# Can exit pressure of nozzle be less than atmospheric pressure?

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?

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

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.

• 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. Aug 31 '20 at 16:30
• @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.
– SF.
Aug 31 '20 at 16:59
• @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.
– SF.
Aug 31 '20 at 17:11
• Doubt it all you want, it's a fact. Sep 1 '20 at 2:56
• See page 3-18 here gandalfddi.z19.web.core.windows.net/Shuttle/… Sep 1 '20 at 3:02