Eight X-15 pilots flew above the 50-mile altitude the United States used to define the edge of space, but their reward depended on the flag on their flight suit. The five US Air Force pilots — Robert White, Robert Rushworth, Joe Engle, Pete Knight and Michael Adams — were awarded military astronaut wings. The three NASA civilians who flew just as high — Joe Walker, Jack McKay and William Dana — received nothing at the time. NASA finally corrected the injustice in 2005, awarding civilian astronaut wings to all three — two of them, Walker and McKay, posthumously. To muddy it further, the world air-sports body (the FAI) drew the line higher, at 100 km; only two X-15 flights, both by Walker in 1963, crossed that mark.

A rocket engine is monstrously powerful and monstrously thirsty. The XLR99 could deliver about 57,000 pounds of thrust, but it emptied the X-15’s propellant tanks in only a minute or two — an aircraft carrying enough ammonia and liquid oxygen to climb to test altitude Ve accelerate past Mach 6 could never have lifted off a runway under its own power. The answer, inherited from the Bell X-1, was to make the X-15 a parasite: two modified B-52s (an NB-52A and an NB-52B) carried it to about 45,000 feet and released it at roughly 500 mph. Only after it fell clear did the pilot light the rocket — and the clock, measured in seconds of burn time, began to run.

Once the rocket burned out, the X-15 became the world’s fastest glider. Every flight ended the same way: a long, unpowered, high-speed descent to a dead-stick landing on the vast hard clay of Rogers Dry Lake at Edwards Air Force Base, the pilot dropping a pair of rear skids and a nose wheel and sliding to a halt. There was no engine to wave off with and no second chance. Learning to bring a hypersonic vehicle home unpowered, and to hit a precise point after a ballistic arc, was one of the X-15’s most direct gifts to the Space Shuttle, which landed exactly the same way twenty years later.

The X-15 was never priced like a service aircraft. It was a government research program of three hand-built airframes, funded jointly by NACA (later NASA), the US Air Force and the US Navy, flown a few dozen times each purely to gather data. There is no meaningful flyaway unit price or cost-per-flight-hour for a machine that existed only to probe hypersonic flight and the edge of space, that was launched from a converted bomber, and that landed as a glider. The whole program is usually reckoned in the low hundreds of millions of 1960s dollars — but any single-aircraft dollar figure is an accounting estimate, not a market price.