Regarding boosters having one single liquid fuel engine, such as Delta 4 for example.

Would some sort of annular aerodynamic device, attached to the skirt and placed around the bell nozzle, help reduce or delay overexpansion by compressing a column of air around its exhaust?

This ring would act like a static compressor blade.

As the booster gains altitude and airspeed, would having a localised higher than expected pressure zone around the nozzle decrease overexpansion, and therfore mitigate inherent lowering of efficiency due to this overexpansion?

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  • 1
    $\begingroup$ Gonna guess the drag of the compression ring will always equal or exceed any gains from preventing overexpansion. $\endgroup$ Commented Mar 14, 2023 at 16:06
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    $\begingroup$ The entire acceleration comes from interaction between the exhaust gas and the bell nozzle and combustion chamber (Newton's third law - this is where the two "bodies" interact with mutual, equal, opposite forces.) The exhaust is supersonic so whatever happens to it after it exits the nozzle has zero bearing on the rocket - interactions between air particles travel no faster than speed of sound, so any compression, turbulence, whatever, from the point of view of the rocket is a bygone story. $\endgroup$
    – SF.
    Commented Mar 15, 2023 at 9:59
  • $\begingroup$ @SF this makes sense thanks $\endgroup$
    – user19132
    Commented Mar 15, 2023 at 10:43
  • $\begingroup$ Additionally the thrust of any given engine only increases with altitude, despite underexpansion. You can, of course, achieve better performance with larger area ratios, but not because of the expansion state of the flow, but the larger exit plane. $\endgroup$
    – A McKelvy
    Commented Mar 18, 2023 at 20:57
  • $\begingroup$ So to keep this engine perfectly expanded with your guide veins would definitively reduce the performance by causing the pressure drag to persist rather than drop off at lower ambient pressures. $\endgroup$
    – A McKelvy
    Commented Mar 18, 2023 at 20:59

1 Answer 1


This would just increase drag, as the compressed/redirected air stream only interacts with the exhaust after both have stopped interacting with the vehicle. It may technically delay expansion, but can not produce any additional work from that expansion.

However, this is very close to being an air-augmented rocket, specifically the shrouded or ducted version: https://en.wikipedia.org/wiki/Air-augmented_rocket

The difference is that the latter has additional surfaces downstream to take advantage of the continued expansion of the mixed exhaust+air stream for generating thrust. This can potentially provide a specific impulse benefit and has been applied to long-range air-to-air missiles, but it's not clear it provides a worthwhile benefit for launch vehicles.

  • $\begingroup$ got it, thanks and for the links too $\endgroup$
    – user19132
    Commented Mar 15, 2023 at 10:43

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