What is the correct why to calculate dry mass?

[kUSer]

I have been struggling a little recently with KSP. When I do the math my landers have about 5000 Dv, but I tend to run out of fuel way half way back home or even before I have even landed on the surface. After a lot of trials and fails I narrowed my problem down to two things: 1) I am calculating my dry mass wrong and thus think I have more Dv than I actually have. 2) My version of the game is seriously bugged.

The way I always calculate my dry mass of a craft is I just remove the stage and the have the new mass of the craft. E.G:

A craft consisting of these parts:

Stage 2:

1 x Command Pod Mk1

Stage 1:

1 x TR-18A Stack Decoupler

1 x FL-T800 Fuel Tank

1 x LV-T45 "Swivel" Liquid Fuel Engine

This ship has a full mass of 6.89 tons. With the TR - 18A Stack Decoupler, FL-T800 Fuel Tank and LV-T45 "Swivel" Liquid Fuel Engine removed it has a dry mass of: 0.89 tons. We also have an Isp of 320 in a vacuum.

Using the Dv formula: dv = ln(FullMass/DryMass) * Isp * 9.81 we get that our Dv is about 6 424.702, that's plenty to get to the moon and back (if already in a stable 70 KM orbit around Kerbin).

This ship only gets to surface and not back. So what am I doing wrong?

[andrewas]

You're not including the mass of the empty tanks and the engines on each stage.

[kUSer]

So my dry mass is basically my FullMass - WeightOfFuel?

[shynung]

Pretty much, yes.

[kUSer]

Thanks guys.

[LN400]

Not sure if the mass has changed in the latest version (I haven't got around to try it yet) but the numbers I get are

Pod 0.840

TR-18A 0.050

FL-T800 4.5 wet/0.5 dry

T-45 1.5

Total 6.89 wet/2.89 dry

320*9.81*ln(6.89/2.89) = 2727.38 which is considerably less than 6424.70. 2727 sounnds about right for a mun landing w/o return ticket.

EDIT: A bit late.