Once you've designed your payload (or any combination of upper stages), and plan your manoeuvres, you'll know how much mass you need to get through a minimum deltav and how fast you'd like to be able to do it (to avoid missing flight windows).
To work out your engine and fuel requirements, you'll either need to do a lot of trial and error or solve the ideal rocket equation with some educated guesses. This calculator will do all that for you.
The calculator is available at http://fommil.github.io/kerbal
To use, clone and run like so
sbt "run-main Solve 1200 10 50 false Large"
(You'll need sbt and a Java Runtime)
Input parameters being:
- minimum deltav
- payload mass
- minimum acceleration
- payload size
Results will be ordered by the minimum initial mass of the engine stage.
e.g. the above returns
Rockomax "Mainsail" with 6.9t (86%) in a Rockomax X200-16 [a = 62.8, dv = 1200, cost = 8747, mass = 13.9t] Rockomax "Mainsail" with 7.0t (87%) in a Rockomax X200-16 [a = 62.6, dv = 1212, cost = 8755, mass = 14.0t] Rockomax "Mainsail" with 7.0t (88%) in a Rockomax X200-16 [a = 62.4, dv = 1224, cost = 8762, mass = 14.0t] Rockomax "Mainsail" with 7.1t (89%) in a Rockomax X200-16 [a = 62.2, dv = 1235, cost = 8769, mass = 14.1t] ... S3 KS-25x4 Cluster with 33.8t (47%) in a Kerbodyne S3-14400 [a = 50.3, dv = 2683, cost = 51697, mass = 53.6t]