Hello MJR,
One way to quickly get your feet wet with multistage rockets, is to simply start filling in values in the ini file you'll have to make if you use Multistage2.dll by Vinka. Be sure to read the documentation for it, it will tell you what each of the values must be.
One way to gain familiarity with this process, and hopefully develop some intuition about real rockets, is to look up values for existing rocket designs, which sure enough, tend to work in Orbiter almost as they do in real life, provided you have good, correct data.
CAUTION: Simply feeding in numbers from one source is usually not sufficient. I find that the article writers for places like Wikipedia and other such online sources don't always "check" their work, and you sometimes see numbers that simply don't add up.
One way to check the numbers on your own is to use the following key relationship between the important variables for any rocket stage:
Isp = (F * Delta T) / (g * Delta M)
Where
Isp is specific impulse (given in seconds). This is a measure of how good or efficient the engine and fuel combination are.
g is acceleration of gravity (let's use 9.86 m/s/s )
Delta T is the burn time, which is how long the stage takes to burn its fuel mass with engines at max. thrust.
Delta M is the amount of mass that gets used during Delta T. In other words, the fuel mass.
Finally, an example. Let's start with the config information needed by Multistage2.dll, for stage 1 of the as yet unflown Falcon 9 rocket by SpaceX (
http://www.spacex.com/falcon9.php)
[Stage_1]
MESHNAME="directory\meshname" (fill these in with whatever you named your mesh for stage 1)
OFF=(0,0,15) (You'll probably want Z to equal the center of your stage mesh. For instance, for a 30 meter-long stage 1, Z will be about 15)
HEIGHT=30
DIAMETER=3.6
THRUST=4400000 (This can be quite tricky, as this isn't modeled quite right in multistage2. Mass flow is almost constant, but Isp and thrust are not, because the rocket rises from sea level to a near vacuum. More about this later)
EMPTYMASS=20000 (This is a guess. I assume [guess] the stage 1 mass to be 200,000 kg, and further assume 10% is "empty mass," which consists of the fuel tanks, engines and anything else that is not fuel. Usually, about 90% of a stage is fuel.)
FUELMASS=180000 (see above)
BURNTIME=116 (This was derived using guessed-at average thrust and Isp values...averaging between sea level and vacuum values...and the other data above, using that equation, above. This might be close to the real value, but don't know yet.)
SPEED=(0,0,-2) (Optional value...speed that stage 1 separates from stage 2)
ROT_SPEED=(0.05,0.0) (Again, optional--you can omit this if you want. 1.0 is one radian, or about 57 degrees rotation, per second, along the given axis.)
ENG_1=(1,1,-15)
...
ENG_9=(-1,-1,-15) (These are the x,y,z coordinates, relative to the center point of your stage 1 mesh, for the "flame" animations to be placed. Do this well and it will look like 9 Merlin engines are burning correctly.)
ENG_DIAMETER=0.5 (Diameter of the exhaust texture)
ENG_TEX=Exhaust2 (Engine exhaust texture)
ENG_PSTREAM1=something (Particle stream. Never mind for now)
Cheers-
MT
