Hybrid Rocket Technology: An Overview

Introduction In hybrid rocket motors, one of the two propellants (oxidizer or fuel) is in liquid phase and the other is in solid phase. The combination of a solid fuel and a liquid oxidizer is the most common one, Fig. 1. 1 Even though the idea of hybrid rocket motor was conceived as early as 1937, it was only after 1960s they were built successfully with good combustion efficiency. A discussion of the history of hybrid rocket development is presented by Altman. After a lull of about two decades in hybrid rocket research, quite a few research and developmental activities have been reported in recent years. 4 The motivation for this has been the growing interest towards 1) lower developmental and operational costs without much loss in specific impulse and density specific impulse, 2) safer operational characteristics, and 3) better environmental friendly exhaust. Hybrid rocket motors offer the following advantages over their solid and liquid counterparts: 1) solid fuels are safer than solid propellants on the points of view of manufacture, transportation, and storage; 2) compared to solid rockets, the performance of hybrid rockets is much less sensitive to cracks and debonds in fuel grains; 3) unlike solid rockets, hybrid rockets have the ability to change thrust over a wider range, and to shutdown and restart; 4) relative to liquid engines, hybrid rockets require only half as much feed system hardware and therefore display higher reliability; 5) hybrid rockets generally have the values of specific impulse higher than solid rockets and of density specific impulse greater than liquid bi-propellant rockets, Fig. 2. On the other hand, the disadvantages of hybrid rockets are: 1) the variation of mixture ratio and hence that of specific impulse during steady state operation and throttling; 2) the lower combustion efficiencies (0.93 to 0.97) compared to liquids or solids; 3) the lower density specific impulse and thus a larger system volume than solids; 4) the higher sliver fraction; and 5) low regression rate of solid fuel resulting in rather awkward envelope of propulsion system that degrades the application of hybrid motors. The survey of the literature in the recent ten years on the research and development of hybrid rockets indicates the clear maturity of the hybrid rocket technology. The hybrid rocket propulsion system has been endorsed for flight in many of the spaceand defence-applications. Many sounding rockets, powered by hybrid rockets, have been flown. The ground firings of large hybrid booster-motors (1.8-m diameter) of launchvehicle class have been successfully conducted. The application of hybrid rocket motors as propulsion modules for microsatellites is seriously being investigated. In support of the above observation, the present paper surveys globally the various developments in hybrid rocket technology in the last decade. Also, the paper discusses the methods being investigated for enhancing the regression-rate of hybrid-rocket fuels. The disadvantage of low regression rate of solid fuel has attracted the attention of many researchers and means to enhance the regression rate are being tried through various routes. The disadvantageous effect due to low fuel-regression rate can be understood by considering the thrust equation and its variations, Eqs. (1) (3).