The Rocketdyne RS-25, a large hydrolox engine best-known in its role as the main engines (SSMEs) of the space shuttle orbiter, was originally certified to a thrust level of 1.67 meganewtons (mN) at sea level, and 2.09 mN in vacuum; as this was originally its maximum certificated thrust rating, it became known as “100% thrust”, or simply “100%”.
Further testing, however, allowed the engine to be uprated to a 1.75 (sea level)/2.17 (vacuum)-mN thrust rating without any changes in hardware, it having demonstrated its ability to operate at the higher thrust level with very little additional risk of spontaneously disassembling. As this new thrust rating was approximately 104% of the original, it became known as “104% thrust” or “104%” (slightly increased to 104.5% in the late 1990s). The higher thrust rating allowed the shuttles to carry heavier payloads into orbit, and it was used routinely for essentially all shuttle launches thereafter.
Even more engine testing was performed, allowing the engine to be uprated once again, to 1.86 (sea level)/2.28 (vacuum) mN, which, being approximately 109% of the original maximum certificated thrust, became known as “109% thrust” or “109%”. Operating at 109% thrust did mean operating with thinner margins of safety, and increased the wear and tear on the engines every time it was used, so the 109% thrust setting was not commonly used (as the vast majority of missions were completely doable at 104%, thus eliminating the need for going to the riskier 109%); it was reserved for very-high-weight missions (where the additional thrust was needed in order to reach orbit)1 and for ascent aborts following a complete or partial failure of one or two engines (where increasing the thrust on the remaining engines would make up a small part of the lost thrust, increasing the orbiter’s chances of reaching orbit, or at least a runway, or of, at a minimum, maintaining control and staying intact long enough to get down to subsonic speeds and allow the crew to bail out safely). The last few RS-25s were uprated even further, to 111% (2.32 mN in a vacuum), but operation at this thrust level was strictly prohibited except during an abort.
Was 109%/111% the absolute maximum thrust the RS-25 could produce without a high risk of exploding, or could it potentially have been pushed further if the shuttle’s FADECs allowed it?
1: For example, Shuttle-Centaur missions, had they not been cancelled following the loss of OV-099 (“Challenger”), would have required the use of 109% thrust even for a nominal ascent, due to the extreme weight of the Centaur upper stage and its payload.