I just hear the term "non-propulsive vent". Is it a thing, a vent that does not cause propulsion?

Or is it an event, where a vent of some unwanted gas has occurred which did not result in a propulsive maneuver?

Does it refer to resulting in zero torque (zero rotation change) or zero thrust (zero acceleration) or either one, or both?

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    $\begingroup$ -1 for not looking up something that's pretty easily Googlable. $\endgroup$
    – RonJohn
    Commented May 11, 2018 at 16:12
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    $\begingroup$ @RonJohn SE is supposed to be a repository of questions and answers. That something can be googled is absolutely not a reason to discourage a question here or on any other SE site. $\endgroup$ Commented May 11, 2018 at 16:34
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    $\begingroup$ @RonJohn I'm afraid of the intimidating message you're telegraphing to new users, perhaps making them hesitant or afraid of asking a question because they might receive a similar comment. To new users: if you ever receive a comment like that on your question, click the flag button because it's not nice, helpful, or correct. If you get a nicely-written message suggesting you should do further research first, that's different. Most know I usually research, so this unusually short one is just a chance to get this term defined so searches will find it and it can be linked to in the future. $\endgroup$
    – uhoh
    Commented May 11, 2018 at 17:14
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    $\begingroup$ @RonJohn It's kind of rude. ohoh is a veteran and probably not intimidated by it, but many newcomers would be. I would be if I were new. Picking on the exactly language ohoh used ("afraid") is needlessly pedantic. $\endgroup$ Commented May 11, 2018 at 17:22
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    $\begingroup$ Here's are some links explaining why googlable questions are on-topic: meta.stackexchange.com/questions/8724/… , joelonsoftware.com/2008/09/15/stack-overflow-launches $\endgroup$ Commented May 11, 2018 at 17:26

2 Answers 2


It's simply a device that emits fluid in such a manner that there's no net force. Simple example, a T-shaped pipe where gas travels up the stem of the T and exits out both sides of the top bar.

A propulsion example: Shuttle would dump excess propellant by firing opposing pairs of RCS jets.

Shuttle's flash evaporator had vents on opposite sides of the aft boat-tail for the same reason.

Here's a Shuttle airlock schematic showing the non-propulsive vent for the External Airlock depress valve.

enter image description here

(this schematic is from the Shuttle ECLSS training manual. This book is online here but when they redrew this image to remove the internal airlock they screwed it up (it's figure 6-19 in the online version) This is a scan of my paper copy which shows the vent properly.)

  • $\begingroup$ I just wrote this ;-) $\endgroup$
    – uhoh
    Commented May 11, 2018 at 15:13

A non propulsive vent is an object designed to redirect venting gases into two opposing directions, such that the propulsive effects cancel each other out.

Here is a link to a photo of one use on ISS.

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    $\begingroup$ Ha! I thought OrganicMarble was pulling my leg, like a "left-handed smoke shifter." This would be quite an unusual behavior for an ion engine, though a pair would do it. Thanks for the speedy answer! $\endgroup$
    – uhoh
    Commented May 11, 2018 at 15:12
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    $\begingroup$ I would never send you after 100 feet of chow line. $\endgroup$ Commented May 11, 2018 at 15:16
  • $\begingroup$ @uhoh: Add a wedge-shaped obstruction at the end of the nozzle, splitting the ion stream in two and deflecting them in two opposite directions and you're done. No need for two ion engines. $\endgroup$
    – SF.
    Commented May 15, 2018 at 15:38
  • $\begingroup$ @SF. Ion beams with this kind of energy do not deflect like a gas. While a few may scatter, most will implant themselves inside whatever they hit. $\endgroup$
    – uhoh
    Commented May 15, 2018 at 19:48
  • $\begingroup$ @uhoh: Doesn't seem like a problem as long as the block doesn't melt away. $\endgroup$
    – SF.
    Commented May 15, 2018 at 21:33

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