On the Evolution of Advanced Propulsion Systems for Spacecraft

In a current research and advanced development program, the use of Earth-storable propellants in unmanned-spacecraft propulsion systems is under investigation. The term Earth-storahle is applied to hypergolic propellants which exist in liquid form at 70 - 30 deg F. Achievement of simplicity in propulsion-system design and operation is facilitated by the use of these propellants for flights to the near planets and the Moon. Propulsion-system requirements for such missions are briefly outlined, and the major hazards of space flight are discussed in relation to their effect on propulsion systems. The basic concepts of a proposed spacecraft propulsion system are presented, and component design is shown to be integrated with these concepts so that solutions to individual component problems are compatible. In the pursuit of system and component reliability, strong emphasis on simplicity and predictability is placed in opposition to the use of redundancy. The scope of this advanced development program has been delineated by three basic design choices, related to the use of 1 gas-pressurized tankage for propellant pumping, 2 flexible, impermeable barriers at the liquidvapor interface in propellant tanks, and 3 thrust chambers of refractory and ablative materials which do not require regenerative cooling. For consistent achievement of stable, efficient combustion processes, compatible with their boundaries, the development of procedures for a priori injector design is shown to be mandatory.