Quite fortuitously, a suitable rocket engine was already available for the X-1 project, a 6,000-pound-thrust rocket developed by Reaction Motors Incorporated.  This engine comprised four separate rocket cylinders, fueled by liquid oxygen and alcohol.  Each cylinder could produce 1,500 pounds of thrust and could be ignited or shut down independently, so that overall thrust could be varied.  The X-1 had no throttle.