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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?
This difference was tracked by the Space Shuttle Program.
Each flight had a Propellant Inventory prepared for it. Here is an example.
The major prelaunch times tracked are
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.
