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I saw the question:

Has in-space refueling been done?

And was actually beginning to wonder, is refueling and transferring propellant dangerous in space? I understand that this would most likely be largely dependent on the propellant type, which is very lacking in that post. Most of the answers dictate that "fuel" is transferred, not what type of fuel or any specifics. I'd like to know if any of the following fuels would complicate transfers between vessels beyond what the current Orbital Refueling System could handle:

Fuels:

  1. LOX Propellant
  2. LOX Propellant + Liquid Hydrogen
  3. Dinitrogen Tetroxide + Hydrazine
  4. RP-1 (Refined Petroleum-1)
  5. Ion Propellants (Hydrogen, argon, ammonia, nitrogen, etc...)
  6. Anything else listed here, as you see fit to touch on.

Monopropellants (Doubtfully dangerous):

  1. Hydrazine
  2. Hydrogen Peroxide
  3. Nitrous Oxide

Yes, I realize it's a lot of Wikipedia links, but that's how I'm slowly learning about different chemical compositions of fuel and specific impulses! Out of all my skill sets my current worst would be biology and chemistry. Feel free to correct any misnomers or other flaws in my post, thanks for taking the time to consider it too!

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  • $\begingroup$ Pumping liquids from one tank to another is difficult in zero gravity. $\endgroup$ – Uwe Jan 9 at 20:14
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    $\begingroup$ There are no additional problems for transfer of pressurized gases in zero gravity. But if you connect two pressure tanks of equal volume, one empty and one full, only 50 % of the gas may be transfered without a pump. The pressure and the mass of gas is equal in both tanks. If you want to transfer 75 % of the gas, the empty tank volume should be three times of the full tank. Pressure after equalization is only 25 % of initial pressure. $\endgroup$ – Uwe Jan 9 at 20:55
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    $\begingroup$ Sadly, the Orbital Refueling System (at least the one linked in the question) is far from current, and in fact the program was never completed IIRC, due to the risk-averse attitudes that prevailed after the STS-51L failure. There is some kind of robotic refueling testing going on at the ISS recently, but I don't know much about it. $\endgroup$ – Organic Marble Jan 9 at 21:09
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    $\begingroup$ If fuel is to be used several days later, cryogenic propeelants should be avoided for refuelling anyway. Loss of propellants by evaporisation should be small. $\endgroup$ – Uwe Jan 9 at 21:38
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    $\begingroup$ You might enjoy "Ignition!" amazon.co.uk/dp/B076838QS2/… (also widely available online) which gives a history of rocket propellants and some of the trials and tribulation of testing and using them on the ground. Many of the hazards (volatile, corrosive, explosive if bumped, or in the presence of water or grease,.... ) are obviously the same in space. Toxicity is only an issue if there are, or will be, humans on one of the vehicles involved. $\endgroup$ – Steve Linton Jan 10 at 20:19

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