This is kinda dumb question .I got 5psi pressure at outlet of the nozzle from open motor software. Is that possible because its less than atmospheric pressure 14.7 psi? Am i missing out something?

  • $\begingroup$ Related: Mach Diamonds $\endgroup$
    – SF.
    Aug 31, 2020 at 17:02

1 Answer 1


Sure! That's called "overexpansion" because the flow is expanded too much to match the ambient pressure.

enter image description here

ME 239: Rocket Propulsion: Over- and Under-expanded Nozzles and Nozzle Configurations

It has a negative effect on the thrust shown in this equation: $$Thrust = \dot{m}V_e +(p_e-p_0)A_e$$

if exit pressure ($p_e$) is less than ambient ($p_0$) the 2nd term in the thrust equation is negative.

As shown in the bottom right of the picture, things can get even worse if the pressure difference is large enough.

  • 2
    $\begingroup$ I guess it depends on what you mean by inefficient. The usual figure of merit for engines is specific impulse. Since overexpansion reduces the thrust, it will reduce the specific impulse. To me, that makes it less efficient since you are burning the same amount of fuel and getting less thrust. $\endgroup$ Aug 31, 2020 at 16:30
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    $\begingroup$ @Auberron: typically good engines will be overexpanded after ignition, and underexpanded around flameout, giving them optimal expansion (and performance) throughout most of the flight altitudes as air pressure drops. $\endgroup$
    – SF.
    Aug 31, 2020 at 16:59
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    $\begingroup$ @Auberron: Depends on how deeply you overexpand - I can't give you the exact metrics, but in general you should be maintaining near optimal expansion for most of the flight, so if you're still overexpanded at flameout altitude, that definitely needs tweaking. $\endgroup$
    – SF.
    Aug 31, 2020 at 17:11
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    $\begingroup$ Some engines intended for vacuum-only operation have radiatively cooled nozzles, so the nozzle skirt can get rather thin. In underexpansion the interior pressure is higher than the exterior, so the nozzle behaves like a pressure vessel: the stress in the nozzle material is tensional, so the material thickness doesn't have to be sized for prevention of buckling. If you try to test such an engine under significant exterior air pressure the nozzle overexpands the flow, putting the skirt in compression, and buckling failure can occur. $\endgroup$ Aug 31, 2020 at 20:13
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    $\begingroup$ @TomSpilker agree, the shuttle OMS engines had a altitude limit of > 70K feet; below that their radiation-cooled nozzles could collapse. $\endgroup$ Aug 31, 2020 at 21:51

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