Mad about Science: Hypersonic Speeds

By Brenden Bobby
Reader Columnist

Extremes in science are fun. Really wild things begin to happen when matter reaches extreme temperatures or speeds. Fast-moving objects are relatively common in the universe, but we’re relatively speed-capped here on Earth due to the presence of our atmosphere.

Though we can’t easily see it, we’re surrounded by air at all times. Moving through air causes resistance, as you’re pushing through a physical object just as if you were pushing through water on the lake. If you’ve spent any time horsing around in a body of water, trying to “push” a large amount of it with the flat of your hand or a pool noodle causes resistance that requires more energy to move the longer you go. This is effectively the same thing that happens when an object travels through air.

Sound is a vibration that travels through the air in the form of a wave. The speed in which sound travels ranges based on the medium through which it’s traveling — for air, it is somewhere between 740 and 767 miles per hour. That’s pretty darn fast, and if we were to be launched at that speed with just our bodies and no protective equipment, it’d probably turn our blood into a thick paste and kill us pretty fast. Luckily, some humans are intelligent enough to create vehicles that can achieve these kinds of speeds without turning our organs into cherry-colored Kool-Aid.

The X-15 was the first aircraft to achieve hypersonic speed. Courtesy image

Vehicles traveling at supersonic speeds — that is, speeds faster than that of sound — create shockwaves when they travel. This is where the term “supersonic boom” comes into play. If you’ve ever lived on or near a military air base, it’s likely you’ve experienced at least one fighter jet exceeding the speed of sound and creating a resonant boom in its wake.

Vehicles traveling at hypersonic speeds are traveling at speeds of at least Mach 5 — five times the speed of sound, or 3,836 miles per hour. These vehicles have to be designed to perfection, as even the tiniest flaw could throw the entire structure into catastrophic failure. 

Conventional aircraft are incapable of reaching these speeds, nor are they able to sustain the forces involved at five times the speed of sound.

The first aircraft to achieve hypersonic speed was the North American X-15 developed by the US Air Force. On Oct. 3, 1967, pilot William J. Knight traveled at 4,520 miles per hour. The X-15 is interesting in that it’s not a conventional jet — rather, it’s effectively a human-guided rocket that can burn up to 15,000 pounds of highly explosive fuel in 80 seconds. Being built as a rocket rather than a jet engine, the ship was capable of entering space.

Conventional jets require an intake of oxygen in order to oxidize fuel to initiate combustion. Sealed fuel fed through a rocket engine is already oxidized and doesn’t require an external oxygen source, allowing it to function outside of Earth’s atmosphere, where it also isn’t butting up against air resistance and therefore able to move faster and achieve greater distances with less fuel expenditure.

Try throwing a rock underwater. You can see that the resistance forces the rock to twirl and then sink after traveling a very short distance. Now try throwing a similar rock through the air and watch it glide much farther. This is effectively the same principle.

You may be wondering: Brenden, how is a plane going to fly if it can’t generate lift since there isn’t any air?

Well, my friend, when you have a tin can strapped to a tube of highly explosive rocket fuel, lift no longer becomes an issue. At that point, all you need is a direction in which to point the rocket and you can let physics take care of the rest.

Hypersonic speed on Earth is one thing, but these speeds are trivial in the scope of the universe. As you read this, you are currently perched on a rock hurtling at 67,000 miles per hour through the vacuum of space orbiting the sun. This mind-boggling speed is still nothing compared to the speed at which we’re spinning around the galaxy’s core. Our solar system is whipping through space at a speed of roughly 517,000 miles per hour, completing a full galactic year in roughly 225 million years.

That’s all well and good, but what about the fastest speed ever achieved on planet Earth?

This happened in 1956 in the Los Alamos research facility during Operation Plumbbob, when scientists wanted to test what would happen if a nuclear detonation were to occur underground. There were theories, and there was a lot of expected data that was collected from the test, but there was one unexpected side effect of this detonation. A steel manhole cover was blown from its housing and sent screaming skyward at Mach 196, roughly 145,400 miles per hour. The friction generated by the air resistance likely resulted in a near-instantaneous liquefaction and atomic breakdown of the steel, as no part of it was ever recovered.

It was likely one of the only times in the history of Earth where the adage: “What comes up must come down” could not be applied.

Stay curious, 7B.