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Reporting period: 2016-02-01 to 2018-01-31

Space based systems are today a natural part of our daily lives for communication, navigation and much more. The use of satellites is expected to increase in the future and so is the number of space launches. Solid rocket motors are today the most cost effective, competitive and reliable propulsion technology for space launch systems. State of the art solid rocket propellants are based on the oxidizer ammonium perchlorate, AP (NH4ClO4), and aluminium powder, embedded in a polymer binder. AP has been used since the 1940’s and is in many ways an excellent oxidizer. Unfortunately, AP has a negative impact on the environment and on personal health due to ozone depletion, thyroid gland interference and acid rain formation.

The GRAIL project was granted to investigate if a green, chlorine free, solid propellant could be developed. Developing a chlorine free green alternative to AP is a challenging task. Currently only two useful green oxidizers exist:

• Ammonium nitrate, AN (NH4NO3), and
• Ammonium dinitramide, ADN (NH4N(NO2)2).

AN is very cheap and mainly used as fertilizer. Propellants based on AN have low performance and low burning rate. Consequently, AN based propellants have mainly been used in low performance applications such as gas generators. ADN is a new powerful oxidizer still in the development phase. It provides high performance and high burning rate, but it is costlier and more explosively hazardous (1.1D detonatable) compared to AP. Thus neither AN, nor ADN, can replace AP on their own. However, by combining ADN and AN, it seemed possible to meet the properties of AP with respect to performance, burning rate, sensitivity and cost.

The objective of the GRAIL project was thus to determine if AP could be replaced by ADN and AN. This was done by developing solid propellants based on ADN/AN, aluminium powder and a polymer binder. To improve performance, high energy fuels such as nano aluminium, aluminium hydride and activated aluminium were also studied. However, in the propellant development work it was decided to focus on micrometric aluminium powder due to its maturity and ease of supply.

The green solid propellants developed were compared with state of the art solid propellants with respect to safety, performance and cost. However, the results from the extensive propellant development showed that the combination ADN/AN have undesirable combustion properties. It was also found that ADN is more sensitive than expected and thus not as high amount of ADN can be used in the propellant as desired. To improve the combustion properties a new approach was needed which finally lead to the development of two different types of propellants; one “greener” propellant based on ADN and AP, and one AP free propellant containing an energetic polymer binder. The propellants were evaluated on a launcher system level. The results show that the costlier greener ADN/AP based propellants can be commercially competitive to current propellants, due to higher performance and thus increased payload mass in orbit.
In order to develop a green solid propellant, production, processing and characterization of the chemicals needed were studied. Ways to improve the synthesis of ADN was studied in order to reduce its cost. A new synthesis method was developed and scaled up to pilot scale. The new method has the potential to substantially reduce the cost of ADN, increase its purity and to decrease the amounts of waste. The method to produce spherical ADN particles, prills, was improved and prilled ADN and prilled phase stabilized AN (PSAN) were produced in the amount needed.

Many different polymers, plasticizers and curing systems were studied in order to develop a suitable binder material. Methods to produce activated aluminium and aluminium hydride were investigated and characterized. The synthesis of aluminium hydride was substantially improved and ways to stabilize the product were studied. The results show great promise but in the propellant development it was decided to focus on micrometric aluminium powder due to its maturity.

In the formulation work the chemicals were combined in order to develop a useful propellant. Challenges encountered for the ADN/AN propellants included compatibility and curing issues. Properties that could not be compromised were safety (non-detonatable hazard class 1.3) and low pressure influence on the burning rate (low pressure exponent). A vast number of different additives were studied to reduce the pressure exponent to an acceptable value, but without success. The explosive sensitivity was studied and it was found that in order to obtain a reasonably safe propellant, less ADN must be used than initially expected. Finally two different types of propellants were developed, one greener propellant based on ADN/AP, aluminium powder and polybutadiene, and one AP free propellant based on ADN, PSAN, 1,1-diamino-2,2-dinitroethene (FOX-7), aluminium powder and polyglycidyl azide (GAP). The propellants were characterized with respect to sensitivity, stability and mechanical properties and were also fired in small ballistic test motors.

A launcher system analysis were performed using the P120 rocket motor, which will be used in both Vega C and Ariane 6, as study case. The results show that the propellant based on ADN/AN/FOX-7/GAP/Al has lower performance than current propellant. However, since no water vapour is present in the combustion gases, and due to lower combustion temperature, the nozzle erosion might be substantially lower. This will result in larger average nozzle expansion ratio and thus higher performance. To evaluate if this actually is the case, large motor tests are required.
The ADN/AP based propellant has high performance but are more costly than current propellant. The higher cost is however compensated by increased payload mass in orbit and thus ADN/AP based propellant can be commercially competitive.
Health issues related to AP is in Europe addressed by the European Food Safety Authority, EFSA. Currently, EPA is developing a proposed national primary drinking water regulation for perchlorate, and the European Commission has confirmed that EFSA will be requested to assess the need to establish an Acute Reference Dose for perchlorates. In view of a possible Union legislation following the EFSA opinion, all EU Member States are currently requested to monitor the presence of perchlorate in food. How these regulations will influence the space propulsion industry is not clear. However, the regulations will not improve the public acceptance of using perchlorates.

ESA, with its Clean Space Initiative, has increased its attention to the environmental impact of space activities, including its own operations as well as operations performed by European industry in the frame of ESA programs. In line with this, a green solid propellant alternative might in the future influence ESA’s acceptance of using AP. If found that ADN/AN can’t replace AP, this will also be an important message to ESA, National Space Agencies and to the industry. The results will thus in any case serve as important input for decision makers when considering development of future European launch systems.
Books concerning environmental and health issues related to perchlorate
GRAIL project logo
Launch of Ariane 5. Picture credit ESA