What Happens When a Plane Flies Faster Than the Speed of Sound

airbos_t 38_ed264 — This T-38C, a supersonic US Air Force training jet, was one of the planes that NASA imaged using the schlieren technique. US Air Force

When a plane goes fast enough, it compresses the air it is flying through so much that it can change its density.

NASA and the US Air Force have been trying to visualize this effect for years so they can build better supersonic aircraft and enable them to go faster than the speed of sound.

Until recently, these kinds of tests were contained to wind tunnels on the ground.

There, researchers used the schlieren technique, invented by German physicist August Toepler in 1864, to understand more about how air was traveling around supersonic aircraft.

Schlieren imaging is a way to see the differences in air density, using a particular setup of lenses and cameras.

Now, NASA researchers have adapted this method to visualize supersonic aircraft in flight. Bringing the Schlieren method into the air has been challenging because the aircraft carrying the imaging equipment has to fly right above the plane it's recording, and travel just as fast — which, during supersonic imaging, is faster than the speed of sound.

The T-38C, a supersonic US Air Force training jet that NASA imaged, traveled at a top speed of Mach 1.09 during the tests. (Mach 1 is the speed of sound, which is about 768 mph at sea level.)

But the tricky maneuvering was worth it for these gorgeous images, showing the shock wave of the T-38 flying over the Mojave Desert:

the silhouette of a plane with straight line shockwaves emanating from each side — This schlieren image dramatically displays the shock wave of a supersonic jet flying over the Mojave Desert. Researchers used NASA-developed image processing software to remove the desert background, then combined and averaged multiple frames to produce a clear picture of the shock waves. NASA

As the plane moves through the air at supersonic speeds, it disrupts the air's density, resulting in the different color lines here — the shock waves that are a result of the plane's motion.

These shock waves act just like the wake behind a boat, which happens because it disrupts the water by moving faster than the water waves were moving.

shock waves airplane — A visualization of what sound waves look like around aircraft traveling at different speeds. Chabacano via WikiMedia Commons CC 2.5

Air pressure right at the tip of the cone is normal, while the pressure inside the cone is high because of the plane passing so quickly through it and pushing the atoms of air together.

Here's another schlieren image visualizing the supersonic flow of the T-38 jet in flight:

the straight shockwaves of a plane flying faster than the speed of sound — NASA is using a 21st century version of schlieren imagery, invented by a German physicist in 1864, to visualize supersonic flow phenomena with full-scale aircraft in flight. NASA

Understanding more about how supersonic aircraft affect the air around them could help develop ways to make planes quiet enough for commercial travel, opening the door to make the trip from LA to NYC a whole lot faster.

Here's what it looks like when a plane breaks the sound barrier

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