Why are jet engines' and rocket engines' tails shaped differently, one gets narrower as the other gets wider?

Asked by: Godzilla

Answer

The jet engine and the rocket engine have different nozzles because they are harnessing two different kinds of propulsion. The rocket engine is expelling burning gases out of the back, and these gases are still expanding when they are ejected. The open cone shaped nozzle allows the outward expansion of the hot gas to exert pressure on the rocket as it flies. The jet engine, by contrast, does all of it's burning inside the engine. With the exception of afterburner, nothing is burning once it leaves the tailpipe of the jet. The constricting nozzles on the rear of a jet engine have much the same effect as putting a conical nozzle on a fireman's hose - forcing the same volume through a smaller area must increase it's speed, and therefore, allows the power of the engine to be more focused.
Answered by: Frank DiBonaventuro, B.S., Physics, The Citadel. Air Force Officer