The Physics of Supersonic Free Fall and the Race to Build a Quieter Concorde

If you want to build a rocket with a bold new design, you have to have a way to test its structural integrity without installing an engine. You don’t have a wind tunnel, but you’re not ready to concede. You think to yourself, “What is flight without propulsion?” Then you answer your own question: “Falling.” Simply put, the easiest way to fly without launching is to plummet. Take a prototype up very high, drop it, and you’ll get a sense of its performance at speed.

The world’s foremost practitioner of the art of precision dropping is the Japan Aerospace Exploration Agency, or JAXA, which is basically Japan’s version of NASA. The agency is trying to build a practical supersonic plane, which is no easy thing. Similar efforts in the past created mediocre products, most famously the Concorde.

The Concorde was plagued with problems that prevented other airliners from adopting the same kind of design for their own craft. One of the biggest issues was excess noise. The term “sonic boom” isn’t a misnomer — breaking the sound barrier is an insanely loud phenomenon. Manufacturers had to design the plane to keep passengers heads from exploding, and airliners couldn’t fly the plane over land since no human being on the ground wants to be subjected to such destructively loud sounds. JAXA’s goal is to create a quieter supersonic passenger plane. And its testing it out through drop tests with an experimental model in Sweden.

How the hell does that work? Basically, a balloon lifts the unmanned model plane — JAXA’s Silent SuperSonic Concept Model about 18.6 miles up in the air, and simply drops it. Sensors attached to the plane measure the shockwaves as the plane nears speeds of up to Mach 1.39 in free fall.

The physics of a supersonic free fall are not all that different from how an object moving faster than sound on a horizontal plane operates. Air becomes powerfully compressed in front of the plane, which floods out a wave of high pressure in all directions. This shockwave starts to propagate through the air but gets weaker as it moves further out, becoming a sound wave in the process. This is the loud explosion we hear and call a sonic boom.

To understand what’s special about a supersonic free fall, we should take a closer look at what exactly Mach numbers refer to: the ratio between the speed of object to the speed of sound at a particular place. And the speed of sound is subject to changes in temperature and pressure — at higher altitudes, the speed of sound decreases, so an object doesn’t need to travel at necessarily the same speed to reach Mach 1 a dozen miles in the air as it does at sea level. (The speed of sound at sea level is about 760 miles per hour).

Furthermore, Mach 1 is a highly unstable environment due to the shockwave created by breaking the sound barrier. Even small movements can have very forceful physical effects on the object. The worst place to be is basically between Mach 0.9 and 1.2.

So when an object is moving at supersonic speeds in free fall, it’s in the unusual position of accelerating faster while its Mach number increases at a slower rate. More time is spent in the unstable Mach zone than if it were moving on a horizontal plane. Most planes are designed to move past Mach 1 and enter a safe zone as quickly as possible. You can’t test something like that in a free fall experiment.

The speed also tops off because of drag. This is what happened in probably the most famous instance of an object moving faster than sound by way of gravity: Felix Baumgartner’s jump in 2012 from about 23 miles up in the air, to become the first sky diver to break the sound barrier without the use of an aircraft. When Baumgartner fell down to Earth, he eventually stopped accelerating because of the collision with air molecules, creating ‘drag force’ that built up as air resistance until it became equal and opposite to the force of gravity. At this point, Baumgartner had reached a maximum speed.

In fact, while most objects that reach terminal velocity would simply stay at a constant speed, Baumgartner actually started slowing down, since the surrounding atmosphere starts to get thicker and thicker as an object in free fall moves down. So the terminal velocity starts to decrease — meaning Baumgartner started to slow down as well. The same thing would presumably happen to one of the Silent SuperSonic Concept Model planes JAXA is testing.

Science, like most other things in life, is cooler when its faster.