Fuel lines: How and where to use them

[Ivan Ivanovich]

Fuel lines are often misunderstood and even more often used wrongly. And I guess I, too, didn\'t quite get them fully yet. But this guide should give you a first clue about the dos and don\'ts of fuel lines.

How and where to attach them - the basics

Fuel lines are the yellow thingamajigs in the third ledger, first item (always assuming you didn\'t install any add ons). Fuel lines work a bit like struts, i.e. they have two connection points which have to be 'visible' for each other, unobstructed by any other part of your rocket. If there is something in between them, the connection will end at the blocking item.

Unlike struts, fuel lines have a direction. The tiny arrows on it indicate the direction the fuel will flow. Note that the fuel will ONLY flow in this direction. You decide which way the fuel goes by the order in which you attach them. The first tank you attach them to will be the tank FROM which you want the fuel to flow. The second point (where you attach the rubberbanding fuel line) will be the receiving end.

Note also that fuel lines can ONLY flow from fuel tank to fuel tank. You can NOT attach a fuel line to an engine, to a SAS module or to the capsule. Well, you can attach them, but they\'re useless there. As a rule of thumb, make certain that your fuel lines are connected to a tank. RCS tanks DO work as targets, but afaik they\'re not good as sources. If they are target of a fuel line, whatever is below them will receive the fuel. This is the only way I know of how you can power an engine without an engine fuel tank on top of it.

Whether your fuel line 'works' can be seen when the rocket is flying on the staging list to the left. If the fuel line is lit (in blue, like a powered engine or a not-empty fuel tank), it 'works' (i.e. fuel is running through it). If it is gray, it\'s dry and no fuel is flowing through it. This can of course be the case when the 'from' tank has been drained (then it should be gray), but if fuel should flow but it\'s gray, something is wrong and you should reconsider your setup.

First test

Build this rocket:

Capsule (with parachute, if you insist in your Kerbonauts to survive)

stack decoupler

(ASAS if you\'re lazy)

1 fuel tank

Engine

3 radial decouplers around the fuel tank

2 fuel tanks on every radial decoupler

1 engine under every radially attached fuel tank

1 fuel line FROM the lower radially attached tanks TO the tank in the middle.

Rearrange the stages so all engines fire simultanously (but keep the inside tank separate from the outside tanks so you can see what\'s happening) and launch the rocket. You will notice that four engines are firing, but only three tanks are being drained. The reason for this is that the radially attached tanks also fuel the central engine since the fuel lines funnel fuel from the tanks outside to the engine inside. All the six outside tanks will be drained eventually and the three outside engines shut down while the inside tank is still completely full and the inside engine has a full tank to draw from.

Fuel lines are like real estate. What matters is location, location, location

Let\'s alter the setup a bit. Now, move the fuel line to link FROM the UPPER radially attached tanks to the tank in the middle and launch again.

At first, it will look like the first attempt. Three tanks will get drained, four engines are firing. But as soon as the first outside tank is empty, an important difference gets visible: Now all remaining tanks are being drained.

How the magic works

Think of it as a matter of gravity. As long as there are full fuel tanks 'above' the place where the FROM fuel line link is placed, fuel will flow. As far as I can judge, it does not matter whether you place the fuel link on the bottom or the top of a single tank, what matters is the tank\'s position in the stack.

How to profit from it

There are two things that you can benefit from this: First, to drain throwaway tanks quickly and carry as little dead weight as possible, and second, to level out fuel consumption over your engines.

Sucking few tanks dry to leave the rest full

The first idea is to slap a few 'booster' liquid stages radially to the rocket, pump the fuel from them to the inside and throw them away as soon as they\'re dry, sacrificing an engine but keeping all the remaining engines and tanks full. To do this, attach a fuel line from the bottom outside tanks to the inside tanks. You can stack them radially, i.e. if you have three 'layers' of tanks radially attached, you can pump from the outside tanks to the middle tanks and from there to the inside tanks and fuel ALL engines from the outside tanks. What will happen now is that the tanks will drain VERY fast since they have to supply quite a few engines (I once built a setup where 12 engines sucked from a single tank, it was dry in 5 seconds), but you can easily gain a lot of height that way, dump the outside attached fuel legs and still have a fully fueled rocket at your hands with zero dead weight.

Evening out the consumption

The other idea comes into play when you 'suffer' from engines that drain their fuel at different speeds (like the gimballed engine and the more powerful but less controllable one) but want them all to run the whole length. If you simply place, e.g. 4 tanks on top of a gimballed engine and 4 tanks around it on normal engines, the normal ones will have run dry by the time you still have 1/2 fuel tank left for the gimballed one. Now, you could of course just toss the powerful ones and putter about with the thrust from the gimballed engine, but if you\'re not in orbit yet, this might not be an option and you\'d have to toss the half full tank and stage to your next full complement of engines.

Here, the idea is to even out the fuel consumption over the engines. Now, the gimballed engine consumes 7/8th the fuel of the powerhorses, which means that you COULD in theory put 7 fuel tanks on the gimbal engine and 8 on the powerful ones, but since that couldn\'t even remotely take off, it\'s no option.

The easiest way to even this out is to combine it with the first idea. Build this (as the power stage):

3 fuel tanks on top of a gimballed engine.

6 radially attached fuel stacks, 3x3 and 3x4 fuel tanks tall, each with a powerful engine below.

A fuel line from each of the topmost tank of the 4 fuel tanks tall legs to the 3-fuel tall ones. I.e. 3 fuel lines running from each ot the 'tall' stacks to one of the 'short' stacks besdides it. That means that the topmost of the 4-fuel tall stacks gets drained by two engines, from there downwards, every stack gets drained by its own engine.

That way, each 'powerful' engine has 3.5 fuel tanks to draw from while the gimballed engine draws from 3 tanks, resulting in a simultanous (ok, not quite but almost) shutdown.

Now, what does and what does not work

The obvious idea of drawing fuel from the upper stages to 'even out' fuel consumption or 'top off' the fuel hungry powerful engines doesn\'t work for a simple reason: Tanks are drained top to bottom. I.e. you\'d end up draining your upper stages before the lower stages are even being touched. I haven\'t found a way to get around this, sorry. If it\'s possible, feel invited to post it.

You also cannot use fuel lines directly to engines or SAS modules. So a lander consisting of ASAS with an engine below and two tanks attached left and right does NOT work. You CAN attach fuel lines to RCS fuel tanks, though. Hence, my Mun lander consisting of

parachute

capsule

decoupler

ASAS

RCS

engine

2 tanks attached left and right

2 fuel lines from tanks to RCS tank

DOES work.

Fuel DOES pass through a stack decoupler, so upgrading the above setup by adding a decoupler between RCS tank and engine DOES work. But it gets quite heavy.

That also means that you can jettison spent tanks 'above' the ones you\'re using. I.e. if you radially attach a tank stack at the bottommost tank and stack tanks above it, you can, e.g. insert a stack decoupler every 2-3 tanks and toss them. It is definitely NOT recommended, though, for ascent stages where you can\'t simply shut off your engine for a while to toss them, navigate past them at leisure and continue your flight. Consider: They are tossed ahead of you. Now guess where you\'re going in powered flight...

I would recommend getting a decoupler with negative ejection force for something like that, though. Just a warning...

[Awaras]

Note also that fuel lines can ONLY flow from fuel tank to fuel tank. You can NOT attach a fuel line to an engine, to a SAS module or to the capsule. Well, you can attach them, but they\'re useless there. As a rule of thumb, make certain that your fuel lines are connected to a tank. RCS tanks DO work as targets, but afaik they\'re not good as sources. If they are target of a fuel line, whatever is below them will receive the fuel. This is the only way I know of how you can power an engine without an engine fuel tank on top of it.

Actually these kinds of setups work pretty well:

Fuel tank to decoupler:

Fuel tank to engine directly:

Fuel tank to pretty much anything connected to an engine (anything you connect a fuel line to becomes a \'conduit\' for fuel I believe...)

Even this way (note that THIS setup does NOT work without the fuel line):

[Ivan Ivanovich]

Ah yes, forgot decouplers. That works too, but I\'m a fan of RCS tanks. They really help a lot when landing on the Mun

[Kosmo-not]

You can route fuel through almost anything. Example: fuel tank to sas, sas to another sas that\'s on top of an engine.

[Awaras]

Sorry, I just had to...

MWAHAHAHAHAHAHA!!!! IT\'S ALIIIIIIIIIVEEEEEEEEEEE!!!!!!! ??? ??? ??? ??? ??? ??? ???

[Ferrit]

Good gods! It\'s an abomination!!

[rcoobc]

Thanks, guess I\'ve missunderstood how to use these. Hopefully I can get a little more thrust out of the rocket by choosing which order fuel tanks to use.

[noneyabidnis]

I was looking for an overview exactly like this. Wonderful - thanks!!