Shock Diamonds

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I recently reported on Chinese plans to launch Shenzhou-9, and used a stock image of a Long March-2F rocket blasting off the launch pad. Nafin wanted to know what that diamond pattern trailing behind the rocket was, and ivan3man_at_large posted the answer: they’re called shock diamonds.

Shock diamonds? That term had somehow slipped past me, so I thought I’d dig into it some more.

Shock diamonds (alternatively known as “Mach disks”) occur when gas is exiting a nozzle at supersonic speeds, at a different pressure than the outside atmosphere. At sea level, the exhaust pressure might be lower than the thick atmosphere. And then at very high altitudes, the exhaust pressure might be higher than the thin atmosphere.

So these shock diamonds can appear just as a rocket is taking off, or at high altitude when it shifts into supersonic speed.

A classic example is the space shuttle blasting off, but another famous example is when Chuck Yeager’s X-1 rocket plane reached Mach 1.

Let’s take the example of a rocket blasting off. In this case, the exit pressure of the exhaust is lower than the outside atmosphere, and so you get a situation called “overexpansion”. The gas exits the rocket at a lower pressure, and fans outward from the exhaust nozzle in an “expansion fan”. But the outside atmosphere is higher pressure than the exhaust gas, and so compresses it inward. This difference in pressure forces the gas back together at a specific point – the first shock diamond.

(I’ll spare you all the complex fluid dynamics at this point.)

Then the gas compensates and expands again into a new expansion fan, and then it’s forced back together the same distance further along from the rocket at the next shock diamond, and so on and so on. Eventually atmospheric distortion and friction takes over, equalizing the pressure of the exhaust plume with the ambient atmosphere.

Shock diamonds were originally discovered by Ernst Mach, the famous Austrian scientist who did work on fluid dynamics.

One other interesting side note, shock diamonds aren’t just seen in rocket exhausts. They’ve also been seen blasting out of volcanoes and artillery guns

There are two great articles on Mach diamonds if you really want to understand them more deeply. Check out this article from Aerospace Web and this one from the Allstar Network.