Small craft wobble under SAS

[kananesgi]

I'm curious about this. I've tried doing some research and find lots of info about wobble in large builds, but that has to do with the various parts of the ship flexing. This is not related to that, as my craft is only a small handful of parts. 

So, I've built a small, two-stage, sub-orbital probe using Procedural Parts fuel tanks, with a pair of reaction wheels (one 1.25m on stage one, and one .625m on stage two with the probe core). Stage one is a Swivel engine, stage two is a Terrier. Stage one launches nicely, and it flies quite well. This is actually the first design I've made that I have successfully executed a gravity turn with. I have to play with the throttle all the way up, otherwise it will go unstable and start flipping at around 25,000m. The problem comes in stage 2. For some reason, the SAS causes the craft to wobble endlessly, and I don't know why. It's wasting my electric charge from all the constant control inputs. If I turn off the reaction wheel, the craft just floats wildly out of control because the probe core doesn't have a built-in reaction wheel.

I've had this problem before, but it was usually associated with having multiple active reaction wheels or RCS thrusters that were interfering with each other. This time, I have only the one reaction wheel active. Any ideas?

[Rocket In My Pocket]

SAS causing small craft to wobble is a well known issue.

You can try Claw's stock bug fixes: http://forum.kerbalspaceprogram.com/index.php?/topic/87737-ksp-v105-stock-bug-fix-modules-release-v105a3-27-dec-15/

It fixes a lot of bugs and also gives you the options to tweak the specifics of the SAS module per ship.

Quote

+ PilotRSASFix (Plus) (27 Dec 15) - (STATUS: Fixed StockPlus Options) - Description: The new(ish) pilot abilities cause smaller vessels to jitter and rapidly deplete electrical charge or monopropellant.
     - Currently does NOT try to prevent overcontrol by RSAS when moving between targets. Only the long term response.
     - Dynamically self adjusts to reduce jitter and resource consumption.
     - Somewhat reduces wobble for bendy craft during launch, when using pilot RSAS abilities.
     - (Plus) Gives access to the tweakable options that control the dynamic tweaks.
          -- The "Plus" tweakable options are available in the right-click menu of the first pod/probe core.
          -- "Min Response" sets the lowest reaction response from the RSAS ("how far" is the initial move)
          -- "Min Clamp" sets the lowest clamp value (how much reaction) as the RSAS nears it's target.
          -- "Threshold" sets the angular speed where clamp and response values start to adjust.
     - Still a bit of a work in progress, and needs testing with more types of craft.

 

Edited January 6, 2016 by Rocket In My Pocket

[CitizenVeen]

11 minutes ago, kananesgi said:

I'm curious about this. I've tried doing some research and find lots of info about wobble in large builds, but that has to do with the various parts of the ship flexing. This is not related to that, as my craft is only a small handful of parts. 

So, I've built a small, two-stage, sub-orbital probe using Procedural Parts fuel tanks, with a pair of reaction wheels (one 1.25m on stage one, and one .625m on stage two with the probe core). Stage one is a Swivel engine, stage two is a Terrier. Stage one launches nicely, and it flies quite well. This is actually the first design I've made that I have successfully executed a gravity turn with. I have to play with the throttle all the way up, otherwise it will go unstable and start flipping at around 25,000m. The problem comes in stage 2. For some reason, the SAS causes the craft to wobble endlessly, and I don't know why. It's wasting my electric charge from all the constant control inputs. If I turn off the reaction wheel, the craft just floats wildly out of control because the probe core doesn't have a built-in reaction wheel.

I've had this problem before, but it was usually associated with having multiple active reaction wheels or RCS thrusters that were interfering with each other. This time, I have only the one reaction wheel active. Any ideas?

What kinda wobble? The slow wobbly type that slowly gets worse? Or more like fast shaking. First is a bug, second one, I found, is caused by having a too strong torque, this happen when the reaction wheels are 'too big' for the craft. The wheel generates such a strong torque, that allready with the smallest adjustment that the SAS makes, it 'overshoots' what it wants to do. For example, it wants to cancel a clockwise spin, but adjusts it so much that the craft starts spinning counter clockwise, it tries to cancel again, and the craft spins clockwise again etc. The result is that with sas on it starts shaking.

Is that it what you are experiencing, try adding a smaller reaction wheel, or using a different probe that has a build in one. I believe that you can also limit torque during Vehicle Assembly, but not sure.

[steve_v]

My guess: excessive SAS PID controller gain. IME the stock SAS system does this if you have a lot of control authority on a light vessel.

IIRC, StockBugfixModules only tunes the pilot RSAS system (e.g. hold prograde etc.) If it is excessive PID gain, you may need to mess with the standard "stability assist" too - Unfortunately Regex's PID tuner no longer exists, but PilotAssistant has tunables for the stock SAS - try dropping the gain (P) a bit on all 3 axes.

Edited January 6, 2016 by steve_v

[Snark]

What R.I.m.P. said.

Essentially what's going on here is that you have too much torque available, and the stock SAS isn't well tuned to handle it.

So your options boil down to: either fix SAS with a mod, as previously suggested, or else reduce the amount of reaction torque on the ship.

[kananesgi]

Ok, I tried the first option which helped a lot. It still wobbles a little, but not near as bad. I believe the problem is too much torque. I'm using the smallest reaction wheel available, though. I'll try more experimentation when I get home. 

Thanks for all the help. 

[Snark]

1 hour ago, kananesgi said:

Ok, I tried the first option which helped a lot. It still wobbles a little, but not near as bad. I believe the problem is too much torque. I'm using the smallest reaction wheel available, though. I'll try more experimentation when I get home. 

Thanks for all the help. 

Bear in mind that most (but not all) probe cores contain tiny reaction wheels built in.  These are considerably weaker than the smallest reaction wheel.  So if you have a probe core that has built-in torque, you can remove your reaction wheel if you're over-torqued, and let the probe core do it.

[Rom201]

The problem is too much torque and this has been going on almost since 1.1 update so just put on something that provides less torque and dont worry they will fix this  

[kananesgi]

This question was originally asked back around the first of the year, months before 1.1 was out. From my playing on 1.1.2 recently, it's still the same as always. If you have a small craft like a simple satellite that doesn't have a lot of mass, it's best not to install any additional reaction wheels. If you encounter SAS wobbles in flight, check for any additional wheels and deactivate them. Sometimes makes the craft sluggish, but it works every time for me. 

[bewing]

Well, even with too much reaction wheel, the stability mode SAS still works nicely. So you can also flip back and forth between modes to eliminate wobble, if you are not thrusting.

[jlhill17]

I had a similar problem with a probe that weighed ~1t.  I had no extra reaction wheels, just the probe core's internal wheel.  I stopped the wobble when I set the gimbal on the probe's engine to "locked".

[Snark]

FWIW... the SAS is much improved in 1.2.  No more wobble.  It's great. 

[Hodari]

2 hours ago, Snark said:

FWIW... the SAS is much improved in 1.2.  No more wobble.  It's great. 

Even better, you can set SAS to retrograde during reentry and just leave it like that the whole time without completely killing your batteries