Add mass to gass (similar to inventory mass)
Years ago when planets were introduced to the game there was a staging rocket trailer taking off from planet.
I understand that some realism is sacrified for gameplay purposes such as hydrogen tanks have higher capacity than their size: Large H2 tank has 421875L volume but holds 15000000L gas.
My suggestion is to add mass to gass similar to inventory mass. There's also a mod but it looks like outdated: https://steamcommunity.com/sharedfiles/filedetails/?id=2048290970