Wouldn’t it be nice to fly from Tokyo  to LA in two hours instead of 10?

Such a fast trip seems like  one of those futuristic dreams   our great-grandparents naively  expected to come true by now. The Concord introduced  supersonic flight in the 1970s,   but that jet flew just twice the speed of sound.

The idea of hypersonic  flight, which is much faster,   has been around since the 1930s, and  has been researched since the 40s. But 80 years later, flying across the world   still means a full day in a tiny  seat, jockeying for the arm rest. So what’s stopping us from building  a plane that can go really fast?

As it turns out, part of the  problem is making it also go slow. [♪ INTRO] By traversing the Atlantic Ocean at around Mach 2, the Concord could fly from London  to New York in just 3 hours! What’s a Mach, you say? I’m glad you asked.

The Mach scale tells us how fast  something is traveling relative   to the speed of sound for the material  that something is traveling through. The speed of sound, meanwhile, relates  to how fast all the particles that make   up the material can bump into each  other and transmit a wave of energy. Traveling at exactly the speed of sound  is equivalent to a Mach number of 1.

A Concorde flying at Mach 2 is  moving at twice the speed of sound. And so on. Anything that moves faster than the  speed of sound is said to be supersonic.

The Mach scale is useful because the speed of  sound varies not just for different materials,   but also the same material under  different environmental conditions. In the case of a plane flying through air,   the speed of sound changes depending on  the ambient temperature and altitude. Two planes could be going at the same  speed in terms of kilometers per hour,   but have different Mach numbers if they’re  moving through different parts of the atmosphere.

And unlike measurements like “kilometers  per hour”, which just tells you a speed,   the Mach number will always give you  a good idea of how much the material   is being compressed or shoved out of the way. This leads us nicely into one reason you  can’t fly on a Concorde, anymore: sonic booms. At supersonic speeds, the air a plane is flying  through can’t get out of the way fast enough.

Instead, it gets compressed into a dense cone   that’s powerful enough to break  windows and damage eardrums. So the Concorde was super noisy, on account of  the continuous shock waves following in its wake. Maybe some people would be willing to tolerate  that for a shorter skip across the pond,   but it was also so expensive to develop and  operate that the company never turned a profit.

It closed up shop in 2003. But we aren’t here to talk  about mere supersonic flight,   we’re here to talk about hypersonic  flight…flights exceeding Mach 5. Hypersonic travel has the clear benefit  of being faster, but at high altitudes,   it also creates less atmospheric turbulence.

And yes, this isn’t just a perk for people  looking for shorter vacation commutes. As you might imagine, there’s a lot of  military interest in tech like this. But let’s stick to the physics of it all:   how fast could a plane possibly fly, and  what’s stopping us from getting there?

First, some aviation fundamentals: If  you want your vehicle to be a plane,   you’ve gotta balance four forces:  propulsion, drag, gravity, and lift. Propulsion is the force  that pushes a plane forward. It’s generated by an engine that  accelerates a mixture of air and   fuel backward, creating an equal  and opposite push forward in response.

Thanks, Newton’s Third Law of Motion! Meanwhile, drag is the force that slows  stuff down as it moves through a fluid. It comes from a bunch of different  sources, like the friction between   air molecules and a surface, and the fact  that an object moving through fluid has   to constantly expend energy pushing  some of that fluid out of the way.

But if you’re traveling faster than sound,   there are even more sources of drag  both inside your vehicle’s engines,   and created by the sonic-boom-causing  pressure cone you’re leaving in your wake. So drag is a big deal for any plane, but even  more complicated at super and hypersonic speeds. Next up is our old friend gravity.

The amount of gravitational pull  depends on the mass of the plane,   which is constantly changing  as its engines burn fuel! And finally, there’s lift: the upward  force that keeps a plane in the sky,   and it’s why airplanes need wings. I’m gonna be honest.

Lift is really complicated, and  aerodynamicists are still arguing   over the various effects that contribute  to it, and how much each effect matters. So I can’t give you a perfect definition of lift,  but here’s what you definitely need to know: The shape of a wing creates a difference in  pressure between the top and the bottom sides. A well-designed wing will create  a low pressure zone on the top,   and a high pressure zone on the bottom.

More pressure on the bottom  results in a net upward force. At the same time, as air flows over both sides of  the wing, that wing also pushes some air downward. It changes the air’s  direction by exerting a force.

And as the wing pushes down on the air,  the air pushes back up on the wing,   contributing another source of upward lift. Thanks, again, Newton’s Third Law of Motion! Tweaking the interplay of  these four forces…propulsion,   drag, gravity, and lift… is the prime  directive for any aerospace engineer.

So how far can those engineers push  these principles to reach faster speeds? But before we jet off to jets,  we have to pay the bills. So here’s a quick ad.

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Just go to jmp.com/scishow. Every flight starts by taking off. To get a plane up to altitude,  you need lift and propulsion.

Wings are designed to do  most of the heavy lifting. The shape and area of a wing determines  just how much lift it can provide. Now, fluid dynamics is so complicated,   it’s both really hard and unreliable to  calculate the lift factor for a wing.

Plus, like Mach numbers, a lift factor  also varies as the density of air changes. So usually, it’s just measured experimentally. The amount of lift a plane can  get also depends on its velocity.

At hypersonic speeds, this matters  a lot more than the wing shape does. And velocity depends on both the amount of   propulsion you can create and how  efficiently you can reduce drag. So although we’re currently concerned  with lift, we simultaneously have to   figure out how to streamline the plane so  it can plow through the not-so-thin air.

One goal is to make the plane as smooth as  possible to avoid stirring up the boundary layer. That’s the relatively calm layer of  air directly in contact with the plane,   which acts as a buffer between the plane  and the hypersonic airflow around it. Sharp corners, or other disturbances in  the boundary layer, can create zones with   high pressures and temperatures  that fluctuate incredibly fast.

Any kind of unpredictability or  turbulence equals new sources of drag,   so in general, sharp corners equals bad. Because of this, most hypersonic  vehicles have sleek shapes and do   everything possible to reduce their  volume and cross-sectional area. And with such a slender shape, you have to get  pretty clever if you want to carry a lot of stuff.

But really, a plane like this  shouldn’t carry much stuff. Because the more it carries, the heavier it  is, and the more fuel it needs to defy gravity. Which makes it even heavier, which  means it needs more fuel…and so on… For something large like the Space Shuttle,  you can use rocket boosters to get it to   its target elevation...which as the name  suggests, is above most of Earth’s air.

So despite the wings sticking out of its belly,   the shuttle didn’t really operate like  a hypersonic plane most of the time. Smaller vehicles can also be ferried  by a carrier plane, and then dropped. Once they detach, they accelerate with  boosters or special hypersonic engines.

While those engines are pretty tricky to design,  they’re more all-purpose than rocket boosters. Rockets might be way better at getting a  vehicle up to cruising altitude and speed,   but they burn fuel to accelerate, and have to  carry all of their fuel components with them. Engines, on the other hand, use fuel to  accelerate the air itself and produce thrust.

So they require less fuel when cruising. And compared to some rockets, they have more  flexibility concerning when they turn off. Speaking of fuel, there’s another thing  to consider here: Current fuels are,   well, not exactly good for the environment.

So, maybe you can make something go very fast,   but someone has to decide it’s worth  the economic and environmental costs. But getting in the air and up to  speed is only about half the problem. Once you’re cruising at Mach 5, you’ll  have a different set of worries.

At hypersonic speeds, weird  stuff starts happening to air. For one thing, some of its molecules start to  break apart when the plane plows through it. This means that the temperature  of the air can vary by a lot,   even in a pretty small volume of space.

Remember, lift depends on air density,  and air density depends on temperature! You might not think it…what with the altitude  and all…but hypersonic vehicles can get very hot! Their thermal coatings have to be able  to withstand temperatures up to 2000ºC!

So yeah, if you’re looking to transport  people, you’ll need shielding that keeps   them from roasting in their hypersonic  tin can as it hurtles through the sky. Another weird thing that happens at  hypersonic speed is that wings… you know, the things that make planes  planes…become a structural liability. But reinforcing the wings to keep your  plane from getting torn asunder means   adding more weight for the lift to counteract.

This is why the few hypersonic vehicles that  have been designed don’t look very plane-like,   from the previously mentioned Space Shuttle,   to three experimental X-43As which had neither  pilots nor passengers…nor landing gear. Each intentionally crashed into  the Pacific at the end of their   first and only flight, never to be recovered. As it turns out, it’s one thing to design  a plane that flies at hypersonic speeds.

It’s another thing entirely to  design a plane that can take off   and land at reasonable speeds while  also flying hypersonic in the middle… Since booster rockets aren’t  ready for commercial use yet,   you’d probably want to rely on  some kind of hypersonic engine. But engines that work above Mach 5 don’t  work so well under Mach 1, and vice versa. Realistically, you’re looking for an engine  system that dynamically switches modes,   which would be incredibly costly  and time-consuming to develop.

And if you want to carry people,  safety is an absolute must. So you’ll also need sophisticated control over  the plane’s acceleration, cabin temperature,   and vibrations, just to name a few. In other words, hypersonic technology has a long   way to go before anyone starts  recruiting flight attendants.

Even if there are humans who  have flown in hypersonic planes. Like back in 1967, an American pilot  pushed the X-15 all the way to Mach 6.7. First, he was brought to a high  altitude by a carrier plane.

After they separated, he ignited the X-15’s   rockets and accelerated to more  than 7000 kilometers an hour! Meanwhile, uncrewed hypersonic vehicles have  been used for military purposes for years. Obviously, a lot of the details are classified,   but engineers seem to be making  some pretty significant advances.

In 2021, China flew a prototype plane over  the Gobi desert and managed to reach Mach 6.5! Everything stayed pretty hush hush until  late 2024, when full press reports came out. It isn’t clear how, or even  if their plane overcame the   hurdle of operating at both slow  speeds and hypersonic speeds.

But this is a particularly interesting test flight   because the shape of the  vehicle is round and broad. That sure sounds more passenger friendly,   but there’s a reason why your classic  hypersonic planes are the opposite. A sleek and slim shape prevents a  high pressure zone from forming on   top of the vehicle, which  pushes down on the plane.

Not exactly helpful when you’re  trying to keep a plane up in the air. China’s prototype throws this  philosophy out the window. Its broad cone shape creates that  undesirable high pressure zone,   but it also diverts that pressure over the body  and into the wings that sit atop the vehicle.

The diverted air pushes up on the  wings, which push back down on the air,   converting the pressure into upward lift! A third and final thanks to  Newton’s Third Law of Motion! With far more room inside the vehicle,  and repositioned wings to compensate,   it’s truly a fascinating design.

And it could be the breakthrough  aerospace engineers have been waiting for. But in the end, most of the challenges  with commercializing hypersonic flight   will probably have to do with economics. Supply, demand, cost, and so forth.

Some company may eventually  announce the Concorde 2.0,   but you can be pretty sure it  won’t come at basic economy prices. [♪ OUTRO]