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.