# Could the Space Shuttle launch with two engines?

In a movie, they need to launch the Space Shuttle, but one engine is not ready to go in time, so they decide to launch with the booster and two of the shuttle's engines - Could the shuttle Launch with two engines?

I know that the boosters provide 85% of the Space Shuttle's thrust at liftoff and for the first two minutes of ascent. The Shuttle engines burn for 8.5 minutes.

Each Space Shuttle Main Engine [..] produce a sea-level thrust of 179,097 kilograms (375,000 pounds) and a vacuum thrust of 213,188 (470,000 pounds). The engines can be throttled over a thrust range of 65 percent to 109 percent.

But I really can't find much more information than that, the Shuttle must be providing the 15% from its 3 engines - so each would be 5%, right? (Is it not that simple..) I am curious if corrections for lack of that 3rd engine would be possible, of course, it seems like that would not be the case, or they would have designed it with two engines.

• It wouldn't launch with an engine out, that would have meant an abort.
– GdD
Apr 6, 2022 at 15:49
• Despite that the answers so far made some interesting points I think the spirit of this question is that it was in a movie and so the spirit of answers can be to address artistic licence. In this case the OP is just asking if it could have made it to orbit on an energy and/or control basis. No one has answered the energy bit yet and so far it seems from the deleted answer and comments that even the control aspect wasn't without hope. Is anyone up for answering that? Apr 6, 2022 at 17:47
• We're talking about aerospace, a sector where every kilogram counts. There's really no room for overdesigning things so much that you can just fly with a third of your second-stage engines inoperable. That sort of performance margin would be staggeringly expensive. Apr 6, 2022 at 17:48
• @OrganicMarble: So a movie-plot-inspired reason that you really need to go to space today could justify that, or maybe something close like 107% if 109% has a bigger risk of making the engine fail if run that hard for the full duration. You still figure even a launch with 2 engines at 109% all the way to MECO wouldn't get you to orbit? Maybe with a reduced crew / payload and stripped interior, and/or cutting off the bell nozzle of the failed engine to save mass if you know before launch it's failed, and have hours but not days. (And the other shuttles are already needed or launched.) Apr 7, 2022 at 1:34
• I assume they start an empty shuttle, so they trade their payload to the broken engine. Right? This would rule out all the answers, that it would not work, because it was never designed to start empty. Apr 7, 2022 at 14:18

Let’s see. The heaviest Space Shuttle mission was STS-117 which had 2052t of launch mass and went to an ISS orbit.

Take-off thrust is 2*13MN from the boosters and 3*1.6MN from the RS-25 engines. So the thrust-to-weight ratio at take-off was 1.53. If we remove 19t of payload and 3.2t of engine we get a thrust-to-weight ratio of 1.47 at take-off. If we run the engines at 111% emergency thrust we get 1.49. So getting off the launch pad shouldn’t be a problem.

I can’t find any data on mass after booster separation, here it says that weight immediately before separation was 880t. If we subtract 2*91t for the empty mass of the boosters we get 698t of mass. At that point the RS-25 engines should operate pretty close to their vacuum thrust of 2.09MN which gives us a TWR of 0.916 (sounds about right, the TWR went below 1 after booster separation).

Removing one engine and the payload gives us 675.8t of mass. Running the two engines at 111% gives us a TWR of 0.7 after booster separation.

That’s not great but at the same time our mass is lower and I assume that even a very low orbit would be okay. The Orbital Maneuvering System could achieve 300m/s of delta-v with a 29t payload. Without payload the mass it has to accelerate would be 30% lower.

So with an adjusted launch profile it might just work.

As for control: I don’t think this would have been an issue. Removing the engine furthest from the external tank would actually bring your center of thrust closer in line with your center of mass.

• This is amazing! thank you for that breakdown, the engine weight itself was something I had not thought of at all. Apr 8, 2022 at 3:42
• As discussed in comments, the normal launch profile throttled back the SMEs as propellant mass was consumed, to limit max G. If you don't do that, or not as much, you can maybe "catch up" some towards where a normal launch would be in terms of speed for a given time after launch, despite having spent more energy lifting more fuel higher and faster than if you'd burned it earlier. (Part of the problem with spreading out the same delta-v over more time is losing more energy to gravity. But with a lighter launch mass and longer burn, you have more delta-v total.) Apr 9, 2022 at 10:32

If one of the Space Shuttle Main Engines (SSMEs) shut down before liftoff, the countdown would stop and the vehicle would not liftoff. This happened a few times.

If the SSME shut down immediately after liftoff, the crew would select a Return to Launch Site abort. This never happened.

If the SSME lasted longer, abort landings in Europe could be possible (this never happened), or even a lower orbit could be made if it happened late enough (this last actually happened).

The vehicle could not go to space with one engine out at liftoff, but it was designed to achieve a safe landing with any one engine out at any point in the ascent if it could not achieve a safe orbit.

References:

• I find it amazing now that the program went through with the abort risks they carried.
– GdD
Apr 6, 2022 at 16:10
• It is somewhat funny to put "safe" into a sentence talking about RTLS. From what I have read about that abort mode, its safety profile can pretty much be summed up as "if X happens, the vehicle is toast, for any X". :-) Apr 6, 2022 at 17:54
• @TooTea a lot of analysis and simulation would not agree with you on that. Since it was never attempted, the proof is not there. Apr 6, 2022 at 18:03
• @OrganicMarble I didn't mean that RTLS was impossible, just that it was even less safe than a (by current standards) already quite unsafe nominal STS launch. But I guess I'll just delete my comments as they are pretty tangential to your answer. Apr 6, 2022 at 21:11
• @TooTea agree completely that RTLS was less safe than a normal launch! Apr 6, 2022 at 21:17