Weird thrust issue once game switches to "Orbit" mode.

[InambaGuum]

I've never had a problem like this...I can't tell if I'm doing something wrong with my design or if it's a bug (or possibly weird mod interaction). But I have a spaceplane that flies beautifully on both the air-breathing and closed cycle stages while in the atmosphere, and then as soon as the navball switches to "Orbit" mode, any thrust applied seems to be pointing about 45 degrees above where the nose is headed, causing the nose to pitch wildly down immediately. As you can see from the screenshots, the thrust vector is dead through the center of mass. I've tried different engines, moving the center of mass, moving the center of lift (not sure why that would matter, of course), disabling TAC Fuel Balancer, etc., and the problem persists through everything I've tried.

Help?

http://imgur.com/a/lOHTp#0

edit: Tried disabling every mod except Joint Reinforcement (necessary to keep this thing from flying apart). It's not a mod issue...so I must be doing something wrong with my design.

Edited September 2, 2015 by InambaGuum

[xtoro]

I suspect it's because your wings extend out below your CoM. In the VAB you're seeing your CoM with full fuel tanks. Remove the fuel in the VAB and I suspect you'll see that your CoM has lowered. There is a mod called RCS Build Aid that shows you your CoM with empty fuel tanks (DCoM, DRY CoM). Give that a shot, or empty your tanks and see if that's the problem. A quick fix might be adding another reaction wheel to fight the offset.

[Randazzo]

The key here is the altitude at which the display switches to "orbit", and the effect it's having on your control surfaces, probably combined with shifting CoM. The only real difference between "Orbit" and "Surface" is the navball display, nothing in the game actually changes. It doesn't switch over until ~30km or so (I think, might even be higher), when the atmosphere is getting too thin for wings and control surfaces.

[Vegetal]

Definitely the wings. You have too much weight below CoM, and as xtoro said, when you dried up most of your fuel, the CoM shifted downwards.

I also recommend RCS Build Aid, it's almost essential in my opinion.

[InambaGuum]

The key here is the altitude at which the display switches to "orbit", and the effect it's having on your control surfaces, probably combined with shifting CoM. The only real difference between "Orbit" and "Surface" is the navball display, nothing in the game actually changes. It doesn't switch over until ~30km or so (I think, might even be higher), when the atmosphere is getting too thin for wings and control surfaces.

Yeah...I'm sure it's some kind of boundary altitude thing, rather than the display...that is just where the problem starts.

The thing is -- the problem doesn't happen gradually. Pre-"orbit", it works fine. Post-"orbit", it suddenly thrusts 45 degrees off, without any kind of intervening state. I tried adjusting my wing plane up to better compensate for empty tanks, and I checked the spaceplane in the SPH with all the tanks completely empty (which they aren't when this problem happens), and the thrust vector still goes through the CoM within a degree or two. That doesn't seem to be the problem, as far as I can tell.

[Randazzo]

Hmm. I'd still try adding a reaction wheel somewhere and see if that resolves the issue. I've had plenty of designs that suddenly can't keep themselves right at high altitude and it's always been a lack of control authority in thin atmosphere. Another issue could be asymmetric thrust (which happens in KSP, unless you've placed them in a specific manner or use a mod to fix it) from those jet engines but I'd imagine they've cut out before you get that high.

[InambaGuum]

Hmm. I'd still try adding a reaction wheel somewhere and see if that resolves the issue. I've had plenty of designs that suddenly can't keep themselves right at high altitude and it's always been a lack of control authority in thin atmosphere. Another issue could be asymmetric thrust (which happens in KSP, unless you've placed them in a specific manner or use a mod to fix it) from those jet engines but I'd imagine they've cut out before you get that high.

It already has an Advanced Inline Stabilizer. :/ And yeah, it's on the rocket engines by that point.

I will try RCS Build-Aid...sounds good regardless...but looks like I may have to scrap this fellow.

[The_Rocketeer]

You can manually switch between 'surface' and 'orbit' speed displays by clicking on it (on top of the navball). Try manually switching at lower and higher altitudes and see if the problem really is 'orbit-mode' related. If so, bug (so report it). If not, probably movement of CoM issue as suggested.

[ForScience6686]

I would guess it's the aero forces causing problems, especially if SAS is on follow prograde. When it switches to orbit, you are no longer prograde to the air stream which is still relevant at that altitude. I have had this issue with large payloads and low control authority, immediate loss of control. I tend to go for a steeper ascent when I encounter this issue. I thought the nav ball switch was more relevant to horizontal speed, as I can get the switch over at minimus at extremely low altitudes but almost orbital speeds.

[bigcalm]

I had this on one of my planes -- what's happening is that in the atmosphere your wings / control surfaces are allowing you to steer in the direction you want. However, in space, you don't have this, so it's dictated by the centre-of-thrust vs. the centre-of-mass. From the vessel pic, it looks like the centre of mass is out of line with the centre of thrust, causing the plane to rotate under thrust.

Possible solutions are:

1) Make the centre of mass in line with the centre of thrust

2) Add more reaction wheels

3) As soon as you start coasting towards apoapsis, reduce the thrust of your engines to 25% or so of their maximum. This should then allow your reaction wheels to compensate for the unbalanced centre of mass / centre of thrust problem.

Edited September 2, 2015 by bigcalm