How do I stop a spaceplane from drawing fuel from the wrong tanks?

[Phanixis]

Ok, here is the situation. I have designed the following spaceplane:

The spaceplane is carrying a Mun lander on the top. The spaceplane is supposed to fly out of the atmosphere, disconnect from the Mun lander, and re-enter atmosphere while the Mun lander burns to enter orbit. The problem is, the spaceplane takes its rocket fuel from the tanks on the Mun lander before using its own rocket fuel. This is despite the fact that the Mun Lander is connected only by a radial decoupler and a pair of struts.

How do I stop my spaceplane from siphoning fuel from the Mun lander?

Thanks for the help.

[cybersol]

Right click the tanks then click the green arrow at the end of the fuel bars. That will turn it to red and stop it from taking fuel from there. You will inevitably forget to turn it back to green later, and wonder why you engine is not working.

[Claw]

Cybersol is correct... Unfortunately this is now how jet engines feed now. Even worse, if you have a probe or something attached to a decoupler, it's targeted first for fuel consumption.

It was an intentional change, to "help" with keeping the Center of Mass balanced. Unfortunately, the selected way to implement this isn't very good for this (at all).

Cheers,

~Claw

[Ferdoni]

Even worse, if you have a probe or something attached to a decoupler, it's targeted first for fuel consumption.

~Claw

How is this possible since there is no fuel crossfeed through a decoupler or separator?

And the answer on the question is very simple, dont attach the tanks of the lander and the plane but put a strucural part in between.

If that doesn work, put a fuel duct from the plane to the lander.

Edited June 6, 2015 by Ferdoni

[NathanKell]

Because STAGE_PRIORITY_FLOW doesn't respect crossfeed rules.

[JoCRaM]

Change the lander to the root part - then the main plane becomes the "expendable" stage and it's fuel is used first.

[Keras]

I would really like it if we could choose if jet engines consume fuel by stage or the usual rocket fuel flow. The stage consumtion is great for pure planes but not so much for SSTOs that also carry a payload.

[Sharpy]

You can still transfer fuel back into the lander while in the orbit. Right-click lander's fuel tanks, alt-rightclick plane's fuel tanks, select "out" on the plane's tanks and they will fill the lander's tanks.

[McPoisoned]

You could use the TAC fuel Balancer Mod, it would allow you to constantly pump the fuel back into the Mun rocket.

[Xannari Ferrows]

Because STAGE_PRIORITY_FLOW doesn't respect crossfeed rules.

Isn't liquid fuel STAGE_PRIORITY_SEARCH?

[Chaos_Klaus]

Change the lander to the root part - then the main plane becomes the "expendable" stage and it's fuel is used first.

Wow. This is so counter intuitive. I would not expect fuel flow to change while I change the root part.

[Murph]

Isn't liquid fuel STAGE_PRIORITY_SEARCH?

Not certain about the exact parameter names, without going off to check them, but it's no longer based on fuel type alone. Air breathing engines behave differently to non air breathing engines.

Personally I don't see it as much of a big deal. It's really quick and easy to just turn off tanks on payload, then turn them back on again before deployment; or do a fuel transfer into the payload before deployment. Sure, it's one more thing to do, but not difficult to do and not difficult to remember to do it (once you get used to it).

Was the change needed? Don't know.

Was it a good idea? Maybe, maybe not, don't know.

Was it better the old way? Not sure, the old way wasn't perfect either.

Is it perfect? Probably not.

Is it a big deal? Absolutely not.

Does it really stop you doing anything? Not that I can see.

Does it really restrict craft designs? Not that I can see, just different tricks needed.

Could it be better in the future? Probably.

N.B. Pretty much every single "how to fix it" suggestion will almost certainly have an equivalent downside for some other use case. This is almost certainly one of those situations where it's actually impossible to get it perfect for everyone. Getting it to work for 80% of normal people, with normal simple use cases, is probably about as good as it gets. In that regard, the current implementation really isn't so bad. It works just fine for simple/normal cases, and there's reasonable tricks/workarounds for complex/unusual cases. The current setup does seem to me like it's probably an improvement over jet engines draining forward tanks first in classic style planes.

[Mic_n]

Haven't checked it out myself, but would you be able to do it with fuel lines feeding in?

Personally I do the "click the 'do not use this resource' button" method, but I'm curious now. It's an easy one to forget. As has been mentioned, you can also manually transfer fuel back into it before separating, but if your lifter is suborbital then you may not have a lot of time to do that. Remember, if you want to maintain that fuel balance as well (ie transfer from symmetrical side tanks on your plane back to one on your lander), you can select multiple 'feeder' tanks as well as the target, then just hit "in" on the lander tank, it'll draw from the other tanks equally.

- - - Updated - - -

I understand the change.. not only for fuel balance but for the odd way fuel did actually flow some times.. trying to work out why a particular engine couldn't get fuel from a particular tank could get a bit complicated on spaceplanes where the tanks weren't all strapped together in a nice neat column and having to run fuel lines all over the place to get it moving the way you wanted...

But yeah, it does make payloads a little awkward.

It'd be nice to have an option on separators to prevent that jet crossfeed though.

[NathanKell]

LF and Ox do default to STACK_PRIORITY_SEARCH (which respects crossfeed breaks), but as Murph says engines can override flow mode now, and jets do.

[Red Iron Crown]

It appears the RAPIER uses STAGE_PRIORITY_FLOW even in closed cycle mode:

Root part is the pod, if it was using STACK_PRIORITY_SEARCH the decoupler's lack of crossfeed would prevent drawing from the upper tank.