Mach Diamonds

Rocket engines often feature a distinctive pattern of diamonds in their exhaust. These shock diamonds, also known as Mach diamonds, are formed as result of a pressure imbalance between the exhaust and the surrounding air. Because the exhaust gases are moving at supersonic speeds, changing their pressure requires a shock wave (to increase pressure) or an expansion fan (to decrease the pressure). The diamonds are a series of both shock waves and expansion fans that gradually change the exhaust’s pressure until it matches that of the surrounding air. This effect is not always visible to the naked eye, though. We see the glowing diamonds as a result of ignition of excess fuel in the exhaust. As neat as they are to see, visible shock diamonds are actually an indication of inefficiencies in the rocket: first because the exhaust is over- or under-pressurized, and, second, because combustion inside the engine is incomplete. (Photo credit: Swiss Propulsion Laboratory)

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