Outside of making flying coach exceedingly more and more uncomfortable, commercial flight hasn't had any groundbreaking changes in the last half century.
Well except one. Supersonic flight; meaning an aircraft flying faster than the speed of sound.
That's why you can fly from Washington, D.C. to London over the Atlantic in just 3.5 hours
Except no you can't. But nearly half a century ago,
commercial airline passengers were taking these flights on a short-lived airliner called the Concorde, which could travel at supersonic speeds up to Mach 2.2.
There aren't many cases where people in the past were able to take advantage of technology
no longer available to us in the future.
There are several excellent videos about the incredible Concorde and the brief existence of commercial supersonic flight
So I'll give you the cliff notes on its demise and why a flight from Washington DC to London today
will take you about seven hours. The two main culprits that killed supersonic flight are noise and economics.
You've probably heard of something called a sonic boom. It's basically an explosion like sound made when breaking the sound barrier.
Needless to say this is a problem when flying over land where people live.
That's why supersonic flight is currently banned over land in the United States and Europe.
But not wanting to annoy people is not why we don't have supersonic commercial flight today. As usual,
it's all about the money.
Even the sonic boom problem is about money, because over land flights make up 75 percent of commercial air travel. So that pesky
overland ban of supersonic flights is a huge problem for airlines.
Keeping operating costs low are crucial to airlines, and cheaper tickets are the main selling point to consumers.
Which is why Spirit still exists and the Concorde doesn't.
But perhaps more importantly, the engine needed to fly at supersonic speeds were far less efficient than regular
subsonic airliners at the time.
And they would be about four to five times less fuel-efficient than today's airliners. The explanation for this discrepancy
lies in something called bypass ratios.
Which is the ratio of air going around an engine core to that going through the core. The higher this ratio,
the more efficient the engine. Today's commercial planes use turbofan engines,
which take in more air than they burn in the core. The rest of the air is bypass around it,
and expelled through the back. These engines have bypass ratios upwards of 10:1.
Supersonic jets like the Concorde on the other hand, require the use of a turbojet engine,
which burn all the air they take in through the core.
This is highly inefficient and is known as a zero bypass engine because its bypass ratio is literally 0:1.
This inefficiency and the need to reduce drag means a narrower and smaller plane, which means less seats for passengers,
which further increases the cost of flying it for the airline and therefore the passengers.
Thankfully NASA as well as a few private startup companies backed by existing aviation giants like Airbus and Boeing are working to solve these problems
and bring commercial supersonic flight back. The news headlines about this
worry me, however, because they all seem to be
focused on the reduction of the noise made by the sonic boom. Perhaps
it makes a better headline than the development of a more fuel-efficient turbocharged engine, or more likely
it's just the easier problem to solve. Don't get me wrong though, easier doesn't mean easy.
Sonic booms start with the shockwave formed when an airplane is flying
supersonically. There are two main points where these shockwaves form: at the nose of the plane is the bow shock and just off the tail
is the tail shock. Then there are other points that create shocks – these are the wing leading edges, the canopy, the
engine housing, and more that combine with the bow and tail shocks. The bow shock compresses air above atmospheric pressure
and then the tail shock
immediately decompresses the air back to normal.
These rapid changes in pressure are what create the two loud booms heard on the ground throughout the planes entire flight path.
All this makes for a unique design challenge. The swept delta wing design of many
supersonic planes serves a function of reducing shock waves produced by the wing, and more importantly
tricking the airflow over the wing into traveling at lesser speeds by disrupting its flow over the wing.
Unfortunately while this does reduce the sonic boom, it also immensely increases drag,
which makes the plane burn more fuel. To get around this issue, NASA has developed Distributed Roughness Elements,
or DREs, to disrupt the turbulent airflow
created on swept wings, and smooth it out toward the preferred laminar flow which just means air is flowing in parallel layers and not creating
significant friction drag. The math behind it is complicated,
but DREs are basically a pattern of bumps on the wing that smooth out the airflow,
which when you think about it is kind of ironic. Some companies have designed windowless cockpits with
360 degree cameras
Displaying the outside to the pilots and others want to get rid of planes' windows
altogether by giving passengers an electronic display of the outside. There are also several other
interesting design changes that should help with making the sonic boom into a sonic thump, and while all this sounds great
I can't help but point out
it does not solve our inefficient engine issue
despite making up for some of it by reducing drag caused by other parts of the plane, the conundrum is that it's not as simple
as making more powerful versions of the high-bypass engines on subsonic airliners, because those engines actually become much more inefficient at
supersonic speeds, because those large engines create massive drag.
But even if the new supersonic airliners wanted to use turbojet engines like the Concorde,
they'd be faced with more noise complaints from the FAA.
This very year, strict new guidelines from the FAA on noise are coming into effect. But while it's
not known if they would be applied to commercial supersonic airliners, it's pretty clear
the tremendously loud Concorde engines would not make the cut.
NASA and General Electric have worked together to potentially solve this using a variable cycle engine, also known as a three stream turbofan.
It's almost like a hybrid engine. Air from the engine's intake floats through the core, is bypassed around the core, and through an additional
pathway. The air flowing through the third pathway
can be combined with the other air streams to allow the pilot to change the engines' bypass ratio as needed. In theory the plane could
have a high bypass ratio at takeoff like current subsonic airliners,
which would reduce noise and increase fuel efficiency, and then switch to a low bypass ratio once
it's reached altitude needed to travel at supersonic speeds.
I hope to hear more about advances in supersonic engines for commercial flight in the coming years,
but this sounds promising.
The companies developing these planes seem bullish on having supersonic airliners become a reality again within the next five years.
There are many more technologies that make this more a possibility today than it was just 15 years ago,
including access to
materials other than aluminum for the body of the plane, which are more resistant to the high heat at supersonic speeds like carbon fiber
composites. And most importantly the advances in computer modeling allow engineers to make more versions and design tweaks than before, when the only
real option was testing a physical model in a wind tunnel.
So, are you ready to fly supersonic?
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