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DIFFERENT DENSITIES of air cause differing refractions of light. This scintillation is what causes shimmering images of hot landscapes on a sunny day. Capturing such images is challenging, but NASA has perfected an advanced method of what’s called Schlieren photography to produce stunning photos of two jets interacting supersonically at 30,000 ft. above California.
A Pair of Talons and a Beechcraft. In “NASA Captures First Air-to-Air Images of Supersonic Shockwave Interaction in Flight,” March 5, 2019, the agency reports that this and other images were captured in the fourth phase of AirBOS, Air-to-Air Background Oriented Schlieren flights.
A Beechcraft B-200 King Air carrying the advanced imaging equipment flew over the pair of Northrup T-38 Talon jets above NASA’s Armstrong Flight Research Center in Edwards, California. The two Talons, traveling supersonically, were approximately 30 ft. from each other, the trailing aircraft about 10 ft. below the leading T-38.
Quoted in Joseph Trevithick’s “NASA Captures Two Jets’ Supersonic Shockwaves by Applying New Tech to an Old Idea,” March 6, 2019, NASA’s J.T. Heineck says, “I am ecstatic about how these images turned out. With this upgraded system, we have, by an order of magnitude, improved both the speed and quality of our imagery from previous research.”
Studies of such aircraft interactions as well as exhaust plumes are topics of interest to researchers. Another AirBOS goal is supporting a Low-Boom Flight Demonstration using NASA’s X-59 QueSST. This Quiet SuperSonic Technology X-plane is designed to replace traditional sonic booms with only a quiet rumble in supersonic flight. It’s hoped that advanced techniques will permit lifting of current restrictions on supersonic flight over land.
Schlieren Photography. The key to capturing these supersonic shockwaves dates from the work of French physicist Léon Foucault, 1819–1869, and German physicist August Toepler, 1836–1912. In 1858, Foucault devised the knife-edge test of measuring optical mirrors to a precision of millionths of an inch. In 1864, Toepler applied Foucault’s knife-edge idea to photograph fluid flow and shockwaves.
Toepler named this technique Schlieren photography, after the German word Schliere,streak. The method is widely used in aerodynamic studies of flow around objects.
And, perhaps, one day, we’ll watch supersonic flights over land producing no more than a quiet rumble. ds
© Dennis Simanaitis, SimanaitisSays.com, 2019