# Why are rocket engines shaped like a bell rather than a gun barrel? [duplicate]

When blasting matter out of a spacecraft engine for thrust, it seems like you'd want as much velocity in one direction as possible. Why do spacecraft engines (e.g., Apollo CSM) have big bell-shaped covers on them, rather than a long gun-like barrel to help direct all the exhaust?

• This is a good question, but it has good answers already. In short, expanding the exhaust reduces the transverse velocity of the molecules (by cooling the gas) and making the motion more parallel.
– uhoh
Feb 7, 2021 at 16:12
• There's more to it than that, @uhoh. The flare in a de Laval thruster increases the exhaust velocity and thereby increases thrust. This increase in velocity as the gas moves further toward the exit goes hand in hand with a decrease in the local temperature and pressure of the gas. Feb 8, 2021 at 5:24
• @DavidHammen I'm not sure if that's more than reducing transverse velocity, or a different and perhaps more specific way to describe the same thing. I see the nozzle as converting a roughly isotropic distribution of velocity vectors of the molecules into a more anisotropic distribution. Ignoring post-combustion chemical reactions, we have to conserve $\Sigma v^2$ of all the molecules, so the nozzle simply makes them all move in more of the same direction. That can be called "cooling".
– uhoh
Feb 8, 2021 at 5:29
• @DavidHammen Balloon of ideal gas pops in vacuo - K.E. per atom unchanged, velocities become locally correlated? I've accepted my own temporary answer but a better one would be most welcomed!
– uhoh
Feb 8, 2021 at 5:31
• @uhoh There's a lot more to it than that. A converging-diverging nozzle wouldn't make sense if the flow was subsonic throughout. With a purely subsonic flow you would want a purely converging nozzle. It's the weird and counterintuitive behaviors of supersonic flow that makes the supersonic gas accelerate after the throat of a nozzle. The flow is subsonic before the throat, Mach 1 at the throat, and supersonic after the throat. Feb 8, 2021 at 6:58