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All pictures of de Laval nozzles I've seen have an hourglass shape where the convergent section is shorter than the divergent section. Is this necessary to attain supersonic exhaust velocities, or would a nozzle that is symmetric about its choke point also have supersonic exhaust?

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The downstream side of the nozzle is much more important to maintaining the efficiency of the nozzle by controlling expansion wave. It also influences the uniformity of the flow exiting the nozzle.

Symmetry would be perfectly fine, but you'd end up making the converging section bigger than it needs to be.

Here's a bit more about the design of the nozzle geometry (At the very end of the chapter).

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  • $\begingroup$ So just to check my understanding, you're saying that a symmetric nozzle would also create supersonic exhaust but it would be overexpanded and therefore less efficient? $\endgroup$
    – chbaker0
    Commented Dec 6, 2014 at 17:10
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    $\begingroup$ Symmetry itself isn't the problem. The expansion process mostly doesn't care about the shape upstream of the throat (as long as it's reasonably smooth). Mirror a good expansion section and you'll still have a good nozzle. Mirror a bad expansion section and you'll have a bad nozzle. The reason to make short converging sections is just to save space/weight/cost etc. because you can do it without hurting the flow (much). $\endgroup$ Commented Dec 7, 2014 at 4:16

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