How does the efficiency of the rocket engine using liquid propellants depend on the throttling setting? (Is it more or less efficient while working on eg.: 85% of its maximum thrust?). The rocket engine is in vacuum.

  • $\begingroup$ I don't quite understand your question. Are you asking whether the throttle setting on a rocket engine (i.e. whether it's operating at 100%, 90%, 80% of its maximum thrust) affects the engine's efficiency? Also, keep in mind that most large rocket engines can be throttled very little (if at all). $\endgroup$
    – Lightsider
    Sep 29, 2015 at 7:29
  • 1
    $\begingroup$ Are you asking about rockets being launched from earth or already in space? $\endgroup$
    – GdD
    Sep 29, 2015 at 8:19
  • $\begingroup$ ".. for rocket engines to work for a long time with very low power?" Perhaps you are thinking of Ion drives and similar. They produce only a small push, but for the mass of fuel, provide a greater end speed. They are good for objects already in orbit, but useless for escaping from the surface of any body that has a higher rate of surface gravitation than the drive can lift against (the craft never rises off the ground). $\endgroup$ Sep 29, 2015 at 10:42
  • $\begingroup$ Re your edit, you'll still have to specify what type of engine do you have in mind. Otherwise, I'm afraid your question is still too broad / unclear. Cheers! $\endgroup$
    – TildalWave
    Sep 29, 2015 at 14:01
  • 1
    $\begingroup$ wedelfach, I re-edited your question and voted to re-open it. Now it doesn't depend on ballistics. $\endgroup$ Sep 30, 2015 at 8:33

2 Answers 2


Corroborating Russell Borogove's answer, some Stennis test data I have from 1987 on three different SSMEs shows a small drop in Isp with power level. From 109% to 100% the Isp dropped about 0.08%. I can't find data at lower power levels but my recollection is that the trend continued, with a small degradation in Isp as you throttled down.

For reference, the original SSME could throttle from 109% to 65% of Reference Power Level. A "bi-stable turbopump anomaly" limited the lower end of the throttle range to 67% in the latter part of the program. The throttle range actually experienced in flight was from 65% to 104.5%.

  • 1
    $\begingroup$ "Historically, some of the issues experienced by the SSME during throttled conditions include nozzle flow separation, side loads and heat loads; preburner boost pump bistability (around 50% RPL); HPOTP preburner vane diffuser stall (prior to Block II upgrade); and HPFTP boilout (stall) at very low (dwell) mixture ratios. The SSME did not exhibit unstable combustion characteristics even at power levels as low as 17%." ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100033271.pdf $\endgroup$ Sep 30, 2015 at 16:17
  • $\begingroup$ I forget the details, but there was a crazy test stand anomaly where an SSME got down to around 30%. I never heard of the 17%, wow. $\endgroup$ Sep 30, 2015 at 16:19
  • $\begingroup$ Yeah, sounds like a lot of different things outside of the injector and chamber would need to change to make it reliably throttle that low, though. $\endgroup$ Sep 30, 2015 at 16:21

It's surprisingly difficult to find a good answer to this question.

Generally, the rated full power level is where the engine is going to be most efficient.

According to Sutton's "Rocket Propulsion Elements", typical deep-throttling engines suffer between 1.5% and 9% reduction in specific impulse (fuel efficiency) at low power levels. It mentions an outlier, the engine in the Lance missile, which has an extraordinary 357:1 throttle range with 15% loss at the low end.

I found a poorly labeled and confusing graph that suggests that the CECE upper stage engine suffers about 5% when throttled far down.

The Apollo Lunar Module Descent Engine is upwards of 97% efficient at 30% throttle.

If you cared to, you could design an "afterburning" rocket engine, dumping extra fuel or oxidizer (or any other working fluid, for that matter) into the nozzle; this would give you a big thrust boost, cooler (and possibly dirtier) exhaust, and a large loss in efficiency. Thus power could be increased beyond the point of peak efficiency.

(Apparently AJR has patented a variation on this, injecting both fuel and oxidizer into the nozzle -- effectively using the upper part of the nozzle as combustion chamber, for a more appropriate expansion ratio at sea level, apparently?)

  • $\begingroup$ Does it explains where does the loss of efficiency comes from ? For exemple with a cryogenic rocket, one must exacerbate fuel evaporation if he don't use it quickly enough $\endgroup$
    – Antzi
    Sep 30, 2015 at 15:59
  • $\begingroup$ Lower chamber pressure and unsteady combustion, mainly. Cryogenic boil off doesn't factor in; ascent engines need moderate throttling at most and finish their work in minutes; descent engines need to hold their fuel for days or weeks depending on where they're descending to. $\endgroup$ Sep 30, 2015 at 16:05

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.