Weird SAS/Autopilot issue

[Sidereus]

I launched two probes today, and for some reason they wouldn't hold on the SAS autopilot points. When I selected one, it would go to the point but instead of staying dead-on it would kind of slowly slide off point. The stability assist worked ok, but nothing else.

I haven't had this problem before, so I'm wondering if it's a craft design issue. The two probes both had the small inline reaction wheels. One had an OKTO2 pod, and the other had the HECS pod. One had no RCS thrusters; the other had six place-anywhere thrusters, with two pointing "up" and the other four pointing out, so they were really only for minute translation, rather than roll.

Anyone have any ideas?

The two craft:

(First one)

(Second one - the small reaction wheel is surrounded by the first toroidal fuel tank).

(Bottom view of second; has a 48-7S engine, with an Oscar-B tank hiding under the two toroidal tanks).

Edited January 27, 2015 by Sidereus

[eddiew]

I've had issues with direction hold when the craft is asymmetrically weighted, but its an inconsistent effect and doesn't show up all the time :/

[Sidereus]

I've had issues with direction hold when the craft is asymmetrically weighted, but its an inconsistent effect and doesn't show up all the time :/

I've had that as well, but usually when that happens my craft will roll consistently towards the over-weighted side. With these two it was fairly random.

[Xannari Ferrows]

This is an issue I have sometimes myself. It is due to the games' system of floating numbers involved in various physics calculations. It has been going on for several versions of the alpha releases of KSP, except those were just regular stability assist issues.

If you find it an inconvenience too much, try hitting Alt+X to reset all of your pitch, yaw, and roll controls. If this doesn't work, It could also be that the probes are pointed the right way, but the information on the navball is slightly off-center. If this is true, just trust what the SAS pointers do.

Also, keep in mind that this is the first time something like this has been in stock KSP, so it's bound to have bugs.

[Tank Buddy]

I found that if a craft has too much torque the autopilot will wobble away from the attitude that you selected. There is a sweet spot where the autopilot doesn't overshoot the and stays on target, but I don't really feel like finding the torque values and mass values for that. I would think Squad will fix this in the next update.

[Sidereus]

This is an issue I have sometimes myself. It is due to the games' system of floating numbers involved in various physics calculations. It has been going on for several versions of the alpha releases of KSP, except those were just regular stability assist issues.

If you find it an inconvenience too much, try hitting Alt+X to reset all of your pitch, yaw, and roll controls. If this doesn't work, It could also be that the probes are pointed the right way, but the information on the navball is slightly off-center. If this is true, just trust what the SAS pointers do.

Also, keep in mind that this is the first time something like this has been in stock KSP, so it's bound to have bugs.

I found that if a craft has too much torque the autopilot will wobble away from the attitude that you selected. There is a sweet spot where the autopilot doesn't overshoot the and stays on target, but I don't really feel like finding the torque values and mass values for that. I would think Squad will fix this in the next update.

Either of these sounds plausible. Could possibly be a combo of the two...craft that small probably don't really need a separate reaction wheel. I'll have to do some experimenting the next time I launch some probes. Thanks for the help!

[makinyashikino]

I found that if a craft has too much torque the autopilot will wobble away from the attitude that you selected. There is a sweet spot where the autopilot doesn't overshoot the and stays on target, but I don't really feel like finding the torque values and mass values for that. I would think Squad will fix this in the next update.

I know how you feel. With strong reaction wheels, a craft will oscillate faster and faster if you don't toggle or disable the wheels.