The interesting aerial phenomena do not substantially impact the fighter jet’s flight—but pilots must be careful with their use due to their distinctive visual appearance and heat signature.

If you’ve ever seen a fighter jet taking off with afterburners, you may have noticed visible “diamonds,” or bright shock cells, in the exhaust. Most visible at night, or in high thrust settings, shock diamonds are a gorgeous result of fluid dynamics and pressure mismatch.

Also called “mach diamonds” and “shock cells,” and formally referred to as “exhaust shock patterns,” shock diamonds appear as alternating bright and dark bands in an exhaust plume. Caused by supersonic exhaust interacting with ambient air, shock diamonds exist where repeating compression and expansion waves exist.

How Do Aircraft Afterburners Work?

Shock diamonds are common in afterburner settings. “Afterburning” takes place when fuel is injected directly into the engine’s exhaust stream, which re-ignites the flow downstream of the turbine. The result is a massive thrust increase, often totaling between 50 and 70 percent more relative to standard engine operations.

There are certain downsides to afterburner use. As one might imagine, they are astonishingly fuel-inefficient—unsurprising, given that the mechanism involves blasting fuel into the exhaust stream. And in the age of advanced sensors, afterburners are also troublesome because they create a massive heat signature, which can compromise the aircraft’s stealth characteristics. The afterburner makes the exhaust flow hotter and faster, and often leads to supersonic flight. 

What Are Shock Diamonds?

Shock diamonds form within afterburners because of a pressure mismatch. Exhaust exits the nozzle at a pressure that is different from the ambient air. Either the exhaust is “underexpanded,” in which its exhaust pressure is greater than the ambient air, or “overexpanded,” in which it is less.

Most afterburner cases are from underexpanded exhaust. The shock structure forms when exhaust expands rapidly, with the pressure dropping below that of the ambient air. The air compresses flow back inward, creating a shock wave. The process then repeats: expansion, compression, expansion, and so forth, producing oscillating pressure zones. The visual result is a diamond shape; the bright regions are high pressure and high temperature, while the darker regions are lower pressure, cooler, and less luminous. The shape is symmetrical due to nozzle geometry and the pattern is diamond-like during the alternating wave structure. 

The spacing of the diamonds is related to exit pressure ratio, mach number, and nozzle diameter. Naturally, variable geometry nozzles adjust exhaust expansion for the purpose of matching exhaust pressure to the ambient air. If perfect expansion is achieved, then there are no shock diamonds. But in reality, there is always some mismatch—which is why shock diamonds are so common in real-world conditions. Designers aim to minimize the mismatch in pressure. But variable conditions make perfect tuning roughly impossible. 

Altitude effects shock diamonds, too. At high air altitude, the ambient air pressure is lower, which leads to larger, more spaced shock diamonds. At low altitude, the pattern is tighter and more compressed.

Does the Shock Diamond Affect the Plane at All?

The diamond-shaped pattern has minimal effect on the direct aerodynamics of flying the plane. Still, from a tactical perspective, pilots must be wary of the visual signature, the infrared signature, and the increased vulnerability that afterburner and shock diamonds create.

The tradeoff is basically boiled down to maximum thrust at the compromise of stealth. But from the perspective of an observer, the afterburner, complete with shock diamonds, is an exciting visual suggestion of a jet’s power.  

About the Author: Harrison Kass

Harrison Kass is a writer and attorney focused on national security, technology, and political culture. His work has appeared in City Journal, The Hill, Quillette, The Spectator, and The Cipher Brief. He holds a JD from the University of Oregon and a master’s in Global & Joint Program Studies from NYU. More at harrisonkass.com.

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