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I've read in a place I can no longer remember that MECO would happen when fuel is depleted or, if the separating stage is returning, when fuel reaches some preset threshold---15%, say.

But now... reading the space shuttle Booster Systems Brief JSC-19041 (1, 2), I've come across the snippet below, which suggests that at least in the case of the space shuttle, MECO would normally happen when velocity, not fuel mass, hits an I-LOAD threshold value.

Nominal MECO is initiated when the vehicle reaches a desired I-loaded velocity. If an off-nominal condition causes the propellant supply to be...

(screenshot)

Can someone confirm this? Is velocity really the variable that triggers MECO? I don't imagine any fuel left would be substantial (it would probably just safety margin of a few percent, tops). But still, it would be nice to know for sure if fuel mass or velocity is the controlling variable for MECO.

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  • $\begingroup$ Does this answer your question? How is rocket engine cutoff controlled? $\endgroup$ Jun 2 '21 at 5:10
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    $\begingroup$ Lower stages may use a fuel level cutoff, but orbital insertion cutoff is normally controlled by velocity threshold. $\endgroup$ Jun 2 '21 at 5:11
  • $\begingroup$ Ah! So MECO for the space shuttle was orbital insertion? That would definitely explain the velocity control variable. I don't know as much about the shuttle as I should by know. $\endgroup$
    – user39728
    Jun 2 '21 at 5:15
  • $\begingroup$ I've converted your screen shot of text to actual text for the reasons stated in the edit. I've also added two links for JSC-19041 but I can't find your quote in either one. Can you find a moire appropriate link and mention the page number? We should require each reader to go hunt down the documentation and read through all of its pages to find (or in my case fail to find) the cited passage. Thanks! $\endgroup$
    – uhoh
    Jun 2 '21 at 5:46
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    $\begingroup$ @uhoh I think you meant we shouldn't require each reader to go hunt down the documentation rather than we should require each reader to go hunt down the documentation. $\endgroup$ Jun 2 '21 at 10:05
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Either.

For the first part, you already answered your question. The main criterion was velocity, because the SSMEs on the orbiter are the second (and orbit insertion) stage of the Shuttle. There's no third stage to compensate for severe under/overperformance (well, there's the OMS, but there's only so much it can do because it doesn't pack quite enough punch to serve as a real stand-in for the SSMEs), so you absolutely need to control the engine cutoff according to the target orbit.

However, you always have to have a fuel level cutoff as well. Given that the turbopumps are powered by megawatt-class turbines and the rotational inertia of the rotor assembly is comparably tiny, the rotor will overspeed and disintegrate the very moment it ingests a big bubble. This needs to be avoided at all costs, even if that means an early MECO and failure to reach orbit.

Indeed, the second half of the sentence you quoted confirms this:

If an off-nominal condition causes the propellant supply to be depleted before the MECO velocity is reached, then the ET LH₂/LO₂ low level cutoff function will shut down the SSME's before the propellant supply to the engines is depleted.

JSC-19041 Booster Systems Brief, section 2.4.1

(It then goes on to explain that there's always more LH₂ loaded in the ET than necessary to avoid oxidizer-rich/engine-rich cutoffs.)

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