The Space Shuttle External tank contains:

  • 629,340 kg of liquid oxygen
  • 106,261 kg of liquid hydrogen.

That's an oxidiser to fuel ratio of 5.92

Meanwhile, the Space Shuttle Main Engine has a constant mixture ratio of 6.03

Where is that extra hydrogen going? (or extra oxygen appearing...?) The Space Shuttle used regenerative cooling and staged combustion, so all the fuel and oxidiser should eventually end up in the combustion chamber despite turbo pumps and engine cooling needs.

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    $\begingroup$ IIRC they always packed a little more hydrogen than oxygen (for that ratio), so that at the end of combustion they wouldn't risk running oxygen-rich (and as result engine-rich). $\endgroup$
    – SF.
    Commented Oct 14, 2020 at 12:38
  • $\begingroup$ That's it! Posting an answer with some details. $\endgroup$ Commented Oct 14, 2020 at 13:04

2 Answers 2


tl;dr as SF said in a comment "...they always packed a little more hydrogen than oxygen (for that ratio), so that at the end of combustion they wouldn't risk running oxygen-rich (and as result engine-rich)."

If you are interested in details, please refer to this propellant loading table for the following explanation.

enter image description here

The numbers indicated by the purple arrow were the masses book-kept in the propellant load to guard against contingency situations. FPR was Flight Propellant Reserve and was extra propellant booked to guard against certain off-nominal cases. FUEL BIAS was extra hydrogen loaded to make sure that the system never had a oxygen-rich cutoff.

From the answer to this question: Is "liftoff mass" = "ignition mass"?

An important thing to know when interpreting this information is that there is not just one mixture ratio for the engines.

This is because the shuttle main propulsion system used autogenous pressurization. Some of the propellant entering the engine was vaporized and returned to the tank as pressurant - it did not flow overboard.

So there are two mixture ratios in play:

  • "inlet mixture ratio" (MR) - the ratio between the flows of propellant that enter the engine
  • "overboard mixture ratio" (OBMR) - the ratio between the flows of propellant exiting the nozzle

Since the pressurization flow of both propellants was ~ 1 lbm/s, but the inlet flow of LH2 was much less than the LO2, the OBMR was always higher than the MR. You can see these numbers in the upper right corner of the chart.

(Also note that the mixture ratio was programmable for the SSMEs and changed over the course of the program. This chart is for STS-5, which was the last flight of the original version of the SSME. See Taxonomy of the SSME.)

So if you wish to match the overboard mixture ratio, the line in the chart is USED AT OBMR, defined as


The FPR and FUEL BIAS are included in the USABLE RESERVES as propellant that is not planned to be used but is available for off-nominal situations. A lot of work went into safely minimizing the usable reserves because every pound of this propellant was a pound that could not be used for payload.

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    $\begingroup$ "they wouldn't risk running oxygen-rich (and as result engine-rich)" What does "engine-rich" mean? $\endgroup$
    – nick012000
    Commented Oct 15, 2020 at 7:13
  • 26
    $\begingroup$ @nick012000 It's a cheeky way of saying "the engine would be the thing burning" :) If you sprinkle liquid (or atomic) oxygen on materials like iron or aluminium, they burn. It's far safer to have excess hydrogen, which is essentially harmless in open space (with no oxygen to combust). $\endgroup$
    – Luaan
    Commented Oct 15, 2020 at 7:28
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    $\begingroup$ @Luaan And doubly so when the engine bell is searing red-hot from operation. Pouring LOx on cold metal is reasonably safe, but when the thing is screaming hot it's just begging to go up like a roman candle. $\endgroup$
    – J...
    Commented Oct 16, 2020 at 20:09

Liquid hydrogen is much colder than liquid oxygen, the heat of vaporisation is much lower too, therefore more volume of hydrogen than oxygen is evaporating in the tanks from ignition until engine shutdown.

It is difficult to compensate the evaporation with more insulation for the hydrogen tank, the volume is about three times the volume of the oxygen ( 1,497,440 to 553,358 l), so also the tank surface is bigger. Both tanks have the same diameter (8,4 m), but the hydrogen tank is much longer (29.6 to 16.6 m). The insulation should not weigh much more than the saved hydrogen.

So the ratio of hydrogen to oxygen in the tanks at launch is bigger than the ratio used for the engine. At engine shut down hydrogen is feeded longer than oxygen to protect the engine from oxidation. So there should be more hydrogen left just before shutdown.

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    $\begingroup$ The table posted by OrganicMarble tells something else. There are 100 lbs of LH2 listed for "Boiloff" but 10000 lbs of LO2. $\endgroup$
    – asdfex
    Commented Oct 14, 2020 at 16:49
  • $\begingroup$ @asdfex We should compare volumes, not mass. There are 629,340 kg and 533,358 l of LOX but 106,261 kg and 1,497,440 l of LH2 in the tank. Hydrogen is much lighter than oxygen. $\endgroup$
    – Uwe
    Commented Oct 14, 2020 at 17:43
  • 1
    $\begingroup$ @asdfex there's more than boiloff in that (as the label says) but the explanation is too long for a comment. $\endgroup$ Commented Oct 14, 2020 at 18:05

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