help! the kraken is eating my fuel! (bug)

[king of nowhere]

so, i have this reusable mun lander. when some tourist pay to go to mun, i send a transfer vehichle, dock it to the lander, land, and send back the transfer vehicle with the science data. it significantly reduced the cost a mun missions.

until the last mission, when i had this strange mishap. on top is the lander coupled, on the bottom are the lander and the transfer module separated. as you can see, the masses do not add up.

from the two separate masses, i should now have 19 tons of ship, of which 9 tons dry weight. i calculated deltaV using the rocket equation, and came out with around 2500 m/s, more than enough to land on mun, take off again, and send the transfer module on kerbin intercept.

but, as you can see on the higher picture, it only gives me a nominal 600 m/s. which, granted, is still a bug. i actualy have more, i tried landing and i did land, which i could not do with only 600 m/s. but landing consummed all my fuel, i checked the tanks and i was left with 20%, not enough to take off again. i'm definitely losing fuel here.

i tried to tinker with the craft, and i eventually found the source: the command moduel i used has an in-built decoupler, and the game thinks that i will engage it before using the engines. this explains why it does not see most of my fuel. however, fuel from the lower tanks is still being drained, and still far faster than it should.

what can i do?

(i have half a mind to activate cheats for infinite fuel. it's not like i don't actually have the fuel)

 

EDIT: another piece of the puzzle came into place; i tried to exclude completely the landing module from the manuever, and i discovered that the two engines on the transfer module were not producing any thrust. probably they are too close to the tanks of the lander, so their exhaust just slams on them and the push gets canceled.

after i disabled those two engines, the ship is flying as it should be, consumming the right amount of fuel. granted, it still does not see that fuel; according to the staging sequence, i have no fuel left whatsoever, yet my rocket is working just fine.

Edited July 10, 2020 by king of nowhere

[AHHans]

Well, the stock dV display is known to be easily confused when the situation becomes more complicated. In particular when different engines only have fuel to run for different amounts of time. (Do you have fuel crossfeed enabled on the docking ports?) But it doesn't actually steal your fuel, it is just the dV display that is wrong.

5 hours ago, king of nowhere said:

EDIT: another piece of the puzzle came into place; i tried to exclude completely the landing module from the manuever, and i discovered that the two engines on the transfer module were not producing any thrust. probably they are too close to the tanks of the lander, so their exhaust just slams on them and the push gets canceled.

Yes, that can easily happen with a pull configuration. And unfortunately it isn't displayed anywhere until (if!) the exhaust heats the "offending" part enough that the increased temperature becomes visible.