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[Jarhead0331]

So I've been dabbling in this again since version 1.0 was released and I've started to try to design multi-stage rockets. I've designed one, the KW-I, that on paper, at least, looks like it would get me into a stable orbit. However, for some reason, around 100,000 KM, the rocket starts doing cartwheels and the spin has proven to be unrecoverable. Even Mechjeb cannot keep it stable.

I'm assuming the sudden loss of control has something to do with a change in the atmosphere. Any of you experts have any idea what I can do to keep this roman candle stable? Does it need some fins or control surfaces? Is it an unbalanced weight problem, ie. the third and final stage in the nose is too heavy?

Attached are blue prints of the KW-I so you can see all the parts. As you can see, its a rather simple design with not much to go wrong. The loss of control occurs after the boosters have separated and after the rocket has been flying straight up under main booster control.

[Nefaro]

I'd put some stabilization fins on the bottom of the main rocket, JH. 

I seem to recall similar builds, which had a larger section on the top of the stack, spinning after the boosters were jettisoned and three or four of those helped stabilize it.   If it's still in the atmosphere when that happens, anyway.  I believe the single vector thrust of the nozzle couldn't keep up with the yaw/spin in a long stack with a tip that was of a larger circumference, or it would just make it worse at times.

Took a little doing to get them evenly spaced on the main engine using the appropriate setting, in combination with the boosters around it, but that's probably worth checking out. 

If it isn't enough with plain 'ole stabilizers, then add some control planes that can help you steer better.  Those weigh more, I'm sure, so the non-moving stabs would be the better first test.

[Jarhead0331]

I think the problem is definitely a weight imbalance in the nose. Once I reach altitude and the atmosphere starts to thin, the heavy nose drops and there is nothing to counteract that force. I tried adding some canards, but it wasn't enough and I had the same problem. I think I either need to add weight to the bottom of the rocket to balance it out, or add some kind of thrust vectoring capability. I rather not add weight, as this will obviously require more thrust to get into orbit. I like the design so I'm going to keep experimenting,

[Capn Darwin]

have you tried to spin stabilize it?

[jomni]

Needs two balls for stabilization.

[Jarhead0331]

have you tried to spin stabilize it?

Yes. Spin has no effect on this issue.

[Capn Darwin]

Bummer. Small set of thrusters near the top? Pointed outward? Is it possible the load is shifting in flight and that is adding to the CG issue? Just thinking out loud.

[Jarhead0331]

Is it possible the load is shifting in flight and that is adding to the CG issue? Just thinking out loud.

Hmmmm...I suppose the command module could be wobbling inside of the fairing, but it looks totally solid. Again, I really think it is a weight imbalance issue. I just need to figure out the best way to counteract this.

[Kushan]

At 100,000km the atmosphere should have no effect on your aerodynamically since your in space once you reach 70,000km and above. Not sure why it would start spinning at that point. You can try hitting F12 in flight and it will display an aerodynamic overlay on your craft. It will show you what parts on your craft are causing drag, thrust, etc. Might help you determine whats causing it to spin.

I've been experiencing end over end tumbling much lower in the atmosphere, ~10km, but nothing that high.

[Yskonyn]

Its definately a weight and balance issue. The game tracks fuel levels and shift in center of mass because of these weight changes.

[bbmike]

Hopefully all of your Kerbals are surviving all of these tests.  ;D

[Jarhead0331]

At 100,000km the atmosphere should have no effect on your aerodynamically since your in space once you reach 70,000km and above. Not sure why it would start spinning at that point. You can try hitting F12 in flight and it will display an aerodynamic overlay on your craft. It will show you what parts on your craft are causing drag, thrust, etc. Might help you determine whats causing it to spin.

I've been experiencing end over end tumbling much lower in the atmosphere, ~10km, but nothing that high.

I meant to correct that. It's meters, not kilometers.

[Jarhead0331]

Hopefully all of your Kerbals are surviving all of these tests.  ;D

Actually, they are. Once the tumble begins, I'm able to disengage the engine and separate the command module from the tumbling rocket. The command module is then able to recover stability and make it back to land, or sea, in one piece. I do feel bad for jebediah kermin though. He has tested this failure of a design at least 8 times.

[steve58]

Try adding an Advanced Reaction Wheel module/Advanced Inline Stabalizer (or 2) between the payload/capsule and the top fuel tank. That and MechJeb should keep it under control...

[Jarhead0331]

Try adding an Advanced Reaction Wheel module/Advanced Inline Stabalizer (or 2) between the payload/capsule and the top fuel tank. That and MechJeb should keep it under control...

Interesting point. I have one of those on the command module, but not the main rocket. I'll give it a shot.