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?

  • $\begingroup$ 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. $\endgroup$
    – uhoh
    Feb 7, 2021 at 16:12
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    $\begingroup$ 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. $\endgroup$ Feb 8, 2021 at 5:24
  • $\begingroup$ @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". $\endgroup$
    – uhoh
    Feb 8, 2021 at 5:29
  • $\begingroup$ @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! $\endgroup$
    – uhoh
    Feb 8, 2021 at 5:31
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    $\begingroup$ @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. $\endgroup$ Feb 8, 2021 at 6:58