When commercial suborbital flights begin, they will follow a trail blazed fifty years ago by the X-15 research aircraft.
Between 1959 and 1968, three X-15s flew a total of 199 times. On two occasions, the aircraft exceeded an altitude of 62 miles (100 kilometers), the Kármán Line, which is the internationally recognized boundary of space. On 11 other flights, X-15 pilots topped 50 miles, which is regarded as the boundary of space by the U.S. Air Force.
The X-15 began as a joint venture between the National Advisory Committee for Aeronautics (NACA), Navy and Air Force. (NASA replaced the NACA in 1958.) North American Aviation, Inc. built three X-15s. North American engineers selected the Reaction Motors, Inc. (RMI) XLR-99 rocket engine to boost the X-15. The X-15 was not a large airplane: just 50 feet long with short, stubby wings that spanned 22 feet, 4 inches.
For control at extreme altitudes, where airplane controls were ineffective, the X-15 had hydrogen peroxide thrusters in the wings and nose. The 100-pound thrust nose rockets controlled pitch (up and down) and yaw (side to side) motion. Thrusters on the wings, which produced 40 pounds’ thrust, controlled roll. A control stick on the left armrest controlled the thrusters. The pilot had two other control sticks. The central, traditional airplane style stick was intended for use during landing. During acceleration, one on the right armrest only required wrist movement. The pilots often considered it an indication of their prowess and flying skill to use only the left and right sticks throughout the entire flight.
The X-15 landed on a conventional nose wheel and a pair of tail skids that were extended just before touchdown. It made a high speed unpowered landing.
The X-15 first flew on June 8, 1959, with North American test pilot A. Scott Crossfield at the controls. This was an unpowered glide test of the airplane’s handling characteristics. Crossfield made the first powered flight on September 17, 1959. Development of the XLR-99 took longer than expected, so the first 25 powered flights were made with a pair of less powerful RMI XLR-11 engines. The two XLR-11 engines, which together had eight separate combustion chambers, produced a combined thrust of only 16,000 pounds. This was enough to propel the X-15 to 1,393 miles per hour, or Mach 2.11 on the first powered flight.
A year after the first flight, X-15 #2 made the first flight with the XLR-99 engine. The XLR-99 produced a maximum thrust of 57,850 pounds at 100,000 feet. The engine could be throttled down to 25,000 pounds thrust. Most of the fuselage comprised fuel and oxidizer tanks that held 1,034 gallons of liquid oxygen and 1,445 gallons of anhydrous ammonia for the engine. At full throttle, the XLR-99 consumed these propellants in 83 seconds. Like earlier rocket-powered X-planes, the X-15 was air launched. It was carried to its drop altitude of 45,000 feet beneath the wing of a Boeing B-52 bomber.
There were two basic trajectories: one for altitude and one for speed. Flying the parabolic arc of an altitude trajectory, the pilot ascended more than 50 miles above the earth. On such a flight, after being dropped from the B-52 carrier plane, the pilot ignited the XLR-99 then pitched the nose up. About halfway through the ascent, the engine consumed all the propellants and quit. The pilot was then weightless as he coasted to apogee, about three and a half minutes after release. The pilot could see from Monterey Bay near San Francisco to the Gulf of California. To reach hypersonic speeds, a lower, more horizontal path was flown. Both trajectories resulted in flights lasting 9 – 11 minutes. For a particular flight, the pilot usually spent about 50 hours in the simulator.
A significant milestone came on the 62nd flight of the program when test pilot Robert M. White reached 314,750 feet – over 59 miles. This flight, on July 17, 1962, earned Air Force Astronaut Wings for White. On July 19, 1963, Joseph Walker reached 347,800 feet (106 kilometers); a month later, on August 22, he reached 354,200 feet. These were the only flights to exceed the 100-kilometer mark.
On November 9, 1962, NASA test pilot John McKay discovered a problem with the throttle control valve and could only bring the XLR-99 up to 35% thrust. He shut the engine down after 71 seconds and initiated an emergency landing at an alternate site. He could not dump all the remaining propellants and the landing flaps failed to deploy. Coming in too fast and too heavy, the left landing gear collapsed and X-15 #2 flipped over. McKay suffered three crushed vertebrae and lung damage from inhaling ammonia fumes. (He returned to flying duty within six months, but his injuries proved more serious than originally thought and led to his early retirement from NASA.)
The aircraft suffered extensive damage to the fuselage, wings, and tail surfaces. It was returned to North American for rebuilding and upgrade. Over the next year and a half, project engineers lengthened the fuselage by two feet and added two external propellant tanks to increase the burning time of the XLR-99 from 83 to 150 seconds. In its modified form, it became known as the X-15A-2. The X-15A-2 returned to service in June, 1964, although it did not fly with fully fueled external tanks for another two years. When it did, it encountered extreme temperatures from aerodynamic heating and needed a protective coating.
Martin Aircraft developed a spray-on silicone ablative coating called MA-25S. The coating itself was a dull terra-cotta color. A coat of white paint was applied over the ablative material. One of the drawbacks of the material was that it would leave a film over the windows, so engineers added a mechanical “eyelid” over the left window. After the airplane was through the hottest part of the flight and the coating was no longer outgassing, the pilot opened the eyelid.
On flight #188, William J. “Pete” Knight reached Mach 6.7, more than 4,500 miles per hour, with a dummy ramjet attached to the ventral fin. NASA hoped to later test a real ramjet at Mach 8. Despite the ablative coating, aerodynamic heating burned a hole in the lower fin and charred the leading edges of the wings and nose.
Heating caused by the wake of the ramjet was much higher than predicted, and there was substantial damage to the structure and systems in the ventral fin. It was the last flight for the airplane and the spray-on coating.
The only flight fatality in the X-15 program occurred on November 15, 1967. Michael Adams took aircraft #3 to an altitude of 50 miles when it began an uncontrolled yaw that led to a Mach 5 spin. After falling nearly 30 miles, Adams recovered from the spin but was in an inverted dive at Mach 4.7. The aircraft began pitching and broke apart.
Eight more flights were made using X-15 #1, the last on October 24, 1969. Throughout the nine years they were in use, the X-15s had been workhorses for research. Besides their aeronautical missions, the aircraft carried such payloads as solar instruments, micrometeorite collectors, infrared detectors and ultraviolet cameras to support the Orbiting Astronomical Observatory. One of the most important lessons learned during the X-15 program was that it was possible to guide a large, unpowered aircraft to a precision landing from two hundred miles out by carefully managing the flight path. This experience was particularly important when it came time to design the Space Shuttle.
At a conference held by NASA in 1989 to celebrate the 30th anniversary of the first X-15 flight, former pilot Joe Engle said: “The X-15 was the greatest airplane I’ve ever strapped my butt into and I just say that right up front, it is the neatest machine to fly, the most professionally rewarding airplane to fly and climb out of that I’ve ever been in.”