Uploaded March 2017 | Updated September 2026, 2 weeks ago

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Ever wonder why rocket nozzles have an hourglass shape, or why fighter jets use something called a converging-diverging nozzle? This video goes through one of the most fundamental concepts in compressible flow, and shows you why you need to first shrink down the area, and then expand the area to get supersonic flow.

===== NOTES =====
► When I talk about the choked flow, I didn't make it clear that you can increase the choked mass flow rate through the nozzle by increasing the stagnation pressure upstream of the nozzle. The mass flow rate will not increase only if you keep the same geometry and upstream stagnation conditions. That is, you can't increase the mass flow rate when the flow is choked by only decreasing the throat area and/or decreasing the back pressure.
► The flow will be sonic at the throat for isentropic flow (see the assumptions used in the derivation). This is not the case for real flows.

===== RELATED VIDEOS =====
→ 1D Mass Conservation
goo.gl/Wo8mcV

→ 1D Momentum Conservation
goo.gl/YJypbz

→ 1D Mass Conservation in Differential Form (Blog Post)
goo.gl/Q4ac2t

→ Speed of Sound
goo.gl/HPtH16

→ Sonic State
goo.gl/eou3aC

===== REFERENCES =====

► Notes by Matt MacLean
► Modern Compressible Flow, Anderson
► Elements of Gasdynamics, Liepmann and Roshko
► Gas Dynamics, Zucrow and Hoffman

===== THUMBNAIL PHOTO CREDIT =====
By NASA [Public domain], via Wikimedia Commons