The XLR99 was the first large man-rated rocket engine that was capable of being throttled and restarted in flight. This complexity resulted in many aborted missions (approximately one-tenth of all mission aborts) and significantly added to the development cost of the engine. When the X-15 program ended, many felt that the throttleable feature might have been a needless luxury that complicated and delayed the development of the XLR99.
But in the mid-1960s these attributes were considered vital to the development of a rocket engine to power the Space Shuttle. At the time, Shuttle was to consist of two totally reusable stages—essentially a large hypersonic aircraft that carried a smaller winged spacecraft much like the NB-52s carried the X-15s. The same basic engine was going to power both stages; the pilots therefore needed to be able to control its thrust output. At some points in the early Shuttle concept development phases, the same engines would also be used on-orbit to effect changes in the orbital plane. So the original concept for the Space Shuttle Main Engines (SSME) included the ability to operate at 10 percent of their rated thrust, and to be restarted multiple times during flight.11
In the end, the production SSMEs are throttleable within much the same range as the XLR99—65 to 109 percent, in one percent increments. In actuality about the only routine use of this ability is to throttle down as the vehicle reaches the point of maximum dynamic pressure during ascent, easing stresses on the vehicle for a few seconds on each flight. Even this would not have been necessary with a different design for the solid rocket boosters.12 So the complexities required to enable the engine to throttle may, again, have been a needless luxury. Nevertheless, the development pains experienced by Reaction Motors provided insight for Pratt & Whitney and Rocketdyne (the two main SSME competitors) during the design and development of the SSMEs.