Another "rocket doesn't track" problem.

[EmanonP]

I'm using a stock lifter from my subassembly mod, that has been used to put a few landers/mun base parts into orbit. Everything it has lifted has been symmetrically designed so as to

distribute weight evenly. Have never had an issue with control before this design.

I'm trying to get this "lander/forklift" to the Mun to move base parts around, and here is where my troubles begin.

It tracks just fine on launch (either with mecjeb on and ascent autopilot on, or mecjeb off and Sas turned on. However, as soon as the 1st stage sheds, it begins a turn towards the side the upper tank is on...as if

the weight of that fuel tank is enough to pull it off line.

If I swap the upper tank 180 on the lander (did this just to see how it would fly) it tracks just fine, no turning etc.

I've checked all my fuel feeds, and such, and everything is connected and draining evenly. If you look, my center of gravity is pretty well dead center, and you can't see it, but the center of thrust is centered as well.

Anyone see something I'm missing? Can the weight of the upper tank be heavy enough to overcome the control of my fins? If this is the case, why does it track just fine if I put the upper tank 180 from its planned location.

Wouldn't the lifter still start to turn towards that extra weight?

Thanks for any input. Going to try to attach pics...one shows the payload and tank in question, 2nd just shows layout of lifter, and 3rd showing CG of rocket.

http://img.photobucket.com/albums/v402/emanonpettis/Kerbal/screenshot7.png' alt='screenshot7.png'>

Edited May 11, 2013 by EmanonP

[EmanonP]

Oh, I've also checked to see if any damage is done to the remaining stages after the 1st one separates, and nope...it just confirms separation...no collisions with other tanks, engines etc.

[Atanar]

If you look, my center of gravity is pretty well dead center[...]

Well, it isn't. The parts you have hanging on the one side are definitly heavier than to the other side. The center of mass ball just isn't accurate enough because he also considers all the mass of the heavy tanks.

[omegamatrixhh]

If you want to balance your ship you'll have to add the masses of the individual pieces, but like Atanar said, it's pretty apparent that your fuel tank is tipping your rocket over in flight. The gimbals in the first stage, or even just the thrust, are preventing the top heavy lander from toppling til that stage drops. You even tried removing that tank to see what happens, and it worked, so you should probably redesign or at the very least make it symmetrical.

[EmanonP]

Does this mean all payloads need to be pretty much symmetrical weight wise? I can redesign and align the tanks easily enough, but I'm wondering about future endeavors as well.

[kahlzun]

Just add something to the otherside that you can decouple.

Alternatively, lots of RCS.

[sal_vager]

Actually I think this is due to drag, the tanks mass means it has more drag (the KSP drag model is strange)

If I remember correctly, when the tank is the right way round it has the lowest drag, every degree off to the side though and the drag goes up till it reaches maximum (backwards)

That'd explain why it's fine one way and crud the other.

[EmanonP]

Just add something to the otherside that you can decouple.

Alternatively, lots of RCS.

Okay...both sound like good solutions. Bugs me to have to add weight that I'm just going to throw away, and or add weight in the form of RCS and the fuel for them, but I guess everything has

it's trade offs. Thanks all for the replies.

[omegamatrixhh]

Okay...both sound like good solutions. Bugs me to have to add weight that I'm just going to throw away, and or add weight in the form of RCS and the fuel for them, but I guess everything has

it's trade offs. Thanks all for the replies.

If you have to add the extra weight anyway, why not just give it some utility? Whenever I have a counter balance issue I'll toss a probe or satellite or something on so I don't feel like I'm wasting my effort. There's never enough probes.

[SciMan]

well, you might have to throw away the weight, but that doesn't mean it has to be "dead weight" to the rest of your space program!

if you have a significantly asymmetrical rocket, you could use a combination of fuel tanks for most of the mass, then add a probe core, rtg, and a docking port to get a fuel stash. Also, you could add a LV-1 engine to get it close enough to your other stuff to get a tug out to it to suck the fuel out of it.