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The Apollo service module carried a lot of propellant for performing the following tasks on a Moon mission:

  • Mid-course correction manoeuvres to and from the Moon.
  • Braking the entire CSM/LM stack into orbit around the Moon.
  • Escaping low Moon orbit to transfer the crew back to Earth.

Especially the two later are relatively expensive manoeuvres, and the same propellant in a CSM configuration without the LM sums up to multiple $km/s$ of $\Delta v$.

What was all that propellant used for when the Apollo spacecraft did not travel to the Moon? Examples are the early hardware tests in LEO before the Moon missions, the three missions to Skylab and finally the Apollo-Soyuz test project.

Was the SM close to empty? Was it used as a last stage to reach orbit? Did the Apollo spacecraft perform some expensive plane change manoeuvre? Was a lot of fuel launched just to be ditched at re-entry?

None of those possibilities makes very much sense.

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up vote 9 down vote accepted

According to Apollo By The Numbers, the loaded weight of the CSM was substantially lower for the LEO Apollo 7 and Apollo 9 missions.

Apollo 7 (SM loaded: 19,730 lbs / 8950kg) was a no-LM mission launched on a Saturn 1B; Apollo 9 (SM loaded: 36,159 lbs / 16400kg) was CSM+LM launched on a Saturn V.

The translunar missions by comparison had loaded SM weighing upwards of 51,000 lbs / 23000kg. The full SPS propellant load is 40,590 lb / 18,410 kg.

Both 7 and 9 were 10-day missions, so would have had a full load of consumables other than fuel; the mass difference must be almost all SPS propellant.

Apollo 9 did 7 test burns of the SPS, some quite lengthy, in addition to the deorbit burn, so that accounts for a great deal of fuel usage.

Likewise Apollo 7 did several SPS burns.

So in each case they loaded a reasonable amount of fuel for the mission they intended to fly.

The Saturn 1B didn't have the capability to lift a fully loaded SM, so the Skylab and ASTP missions actually removed some tankage and other unneeded components to save weight.

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