Dual-plane stage separation?

[Kurld]

I've been wondering how one might be able to simulate an Apollo-style dual-plane separation, where the interstage between S1 and S2 remains coupled to the upper stage, then is jettisoned after the second stage has assumed stable flight.

I created a decoupler to serve as the interstage, and set the explosiveNodeID = bottom in ModuleDecouple. This leaves the decoupler attached to the upper stage, but that leads me to ask, is there any way then to detach it from the upper stage?

The decoupler I created also has ullage motors that fire at separation, which I suppose complicates things.

Cheers!

[Supernovy]

I can think of two ways:

Use two decouple modules, with different explosive node IDs (could be difficult as spacebar will activate both).

Or use a regular decoupler and set the engine above's fairing jettisonmodule to "isfairing = false" and set its jettison direction to downward. if you separate the engine and decoupler in staging, the lower stage should decouple on the first hit of spacebar and the interstage should fall off on the second. I'm not 100% sure of how the fairing colliders interact, though.

[Kurld]

I tried the dual-decoupler idea and it caused very strange behavior with the kerbal joint reinforcement mod installed.

I'm moving down the engine fairing path now. I think simming the ullage motors is going to be un-doable with this approach though.

[Kurld]

OK, so this does work, sort of. When the fairing jettisons, it kind of pops off the engine instantly, travelling retro the extent of the engine itself, then sort of stops and falls away naturally after that. It does not appear to be colliding with anything. I've reduced the jettisonForce by a factor of 10 and it didn't make any discernible difference. I will experiment with a stock engine and see if it does the same thing.

For the ullage motors, I think I'll come up with a small part that remains attached to the main rocket body, and translate the trust transform or the fx to the correct visual location.

[Kurld]

When I tried setting the isFairing attribute of ModuleJettison to true for the skipper engine, it cause catastrophic failure and the same error "Ignore collision failed". I am assuming this is because the skipper model has a collision mesh. I was getting the same catastrophic failure until I removed the collision mesh from my model.

[Guest]

Well, there's actually one, quite simple way: use two decouplers. First one should be a normal, slim decoupler and the other should consist of an interstage shroud, KW-style. The first one would decouple the fuel tank (if you're modelling the S-I you could even integrate it into the tank, as long as the engines are split off, which you should do anyway) and the second one decouples itself. Ullage motors can be radially attached to the upper decoupler.

[MOARdV]

If I understand what you're describing correctly, I think KOSMOS URM interstage behaves the way you want. You attach an interstage part (a cylinder that surface attaches) to the fuel tank above the engine, and then you attach a regular decoupler below the engine. You can trigger the in-line decoupler to separate the lower stage, and you can separately trigger the interstage decoupler, which will then fall clear of your rocket if you are accelerating.

[Guest]

Yeah, pretty much what I meant. KW shroud+slim decoupler will do that, too. Any combo like that would do, really.

[Jaybtlr]

not entirely if this is what you mean, but a stock solution i use is simply use two standard decouplers ontop each each other, with the bottom one upside down.... that way, the bottom decoupler detatches from the stack, with ullage motors also, then the top one detatches with the interstage fairing thingy off the engine at the same time, or just before next stage ignition.