Communal Fuel

[DChurchill]

Hypothetical:

I have one center engine with it's own fuel tank. I have two more outboard engines which are parallel the first. So basically I have 3 engines, each with their own tank. Not a problem if the engines and tanks are identical. Say I want to have a large center tank and two auxiliary tanks, but I don't want to jettison them, but I want the fuel to burn equally from all tanks.

Do I use 2 fuel lines pointed both ways?

What I'm trying to get to is more fuel for 3 engines equally distributed as it's burned so that they would all run out at the same time, regardless of the tank/engine configuration.

[Razor235]

Don't put any fuel lines. There's no way to solve the problem you propose AFAIK.

[nhnifong]

Pssshhh, there's got to be a way. For one, you could route two lines from the center tank to a docking port somewhere isolated with a decoupler. then route lines from that decoupler to the side tanks. with an action group you can toggle the fuel flow crossfeed of that decoupler. If, in flight you toggle the decoupler at the right rate, it will alternate between draining and refilling the side tanks.

[UmbralRaptor]

It's doable provided either the center or outboard engines are not directly connected to the tanks (say, separated by decouplers). Then it's possible to run fuel lines from the unconnected tanks to the connected ones, and from the connected tanks to the unconnected engines. I ran into similar issues when designing an Atlas clone.

[CaptainKipard]

If you download TAC fuel balancer you can keep your fuel balanced in terms of percentages. Even as you burn, equal percentages of fuel will be drawn from ALL tanks. Same for oxidizer of course but you have to set both resources separately on all the engines.

[numerobis]

Hypothetical:

I have one center engine with it's own fuel tank. I have two more outboard engines which are parallel the first. So basically I have 3 engines, each with their own tank. Not a problem if the engines and tanks are identical. Say I want to have a large center tank and two auxiliary tanks, but I don't want to jettison them, but I want the fuel to burn equally from all tanks.

Do I use 2 fuel lines pointed both ways?

What I'm trying to get to is more fuel for 3 engines equally distributed as it's burned so that they would all run out at the same time, regardless of the tank/engine configuration.

If you cross-link fuel lines, you won't draw evenly from all three tanks, but all three engines will have fuel until all the fuel is exhausted. The left engine will draw from the right tank, then the middle, then the left. The right engine will be symmetric. The middle engine will draw equally from both sides, then the middle.

So what you'll see is the side tanks get consumed first, and equally; then the middle.

[bsalis]

If this was an orbiter rather than an ascent stage, then another possible workable solution would be to...

- connect fuel lines from the large inner tank to the small outer ones.

- manually transfer fuel from the smaller outer tanks to the inner ones between burns.

It's not ideal but it's simple in terms of construction.

[sal_vager]

Fuel lines both ways does work, I have used that setup on a few rockets that had mixed engines on a stage but I wanted them all to keep running till every drop of fuel was depleted.

Your inner or outer tank may run out early depending on the engines you use, but they'll just draw from the remaining fuel on the stage.

[jagfour]

It's doable provided either the center or outboard engines are not directly connected to the tanks (say, separated by decouplers). Then it's possible to run fuel lines from the unconnected tanks to the connected ones, and from the connected tanks to the unconnected engines. I ran into similar issues when designing an Atlas clone.

This is how I connect engines to tanks if I want to collect all fuel and share it to all connected engines.

I usually seperate satelite engines from their tanks. Connect satelite tanks to the centre tank with fuel lines. The centre tank engines can be directly connected to it, but the satelite engines are fed with fuel lines from the centre tank. The satelite tanks empty first but the engines remain fed from the centre tank and it allows you to eject the empty satelite tanks and engines when or if desired.

Just as an option, I call the centre tank the collector tank in the case above. You could use more than one collector tank, but if you want equal sharing, only connect engines via collector tanks.

[Black_Tomcat]

You could simply connect the outboard fuel tanks to the center engine and the center fuel tank to the outboard engines (or the other way around).

I use this technique on many of my design and it always works.