Why won't this thing fly?

[Colonel_Panic]

I've completely rebuilt this plane into different though similar variations and configurations seven times, but it still does the same thing... shortly after takeoff, it starts pulling to the left or to the right harder than I can compensate for, or starts rolling and yawing back and forth violently until it enters a flat spin. Every. Time.

This is the longest flight I have yet accomplished with this plane:

Any idea how I can fix this, or is the physics engine just incapable of handling a double fuselage design?

[Tex_NL]

Screenshot #6 probably explains best why. Having your CoL that close to your CoG isn't necessarily a bad thing, iIt makes planes easier to fly without (A)SAS/avionics. But yor CoL is not slightly behind the CoG, it's on top of it. You might want to lower the CoL to bring it in line with the CoG and LoT.

[MBobrik]

Did you lock the gimbal of your front engines ? thrust vectoring and being in front of the center of mass don't work properly together in KSP.

[UmbralRaptor]

Turn off thrust vectoring for the (jet) engines in front of your CoM? The current TVC implementation is... bad with such parts.

[Tex_NL]

MBobrik has a good point. The engines might be trying to push the plane in the exact opposite direction they should.

Looking at your video gave me another possible clue. Your plane has a lot of drag and is quite heavy and you fly full throttle. Try flying your plane a bit slower at low altitude and only speed up when you get a lot higher.

[Colonel_Panic]

Wow, you were right about the engine gymballing. I'm kicking myself now that this was the problem all along.

also yeah I know about the cog/col thing, some of my later versions had col far far aft, still no difference...

[Tex_NL]

... some of my later versions had col far far aft, still no difference...

You don't want your CoL too far behind your CoG either. Making a plane 'nose heavy' is just as bad as 'tail heavy'.

[Giggleplex777]

The main problem is that your rockets are draining fuel pretty fast making the plane progressively front heavy. Making the plane shorter might help.

[Colonel_Panic]

Hey guys,

No really, locking the gimbals really did solve the problem. She flies perfectly now... well, about as perfectly as one can hope for in a gargantuan KSP spaceplane, but definitely well enough to reach SSTO reliably and assemble my payloads in orbit.

The way I have the current model has the starting com a fair bit ahead of the col, since the plane burns up that front tank almost completely on takeoff. After that it levels out and I drop down to just the turbojet engines, and can hit 23km before firing up the rockets again for a final burn to orbit.

It's still a bitch on re-entry though, and doesn't like to climb in thinner atmosphere, because it's heavy and doesn't have much lift... but it has enough.

I have her designed to have enough dry weight in the front that emptying the front tanks doesn't affect her handling on ascent (the side tanks remain full until the rockets are fired leaving the atmosphere, and by then the front-rear weight distribution no longer matters as much), then I just refill the front tanks with the last of the liquid fuel after the re-entry burn to bring some weight back to the front for landing, and to give the jet engines something to burn.

Edited May 17, 2013 by Colonel_Panic