Supersonic is back in style

Well, it will be in 2029.

Jun 09, 2021

After an almost 20 year hiatus, supersonic commercial travel is back in mainstream media and, as you’d expect, I couldn’t be more excited. Boom Supersonic has stepped into the limelight over the past few days in its bid to bring faster-than-sound transport back to consumers. Just last week, United Airlines announced an order for 15 planes, with the option of 35 more. This would, by 2029, give you the opportunity to fly from New York to London in three and a half hours at 60,000 feet. That gives you three extra hours to tour the Big Ben, eat some Fish and Chips, or ponder how that plane even managed to go that fast. But don’t fear, just like BOOM, I’ll try to save you some time.

Quite simply, there are two main hurdles we run into when designing for supersonic flight. Fuel efficiency and noise. When you’re moving that fast, you experience a LOT of drag. We can calculate it with this:

In this case, the only variable we care about is our speed, V, and in this equation, it’s squared. This means for every 2x increase in speed, drag increases by 4x. Take Boom’s plane, for example. It flies at Mach 1.7—1,300 mph. This is roughly twice as fast as typical commercial airliners, which fly at around Mach 0.78 to 0.85 (500-650 mph). As a result, the supersonic aircraft has to generate 4 times more thrust when cruising than the competitors. To be fair, a good deal of this drag is offset by flying at higher altitudes. Also, supersonic drag is a lot more complicated than our friendly neighborhood drag equation above. It’s a lot meaner and differential-equation-ey-er. Regardless, this all boils down to one fact: there’s a lot more drag when flying this fast.

We have engines that can overcome this, military planes can reach these speeds relatively easily, but only for a short period of time. Current production engines can only run so hot for so long without needing to be serviced. Not to mention burning all of their fuel. Flying this fast means running the engine like you’re taking off—for 4 straight hours. This is exactly why the Concorde ended up going out of service. It had to carry an absurd amount of fuel and constantly needed to be repaired. As such it was too expensive to provide long-term business value. Not to mention it could only fly over water.

This brings us to our other hiccup: noise. Pushing the engines this hard is really, really loud. Which would be fine up at 60,000 feet, because it would dissipate decently well, but not well enough for shock waves.

Shock waves form when you travel so fast that the sound that you’re creating falls behind you. The “waves” merge together and become much more powerful. This propagates more effectively through the air, meaning it can travel all the way down to the ground and cause audible and even physical damage to people and property. The strength of these shocks is dependent on your Mach number. We’re trained to think this is your speed but it’s slightly different. Mach number is the speed you’re going divided by the speed of sound where you are.

V is your speed and c is the speed of sound in the medium you’re traveling through. As you get higher up in altitude, the air is thinner, meaning there are fewer air molecules to bounce into each other, and less opportunity for sound to travel. Therefore, the speed of sound at higher altitudes is slower. This means, if you’re going 1,000 mph on the ground, your Mach number is much higher than it would be if you’re at 60,000 feet. This is why we can calculate shock wave energy using Mach number, because the higher you are, the lower your Mach number, which corresponds to a less powerful shock wave. Did that make sense? I hope so, just be glad I didn’t try to explain it with math: [2]

Yeah, you’re welcome.

Unfortunately, even though noise reduction technology has improved dramatically since the Concord era, shock waves still aggressively violate FAA noise regulations. So, either the plane has to fly subsonic over land, which it isn’t designed to do, or it can only fly over water. Nonetheless, if Boom can get it done as fuel efficiently and with net-zero emissions as advertised, they have a real shot at giving us a cheap-ish opportunity of flying round-trip internationally and being home for dinner.

Thank you for reading this edition of It’s Not Rocket Science, see you next week!

It's Not Rocket Science

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