What is a non-propulsive vent?

Question

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?

-1 for not looking up something that's pretty easily Googlable. — RonJohn, May 11 '18 at 16:12
@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. — Wayne Conrad, May 11 '18 at 16:34
@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. — uhoh, May 11 '18 at 17:14
@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. — Wayne Conrad, May 11 '18 at 17:22
Here's are some links explaining why googlable questions are on-topic: https://meta.stackexchange.com/questions/8724/how-should-we-deal-with-google-questions , https://www.joelonsoftware.com/2008/09/15/stack-overflow-launches/ — Wayne Conrad, May 11 '18 at 17:26
@WayneConrad "I would be if I were new. Picking on the exactly language ohoh used ("afraid") is needlessly pedantic." Not being pedantic in online conversation has burned me more than once due to assuming someone was joking/being sarcastic when they weren't. Thus, I now take people at their word. — RonJohn, May 11 '18 at 17:38
This question flagrantly demonstrates a lack of research effort, which is explicitly given as a reason for downvoting in the downvote button tooltip. — user2357112, May 11 '18 at 18:53
Even for cases where googlable questions are on topic, you are still supposed to google first. In your second paragraph, you promote asking questions as a way to avoid looking things up, which is absolutely not what Stack Exchange is for. If your intent here is to create a resource people will find when they try to look up "non-propulsive vent", then please do not encourage people to not look things up. — user2357112, May 11 '18 at 19:03
@user2357112 It's a really well-written comment, thank you! It's quite a valid argument and one I've used more than once myself; that we shouldn't skip following best practices because then it makes it harder to then tell new users to follow them. The field should remain flat. I see your other points as well. Poking around meta I find a variety of carefully considered and reasonable views. cf. answers to this just for example, which tend to recommend downvote not close research-lacking questions which lead to good, well received answers. — uhoh, May 12 '18 at 00:59
Related: Do spacecraft ever release unneeded gases into space? Related because its generally better to get rid of (i.e., vent) those unneeded fluids non-propulsively. — David Hammen, May 12 '18 at 15:02

Organic Marble · Accepted Answer · 2018-05-11T18:19:13.540

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.

(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.)

score 7 · Answer 2 · answered May 11 '18 at 15:07

7

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.

answered May 11 '18 at 15:07

Tristan

17,263
1
63
83

1

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! – uhoh May 11 '18 at 15:12
2

I would never send you after 100 feet of chow line. – Organic Marble May 11 '18 at 15:16
@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. – SF. May 15 '18 at 15:38
@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. – uhoh May 15 '18 at 19:48
@uhoh: Doesn't seem like a problem as long as the block doesn't melt away. – SF. May 15 '18 at 21:33
@SF. You'll sputter your beam stop as well and some of those sputtered neutrals can travel straight back into the running ion engine and deposit on surfaces (there's no "wind" to slow them down). The name for the thing that stops a charged particle beam without letting a bunch of stuff back out again (charged, but usually uncharged as well) is Faraday cup. It's usually used to measure total charge, but sometimes just grounded and used as a beam stop. – uhoh May 16 '18 at 02:00

What is a non-propulsive vent?

2 Answers2

Linked