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A closeup of the X-15's remarkable XLR-99 rocket engine. Its 57 000-lb. maximumthrust is equivalent at burnout to 600 000 hp. The engine can be throttled from40-percent to 100-percent thrust. Its propellants flow at the rate of 13 000 lb. perminute at maximum thrust, exhausting the entire 18 000-1b. fuel supply in 85seconds. The engine's nozzle diameter is 39.3 in.; its over-all length, 82 in.; itsweight, 1025 lb.