Oscillating ship when SASing

[seyss]

Hello,

For some reason my ship shakes and oscillates when I burn using SAS pointed to prograde/retrograde/normal/anti-normal/radial/anti-radial as shown in the video below. (when pointed to Stability Assist there's no shake)

Can anyone give me any info about this oscillation? It ends up screwing my injection burns. Thanks.

Note: between the big fuel tank and the other engine (next stage), I have an Advanced Reaction Wheel. So there are 3 torques here being used by SAS: engine gimbal, adv. reaction wheel, and the reaction wheel in the command pod.

Edited April 13, 2015 by seyss

[Slam_Jones]

You might need to put some struts from the tank up to the trusses. If you don't have it yet, I recommend Kerbal Attachment System, which gives you (among other things) struts that can be placed and moved while in flight.

Edit: Another option might be to add SAS to the ends of the trusses to keep em stable.

[WuphonsReach]

Or add the Kerbal Joint Reinforcement mod.

[Reactordrone]

Yeah, even with small ships the the SAS direction holds cause vibrations as they try to constantly adjust the ship heading.

[Deutherius]

It's not a problem with your design, the fault lies in stock SAS. Any craft told to point pro/retrograde (or other directions determined by orbit) will wobble, as the SAS is overcompensating for its own movement - unless being pointed exactly in the direction you want it to and having no thrust offset torque whatsoever. And even then it's not going to last, because when you thrust, you change the orbital parameters and therefore the precise position of the node itself, which the ship is going to try and follow - and overcorrect itself.

You can observe this effect more clearly when you tell your craft to point somewhere and it has to turn around on its own - it's going to go full reaction wheel/RCS power until it reaches the correct position, and ONLY THEN it's going to try correcting any momentum that's left (which is a lot, usually). This causes the craft to wobble back and forth over the chosen node as it overcorrects a little less each time.

The reason why simple stability assist doesn't wobble is it's not trying to point anywhere in particular, it's just trying to kill any rotational momentum by producing torque in the opposite direction, until the ship is steady.

The solution is simple - use less thrust (so the ship has less momentum to overcorrect), turn on smooth controls (caps) or use stability assist, and pray for SQUAD to implement a better node tracking method.

[Mad Rocket Scientist]

Turning off one of your control systems should help.

[bakanando]

The gimbaling engine will give you the worst wobble so locking it will help.

But as was mentioned earlier, this is an implementation bug (feature?) that is better fixed by not using it. Struts and disabling one or two control parts will definitely help though.

[seyss]

I tried disabling other control parts and the effect was undesirable.. I put a couple of struts as Slam_Jones said [in Scott Manley's voice] and....... [/in Scott Manley's voice] problem fixed! Thanks.

[Slam_Jones]

I tried disabling other control parts and the effect was undesirable.. I put a couple of struts as Slam_Jones said [in Scott Manley's voice] and....... [/in Scott Manley's voice] problem fixed! Thanks.

Nice! Glad I could help