The answer to How much propellant is used up until liftoff? makes me wonder whether the terms "ignition mass" and "liftoff mass" have widely accepted precise meanings.

Does anybody have an authoritative reference to explicit definitions of these terms, or some sort of evidence for their (un)ambiguity?

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    $\begingroup$ By "moment of ignition" I mean the beginning of the ignition sequence. $\endgroup$ Commented Mar 18, 2020 at 11:10
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    $\begingroup$ Consider also linking to Do Blue Origin's BE-3 engines need to run for 7 seconds to “warm up”? It's probably going to be hard to find an authoritative source to defend any particular answer to this. If you can link to one or two specific examples of how these terms are used, that might be better, but "what do they mean when they say..." followed by something ambiguous probably can't be answered definitively. If you just ask "Do the following terms have generally accepted, unambiguous and distinct definitions?" that might more answerable. $\endgroup$
    – uhoh
    Commented Mar 18, 2020 at 11:54
  • $\begingroup$ I certainly use "liftoff mass" to mean "ignition mass" in casual use. $\endgroup$ Commented Mar 18, 2020 at 20:02
  • $\begingroup$ Close vote retracted after edit. $\endgroup$ Commented Mar 19, 2020 at 21:22

1 Answer 1


This difference was tracked by the Space Shuttle Program.

Each flight had a Propellant Inventory prepared for it. Here is an example.

enter image description here

The major prelaunch times tracked are

  • LOADED (blue arrow)
  • ENGINE START COMMAND (green arrow)
  • SRB IGNITION COMMAND (red arrow)

The mass at SRB IGNITION COMMAND is the mass that was important for ascent performance. (Note: liftoff happened virtually instantly after SRB IGNITION). The LOADED mass is what was worked to by the launch pad system that loaded propellants into the tank.

Part of the question might be "Was the difference between these numbers significant"? The answer to that is also yes.

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. Notice that the magnitudes of these masses are less than the difference expended between LOADED and ENGINE START COMMAND.

Source Shuttle Systems Weight And Performance

  • $\begingroup$ Do I recall correctly that SRB ignition command = liftoff? If so, you could mention this fact in the answer because it is relevant to the question. $\endgroup$ Commented Mar 20, 2020 at 20:08
  • $\begingroup$ Thanks, will do. $\endgroup$ Commented Mar 20, 2020 at 20:17

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