Fuel drawn from all tanks

[milosh]

Hello,

it's been a while since I played the game but I'm pretty sure that something profound was changed: jet engines draw fuel from my rocket tanks which kinda sucks big time. On a typical SSTO I'd have some rocket fuel tanks attached to a rocket engine and some jet fuel tanks with whipsplash jet engines attached. I calculate the fuel for the rockets so I will have enough D/V to get into orbit once I'm in flame out height of the jet engines, around 22-24km. But when I disable the jet engines now I find my rocket tanks only slightly over half full with no way to reach a high orbit around at least 130k km, the jet engine draw fuel evenly from all my tanks as if I'd have fuel links between the tanks. This makes no sense to me, if I have a jet engine attached to a jet tank I want it to only draw fuel from that tank, anyone knows if and how I can achieve that? Thanks

Edited October 6, 2015 by milosh

[Red Iron Crown]

This is a new behavior introduced with 1.0, the idea is that the CoM changes less when draining and so makes for a more stable plane. Opinions are definitely mixed on this feature; personally I find it helpful sometimes and harmful in others.

If you'd like to revert to the previous behavior, edit the configs for the basic jet, turbojet, and Rapier and remove the line "resourceFlowMode = STAGE_PRIORITY_FLOW" wherever you see it (once for each of the two jets, three times for the Rapier).

[Sampak]

I have a similar issue with my space plane also drawing liquid fuel from my probe in the cargo bay, which is attached with a small decoupler...!!! I thought decouplers did not allow fuel flow... yet when I reach orbit with my spaceplanes the first few times messing around, I decoupled my satellite only to find the poor thing was drained of liquid fuel with lots of Oxidizer left over for.. explosions LOL.

As for the rocket tanks being drained, yes, you'll have to manually "disable" them by clicking the the little green arrows next to Liquid fuel value in the right click menu for each Rocket fuel tank. OR you can simply pump fuel from where ever you need back into the rocket tanks, by using the ALT + left click on all involved (or 2 at a time) tanks and balance out your fuel as needed.

Generally though, if your jet engine can sip fuel from rocket tanks, your rockets *should* be able to take fuel from the jet tanks automatically as well, UNLESS there is a bug (or feature") that I'm unaware of preventing this.

[Red Iron Crown]

Generally though, if your jet engine can sip fuel from rocket tanks, your rockets *should* be able to take fuel from the jet tanks automatically as well, UNLESS there is a bug (or feature") that I'm unaware of preventing this.

That's not so, only the Rapier and Puff engines use the same fuel flow rules as jet engines; all other rocket engines use STACK_PRIORITY_SEARCH which honors no crossfeed parts and fuel lines.

[Mastikator]

Fuel ducts lets your rockets draw from all tanks. Use of fuel ducts basically solves this problem.

[milosh]

Thank you all for clarifying this, I wasn't sure anymore if my mind plays tricks on me and this was always the case. I see the point somewhat about the shifting CoM but it never affected my SSTOs severaly because once the jet fuel is half drained I'm in high alt with little lift anyway and switching to rockets. Rebalancing fuel during rocket ascend is thus not possible because the weight of my plane would shift to one side which does affect ascend. Tempering with the cfg would be a possibility but I'd rather first try to work with this new "feature", maybe use rocket fuel tanks all around even for the jet engines as I did in some earlier SSTOs with reduced oxydizier. But I sure wished there'd be a box to tick in the editor to enable or disable this behaviour.

[FlyingPete]

I built a TSTO plane with detachable side pods for extra fuel/thrust. I just ran fuel lines from the side pods to the main orbiter, which seems to drain the side pods' fuel first, leaving the orbiter's tanks full when they're detached.

[Padishar]

The flow mode used for jets groups all the tanks according to how many decouplers there are between the tank and the root part of the vessel and then draws evenly from the ones "furthest away". This allows drop tanks to be made without using fuel lines (in fact, fuel lines have no effect on jet engines anymore, they work just like monoprop now) but it doesn't really help with draining the tanks on the payload. One thing you can do is to set the root part of the vessel to be a part of the payload, then it will first drain from the drop tanks which are two decouplers away from the root and then will drain from the plane's tanks which are one decoupler away.

Unfortunately, there is a bug in the editor that can cause headaches for this. When you change the root part the "decoupler count" is not recalculated for the parts in the rocket so if you change root and immediately launch you wont see any difference in jet fuel flow. The count is only recalculated when a tank is attached to the vessel so you need to detach all the fuel tanks from the new root part and reattach them. Choosing a sensible root part usually means you can detach all but the root part in one go which makes this very easy.

It also doesn't help with jets draining fuel from tanks that you intend for the rockets, e.g. if you built a plane in 0.90 that had jet engine nacelles complete with fuel mounted on the wings and a rocket engine and fuel on the main fuselage then the jets would have only drawn from the wing nacelles but now they will draw from the fuselage too. This results in less fuel for the rocket because it can't pull the fuel from the wings due to the fuel flow rules for rockets. You will have to either change the design so none of the fuel is where the rocket can't get at it or, as Mastikator said, add fuel lines so the rocket can access the inaccessible fuel.

[tSpecR]

I still find this really annoying. I just want my jet engines to use my wing mounted tanks only and leave my main fuselage rocket tanks alone. I understand how it makes spaceplanes easier to handle but surely it wouldnt be hard to add an option as to which tanks my jet engines use or I think it should just use fuel evenly from parts that are in the same cross fuel circuit and not use any at all from parts that arent part of the cross fueled section. Basically so I can isolate certain tanks from being used at all without having to disable them altogether. Sometimes I want my rockets and jets firing at the same time without jets syphoning off my rocket fuel so disabling the tank isnt an option. I dont see why I should need to use a workaround when there is a crossfuel option there.

Edited May 15, 2016 by tSpecR

[AeroGav]

#signed

This "feature" is driving me crazy as well.

The rocket engine "staged" drain mode causes the centre of gravity of your plane to shift aft as it drains the front tanks first.  But the rocket engines are nevertheless too dumb to drain from the wings etc.

Please for the love of god make this a tweakable setting in the SPH/VAB,  each engine can be set up to drain like a jet or a rocket.It's like the devs never even considered you might want to build a spaceplane with anything other than RAPIER engines.

Quote

 

Fuel ducts lets your rockets draw from all tanks. Use of fuel ducts basically solves this problem.

 

 

Pete, Mastik -  the drag from Ducts is currently set to an excessive value.  On a spaceplane you're going to have little jet fuel tanks all over the place,  big S strakes (100 fuel),  ncs adapters (80 fuel),  big s delta wings (300 each) - hooking all of those up is going to give it the aerodynamics of a barn door.   Probably doesn't help that the stock wing parts are small so you often have multiple sets of wing on the same ship.

 

With Ducts - max altitude 44,058.

Without ducts - max altitude 67,486

Also, ducts don't drain tanks proportionately according to their size.   A jet engine will pull fuel 3 times faster from a tank that's three times larger, so that both the large and small tank reach 50% together.   Ducts won't, so the small tank will empty after time N  and the large tank will empty after time = N x 3.  This too can shift your CG around.

Like I say, this "feature" of the game is a great source of artificial difficulty when trying to build something for a "no mods" game challenge or put a craft on Kerbal X that anyone can use without needing to install a mod , transfer fuel around manually (isn't flying a spaceplane hard enough, if hand flying i need both hands for the flight controls pretty much the whole way up), or become backflip-happy the longer you run the rockets (ditto!).

To the OP - Milosh, recommend a mod like GPOSpeedFuelPump.     What this does is give every tank two tweakables in the assembly building.  One is "Pump - off/on".   The other is "tank priority level".   With the pump set to OFF, the game reverts to stock behaviour.   With the pump ON  , the tank contents are automatically transferred to other tanks on the vessel which were set to a higher priority level, as space becomes available.  So you can set the rocket tanks to have the highest priority and the wing tanks to pump automatically into them to keep them full.

Other fuel tank mods you might like - Tanklock (lets you use action groups to configure the locking of groups of tanks ),  JettisonFuel,  and ModularFuelTanks (lets you swap contents of stock tanks - eg. the Ft800 tank can be swapped from holding 360LF 440OX to  800LF 0 OX or any fuel /oxidizer ratio you like )

Even if the mods sort out the plumbing,  I find these kind of Liquid Fuel + Oxidizer space planes difficult to design because the two engines use different resources.    If you don't bring enough jet fuel, you might not reach max airbreathing speed/height before transitioning to rocket power.  Worse, your rocket engines may quit with leftover oxidizer still in the tanks on no LF remaining.   Or you bring too much jet fuel and it's just dead weight when in orbit.

That's why I like building spaceplanes with NERV as their only closed cycle engine.  One fuel type for all engines on the craft, that i can spend as i see fit.

https://kerbalx.com/AeroGav/Astrojet-Citation