Rocket roll

[JaySmoka]

I don't know if it's me (my designs) or my ASAS, but my rocket rolls a lot. I notice that my ASAS over correct the roll, but i don't know why my rocket roll. Is this because the fuel tank emptying and that lowers my CoM of I have too powerful engine to push my rocket up?

What makes a rocket roll?

[sporkafife]

Is the roll usually encountered on asparagus staged rockets? I have noticed with my own rockets that, because it is impossible to perfectly place struts, the engine and strut placement brings in a little bit of torque. This effect then increases as the outer stages lose fuel, like a spinning ballerina pulling their arms in makes them spin faster.

The only real solution is to try and place struts as evenly as possible I guess...

[JaySmoka]

I noticed on my asparagus design, but i made a test: an unmanned pod, ASAS, orange tank and the skipper engine. I attached radially 4 more orange tank with skipper engine under them (no struts, no decoupler). Rocket still rolls...

[Tw1]

Could be parts clipping. They don't like to be inside each other.

Hold on, do you mean spinning or tipping over?

Edited June 17, 2013 by Tw1

[JaySmoka]

Could be parts clipping. They don't like to be inside each other.

I double-check that an its seems not to be the problem...

I think it's the ASAS that over correct roll... If I turn it off, the rocket roll a little bit, and roll faster as the tank are emptying... If I turn it on, the rocket roll less, but the roll indication (at bottom left) stay completely at left position (pitch and yaw act more normal)

[Tw1]

What about fins? They can cause problems if placed badly, but maybe some well placed ones could do a bit of good.

[The Deep Space Kraken]

To fix this turn off gimbaling on the outer four engines.

[JaySmoka]

To fix this turn off gimbaling on the outer four engines.

OK I'll try.

More generally what could explain the roll (physics) ?

[zanzibar196]

A picture of the craft and/or video showing the roll would be best to help you!

[Guest]

To fix this turn off gimbaling on the outer four engines.

Just to reinforce this point, having non-gimballed engines on booster stages helps immensely. I generally only gimbal the central engine, whether it be a Mainsail or an LVT-45 in the middle of six -30's or whatever. I also find that a small number of fins (4) can correct for pretty much anything else during the ascent, but they aren't often needed.

[falofonos]

A slight offset of mass or drag can cause roll and / or the whole rocket to pancake.

KSP is happy to calculate drag or thrust imbalances during flight but not quite so helpful in the VAB for placing parts in a way which would avoid them.

[Dedjal]

I only tend to get an overreacting asas when using mod parts that hasn't the right weight etc. In the right places. Mechjeb loves those aswell, keeps slamming the rocket to the ground until the darn piece is found and thrown out of the garage.

[Guest]

KSP is happy to calculate drag or thrust imbalances during flight but not quite so helpful in the VAB for placing parts in a way which would avoid them.

RCS Build Aid helps with the weight imbalances.

[strongest_2hu]

I think read somewhere that roll on rockets with radially attached boosters is caused by flexing of the attachment points allowing the boosters to twist just slightly off-axis under stress (i.e. boosting) which causes a significant roll force when they all twist in the same direction.

It may have something to do with the bug about parts created by the symmetry feature having weaker attachments than the original part.

[SDIR]

What I do if the rocket starts rolling is placing four fins near the center of mass so that the fins correct for roll but do not go into an over-correcting effect for pitch/yaw. Usually when you have reached about 15,000m or so the fins lose effect but the roll has stopped anyways. Hope that helped!

[JaySmoka]

Thanks for answering. With all your comments i think i figured what's happening. Sorry I have no pic of my design to share, ask me if interested.

When the "exterior" tanks are emptying and gimbal is active on all engines, it induce the roll that the SAS or mechjeb try to compensate. Since my rocket is pretty heavy (around 800 tons), it need a lot of force to counter the effect.

To reduce the roll, I'll try to split my stages to have less dead weight while going up. Pretty confident that will work.

[KerbMav]

RCS Build Aid helps with the weight imbalances.

What also comes in handy is if you want to built boosters around the main engine(s tank e.g.) it is easier to balcance things out by first building one booster from decoupler outwards, then to ALT-LeftClick the decoupler to make a copy of it, set it beside the rocket (semi-transparent in midair), remove and delete the original, pick up your copy again and then place it with the symmetry function back onto the rocket.

Trying to set four more big tanks on the already placed decouplers, then adding boosters, fins etc. --- can lead to asymmetrical designs, clipping ... problems.