Fig. 20. Air-augmented HTO-SSTO design features. Fig. 21. EOTV configuration. A horizontal takeoff, single-stage-to-orbit concept that carries 91,000 kg into a 550-km geosynchronous orbit also was studied. This concept takes off from an airport, using airbreathing engines similar to a supersonic transport as shown in Fig. 18. After climbing to 20,000 feet, Fig. 19, the airplane cruises to the injection point at Mach = 0.85. Following a climb to 45,000 feet, a dive to 37,000 feet is accomplished to increase velocity to Mach = 1.2. Still using the airbreathing engines, acceleration to a velocity of 6200 ft/s is accomplished. Between 6200 ft/s and 7200 ft/s, both airbreathing and rocket engines are used for acceleration to an altitude of 107,200 feet. Final injection into low orbit is accomplished with rocket engines. Some of the design features of this concept are shown in Fig. 20. The wings contain propellant tanks to effectively utilize the available volume. This concept requires considerably more advancement in technology than the rocket-propelled concept (air-breathing engines and low-mass structure), but it offers significant operational cost savings. The EOTV concept and its characteristics are shown in Fig. 21. Trip time from LEO to GEO is 100 days with 5.26 x 106 kg of payload. Return trip time is 22 days with
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