Three airframes ยท One rocket engine
The X-15 aircraft
The X-15 was a small, heavy, black aircraft: 49 ft long with a 22 ft wingspan and thin, stubby wings. Almost everything about it was built to survive heat and to be controlled where air no longer works.
The three airframes
| Aircraft | Serial | Free flights | Fate |
|---|---|---|---|
| X-15-1 | 56-6670 | 81 | Donated to the Smithsonian; on display at the National Air and Space Museum in Washington, D.C. |
| X-15-2 / X-15A-2 | 56-6671 | 53 (31 as X-15-2, 22 as X-15A-2) | Retired to the National Museum of the US Air Force in October 1969 |
| X-15-3 | 56-6672 | 65 | Destroyed on flight 191, 15 November 1967 |
Size and weight
The standard X-15 was 49 ft 2 in long, 13 ft 1 in high and had a wingspan of 22 ft 4 in, with 200 sq ft of wing. It weighed about 14,600 lb empty and 33,500 lb at launch.
Inconel X skin and a wedge tail
At high Mach numbers the skin of an aircraft gets hot enough to weaken ordinary aluminium. The X-15 used a nickel alloy, Inconel X, for parts of its outer skin. NASA counts it as the first reusable superalloy structure able to withstand hypersonic re-entry temperatures. A thick wedge-shaped tail kept the aircraft stable at hypersonic speeds, and panels on the tail could open as air brakes.
Engines: XLR11, then XLR99
The first 24 powered flights used two Reaction Motors XLR11 engines, about 16,000 lbf in total. The XLR99 was delivered by November 1960 and first flew on 15 November 1960. It produced up to 57,000 lbf, could be throttled down to about 28,000 lbf, and powered the remaining 175 flights. It burned roughly 15,000 lb of propellant in about 80 seconds of full-power running.
Controls for the edge of space
In thin air, wings and tail surfaces stop working. The X-15 steered with small hydrogen-peroxide thrusters in the nose and wings. NASA lists the first use of reaction controls for attitude control in space among the program's key accomplishments. Lower on the airframe, a ventral fin helped stability at speed and was jettisoned before landing, and the aircraft touched down on a nose wheel and two rear skids.
The pilot flew in a full pressure suit, which NASA lists as the first practical one, and sat on an ejection seat that was never used.
The X-15A-2
On 9 November 1962, Jack McKay crash-landed X-15-2 at Mud Lake after an engine failure. The airframe was rebuilt as the X-15A-2 with a longer fuselage, external fuel tanks under the fuselage and wings, and a full ablative heat-protection coating. It first flew in that form on 25 June 1964 and, on 3 October 1967, flew Mach 6.7.
The B-52 carrier aircraft
Two modified B-52s carried the X-15 under the wing and released it at about 45,000 ft. The NB-52A (52-003), nicknamed "The High and Mighty One", is displayed at the Pima Air and Space Museum. The NB-52B (52-008), known as "Balls 8", launched most flights and now stands outside the north gate of Edwards Air Force Base. See how a flight worked from release to landing.