Shock diamonds: supersonic heat nuggets

An SR-71 Blackbird takes off from the Dryden Flight Research Centre, California, on March 9, 1993. Shock diamonds are visible in its exhaust.
| Photo Credit: NASA

Sometimes when a rocket or a jet takes off, its exhaust has an alternating pattern of light and dark patches (see image). The bright patches in this formation are called shock diamonds, a.k.a. Mach diamonds. Shock diamonds are formed when an engine releases its exhaust into the atmosphere at a supersonic speed.

Just as it leaves the engine, the exhaust can be at a lower pressure than atmospheric pressure at the same altitude. As the exhaust flows out, the atmosphere compresses it until the two pressures are equal. It’s also possible that the exhaust becomes over-compressed, at which point it will expand outward again to drop its pressure. This seesawing process may repeat itself multiple times until the exhaust pressure is close to the atmospheric pressure. This entire process generates waves in the exhaust plume, leading to the formation of shock diamonds.

When the atmospheric pressure bears down on the plume, it causes exhaust that’s diverging outward to bend inward instead, before its pressure causes the exhaust to bend outward again and so on. When it flows inward, the pressure in that portion increases, hiking the temperature there and causing any fuel passing through that area to burn. The combustion creates a bright spot at that location, i.e. a shock diamond. The bending of the exhaust outward and inward produces shock waves that flow through the plume, creating the shock diamond pattern throughout.