wobbly stations

[ohlookabirdie]

i have a space station in orbit (3000km) and its big by my standards. up until now i have not had any problem with wobbles, but i just another ship to it and it starts wobbling all over the place. whats up with that?

Edited July 9, 2014 by ohlookabirdie

[GoSlash27]

Loose connections and distributed reaction wheels/ controllers. They can set up a parasitic oscillation wherein they detect the wobble and make torque corrections that make it worse instead of better.

Your station is already built, so larger clamp-o-trons and additional bracing are no longer an option.

Your best bet is to therefore go around the station and disable any reaction wheels that are on the opposite side of a joint from the controller. Likewise, disable any unused controllers.

When docking a ship, disable SAS just as soon as the connection is established.

Best,

-Slashy

[zekes]

yes, a lot of times too much SAS will tear it apart. Try turning them all off except one near the center, then the station will be able to maintain attitude without wobbling.

[ohlookabirdie]

thanks guys. i'll try these things when i play later.

when you dock does the point of control automatically go back to the main station controler?

[Cerberus738]

not sure about point of control. i have a action key set to disable all torque on all my ships except for 1 reaction wheel on the main module of a station. so any time i dock i just push 0 and all ships/modules docked have their torque turned off. i usually set engines to be shutdown on the same action key that way no silly shift mistakes happen. ha ha. then when i undock i just push 2 and everything turns back on again.

[GoSlash27]

I don't know how the program decides that, but in my experience it seems to work out that way. I'd guess that it's order of creation in the save file.

-Slashy

[ohlookabirdie]

ok, on a related note... i launched a ship today which had several reaction wheels because of the lander attached. all were in a line with the CoM, none were attached radially. the ship still wobbled... why? this never happened me before

[LordFerret]

My station has grown quite large, and I've encountered the wobble. From Tracking, I went 'to' the station and turned RCS and SAS off, then went back to Tracking, and then back to the station - wobble be gone.

For the ship wobbles, I added additional ASAS units evenly distributed the entire craft, and used fins. That only partially solved my problem though. I found the final solution was to throttle back a little (too much thrust) and nurse the craft into orbit. I've lifted some heavy structures for my space station (top-heavy rocket shall we say), and that's how I got them up.

[Cerberus738]

you could try turning off SAS and then fast forward time. Everything should stop moving and then when you come out of time warp SAS won't start moving things again.

I've found that you can keep pressing the fast forward button when it says can't warp because "the craft is under acceleration" and eventually it will go.

Edited July 9, 2014 by Cerberus738

[weezl]

My station has grown quite large, and I've encountered the wobble. From Tracking, I went 'to' the station and turned RCS and SAS off, then went back to Tracking, and then back to the station - wobble be gone.

For the ship wobbles, I added additional ASAS units evenly distributed the entire craft, and used fins. That only partially solved my problem though. I found the final solution was to throttle back a little (too much thrust) and nurse the craft into orbit. I've lifted some heavy structures for my space station (top-heavy rocket shall we say), and that's how I got them up.

Wobbles during takeoff seem to be induced by a feedback loop between the SAS and the vectoring on the rockets driving your craft. Things will shift one direction, the engine will vector, then it shifts back, the engine compensates (kinda), and the loop is going.. Its a resonance wobble induced by the SAS control system.. and the only way I've found to get rid of it is to use struts on the connections between tanks/modules/etc. to reduce wobble. Creative use of girders and struts is the trick to longer rockets to keep them from resonating apart.

The reason that reducing throttle reduces the wobble is because there is less force working to bend the connections out of place during each resonance cycle. The only real solution in game is to reduce wobble with struts, since we don't have control over how fast the SAS reacts to attitude/thrust measurements(anyone who has worked with industrial control PID loops will 'get it' regarding this issue.. PID loops are tuned based on the portion of the system with the most inertia... i.e. the thing that is the slowest to change in reaction to a control change.. you tune your loop specifically to avoid these types of feedback resonance issues.. you can always tell when someone did something wrong with a PID, since a motor (typically the slowest thing in a control system) will speed up then slow down in an even time cycle.. a feedback resonance. In a rocket the bending changes the center of mass, which changes the rate of turn for a given vector position on the engine.. and its changes slowly in comparison to many other things in the system). Note that in all this there is a trade off.. if you slow down SAS reaction speed, you could make the rocket uncontrollable because of the speed needed to overcome changes in attitude in the last control cycle.. so (IMO) I think its better to strut up anyways..

Edited July 9, 2014 by weezl

[Blind Dead McJones]

There's a mod called Kerbal Attachment System that allows you to add struts to a structure that's already built and launched.

[LordFerret]

Wobbles during takeoff seem to be induced by a feedback loop between the SAS and the vectoring on the rockets driving your craft. Things will shift one direction, the engine will vector, then it shifts back, the engine compensates (kinda), and the loop is going.. Its a resonance wobble induced by the SAS control system.. and the only way I've found to get rid of it is to use struts on the connections between tanks/modules/etc. to reduce wobble. Creative use of girders and struts is the trick to longer rockets to keep them from resonating apart.

The reason that reducing throttle reduces the wobble is because there is less force working to bend the connections out of place during each resonance cycle. The only real solution in game is to reduce wobble with struts, since we don't have control over how fast the SAS reacts to attitude/thrust measurements(anyone who has worked with industrial control PID loops will 'get it' regarding this issue.. PID loops are tuned based on the portion of the system with the most inertia... i.e. the thing that is the slowest to change in reaction to a control change.. you tune your loop specifically to avoid these types of feedback resonance issues.. you can always tell when someone did something wrong with a PID, since a motor (typically the slowest thing in a control system) will speed up then slow down in an even time cycle.. a feedback resonance. In a rocket the bending changes the center of mass, which changes the rate of turn for a given vector position on the engine.. and its changes slowly in comparison to many other things in the system). Note that in all this there is a trade off.. if you slow down SAS reaction speed, you could make the rocket uncontrollable because of the speed needed to overcome changes in attitude in the last control cycle.. so (IMO) I think its better to strut up anyways..

Very true, good points! I should have mentioned struts, as I do use them, a lot. I put radial decouplers up on some of my larger builds, which allows me to place struts and build a support 'cage' around stuff. After launch and leaving the atmosphere, before orbit, I stage the decouplers and release the cage... debris falls back.