Fall 2023 Best of Web – Patrick Watson

Fall 2023 Best of Web – Patrick Watson

Mach Diamonds are a complex flow phenomena where high ambient pressure forces an over expanded plume of a rocket engine (or jet engine) back in on itself. The ambient pressure squeezes the supersonic plume into a localization, where the pressure will be higher, and then expand outward again and so on to form a series of mach diamonds. Mach diamonds are most apparent when testing or flying a rocket engine near sea level when the rocket engine itself is designed to be most efficient at higher altitudes or in the vacuum of space. Mach diamonds are a sign of inefficiency at the expense of thrust. Launcher Space’s E2 rocket engine (above) displays this flow characteristic when tested at the NASA Stennis Space Center in Mississippi.

AeroSpaceWeb.org Diagram Showing Shock Wave Formation

If the nozzle of a rocket engine that is designed for the vacuum of space is tested at atmospheric pressure, the over expansion becomes more severe. The over expanded flow can separate from the hot gas walls, further reducing efficiency, and can cause damage to the nozzle in the form of transient loading. A good first stage rocket engine will be designed to be over expanded at sea level, with no flow separation at steady state, and be under expanded in space. A vacuum optimized version of the engine will utilize a larger expansion ratio (bigger nozzle) and would be flown on the second stage of the rocket.


Kraus, J. (2022, May). Launcher engine-2 [May 2022]. John Kraus. https://www.johnkrausphotos.com/Galleries/Space/Launcher-Engine-2-May-2022/i-qSV4BZD/A

“Ask US – Nozzle Overexpansion & Underexpansion.” Aerospaceweb.Org | Ask Us – Nozzle Overexpansion & Underexpansion, aerospaceweb.org/question/propulsion/q0220.shtml. Accessed 6 Sept. 2023.
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1 Comment. Leave new

  • Kenny Olavarria
    Sep 10, 2023 20:13

    First prize – I’ve never seen such a translucent kerolox flame in my life. You can even see the orange-hot nozzle exit through the exhaust plume!
    This translucency also helps give more prominence to the beautiful Mach diamonds here, which are often hidden in other kerolox engines.
    I love how much more chaotic and “dirty” this appears compared to other propellant mixtures – it makes obvious the true turbulent and chaotic nature behind rocket engines.

    Also – good old E1 stand. I saw one of these engines test fire earlier this summer :)


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