the silicium route are more severe compared to the other one, since highly explosive and toxic vapours are produced during the synthesis and this detail must be carefully considered when scaling up. Notwithstanding the accomplishments, the stability of the AlH3 produced with the silicium tetrachloride route has to be assessed, also in contrast with the results obtained from the conventional route.
Since aluminium hydride was not available for propellant testing, the sole option left for the improvement of the delivered specific impulse was the reduction of two-phase nozzle flow losses generated by the presence of agglomerates. Further combustion studies thus focused on a set of three ingredients: one type of nanometric aluminium, one type of activated aluminum, and a baseline micrometric aluminum. Conclusions from the investigations suggested that the use of multimodal powders obtained by mixing nanoaluminum and standard micrometric aluminium would lead to a good compromise between burning rate, metal content, agglomeration, and processability.
The optimal micro-nano aluminium blend was investigating by developing some additional propellants. In the compositions a part of a standard micrometric aluminium fuel was replaced by a nanopowder. The formulations consisted of standard AP/Al/HTPB propellants where 1.5%, 3.0%, and 4.5% of the baseline aluminium fuel was replaced with nanoaluminum, testing the materials for ballistics, ignition delay, and agglomeration attitude. A procedure for the collection of the condensed combustion products was developed and applied to the propellant set. The collection facility enabled the detailed measurement of the combustion products through laser granulometry, computing relevant statistical parameters.
The fine tuning of propellant composition was accomplished using the reference AP/Al/HTPB composition, performing a partial replacement of micrometric baseline aluminum with coated nanoaluminum. The progressive introduction of small fractions of nano Al produced a monotonic increase of burning rate. As expected, the increment was not proportional and the effect tended to be lower as the amount of nanometric powder increased. Halfway improvements have been obtained through the replacement of 1.5% of µAl with nAl. Gains with respect to the baseline have been found in the order of 20-30% depending on the operating pressure.
Once nAl content was increased to 3%, the gain in burning rate ranged between 40-60% while, for 4.5%, propellant ballistics approached the behaviour of composition, containing nAl only. Experimental data revealed gains in the order, respectively 55-75% for the former composition and 60-80% for the latter.
Similar trends were observed for propellant ignition delay, measured by exposing the energetic material to a CO2 laser. With respect to the baseline, all propellants featured a decrement in the time required to come to ignition, thanks to the addition of even small quantities of nAl. Also in this case, a saturation effect was observed for the formulation containing 3% of nAl.
The collection of the condensed combustion residues was performed in an experimental rig where the propellant strand was burned upside down. The condensed combustion products (CCPs) originated from the specimen combustion were quenched in a pool filled with proper liquid. The collection was then treated before the analysis in a laser granulometer. Combustion tests have been performed at 10 bar.
The result showed that nAl decreased the size of condensed combustion products. Agglomerates for featured about 3 µm of mass-weighted mean diameter which is sensibly lower than the value measured for the baseline propellant, which was around 20 µm. The presence of small fractions of nAl did not carry out immediate advantages at the tested conditions, in terms of mass-weighted mean diameter and only when nAl was set to 4.5%, the distribution curve became very similar to that obtained for the propellant containing only nAl.
From the ensemble of the results available for the mixed HEDF fuelled propellant series it appears that the step-up performance gain was obtained for a nAl content between 1.5 and 3%. Further increase above 3% of nAl leaded to reduced gains in both burning rate and ignition delay. Moreover, the performance of the formulation containing 4.5% nAl was almost levelled with behaviour of propellant the propellant fully fuelled with nAl. Regarding agglomeration attitude, a qualitative leap was obtained only when the nAl content was set to 4.5% or above.
Results obtained showed that an optimal replacement fraction can be identified on the basis of performance parameters (burning rate, agglomeration, ignition delay). Also the metal content must enter the definition of the optimal µAl-nAl fuel blend since nano-sized aluminium particles contain a consistent fraction of metal oxide which, in turn, reduces the ideal attainable performance.
Propellant formulation and characterization
The selection of propellant compositions was based on thermodynamic calculations, small scale propellant preparation, recommendations from the system analysis study, the evaluation of the different high energy density fuels (HEDF) considered and results from the GAP binder development performed.
As a result AVIO, TNO and ICT selected aluminised ADN/GAP propellant formulations. FOI decided not to use HDEF and develop an ADN/GAP-propellant with high filler loading. Initial formulation assessment of small batches showed which calculated composition could be prepared in reality. The propellant formulations were scaled up and characterized with respect to processability, physical properties, small-scale hazard testing, mechanical properties and thermal stability. The progress was slowed down by problems regarding the formation of voids during propellant curing, unsatisfying mechanical properties and the removal of a curing agent from the product range.
The ideal specific impulse (Isp) for an Al/ADN/GAP or ADN/GAP propellant was unrealizable. The compositions selection had to be compromises between Isp and processabilty. The following propellants were formulated and characterized:
• ADN/GAP (70/30)
• Al/ADN/GAP (18/56/26)
• Al/ADN/GAP (16/60/24)
Mechanical properties of ADN/GAP-based propellant were the most challenging properties. However they could be improved significantly with a maximum elongation of approximately 15%. Interestingly the elongation at break increased at lower temperatures.
The formation of bubbles during propellant curing created problems which lead to a large deviation of the densities compared to the maximum theoretical density. This effect, especially with the propellant preparation at AVIO and ICT delayed the development but was eventually solved.
As opposed to the gas formation during curing stability investigations of the cured propellants revealed no suspicious behaviour and the cured propellant therefore could be considered stable. Self-ignition temperature and adiabatic self-heating rate is dominated by the ADN properties. A deterioration of the mechanical properties was found by aging of the propellant. This is probably due to debonding phenomena affecting the solid particles and the binder system. The ageing tests demonstrated that the GAP binder system is stable at the used ageing conditions.
Pot life time is sufficient for grain preparation for small scale motors but probably not sufficient for casting large motors. Plasticizer were investigated but not used in the final formulation which cause high glass transition temperatures (>-34°C).
The results of the ballistic evaluations of ADN/GAP-based propellant pointed out high burning rates (>19 mm/s @ 7MPa) and reasonable pressure exponent similar to traditional propellants. The burning rate is slightly lower for the aluminized formulations but still too high for the desired application. Additional metal agglomeration experiments were performed at ICT in cooperation with POLIMI. The agglomerates measured for Al/ADN/GAP propellants are about 30 to 50% larger over the tested pressure range of 0.1- 5 MPa than the ones of commercial formulations of AP/HTPB/Al.
Impact sensitivity and friction sensitivity are in the range of AP/HTPB/Al composite propellants and double base propellants. Different small gap test were performed. ADN/GAP propellants should belong to hazard class division 1.1 whereas for Al/ADN/GAP propellants it might be possible to reach a hazard class division 1.3. The Koenen test exhibits no evidence for a detonation or deflagration. Fast burning was the observed reaction only and was evaluated as “explosion”. No reactions were observed at the electrostatic discharge tests (EDS).
Rocket motor testing
The propellants developed have been fired in rocket motors by several project partners.
Six test motors were prepared at AVIO and fired at two temperatures 20 and 60°C. In this occasion a 4 kg scale mix was performed using the composition and the mixing procedure indicated in D6.3. No problem was encountered in mixing and casting of the propellant which was free of bubbles and with a very good density, not less than 99 % of the theoretical maximum density. The firing tests showed regular combustion of the propellant. A slight erosion of the nozzles was observed. The final diameters were about 0.1 mm larger than initial ones. The measured specific impulse (299.4-306.0 s) was lower compared to the calculated value (327.7 s) probably due to the small size of the test motors used. A higher thermal sensitivity of the propellant was noted with respect to that of HTPB/AP/Al propellants: 0.0064 against 0.0015 1/K.
At ICT four end burning Al/ADN/GAP propellant grains have been casted. The manufactured propellant cured well and had a density between 99.8-100% of the theoretical maximum density. Computer tomograph inspection showed that two grains from the first casting process revealed sporadic voids and two grains from the second casting process contains few voids. The four test motors were successfully fired at different pressures between 20 and 100 bar. The propellant burned 0 - 14% faster compared to the results from the strand burner tests and showed a decreasing burning rate at higher pressure range. Ispexp/IspICTCode is relatively low, between 79-87%. The reasons are the two phase losses, the small motor design and the short burning times. The c* efficiency ηC* (combustion efficiency) was found to be between 92 and 100%.
At FOI four ADN/GAP propellant grains have been casted from a 2.7 kg batch. The manufactured propellant cured well and had a density between 99.0-99.5% of the theoretical maximum density. X-ray inspection of the cured grains showed one void in one grain and possibly small voids in two other. The four test motors were successfully fired at different pressures. The propellant burned 12 - 24% faster compared to the results from the strand burner tests. The combustion efficiency (c* efficiency) was between 93 and 95% which is in line with previous experience using the selected test motor size. There were irregular fluctuations in the pressure-time and the thrust-time records. Possible causes of the irregularities are believed to be:
• de-bonding between propellant and bakelite tube
• voids or other inhomogeneities in the grain
• igniter flow affecting or causing damage to the grain
• combustion dynamics of the propellant
At TNO one large grain of ADN/GAP/Al propellant was casted, which was subsequently machined in slices of varying diameter between 10 and 30 mm. The manufacturing process of the propellant required attention as the initial mixing did not create a homogeneous mixture. By increasing the temperature, the mixing process was improved. The casting of the propellant was smooth. The density of the propellant was in the order of 97,3 ± 0.1% of TMD, which is lower than generally accepted. A total number of 10 slices were tested at different pressures by mounting different nozzles on the L* test rig. With only one propellant slice stable burning behaviour was achieved and only for a short period of time. As the density of the propellant was 97,3 % of TMD this may have an influence on the burning behaviour, as the propellant will be compressed by the pressure generated during the combustion process, herewith interfering the formation of a stable, cigarette burning combustion front. Another factor may be the formation of slag material depositing in the nozzle throat, hereby increasing the pressure inside the chamber. At a certain point the pressure becomes that high that it blasts the slag away, increasing the throat opening and herewith reducing the pressure in the chamber. For the propellant slice that shortly burned in a stable manner at a pressure of 59,1 bar, the burn rate was 26,5 mm/s.
Potential Impact:
The objective of HISP was to develop solid propellants with higher performance which will enable increased scientific return from future space exploration missions by significantly reducing time, cost and mass required for spacecrafts to reach their destinations. Propulsion and propellants are key technologies for all space missions. Developing new propellants will strengthen the European space propulsion industry and increase its competitiveness, and will increase the effectiveness of future European space exploration missions. Increased scientific return from space exploration missions will increase the knowledge of the solar system and the universe and, in the long run, the knowledge of ourselves.
Sustainable Development has become a top priority on the European and international agendas. With ever increasing environmental concerns, the industries in Europe need to adapt to more restrictive environmental legislation in order to stay competitive and to enhance social acceptance. The space industry is in this case no exception which is reflected by ESA´s Clean Space Initiative and ESA´s Green Propulsion Harmonisation Process.
Replacing current AP-based propellants with green alternatives, such as ADN, are thus very interesting. The results from the HISP project are very valuable for paving the way for development of more environmentally friendly solid propellants for launchers, and future successful development of a high energy green propellant will lead to a breakthrough in solid rocket propulsion technology with respect to performance, competitiveness and environmental impact.
Main dissemination activities
Dissemination of HISP and its results have included several key elements, such as:
1. HISP public website
2. ESA green propellant harmonisation process
3. ESA Green Propellant Working Group
4. Publication in Scientific Journals
5. Participation in conferences
6. Dissemination meetings with relevant industries, institutes and agencies
The official HISP website provides general background information of the HISP project as well as the results. Due to the scientific nature of the HISP project a continuous exchange with groups and researcher not directly involved was essential. Presentation of the HISP results at relevant conferences and in journals has thus been encouraged.
Beyond conferences and publications, an important element of the dissemination efforts is the participation in meetings with the relevant decision makers from industry and research agencies. Particularly important was the ESA organised Green propellant Working Group, and the ESA Green propellant Harmonisation Process. During these meetings the HISP elements and project goals was presented and discussed.
In the moment a newly introduced technology is starting to become accepted by the industry, people are needed who understand this technology and can further develop it. To the same token a new technology can only be integrated into the industrial world if highly knowledgeable and skilled workforce is available.
Beside the above, HISP intends to exploit other means of dissemination whenever possible. This includes workshops, specially organised sessions at conferences and others.
1. HISP public website
On the HISP public website a short presentation of the project, its goal and results are presented. The website is repeatedly updated to reflect the status of the project and contains the following six subpages:
• Activities and events
• Objectives
• Partners
• Outline
• Facts
• Dissemination
• Results
Apart from giving an overview of the project and the partners involved, accepted public deliverables can be downloaded from the results page. A HISP flyer can also be downloaded from the fact page.
The total number of pageviews of the HISP website was 3855 from April 2011 until January 2014. The number of visits (made by a web browser) on the website were estimated to be around 2900 during this period. The website has been visited from more than 20 countries. The eight most frequent countries have been Sweden, Italy, Germany, France, UK, India, Russia and Iran.
2. ESA green propellant harmonisation process
ADN is seen as a promising green alternative to ammonium perchlorate. Thus the project actively participated in ESA’s green propellant harmonisation process in cooperation with the Swedish Space Board, SNSB. The Swedish view on green propellants and the activities performed in the HISP project was presented by Robert Lundin, SNSB, 2012-02-07. The input was also included in the Technical Dossier of Chemical Propulsion, 2012.
3. ESA green propellant working group
In conjunction to the conference “Space Propulsion 2012”, Bordeaux, France, in May 2012 a Green Propulsion Workshop was organised by ESA. At the meeting the Green Propellant Working Group was founded with the purpose to discuss the development of green propellants for space applications. The Green Propellant Working Group was identified as an important dissemination activity and members from the HISP team have been presented at the following meetings:
• Initial meeting in Bordeaux France, May 2012
• Phone conference, August 2012
• Phone conference, June 2013
• Meeting in Linköping, Sweden, September 2013
Participants from HISP will continue to participate in the Green Propellant Working Group also after the end of the HISP project.
4. Publication in Scientific Journals
Publication in scientific per-reviewed journals is a very important mean of dissemination to the scientific community. Below is a list of papers submitted which still are in the review process. Papers currently accepted for publication are shown in the table “LIST OF SCIENTIFIC PUBLICATIONS” in section 4.2.
• Activated Aluminum Powders for Space Propulsion
Filippo Maggia, et al.
Submitted to Powder Technology July 24, 2013
• Green Space Propulsion: Opportunities and Prospects
Amir S. Gohardania et al.
Submitted to Progress in Aerospace Sciences May 13, 2013
• High Performance Green Propellants
N. Wingborg
Submitted to CEAS Space Journal, autumn 2013.
5. Participation in conferences
The project strived to present as much public results as possible. One of the most effective ways for this is to attend relevant conferences. The project team has been very active in this respect. During 2011 and 2014 a total number of 23 papers were presented at 14 different conferences. A more detailed description of respective paper is presented in HISP deliverable D9.5 and an overview of the papers presented are shown in the table “LIST OF DISSEMINATION ACTIVITIES” in section 4.2.
6. Dissemination meetings with relevant industries, institutes and agencies
Direct contact with the industry, relevant research entities and space agencies is considered to be essential for dissemination. Subject of such meetings is the presentation of HISP in general, its goals and results as well as to exchange of information. A few of the more important dissemination meetings were:
• Swedish Society of Aeronautics and Astronautics, May 2011
• NASA and ESA ,September 2011
• OHB Sweden and NASA, September 2011
• Stefan Schlechtriem. DLR, Director Space Propulsion. August 2012
• Ferran Valencia-Bel. ESA / ESTEC, September 2012
• OHB-Sweden. September 2012
• Swedish National Space Board, February 2013
Exploitation of results
Exploitation of the results generated by the HISP project is mainly of interest to the industrial partners involved in the project, which were:
• EURENCO France
• EURENCO Bofors
• AVIO
EURENCO France will benefit directly of the HISP results by the improved GAP production methods developed in the project. This will enable improved productivity and more environmental benign production methods and thus make EURENCO France to a more competitive provider of energetic polymers.
EURENCO Bofors will exploit the improved ADN production methods developed in the project. This will lead to future reduction of the cost of ADN, increase the competitiveness of the company and manifest EURENCO Bofors as the worldwide leader in ADN production. The analysis of the future cost of ADN performed in the project will also help in marketing.
The technology readiness level of the propellants developed in the project needs to be increased before put into production. However, AVIO will benefit from the project results by the increased knowledge concerning green high performance solid propellants and to show that they are a forerunner in environmental benign propulsion technology. To show that they are concerned about environmental issues will also improve their public relation.
The project was coordinated by:
FOI, Swedish Defence Research Agency
SE-164 90 Stockholm
Phone +46 8 555 030 00
Fax +46 8 555 031 00
Contact:
Niklas Wingborg
Phone : +46 8 5550 4181
Fax : +46 8 5550 3949
e-mail: niklas.wingborg@foi.se
Spacecrafts are propelled using chemical or electrical propulsion systems. Electrical propulsion provides superior specific impulse but very low thrust. If high thrust is required, chemical propulsion is the only possible alternative. Today the majority of spacecrafts use chemical propulsion and this is also expected in the foreseeable future. The only way to drastically increase the performance of chemical propulsion systems is to improve the propellants. Depending on the propellants used, chemical propulsion systems are divided in two major categories:
• Liquid propellant rockets (monopropellant and bi-propellant systems)
• Solid propellant rockets
Liquid rockets provide high performance and adjustable thrust but are complex, costly and use toxic propellants. The benefits of solid rockets are their storability, compactness and simplicity. No propellant delivery system is required which enables a huge improvement in reliability and cost. One disadvantage with solid propellants is however their relatively low performance.
The aim of the HISP project was to improve the performance of solid propellants by using the new high energy density oxidizer ammonium dinitramide, ADN, an energetic binder based on glycidyl azide polymer, GAP, and high energy density fuels such as aluminium hydride, nano-aluminium and activated aluminium. These materials have all been known for many years but using them has shown to be a challenging effort due to problems concerning manufacturing, chemical stability and compatibility.
The project has substantially increased the technology readiness level of these materials and shown that solid propellants with substantially higher performance, compared to state of the art solid propellants, can be developed. Unfortunately, the burning rate of propellants based on the energetic polymer GAP seems high for many space applications. However, propellant based on ADN and non-energetic polymers is as a very interesting green alternative to current perchlorate based solid propellants.
Project Context and Objectives:
Solid propellant rocket motors have been used to propel spacecrafts in numerous missions since first used in the upper stage of the first U.S. Satellite Explorer I in 1958. They have been utilized in many different in-space applications such as:
• Large kick motors for orbital insertions
• Retrorockets for de-orbital manoeuvres
• Retrorockets, in combination with airbags, for soft landings
• Small spin rockets for spin stabilized spacecrafts
One well known spacecraft is the Magellan Venus probe launched from the Space Shuttle in 1978, using a solid rocket motor. On arrival at Venus, after a 15 month journey, a second solid propellant motor was fired to place the spacecraft in its orbit around the planet. The same type of motor has been used in several other missions until 1997, among them the Jupiter spacecraft Galileo in 1989. Another example is the Mars Observer, launched in 1992, which used a solid rocket motor to boost its way to Mars. More recently solid propellant rocket motors are considered for deorbiting in the ESA Clean Space Initiative and by NASA for the ascend module of the Mars sample return mission.
There is an ever increasing objective to significantly reduce the time, cost and mass of spacecrafts and to increase the scientific return of space exploration missions. The time and mass required for a spacecraft to reach its destination is directly linked to its propulsion system. The only way to significantly improve the performance of a propulsion system is to develop propellants with higher specific impulse and higher density. Using propellants with higher performance enables increased payload. A typical interplanetary transfer stage may contain 10 tons of solid propellant. If the specific impulse is increased by 10%, the mass of the spacecraft can be reduced significantly, or its scientific payload can be increased by several hundred of kilograms.
State of the art solid propellants are based on the oxidizer ammonium perchlorate (AP), NH4ClO4, and aluminium powder embedded in a hydroxyl terminated polybutadiene (HTPB) polymer binder matrix. AP is in many ways an excellent oxidizer due to its relative low explosive hazard and the possibility to tailor its ballistic properties. However, propellants based on HTPB/AP/Al have low specific impulse compared to current storable bi-propellants.
HISP stands for High performance solid propellants for In-Space Propulsion, or solid propellant with High Isp (specific impulse). As the name indicates, the objective of the project was to develop solid propellants with substantially higher performance compared to solid HTPB/AP/Al based propellants.
To accomplish this, the new high energy density oxidizer ammonium dinitramide, ADN, an energetic binder based on glycidyl azide polymer, GAP, and high energy density fuels such as aluminium hydride (AlH3), nano-aluminium and activated aluminium has been used in the project. Using these materials has in the past shown to be a challenging task due to problems concerning manufacturing, chemical stability and compatibility. In order to develop high performance solid propellants the technology level of these materials needed to be improved substantially.
Project Results:
Improving the technology readiness level of solid propellants containing new high energy density ingredients require not only propellant development as such, but also to improve the production methods of these materials. In the HISP project substantial improvements have been made concerning the following topics:
• ADN production improvement
• GAP production improvements
• GAP binder development
• High energy density fuels
• Propellant formulation and characterization
• Rocket motor testing
The results for each of these topics are presented below.
ADN production improvement
Ammonium dinitramide, ADN, NH4N(NO2)2, is an environmentally benign high energy inorganic oxidizer salt, first synthesized in 1971 at the Zelinsky Institute of Organic Chemistry in Moscow. It is today produced by EURENCO Bofors in Sweden. The material is of high purity but the cost is high. In order to implement ADN in future propulsion applications, the production method needs to be improved to reduce the cost. To some extent the cost can be decreased by improving the current production method. However, to drastically reduce the cost new production methods are required and production in larger scale is needed.
The current used ADN production method is a three step synthesis starting from ammonium sulfamate. The production improvement on ADN has concentrated on; recirculation of chemicals used in the process, finding a new solvent for ADN crystallization for higher production rate, a study of ADN properties relevant for drying, handling and packaging operations, and a new synthesis route.
Studies of the current ADN production process have shown it is possible to recover chemicals from the process. Guanylurea salt recovered from the second process step was recycled and tested for production of GuDN. The yield was as expected, after losses accounted for by the solubility of GuDN in the acid/water. Ammonium sulfate from the final process step was recovered in good yield and ADN produced from this recovered ammonium sulfate fulfilled the specification.
ADN is crystallized by removing water from the reaction mixture by azeotropic evaporation with a solvent, propanol. Propanol occurs as 1-propanol and 2-propanol. Today 2-propanol is used, but this work shows that 1-propanol is to prefer. ADN of good quality has been produced using 1-propanol and a method suitable for large scale production has been established. 1-Propanol is technically possible to recover with a purity of at least 99 mole-percent from an azeotropic water mixture, by extractive distillation. The optimum results are achieved with a distillation column operating at low pressure (500 mbar). The yield of this process is estimated to be 80% but the designed process will recover 94% of the initially entered propanol. In this work benzene was used for the extractive distillation but if the process is to be scaled up to industrial scale another solvent should be used.
The drying and handling operations could be improved if the ADN crystals are washed with a non-solvent prior to drying, which diminish the crystal lumping tendency. The dried product should be stored and handled at a humidity no higher than 45 % RH, but not at super-dry condition.
A new synthesis route, a two-step nitration of urea with preliminary separation of nitramide has been investigated. Using this process, the problem of ammonium ions in the spent acid would be avoided. A total yield of 40% of dinitramide has been achieved. This is rather low but since urea is very cheap this is not considered a problem. The process is still not ready for scale up, especially the hydrolysis of dinitrourea into nitramide requires more efforts.
The ADN production method was evaluated to make a prospect for industrialization, including estimation of future prices. A conceptual process design (CPD) was made for the improved ADN production process.
To assess the economic feasibility of large scale production of ADN, the Aspen program was used to simulate the production process and the results of the simulation were used as inputs for an economic feasibility analysis. Separation, crystallization and acid recovery sections were not included in the study; neither were storage facilities including piping for raw materials, intermediates and final product. This was related to the lack of precise kinetic data and thermodynamic properties of several substances that made the modelling of the reaction section of the plant not straightforward. The design of the chemical process and its specifications were based on an annual production rate of ammonium dinitramide of 2500 tonnes/year.
Investing in a facility for production of 2500 tonnes of ADN per year is a long term project. To give a view of future prices in medium term perspective, a process for a medium scale ADN production was defined, based on current production know-how and possible improvements established within this project. Then prices for ADN produced using the improved process in plant with capacities of 40-400 ton ADN/year were estimated.
According to the CPD an estimated future cost for large scale production of ADN could be in the order of 15 €/kg. This price does not include profit or return on investments. Neither are costs for separation, recrystallization, acid recovery, waste treatment etc. included. Some of these items will increase and some decrease the cost of ADN, but all together will have a small effect on the price of ADN.
The estimated price for ADN produced according to the EUB defined process described in the deliverable D2.3 is approximately 62-65 € for 40-400 tonnes per year, when all investments are paid off and with costs for media, personnel, chemicals etc. at the 2013 cost level. The bottle-necks in the described process are the crystallization and the nitration operations.
In general, the performance of a solid propellant increases with increased solid loading (oxidizer particles and aluminium powder). If too much is added, the viscosity of the uncured propellant slurry becomes too high. To obtain a castable propellant formulation with sufficient low viscosity and high solid loading, particles with minimum spatial extension are required. For this reason spherical particles are preferred. The particle shape of ADN as received from the synthesis is needle shaped and thus not suitable for formulation. Methods to produce ADN particles with suitable morphology have thus been studied.
Several different particle production methods were evaluated in order to select the method to be scaled up. The selection was based on the maturity of respective particle production method and the approximate particle size produced. Usually bimodal particle sizes are used in propellants. The desired sizes of respective fraction are in the range of 200 – 250 µm and 20 – 50 µm respectively. The results from the study show that:
• Spray prilling can produce spherical particles of desired size and shape. The method is mature for
up scaling
• Recrystallization
Cool crystallization gives particles with a high aspect ratio and broad particle size distribution.
Currently this method is not mature for up scaling
Solvent/non-solvent technique gives a small particle size distribution and small particles with
a favourable aspect ratio
• Jet milling seems as a promising method but the particles produced are too small to be used in the
project
• Emulsion prilling is currently not mature for up scaling since the process- or material-parameters
need to be modified
The two most suitable methods were spray prilling and solvent/non-solvent crystallization. In a propellant more large particles are used than small. Due to this and the reason that solvent/non-solvent crystallization of ADN in larger quantities has not been performed it was decided to select spray prilling as the method to be scaled up to a capacity of producing at least 10 kg ADN particles per day. The apparatus is further described in deliverable D3.3 “Scaling up of selected production method”.
The capacity of the spray prilling apparatus built is reported in HISP deliverable D3.4 “Production of ADN particles for WP7 using new method”. The capacity of the up scaled spray prilling apparatus has been verified. During four days a total amount of 45 kg prilled ADN was produced and the maximum amount of prills produced during one day was 16 kg. It was also found that jet milling was a convenient method to obtain small ADN particles.
A theoretical study has been performed on future industrial scale production to estimate the future cost of ADN particles. With an annual production of 40-400 ton ADN/year, the cost for spray prilled ADN is estimated to be in the range of 65 to 85 €/kg and the cost for jet milled ADN is estimated to be approximately 60 €/kg, at 2013 cost level. Costs for investments are not included in the cost estimates and thus the actual costs will be higher during the first years of production.
GAP production improvements
Glycidyl azide polymer, GAP, is an energetic pre-polymer manufactured by EURENCO France. Production of GAP is made using a two-step process. First an intermediate (PECH) is produced from polymerization of epichlorhydrinein dichloroethane, then GAP is formed by azidation. The azidation reaction is quite time consuming.
A theoretical study of possible process improvements and validating lab studies have been carried out. Dichloroethane (DCE) used as a solvent in the GAP production process most probably will have to be replaced in the future due to its environmental impact. The studies of the GAP production process has shown that replacement of DCE by dichloromethane as a solvent for the polymerization reaction can be considered as an intermediate solution but a long-term solution is needed.
A new analytical method to follow the azidation reaction has been developed and applied to follow GAP production in industrial scale. The azidation reaction time could not be decreased, despite more efficient stirrer or larger quantity of catalyst. Washing/decantation is possible improve at lab scale by adding a solvent. This improvement will be validated at industrial scale production.
GAP binder development
GAP in combination with ADN provides high specific impulse and good ballistic properties. However, the mechanical properties of GAP needed to be improved in order to obtain a propellant with sufficient elasticity and strength. In HISP delivery D4.2 “Development of curing systems” the mechanical properties of GAP were studied by using isocyanate free curing systems (ICT) and more conventional curing systems (FOI) and in delivery D4.3 “Improvement of mechanical properties” it was shown that the mechanical properties of GAP could be improved by optimizing the curing systems used.
The work performed at ICT showed that all binder formulations and ingredients were compatible with ADN. The mechanical properties obtained were however not in the range as desired but it still seemed possible to obtain a propellant formulation with acceptable mechanical properties without or with a low content of plasticizer. The most promising binder formulations was selected and characterized.
At FOI several types of GAP based binders were investigated with respect to mechanical properties. The binder with best mechanical properties was based on GAP, cured with mixture of Desmodur N3300 and Desmodur W, using 1,4-butanediol as chain extender. This material had an excellent strength (2.56 MPa) and elasticity (730%). The second best material was GAP, cured with Desmodur N3400. It had a strain of 630% and a strength of 0.59 MPa. Unfortunately, none of these materials cured properly in the presence of ADN. Successful curing and reasonable mechanical properties were however obtained by omitting the chain extender. The influence of four different bonding agents was also studied. The preferred bonding agent resulted in increased strength and elasticity of the selected binder, but adding more than 0.5 phr resulted in gassing problems during mixing.
High energy density fuels
In current solid rocket propellants aluminium is used as fuel. It has a relatively high specific density, abundantly available, burns with a high temperature (which is good for generating high pressures and hence high thrust levels) and is compatible with the other ingredients in the formulation.
The standard Al used is of micrometric size, typically spanning over the range 10 to 50 μm. It is a well-known fact that combustion of micro-sized Al implies two-phase flow losses in supersonic gasdynamic nozzles and a series of additional possible problems (throat erosion, slag accumulation, incomplete combustion, etc.) depending on the specific operating conditions and motor configuration. For example, incomplete combustion is mainly associated with insufficient residence time in the combustion chamber (i.e. small motors). At any rate, two-phase flow losses are the main reason for the poor combustion efficiency generally observed in metalized formulations. The elusive agglomeration phenomena, leading to burning particles much larger than the initial Al powder, are the key factor for this effect. By decreasing the Al ignition temperature using activated Al or nano-Al, the agglomeration effects can be mitigated, although not totally eliminated.
Another way to increase the specific impulse is by replacing aluminium with aluminium hydride. Although the density of aluminium hydride is lower than that of aluminium (meaning that one can take less aluminium hydride per unit volume compared to aluminium and hence less energy is available), the combustion energy released by aluminium hydride more than compensates for the difference in specific density. However, its use has been limited due to its incompatibility with several components and its apparent instability in general. Research was performed in the past on this latter issue. The literature mentions that research was conducted on the effect of air, impurities, heat, radiation and water on aluminium hydride and no decisive conclusions were given for all cases.
Hence, in order to obtain the desired performance two main roadmaps were followed; the enhancement of ideal performance on one side, and the reduction of specific impulse losses (mainly derived from two-phase flow) on the other. An initial basket of high-energy density fuels comprising three types of nanoaluminum powders, three chemically activated batches of micrometric aluminium particles and a sample of aluminium hydride was available at the beginning of the project. In addition other standard metal powders were tested for comparison.
The ingredients under examination were shortlisted pursuing a compromise between high active metal content and powder reactivity. The project addressed also the properties of powder blends aiming at a compromise among the properties of the individual ingredients. After a preliminary characterization of the fuel powders alone and as ingredients of standard propellants, a relative grading among the aluminium powders was obtained.
During the first two years of the project, the production of aluminium hydride was attempted in batches of 20 – 30 grams which turned out to be impure, necessitating more work on smaller scale methodology. Research activity carried out on small scale batch production identified the cause for the failure. Before continuing with larger scale production, some vital tests with the nearly pure AlH3 were performed, being the long term stability of the alane and its compatibility with the other propellant ingredients classified as major issues for the final application. It turned out that AlH3 was compatible with all ingredients, including ADN, but that the long term stability was insufficient for rocket propulsion applications.
Alternative routes to produce alane were investigated and the route using silicium tetrachloride was reproduced from literature. After some fine tuning of the heating program also needed in this route yielded the same purity of AlH3 compared to the conventional synthesis (i.e. using lithium aluminium hydride with aluminium chloride) route. From a simplicity point of view, there is not that much difference between the two routes. However, safety issues associated with the silicium route are more severe compared to the other one, since highly explosive and toxic vapours are produced during the synthesis and this detail must be carefully considered when scaling up. Notwithstanding the accomplishments, the stability of the AlH3 produced with the silicium tetrachloride route has to be assessed, also in contrast with the results obtained from the conventional route.
Since aluminium hydride was not available for propellant testing, the sole option left for the improvement of the delivered specific impulse was the reduction of two-phase nozzle flow losses generated by the presence of agglomerates. Further combustion studies thus focused on a set of three ingredients: one type of nanometric aluminium, one type of activated aluminum, and a baseline micrometric aluminum. Conclusions from the investigations suggested that the use of multimodal powders obtained by mixing nanoaluminum and standard micrometric aluminium would lead to a good compromise between burning rate, metal content, agglomeration, and processability.
The optimal micro-nano aluminium blend was investigating by developing some additional propellants. In the compositions a part of a standard micrometric aluminium fuel was replaced by a nanopowder. The formulations consisted of standard AP/Al/HTPB propellants where 1.5%, 3.0%, and 4.5% of the baseline aluminium fuel was replaced with nanoaluminum, testing the materials for ballistics, ignition delay, and agglomeration attitude. A procedure for the collection of the condensed combustion products was developed and applied to the propellant set. The collection facility enabled the detailed measurement of the combustion products through laser granulometry, computing relevant statistical parameters.
The fine tuning of propellant composition was accomplished using the reference AP/Al/HTPB composition, performing a partial replacement of micrometric baseline aluminum with coated nanoaluminum. The progressive introduction of small fractions of nano Al produced a monotonic increase of burning rate. As expected, the increment was not proportional and the effect tended to be lower as the amount of nanometric powder increased. Halfway improvements have been obtained through the replacement of 1.5% of µAl with nAl. Gains with respect to the baseline have been found in the order of 20-30% depending on the operating pressure.
Once nAl content was increased to 3%, the gain in burning rate ranged between 40-60% while, for 4.5%, propellant ballistics approached the behaviour of composition, containing nAl only. Experimental data revealed gains in the order, respectively 55-75% for the former composition and 60-80% for the latter.
Similar trends were observed for propellant ignition delay, measured by exposing the energetic material to a CO2 laser. With respect to the baseline, all propellants featured a decrement in the time required to come to ignition, thanks to the addition of even small quantities of nAl. Also in this case, a saturation effect was observed for the formulation containing 3% of nAl.
The collection of the condensed combustion residues was performed in an experimental rig where the propellant strand was burned upside down. The condensed combustion products (CCPs) originated from the specimen combustion were quenched in a pool filled with proper liquid. The collection was then treated before the analysis in a laser granulometer. Combustion tests have been performed at 10 bar.
The result showed that nAl decreased the size of condensed combustion products. Agglomerates for featured about 3 µm of mass-weighted mean diameter which is sensibly lower than the value measured for the baseline propellant, which was around 20 µm. The presence of small fractions of nAl did not carry out immediate advantages at the tested conditions, in terms of mass-weighted mean diameter and only when nAl was set to 4.5%, the distribution curve became very similar to that obtained for the propellant containing only nAl.
From the ensemble of the results available for the mixed HEDF fuelled propellant series it appears that the step-up performance gain was obtained for a nAl content between 1.5 and 3%. Further increase above 3% of nAl leaded to reduced gains in both burning rate and ignition delay. Moreover, the performance of the formulation containing 4.5% nAl was almost levelled with behaviour of propellant the propellant fully fuelled with nAl. Regarding agglomeration attitude, a qualitative leap was obtained only when the nAl content was set to 4.5% or above.
Results obtained showed that an optimal replacement fraction can be identified on the basis of performance parameters (burning rate, agglomeration, ignition delay). Also the metal content must enter the definition of the optimal µAl-nAl fuel blend since nano-sized aluminium particles contain a consistent fraction of metal oxide which, in turn, reduces the ideal attainable performance.
Propellant formulation and characterization
The selection of propellant compositions was based on thermodynamic calculations, small scale propellant preparation, recommendations from the system analysis study, the evaluation of the different high energy density fuels (HEDF) considered and results from the GAP binder development performed.
As a result AVIO, TNO and ICT selected aluminised ADN/GAP propellant formulations. FOI decided not to use HDEF and develop an ADN/GAP-propellant with high filler loading. Initial formulation assessment of small batches showed which calculated composition could be prepared in reality. The propellant formulations were scaled up and characterized with respect to processability, physical properties, small-scale hazard testing, mechanical properties and thermal stability. The progress was slowed down by problems regarding the formation of voids during propellant curing, unsatisfying mechanical properties and the removal of a curing agent from the product range.
The ideal specific impulse (Isp) for an Al/ADN/GAP or ADN/GAP propellant was unrealizable. The compositions selection had to be compromises between Isp and processabilty. The following propellants were formulated and characterized:
• ADN/GAP (70/30)
• Al/ADN/GAP (18/56/26)
• Al/ADN/GAP (16/60/24)
Mechanical properties of ADN/GAP-based propellant were the most challenging properties. However they could be improved significantly with a maximum elongation of approximately 15%. Interestingly the elongation at break increased at lower temperatures.
The formation of bubbles during propellant curing created problems which lead to a large deviation of the densities compared to the maximum theoretical density. This effect, especially with the propellant preparation at AVIO and ICT delayed the development but was eventually solved.
As opposed to the gas formation during curing stability investigations of the cured propellants revealed no suspicious behaviour and the cured propellant therefore could be considered stable. Self-ignition temperature and adiabatic self-heating rate is dominated by the ADN properties. A deterioration of the mechanical properties was found by aging of the propellant. This is probably due to debonding phenomena affecting the solid particles and the binder system. The ageing tests demonstrated that the GAP binder system is stable at the used ageing conditions.
Pot life time is sufficient for grain preparation for small scale motors but probably not sufficient for casting large motors. Plasticizer were investigated but not used in the final formulation which cause high glass transition temperatures (>-34°C).
The results of the ballistic evaluations of ADN/GAP-based propellant pointed out high burning rates (>19 mm/s @ 7MPa) and reasonable pressure exponent similar to traditional propellants. The burning rate is slightly lower for the aluminized formulations but still too high for the desired application. Additional metal agglomeration experiments were performed at ICT in cooperation with POLIMI. The agglomerates measured for Al/ADN/GAP propellants are about 30 to 50% larger over the tested pressure range of 0.1- 5 MPa than the ones of commercial formulations of AP/HTPB/Al.
Impact sensitivity and friction sensitivity are in the range of AP/HTPB/Al composite propellants and double base propellants. Different small gap test were performed. ADN/GAP propellants should belong to hazard class division 1.1 whereas for Al/ADN/GAP propellants it might be possible to reach a hazard class division 1.3. The Koenen test exhibits no evidence for a detonation or deflagration. Fast burning was the observed reaction only and was evaluated as “explosion”. No reactions were observed at the electrostatic discharge tests (EDS).
Rocket motor testing
The propellants developed have been fired in rocket motors by several project partners.
Six test motors were prepared at AVIO and fired at two temperatures 20 and 60°C. In this occasion a 4 kg scale mix was performed using the composition and the mixing procedure indicated in D6.3. No problem was encountered in mixing and casting of the propellant which was free of bubbles and with a very good density, not less than 99 % of the theoretical maximum density. The firing tests showed regular combustion of the propellant. A slight erosion of the nozzles was observed. The final diameters were about 0.1 mm larger than initial ones. The measured specific impulse (299.4-306.0 s) was lower compared to the calculated value (327.7 s) probably due to the small size of the test motors used. A higher thermal sensitivity of the propellant was noted with respect to that of HTPB/AP/Al propellants: 0.0064 against 0.0015 1/K.
At ICT four end burning Al/ADN/GAP propellant grains have been casted. The manufactured propellant cured well and had a density between 99.8-100% of the theoretical maximum density. Computer tomograph inspection showed that two grains from the first casting process revealed sporadic voids and two grains from the second casting process contains few voids. The four test motors were successfully fired at different pressures between 20 and 100 bar. The propellant burned 0 - 14% faster compared to the results from the strand burner tests and showed a decreasing burning rate at higher pressure range. Ispexp/IspICTCode is relatively low, between 79-87%. The reasons are the two phase losses, the small motor design and the short burning times. The c* efficiency ηC* (combustion efficiency) was found to be between 92 and 100%.
At FOI four ADN/GAP propellant grains have been casted from a 2.7 kg batch. The manufactured propellant cured well and had a density between 99.0-99.5% of the theoretical maximum density. X-ray inspection of the cured grains showed one void in one grain and possibly small voids in two other. The four test motors were successfully fired at different pressures. The propellant burned 12 - 24% faster compared to the results from the strand burner tests. The combustion efficiency (c* efficiency) was between 93 and 95% which is in line with previous experience using the selected test motor size. There were irregular fluctuations in the pressure-time and the thrust-time records. Possible causes of the irregularities are believed to be:
• de-bonding between propellant and bakelite tube
• voids or other inhomogeneities in the grain
• igniter flow affecting or causing damage to the grain
• combustion dynamics of the propellant
At TNO one large grain of ADN/GAP/Al propellant was casted, which was subsequently machined in slices of varying diameter between 10 and 30 mm. The manufacturing process of the propellant required attention as the initial mixing did not create a homogeneous mixture. By increasing the temperature, the mixing process was improved. The casting of the propellant was smooth. The density of the propellant was in the order of 97,3 ± 0.1% of TMD, which is lower than generally accepted. A total number of 10 slices were tested at different pressures by mounting different nozzles on the L* test rig. With only one propellant slice stable burning behaviour was achieved and only for a short period of time. As the density of the propellant was 97,3 % of TMD this may have an influence on the burning behaviour, as the propellant will be compressed by the pressure generated during the combustion process, herewith interfering the formation of a stable, cigarette burning combustion front. Another factor may be the formation of slag material depositing in the nozzle throat, hereby increasing the pressure inside the chamber. At a certain point the pressure becomes that high that it blasts the slag away, increasing the throat opening and herewith reducing the pressure in the chamber. For the propellant slice that shortly burned in a stable manner at a pressure of 59,1 bar, the burn rate was 26,5 mm/s.
Potential Impact:
The objective of HISP was to develop solid propellants with higher performance which will enable increased scientific return from future space exploration missions by significantly reducing time, cost and mass required for spacecrafts to reach their destinations. Propulsion and propellants are key technologies for all space missions. Developing new propellants will strengthen the European space propulsion industry and increase its competitiveness, and will increase the effectiveness of future European space exploration missions. Increased scientific return from space exploration missions will increase the knowledge of the solar system and the universe and, in the long run, the knowledge of ourselves.
Sustainable Development has become a top priority on the European and international agendas. With ever increasing environmental concerns, the industries in Europe need to adapt to more restrictive environmental legislation in order to stay competitive and to enhance social acceptance. The space industry is in this case no exception which is reflected by ESA´s Clean Space Initiative and ESA´s Green Propulsion Harmonisation Process.
Replacing current AP-based propellants with green alternatives, such as ADN, are thus very interesting. The results from the HISP project are very valuable for paving the way for development of more environmentally friendly solid propellants for launchers, and future successful development of a high energy green propellant will lead to a breakthrough in solid rocket propulsion technology with respect to performance, competitiveness and environmental impact.
Main dissemination activities
Dissemination of HISP and its results have included several key elements, such as:
1. HISP public website
2. ESA green propellant harmonisation process
3. ESA Green Propellant Working Group
4. Publication in Scientific Journals
5. Participation in conferences
6. Dissemination meetings with relevant industries, institutes and agencies
The official HISP website provides general background information of the HISP project as well as the results. Due to the scientific nature of the HISP project a continuous exchange with groups and researcher not directly involved was essential. Presentation of the HISP results at relevant conferences and in journals has thus been encouraged.
Beyond conferences and publications, an important element of the dissemination efforts is the participation in meetings with the relevant decision makers from industry and research agencies. Particularly important was the ESA organised Green propellant Working Group, and the ESA Green propellant Harmonisation Process. During these meetings the HISP elements and project goals was presented and discussed.
In the moment a newly introduced technology is starting to become accepted by the industry, people are needed who understand this technology and can further develop it. To the same token a new technology can only be integrated into the industrial world if highly knowledgeable and skilled workforce is available.
Beside the above, HISP intends to exploit other means of dissemination whenever possible. This includes workshops, specially organised sessions at conferences and others.
1. HISP public website
On the HISP public website a short presentation of the project, its goal and results are presented. The website is repeatedly updated to reflect the status of the project and contains the following six subpages:
• Activities and events
• Objectives
• Partners
• Outline
• Facts
• Dissemination
• Results
Apart from giving an overview of the project and the partners involved, accepted public deliverables can be downloaded from the results page. A HISP flyer can also be downloaded from the fact page.
The total number of pageviews of the HISP website was 3855 from April 2011 until January 2014. The number of visits (made by a web browser) on the website were estimated to be around 2900 during this period. The website has been visited from more than 20 countries. The eight most frequent countries have been Sweden, Italy, Germany, France, UK, India, Russia and Iran.
2. ESA green propellant harmonisation process
ADN is seen as a promising green alternative to ammonium perchlorate. Thus the project actively participated in ESA’s green propellant harmonisation process in cooperation with the Swedish Space Board, SNSB. The Swedish view on green propellants and the activities performed in the HISP project was presented by Robert Lundin, SNSB, 2012-02-07. The input was also included in the Technical Dossier of Chemical Propulsion, 2012.
3. ESA green propellant working group
In conjunction to the conference “Space Propulsion 2012”, Bordeaux, France, in May 2012 a Green Propulsion Workshop was organised by ESA. At the meeting the Green Propellant Working Group was founded with the purpose to discuss the development of green propellants for space applications. The Green Propellant Working Group was identified as an important dissemination activity and members from the HISP team have been presented at the following meetings:
• Initial meeting in Bordeaux France, May 2012
• Phone conference, August 2012
• Phone conference, June 2013
• Meeting in Linköping, Sweden, September 2013
Participants from HISP will continue to participate in the Green Propellant Working Group also after the end of the HISP project.
4. Publication in Scientific Journals
Publication in scientific per-reviewed journals is a very important mean of dissemination to the scientific community. Below is a list of papers submitted which still are in the review process. Papers currently accepted for publication are shown in the table “LIST OF SCIENTIFIC PUBLICATIONS” in section 4.2.
• Activated Aluminum Powders for Space Propulsion
Filippo Maggia, et al.
Submitted to Powder Technology July 24, 2013
• Green Space Propulsion: Opportunities and Prospects
Amir S. Gohardania et al.
Submitted to Progress in Aerospace Sciences May 13, 2013
• High Performance Green Propellants
N. Wingborg
Submitted to CEAS Space Journal, autumn 2013.
5. Participation in conferences
The project strived to present as much public results as possible. One of the most effective ways for this is to attend relevant conferences. The project team has been very active in this respect. During 2011 and 2014 a total number of 23 papers were presented at 14 different conferences. A more detailed description of respective paper is presented in HISP deliverable D9.5 and an overview of the papers presented are shown in the table “LIST OF DISSEMINATION ACTIVITIES” in section 4.2.
6. Dissemination meetings with relevant industries, institutes and agencies
Direct contact with the industry, relevant research entities and space agencies is considered to be essential for dissemination. Subject of such meetings is the presentation of HISP in general, its goals and results as well as to exchange of information. A few of the more important dissemination meetings were:
• Swedish Society of Aeronautics and Astronautics, May 2011
• NASA and ESA ,September 2011
• OHB Sweden and NASA, September 2011
• Stefan Schlechtriem. DLR, Director Space Propulsion. August 2012
• Ferran Valencia-Bel. ESA / ESTEC, September 2012
• OHB-Sweden. September 2012
• Swedish National Space Board, February 2013
Exploitation of results
Exploitation of the results generated by the HISP project is mainly of interest to the industrial partners involved in the project, which were:
• EURENCO France
• EURENCO Bofors
• AVIO
EURENCO France will benefit directly of the HISP results by the improved GAP production methods developed in the project. This will enable improved productivity and more environmental benign production methods and thus make EURENCO France to a more competitive provider of energetic polymers.
EURENCO Bofors will exploit the improved ADN production methods developed in the project. This will lead to future reduction of the cost of ADN, increase the competitiveness of the company and manifest EURENCO Bofors as the worldwide leader in ADN production. The analysis of the future cost of ADN performed in the project will also help in marketing.
The technology readiness level of the propellants developed in the project needs to be increased before put into production. However, AVIO will benefit from the project results by the increased knowledge concerning green high performance solid propellants and to show that they are a forerunner in environmental benign propulsion technology. To show that they are concerned about environmental issues will also improve their public relation.
The project was coordinated by:
FOI, Swedish Defence Research Agency
SE-164 90 Stockholm
Phone +46 8 555 030 00
Fax +46 8 555 031 00
Contact:
Niklas Wingborg
Phone : +46 8 5550 4181
Fax : +46 8 5550 3949
e-mail: niklas.wingborg@foi.se
