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.
PHOTO CREDIT: John Kraus
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.