During the golden age of flight test, the name of the game was higher, faster and louder. No manned x-plane ever exceeded the X-15 in any of these categories. X-15 66670 (Ship 1) flew 81 times, all originating from the undisputed home of experimental flight test – Edwards Air Force Base, California.
The flights were first operated by its manufacturer North American Aviation (NAA), then as a joint NASA/USAF venture. Together, they would embark on the most successful and exciting flight test program in history.
Before any of this could start, lots of strange new problems had to be overcome. One of the big unknowns was the thermal properties of operating in the hypersonic speed regime. Robert Hoey (X-15 Primary Flight Test Engineer) recently offered insight into these early stages.
“At the time it was all being put together, there was nothing on the ground that would come close to duplicating the flight conditions with respecting to heating. Just nothing. You couldn’t get the combinations of Mach number and Reynolds number to even come close to duplicating flight conditions, so you had a bunch of different theories. There were about six of them, as I remember. Six different theoretical calculations that were to predict what the temperatures would be at various parts of the airplane. And we didn’t know which one was going to be right. At that time, the proper approach was to over-design for the worst case. We had an airplane that had all kinds of built in backup structure to handle the high temperatures. As soon as we got into it, it became pretty obvious that there were a couple equations that were better than the others.”
I have learned from the personal experience of handling large pieces of X-15 structure that the plane is heavy. Rather than using lightweight aluminium, the X-15 skin and structure is primarily made of Inconel steel. This material acts as a heat sink, keeping structural rigidity at temperatures of over 2,000 ºF.
The X-15, unable to take off under its own power, was carried aloft by a B-52H mothership, then dropped to begin its flight. Four captive flights were made before the first free flight of Ship 1, which NASA test pilot Scott Crossfield bravely flew on June 8, 1957. This was an unpowered flight, testing the glide characteristics of the new airframe. Its optimization for high speed flight was shown as it glided to a speed of Mach .79 before landing safely on Rogers Dry Lake at Edwards Air Force Base. Hoey remembers this NAA orchestrated flight,
“For the other North American flights, we were in the control room but we weren’t controlling anything. We were just watching. But for that flight, of course, we wanted to go out on the roof and watch.”
After the successful glide flight, Ship 1 met the next milestone by making its first powered flight on January 23, 1960, again flown by Crossfield. He fired the propulsion system, reaching a speed of Mach 2.53 and an altitude of 66,843 ft. This flight was propelled by two XLR-11 rocket engines, totalling 16,000 lbf of thrust. Later flights would use a much more powerful XLR-99 engine, throttleable from 15,000 lbf to 57,000 lbf of thrust. The fuel and oxidizer tanks only held enough fuel for to burn for 80 to 120 seconds. As the propellant level decreased, the aircraft quickly became lighter, but the force of the engine remained the same. This subjected pilot to 4 gs near the end of the burn. NASA pilot Milt Thompson was once said
“The X-15 was the only aircraft I ever flew where I was glad when the engine quit.”
However, things would become even more dangerous if the engine failed to light in the first place. At the end of a typical mission, the fuel tanks would be empty resulting in a safe landing weight. If the engine didn’t fire, the X-15 was too heavy to land safely. In this case, a fuel dump was initiated, but the aircraft plummeted to the ground so quickly that there wasn’t enough time to make a dent in the heavy on-board fuel supply. This resulted in a very dangerous overweight landing on Mud Lake, a dry lakebed in Nevada positioned under the launch zone. The first time Ship 1 met this fate, Marine pilot VADM Forrest Peterson was at the controls on January 10, 1962. Peterson came out unscathed, but in other incidents, these forced landings at Mud Lake severely injured man and machine.
After landing so far away from home base, the X-15 had to be loaded on a flatbed trailer and transported back to Edwards. The aircraft was fairly wide, so the convoy moved at night with safety vehicles leading and following the flatbed. This could sometimes prove just as harrowing as the emergency landing itself. Hoey recalls one such incident;
Source: YouTube