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Throughout much of a space shuttle ascent, even a single SSME failure would have left the mission’s intended orbit out of reach and forced some sort of abort; a two-SSME-out situation would have left the shuttle unable to achieve any orbit at all unless it occurred fairly close to the time of nominal MECO, and the loss of all three SSMEs would, unless it occurred during one of a few short windows, force the crew to bail out over the Atlantic and leave the orbiter to crash. An engine failure during an RTLS abort (where the performance margins would be even tighter than during a nominal ascent - especially given that one of the main reasons for performing an RTLS abort in the first place was if one SSME had already failed) would be even worse; the orbiter would still probably have been able to make it back to land on two engines, but going down to a single engine would have forced a bailout unless it occurred either very late in the RTLS sequence or (for high-inclination flights) during a short window prior to powered pitcharound, and a three-out during an RTLS would, for much of the RTLS sequence, result in a loss of control and the orbiter subsequently disintegrating in flight. And, yet, nowhere in the shuttle engine-out and abort checklists is there any mention of attempting to restart the failed engine, despite the fact that many of the situations that could cause an SSME to fail would still leave it safe to restart - for instance, a hiccup in the combustion process resulting in a brief overheat, or a bubble entering one of the fuel lines and causing a momentary drop in fuel flow, or a failed sensor causing a good engine to spool down before the sensor could be taken offline.

Given the potentially dire consequences of an SSME flaming out, why didn’t the shuttle’s engine-out procedures call for attempting to relight the failed engine?

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    $\begingroup$ Can't remember the source (perhaps someone will chime in here), but I know that it took months of testing at Stennis before the SSME's were able to reliably make it through their six-second start sequence without (sometimes spectacular) failure. A lot of us felt that, when flying a Space Shuttle mission, if we made it through those first six seconds, a good portion of the total risk involved was behind us...I think relight capability would have been a bridge much too far. $\endgroup$ – Digger Feb 24 at 20:22
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    $\begingroup$ Hey, your gigawatt, thousands of degrees engine just suffered an unexpected major failure. Do you want to attempt pouring more fuel and oxidizer to see if it might work again ? $\endgroup$ – Antzi Feb 25 at 8:52
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    $\begingroup$ @Antzi you are so right. There were 30 years of arguments about how to manage the auto-shutdown switch, imagine the arguments about when to restart (if you could have). $\endgroup$ – Organic Marble Feb 25 at 13:39
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It was impossible to restart the engines in flight.

If for no other reason, the pressure/temperature conditions for the "start box"1 were not met in flight. Also, Post Shutdown to Engine Ready was not a legal transition in the controller flight software. There were also considerations of engine drying (removal of residual water after shutdown), prelaunch purge sequences that had to be done, and other criteria that couldn't be met. There is some discussion here, but largely unsourced.

The LH2 inlet pressure was never as high as even 40 psi after liftoff and for the majority of ascent was around 28 psi, placing it well out of the start box. Similarly, the LO2 inlet temperature was never high enough during flight to get into the start box - it relied on the temperature rise due to prelaunch drainback to get into the box.

This diagram from the Pocket Data Book shows the start box.

enter image description here

Work was done to expand the start box with the hope of using the SSME for other vehicles but it never reached the stage of flight readiness. (source)

The impossibility of restarting the SSME in flight was a major reason for the failure of the Ares 1 design. A single air start might have been doable, but not restarting for any orbit adjustment burns. Once this was realized, the substitution of the less efficient J2 engine started, with subsequent vehicle weight growth etc, etc.

1 The "start box" is the pressure-vs-temperature conditions at the engine inlet that the engine is certified to start under.

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    $\begingroup$ You could make clear that the "start box" is not a box... Took me a moment to understand that it's just a rectangle in the temperature/pressure parameter space. They actually used Réaumur for the temperature. First time I see it in actual use... $\endgroup$ – asdfex Feb 24 at 17:39
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    $\begingroup$ Good comment. Editing. $\endgroup$ – Organic Marble Feb 24 at 17:39
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    $\begingroup$ @asdfex, those numbers look really high to be Reaumur. 170 Reaumur is hotter than boiling. I'd expect them to be Rankine. $\endgroup$ – BowlOfRed Feb 24 at 22:35
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    $\begingroup$ Rankine is what I knew them as. It was easier to write 40 than -420. $\endgroup$ – Organic Marble Feb 24 at 22:56
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    $\begingroup$ @BowlOfRed That's what I meant. Not familiar enough with all those legacy units... $\endgroup$ – asdfex Feb 25 at 8:21

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