How Fast Is Hypersonic Speed, Really?
If you have a need for speed, the next frontier is hypersonic. The United States is currently developing hypersonic missiles that travel in excess of 6,000 miles per hour. The hypersonic SR-72 is reported to be making great gains in secret. It's all very impressive, but it can be hard to wrap your head around in the abstract, so let's do a little bit of napkin math to really bring it home.
First, let's establish some terms. Hypersonic is, obviously, supersonic on steroids. But while "supersonic" has the clear cut definition of being faster than the speed of sound (Mach 1), hypersonic is a little fuzzier. Generally, hypersonic speeds are the point at which the molecules of air that surround the aircraft start to change by breaking apart (dissociation) and/or picking up electrical charge (ionization). These things don't happen at one particular speed, so the term "hypersonic" instead refers to the point at which they start to meaningfully affect the mechanics of flight-generally accepted to be Mach 5, or 3,836.35 mph in conditions of 20 degrees Celsius at sea level.
Hypersonic speeds have been achieved before, most notably by the U.S. Air Force and NASA when test pilot William J. Knight set the record for fastest crewed flight in 1967 when he piloted the North American X-15 to Mach 6.72 (4,520 mph) at an altitude of 102,100 feet. In 2004, the uncrewed NASA X-43A set a speed record at Mach 9.6, or 7,310 mph at an altitude of about 110,000 feet.
The top speed of military aircraft like the SR-72 is estimated and unsurprisingly classified, with officials saying only that it will be able to hit at least the minimum hypersonic speed of Mach 5, so we'll use that as our baseline. The Air Force and NASA's two record flights can be used to extrapolate what even higher speeds might look like. And even though the speed of sound varies based on altitude and temperature, we'll set it to a fixed 660 miles per hour, which is what you would expect at a cruising altitude of 40,000 feet and temperature of -70 degrees Fahrenheit. This is napkin math, after all.
So without further ado, hypersonic speeds in more practical terms:
Given that the circumference of the Earth is 24,901 miles, a hypersonic jet could fly around the world in just over seven and a half hours. The X-15 could do it in just over five and a half, and the X-34A in just 3 hours and 55 minutes. At a paltry speed of Mach 1, it would take over a day and a half-about 37 hours and 45 minutes.
The longest commercial flight in the contiguous United States is Miami to Seattle at 2,724 miles or about six hours and 40 minutes in typical flight time. A hypersonic speeds that takes about 50 minutes-and only 37 minutes in an X-15. The X-43A meanwhile, could make the trip in 26 minutes.
The flight from Dubai to New Zealand recently became the longest non-stop scheduled commercial flight by distance at 8,824 miles. The Airbus A380 taking the inaugural flight landed after 16 hours and 24 minutes. At hypersonic speeds, you could expect to make the trip in a cool two hours and 40 minutes. The X-15 could swing it in about 30 seconds shy of two hours. The X-34A? An hour and 24 minutes. Not even enough time to watch Dunkirk, Christopher Nolan's shortest movie in years.
The world's shortest passenger flight, Westray to Papa Westray in Scotland, is just 53 seconds and covers 1.7 miles. Yes, this example is ridiculously, almost pointless unrealistic and abstract BUT-hypersonic speeds cut that down to about 1.9 seconds. The X-15 to blistering 1.4 seconds, and the X-34A to 0.97 seconds.
This is admittedly a simple, silly, and theoretical exercise-one that ignores not only large swaths of physics (the time it would take to get up to speed, most egregiously) but also gigantic practical engineering concerns like the max duration of sustained high-speed flight and the fuel requirements to get up to those hypersonic speeds.
Still, considering how profoundly hard it is to practically imagine things moving at such ridiculously high speeds, abstractions like this can be a helpful tool in wrapping your mind at least halfway around the sheer magnitude of the velocities at play. You probably won't ever know what it's like to travel Mach 5, and maybe you can't even adequately imagine it, but at the very least this should help us all appreciate the borderline suicidal bravery it takes to hop behind the stick of something that promises to scream up to hypersonic and beyond.
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