How to build droptanks for a jet plane?

[Gooru]

I´d like to try a spaceplane with droptanks, but there are some problems.

1) Jets drain fuel from all tanks, but i want them to drain the droptanks as fast as possible.

2) Where/how should i place the tanks? it´s hard to do it symetrical. i´m running out of ideas before i really start it seams.

[Gaarst]

AFAIK it is not possible to do such droptanks in stock KSP as jet fuels drain evenly from all tanks, regardless of staging, or fuel lines.

According to comments below, jet engines drain tanks attached to decouplers first.

If you use a mod to manage fuel drain, I'd suggest putting the droptanks under the wings, or under the fuselage itself if you have enough clearance at takeoff for doing so. Although placing them depends on the structure of your plane: shape and position of wings, CoM...

Here you can see a droptank under a P51's wing:

Or here under a MiG 29's wings:

Here under a Saab Viggen's fuselage:

Edited October 17, 2015 by Gaarst

[Alshain]

Yeah, as Gaarst said the new fuel flow rules have kinda broken this one little design option. You can still do it but you have to constantly move fuel around. The big problem is it is kind of half-way done in stock. They changed the fuel flow rules but didn't improve the interface.

You could make this work by shutting off tanks, but that is a pain and would work better if they would let us shut off tanks on an action group. There are mods to let you do this, if you want to go down that route, but now I can't find them. I know they are there, someone help me out on this.

[Red Iron Crown]

Placing a decoupler between the droptanks and the main body of the plane should cause them to drain first without touching the fuel in the plane itself.

[Alshain]

Placing a decoupler between the droptanks and the main body of the plane should cause them to drain first without touching the fuel in the plane itself.

I don't think it does, unless they changed something. I tried this right after the fuel flow change but when using a single engine (or single set of engines) it doesn't work that way. The problem is the tanks aren't part of staging, only the engines.

[Yukon0009]

Placing a decoupler between the droptanks and the main body of the plane should cause them to drain first without touching the fuel in the plane itself.

Yup, I can confirm this, works with RAPIERs on closed-cycle mode as well.

[Alshain]

Well I just tried it again and it does in fact respect the decouplers now. Good to know.

[Red Iron Crown]

For jet fuel flow purposes, stage is determined by the number of decouplers between the tank and root part; tanks of the same stage are drained evenly. It does work, check out this rocket I made using airbreathers and staging. The tanks with the turboramjets supply all engines while they have fuel in them, those tanks have doubled decouplers to make them a different stage than the rapier tanks. Fuel lines are there for the LV-N, if there was an airbreather in the center they would not be necessary.

Basically, it works like monopropellant, which also permits staging through decoupler count (silly example).

Edit: Double ninja'd!

[Majorjim!]

Placing a decoupler between the droptanks and the main body of the plane should cause them to drain first without touching the fuel in the plane itself.

But you disagreed with me on this exact point... So it does work with Jets.

[Red Iron Crown]

But you disagreed with me on this exact point... So it does work with Jets.

I did? When? It works this way as long as the root part is in the main body of the plane somewhere.

It gets tricky when you have a payload that does not contain the root part (common for spaceplanes with a payload in a bay). Or multiple payloads, I have yet to find a way to prevent a plane from draining fuel from multiple LFO-powered missiles short of disabling their tanks.

For clarity, the engines that use this flow mode:

Basic Jet Engine

Turboramjet

Rapier (both open and closed cycle)

O-10 Monoprop engine

Ion engine

[Alshain]

For jet fuel flow purposes, stage is determined by the number of decouplers between the tank and root part; tanks of the same stage are drained evenly. It does work, check out this rocket I made using airbreathers and staging. The tanks with the turboramjets supply all engines while they have fuel in them, those tanks have doubled decouplers to make them a different stage than the rapier tanks. Fuel lines are there for the LV-N, if there was an airbreather in the center they would not be necessary.

Basically, it works like monopropellant, which also permits staging through decoupler count (silly example).

Edit: Double ninja'd!

Your test was slightly different than mine or the OP's drop tank scenario. A plane would not have engines on the drop tank. The result ended the same, but logistically the fuel flow on your rocket and a drop tank design is backward from each other.

Engine -> Tank -> Decoupler -> Drop Tank (w/ Fuel Line to main)

Engine -> Drop Tank -> Decoupler -> Main Tank

The concern is that in the first scenario, the fuel lines make all the tanks the same and the engine would always drain from all the tanks in it's domain, which included the center stack. My testing however did show that not to be the case (I'm almost certain that was not always so, I suspect a change between 1.0 and 1.0.4 to correct this).

Edited October 17, 2015 by Alshain

[Red Iron Crown]

The Rapiers are drawing from the LF tanks with the turboramjets, so for the Rapiers those tanks are essentially drop tanks. I'm not at my ksp computer at the moment, if I have time this evening I'll put together a more explicit example craft.

It does not matter where the engines are at all, attached to the tank or the main body. All that matters is the decoupler count between root part and tank.

[Alshain]

The Rapiers are drawing from the LF tanks with the turboramjets, so for the Rapiers those tanks are essentially drop tanks. I'm not at my ksp computer at the moment, if I have time this evening I'll put together a more explicit example craft.

It does not matter where the engines are at all, attached to the tank or the main body. All that matters is the decoupler count between root part and tank.

I put mine in the last post. I think we are in agreement though, no need to go to the trouble.

[Majorjim!]

I did? When? It works this way as long as the root part is in the main body of the plane somewhere.

It gets tricky when you have a payload that does not contain the root part (common for spaceplanes with a payload in a bay). Or multiple payloads, I have yet to find a way to prevent a plane from draining fuel from multiple LFO-powered missiles short of disabling their tanks.

For clarity, the engines that use this flow mode:

Basic Jet Engine

Turboramjet

Rapier (both open and closed cycle)

O-10 Monoprop engine

Ion engine

I could be wrong here but you said that you cannot isolate tanks with decouplers for jets. I may have previously misspoken though. When I wrote about isolating tanks in the other thread by using the node twice I meant by putting a decoupler there.

As I said I could be wrong, I am not up to speed on Jets post 1.0+ versions.

[Red Iron Crown]

You can't isolate them, that is accurate. The only way to keep a jet (or other engine using these flow rules) from eventually draining a tank is to disable it.

You can choose which order they drain in, though, through strategic use of decouplers. If you look at the example craft I linked earlier, you'll see that I have the fuel tanks' context menus open during ascent; the reason is that all engines will keep happily burning away once that stage has no fuel left so I have to keep an eye on it to know when to stage.

Hope that makes some sense.

[pTrevTrevs]

I suppose you could always switch off the main fuel tanks in the plane, which would cause the drop tanks to be emptied first, but you would have to remember to switch them back on shortly before dropping the tanks. This wouldn't be practical for a jet with lots of fuel tanks, but if you only have one or two tanks (most good plane designers can get by with one or two anyway), it ought to be a workable solution.

For placement, you have a couple of different options. Under the wings, under the fuselage, or on the wing tips are the three most obvious. The problem that may arise with placing them on the wing tips is getting the fuel to flow correctly.

[Claw]

I could be wrong here but you said that you cannot isolate tanks with decouplers for jets.

Yeah, there's some confusions still about Jet fuel flow.

So anything behind decouplers is drained first. However, you can't stop it from draining. (As Red is saying.) So the real (quite annoying) problem is that jets happily drain the payload's fuel. And in fact, if the payload is attached via decoupler, it will drain it FIRST. This is optimal for drop tanks, and a complete pain to deal with for payloads.

Cheers,

~Claw

[DancesWithSquirrels]

If you are using the payload as the root (for example, pulling a subassembly SSTO out as a launcher), just re-root to the SSTO cockpit before launch, and your drop tanks should work properly.

[satnet]

If you're not opposed to using mods you could use Tank Lock, lock all your non-drop tanks, and add an action group to decouple and unlock. It isn't pretty, but it should work.

[Gooru]

Did 1.0.5 change the plane fuel drain?

My droptank shuttle now drains the tanks in the main hull before draining the droptanks.

[Red Iron Crown]

Did 1.0.5 change the plane fuel drain?

My droptank shuttle now drains the tanks in the main hull before draining the droptanks.

Yes! Airbreathers now honor no-crossfeed parts. You'll need to turn on crossfeed on the decouplers in the right-click menu to get your droptanks to drain.