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In a discussion associated with this answer a comment includes the following:

The ISS is the only spacecraft in the world that has experience with regular on-orbit refueling.

What is the extent of on-orbit refueling experience at the ISS? Which fuels? which spacecraft?

uhoh
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2 Answers2

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The propulsion system of the Service Module (public relations name: Zvezda), part of the Russian side of the ISS, is refueled by Russian Progress spacecraft, and formerly by European Automated/Ariane Transfer Vehicle (ATV) spacecraft.

Both Progress and ATV spacecraft deliver approximately 850 kg of propellant to the ISS.

The Service Module uses storable dinitrogen tetroxide and unsymmetrical dimethylhydrazine propellants.

The propellant is transferred through ports on the docking interface.

This schematic drawing shows the tankage in the midsection of the Progress.

enter image description here

Organic Marble
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  • If progress is retired or stops going there is there another option for refueling? – ikrase Dec 14 '20 at 01:06
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    @ikrase Russia owns their side of the ISS and the Progresses. It's up to them to manage it; it seems unlikely they would cripple themselves. – Organic Marble Dec 14 '20 at 05:04
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    It doesn't seem impossible that Russia might back out of the ISS someday before the USA and European-aligned parts. – ikrase Dec 14 '20 at 05:21
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    @OrganicMarble You mean, like the US did for almost a decade? – Asteroids With Wings Dec 14 '20 at 12:26
  • @ikrase if Russian "backed out of the ISS" it would have to be deorbited, barring some highly implausible intervention. The US cannot control the Russian segment so the ISS would be left without any propulsion other than the meager amount visiting capsules can provide. – Organic Marble Dec 14 '20 at 14:02
  • I wouldn't be surprised if SpaceX came up with a Cargo Dragon that instead of being returned to Earth would instead have an engine and refillable tanks in the trunk, so that it could be permanently docked and used for station-keeping. – manassehkatz-Moving 2 Codidact Dec 14 '20 at 15:54
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    @manassehkatz-Moving2Codidact Yeah, in "highly implausible intervention" I was imagining Elon ripping his shirt off to reveal the Superman logo and doing something like that. However, I would be surprised if that happened. – Organic Marble Dec 14 '20 at 15:55
  • @OrganicMarble Actually, even simpler would be if they could juice up a 2nd-stage enough that instead of just the Cargo Dragon getting to ISS, the 2nd stage would get there too. That takes care of engine development - just need to figure out refueling. – manassehkatz-Moving 2 Codidact Dec 14 '20 at 16:00
  • @manassehkatz-Moving2Codidact doesn't the 2nd stage use cryogenic oxidizer? – Organic Marble Dec 14 '20 at 16:01
  • @OrganicMarble Just checked and yes, it does. Which I suppose complicates things a bit...but I'm not a rocket scientist. – manassehkatz-Moving 2 Codidact Dec 14 '20 at 16:02
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    @manassehkatz-Moving2Codidact: The MVAC, even at minimal thrust (and I don't even know off the top of my head whether it is throttlable at all), would rip the station apart. When the Shuttle reboosted the station, they didn't even use the OMS, they only used the RCS thrusters. For Cargo Dragon, they'd use the Dracos, I guess, even the SuperDracos would be too powerful. – Jörg W Mittag Dec 14 '20 at 17:06
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    Fascinating. I had no idea the difference in power. The 2 rarely-used main engines on Zvezda are 3,070 N each. Wikipedia doesn't say how big the small thrusters are, but with Draco @ 400 N each and SuperDraco @ 71,000 N, and Merlin more than 10x, yeah Draco is all you need. Which means that a Cargo Dragon with refillable tanks could probably do the job quite well using Dracos. Not a Superman job at all. – manassehkatz-Moving 2 Codidact Dec 14 '20 at 17:42
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In addition to the standard operational refueling missions that are part of normal station-keeping operations, there have also been experiments performed that studied in-space refueling.

In particular, three phases of the NASA Robotic Refueling Mission have been completed, which simulated on-orbit refueling of a satellite not designed for refueling. A module was installed on the outside of the ISS that mimicked the fueling connections on a typical satellite. Note that "fueling connections on a typical satellite" here means the kinds of connections, valves, seals, etc. that you would find on a satellite not designed for refueling, to be only used once, on the ground, for initial fueling of the satellite.

Using these connections that were never intended to be re-used and never intended to be used in-flight, the RRM successfully demonstrated on-orbit refueling of hypergolics, oxidizer, and cryogenics.

Jörg W Mittag
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