Skycrane-ish rocket won't fly straight.

[painking]

So I'm trying to make a Skycrane like thing to lift my big 16K kethane tank and accompanying power source and drills onto the mun. drop it off with the sky crane and decouple it so it flies off to wherever. I'm testing its stability on Kerbin but it just won't fly straight and I don't wanna bring it to the mun knowing I can't keep a handle under it.

The LV-45s are just stand-ins for nukes cause they don't have enough thrust on Kerbin to lif that. The CoM is centered right at where the girder and tank connect, and is equal on all sides. The CoT is -slightly- off with the CoM but it's so negligible I didn't think that it'd matter, you can't even tell the fuel tanks and engines are without really closely inspecting it. Is there something I'm doing wrong or should I try a different design?

Edited August 1, 2013 by painking

[stupid_chris]

Your problem is that your CoT is over your CoM. That causes the gimbals to be inverted and renders your craft uncontrollable. Try disabling the gimbals and controlling with torque instead.

[painking]

I lowered my CoT below the CoM and tried flying it with and without gimbals locked, no dice. My ability to control it however, is much better at low thrust, up to about 60% throttle where I no longer have control over it. I don't know how that's going to translate over to non-atmospheric flight with the nukes.

[Sumyunguy]

After looking, I think a small change to it would be to shift the rockets to the corners of the craft instead on the x/y axis of the fuel tank.

If that doesn't work, try adding either more reaction wheels or RCS with a high thrust output to the craft. This might help controlling the craft with more control authority(?).

And it THAT doesn't work, try flipping the kethane tank vertically and attach the framework/rockets to the center. That should eliminate any CoM balance issues, but you might need to readjust the legs and docking ports to the sides.

Hope it works out. But if it doesn't, I'll keep brainstorming new ideas to help.

[Specialist290]

Looking at your Build Engineer window, you have a TWR of 2.36. I don't know if that's set up to display for Kerbin or the Mun, but either way that's well within the amount of thrust you'd need to safely land. The Mun's gravity is only about 1/6th that of Kerbin, so if you can take off from Kerbin's surface with that setup, you can almost certainly land it on the Mun.

[Bone White]

You could also try adding more reaction wheels to apply more torque. I've found that side-mounted rockets are much more stable and reliable than a sky-crane apparatus, and require a lot less parts too.