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In this static test of XCOR Aerospace’s Lynx rocket engine, Mach diamonds (shown at the top of the frame) are visible in the rocket exhaust. The distinctive pattern is a result of the over- or under-expansion of the exhaust jet with respect to the ambient air; in other words, the gases exiting the rocket are either too high or too low in pressure relative to the surrounding air. A series of shock waves and expansion fans forms in the exhaust jet until the pressure is equalized to ambient. It is these compressions and expansions that form the diamond pattern. (Video credit: XCOR Aerospace)
