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The Forefront of Space Science

Ever-Evolving Solid Rocket Propellant

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For solid rockets, Japan has concentrated its focus on basic technology development for the last half century. An array of technologies accumulated by R&D has been advanced through flight-demonstration opportunities and came to fruition as the well-known M-V rocket. Solid-rocket propulsion technology is now a basic infrastructure to support space activities. However, there is a change in the trend for solid-rocket technology, which has so far been driven simply by an R&D-oriented approach to improve performance. In addition to high-performance and high-function requirements, economy of space-related technologies, which is expected to encourage space utilization, is also emphasized now. This emphasis is probably not limited to the propulsion system.

In this article, I would like to introduce two futuristic research topics - one aiming at cost reduction and another aiming at both high-performance and environmental-impact reduction Econcerning solid propellant, which is unseen but is crammed into the solid rocket.

Making propellant cheaper to contribute to the expansion of space utilization

The development concept of the EPSILON Launch Vehicle, which will be a mainstay in the new era of solid rockets in Japan, requires the further evolution of launch system. The solid rocket motor used in the M-V boasts high-thrust performance which is still unparalleled by any motor in the world. The next-generation solid rocket motor to be employed in the EPSILON is designed to be even more economical to increase its international competitiveness while, based on the M-V, improving Japan’s high-level basic technology for space propulsion.

Solid rocket propellant comprises three main constituents: Hydroxyl-Terminated Polybutadiene (HTPB), used as a binding agent, ammonium perchlorate (AP) and aluminum (Al). Since propellant generally accounts for 90% of the total weight of rocket motor, it is a key factor in cost analysis. To reduce the cost, we need to utilize fundamental technologies including replacement of material with inexpensive alternatives and introduction of new more efficient production processes. Examination from the aspect of manufacturing technology is also required to maintain quality. In the meantime, we have another option: continued use of the current propellant without changing its composition from the standpoint of maintaining/improving the reliability of the solid rocket motor. With this option, no initial investment is necessary and we can minimize the initial risk of technological development. To our regret, this approach deprives researchers of the opportunity for technological development. In fact, however, using existing technology for a long time poses the risk of “material depletion,Ewhich is an unavoidable problem as time goes by. We frequently face the problem resulting from discontinuation of products or changes in specifications. Since all materials used for rockets were fully examined in the design stage, we cannot change easily any of them, even if there is an equivalent or higher-performance material according to the catalog. The problem becomes more serious if material is imported from overseas.

Therefore, we determined to initiate research on solid propellant that would hardly cause a material depletion problem and, at the same time, would realize a cost reduction.

Currently, two main materials are imported for the solid propellant now used. One is an agent for lowering combustion temperature, which is used for the Gas Generator Propellant (GGP). Japan is one of the chief countries using the agent and we import it from foreign manufacturer every time we use it. Due to limited production volume, its unit price is very high. When M-V launchers were operated, the material was also continuously used. There were no particular problems at the time, but it became evident that, once import stopped, troublesome and time-consuming work is required to resume. When I participated in the development of the next-generation solid rocket, I thought that this problem should be settled as soon as possible. The sub-propulsion system is small when compared to the entire rocket, so it is not conspicuous. Nonetheless, if the material were not produced, a serious problem would occur. For this reason, we are studying the introduction of inexpensive materials that can be procured in Japan in terms of economy and risk management.

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