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Occasionally one reads about a spacecraft "dumping fuel". It may mean expelling the propellant(s) as a liquid, or depleting them by burning them up. Although usually only fuel is mentioned, often the oxidizer is also eliminated at the same time. In any case, the propellants are eliminated from the spacecraft.

How does one do this without affecting the attitude or trajectory of the spacecraft?

Related, but about health/safety of a fuel dump: Any risk from Soyuz 22-ton fuel dump?

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    $\begingroup$ related but not the same question (it doesn't address attitude): What is a non-propulsive vent? and, slightly related: What is “propulsive passivation” and why will the SpaceX STP-2 mission do it? $\endgroup$ – uhoh May 28 '19 at 0:09
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    $\begingroup$ If i had to do it, i would eject equal amounts of mass in opposite directions. $\endgroup$ – Paul May 28 '19 at 4:31
  • $\begingroup$ @Paul: That is basically what RussellBorogove said in his answer below. $\endgroup$ – DrSheldon May 28 '19 at 5:11
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    $\begingroup$ Interestingly, a similar solution is applied to the vacuum cleaner used on ISS. $\endgroup$ – SF. May 28 '19 at 6:46
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In many cases, propellant is only dumped when the spacecraft’s mission is complete, so any minor changes to trajectory caused by the dump are unimportant.

If you must avoid any trajectory or attitude change due to a propellant dump, the most straightforward way is to have multiple vents pointing in opposite directions, so the propulsive forces cancel out.

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    $\begingroup$ Another approach I've seen done for launch vehicle upper stages (say, for blowdown of the main propulsion system through the engines themselves) is to set the vehicle up in an end-over-end tumble, so that the resulting thrust from the venting cancels out over time. $\endgroup$ – Tristan May 28 '19 at 13:30
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It will depend on many factors, but so long as you are NOT using a mono-propellant, then you can simply allow your fuel to run through the system normally (but without lighting it/mixing with oxidiser). This will still result in a slight thrust, but several orders of magnitude less than a 'proper' burn.

If this is done through thruster nozzles, it should be trivial to 'balance' them (even if you are forced to 'light' the fuel, as you would with a hypergolic fuel) so that the net effect is zero.

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    $\begingroup$ This sounds very plausible theoretically, but do you know any actual engines that allow for this? $\endgroup$ – Hohmannfan May 28 '19 at 21:07
  • $\begingroup$ Good point @Hohmannfan. Since the pumps are typically powered by combustion I'd imagine you couldn't pump the contents out actively. You'd have to just open the valves and let it outgas. $\endgroup$ – Ingolifs May 28 '19 at 21:51
  • $\begingroup$ @Hohmannfan, the J-2 engine used by the Saturn V third stage. Venting was sufficiently propulsive to turn a lunar free-return trajectory into either a lunar impact or a gravity assist into heliocentric orbit. $\endgroup$ – Mark May 28 '19 at 22:57
  • $\begingroup$ The shuttle dumped residual LOX through its engines. The LH2 took a different path. There was a significant propulsive effect. $\endgroup$ – Organic Marble May 30 '19 at 1:55

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