The concept of varying the nozzle of jet streams is very simple. You may wish to ask yourself why space rockets have fixed nozzles of the shape most often seen....that convergent-divergent thingy. It absolutely has everything to do with speed. what do you think would happen if the exhaust is exiting the nozzle at mach 1 while the aircraft flies at mach 1? what about wanting to accelerate? there is a point of diminishing returns and just as you ask for more power from your equipment, you have to keep an eye on its vital stats (like the egt) so you don't toast things.
Perhaps this equation may be of help:
An approximate equation for calculating the net thrust of a jet engine is:
Fnet = m(vjfe - va )
m = intake mass flow
vjfe = fully expanded jet velocity (in the exhaust plume)
va = aircraft flight velocity
"While the mÂ·vjfe term represents the gross thrust of the nozzle, the mÂ·va term represents the ram drag of the intake. Most types of jet engine have an air intake, which provides the bulk of the gas exiting the exhaust. There is, however, a penalty for picking this air up and this is known as the ram drag. Conventional rocket motors, however, do not have an air intake, the oxidizer being carried within the airframe. Consequently, rocket motors do not have ram drag; the gross thrust of the nozzle is the net thrust of the engine. Consequently, the thrust characteristics of a rocket motor are completely different from that of an air breathing jet engine"
Cullled from: http://en.wikipedia.org/wiki/Jet_engine
If the Nozzle Pressure Ratio (Nozzle Entry Pressure/Ambient Pressure) is very high, to maximize thrust it may be worthwhile, despite the additional weight, to fit a convergent-divergent (de Laval) nozzle. As the name suggests, initially this type of nozzle is convergent, but beyond the throat (smallest flow area), the flow area starts to increase to form the divergent portion. The expansion to atmospheric pressure and supersonic gas velocity continues downstream of the throat, whereas in a convergent nozzle the expansion beyond sonic velocity occurs externally, in the exhaust plume. The former process is more efficient.
So you see, that's why your f-15 traveling at supersonic speed needs a convergent-divergent nozzle. for model purposes, it's USELESS!!....well it'd look cool....and perhaps increase performance by a smidgen but at what cost to weight?
afterburning is also cool and purely has flameholders, spray nozzles and a variable nozzle for matching to engine etc to add addition energy to the hot stream of gases...., i'd leave it out of nozzle design for now (simpler i think). i think it'd work on model airplanes tho....