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Follow up to this question: Why will Soyuz TMA-18M take two days to reach the ISS?.

Clearly the expedited rendezvous is cheaper in terms of time. However I was wondering how fuel usage compares.

Is the fuel usage:

  • about the same (after all the same orbital parameters are being targeted either way)?
  • more for the expedited rendezvous (perhaps to get there faster, more fuel is needed)?
  • less for the expedited rendezvous (perhaps shorter time to rendezvous means less fuel spent)?
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My initial thought was that it should be about the same. My understanding is that most of the difference between the profiles is getting the phasing correct prior to launch. But fuel isn't consumed to correct phasing in a standard approach, just loiter time at a lower altitude.

The only thing I found was this Murtazin/Petrov paper (linked to from another question). I don't have access to the paper itself, but from the notes in the abstract ("test in 2012") it may be describing the current "short profile" scheme. One of the highlights it mentions is "The fuel consumption for short profile corresponds to the 2-day rendezvous mission"

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Theoretically, the fuel usage should be the same. In practice, here are a few things that might change the fuel usage each way:

More Fuel for quick

  • More orbital uncertainty might result in an incorrect thrust.
  • Higher thrust is required from early on. There is little time to correct any errors. If a small error happens, it will have to be corrected. As the time of approach is only a few orbits, the corrections will require substantially more thrust than others.
  • Some burns might have to happen at non-optimal times. This should be a small effect for LEO, but isn't negligible.

More Fuel for 2 day

  • A certain amount of fuel is required for station keeping. This fuel usage will increase.

Overall, I believe it would be close, but slightly higher to do the quick burn, unless the errors are brought down to something negligible.

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