Boiloff? Or: where did my fuel go???

[lugge]

So I  managed to land some kerbals on Duna.
Also, my transfer stage had an unmanned probe attached which is going to Ike to get some science.

Well, at least, that was the idea...

My kerbals are on Duna surface now, happy gathering science.
My probe is still on low Duna orbit, waiting for command.

Now, I switched to my probe, wanted to go for Ike... when I saw my probe had almost no fuel left.

So I checked a few quicksaves.

I found a savefile where my transfer stage, my lander stage and my probe are still assembled in Duna orbit, you can see it here:

The part on top is, well, my probe. And as you can see, my probe is already missing some fuel.

There are three possibilities:
- transfer staged burned my probe's fuel. Thats not possible because of the landing can between my probe and the transfer and lander engines.
- my probe accidentally activated its engines. Well, unlikely. I checked the engines, they are deactivated. Didn't activate them.
- Boiloff? Kraken?

Anyone encountered a similar issue?

Oh, I checked some older saves when my craft was still in LKO. My probe had all its fuel left. So, no way I accidently reduced the fuel in the VAB.

Edited June 7, 2016 by lugge

[Violent Jeb]

Your fuel will transfer "around" the command pod no problem. It's option 1; Your rendezvous stage took the fuel from your probe

You want to click "disable crossfeed" on your decoupler or docking port.

[lugge]

But there's also the Terrier in my lander stage?

Does my fuel also transfer "around" an engine?

[bewing]

Yup, everything crossfeeds fuel except for heatshields, unless you disable it manually on a decoupler.

[lugge]

Wow, wow... I though I know a lot about KSP, playing for over a year now...
But this is a "feature" I never encountered...

OK, last question:

the fuel is drained from the big tank of the transfer stage and the small and the round tanks of the probe. Simultanously. Why?

When I just stack a couple of tanks, only one at a time is drained. Why is my probe drained even if the transfer stage is not empty yet?

[cantab]

Normally the decouplers will block fuel flow by default. But there can be situations that doesn't work as expected, perhaps if you constructed the ship in an order other than "top down". And if you used either jet engines, the Rapier (in either mode), or Vernor thrusters, they all consume fuel in a different way to normal rocket engines.

[lugge]

Many thanks guys :-)

[DancesWithSquirrels]

I've hit this a few times, and I find it most useful to disable the fuel tanks for the probes in the Assembly Building - just re-enable them when you launch the probe, and you're good to go. 

[Pecan]

19 hours ago, lugge said:

... Why is my probe drained even if the transfer stage is not empty yet?

By default rocket engines take fuel from the furthest available tank first so yours should take all the probe fuel before any of the transfer stage; complex engine use aside as noted by cantab.