?? theoretical efficiency of hot-air balloon ....

?? "dropping passengers / freight off ... " .... ??? ....

?? slight subtlety here about gravitational potential energy vs "gravitational-hydrostatic potential energy" ???? .... ??? is that going to screw things up ??? .... ???? ..... ?? definition of potential energy difference here coicncide with definition of .... ???? .....

??? trying to keep "hydrostatic" aspect straight from "thermodynamic" aspect ?? .... ??? despite .... ???? .... ?? "interaction" ... ??? ....

?? hmmm, this whole issue pretty much avoided by taking the freight to be infinite density ... so their potential energy is purely gravitational instead of hydrostatic ... ??? ....

?? "re-(?? ...)use any waste heat" .... ???? ....

?? "reversibility" .... ??? ... ?? "piston" .... ???? .... ??? ....

?? "one person's freight (or in extreme case one person) as another person's ballast" ... ?? ....

?? limit where starting (not ending .....) volume of balloon approaches zero ..... ??? ......