Supersonic (traveling faster than the speed of sound) flight has been possible for about 70 years. The auditory results of cracking the sound barrier are sometimes heard (and felt) in the form of sonic booms. But what does it look like when an object reaches Mach 1? Often, aircraft are photographed with a visible plume of moisture erupting around them when they reach supersonic speeds. But that’s just part of the picture, as NASA observed with a recent photographic experiment.
NASA used an unlikely process to capture images of supersonic shockwaves created by an Air Force T-38C test plane. Researchers employed a 150-year old technique called schlieren photography. Wikipedia describes this style of photography as:
…a visual process that is used to photograph the flow of fluids of varying density. Invented by the German physicist August Toepler in 1864 to study supersonic motion, it is widely used in aeronautical engineering to photograph the flow of air around objects.
Having access to plenty of modern technology and techniques, NASA used an updated version of this technique called background oriented schlieren (BOS):
First, researchers obtain an image of a speckled background pattern. Next, they collect a series of images of an object in supersonic flow in front of the same pattern. Shock waves are deduced from distortions of the background pattern resulting from the change in refractive index due to density gradients. This method requires very simple optics and a variety of background patterns, including natural ones, may be used. The complexity with this method is in the image processing and not the hardware or positioning, thus making BOS an attractive candidate for obtaining high-spatial-resolution imaging of shock waves in flight.
Detailed explanations of the project (and a few more images) are available at the link above.
Image credit: NASA