Reciprocating Feed System for In-Space Propulsion Systems

The Reciprocating Feed System (RFS) offers a means of reducing the cost and weight, and improving the performance of in-space propulsion systems that typically rely on conventional pressure fed systems or that may require development of new pump systems. The RFS uses relatively low mass, low-pressure main tanks for the fuel and oxidizer, connected with two or three small, high-pressure propellant tanks. The small tanks alternately expel propellant, vent, and are then refilled and pressurized in sequence, to maintain steady and/or modulated flow to the engine. This system appears to be substantially cheaper than turbo-pump systems, with about the same total system mass, and significantly lighter than conventional pressure fed systems. With the representative in-space propulsion system analyzed herein, having 5000 lbs of liquid hydrogen and liquid oxygen, the inert mass for the 250 psi system is reduced from 1771 lbs for a pressure fed system to 601 lbs for one of the RFS options at the same pressure; this savings of 1170 lbs would substantially increase the payload. The weight savings is even greater at higher tank pressures. The RFS also offers the additional benefit of allowing higher chamber pressure engines than with conventional pressure fed systems, with attendant improvements in inert mass, propellant mass and volume, and system performance. Other benefits include built-in redundancy and fail-operational modes, and improved propellant management in a micro-gravity environment. The system can be used with cryogenic and storable propellants for both long and short-term missions. TECHNICAL DISCUSSION The basic concept is based on Patent Number 6,314,978 B1 (Reciprocating Feed System for Fluids), dated November 13, 2001 and assigned to The Boeing Company. Other work related to this approach is treated by Knight 2 and by Flowmetrics . An example system is shown in Figure 1; although this design is for a hybrid propellant rocket, the principles are the same for an in-space propulsion system. Figure 1: RFS Conceptual Layout with RFS Tanks External to Main Propellant Tank Compared to a turbo-pump system, the cost is expected to be substantially lower, in part due to the expectation of a high development cost for the turbopump and engine, especially since there are no spacequalified turbo-pump systems available for relatively small in-space propulsion systems (i.e., thrust of the order of 1000 lbs). Since conventional pressure fed systems are typically limited to about 300 psi, the rocket engine must be operated at a relatively low pressure. With the RFS, pressures of the order of 250 to 2500 psi are achieved with relatively small increases in system mass. Thus, a total system optimization could show that smaller, higher pressure engines can be used, which could further reduce overall system weight, increase the propulsion system performance, reduce the overall spacecraft volume, and further increase the payload mass compared to a conventional pressure fed system. The RFS contains propellant in both the main tank, and in two or three of the small, high-pressure tanks. 39th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit 20-23 July 2003, Huntsville, Alabama AIAA 2003-4500 Copyright © 2003 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.