UI/UX "Up" no longer selectable once SAS auto switches from surface to orbit velocity

[Davidian1024]

No mods.

While ascending, the velocity display will automatically switch from surface to orbit.  This happens at different altitudes for different bodies.

When this switch happens, the targeting options available to SAS switch accordingly.

If "up" was selected as the direction for SAS to target when the switch occurred, "radial out" will now be selected.  At this point attempting to switch back to "up" by first switching back to surface doesn't work.  The button isn't clickable.

You have to either turn SAS off and on again, or select a different SAS target and then select surface: up.

Edited June 1, 2023 by Anth12

[RaccoonTOF]

"Radial Out" basically IS "Up" when in orbit mode.  Other than the  name being different, is there a particular instance where setting SAS to "Radial Out" doesn't perform the same maneuver you would expect from "Up"?  (I suppose there might be some strange edge case, possibly involving spaceplanes with 90 degree rotated probe/cockpit controls?)

[Jason_25]

This sudden transition itself to orbit mode is devastating to large missions.  Huge launches need to clear the atmosphere before making any turns and this does some foolishness where it tries to sabotage the mission around 30km altitude. 

Radial out and in are slightly different than up and down depending on your orbital speed I think.  Regardless, I think it switches from "up" to another mode that is not "radial out" but the net result is flipping at high altitude among the many other things that can cause high altitude flipping.

Also I am not seeing the issue with up not being selectable due to my specific "workflow".  I watch the navball like a hawk and immediately engage hold mode with a double tap when it switches to "mission sabotage mode".  I then click the indicator to change back to surface and specify surface mode again and it works fine.

[Davidian1024]

18 hours ago, RaccoonTOF said:

"Radial Out" basically IS "Up" when in orbit mode.  Other than the  name being different, is there a particular instance where setting SAS to "Radial Out" doesn't perform the same maneuver you would expect from "Up"?  (I suppose there might be some strange edge case, possibly involving spaceplanes with 90 degree rotated probe/cockpit controls?)

The easiest way to reproduce this is to fly a rocket straight up off the launchpad with SAS on, velocity set to surface, and "up" set as the SAS target before reaching the autoswitch altitude of ~36K above sea level.

Note; you can't select "up" as the SAS target until your craft is off the ground.

[Turbo Ben]

21 hours ago, RaccoonTOF said:

"Radial Out" basically IS "Up" when in orbit mode.  Other than the  name being different, is there a particular instance where setting SAS to "Radial Out" doesn't perform the same maneuver you would expect from "Up"?  (I suppose there might be some strange edge case, possibly involving spaceplanes with 90 degree rotated probe/cockpit controls?)

 

11 hours ago, Jason_25 said:

Radial out and in are slightly different than up and down depending on your orbital speed I think.

 

Radial in/out (and normal/anti-normal) are perpendicular to your direction of travel (prograde), irrelevent of speed. They move as you thrust toward them because you are changing your direction of travel, unlike thrusting prograde/retrograde where there is acceleration but no change in direction. Radial in/out is only the same as up/down in a circular orbit, or at AP and PE in an eliptical orbit. In a highly eliptical orbit, away from AP and PE (such as during launch), radial out and up are very, very different.

If you launched using the up setting, your direction of travel is, well, up. When it switches to radial in/out at 36K, SAS immediately tries to turn the rocket 90 degrees from prograde to almost horizontal (almost because when it switches to orbital, the momentum you carried from the rotation of the planet is taken into account. That's why prograde moves when switching between ground and orbit modes, and why your orbit line in the map view isn't a straight up and down line.)

I usually set SAS to hold before 36K, switch back to ground mode and switch SAS back to up. It's also possible to just switch to prograde before 36K (as long as your rocket is stable enough to handle the ground/orbital prograde shift and the inevitable SAS overshoot when moving to it). It can be a pain and it's easy to forget if you're concentrating on a staging event though.

[RaccoonTOF]

I realize that once actually having an established orbit, "Radial Out" and "Up" can be vastly different, as you described.  I was thinking of during the ascent specifically though - I've not encountered the nearly 90 degrees swap you've described before - "Up" and "Radial Out" are usually within a few degrees of each other when it transitions from surface to orbit mode (much like the "Prograde" changes during the surface/orbit shift as well, and usually by about the same amount.)  I suppose I've just never flown whatever specific ascent profile you are looking to do, and so I've never really noticed any extreme differences as you have described (again - only referring to the ascent here; as you've described already they are quite different in an actual established orbit).

[Turbo Ben]

1 hour ago, RaccoonTOF said:

I realize that once actually having an established orbit, "Radial Out" and "Up" can be vastly different, as you described.  I was thinking of during the ascent specifically though - I've not encountered the nearly 90 degrees swap you've described before - "Up" and "Radial Out" are usually within a few degrees of each other when it transitions from surface to orbit mode (much like the "Prograde" changes during the surface/orbit shift as well, and usually by about the same amount.)  I suppose I've just never flown whatever specific ascent profile you are looking to do, and so I've never really noticed any extreme differences as you have described (again - only referring to the ascent here; as you've described already they are quite different in an actual established orbit).

Radial out is always 90 degrees from prograde whether in orbit or during ascent. If your prograde is up, radial out is horizontal.

Just try launching a ship using the up setting and watch it all go wrong at 36Km if you don't believe me.

[RaccoonTOF]

No I get that, I just have never had an ascent profile (other than suborbital sounding rockets) that just burned straight up for long enough that it would matter (nor can I see an actual use case for such, since that is just about the LEAST efficient means of achieving orbit - even a very draggy craft will still benefit from having a significant portion of the delta-V spent being "forward" rather than "up", even if that pitchover doesn't occur until later in the launch than a streamlined craft).  Basically, I can see the theoretical issues involved - I just don't see any actual practical scenario where it actually makes much difference for a real ascent profile...

[Jason_25]

When you are launching a gigantic winged vehicle at the top of a rocket the center of lift is near the top so It is best to just clear the atmosphere before trying to make a turn in that case because no practical amount of fins at the bottom will offset that.  Or if you are launching something like a space cruiser or a space station all in one go and you have decided to make a long tall rocket for whatever reason instead of a "cocoon" type launch with boosters wrapped around the upper stage.

With the first rocket any deviation from up is a really bad thing for stability and with the second rocket wobble becomes a factor which is better dealt with in space.  If you are like me and build long and tall and with wings at the top it is a necessity to use the up mode for a long time.  Really this is because we are not paying for the cost of the launches so it is better to build reliable than to build efficient.  Also the cost of the upper stages I build would be in the many trillions of dollars so again favoring reliability is best.