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.
