Why does the SAS target hold not always point to the target?

[THX1138]

Often my craft do not hold their target correctly but position some distance below the target on the navball. Please see the screenshot for clarification. Why might this happen?

[Stoney3K]

Because the SAS action is usually too much to hold the exact point where you want it to go.

That's not limited to the 'Target' setting, the same thing happens with prograde, retrograde, normal and radial settings, to a certain degree. You can notice it most when you're re-entering and you have SAS set to retrograde -- it will jiggle, wobble and drift all over the place while it stays perfectly stable using the 'Stability Assist' setting.

I do believe it's caused by some minor bugs in the SAS code that cause the control loop to overshoot if you want it to track something.

[Guest]

Long standing bug. This usually happens with docking ports mounted on fuel tanks. Mechjeb can target the ports on tanks just fine. Stock cannot.

[THX1138]

I've never noticed this problem with any of the other markers (prograde, anti-normal etc); only target. At least I know it's not just me.

[ibanix]

This problem is more visible when in physical time warp, but it is indeed present. I'm guessing that what @Stoney3K is correct, and there is a need to tweak the variables of the those modes. I'm guessing it's based on a PIC controller, but a Squad dev would know best.

[Stoney3K]

9 hours ago, THX1138 said:

I've never noticed this problem with any of the other markers (prograde, anti-normal etc); only target. At least I know it's not just me.

It depends a lot on your ship's mass and the amount of reaction wheels / RCS thrusters that are on the craft.

Part of the problem is that the reaction wheels and RCS are always running flat out, no matter what distance they are from the CoM of the craft, which causes the outer part of the craft to move more when compared to the inner parts of the craft. This is also what causes the craft wobble and flex (which is especially noticeable when you have a station). Depending on the location of your control unit, this causes a feedback in the SAS control loop which in turn causes oscillation.

SAS needs to be fixed in a future version, as @ibanix said, MechJeb's Smart A.S.S. can usually track a target marker or maneuver node just fine. For starters, the reaction wheel torque and RCS thruster strength should be proportional to the part's distance relative to CoM.

[ibanix]

Also, I believe the GimbalPlus module in the Stock Bug Fix Modules mod (see below) allows for tweakable strengths and rates on SAS, which may allow for individual craft fixes.

 

[A_name]

14 hours ago, ibanix said:

Also, I believe the GimbalPlus module in the Stock Bug Fix Modules mod (see below) allows for tweakable strengths and rates on SAS, which may allow for individual craft fixes.

 

To add, although not compatible with Claw's GimbalPlus module, this solves all your SAS problems forever, I cannot state how well it works for planes as well as spacecraft despite it being presented as an atmospheric tool: