Thanks for the replies everyone! I'm at work right now, but i've started plugging away at figuring out Tsiolkovsky's rocket equation.... It's hurting my noggin! @purpletarget, that's how I usually build my rockets (with help from MechJeb to check my TWR and Dv stats) - Payload first, and then build the lifter underneath as needed until I get the desired Dv and TWR. I'm more specifically trying to figure out what the practical lifting capacity of my lifter would be, so that I can start creating sub-assemblies and just picking the right lifter for the given payload instead of rebuilding a lifter for every launch... @Th3F3aR, thanks for your slight simplification of the rocket equation. It's really helped me to start understanding it a bit more... I think I'm now on the right track to getting there. Just have to work out the Isp thing. When it comes to g0, is that relative to which celestial body you're taking off from? so, for example, on Kerbin it would be 9.81 (m/s), whereas on Minmus it would be 0.491m/s? Also, are the Isp values quoted in-game specific impulse in seconds, or specific impulse in meters per second? @capi3101, my configuration is as follows: 2x Kerbodyne S3-14400 tanks stacked 1x Kerbodyne KR-2L Advanced engine (2500kN thrust) Outboard on 2x radial decouplers: 4x Rockomax Jumbo-64 Tanks (2 per side) 2x Rockomax X200-16 Tanks (1 per side) 2x Mainsails (1500kN Thrust Each, 3000kN together) Total Maximum thrust is 5500kN It can get to a 75km orbit without having to stage the outer engines (there's a spot of fuel left in them once it gets to 75km orbit). I hope that helps you! Once again, thanks for all the help all!