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The tanks of the Space-X rocket should be emptied after a launch abort. I assume the liquid oxygen is pumped back to the storage tanks at the launch pad.

But how much oxygen is lost by evaporation?

Is the loss replaced by tanker trucks or by a liquid oxygen plant near the launch site?

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Calculating how much LOX is lost is going to be an interesting calculation, but I'm not sure we have the data for that. Instead, I'll be focussing on the other part of your question:

Is the loss replaced by tanker trucks or by a liquid oxygen plant near the launch site?

The ball-shaped structures near pad 39A are storage tanks for liquid propellants. It's pumped from there to the pad by pumps capable of pumping 1300 gallons per minute. I imagine the loss per minute would be substantially less than that.

Overview: Liquid oxygen used as an oxidizer by the orbiter main engines is stored in a 900,000-gallon tank on the pad's northwest corner, while the liquid hydrogen used as a fuel is kept in an 850,000-gallon tank on the northeast corner. The propellants are transferred from the storage tanks in vacuum-jacketed lines that feed into the orbiter and external tank via the tail service masts on the mobile launcher platform.

LOX and LH2 storage (NASA)

No tanker trucks are required to refill the loss by evaporation, directly. Naturally, something has to refill the storage spheres eventually and this is done by truck (thanks Organic Marble).

For more on LOX consumption, see this answer. 1kg of LOX is roughly 0,23 Gallon.

A picture of KSC LC-39A with Crew Dragon Demo-2 and highlighted ball-shaped storage tank (one of the cryogenic liquid propellants tanks):

KSC LC-39A with Crew Dragon Demo-2 and highlighted ball-shaped storage tank

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  • $\begingroup$ The storage spheres are replenished by tanker trucks. blogs.nasa.gov/kennedy/2017/10/03/… $\endgroup$
    – Organic Marble
    Commented May 27, 2020 at 22:03
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    $\begingroup$ @OrganicMarble Better? $\endgroup$
    – Mast
    Commented May 27, 2020 at 22:07
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    $\begingroup$ The storage tank for LOX holds 900,000 gallons or 3,913 t. The Falcon 9 Full Thrust needs 287,400 kg (633,600 lb) for the first stage and 75,200 kg (165,800 lb) for the second, together 0.362 t. So less than 10 % of the storage capacity are needed for the rocket. Boil off losses up to 50 % would be no problem. $\endgroup$
    – Uwe
    Commented May 28, 2020 at 10:43
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    $\begingroup$ @Alexander-ReinstateMonica See this answer with an image of launch site with storage tanks at some distance. $\endgroup$
    – Uwe
    Commented May 28, 2020 at 18:44
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    $\begingroup$ @Alexander-ReinstateMonica If you manage to hit one of those, you're in big trouble already anyway. $\endgroup$
    – Mast
    Commented May 28, 2020 at 19:01
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The evaporation of LOX under supercritical and subcritical conditions was studied experimentally Research Paper: Evaporation of LOX under supercritical and subcritical conditions Vaporization rates will vary and may be as low as 0.4% or as high as 3% in the span of 2-3 hours.

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    $\begingroup$ Please expand your answer, at least adding a link to the paper and possibly some discussion on how it applies to this case specifically. $\endgroup$
    – TooTea
    Commented May 28, 2020 at 12:36
  • $\begingroup$ -1 for citing information from a source that is unmentioned. What research paper? $\endgroup$
    – uhoh
    Commented Sep 17, 2020 at 6:23

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