[SciShow intro]

Hank Green: Commerical jets are amazingly convenient and super fast, but if you look at their history, you will notice something a little bit weird. The first passenger jet, called the de Havilland Comet, entered commercial service in 1952 and it flew at about 740 kilometers per hour. Next came the Soviet Union's Tupolev Tu-104, which maxed out at about 950 kilometers per hour. And then there was the Boeing 707 in 1958, which had a top speed of 965 kilometers per hour. Which, for the record, is about the same speed as commercial jets flying today.
So what gives? Why have passenger jets been stuck at this speed for more than 60 years? I want to go places faster!

Well, if you're looking for somebody to blame, and I am, don't point your fingers at the engineers. You can instead blame air molecules. To understand this speed limit, you have to know a little bit about airplane wings. If you were to slice off the end of a wing and look at the cross-section, you would see a shape like a squished teardrop. That's called the airfoil, and it's a major part of what gives planes lift. As the plane moves forward, air travels under and over the wing, and the teardrop shape creates two distinct regions: an area of high-pressure air below the wing, and one of low-pressure air above it. Ultimately, the high-pressure air ends up pushing upward, which pushes on the wing and keeps you chilling in the sky eating that free bag of slightly stale pretzels.

But lift isn't the most important thing in this conversation. After all, while this is happening, the plane also is hurtling forward at hundreds of kilometers per hour. That means air isn't just sitting around in these high and low-pressure bubbles, it's flowing over the wings at incredibly high speeds. And here's the really important thing, in a low-pressure environment, fluids like air move even faster. So even though your plane might only be going three-quarters of the speed of sound, the air molecules zooming over the wing get a kick, and they get going so fast they start to break the sound barrier. And that is where the trouble happens. See, even though those first air molecules broke the sound barrier, the molecules behind them did not. And the thing about molecules is that they really want to fit in - they want to reach an equilibrium.

Once the first air molecules break the speed of sound, they immediately want to slow down again and match the molecules around them. That creates a sudden pressure difference, called a shock wave. And if that sounds like a bad thing to you, you're right. Right behind this shockwave, the air expands and that uses up energy that could have contributed to the plane's lift and speed. This phenomenon is what's called wave drag and ultimately it slows a plane down, forcing it to use significantly more energy and fuel to maintain the same speed. For the record, of course, anything moving through air has some drag, but wave drag is a separate problem. At an altitude of 10 kilometers, it's strongest from roughly 850 to 1300 kilometers per hour, and it's worst exactly at the speed of sound. So planes are optimized to go as fast as they can go without running into wave drag. And that means passenger jets are topping out with a flight speed of about 800 to 950 km/h, just like they did in the fifties.

Of course, there is a loophole to this rule. Technically, once you get above about 1300 kilometers per hour, you don't have to worry about wave drag because the airflow around the plane stabilizes. That was one of the benefits to flying on the Concorde - a sleek, futuristic-looking passenger jet that hit the skies in the late sixties. This thing could fly at double the speed of sound, so it could take off from London at 9 AM and get you to New York at 7:30 AM the same day. That's of course partly thanks to timezones, but the plane did help out a lot.

Still, this thing had problems, many of which were insurmountable. Most notably, the Concord flew so fast that air couldn't get out of its way quickly enough. Instead, it started to pile up, compress around the plane and make it heat up. That created a lot of drag which required a lot of extra fuel to counteract.

The Concord used about one metric ton of fuel per person it flew across the Atlantic, and as a result, round-trip tickets cost more than 50,000 U.S. dollars in today's money. In the end, the plane cost so much to develop and guzzled so much fuel, even 50,000 dollar tickets couldn't make it financially viable, and the Concord's last flight was in 2003.

That means that unless someone has some engineering breakthrough that somehow fixes wave drag, we're probably going to be stuck at our measly 950 kilometers per hour speed limit for the foreseeable future.

But that's not actually necessarily a bad thing. These days, instead of focusing on speed, engineers are designing new jet engines that improve efficiency so our flights can be cheaper and more friendly to the Earth. So yes, I might not be able to fly from London to New York in 4 hours, but at least I'll be saving some cash and helping the planet, which, in the grand scheme of things, is pretty alright. 

This episode of Scishow couldn't have happened without our Patrons on Patreon. They are the people who help keep Scishow free for everyone and allow us to explore complicated topics like aerodynamics and airfoils. So, to all of our Patrons, thank you. And if you like Scishow, those are the people you should be thanking. If you aren't a Patron and want to help us keep making episodes like this, we'd love to have you as part of our Patron community. You can learn more at patreon.com/scishow

[SciShow end credits]