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North American X-15


The X-15 was a hypersonic research aeroplane, a rocket-powered type air-launched by an adapted B-52 bomber within a programme that yielded important results in flight at very high speed and extreme altitudes. The objective was an aeroplane capable of flying at 4,500 mph (7,240 km/h; nearly Mach 7) and reaching an altitude of 250,000 ft (76,200 m). North American Aviation won the design contest and was awarded a development contract on 30 September 1955.

The aircraft was designated X-15 and was designed around a Thiokol (Reaction Motors) XLR99-RM-2 single-chamber throttleable XLR99 liquid-fuel rocket engine capable of delivering a thrust of more than 60,000 lb (27,215kg) for a period of several minutes. Like X-1, the X-15 was to be air-launched, since fuel could not be expended in getting the X-15 off the ground and up to operating altitudes.

The X-15 was built largely of titanium and stainless steel, covered mostly with a so-called ‘armour skin’ of Inconel X nickel steel alloy to withstand temperatures ranging from 1200 deg F to –300 deg F, with heating rates of 30 BTU per sq ft of surface area per second. Far higher temperatures were recorded by the X-15A-2 after this type had been fitted with Emerson Electric T-500 ablative material to provide a capability for comparatively steep (and therefore high  friction) angles of re-entry after apogee in the interface between the troposphere and space. This capability to operate on the edges of space demanded a reaction control system for orientation of the aeroplane in these virtually airless regions: a rocket system was used for this stem, with four nozzles in the wingtips and eight more nozzles in the nose to provide three-dimensional manoeuvre capability.

Two Boeing B-52s were modified as carriers for the X-15s, three of which were ordered. With a span of only 22 ft (6.70 m) for the slightly swept-back trapezoidal wings the X-15 had a gross weight at launch of over 31,000 lb (14,060 kg), of which more than 18,000 lb (8,165 kg) was accounted for by the liquid oxygen and anhydrous ammonia rocket propellants.




When the first X-15 free flight was made on 8 June 1959 the aircraft was fitted with lower-powered engines, as the XLR99 was not then ready. Carried aloft by an NB-52, the X-15 was piloted by Scott Crossfield. With two XLR11-RM-5s, giving a combined thrust of about 33,000 lb (15,000 kg), the first powered flight was made by the second X-15 prototype (56-6671) on 17 September 1959, a speed of Mach 2.11 and altitude of 52,341 ft being achieved. From that point on, both the speed and the altitude reached by the X-15s climbed steadily, with Mach 3 being reached in November 1961 after the XLR99 engine had been fitted in the second proto-type. By December 1963 the X-15s had reached a speed of Mach 6.06, had encoun-tered a skin temperature of 1,320 degrees F and reached an altitude of 314,750 ft (95,936 m).


YLR99 rocket engine
The X-15A-2 propulsion system is Thiokol Chemical’s Reaction Motors Division’s YLR99 rocket engine. Even its 58,000 lb thrust can be upped through externally-mounted ramjet engines. The YLR99 operates on liquid oxygen and anhydrous ammonia which is fed into the thrust chamber by a turbopump driven by hydrogen peroxide. The ball above the engine exhaust chamber (tail slot) will contain pressurised helium which will by used to expel liquid hydrogen fuel in testing of the ranjet engines.


In an experiment in 1964, an X-15 attempted a photo mission from 100,000 ft while speeding at 3290 mph over Edwards AFB (Calif). The purpose was to determine effect on camera and film of extremely high temperatures encountered at that speed, and clarity of photos for reconnaissance use.

With Major William J. “Pete” Knight at the controls, the modified X-15A-2 set an unofficial speed record of 4,520 mph (Mach 6.70) on 3 October 1967. This would be the fastest flight of the X-15 program.


Before the end of 1961, the X-15 had attained its Mach 6 design goal and flown well above 200,000 feet; by the end of 1962 the X-15 was routinely flying above 300,000 feet. The X-15 had already extended the range of winged aircraft flight speeds from Mach 3.2 to Mach 6.04, the latter achieved by Bob White on 9 November 1961.

On 9 November 1962, the second X-15 crashed while executing an emergency landing on Mud Lake near Edwards AFB. Pilot Jack McKay was seriously injured but later returned to flight status. The X-15 itself was nearly a write-off, but eventually the Air Force and NASA decided to rebuild it to a slightly different configuration. The fuselage was lengthened and external drop tanks were added to accommodate additional propellants. It was hoped this would allow the X-15A-2 to achieve at least Mach 7 while testing experimental scramjet engines. This first flew on 28 June 1964.

Using an ablative coating to provide additional heat protection, Major Pete Knight took the X-15A-2 to Mach 6.72 (4,520 mph) and an altitude of 354,200 ft / 107,960m on 3 October 1967, the fastest piloted flight of the X-Plane program. This is the highest recorded speed yet achieved by man in an aeroplane capable of being controlled in normal flight. Due to damage resulting from this flight, the aircraft was retired and subsequently transferred to the Air Force Museum.

The aircraft proved remarkably flexible as a research tool. In fact, most of the later flights used the X-15 as a carrier vehicle for other experiments rather than as a research aircraft in its own right. An assortment of experiments were carried, including micrometeorite collection pods, missile detection systems, samples of insulation destined for the Saturn launch vehicle, and a wide variety of others.

After 177 flights, the last on  24 October 1968, the X-15 programme was terminated in 1968. Of the three X-15s manufactured, one crashed while returning from space, killing test pilot Major Michael J. Adams, and one survives in the National Air and Space Museum.




Engine: 1 x Reaction Motors XLR-99 rocket engine, 253.7kN
Max take-off weight: 15422 kg / 34000 lb
Wingspan: 6.7 m / 22 ft 0 in
Length: 15.8 m / 51 ft 10 in
Height: 4.1 m / 13 ft 5 in
Wing area: 18.6 sq.m / 200.21 sq ft
Max. speed: 7297 km/h / 4534 mph
Ceiling: 107960 m / 354200 ft
Crew: 1


North American X-15



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