Spaceplane balance question: Atmo transition

[dom849]

I'm attempting to build a heavy lift SSTO. Getting up to +23KM is not an issue. The beast handles incredibly well in atmo. Transitioning from atmosphere to orbit is....well....a problem.

When igniting the rocket motors the craft will immediately begin to pitch up. Obviously, or not so obviously, balance is the issue...I'm pretty sure. Even as the craft is coasting to +38km with the motors off I'm able to correct the attitude. I start the engines up and again its pitching in the same direction.

Examining the line of thrust and the CoM they appear to be aligned. I will admit that viewing the CoM and line of thrust is a fair guestimation within the VAB. Does anyone have any tips for getting it just perfect?

If you're thinking uneven fuel burn might be the culprit, I've made sure that the fuel tanks are along 4 axis and burn counter to each other. So, tank left top burns front to back, right top back to front, left bottom back to front, right bottom front to back. Jet fuel is set up similarly and opposite to rocket fuel.

For your advice, thank you in advance!

Mantis 1

Mantis 2

[breakthrough]

The RCS Build Aid plugin has an 'engine' mode that displays the offset with great precision.

[capi3101]

How bad of a pitch up are you experiencing?

My own experience, by the time you have to light the rockets you're at least close to orbital velocity (1900-2000 m/s or so; I start noticing it around 1500). If that's where you're at, there's nothing wrong with your design - what's happening is that Kerbin is rotating underneath you and you're traveling fast enough that your pitch relative to the planet is changing. Nothing you can do about it, unfortunately.

Now, if it's happening earlier than that, or you're talking about a serious pitch up (20 degrees instantaneously, for instance), then there may be a problem. To help diagnose it, we'd need to see a picture of your plane.

[Claw]

Try removing the landing gear and see how much your CoM moves. With landing gear installed, the location of the CoM in the SPH is not where it will actually be in flight. The landing gear is ignored by the in-flight physics engine.

Also check to see if the fuel is actually feeding how you planned it. If one of the tanks isn't hooked up right that might be causing your problem.

You do have more mass on top with those vertical wings. Not a lot, but it might be enough given how big your plane is and that it looks like you're using a mainsail. Especially after you burn off some of that fuel. (And like I said earlier, the landing gear will not actually balance it out in flight.)

Depending on how bad the pitch up is, you can also try adding a reaction wheel to see if that helps enough.

Edited February 1, 2014 by Claw
Clarification