My rocket leans to one side when I apply a circular fuel supply system!

[pig2000]

I'm building a weird giant rocket these days, trying to put all my engines at the top and the fuel-tanks at the bottom so that I can decouple the fuel-tanks as soon as they are empty.

This is what it look like.

My first prototype of the rocket uses a normal fuel supply system which decouples the first stage(all the Jumbo fuel tanks at the bottom) after all of them are empty and then continues to burn fuel taken from the second stage.

Of course this design made it hard to steer, but adding lots of SAS modules perfectly solved this problem and the first prototype can send my payload into orbit as wanted.

However when I apply a circular fuel supply system(exactly as the method used on the Kerbal X rocket, which takes fuel from only two fuel tanks at a time and then decouple them, so that more fuel can be saved by quickly getting rid of the empty tanks), the rocket starts to lean to one side after launch. Even a huge amount of SAS module can't stop that.

I'm quite sure that the fuel is consumed symmetrically. So I guess that this problem may be caused by the flow of the fuel, which adds extra momentum to the rocket.

But how? I can't explain why the fuel that flows in a circle and then pumped up can make the rocket lean.

Can anybody tell me exactly why did this happen or help me solve the problem? Thanks a lot!

Edited December 30, 2013 by pig2000
Image fix.

[vexx32]

The Kerbal X doesn't use a circular fuel system... does it? At best, a circular fuel flow will simply do nothing. At worst, it's incredibly unpredictable. The algorithm KSP uses to determine where to draw fuel from cannot handle loops of any kind. I don't recall the Kerbal X using a circular fuel flow system... It does asparagus staging, draining the fuel from the outer tanks first by setting lines between them and the inner one, to allow the inner engines to draw from the outer tanks, but... I don't recall it having fuel lines enough to have a circular flow.

Avoid loops with fuel lines. It never ends well.

[pig2000]

Sorry for the confusion of names. I was talking about asparagus staging but I simply didn't know that it is called a asparagus, not a ‘circular supply’. Terribly sorry for that!

And thanks for your advice!! I've checked it again and again and it appeared that the problem is indeed caused by the loop of fuel lines. I really appreciate your help!!

[Mystique]

I'd suggest using nice little mod called RCS Build Aid, it'll show you where is your rocket's CoM and dry CoM and how thrust is applied (and whether it causes unwanted rotation). Also check you CoL relative to CoM, while in atmosphere you may need to use wings with control surfaces to keep it stable.

P.S. Screenshot isn't showing, at least for me.

[Kasuha]

At best, a circular fuel flow will simply do nothing. At worst, it's incredibly unpredictable. The algorithm KSP uses to determine where to draw fuel from cannot handle loops of any kind.

The fuel drawing system is 100% predictable and it can handle fuel cycles just well. The matter is, unless you really understand how it works and know what you are doing, it will most probably draw the fuel differently than how you think it should.

Kerbal X uses asparagus fuel supply with no cycles involved:

Stage 3 -> Stage 2 -> Stage 1 -> Central <- Stage 1 <- Stage 2 <- Stage 3

Of course this design made it hard to steer, but adding lots of SAS modules perfectly solved this problem and the first prototype can send my payload into orbit as wanted.

Putting engines in the front means gimbal will go the wrong way. By adding more SAS modules you're just battling the gimbal with torque. The correct solution is lock gimbal on engines.

I'm quite sure that the fuel is consumed symmetrically. So I guess that this problem may be caused by the flow of the fuel, which adds extra momentum to the rocket.

I'm perfectly sure the fuel flow does not generate any momentum at all. Right-click on your fuel tanks in flight to watch how the fuel is consumed in them. Or click on the Resources button in the upper right and click to the square next to Liquid Fuel line to see all tanks at once. I'm quite sure you'll notice fuel is not consumed symmetrically.

Your image is rather small and unclear but it looks like you have fuel pipes laid this way:

Stage 1 -> Stage 1 -> Stage 1 -> Stage 1 -> Stage 2

Instead of that, put fuel pipes just from each Stage 1 tank right to the Stage 2 tank.

Edited December 30, 2013 by Kasuha