Essentially Non-explosive Propulsion Paving a Way for Fail-Safe Space Transportation

In this paper, among the technological innovations for sustainable development of future space transportation, the safety aspects of rocket propulsion are discussed to show that the achievement of fail-safety or fail-tolerance is equivalent to the realization of safety resilience in the space transportation system. Further, it is shown that such realization is extremely difficult with the conventional essentially explosive propulsion systems (EEPS) and that essentially non-explosive propulsion systems (ENEPS) are indispensable to obtain the required safety characteristics in space transportation systems. Specifically, the safety characteristics of the EEPS and ENEPS are analyzed under both the current Safety-I and the Safety-II conditions advocated by Hollnagel. As a result, it is shown that the safety characteristics of EEPS are in the category of Safety-I, and it is difficult to shift to Safety-II, while the safety characteristics of ENEPS allow achieving the category of Safety-II. The technical challenges of hybrid rockets, which are representative of ENEPS, are expected to be overcome by devising the Altering-Intensity Swirling-Oxidizer-Flow-Type method or similar ones. Thus, ENEPS will pave the way for future failure tolerance of space transportation.