Inconsistent displayed dV

[OJT]

So Imma get straight to the point. I am doing a Jool 5 Mission. I am currently on a refueling session near Pol. I also have another ship of the same configuration sitting in VAB, where I can play with fuel levels and see what dV I get. However, dV shown in VAB and dV shown in flight differ, sometimes quite substantially. I even had in flight dV to be double of the VAB dV once. Obviously this causes issues because I can't be sure how much do I need to refuel my ship. Before you ask, I do have vacuum setting on in VAB, and KER isn't of much help because it outright doesn't show the dV I have. I tried to play around in VAB with the ship. I switched up the engines on my landers with ballast weights incase those mess up the dV numbers, I checked that crossfeed on docking ports is off, and still numbers are different. Any fixes for this? Or do I just have to deal with it and take more conservative dV value of the two (the VAB value in this case). Because at this point I don't even know which dV number is the correct one

Screenshots:

[jimmymcgoochie]

Delta-V readouts are easily confused when you have engines in multiple stages but no decouplers, as in the case of large interplanetary ships- the calculations don’t know that you’re planning to undock stuff.

Try disabling fuel feed from all the tanks on your landers, leaving only the main ship’s tanks active (click the little green triangles beside their fuel bars to turn them to red crosses), this removes the contents of those tanks from the equations and should give you a more accurate delta-V number.

[OJT]

7 hours ago, jimmymcgoochie said:

Delta-V readouts are easily confused when you have engines in multiple stages but no decouplers, as in the case of large interplanetary ships- the calculations don’t know that you’re planning to undock stuff.

Try disabling fuel feed from all the tanks on your landers, leaving only the main ship’s tanks active (click the little green triangles beside their fuel bars to turn them to red crosses), this removes the contents of those tanks from the equations and should give you a more accurate delta-V number.

This did help. dV values now match both in VAB and in flight.

Unfortunately it also means that I need to do many more refuel runs, but that's more of a personal problem lmao. Appreciate the help

[king of nowhere]

i always found that, when dealing with complex motherships with landers, it's a lot easier to just run the manual calculation using the rocket equation