An incredible look into the supersonic flight and impact of a baseball, by @smartereveryday and team!

Also, did anyone else catch that pre-impact flame? I did too! https://twitter.com/smartereveryday/status/1326906621189099524

If you watch closely at 10 mins 50 sec into the video, and slow it down, you’ll see a tiny glimmer of orange trailing the baseball *before* it impacts the glove

What’s happening here?

Here’s a nice cross section of a baseball for reference

The exposed outer layer materials are alum-tanned cowhide and 100% cotton thread for stitching

At what temperature will these materials burn?

—> The ignition point for COWHIDE is ~800+ def F

—> The ignition point for COTTON is ~200+ def F

So that cotton stitching is sus

So how is the outer layer of that baseball reaching those higher temps?

See that nice #shockwave trailing the baseball?

Shock waves are very small regions in a medium - air here - where the gas properties change by a LARGE amount.

Shock waves are fascinating! And a challenging engineering phenomenon

Across a shock wave, the static pressure and temperature increases almost instantaneously.

Here’s a short explanation of (oblique) shock waves, courtesy of @NASA —> https://www.grc.nasa.gov/www/k-12/airplane/oblique.html

Thankfully, @smartereveryday also took some video of a supersonic baseball using the #Schlieren technique, here: (fast forward to ~16 mins)

Here’s a screen grab of that video

#Schlieren imaging is a technique by which the flow of fluids with varying densities can be photographed.

In other words, this image highlights where a change in air density is happening, making it easier to visualize shock waves and general density gradients around the ball

So, shockwave

But is the air heating up high enough to burn the cotton stitching?

Hmm... math on a weekend... here we go...

Here's a rough outline of supersonic flow around a sphere

The front face of the ball heats up as it moves thru air due to it's kinetic energy being converted to heat via adiabatic compression, and (less significantly) by skin friction heating on the surface of the ball.

The shock wave ahead of the ball is curved in general but
DIRECTLY in front of the body, the shock is effectively
normal to the flow. Conditions across the shock can be determined here (circled) using the normal shock relations...