Technical Issues Related to Propulsion Systems for Launch Vehicles

Abstract

provide sufe cient margin, and use a balance of inherited technology and advanced technology. Multiple-engine vehicles can rely on active redundancy, in which the failure of a single engine is overcome by operating the other engines at higherthan-nominal thrust levels. Single-engine vehicles require higher design margins, more extensive system-level tests, and pad holddown with engine shutdown capability at launch. To enhance reliability and safety, a health-monitoring system can be used to measure and diagnose engine operation. Health monitoring also provides an opportunity to shorten maintenance time and reduce costs. The development of the healthmonitoring system should be concurrent with the development of the propulsion system, at which time basic data on engine characteristics and operating parameters are obtained. The most important technology requirements for launch vehicles are specie c impulse and engine thrust-to-weight ratio, which is related to mass fraction. Specie c impulse and mass fraction fundamentally characterize the performance of a rocket engine and, therefore, improvements in their values have been sought for years. The maximum specie c impulse that can be obtained with a rocket engine is ultimately dependent on the propellant combination. The propellant combinations under consideration today must also meet increasingly stringent cost and environmental requirements. For a given propellant combination, improvements in combustion efe ciency, engine dry weight, and system operating pressure are needed. Combustion efe ciency depends to a great extent on injector patterns, propellant physical conditions, and propellant properties. The propellants entering a combustion chamber can be generally categorized as being in one of three modes: gas/gas, gas/liquid, or liquid/liquid. The mode is determined by the power cycle employed. In theory and practice it has been shown that the combustion efe ciency of injection systems that use gas/liquid propellants is higher than that of liquid/liquid injection, and that gas/gas injection offers the highest efe ciency when all of the other conditions remain identical. Table 1 lists the characteristic exhaust velocity (c*) efe ciency of several engines with different injection modes and propellant combinations. It is clear that improvements in combustion efe ciency should be sought, particularly for engines that use liquid/liquid propellants. The reduction of engine dry weight, which should result in an increase in the engine thrust-to-weight ratio, has particular signie cance for SSTO vehicles. Table 2 lists the thrust-to-weight ratios of several representative engines. The thrust-to-weight ratios for cryogenic propellant engines are generally lower than those of storable engines, and gas-generator (GG) cycle engines