In today's space initiative there appears to be only one propulsion system of choice, the liquid or solid rocket. In fact since the early 1950s a wide variety of space launcher propulsion systems concepts that were built and tested. These systems had one goal, that of reducing the carried oxidizer weight, so a greater fraction of the gross weight could be payload. Another need was for frequent, scheduled launches to reduce the costs required to reach LEO from the surface of Earth. Without that frequency launches would remain a one-of-a-kind event instead of a transportation infrastructure. Figure 3.3 and 3.4 give two representations for the single-stage-to-orbit (SSTO) mass ratio (weight ratio) to reach a 100 nautical mile orbit (185 km) with hydrogen for fuel. In Figure 3.3 the mass ratio is a function of the maximum airbreathing Mach number. Six classes of propulsion systems are indicated: Rocket derived, Airbreathing rockets, KLIN cycle, Ejector ramjet/ scramjet, Scram-LACE, and Air Collection and Enrichment Systems (ACES). These and others are discussed in Chapter 4 in detail. The trend clearly shows that to achieve a mass ratio significantly less than rocket propulsion (about 8.1) an airbreathing Mach number of 5 or greater is required. This can be calculated by the equations that follow:
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