This Week in Aviation History: The End of an Era

The flight made on October 24, 1968, was number 199, and the last for the incredible North American Aviation, X-15. Nearly forty years ago, this aircraft flew faster and higher than any other aircraft ever has — before or since. To beat the X-15 at either game, you’ll have to hitch a ride on the Space Shuttle.

The X-15 was 50 feet long and had a wingspan of 22 feet. It was an air-launched aircraft that had no provision for a conventional takeoff. A Boeing NB-52 Stratofortress mothership would takeoff with the X-15 nestled under her starboard wing, climb to a drop altitude of between 36,000 and 45,000 feet and drop the X-15. Only then were the X-15’s rocket engines ignited.

When it first flew, the engine it was design for was not ready, so a pair of Reaction Motors XLR-11 rocket engines was used. This was the same engine used on the Bell X-1, but as two engines were coupled, it had eight exhaust chambers vs. four on the X-1. Combined thrust for the eight-chamber package was 16,000 lbs., but once the teething problems of the Thiokol XLR-99 rocket engine were solved, the X-15 was endowed with 57,000 pounds, total thrust. The first flight with the XLR-99 package was flight actually #26 and this powerplant would take the rocket plane to speeds and altitudes never seen before by any living thing.

The structure of the X-15 was built solidly for essentially two types of flight: high altitude and high speed. The X-15’s flight envelope produced awesome stresses on the airframe. At the aircraft’s maximum speeds, the friction of air molecules passing over its caused scorching temperatures on the aircraft’s skin. To compensate, engineers built the rocket-plane’s skin from Inconel-X — the same alloy used for turbine blades operating at the core of jet engines. By 1963, through modifications, the X-15’s skin was able to withstand temperatures of 1,300 degrees Fahrenheit — and it had to.

The X-15 routinely flew at altitudes much greater than 200,000 feet. Even well below those altitudes, air becomes so thin that aerodynamic controls cease to function. So, the X-15 was equipped with hydrogen peroxide controls in the nose and wings, to provide positive pitch, yaw and roll control.

For high-altitude the pilot would pitch up, after release and safety checks, and put all the engine power behind a pre-planned arcing flight profile. The flights had to meet certain objectives, such as testing a star tracker or optical sensor. For high-speed flight, after release the X-15 would drop using gravity and the engine to gain speed to test materials or attached experiments like a dummy scramjet.

William J. ‘Pete’ Knight, was carrying a scramjet when he took the aircraft to 4,520 miles an hour (6. 7Mach) on October 3rd, 1967. That flight set a world speed record and made Pete Knight the fastest man alive — it also created so much aerodynamic friction, that the aircraft’s skin reached 3,000 degrees, the scramjet was burned off and a hole was melted into the fuselage. Pete returned the aircraft — badly damaged — but safely.

Eleven other men have flown the X-15, starting with factory pilot A. Scott Crossfield. Two pilots of the black aircraft would go on to be astronauts, Neil Armstrong and Joe Engle. Five Air Force pilots were awarded USAF Astronaut Wings for taking the craft to more than 50 miles high. Joe Walker, a NASA pilot, went to 67 miles, the highest altitude achieved by a winged aircraft (other than the Space Shuttle).

The last man to have ever flown the X-15 is William H. ‘Bill’ Dana, who made the flight this week in 1968, on October 24. He climbed to 255,000 feet and reached 3,716 miles per hour, (5.38 Mach).

As the program ended, just shy of 200 flights, the team behind the North American Aviation X-15 made a push, in December of 1968, for one last flight — to make the 200 flight mark for the three rocket aircraft built. But it was not to be. Weather and other factors stopped the NASA and Air Force team short of that one goal. Still, there was much to celebrate, and much to mourn. Ship number three, on flight 191, crashed on November 15, 1967, killing its pilot, Michael J. Adams. As an indication of the regimes the X-15s flew in, Adams was posthumously awarded USAF Astronauts Wings.

X-15 number two; serial 56-6671 is at the USAF Museum. The first X-15, serial number 56-6670 was also the last to fly. It now hangs in the National Air and Space Museum in Washington as a monument to aspiration, dedication and all that is flight research.