Profile – North American X-15

Meanwhile, NACA were drawing up specifications for a new generation rocket plane capable of reaching speeds up to Mach 7 and heights of 280,000 feet. This machine was planned as the stepping stone to a viable ‘spaceplane’. Tenders were called for in December 1954. The North American/Reaction Motors proposal was chosen over three rival bids, and the X-15 was born. The aircraft was designed to be air launched from a B-52, and powered by a single chamber rocket capable of being throttled between half and full power. North American would develop the aircraft itself, and Reaction Motors would develop an entirely new rocket engine. This new engine, designated XLR-99, would develop 25. 8 tonnes of thrust (compared with the X-1’s 2. 7 tonnes) and would burn for 90 seconds (compared with 100 seconds for the X-1). A special tracking and communications network was developed at Edwards Air Force Base in California to monitor flight trials.

North American commenced manufacture of the first of three air frames in September 1957. By January 1958, it was apparent that Reaction Motors would not have theXLR-99 ready on schedule, so it was decided to fit two XLR-11 (a four chamber rocket of 3. 63 tonnes) engines to the first machine, X-15-1, to investigate the aerodynamics of the new aircraft. The rollout ceremony took place in October 1958. The deep blue machine was allocated the serial 56-6670.

Technical Description

The structure of the X-15 was for many years the most technologically advanced in the world. Like the SR-71 Blackbird, which was designed for lengthy periods of high speed flight, the X-15, designed for short periods of ultra high speeds, required special construction techniques to cope with the variation in skin temperatures. Temperatures from -185 to 650 degrees Celsius were expected, due to the extreme altitudes and tremendous air friction.

The primary load bearing structure of the aircraft was of stainless steel and titanium construction. However, the outer skin, propellent tanks and wing leading edges were fabricated in the high nickel content ‘lnconel X’ nickel/chromium alloy, normally used only for turbine rotor blades.

Three quarters of the structure was welded – not riveted.

Once welded, the whole aircraft structure had to be annealed at temperatures up to 1095°C, then aged at 705°C, in huge, specially constructed furnaces. Fuel and hydraulic pipes were welded or brazed to avoid conventional joins.

The complete aircraft had a span of 6. 70 metres (22ft), height of 4. 15 metres (13ft 6in), and length of 15. 24 metres (50ft). The stubby wing had an aspect ratio of 2. 5:1, with an area of 18. 6sq metres (200sq ft). Loaded weight was 22,512kg (49,640lb).

Aerodynamic control was to be effected by elevons on the tailplane and an all-moving fin. Airbrakes were also fitted to the fin. For control in the upper reaches of the atmosphere, where the X-15 would actually become weightless for over 30 seconds and virtually a spacecraft, control was effected through three a xis hydrogen peroxide compressed gas jets. Thrusters in the nose provided pitch and yaw control, while wingtip thrusters provided roll control.

Being air launched, the undercarriage was only required tor landing. As the X-15 was to be flown from Edwards Air Force Base, it was decided to use the endless dry salt lakes as a landing field, so the X-15, like its predecessors, was fitted with a skid type undercarriage. Touchdown was achieved on the twin skids at the rear of the aircraft, and after the aircraft slowed considerably the nose was lowered onto conventional twin wheel nosegear. To facilitate ground handling, rear wheels were attached after landing.

The rocket motor used a Liquid Oxygen (LOX) and anhydrous ammonia fuel mix, stored in tanks occupying half of the fuselage length. The LOX tank was forward of the anhydrous ammonia tank.

Flight Testing Begins

Flight testing did not commence until 19th March 1959, when a series of captive flights (under a B-52) were initiated. Also in March, the second machine, X-15-2 (56-6671), was delivered to Edwards. It was notuntilthe 8th June that a gliding tree flight was made, the pilot being North American Aviation test pilot Scott Crossfield. (Crossfield was a career civilian test pilot, having previously worked for NACA. ) The XLR-99 was still not ready, and X-15-2 was also fitted with two XLR-lls; this machine made the first powered X-15 flight on 17th September. Again, Crossfield was the pilot.

However, disaster struck the program in November, when X-15-2 landed heavily and snapped the fuselage. Repairs took some three months. Still the XLR-99 was not ready.

The third and final machine, X-15-3 (56-6672), was nearing completion when the XLR-99 became available at last. X-15-3 was fitted with the new engine and delivered to Edwards, but during ground testing the engine disintegrated, blowing the tail unit off the aircraft. X-15-3 was consequently returned to North American for repairs.

In April 1960, X-15-2’s XLR-99 engine arrived at Edwards and was installed. The re-engined aircraft flew for the first time on 5th November. Soon after began a series of record breaking flights, with faster and taster speeds being achieved. The XLR-11 powered X-15s had already pushed the record from Mach 3. 19 (achieved by an X-2 minutes before its fatal crash in 1956) to Mach 3. 31. But the XLR-99 engine soon had the Mach 4 milestone tumbling, and then Mach 5.

The X-15s were also progressively increasing the altitude record, culminating in an August 1963 flight by Joe Walker in X-15-3 reaching 354,200 feet. Pilots reaching altitudes above 50 miles (280,000ft) earned USAF Astronaut wings.

Valuable data on heat build-up and stability was also being collected, which would prove to be the basis of all later commercial and military hypersonic aircraft designs. The highest skin temperature recorded was an incredible 715°C.

The Birth of theX-15A

On 9th November 1962, X-15-2 had suffered a landing accident in which the pilot was injured. Before repairs were completed, NASA decided to modify the aircraft to carry more fuel, thus increasing the burn time from 90 seconds to 135 seconds. The extra propellent was to be stored in two drop tanks carried beneath the fuselage fairings, increasing the overall length by 63cm (29in). The windscreen design was also improved, as was the undercarriage. Provision was made for the fitment of external ramjets, but these were never applied. The modified aircraft was designated X-15A-2. The first flight took place on 25th June 1964, by which time the other two X-15s were concentrating on collecting data rather than breaking records. Interestingly, a number of X-15 test pilots were later to become space program astronauts, including the first man on the moon, Neil Armstrong.

In mid 1967, to cope with the higher temperatures likely to be encountered with the longer motor burn time, X-15A-2 was further modified with the addition of a heat ablative paint finish similar to that adopted by the Lockheed Blackbird. X-15A-2 was thus able to drastically boost the speed record up to the incredible speed of Mach 6. 72, achieved on its last flight in October 1967. This record stood until the Space Shuttle revolutionised the aerospace world and broke the barrier between aircraft and spacecraft.

The two official records attributed to the X-15A-2 were as follows:

Tragedy however struck the X-15 program on 15th November, 1967. Major Mike Adams was carrying out the 191st X-15 flight when things went dreadfully wrong. The aircraft re-entered the atmosphere in the wrong attitude and went out of control at Mach 5. 2 and 260,000 feet, with the nose yawing through 90 degrees to the flight path before the machine disintegrated at 62,000 feet. The wreckage was strewn over some 16km. The pilot, who chose to fly the X-15 instead of joining the space program, never ejected and was killed.

The Project Winds Down

On 24th October1968, X-15-1 completed its last test flight,and the X-15’s domination of the aerospace research world ended. The other surviving machine, X-15A-2, had been retired a year earlier. 199 flights had been achieved in what were arguably the most exciting times in the history of test flying.

It had been intended that the X-15 program be the forerunner of a true reusable spacecraft (the X-20 ‘Dyna-Soar’) but politics resulted in the Mercury project ‘manned rocket’ winning funding and priority. The X-15 project did spawn further ‘lifting body’ reentry vehicle experiments, laying the groundwork for the eventual advent of the Space Shuttle.

X-15-1 now resides in the Smithsonian National Air and Space Museum, while X-15A-2 takes pride of place in the USAF Museum in Dayton, Ohio.