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.

North American X-15 Article

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.

In September 1957, the first one rolled out of the factory. Six months later, in March 1959, the X-15 made its first captive flight and, three months after that, its first glide flight.

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 prototype. By December 1963 the X 15s had reached a speed of Mach 6.06, had encountered a skin temperature of 1,320 degrees F and reached an altitude of 314,750 ft (95,936 m).

In June 1959, in the X-15s very first free flight, Crossfield’s landing was a little touchy due to the pitch damper failure and pilot-induced oscillation.

On 17 September 1959, Scott Crossfield took the X-15 through the paces of its first powered flight.
In its second powered flight three months later, also flown by Crossfield, the vehicle’s nose gear door failed due to a rough landing on Rogers Dry Lake. According to the official report, the structural failure occurred on landing “due to design flaw and excessive propellant weight,” but the NASA engineers at Edwards knew otherwise and North American then adopted the special approach theory for landing. This involved a 360 degree spiralling descent starting at about 40,000 ft, right above the desired touchdown point on the runway. From that ‘high key’ position, the pilot moved into a 35 degree bank (usually to the left) while maintaining an airspeed of 285-345 mph. At roughly 20,000 ft after some 180 degrees of the spiral had been completed, the X-15 reached the ’low key’. At this point, the aircraft was headed in the opposite direction of the landing runway and was about four miles abeam of the touchdown point. From the low key, the turn continued through the other 180 degrees until X-15 lined up with the runway at about a five mile distance. The rate of descent through the spiral averaged over two miles per minute, which meant it took on average about three minutes to go from high key to that point where the X-15 was ready to head straight for landing.

Neil Armstrong’s second X-15 flight, and his first for research purposes, came just before noon on Friday 9 December 1960, also in the number one airplane. Flight 1-19-32 first tested the X-15’s newly installed ‘ball nose’. Until this flight, the X-15 had a front mounted boom with vanes to sense airspeed, altitude, angle of attack, and angle of sideslip in a free aerodynamic flow field. At such high altitudes and high speeds, the X-15 would melt its nose boom, destroying measurement data. The ball nose sphere could be cooled from the inside by liquid nitrogen.

Equidistant from the circumference were sensor ports in the middle of the ball as well as on the top and bottom. The ball moved automatically in pitch and yaw to keep the pressure equal on both of the ports, pointing the centre hole directly into the free flow. The angle of the ball movement amounted to the airplane’s angle of attack. Similarly, the ball nose received precise indications of angle of sideslip and dynamic pressure, which then gave airspeed.

Flight 3-4-7 piloted by Neil Armstrong on 5 April 1962, reached Mach 4.12 and 180,000 ft to test the MH96 reaction controls. The test flight spanned 181.7 miles in a little over 11 minutes before landing at Rogers Dry Lake.
Flight 3-4-8, on Friday 20 April 1962, by Neil Armstrong, was to test the MH-96 system limit, or ‘g limiter’, to prevent the pilot from exceeding 5g. The flight reached 207,500 ft. Ballooning creates some control problems at the altitude and made an extended trip back.
Neil Armstrong’s X-15 flight on 27 June 1962 resulted in the highest Mach number every attained in the X-15 program – Mach 5.74 or 3989 mph.

North American X-15 No.2, damaged in November 1962 in a hard landing at Mud Lake, Nevada, was completely rebuilt, ready to fly in May 1964. Most noticeable difference in X-15-2 from other models is the addition of two 22 ft fuel tanks of 38in diameter on the lower sides of the fuselage. Carrying liquid hydrogen and anhydrous ammonia, they extend burning time of the Thiokol 58,000 lb thrust YLR-99 rocket engine from 88 seconds to 146 seconds. With additional length of powered flight X-15-2 should top previous marks of 4104 mph and 354,200 ft altitude. Both set by NASA Chief Test Pilot Joe Walker.

The tanks drop off when the plane reaches Mach 2, are recoverable by parachute. Extra fuel, weighing 13,500 lb, plus other modifications, bring X-15-2’s take-off weight to 25 ton, eight ton more than the others.

The outer half of the right wing is detachable so that various structural material can be tested in flight. The plane will carry a ramjet engine slung from the tail to test hypersonic airbreathing propulsion. Liquid hydrogen ramjet fuel is stored in two internal tanks.

Cameras have been installed for ultra-violet star photography at altitude above 40 miles, beyond the ozone layer which filters out most ultra-violet rays.

The fuselage is 29in longer than other X-15’s, nose and main landing gear has been lengthened to 39.5in ground clearance, oval windshields installed to withstand higher temperatures, and ablative material added to skin surfaces to suppress heating of the basic structure.

Projects to be performed by all three X-15’s were expected to require another 100 flights running well into 1968.

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 be used to expel liquid hydrogen fuel in testing of the ramjet 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 29 inches 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.
On 22 August 1963 Joseph A. Walker, NASA test pilot, took the o.3 X-15 for a world altitude record of 351,000 ft. It was the fifth flight into space for the plane and the third for Walker. He covered 315 miles in 10 minutes. Walker released a 30-inch balloon from the plane’s tail then towed it 100 yards behind to measure air density in space. The aircraft reached a speed of 3614 mph and a climb angle of 48 degrees, the steepest yet. It used 18,000 lb of fuel in 83 seconds. Walker and the X-15 then held the world speed record of 4104 mph for winged aircraft.

In 1965 Joe Engle flew an X-15 to 78,000 ft and 3511 mph on an 8 min flight to simulate surface heating characteristics. A sheet of brown silicon rubber glued on the lower tail was expected to reduce temperature at that point from 800deg to 400 deg F.

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 (some report 199), 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.

Crossfield flew the X-15 a total of 13 times before North American turned it over to NASA-Air Force-Navy partnership. Two of Crossfield’s flights were in the number one airplane, the rest in number two. The highest speed he reached in any of them was Mach 2.9, the highest altitude 88.116 ft, and the furthest distance was 114.4 miles.

The NASA and other pilots were Joe Walker, Jack McKay, Robert White USAF, Neil Armstrong USN, Cmdr Forrest Petersen USN.

NASA pilot Bill Dana flew the X-15 sixteen times.

Gallery

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