When a cargo mission is flown toward the ISS, there are at least four different kinds of liquids that may need to be moved into/out of the station:

  • Propellant for Zvezda's thrusters
  • Water
  • Wastewater
  • Oxygen

How is this managed? Are the docking ports specially equipped for each sort of liquid? How is the liquid moved in the station (i.e. is there plumbing for wastewater)? What drives the liquids? Are there pumps on the station, the vehicle or both?

Finally: How is this system maintained in microgravity? While you can pump liquids, how do you drain the pipes in case you need to work on them?

  • 1
    $\begingroup$ Draining should be easy enough; open plumbing to space and the contents should rapidly evaporate and/or sublimate. $\endgroup$ Commented Aug 5, 2016 at 15:49
  • $\begingroup$ On Mir, the back docking port had fuel ports, but Kvant blocked it so Progress had to use teh fuel for its engines to do reboost. Zvezda is basically Mir-2 so I assume it has the same. $\endgroup$
    – geoffc
    Commented Aug 5, 2016 at 18:02

1 Answer 1


With the exception of the propellant for Zvezda's thrusters, which is fairly simple and fairly short plumbing, you are asking about the Space Station's Environmental Control and Life Support Systems (ECLSS). The plumbing for propulsion is quite simple. This plumbing has existed since the dawn of spaceflight. The plumbing needed for life support is anything but simple. A simplified diagram follows:

enter image description here
Source: NASA (so no copyright), via wikipedia.

Providing fuel and oxidizer to the rockets on the Space Station is extremely simple compared to the problem of providing usable water and breathable atmosphere to people on the Space Station. The Space Station has leaks, requiring regular resupplies of breathing atmosphere. People consume water and oxygen and produce carbon dioxide. That CO2 must be continuously removed (watch Apollo 13) from the breathing atmosphere lest the crew be poisoned. The crew can also be poisoned by trace contaminants in the breathing atmosphere.

Water from human waste in the Space Station toilets (both #1 and #2) is carefully scrubbed using specialized equipment. This waste water is then electrolyzed to form hydrogen and oxygen. The hydrogen is vented overboard as waste while the oxygen goes back into the breathing atmosphere. The desiccated products of human #1 and #2 waste are either sent overboard or placed in modules that will burn on reentry into the Earth's atmosphere.

Carbon dioxide is removed from the breathing atmosphere via a complex and somewhat failure-prone set of machinery in both the Russian and US segments of the Space Station. Both use a two segment chamber in which one chamber selectively collects carbon dioxide from the breathing atmosphere while the other vents accumulated carbon dioxide into space. Eventually, the collecting chamber becomes saturated with carbon dioxide and the venting chamber has little left to vent. Valves are repositioned at this point to make the old collecting chamber become the new venting chamber and the old venting chamber become the new collecting chamber.

This process of course leaks. Some of the good stuff (nitrogen and oxygen) is vented along with the carbon dioxide. Moreover, the vented carbon dioxide is mostly oxygen. Experiments have been tried that used equipment applicable to Mars that converts that carbon dioxide into methane (which was vented), but those experiments were just that, experiments. (Aside: That these experiments have not become operational and have instead been cancelled says something about the prospects sending humans to Mars.)

The Space Station is a leaky vessel. In addition to those expected leaks from hydrogen and carbon dioxide venting, the station leaks fluids with every docking and undocking, and it also leaks at every joint. This latter problem is is becoming more severe as the station ages. Because of this, the Space Station is regularly resupplied with water, nitrogen, and oxygen.

  • $\begingroup$ I'm not sure the "methane production" (Sabatier reaction) is just an experiment; we already had a question on that: space.stackexchange.com/questions/67/… $\endgroup$
    – MSalters
    Commented Aug 8, 2016 at 9:29
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    $\begingroup$ @MSalters - AFAIK, the ISS Sabatier module is just that, an experiment rather than an operational piece equipment. As an experiment, it has proven itself to be quite successful. It definitely has produced methane from CO2 and hydrogen, thereby theoretically reducing the amount of resupply oxygen that would be needed. If it was instead viewed as an operational piece of equipment, it would have to be deemed as an abject failure due to numerous shutdowns, clogs, etc. It's close, but no cigar. $\endgroup$ Commented Aug 8, 2016 at 10:10

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