Using Action Groups to Jettison Engines?

[starbork]

Hi all,

When creating an action group, if an engine is selected one of the options is "jettision". Presumably, if this option is assigned to an action group, then when in-flight the respective action group should detach the engine, right? I've tried this a few different ways, and it does not seem to work. Can anyone here shed any light on what the "jettison" option actually does, or is supposed to do?

Thanks!

[sal_vager]

The jettison option will drop the engine shrouds, not the whole engine

[EndlessWaves]

I believe decouplers have actions though, so you can jettison engines through action groups that way.

[starbork]

The jettison option will drop the engine shrouds, not the whole engine

Ah, I see, thanks! Just to be clear, the engine shrouds are the cylindrical coverings that appear when you connect (say) a decoupler to the bottom of the engine, correct? What is the use of jettisoning those, I wonder?

Anyhow, thank you for the answer!

[Specialist290]

The jettison option will drop the engine shrouds, not the whole engine

They really should clarify that in a future update. I've spent this whole time never using the option, because I was worried I'd be dumping my engines with a misplaced keystroke!

That said, I probably still won't use it, since decouplers jettison the shrouds automatically as well.

[tg626]

Flip the decoupler over and the previous stage disconnects but the shroud stays. I think then the jettison action drpos the shroud, like in that one famous Saturn v film where it goes gently tumbling away from a camera that must have been up next to the engine.

[Roastduck]

that one famous Saturn v film where it goes gently tumbling away

You're thinking of Apollo 13, which I think is an accurate portrayal of many Mun missions in KSP >.>

[Taki117]

Flip the decoupler over and the previous stage disconnects but the shroud stays. I think then the jettison action drpos the shroud, like in that one famous Saturn v film where it goes gently tumbling away from a camera that must have been up next to the engine.

That's not actually an engine shroud. (It does serve that purpose though) It is the separator ring containing the Ullage motors for settling the fuel immediately prior to second stage ignition.

[Sirrobert]

The jettison option will drop the engine shrouds, not the whole engine

THAT's what that does?

I always asumed it simply detatched the engine, never used it because of that.

Sounds like a pritty useless function than, since the shroud already comes off

[DrD]

Ha I know this one! (I'm an Apollo nut)

So if you think about staging, you can see that you can either break the stage off such that the "lip" or shroud stays with the first stage such that the second stage engines are "exposed." The problem with that is if the first stage comes off at any small angle the "lip" or shroud could impact one of the second stage engines. So it would be safer to break off the 1st stage so the shroud stays on the second stage. But the problem with that is for some reason that I don't know having the engine bells shrouded like that reduces efficiency. It also adds weight.

So they decided to have it both ways. First they stage off and left the shroud on the second stage so they didn't have to worry about damaging the engines. Then after about 30 seconds the shroud (actually called interstage ring) stages off. Since it's only a small ring the chance of an impact with engine bell is much much lower.

They did put the ullage motors on it also for additional weight savings.

Interestingly on one mission (I think one of the unmanned) the interstage ring never sepearated and they still made orbit just fine, so like so many other things in Apollo it turned out to be overkill.

A great awesome super book on all this is called "How Apollo Flew to the Moon." I give it my very highest recommendation.

In that famous video you can see the interstage ring look like it catches fire. This is because the second and third stage engines burned hydrogen and the flames were invisible. Although it doesn't look like it the second stage is thrusting in that video. The flaming appears when the interstage ring enters the flame.

Also do you know what sophisticated method they used to separate the stages? It was basically a line of plastic explosive. They stages were welded together then literally blown apart.

- - - Updated - - -

You're thinking of Apollo 13, which I think is an accurate portrayal of many Mun missions in KSP >.>

Actually the footage is from one of the early unmanned Saturn V launches. They only made that video of the interstage ring staging off once. They used the footage in the movie Apollo 13.

[Superfluous J]

I actually assumed it jettisoned all the fuel that the engine could use, like some planes do before landing.

Never tested it, though. I agree it seems a bit useless