X-15 – A ROCKETPLANE TO SPACE

During the 1940s and 1950s, proposals for rocket-propelled aircraft designed to exceed the speed of sound were instigated in several countries. The most successful were the American X-series of aircraft, which included the X-1 that broke the sound barrier (Mach 1) in October 1947 and the X-15 hypersonic research aircraft. The X-20 was a USAF proposal for a one-man orbital space plane which would have been rocket-launched, but would have landed on a runway utilising the technique of dynamic soaring, hence its nickname – the DynaSoar. The concept never flew. Similar designs were developed in the Soviet Union and both countries eventually used the technology to develop a manned reusable space shuttle, though the Soviet craft (Buran) only flew one short unmanned mission in 1988.

X-15 – A winged marvel

The X-15 programme began in 1954 and three aircraft were constructed. The pro­gramme was designed to provide data on aerodynamics, structural and control prob­lems, and the physiological aspects of high-speed, high-altitude flight. Once these primary objectives had been met and exceeded, the programme began to gather data from various experiments and materials carried by the X-15 to the fringes of Earth’s atmosphere, many of which helped in the development of techniques and material for future programmes such as Apollo and the Space Shuttle. To assist in meeting this new objective the second aircraft was modified (after a serious accident in 1962), improving its performance to attain speeds of Mach 8 and support the development of supersonic combustion ramjet engines (scramjets).

Measuring 50 feet (15.24 metres) long and with a short stubby wingspan of 22 feet (6.70 metres), the X-15 featured a wedge-shaped vertical tail. The vehicle weighed about 14,000 lbs (6,350.4 kg) unfuelled and 34,000 lbs (15,422.40 kg) with a full fuel load. The high speeds that the X-15 would attain posed a problem for protecting the structural skin of the aircraft. This was solved with a heat-resistant skin made from Inconel-X nickel-steel alloy over a titanium and stainless steel structure. Control during atmospheric flight was by conventional aerodynamic control surfaces, but “in space’’, eight hydrogen-peroxide thrusters on the nose controlled the pitch and yaw, with a further four on the wings used for roll control. Difficulties with the X-15’s primary propulsion system (Thiokol XLR99) resulted in the decision to use two Reaction Motors XLR11 rocket engines for the initial powered test flights. Each XLR11 engine produced 2,000 lb (907.2 kg) of thrust. The manually-controlled XLR99 rocket engine had a thrust of about 60,000 lbs (27,216 kg).

Air-launched from under the wing of a B-52 at about 45,000 ft (13,700 metres) at an air speed of 500 mph (804.5 kph), the rocket motor was ignited by the pilot after dropping from the B-52 wing and burned for about 80 seconds. There were two flight profiles. The one for high altitude included a steep rate of climb while the other was used for level attitude high-speed objectives. Each free-flight lasted for about 10-11 minutes, mostly unpowered, and at the peak altitude of the astro-flights, only two or three minutes of “weightlessness” was experienced. However, the pilots did get a clear view of the curvature of the Earth’s surface and the blackness of space above the thin layer of the atmosphere. The X-15’s landing gear consisted of rear skids and a nose wheel, and landings were usually at about 200 mph (321.8 kph).