Originally posted by ZippyOgiveHead
Would an AeroTech composite motor benefit from having a scale like shroud aft of the nozzle?
Motor nozzles are already designed with throats and (very) short lengths of divergent nozzle. Attempts to add more external nozzle expansion chamber length to a hobby-quality motor would not likely be successful, and could significantly decrease performance.
The motors available to us have a wide range of operating conditions, from the initial thrust spike through the rest of the burn. Even the 'sustainer' burn rarely occurs at constant temperature, constant mass flow, and constant pressure. The same problem with optimization of bell nozzles for NASA-sized launch vehicles applies here: even if you get the nozzle shape right for one part of the burn, it will be wrong (and probably badly wrong) for all the rest.
You also run into the proverbial Krushnik effect in your model or sport rocket. The thrust is killed by swirling flow outside the motor nozzle, inside your added bell nozzle. You end up with less instead of more.
Even if our motors burned at a nominal constant thrust, we still have the problem of sputtering and spitting combustion debris through the nozzle; this problem is proportionally bigger for our motors than for the 'big' ones. These events cause momentary pressure pulses that further complicate nozzle design.
Finally, if you really do somehow achieve a useful containment of exhaust flow, you will have grabbed hold of some pretty hot, high pressure flow. What structural material will you use to build your bell nozzle? (a cardboard model of a nozzle won't get you far)
These big bell nozzles can certainly be modeled for static display, but there are many reasons why they are removed for flight. Anyway, from 500 feet below, where we are watching your rocket, no one can see the nozzle anyway?