Need advice on why I can't control attitude during asteroid ejection

[THX1138]

I'm having a problem moving a class E asteroid. It had no problems ejecting a class C asteroid from solar orbit using a similarly-designed craft but with this asteroid, I can't maintain control of the attitude.

There are a total of three craft attached to the asteroid. One is at the back (failed attempt one which holds most of the fuel) and two are at the front - the two at the front are attached to each other by a claw.

There are a total of six large reaction wheels on the two craft on front of the asteroid and three on the craft at the back. There are four vernor engines on the front-most craft and there are eight nuclear engines around the outside of the circular frame. All the other engines are shut down (edit - the four at the back are not shut down but I've tried shutting them down and I'm not sure whether it helps).

No matter where I select to "control from here", the damn thing just starts spinning when I attempt to apply thrust. The goal is to move it out of solar orbit, or at least try.

Could someone tell me whether the three reaction wheels at the back are helping or hindering me? I don't know if the software has them working against the others.

I uploaded the save file in case anyone wanted to investigate the situation more closely.

Edited January 29, 2015 by THX1138

[Starwhip]

Kill the rear NERVA engines and see what happens: I have a sneaking suspicion that your Center of Thrust is out of alignment...

[StrandedonEarth]

Most Likley your center of thrust is not in line with your center of mass. Disable the gimbals on the pulling engines, as I've read that they don't steer properly when ahead of the CoM. If it still spins, use the thrust limiter to reduce thrust on the engines on the side away from the direction of spin, I.e. If it's spinning to port, reduce thrust on the starboard side.

Edit: having the engines spread out like that will multiply any torque generated by being off center ( greater leverage). You may have to completely shut down an engine

Hope that helps!

Edited January 28, 2015 by StrandedonEarth

[THX1138]

I tried that but it's still unstable. I think I know what the problem is now. I think I'm going to have to shut down six of the eight engines because three of them are partially blocked by the asteroid.

I knew this was a risk when I launched but I couldn't build the craft any larger within the confines of the VAB, otherwise I would have. If I was starting over, I guess I'd build something which I'd construct in orbit but geewhiz, that's a going to be a kerfuffle if it comes to that.

- - - Updated - - -

Sigh. It's an irredeemable failure.

[Red Iron Crown]

The puller engines need their gimbals locked, they move in exactly the wrong direction when ahead of the CoM.

[sal_vager]

Your design could work with some adjustment, specifically the angle of the LV-N's on the puller vessel.

By directing thrust outwards slightly you can avoid thrusting at the asteroid, you will lose some efficiency but it's better then the issue you have now.

[THX1138]

The puller engines need their gimbals locked, they move in exactly the wrong direction when ahead of the CoM.

Thanks. It's a shame the software can't simply take into account the positions of the components. I have a computer core at the very front of the whole mess so I could just use that if it weren't for the asteroid partially blocking some of my engine plumes.