Fighting COM movement?

[SpaceToad]

Hey all,

How do you guys fight the change in center of mass from fuel drain? I'm trying not to add tons of extra mass though.

[sal_vager]

Gameplay question moved to Gameplay Questions

Jets shouldn't suffer much from this as they will draw fuel equally per stage, much like RCS does.

If using rockets it's possible to separate fuel tanks with decouplers, and control fuel flow with fuel lines.

Fuel tanks can also be arranged around the CoM so they don't affect its position when they drain.

Though if you can make your craft stable when both full and empty, it won't matter that the CoM moves.

[Gaarst]

Are you talking about planes or rockets ?

For rockets, adding fins at the bottom will make your rocket stable enough to not flip is piloted correctly. For shuttles, you'll have to make sure the CoT vector is aligned with the CoM when dropping the boosters; in the phase with the boosters, simply modifying your shuttle engines should balance the craft.

For planes, the best way is to build your plane so that the CoM moves as least as possible. Jet engines drain fuel evenly from all tanks, so if you approximately even your fuel tanks in front and behind you CoM, it shouldn't move too much. You can also try to add fins/canards to have better control to compensate the CoM shift.

[Superfluous J]

The easy way is to separate your launcher into multiple stages. Usually 2 is enough.

Another trick is to disable the top fuel tanks until the bottom ones have drained. That's a bit harder because you need to use the right-click interface on the (literal) fly.

These are of course general answers, as I don't know what your rocket looks like. Or (as sal_vager implies) if it's a rocket at all

[Clipperride]

Sorry to butt in on this thread, but I have been using 5thHorsemans solution (stopping fuel flow from certain tanks until I want them) and wondered if there is any way to link toggling fuel flow to a hotkey? Or any other way (mod?) to toggle multiple tanks at the same instant?

[cantab]

Depends what sort of mass shift.

For launch rockets the fairing is ideally flush with the stage below it, or at least only slightly wider, and I usually add three or four of the little basic fins at the bottom just to make sure. I'm careful not to deviate too far from prograde during the ascent. As 5thHorseman says having an upper stage can help keep mass up front, but it can also hinder - if the lower stage drops while the rocket is still in noticeable atmosphere it may well flip on the upper stage. In FAR I've had aerodynamic flips at 30 km.

To handle lateral mass shift in orbital spacecraft I use engine clusters. Individually adjusting the thrust limiter on each engine in space lets me balance things out. KER can display "thrust torque", I just tweak the limiters until it's close to zero. My recent space station was able to make orbital manoeuvres quite easily in this way despite having varying ships and modules docked to it.

For RCS I generally try and put the monopropellant tanks at or around the CoM, so monoprop drain doesn't shift things. Fuel drain is harder to deal with though, I'll typically have to think about how much fuel the craft will have when I dock it.

For aircraft I ensure the plane will be stable in pitch with the CoM at its aftmost point, and further seek to minimise fore-aft shift.

[Feradose]

If my CoM seems to move too far to the back, l'll move all my sciency bits, and misc stuff forwards. Cargo bay will be centered on dry CoM as much as possible.

[Superfluous J]

Sorry to butt in on this thread, but I have been using 5thHorsemans solution (stopping fuel flow from certain tanks until I want them) and wondered if there is any way to link toggling fuel flow to a hotkey? Or any other way (mod?) to toggle multiple tanks at the same instant?

My mod of choice is GPO Speed Fuel Pumps. Set the top tank to 0, the next down to 1, then 2, etc, and then turn "pump" on for all of them. Don't pump out of the bottom tank, though, or the engine will sputter because it thinks its tank is empty. Bonus: You can do this in the VAB so it remembers it for every launch.