15

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.

End Anti-Semitic Hate
  • 233
  • 3
  • 12
nycynik
  • 763
  • 7
  • 12
  • at least one mission had an engine shutdown in flight; STS-51F – BrendanLuke15 Apr 06 '22 at 15:48
  • 3
    It wouldn't launch with an engine out, that would have meant an abort. – GdD Apr 06 '22 at 15:49
  • 7
    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? – Puffin Apr 06 '22 at 17:47
  • 10
    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. – TooTea Apr 06 '22 at 17:48
  • 2
    @Puffin The whole reason that they had to abort RTLS for an engine out at liftoff was because of the energy problem (including hauling a bunch more prop uphill) – Organic Marble Apr 06 '22 at 18:18
  • 1
    @OrganicMarble: couldn't the remaining two engines use all the propellant, so you get the same total delta-V out of it, unless thrust vectoring to maintain control hurts efficiency. Of course with less thrust, gravity has more time to eat away at your vertical speed costing energy while you're still thrusting upwards, so early engine-out is clearly bad for energy. But it seems to me what you're hauling uphill is a dead engine, not propellant? Or you mean because the propellant doesn't get burned as quickly, you're hauling more of it higher with energy from the SRBs? – Peter Cordes Apr 07 '22 at 00:50
  • 1
    @PeterCordes yes, you can burn it up by running the remaining engines longer. That's exactly what happened in the late-ascent abort to orbit cases. But at time "t" in the trajectory (after SRB sep) you have ~ 2/3 the thrust along with extra weight due to the prop that should have been burned by the other engine all of which hurts your acceleration. If the engine failed at liftoff, at SRB sep there would be ~120,000 lbm of prop in the tank above what was planned for. – Organic Marble Apr 07 '22 at 00:52
  • 1
    @OrganicMarble: Thanks for expanding on that. I guess another thing that makes late failure less bad is that the SSMEs were throttled back gradually to ~70% to limit G forces later in the flight, but with only 2 active you could throttle back up to 100% (or even the absolute max 109%). – Peter Cordes Apr 07 '22 at 01:03
  • 2
    @PeterCordes agreed, in the Ascent Checklist https://www.nasa.gov/centers/johnson/pdf/567068main_ASC_135_F_1.pdf you can find the callouts for MAX THROT (meaning allow 109%) but they are typically in the "hail mary" type cases. – Organic Marble Apr 07 '22 at 01:07
  • 3
    @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.) – Peter Cordes Apr 07 '22 at 01:34
  • 2
    "You still figure even a launch with 2 engines at 109% all the way to MECO wouldn't get you to orbit? " I don't think so, or they would have done it that way in the shuttle program. As far as shedding mass - could be. We need to make an ARD (abort region determinator) run! – Organic Marble Apr 07 '22 at 01:55
  • 1
    @PeterCordes It's not your vertical speed that matters--it's the horizontal speed. However, you are right that it's gravity eating away--they allocated a certain amount of delta-v for gravity loss. Riding two engines means more will be lost to gravity, when the engines shut down it won't be going fast enough. – Loren Pechtel Apr 07 '22 at 03:59
  • 4
    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. – usr1234567 Apr 07 '22 at 14:18
  • Between the 2 SRBs providing 85% of total thrust and 2 SSMEs providing 5.5% each for 11% of total... there could be 96% of nominal thrust available at launch. Sounds within the margins of possible to me! Then just do a long SSME burn. Yeah, you get off to a slow start but make it up later after SRB jettison. ( 100% thrust - 85% from SRBs leaves 15% split between 3 SSME. 15%/3 = 5% nominal from each SSME. If SSME pushed to 110% output (compromise between 109% and 111%) you get 5.5% SO 85% + 5.5% + 5.5% = 96% of nominal thrust) – BradV Apr 07 '22 at 16:28
  • 2
    Is nobody going to ask what movie this was? – N. Virgo Apr 08 '22 at 13:44
  • @N.Virgo I was wondering that myself! – AJM Apr 08 '22 at 18:10
  • The movie was MoonFall - From Wikipedia: "audiences "questioned the use of the Space Shuttle which has been out of commission since 2011" – nycynik Jul 29 '22 at 13:51
  • @GdD the space shuttle had no launch abort system, you launch, and you're stuck. – Deko Revinio Aug 24 '22 at 16:50
  • If the three engines hadn't lit the launch would have been aborted before lighting the solid boosters @DekoRevinio. It happened at least once. – GdD Aug 25 '22 at 15:15

2 Answers2

28

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).

enter image description here

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:

Organic Marble
  • 181,413
  • 9
  • 626
  • 815
  • 8
    I find it amazing now that the program went through with the abort risks they carried. – GdD Apr 06 '22 at 16:10
  • 7
    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". :-) – TooTea Apr 06 '22 at 17:54
  • 5
    @TooTea a lot of analysis and simulation would not agree with you on that. Since it was never attempted, the proof is not there. – Organic Marble Apr 06 '22 at 18:03
  • 6
    @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. – TooTea Apr 06 '22 at 21:11
  • 4
    @TooTea agree completely that RTLS was less safe than a normal launch! – Organic Marble Apr 06 '22 at 21:17
  • 1
    Isn’t OP talking about a theoretical scenario? The later versions of the engine could go to 111% thrust. Two engines running at 111% is only 26% less thrust than 3 engines at 100%. If you removed one engine and all payload from the Shuttle, would it really be unable to launch? Especially if we don’t need it to go to orbit, only achieve a Thrust To Weight ratio greater than 1. – Michael Apr 07 '22 at 12:43
  • @Michael no engine on the shuttle was certified for use at 111%. Feel free to post your own answer. – Organic Marble Apr 07 '22 at 13:33
  • 1
    @OrganicMarble: The Wikipedia says: “Block II engines could throttle as high as 109% or 111% in an emergency, with usual flight performance being 104.5%.”. Again, this is a theoretical question, so using emergency thrust (maybe making the engines unsuitable for re-use) would totally be fine. – Michael Apr 07 '22 at 15:01
  • 3
    @TooTea: John Young described it as "RTLS requires continuous miracles interspersed with acts of God to be successful.". – Jörg W Mittag Apr 07 '22 at 18:02
  • 1
    Just reading the Wikipedia summary is pretty harrowing.... https://en.wikipedia.org/wiki/Space_Shuttle_abort_modes#Return_to_launch_site – adam.baker Apr 08 '22 at 07:19
  • 1
    I love the description of RTLS as an "unnatural act of physics", personally. – Sebastian Lenartowicz Apr 08 '22 at 09:08
8

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.

Michael
  • 472
  • 4
  • 10
  • 1
    This is amazing! thank you for that breakdown, the engine weight itself was something I had not thought of at all. – nycynik Apr 08 '22 at 03:42
  • 1
    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.) – Peter Cordes Apr 09 '22 at 10:32