X-15/B-70

It had been proposed that X-15 number 3 would be reworked to install a delta wing and designed to reach Mach 8.

The wing's leading edge would be of columbium, with Rene-41 alloy on the upper and lower surfaces. The rocketplane could have been air-launched from the top of an XB-70, but even when dropped from a subsonic B-52 would be capable of reaching Mach 8. The project was cancelled when X-15 number 3 was destroyed in a crash.

Length: 19.05 m (62.49 ft). Span: 6.80 m (22.30 ft). Mass: 25,000 kg (55,000 lb). Main Engine: XLR-99. Main Engine Thrust: 262.445 kN (59,000 lbf). Main Engine Propellants: Lox/Ammonia. Main Engine Propellants: 20,000 kg (44,000 lb). Main Engine Isp: 276 sec. Spacecraft delta v: 2,400 m/s (7,800 ft/sec). Electrical System: Batteries.