Monopropellant (RCS) use from the lower stages UP

[E. F. Kranz]

I'm thankful for the ability now to turn RCS tanks (and nozzles!) off. That has been a big advancement.

But sometimes, my tanks are hidden... behind or "inside" other parts, so it's impractical to shut them off.

But I got to thinking....

I can't really think of a typical case where you would want to use the monoprop in an upper stage, while some is still available on a lower stage.

Now certainly, if that's what you wanted to do, it should be (and is) possible.

But wouldn't it be a safe assumption for the system to automatically prioritize fuel use from the lowest stages?

[Beeman]

Yeah, I always thought it a bit odd that monopropellant drains the way it does. I'd much rather have it pulling fuel from the lowest possible stages first /then/ moving up the rocket as you go.

[Captain Sierra]

Monopropellant is a universal resource, meaning it doesn't obey flow rules like rocket fuel. Kethane and electricity are also universal resources. The way these are coded means they draw from all sources evenly. I agree, this way is not the most convenient and bottom up would be better but unfortunately, there isn't much you can do because of the nature of how the game handles different resource types.

[dlmarti]

Think of mono propellant as a tank of compressed gas (which is pretty much what it is). Once the tanks are connected, you drain from all of the tanks equally.

[Beeman]

It'll probably change once they rework the way fuel tanks are handled. I mean, if we're going to be able to fill tanks with whatever fuel we like in whatever proportions we like, I would expect them all to flow the same way. More or less, anyways.

[Diche Bach]

Until something changes in the game, wouldn't simply turning off the tanks on the upper stages that you want to conserve while the vessel is in the pre-launch work? I have done something like this a few times during mid-ascent orbital maneuvers in order to use up the gas in my bottom stage and preserve it for the upper stages.

[E. F. Kranz]

Until something changes in the game, wouldn't simply turning off the tanks on the upper stages that you want to conserve while the vessel is in the pre-launch work?

That does work, assuming you have easy access to your monoprop tanks. But I can't be the only one who tucks those into places where they become impractical to manually shut on/off.

Monopropellant tanks should be assignable in Action Groups, to be toggled in symmetry.

Either that or, an $ALT-RIGHT_CLICK on a tank in flight mode should result in the selection of that tank and all symmetried tanks. (This would have utility beyond the monoprop discussion.)

[Supernovy]

The game used to do this, the only problem being it wasn't so good at it. In 0.18 I believe they changed it to the current system of all vessel flow.

What would be nice is being able to disable fuel flow with action groups.

[Captain Sierra]

There are multiple suggestion threads for stuff like that. I found one earlier that had pretty much everything I want to see done with fuel flow. Action group tank toggle really oughta get bumped up on the list of things to do. It can't be that hard to do so I think it's worth the devs' time to listen to all the asking for it and just take a few hours and put it together.

[Diche Bach]

I very much agree that not being able to easily get access to parts that are obscured is a general problem, not just the tanks. But yes, having the system by default take from the bottom first and then the top after bottom empty would be better. Even better would be a system that allowed the user to choose from say three options:

1. Even draw from all tanks

2. Draw from lowest tanks first

3. Draw from highest tanks first