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My rockets keep rolling, preventing me from doing equatorial orbits with just the s key. How do I fix this?


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Maybe. Tbh it isn't that annoying, but I would like to be able to not have to constantly yaw and roll to stay equatorial. What's strange is my planes don't do it, and I usually use Jebediah for any of my flights, rocket or plane.

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A bit of roll during ascent doesn't have to be an issue, as long as the rocket keeps following the correct trajectory.  In a proper gravity turn, you really shouldn't need to steer at all - at most, just the very initial nudge a few moments after lift off, to initiate the turn. KSP doesn't model weather or wind, so there's no unpredictable forces during ascent that require us to be 'on the stick'. Gravity and aerodynamic forces should keep the rocket on track. A tiny bit of gimbal from the engines and SAS set to follow prograde take care of any corrections.

In fact, the less you try to steer manually, the better. It's very hard to be precise in either timing or force, and you end up causing more deviation than you correct. Which then needs more correction, etc etc.

 

All that said, it is annoying when the rocket starts rolling when it's not supposed to. A few common causes:

Asymmetry: this should be easy to spot and avoid, just try to keep your rocket symmetrical around its long axis.

Joints: generally, try to use the biggest diameter connections available between parts/sections of your rockets. The smaller a joint, the more flexible it is. Any form of steering input can cause the rocket to wobble around a 'bendy' joint, and this wobble can veer the rocket off course. If you can't avoid narrow sections, try adding some struts.

Fins: if you use fins at the bottom, and SAS (usually you'll want to), those can be the cause. The current SAS code sometimes does a very regular 'flutter' of control surfaces, small enough that it's only visible when zooming in, but enough to cause an ongoing deviation.... which we experience as a tendency to slowly roll/yaw/pitch in one direction. This is more noticeable in planes than rockets, but it can cause this unwanted roll. Rockets can be flown without fins, so I highly recommend trying to do without them to avoid this issue. If you absolutely need fins to add drag to the bottom, try lowering their control authority to minimize the effect, or use fixed fins.

Gimbal: almost all rocket engines have a bit of gimbal capability, often more than is really needed. To a lesser degree than fins, they too can be affected by the SAS flutter, so try to tune them down to lessen the effect.

 

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3 hours ago, Nicodo123 said:

SAS does absolutely nothing

Just a FYI: SAS only cares about keeping the nose/front of a craft (*) pointed at the marker. It doesn't at all care about the craft's orientation around the axis towards that marker, so it literally does indeed do nothing to stop or avoid rolling.

(*: more accurately, it cares about pointing the control direction of the active probe core/command pod/docking port at the marker. Generally that will be the nose/front of your rocket though.)

Edited by swjr-swis
*
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  • 2 weeks later...
On 4/13/2021 at 8:40 PM, swjr-swis said:

Joints

This. (Rest of info is excellent as well).

I had issues with a Soyuz rocket constantly spinning on ascent. Went to dark side and started using autostruts and it doesn’t do it anymore.

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  • 4 weeks later...

I had very similar issue - the problem was joystick connected to the PC.

I didn't use it for Kerbals, but it was not perfectly center (I think default deadzones in kerbals are tight) and so my rockets had constant change of course...

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On 4/13/2021 at 2:55 PM, swjr-swis said:

Just a FYI: SAS only cares about keeping the nose/front of a craft (*) pointed at the marker. It doesn't at all care about the craft's orientation around the axis towards that marker, so it literally does indeed do nothing to stop or avoid rolling.

(*: more accurately, it cares about pointing the control direction of the active probe core/command pod/docking port at the marker. Generally that will be the nose/front of your rocket though.)

Is that for the hold-prograde mode? In the stability-assist mode, SAS does damp out roll. I will also note that the Smart A.S.S. module of MechJeb lets you specify the roll, plus offsets to roll+pitch+yaw from surface-prograde.

To the OP: try whacking alt-X in case, for some reason, you have some trim set. Otherwise, roll can be caused by flexible joints (both axial and on radial boosters) and SAS overcompensation. To avoid the SAS overcompensation, as mentioned by @swjr-swis, reduce your control authority: limit engine gimbal and roll control on aerodynamic surfaces. To minimize flexibility, try to avoid wide-narrow-wide joints, and consider using struts, particularly for radially attached boosters. I usually attach an extra strut from the top of the booster to the core stage to help keep them pointed straight and avoid inducing roll.

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