Overview

Current air-breathing propulsion systems operate routinely at flight Mach numbers up to 3 and at altitudes as high as 90,000 ft. Between these and the orbital operating conditions of spacecraft is a huge range of speed and altitude over which the air-breathing system-because it draws its oxygen from the atmosphere-is in principal capable of much higher specific impulse than is rocket propulsion. Some idea of the potential of high-speed air-breathing propulsion can be gained from figure 10.1, where values of specific impulse are plotted for a number of engine types over a wide range of M ₀ and where the parameters of each engine are selected as they would optimize for its usual mission. The values for the turbojet, the turbofan, and the ramjet were taken from chapter 2; those for the supersonic combustion ramjet were estimated by techniques to be explained below and are a bit speculative. But the main point, the potentially high specific impulse of air-breathing relative to rocket propulsion, seems clear. As the enabling technology for the National Aerospace Plane (NASP) concept, the supersonic combustion ramjet (scramjet) has received a good deal of attention since the first edition of this book was published. First seriously considered for propulsion of transatmospheric vehicles in the 1960s, it was studied at a low level of effort during the 1970s and 1980s, and has since been examined intensively in the context of NASP. (For reviews of the earlier work see references ^{[10.1] [10.2] [10.3]}; for the more...