Why "shock diamonds" shoot out of jets and space shuttles

When you see what look like strobe lights shooting out behind a jet or a space shuttle, blame it on our old friend physics. Those lights called are shock diamonds, or mach disks.

Sometimes they look like puffs of compressed smoke behind aircraft. At other times, they look more like glowing disks or diamonds popping up at regular intervals in the stream of exhaust behind an aircraft or spacecraft. They're an impressive little show, and they're the result of a winning, and then losing, and then winning again battle between the gas being shot out of the jet and the air that's already hanging around the sky.

Why "shock diamonds" shoot out of jets and space shuttles

Often the exhaust from a jet is at higher than air pressure. Because it is high, it expands smoothly into the atmosphere, shoving the regular air out of the way as it expands. Sometimes, when the aircraft is very close to the ground and the atmospheric pressure is at its highest, the gas coming out of the jet is at a lower pressure than the surrounding air. Immediately, the atmosphere takes advantage. The high-pressure air presses in on the gas from all sides, compressing it.

Ah, but when the gas is compressed too much, the shoe is very much on the other foot. By compressing the gas quickly, the air has pushed it so hard that the gas is suddenly at a higher pressure than the air. It Hulks out, and pushes out at the surrounding air. Like the Hulk, it overreacts, pushing out so far that it is again at a lower pressure than the atmosphere around it, and gets pushed inward again. The cycle continues until the two pressurized gases at last equal out.

All this pushing out and in creates shock waves within the flow of the exhaust that move at angles to the backward push of the gas. When the gas is getting pushed inwards by the surrounding atmosphere, and the shock waves move towards that backwards flow, putting a lot of pressure on it, some of the extra fuel can ignite and glow. Since the entire production is cyclical, it keeps going down the exhaust, putting on a pretty, if unnerving light show for anyone watching.

Top Image: NASA

Second Image: NASA

Via Aerospaceweb.org