A very informative article on the RS-25 (SSME) indicates that it could be throttled from 67% to 109%. (http://en.wikipedia.org/wiki/Space_Shuttle_Main_Engine) I know on 51L the data indicates the throttle setting was being reduced to 65%. My question is what was the minimum setting for the SSME in flight? In an off nominal situation could they be throttled like any airliner engine?

  • $\begingroup$ What’s your source for the 65% on 51L? $\endgroup$ Commented Nov 4, 2019 at 16:29
  • $\begingroup$ @RussellBorogove 65% is correct: space.stackexchange.com/a/20332/6944 But not the time. At 35 seconds the SSMEs were at 94% on 51-L. $\endgroup$ Commented Nov 4, 2019 at 16:40
  • $\begingroup$ T+35.379 The three main engines begin throttling down to 65 percent power as planned. spaceflightnow.com/challenger/timeline $\endgroup$ Commented Nov 4, 2019 at 16:44
  • $\begingroup$ @ChallengerTruth check the reference in the link in my comment. 51-L had 2 stage bucket; it first throttled down to 94% at 24 seconds, then to 65% at 42 seconds, then back up at 65 seconds. Note that even in your link the PAO person makes the comment at 28 seconds "Engines beginning throttling down, now at 94 percent." The 35 second reference there is incorrect. $\endgroup$ Commented Nov 4, 2019 at 16:49
  • 1
    $\begingroup$ I accept your data, I was simply looking for a reference to the 65%, not so much the specific time it occurred which was not relevant to the question asked. Thank you for the clarification. $\endgroup$ Commented Nov 4, 2019 at 16:56

1 Answer 1


The original specification minimum power level was 65% as seen in this page from the 1989 Rocketdyne SSME Pocket Data Book.

enter image description here

Later a "bi-stable turbopump" issue caused the lower limit to be raised to 67% as seen in this slide from a June 1998 presentation.

enter image description here

The SSMEs could be manually throttled by the pilot using the Speedbrake/Thrust Controller (SBTC) mounted to the left of their seat.

This procedure from the Ascent/Entry Systems Procedures checklist references that capability.

enter image description here

Use of the SBTC is described in the Shuttle Crew Operations Manual page 2-13.38.

In the case of the engine thrust-level setting, the top half (AUTO) of both SPD BK/THROT pushbutton indicators on panels F2 and F4 are illuminated nominally. Only the pilot's SBTC can be enabled for manual throttle control. The pilot depresses the TAKEOVER pushbutton on the SBTC, causing the GPC throttle command to be frozen at its current value. While depressing the TAKEOVER button, the pilot moves the SBTC to match the frozen GPC command. Manual control is established when the SBTC command matches within four percent of the GPC command. When the match is achieved, the pilot's SPD BK/THROT MAN pushbutton indicator on panel F4 is illuminated, and the AUTO light is extinguished on both panels F2 and F4. A manual throttle indicator also appears on the Ascent/Entry Flight Display (AFD).

At this point, the pilot will have manual control of the throttles and the TAKEOVER pushbutton is then released. If the TAKEOVER pushbutton is released before a match is achieved, the system reverts to GPC auto commands. Under manual throttle command, depressing either or both pushbutton indicators on panel F2 and F4 causes the system to revert to the GPC auto commands. Transferring back to auto leaves the throttle at the last-commanded manual setting until a new command is issued.

  • $\begingroup$ So below 65% the engines would suffer damage or fail? $\endgroup$ Commented Nov 4, 2019 at 16:02
  • $\begingroup$ Certified operational limits were as stated, and the crew had no ability to command the throttle below the software lower limit. There are comments on this site about the engines running stably at lower power levels in malfunction cases, but searching for them is failing me at the moment. $\endgroup$ Commented Nov 4, 2019 at 16:32

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.