Spinning rocket

[spacedoctor42]

I'm planning a manned (kerbaled?) mission to Duna. I have the main ship in orbit and plan to dock the lander at the top of this rocket before departing. However, this rocket spins uncontrollably during launch. Can someone tell me why?

Edited March 13, 2015 by spacedoctor42

[Deutherius]

It would be best to provide us with a craft download so we can test it, or at least provide more info (when exactly does it spin out of control? In what manner? Screenshots would help). The only thing I can tell you from looking at the screenshot is check your fuel lines. If one or more of the side tanks drain asymmetrically it will become a problem.

[HoloYolo]

It could be the wings...

[Slam_Jones]

If you put wings anywhere on your rocket, put em at the bottom.

Even then, they're unnecessary. With the proper application of SAS, a wingless rocket will fly perfectly.

[spacedoctor42]

It was spinning before the wings. I put the wings on to try and stabilize it.

[Kryxal]

I'd use fins on the lower part of the rocket if anywhere, and the XL variant (and if you're doing that, angle the launch clamps). By the way, WHEN does the rocket start spinning?

A further thing that might be a good idea is to disable gimbals on the initial engines except the center one.

[Reactordrone]

Are the boosters strutted properly? If they're only strutted on one side they might be tilting and causing a rotation when thrust is applied. The basic design should fly ok although it's a little expensive and overpowered to get that little lander into Kerbin orbit.

[LordFerret]

I'd strut the boosters closer to the top of the booster, and at the bottom. I'd also question the fuel lines... I've issues before with them introducing spin. Can you move them in more towards the center tank?

[ScarM]

Three way symmetry of the lander, over the four way symmetry of the booster creates a thrust imbalance. Draw an imaginary vertical plane from the VAB door though the center of you rocket. Notice the boosters are symmetrical on the left and right of this plan, however the lander has that two engines on the right while only one is on the left. Thus, imbalanced.

The rocket probably pitches south immediately at launch, the thrust vector boosters spin the rocket trying to compensate.

[spacedoctor42]

I managed to fix it by moving the fuel lines around. Now I don't have enough thrust to get into orbit but that I do know how to fix.

[Leszek]

Three way symmetry of the lander, over the four way symmetry of the booster creates a thrust imbalance. Draw an imaginary vertical plane from the VAB door though the center of you rocket. Notice the boosters are symmetrical on the left and right of this plan, however the lander has that two engines on the right while only one is on the left. Thus, imbalanced.

The rocket probably pitches south immediately at launch, the thrust vector boosters spin the rocket trying to compensate.

I very much doubt that is the issue. The two on the one side have less moment arm from the chosen datem. In fact if that was the case then ANY 3 way symmetry would be unbalanced.

If moving the fuel lines made a difference then what was happening is you were draining fuel from tanks in a non-symmetrical way. Probably one of the lines where wrong. I have heard (but never tested) that it is possible to have non-symmetrical fuel drain even from a symmetrical rocket under certain circumstances but I don't know what they were. If you eliminated a wrong fuel line then I would check out that possibility. I wonder if anyone here knows the details?

[Deutherius]

I very much doubt that is the issue. The two on the one side have less moment arm from the chosen datem. In fact if that was the case then ANY 3 way symmetry would be unbalanced.

Also... AFAIK KSP doesn't care about your symmetry, it just cares about the center of mass. You can make an assymetrical craft and it will fly just like a symmetrical one, provided that the CoM and drag coefficients on both sides are equivalent. (Yes, OP uses FAR which would make a problem. But his payload is in a fairing, so no possible problems there)

[leopardenthusiast]

OP, since you're using FAR, you might want to put some nose cones on the boosters. It'll make the rocket more aerodynamic.

[Blondai]

I jad this problem with my larger rockets in a similar style to that. I find tail fins on each stage helps. The earlier stages have bigger fins to keep lift on the arse. Also lots Well placed struts helps. You wanna make triangles where ever possible with them. Any looseNess at the bottom will cause enines to swing out. Causing as symmetric thrust. If this happens mid flight. Throttle back. I will upload pictures of my large lifter soon