Lockheed X-7

The Lockheed X-7 (dubbed the "Flying Stove Pipe") was an American unmanned test bed of the 1950s for ramjet engines and missile guidance technology.

Design and development
The X-7 was carried aloft by a B-29 or B-50 Superfortress carrier aircraft. A booster rocket ignited after launch and propelled the vehicle to a speed of 1,000 mph (1,625 km/h). The booster was then jettisoned, and the underslung ramjet took over from that point. The X-7 eventually returned to Earth, its descent slowed by parachute. A maximum speed of 2,881 mph (4640 km/h / Mach 4.31) was attained, setting a record for fastest air-breathing aircraft at the time. A total of 130 X-7 flights were conducted from April 1951 to July 1960.

The X-7 was also used to test HEF-2 and Hi Cal-3 zip fuel, which has a superior heating value of 26,500 Btu/lb. compared to hydrocarbon fuel with 18,400 Btu/lb.