Confused about plane behaviour

[Two]

So I had this brilliant idea last night to attach wings to my rocket, so it could gently drift on the available air towards a stable orbit to save some fuel, however I soon had to learn that the game behaves very strange when attaching rocket engines to planes, and I can't seem to figure out whether this is a game bug or actually working as intended.

So what I did in the end to test this was to build a plane in the rocket lab, attached standard jet engines and tested it. As expected it was flying close to perfect, had no issues beside the mysterious wobbling effect if you steer (not sure if that is intended either). The next thing I did was to replace just the jet engines with rocket engines, and suddenly the otherwise perfectly balanced plane rotates like crazy instead of actually flying.

I tried basically everything to get it stable: more wings, stabilizers, different engines, different thrust settings, but whatever I do: with jet engines the plane flies perfectly, with any kind of rocket engine it rotates and crashes like crazy, and I can't figure out why.

Can anyone please enlighten me on this?

Edit: Answer from Red Iron Crown

1. Rocket engines use a lot more fuel, so the CoM will change rapidly during flight.

2. Jet engines drain fuel evenly, while rocket engines drain tank by tank. So rocket engines will empty forward tanks first, again changing CoM rapidly during flight.

Edited May 23, 2015 by Two

[Foxster]

I'm not sure on this one, I'm sure someone will enlighten us soon.

However, I will say that I have noticed the same craft behave quite different depending on whether it is launched from the SPH or the VAB.

[Xannari Ferrows]

What engines are you putting on it specifically? The imbalances could be caused by a short change in velocity over time, as opposed to the gradual spool-up of the jet engine, or perhaps engine gimballing. Also, where's your CoT relative to your CoM?

[Rhomphaia]

Jet engines have offset mass, compare the basic jet with the Swivel engine, both weigh the same but will have very different effects on COM

.

[steve_v]

Odd. Engine gimbals got a buff in 1.0, Does locking them make any difference?

How's your COM/COT alignment?

Bah, dang Ninjas

[KerikBalm]

Wow... when did they give jet engines offset mass?

That's a big change that I didn't know about.

I was thinking this would be a result of the different way fuel drains from stages... rockets become bottom heavy as they burn off fuel.

The Jet engines drain tanks equally.

It could be a combination?

[Chaos_Klaus]

Check this link for some aircraft design tips:

http://forum.kerbalspaceprogram.com/threads/52080-Basic-Aircraft-Design-Explained-Simply-With-Pictures?p=685737&viewfull=1#post685737

[Red Iron Crown]

I think the OP's troubles stem from fuel flow rules. With an airbreathing engine liquid fuel is drawn from all tanks in the stage evenly, minimizing the change in location of CoM and maintaining CoM/CoL relationship. With a rocket engine, the tanks are drained front-to-back in a single stack, causing the CoM to move rearward as forward tanks drain. Once the CoM moves behind the CoL, the plane will flip out.

You can check if this is what is happening by turning on the CoM and CoL indicators in the SPH, and tweaking the fuel out of the tanks one at a time, front to back, to simulate the rocket burning off the fuel.

[Two]

I think the OP's troubles stem from fuel flow rules.

That actually sounds very reasonable, I never thought about that. Thanks!

There is actually even another issue with that: rocket engines drain a lot more fuel than jet engines, so the CoM changes drastically during flight.