Research on HEM has increased recently again in Japan to answer these new requirements. A high-energy material research group (consisting of main members of ISAS, National Institute of Advanced Industrial Science and Technology, University of Tokyo, Yokohama National University, Nihon University, and Fukuoka University) was founded in the Japan Explosives Society. The group is proceeding with basic research on the introduction of Ammonium Dinitramide (ADN), an HEM, to solid propellant. Like AN, AND is a material comprised of only H, N, and O, but it has high hygroscopicity. This is one problem that must be cleared to utilize it fully and flexibly. One solution is to prevent moisture by skillfully processing its crystal particles. Young researchers including graduate students are positively engaged in pioneering new technology fields like this. I strongly wish to develop new solid rocket propellant together with them.
Switching to another topic, space-related scientists all over the world are troubled by space debris in orbit around the earth. We, the users of outer space, need to actively address this problem. With solid rockets, it is believed that slag emitted from upper-stage rocket motors and carbonized heat-insulation rubber (material between motor case and propellant) become debris.
The main component of slag is a mixture of oxidizer alumina and burnt Al embers (Al is one of the main components of propellant). It is thought that a little slag accumulates near the nozzle throat during rocket combustion and is released at the end of combustion. Al is generally blended in fine-particle form to improve combustion efficiency and it is a very important material affecting performance of solid propellant. If we were to eliminate Al as a solution to the debris problem, we would need to compensate for the lowered thrust performance with alternatives.
In this situation, the HEM above emerges as an alternative. Glycidyl Azide Polymer (GAP) is a polymer HEM. For more detail, please refer to ISAS News of August 2000 (No. 233). By introducing GAP in place of HTPB, we can expect to minimize reduction in thrust performance. Based on this idea, we are advancing research on the combustion properties of two-constituent GAP/AP composite solid propellant as a solid-propellant candidate that could reduce debris.
Because of limited space, I am unable to go into more detail. However, as discussed above, new technology is required with the changing era even in the field of solid propellant, which is at mature-level in application. In this article, I introduced the idea of a general-purpose material for cost reduction and HEM for higher performance and environmental-impact reduction. Fundamental research is required in both cases to introduce those new materials to solid propellant. There are still many challenges ahead.