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It used to be that you could get on a Concorde jet and fly across the Atlantic ocean faster than the speed of sound, but what does the future hold for supersonic flight?

Hosted by: Hank Green

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Why can’t I just get on a plane and go from, like, Montana to London, in a couple hours? I just want to experience the thrill of zooming through the sky faster than the speed of sound? Well, if you flew on a Concorde jet back before they were grounded -- or you happen to be a fighter pilot -- then you’ve probably experienced faster-than-sound travel. And, some companies are looking to make supersonic flight a reality again, with new commercial planes that travel faster than the speed of sound. And someday, you might be able to fly over the Atlantic Ocean in an hour -- or even less.

Problem is, most people don’t want to fly on a plane that feels like an out-of-control rocket. And there’s also the problem of faster-than-sound planes becoming ridiculously hot and unbearably loud. So engineers have some developing to do. On the morning of December 17th, 1903, Orville Wright became the first human to successfully pilot an airplane -- a heavier-than-air vehicle that was controlled, powered, and sustained. His flight lasted 12 seconds, and crossed 120 feet of a North Carolinian beach -- with an average speed of almost 11 kilometers an hour. By the end of the day, his brother Wilbur flew the same airplane for almost a whole minute, with an average speed of almost 16 kilometers an hour.

Less than a century later, in the 1970s, commercial planes went supersonic -- faster than the speed of sound. A few dozen supersonic planes were in regular service, available in two models, the Concorde and the Soviet Tupolev. But the Tupolev only made 55 passenger flights, from 1977 to 1978. And after a Concorde crashed in 2000, people started to fly on them less. Eventually, they just weren’t financially worth it anymore, and the planes were retired in 2003. Thirteen years later, there still aren’t any new commercial faster-than-sound planes. But soon, there might be! There are just a couple of improvements companies are trying to make first.

The main challenge comes from getting past what’s known as Mach 1. See, sound usually travels around 1230 kilometers per hour, but that’s not a constant number; it depends on things like the temperature and humidity of the air. So, when it comes to planes, it’s easier to talk about speed in Mach numbers, which take into account the speed of sound in the particular place where the plane is flying. Mach 1 is just the speed of sound. Anything slower than that is called subsonic, and anything faster is called supersonic.

But switching from subsonic to supersonic isn’t easy, because the plane has to overcome the infamous sound barrier. And that can be a problem, because the sound barrier is sometimes strong enough to tear away at planes, and even send them crashing to the ground. The sound barrier exists because of the way sound waves travel: by compressing and stretching the air they travel through. The compressed air ends up at a higher pressure, and the stretched air has lower pressure. As a plane moves, it produces sound waves that shift the air back and forth, creating areas of lower and higher pressure. But as the plane gets faster, it starts to catch up with those waves. New sound waves start to form on top of the old sound waves, causing huge swings between higher and lower pressure air.

Those differences in pressure can rattle and shake planes like toys, and there’s a real danger of them tearing to pieces. Low pressure areas can also lead to drops in temperature, condensing any moisture in the air and forming a visible cloud, sometimes known as a vapor cone. The first plane that could get past the sound barrier was the Bell X-1, built in 1947. It was designed to absorb 18 times the force of gravity, and modeled after a machine gun bullet. It didn’t actually lift off from the ground on its own, though -- it was dropped from a larger mother-ship plane, known as the B-29, so it got a bit of a head start. By the mid-1970s, supersonic planes were ready for commercial use -- with the UK and France designing the Concorde, and the Soviet Union designing the Tupolev.

The Concorde flew passengers from London to New York in about three and a half hours -- about half the time it would take in a plain old subsonic commercial plane. But they only flew that one route, and there’s a reason they spent as much time over the water as possible: the painfully disruptive sonic boom. Like the sound barrier, sonic booms come from a build-up of compressed sound waves, known as a shock wave. The shock wave heads away from the plane, which you hear as a VERY loud boom -- so powerful that they’re sometimes mistaken for earthquakes. And those sonic booms don’t just happen once, like when a plane breaks the sound barrier. They continue throughout the entire supersonic flight. That’s because the sound waves keep bunching up behind the plane, then expanding outwards, creating a cone shape known as the Mach cone. So wherever the plane flies over land, people hear that incredibly loud boom.

So that’s why the Concorde’s supersonic commercial flights only really happened between western Europe and eastern North America. If they flew over land, odds are people would not have appreciated the booms. And even though you can’t fly on a Concorde anymore, you might still be able to fly on a supersonic plane someday. NASA, for example, is looking into how to dampen the effects of the sonic boom. One way to do that might be by moving one, or even two engines above the wings, which would direct shockwaves upwards. So the sonic booms would happen in the sky, rather than on the ground.

Then there’s the Concorde 2, which Airbus is working on. The Concorde 2 would first fly directly upward, to an altitude of about 30 kilometers. Then, the plane would rotate its tail fin in a way that would redirect the shock waves to be horizontal, so you wouldn’t feel them as much on the ground. The Concorde 2 would be able to accelerate up to Mach 4.5 -- and at those speeds, it could take passengers from London to New York in an hour. But maybe that’s not enough, what if you want to go faster?

The Concorde 2 would be very close to going beyond supersonic, and into an even faster category, known as hypersonic. When people talk about hypersonic speeds, they’re generally talking about Mach 5 or higher -- more than five times the speed of sound. Those speeds get their own category, because that’s when the temperature of the plane becomes a bigger issue. The plane is flying through the air so quickly that friction with particles in the air is a real problem, because it makes a lot of heat. At hypersonic speeds, planes need to be able to withstand temperatures over 1,000 degrees Celsius... but almost all of the more typical metals would melt, or at least become very weak, at temperatures below that.

The other challenge is the engine, because a regular jet engine wouldn’t work. Standard, subsonic planes use large rotating blades to compress incoming air, inject fuel, and then let it burn, propelling them forward. At supersonic speeds, it becomes even easier, because the high speeds already compress the air. In that case, the engine doesn’t even need the blades -- that’s what’s known as a ramjet engine. Ram, because the air is just rammed into the engine.

At hypersonic speeds, though, this plan doesn’t work as well. Sure, the air is compressed, but it’s moving so fast that there’s not enough time for it to combust and actually help move the plane. So hypersonic planes need their own fuel and their own oxygen -- which is what NASA used in the X-15, the first plane to reach hypersonic speeds. It used a titanium skin to protect itself from the extreme temperatures, and was able to fly at Mach 6.72. It also flew high enough that some of the X-15 test flights are considered space flights.

But the X-15 is not the kind of plane that could be used commercially. For one thing, it burned through fuel so fast that it would run out in less than two minutes. Also, pilots sometimes experienced 8 times the force of Earth’s gravity, and most people wouldn’t consider that a comfortable business trip. So, until engines become more efficient and practical, commercial hypersonic planes are a long way from reality. And the scramjet might be the answer.

Scramjet engines work kind of like ramjets do, but they’re designed to handle the faster-moving air. In testing, NASA’s found that they could work at speeds up to Mach 15, at least in theory. There’s one big drawback, though: scramjet engines only work at hypersonic speeds. The X-43A, for example, an unmanned test plane that uses a scramjet, has to be accelerated above Mach 5 before it can fly on its own. It’s strapped to a booster rocket, which is then loaded onto a subsonic plane. All right, stay with me... The plane flies up to about 6 kilometers above the ground, then releases the X-43A, along with the rocket, which gets to about 30 kilometers up and to speeds of Mach 5. Then the X-43A can start its flight. So, it might be a while before hypersonic planes are a practical way to get across the Atlantic. But a future where a trip to the other side of the world involves flying faster than the speed of sound, without painful sonic booms for the people on the ground? That might not be so far off.

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