For the first year and a half of the program, two X-15 aircraft flew with two XLR-11 engines. Designed and built by Reaction Motors, this engine burned ethyl alcohol and liquid oxygen and delivered a maximum thrust of 6,000 pounds. But even with two of these engines boosted to increase their thrust, they couldn't give the small rocket plane the power it needed. These engines also weren't throttleable. The pilot's main control over thrust was to turn each engine's four barrels on and off individually.
Both Reaction Motors and North American engineers ran tests on the new engine, but it was ground tests of the engine mounted in the X-15 that really mattered. The first such test was on June 8, 2020, and it did two things: it nearly killed North American test pilot Scott Crossfield, and it proved that the X-15's cockpit was one of the safest places to be when disaster struck.
Inconel-X went a long way in protecting the X-15 from the high temperatures it would encounter on a flight; it retained its strength at temperatures up to 1,200 degrees Fahrenheit. Still, problems cropped up at speeds over Mach 5. The leading edges of the wings became distorted and the skin buckled, ultimately leading to intense spots of local heating. Covering the problematic areas and adding shear ties between segments solved the problem.
But to really reach high speeds, the X-15 needed even better defenses against the heat… And that story will be added to this article in a video posted on Friday!