Rocket performance
Copyright (c) 1995 by Tracy Ratcliff <tratclif@oucsace.cs.ohiou.edu>
   
   
   Detailing rocket performance means introducing some physics.
   
   Fuel use -- the fuel use in pounds per second is given by:
   
   wf = F / ISP
   
   where wf is weight flow, F is the thrust in pounds, and ISP is
   specific impulse in seconds. ISP is defined as ve/g, exhaust velocity
   divided by one earth gravity. ISP is an artifact of the
   foot/pounds/second system of measurement. As you can see here, it
   simplifies fuel-use calculations in English measures; people in the
   civilized world would use the following to get _mass_ flow in
   kilograms:
   
   mf = F / ve
   
   The ISP is a characteristic of the fuel used and to lesser extent the
   design of the engine. While it can be calculated, it's usually easier
   to look it up in a table of ISPs and rocket fuels.
   
   Space performance -- performance for space-based reaction drives is
   "delta vee", the change in velocity caused by the drive. Delta vee is
   given by:
   
   dv = c * ln(m0/m1)
   
   where c is the exhaust velocity of the reaction drive, ln() is the
   natural logarithm, m0 is the mass of the vehicle before the drive is
   started, and m1 is m0 minus the mass of the fuel used. Since it's a
   ratio either true mass in kilograms or Vehicles "mass" in pounds will
   work for m0 and m1.
   
   This is the "free-space" delta vee, with losses due to gravitational
   fields ignored. In free space, the delta vee depends only on the fuel
   burned, not how it was burned. It is the same whether the fuel is
   quickly all at once, or slowly over a long period, or in several off
   and on cycles. However, in most cases you'd find in an RPG,
   gravitational effects would be significant (in planetary orbit, within
   a few AU of a sun), and this is a simplification for game play.