Can/Should liquid fuel transfer through an engine/decoupler into a lower stage? (not sure if a bug or if I'm missing something)

[Pin]

[PC (Steam) complete edition  - version 1.11.1.3066, with making history (1.11.1) and breaking ground (1.6.1)]

Hi there, 

So firstly - I'm new to the game, so if this is just a total newbie screw-up then absolutely let me know

Bought the game maybe a week ago on Steam - new install, no mods or anything. Playing career mode. Just managed my first Kerbin orbit and was trying to send my first mission to the Mun. Got a little greedy - have the contract for a fly-by, but also one for testing a couple of components in orbit (one Kerbin, one Mun) so I figured I'd try to build big and achieve all in the same mission .

The Rocket (stages are grouped, I have decouplers/chutes split out  to make 7 actual stages in the game) 

• Stage 0 - Command module, Bunch of science  Service bay containing a 48-7S 'Spark' LFE (to test in Mun orbit - I checked on the launchpad and I can open the bay door and click "run test"). Heatshield and a TD-12 Decoupler
• Stage 1 - RT-10 'Hammer' SFB (no fuel - to test in Kerbin orbit, again checked and can run test without fuel), FL-T400 Fuel Tank, LV-909 'Terrier' LFE, TD-12 Decoupler
• Stage 2 - 4x FL-T400 Fuel Tanks, LV-T45 'Swivel' LFE, 2x BACC 'Thumper'. All firing at launch, but then the thumpers were radially-mounted with TT-38K Decouplers so I could drop them after burn-out

So I just flew the mission - felt like everything was going ok. Looked like I had a nice efficient transition into Kerbin orbit and almost enough fuel left in the first stage to transfer to the Mun. I planned my transfer manoeuvre, stated the burn on time, and then just before it completed my fuel ran out so I decoupled the first stage intending to finish off the burn with the Terrier. However, after decoupling I realised I had zero fuel left in the remaining FL-T400 so at this point was just left drifting in space!

This was a huge surprise and not at all what I expected so I reloaded the save to check and, sure enough, prior to starting the manoeuvre when I clicked on the individual fuel tanks the stack of 4 in the firs stage were all showing at 0, and the remaining fuel was being drawn out of the tank in the second stage. This means it was being drawn through both the terrier engine and the decoupler to feed the swivel engine at the bottom of the rocket?

is this right/how it's supposed to work? And if so is there someway I can prevent it? I assumed that any fuel tanks split by engines/decouplers would remain separated. I don't have any kind of fuel lines connected that would allow drainage - literally the only connection  between the two stages is the decoupler. What am I missing?

And if not then should I raise a bug? The reason I haven't is it seems like such a huge thing that I would have seen it elsewhere in the forums/wiki/issues log, but I can't find any similar threads.

Thanks in advance for the help. I'm really stumped with what to do 

Edited March 1, 2021 by Pin
adding version info

[4x4cheesecake]

Welcome to the forum

By default, this is not an intended behavior but it's possible to setup your rocket to crossfeed across multiple stages and maybe you did this by accident.

To clarify: By default, fuel will never be transferred/used by other stages of the rocket, if these are connected with a decoupler or separator. Though, the game allows you to "crossfeed" between stages. This means, the previously described restriction will be lifted and engines are allowed to use fuel from tanks in other stages then their own. To accomplish this, you either have to add a fuel line to connect tanks of multiple stages or by enabling crossfeed in the context menu of the decoupler/separator. While it is unlikely to accidentally add an addition part, you probably enabled crossfeed on the decoupler by accident or without knowing what this will do.

You may want to check your decoupler settings for this. There should be a button which says "enable crossfeed" if it is currently disabled, or "disable crossfeed" if it is currently enabled.

[Liam_The_Great]

When you right click the decoupler, there should be an option to either enable or disable fuel crossfeed. Make sure that it's disabled and this won't happen.

[Pin]

Thanks both - really appreciated. No idea how I managed that but sure enough when I looked at the ship I had enabled crossfeed. I guess I'm probably not the first to make such a mistake!

Absolutely loving the game, this makes me very happy that I don't have to completely rethink my staging strategy.  or rather if/when I do need to rethink it the reason will be my own faulty calculations rather than a technical weirdness

[18Watt]

On 3/1/2021 at 7:42 PM, Pin said:

 I guess I'm probably not the first to make such a mistake!

Yep!  KSP is actually quite complex, there are a lot of settings and sub-menus that can trip you up.  You came to the right place.

There are also a lot of bugs and unintended behavior in the game.  If you get frustrated, it is highly likely that someone else has had the same problem, and someone has figured out a solution or workaround.  Welcome to the forums!