how to keep fuel in plane balanced?

[jeancallisti]

So I have this space plane where the fuel is stored in several tanks (some closer to the front, some closer to the back)

As the plane flies, its center of mass changes as the fuel gets consumed (and disappears from the tanks).

Therefore I have this commn issue that after some time flying, the plane gets totally unbalanced.

I can fix that manually mid-flight by using the Alt+click trick on the tanks and transferring the fuel to the front of the plane.

But I was hoping to achieve that automatically by putting a duct fuel line from the back tanks to the front tanks. It doesn't work. No matter what I try, the front tanks are drained first. I tried separating the tanks using some dummy part inbetween (separator, RCS tank, structural girder), and keeping them connected with the said duct fuel line pointing from the rear tank to the front tank, but no matter what, the frontmost tank gets emptied first and the lower (back) tank doesn't start draining fuel until the upper (front) tank is empty.

How do I fix that?

Edited August 16, 2016 by jeancallisti

[eddiew]

I think you're making this harder on yourself than it should be... jet engines automatically pull fuel evenly from all available tanks, unless you have a physical obstruction through which they can't draw it. A simple engine-tank-tank-cockpit plane will empty both tanks equally. Messing with fuel lines and obstructing blocks is very 0.90 and probably causing more pain than good  

Maybe you could provide some screenshots? It's possible that your particular set up is confusing the fuel routing.

Also remember that even if your tanks drain evenly, this doesn't guarantee that your CoM won't change during flight, unless all the tanks are equally distributed around the CoM. You can of course check this in the SPH by draining all the tanks and seeing if the indicator wanders  

[jeancallisti]

I'm not using a jet engine I'm using a rocket engine :-( That's for the later phase of the SSTO liftoff

Edited August 16, 2016 by jeancallisti

[Red Iron Crown]

Rocket engines in general draw from one tank at a time, it won't draw from multiple tanks that weren't placed with symmetry. For rocket planes it works best if all fuel tanks are located at CoM fore and aft with dry mass split evenly ahead of and behind them. 

You can also try transferring fuel manually during the ascent to keep tanks somewhat balanced, this can be both tricky and tedious, though.

KSP 1.2 is getting closer, which will apparently use a unified fuel flow model for both rockets and jets, where they'll both draw evenly from all tanks in a stage.

[jeancallisti]

My issue is that the main weight to counterbalance is the rear engines' weight, which is huge (this is for heavy payload). there is no dry mass to put in front to balance that mass. That's why I was hoping that the fuel tank could be of some help.

[Red Iron Crown]

Not going to be able to give more than general advice without a pic of the craft, preferably in the SPH with the CoM and CoL markers on. 

[eddiew]

41 minutes ago, jeancallisti said:

I'm not using a jet engine I'm using a rocket engine :-( That's for the later phase of the SSTO liftoff

Ah, apologies, misunderstood.

I have seen the 1x1 metal panels being used to block fuel flow in the past, with a line being run from the front tank to the engine. This would result in the rear tank being "furthest away" from the engine and drained first. You could also try putting the engine on a decoupler and ensuring that crossfeed is disabled (don't stage it by accident!) which might look neater.

Pics will help us though  

Edited August 16, 2016 by eddiew

[jeancallisti]

4 minutes ago, eddiew said:

I have seen the 1x1 metal panels being used to block fuel flow in the past, with a line being run from the front tank to the engine.

 

 

Thanks, that might come handy.

I would love to send pics but I don't have an account in any online image hosting. I think I'll manage though, thanks to your help guys.

 

[LN400]

1 minute ago, jeancallisti said:

I would love to send pics but I don't have an account in any online image hosting. I think I'll manage though, thanks to your help guys.

There are free/no account hosts for images online. Tinypic is just one of them.

[eddiew]

8 minutes ago, jeancallisti said:

I would love to send pics but I don't have an account in any online image hosting. I think I'll manage though, thanks to your help guys.

Imgur is completely free and well supported by this forum - just sign up to it    http://imgur.com/

[Rocket In My Pocket]

This mod (which I highly recommend) not only corrects some errors in the stock COL indicator but also provides a very handy chart comparing wet mass aerodynamic stability to dry mass aerodynamic stability in any given craft. Should help with trouble shooting your problem.

[dire]

I have had good luck with simply locking the fuel outflow on tanks until I am ready to spend them. When you right click the tank it'll have a little green arrow next to LFO and Oxidizer. If you click that, the tank's fuel will be blocked until you manually go in and unblock it during flight.

[Spricigo]

TAC fuel balancer may help

[foamyesque]

There's a neat little trick you can do to get pseudo-balanced fuel draw with a rocket engine. Have a central core stack with your engine on it. With symmetry, place a pair (or more, but a pair works) of fuel tanks on the CoM of the core stack. Extend out from fuel tank evenly, front and back, until you have as much fuel as you want. Then run a fuel line from the root tank to the core stack.

The result will be that the rocket engine will draw fuel from the ends of the outboard fuel tanks (front and back) symmetrically, working its way inward to the root tanks. This produces zero CoM movement.

Edited August 17, 2016 by foamyesque

[Red Iron Crown]

20 minutes ago, foamyesque said:

<Good advice snipped> This produces zero CoM movement.

Careful, this is only true if a) the core stack has no fuel or b) the core stack has only one tank placed exactly at the CoM. Other than that, neat trick!

[foamyesque]

5 minutes ago, Red Iron Crown said:

Careful, this is only true if a) the core stack has no fuel or b) the core stack has only one tank placed exactly at the CoM. Other than that, neat trick!

 

It's true even if the core stack does have fuel, right up until that fuel starts being drawn (i.e., the outboard tanks are empty); at that point, standard CoM-balancing and shift-reduction techniques apply. For the most part it can be worked around.

Edited August 17, 2016 by foamyesque

[Red Iron Crown]

1 minute ago, foamyesque said:

 

It's true even if the core stack does have fuel, right up until that fuel starts being drawn (i.e., the outboard tanks are empty); at that point, standard CoM-balancing and shift-reduction techniques apply. For the most part it can be worked around.

Ah yes, I see what you mean. Thanks for clarifying.