UI/UX SAS in maneuver mode gives incorrect outcome when thrusting normal/antinormal.

[Turbo Ben]

When setting up a maneuver node with a normal/antinormal element, SAS in maneuver mode gives the wrong result as the maneuver marker moves as normal/antinormal moves.

Steps to recreate:

To see the problem clearly, it would be best to recreate the following steps in a high orbit with a low orbital velocity. High over Minmus would be ideal but the Mun would be fine.

1. Set up a maneuver node with a purely normal vector.

2. Set SAS to track the maneuver.

3. Perform the burn as instructed.

Expected result - the maneuver marker on the navball should be fixed during the burn and the resulting trajectory should match the maneuver node prediction.

Observed result - the maneuver marker follows the normal marker as it moves during the burn. The final trajectory is not what the maneuver node predicted.

This may also apply to radial in/out maneuvers, but I haven't checked.

Edit:

Just done some testing and can confirm this also happens with radial in/out.

Also, the maneuver doesn't need to be purely radial in/out or normal/antinormal. Any maneuver that contains any of these elements will not be accurate, though won't be noticeable in most cases. The larger the radial/normal element, the less accurate the burn will be.

For example, I set up a 45 degree inclination change in a 400Km minmus orbit. Performing the burn using the SAS maneuver hold and following the burn timer resulted in a 57 degree inclination change. Another sign that something isn't right is the yellow bar below the burn timer never reaches it's end point. It stops at about 1/3 and then starts going up again.

Edited May 3, 2023 by Turbo Ben

[RaccoonTOF]

This can occur with any maneuver, though it is definitely most common with maneuvers that are not pure pro-/retro-grade burns.  The maneuver marker TRIES to always point the direction the burn needs to continue in to result in the expected pre-burn planned outcome.  The issue comes in that it is an instantaneous update, and thrust and actual reorientation of the rocket takes non-infinitesimal time.  The same issues can be observed with extremely long low-thrust standard pro-/retro-grade burns - it's just that the margin of error is usually significantly (sometimes orders of magnitude) smaller for inclination changes. 

[Turbo Ben]

Yes, the marker will try to adjust to keep the burn on track. It's generally pretty stable though except maybe at the very end of the burn.

This is something different. The maneuver marker tracks the normal marker as it moves. In a high orbit at low velocity, this tracking is very rapid. Try setting up a maneuver node to create an escape trajectory using only a normal vector. The node will predict your orbit changing to head north and then leaving the SOI. What will happen is the ship will start to perform loops as it tracks the normal marker, and you'll finish up pointing in a random direction with hardly any speed added.

[Turbo Ben]

I've done some further testing and updated my original post.

[Wetzelrad]

The workaround is to burn during timewarp, or burn with SAS set to lock/hold. As long as you don't rotate to follow the marker, you should stay on course for the manuever as it was planned.