You didn’t ask for this, but we’ve done it anyway
Hi friends,
This week, we had planned to write about a y-combinator avionics start up called Boom. Boom is building commercial planes fit for supersonic travel (which hasn’t really been done successfully before, but would be a huge boon for long-distance travelers).
The next thing we usually try to figure out about a company is: what, exactly, is the challenge that they are surmounting? And why is their approach to that challenge novel?
Those answers for Boom come down to physics and aviation economics and principles of engineering. Lea decided it was time to phone a friend. What resulted was a long story but an interesting one.
Lea: So, commercial supersonic jets. My understanding is that we’ve had the technology to do this for a while, but it’s never been economical. Why is that?
Noah: It takes a huge amount of energy to move something that fast. We do have military aircraft that are designed for supersonic speeds, but most fly at high altitudes where there is less air resistance (because there is less air). This makes it easier to maintain high speeds. But these military aircraft are small and light, so they require relatively less energy to lift than, say, a passenger plane with people and cargo. For that, you need some serious engine hardware...engine hardware that is expensive and heavy and difficult to build robustly.
Lea: Okay, so: because flying fast = more energy, and flying high = less energy…we try to fly high when we fly fast. If you try to do this with passengers and their extra weight, it becomes doubly harder.
Noah: Those are some of the reasons.
I think another factor is that even if you have the energy to fly fast at low altitudes, the higher air density leads to more pockets of varying pressure (a mechanism that causes turbulence, I believe). And when you move supersonic, you cross those pockets very fast… this can cause instability (the plane could flip) or engine malfunctions (not enough intake air = burnout; too much intake = explosion).
Another thing is sonic booms, which can be dangerous to people and structures nearby. Planes need to move sub-sonically until they get far away. This limits supersonic trips to any route that is long enough to warrant the added overhead of moving to and from safe supersonic areas.
Finally, when a fighter pilot’s supersonic craft spontaneously disintegrates, that’s bad. When a plane full of 300 civilians spontaneously disintegrates, that’s a deal breaker.
Lea: So add turbulence, sonic booms, and safety concerns to the odds stacked against anyone wanting to try commercial supersonic travel.
But Noah, what I don’t get is, like, what is “the sound barrier”?
Noah: It is a distinguisher between two sets of speeds: speeds at which the thing making a sound travels slower than the sound it emits, and those at which the thing producing sound is moving faster than the sound waves it emits.
Lea: And then what is a sonic boom? Why is there this invisible barrier of sound?
Noah: The barrier is not one of sound. I don’t want to call it an "imaginary line" because that implies that it is arbitrary, but it is not arbitrary. It’s determined by the physical properties of the medium (air).
Lea: Ok kapeesh, but why is that barrier, that distinguisher, significant?
Noah: When you make a sound, it moves air particles back and forth. This creates a wave. The wave moves through the air almost exactly like a wave moves through water.
Sound waves can only move at certain speeds through things like air. They can’t go any faster because that’s just the speed at which atoms bounce into each other. It’s determined by density and things like that.
Lea: So when you move faster than the speed of sound, then what? Translate this into plane stuff for me.
Noah: The plane emits sound. When it moves faster than the sound waves it emits, the sound waves coming off the plane in the direction of travel can’t move out of the way before another wave is pushed. Right? So the plane is pushing off a sound wave in front of it, and then immediately crashing into that sound wave. And all the sound waves it creates are bumping into those sound waves from before. The sound waves therefore build up and merge into a wave front of more intensity; a shock wave.
We say sonic boom in the singular, but if you were traveling alongside the aircraft and staying in its sound wake, you would notice that the plane is really letting off a singular continuous boom.
Lea: So to summarize, you have this event — “the boom” — and our planes and the regulations governing them must be shaped around it. The cost of doing that has previously been too high; Boom is trying to be the first company to make it work.
Noah: Yes. The sonic boom is exciting to think about but the biggest influence it has is on the economics.
So why is Boom saying that they’re able to do what no one else has done before? They’ve put some innovations together to reduce cost. They claim that “total operating cost per seat-mile is comparable to subsonic business class.” This is afforded by the advantages of modern materials, like carbon fiber and kevlar, and other advances not employed by the Concorde (a commercial supersonic airliner that began flying in the 70s).
Boom is looking for a handful of new employees in engineering, people ops, and finance (ex. recruiting manager, accounting manager, aircraft technician, stress analyst).
Noah is a recent umich grad who knows about pretty much everything and is always willing to answer all of Lea’s questions and/or argue about the pretty-much-everything that he knows.
Okay, you made it: that’s the end of the longest 4CJ ever. See you next Monday for the regular stuff.
Love,
Your Caring Parents
(Dana & Lea)
4cooljobs@gmail.com