Why We Still Don't Have Hypersonic Flights

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.

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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,