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Why We Still Don't Have Hypersonic Flights
Why We Still Don't Have Hypersonic Flights
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คำบรรยาย (209)
0:00
Wouldn’t it be nice to fly from Tokyo to LA in two hours instead of 10?
0:04
Such a fast trip seems like one of those futuristic dreams
0:07
our great-grandparents naively expected to come true by now.
0:11
The Concord introduced supersonic flight in the 1970s,
0:14
but that jet flew just twice the speed of sound.
0:17
The idea of hypersonic flight, which is much faster,
0:21
has been around since the 1930s, and has been researched since the 40s.
0:25
But 80 years later, flying across the world
0:28
still means a full day in a tiny seat, jockeying for the arm rest.
0:32
So what’s stopping us from building a plane that can go really fast?
0:36
As it turns out, part of the problem is making it also go slow.
0:40
[♪ INTRO]
0:43
By traversing the Atlantic Ocean at around Mach 2,
0:46
the Concord could fly from London to New York in just 3 hours!
0:50
What’s a Mach, you say?
0:51
I’m glad you asked.
0:52
The Mach scale tells us how fast something is traveling relative
0:55
to the speed of sound for the material that something is traveling through.
0:59
The speed of sound, meanwhile, relates to how fast all the particles that make
1:03
up the material can bump into each other and transmit a wave of energy.
1:08
Traveling at exactly the speed of sound is equivalent to a Mach number of 1.
1:12
A Concorde flying at Mach 2 is moving at twice the speed of sound.
1:15
And so on.
1:16
Anything that moves faster than the speed of sound is said to be supersonic.
1:20
The Mach scale is useful because the speed of sound varies not just for different materials,
1:24
but also the same material under different environmental conditions.
1:28
In the case of a plane flying through air,
1:30
the speed of sound changes depending on the ambient temperature and altitude.
1:34
Two planes could be going at the same speed in terms of kilometers per hour,
1:38
but have different Mach numbers if they’re moving through different parts of the atmosphere.
1:42
And unlike measurements like “kilometers per hour”, which just tells you a speed,
1:46
the Mach number will always give you a good idea of how much the material
1:51
is being compressed or shoved out of the way.
1:53
This leads us nicely into one reason you can’t fly on a Concorde, anymore: sonic booms.
1:58
At supersonic speeds, the air a plane is flying through can’t get out of the way fast enough.
2:03
Instead, it gets compressed into a dense cone
2:05
that’s powerful enough to break windows and damage eardrums.
2:09
So the Concorde was super noisy, on account of the continuous shock waves following in its wake.
2:14
Maybe some people would be willing to tolerate that for a shorter skip across the pond,
2:18
but it was also so expensive to develop and operate that the company never turned a profit.
2:23
It closed up shop in 2003.
2:25
But we aren’t here to talk about mere supersonic flight,
2:28
we’re here to talk about hypersonic flight…flights exceeding Mach 5.
2:32
Hypersonic travel has the clear benefit of being faster, but at high altitudes,
2:37
it also creates less atmospheric turbulence.
2:40
And yes, this isn’t just a perk for people looking for shorter vacation commutes.
2:43
As you might imagine, there’s a lot of military interest in tech like this.
2:47
But let’s stick to the physics of it all:
2:48
how fast could a plane possibly fly, and what’s stopping us from getting there?
2:52
First, some aviation fundamentals: If you want your vehicle to be a plane,
2:56
you’ve gotta balance four forces: propulsion, drag, gravity, and lift.
3:00
Propulsion is the force that pushes a plane forward.
3:03
It’s generated by an engine that accelerates a mixture of air and
3:06
fuel backward, creating an equal and opposite push forward in response.
3:11
Thanks, Newton’s Third Law of Motion!
3:12
Meanwhile, drag is the force that slows stuff down as it moves through a fluid.
3:16
It comes from a bunch of different sources, like the friction between
3:20
air molecules and a surface, and the fact that an object moving through fluid has
3:24
to constantly expend energy pushing some of that fluid out of the way.
3:28
But if you’re traveling faster than sound,
3:30
there are even more sources of drag both inside your vehicle’s engines,
3:34
and created by the sonic-boom-causing pressure cone you’re leaving in your wake.
3:39
So drag is a big deal for any plane, but even more complicated at super and hypersonic speeds.
3:44
Next up is our old friend gravity.
3:46
The amount of gravitational pull depends on the mass of the plane,
3:50
which is constantly changing as its engines burn fuel!
3:53
And finally, there’s lift: the upward force that keeps a plane in the sky,
3:56
and it’s why airplanes need wings.
3:58
I’m gonna be honest.
3:59
Lift is really complicated, and aerodynamicists are still arguing
4:03
over the various effects that contribute to it, and how much each effect matters.
4:08
So I can’t give you a perfect definition of lift, but here’s what you definitely need to know:
4:12
The shape of a wing creates a difference in pressure between the top and the bottom sides.
4:16
A well-designed wing will create a low pressure zone on the top,
4:20
and a high pressure zone on the bottom.
4:22
More pressure on the bottom results in a net upward force.
4:26
At the same time, as air flows over both sides of the wing, that wing also pushes some air downward.
4:33
It changes the air’s direction by exerting a force.
4:36
And as the wing pushes down on the air, the air pushes back up on the wing,
4:41
contributing another source of upward lift.
4:44
Thanks, again, Newton’s Third Law of Motion!
4:46
Tweaking the interplay of these four forces…propulsion,
4:48
drag, gravity, and lift… is the prime directive for any aerospace engineer.
4:53
So how far can those engineers push these principles to reach faster speeds?
4:58
But before we jet off to jets, we have to pay the bills.
5:01
So here’s a quick ad.
5:03
This SciShow video is supported by JMP.
5:05
Most of the stuff we talk about in SciShow videos comes from academia,
5:08
where researchers collect the data we’re going over right now.
5:12
But none of those people know what their data means until they analyze it.
5:16
That’s where JMP comes in.
5:18
Plus, teachers can use JMP as a pedagogical partner in your classroom.
5:21
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5:30
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5:34
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5:41
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5:45
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5:49
Every flight starts by taking off.
5:50
To get a plane up to altitude, you need lift and propulsion.
5:53
Wings are designed to do most of the heavy lifting.
5:56
The shape and area of a wing determines just how much lift it can provide.
6:00
Now, fluid dynamics is so complicated,
6:03
it’s both really hard and unreliable to calculate the lift factor for a wing.
6:09
Plus, like Mach numbers, a lift factor also varies as the density of air changes.
6:14
So usually, it’s just measured experimentally.
6:16
The amount of lift a plane can get also depends on its velocity.
6:19
At hypersonic speeds, this matters a lot more than the wing shape does.
6:24
And velocity depends on both the amount of
6:27
propulsion you can create and how efficiently you can reduce drag.
6:31
So although we’re currently concerned with lift, we simultaneously have to
6:34
figure out how to streamline the plane so it can plow through the not-so-thin air.
6:39
One goal is to make the plane as smooth as possible to avoid stirring up the boundary layer.
6:43
That’s the relatively calm layer of air directly in contact with the plane,
6:48
which acts as a buffer between the plane and the hypersonic airflow around it.
6:52
Sharp corners, or other disturbances in the boundary layer, can create zones with
6:57
high pressures and temperatures that fluctuate incredibly fast.
7:01
Any kind of unpredictability or turbulence equals new sources of drag,
7:06
so in general, sharp corners equals bad.
7:08
Because of this, most hypersonic vehicles have sleek shapes and do
7:12
everything possible to reduce their volume and cross-sectional area.
7:16
And with such a slender shape, you have to get pretty clever if you want to carry a lot of stuff.
7:22
But really, a plane like this shouldn’t carry much stuff.
7:24
Because the more it carries, the heavier it is, and the more fuel it needs to defy gravity.
7:29
Which makes it even heavier, which means it needs more fuel…and so on…
7:34
For something large like the Space Shuttle, you can use rocket boosters to get it to
7:39
its target elevation...which as the name suggests, is above most of Earth’s air.
7:44
So despite the wings sticking out of its belly,
7:46
the shuttle didn’t really operate like a hypersonic plane most of the time.
7:50
Smaller vehicles can also be ferried by a carrier plane, and then dropped.
7:54
Once they detach, they accelerate with boosters or special hypersonic engines.
7:58
While those engines are pretty tricky to design, they’re more all-purpose than rocket boosters.
8:03
Rockets might be way better at getting a vehicle up to cruising altitude and speed,
8:08
but they burn fuel to accelerate, and have to carry all of their fuel components with them.
8:13
Engines, on the other hand, use fuel to accelerate the air itself and produce thrust.
8:19
So they require less fuel when cruising.
8:21
And compared to some rockets, they have more flexibility concerning when they turn off.
8:27
Speaking of fuel, there’s another thing to consider here: Current fuels are,
8:30
well, not exactly good for the environment.
8:32
So, maybe you can make something go very fast,
8:35
but someone has to decide it’s worth the economic and environmental costs.
8:39
But getting in the air and up to speed is only about half the problem.
8:43
Once you’re cruising at Mach 5, you’ll have a different set of worries.
8:46
At hypersonic speeds, weird stuff starts happening to air.
8:50
For one thing, some of its molecules start to break apart when the plane plows through it.
8:55
This means that the temperature of the air can vary by a lot,
8:59
even in a pretty small volume of space.
9:01
Remember, lift depends on air density, and air density depends on temperature!
9:05
You might not think it…what with the altitude and all…but hypersonic vehicles can get very hot!
9:11
Their thermal coatings have to be able to withstand temperatures up to 2000ºC!
9:16
So yeah, if you’re looking to transport people, you’ll need shielding that keeps
9:20
them from roasting in their hypersonic tin can as it hurtles through the sky.
9:23
Another weird thing that happens at hypersonic speed is that wings…
9:27
you know, the things that make planes planes…become a structural liability.
9:31
But reinforcing the wings to keep your plane from getting torn asunder means
9:35
adding more weight for the lift to counteract.
9:38
This is why the few hypersonic vehicles that have been designed don’t look very plane-like,
9:43
from the previously mentioned Space Shuttle,
9:45
to three experimental X-43As which had neither pilots nor passengers…nor landing gear.
9:52
Each intentionally crashed into the Pacific at the end of their
9:55
first and only flight, never to be recovered.
9:58
As it turns out, it’s one thing to design a plane that flies at hypersonic speeds.
10:02
It’s another thing entirely to design a plane that can take off
10:05
and land at reasonable speeds while also flying hypersonic in the middle…
10:10
Since booster rockets aren’t ready for commercial use yet,
10:13
you’d probably want to rely on some kind of hypersonic engine.
10:16
But engines that work above Mach 5 don’t work so well under Mach 1, and vice versa.
10:21
Realistically, you’re looking for an engine system that dynamically switches modes,
10:25
which would be incredibly costly and time-consuming to develop.
10:28
And if you want to carry people, safety is an absolute must.
10:31
So you’ll also need sophisticated control over the plane’s acceleration, cabin temperature,
10:36
and vibrations, just to name a few.
10:38
In other words, hypersonic technology has a long
10:40
way to go before anyone starts recruiting flight attendants.
10:44
Even if there are humans who have flown in hypersonic planes.
10:47
Like back in 1967, an American pilot pushed the X-15 all the way to Mach 6.7.
10:54
First, he was brought to a high altitude by a carrier plane.
10:57
After they separated, he ignited the X-15’s
11:00
rockets and accelerated to more than 7000 kilometers an hour!
11:03
Meanwhile, uncrewed hypersonic vehicles have been used for military purposes for years.
11:08
Obviously, a lot of the details are classified,
11:11
but engineers seem to be making some pretty significant advances.
11:14
In 2021, China flew a prototype plane over the Gobi desert and managed to reach Mach 6.5!
11:20
Everything stayed pretty hush hush until late 2024, when full press reports came out.
11:25
It isn’t clear how, or even if their plane overcame the
11:28
hurdle of operating at both slow speeds and hypersonic speeds.
11:32
But this is a particularly interesting test flight
11:34
because the shape of the vehicle is round and broad.
11:37
That sure sounds more passenger friendly,
11:39
but there’s a reason why your classic hypersonic planes are the opposite.
11:43
A sleek and slim shape prevents a high pressure zone from forming on
11:47
top of the vehicle, which pushes down on the plane.
11:50
Not exactly helpful when you’re trying to keep a plane up in the air.
11:53
China’s prototype throws this philosophy out the window.
11:56
Its broad cone shape creates that undesirable high pressure zone,
12:00
but it also diverts that pressure over the body and into the wings that sit atop the vehicle.
12:06
The diverted air pushes up on the wings, which push back down on the air,
12:11
converting the pressure into upward lift!
12:14
A third and final thanks to Newton’s Third Law of Motion!
12:17
With far more room inside the vehicle, and repositioned wings to compensate,
12:21
it’s truly a fascinating design.
12:22
And it could be the breakthrough aerospace engineers have been waiting for.
12:26
But in the end, most of the challenges with commercializing hypersonic flight
12:29
will probably have to do with economics.
12:32
Supply, demand, cost, and so forth.
12:34
Some company may eventually announce the Concorde 2.0,
12:37
but you can be pretty sure it won’t come at basic economy prices.
12:41
[♪ OUTRO]
