My friend Mark recently started reading Dan Brown’s Angels and Demons and clued me in last night on some of the rather… interesting… things going on in the book. Now I know that some people have questioned the quality of Brown’s research on the subjects of Renaissance art and secret societies and such, but, frankly, I really don’t know a lot about those subjects. I do, however, know a thing or two about air- and spacecraft; it’s one of those subjects they expect you to know about before they give you a degree in aerospace engineering.

So let’s have some fun, shall we?

The word took a long moment to register. “Switzerland?” Langdon felt his pulse surge. “I thought you said the lab was only an hour away!”
“It is, Mr. Langdon.” The pilot chuckled. “This plane goes Mach fifteen.”

We might as well start at the beginning. Our intrepid Mr. Langdon is being flown from Boston to Geneva, a minimum distance of 3,919 miles, according to Google. The speed of sound at the altitude they’re flying (60,000 ft) is somewhere around 660 mph. So, in fact, if you want to assume a constant speed between Boston and Geneva–which isn’t really possible because supersonic flight isn’t allowed over land and you never fly a straight line between two points anyway–you could get to Geneva in an hour at only Mach 6. I mean, why bother with Mach 15? Sure, if you could go Mach 15, you could cover that distance in about 24 minutes. And wouldn’t that be nice?

However, as some of you have probably guessed, there are reasons that we don’t currently have Mach 15 hypersonic commercial transporters flying all over the place. I’ll hit a few highlights as we go along.

Going Over The Top?

Brown’s pilot describes the plane as a prototype Boeing X-33 aircraft. Presumably, Brown got this from the now-defunct Lockheed Martin X-33 craft that was being developed for NASA. Brown’s description of his X-33 as “reminiscent of the space shuttle except that the top had been shaved off, leaving it perfectly flat” sounds about right in describing the actual X-33 prototype. Here’s the catch, though: the real X-33 was a single-stage-to-orbit (SSTO) reuseable launch vehicle. In other words, it was meant to get people and items into orbit, not across continents. Brown includes no descriptions of weightlessness in the flight, so presumably his X-33 is flying at Mach 15 in the atmosphere and has neither an orbital nor a suborbital trajectory on the way to Geneva. Moreover, the real X-33 was a vertical launch vehicle that landed like an airplane. But, judging from Brown’s text, his X-33 operates just like an airplane. A really, really fast airplane.

There’s no denying that the real X-33 would have gone Mach 15 in its flight to orbit. That’s what rockets do. It’s also what air-breathing engines do not. Right now the fastest reliable air-breathing supersonic transport comes in the form of the ramjet engine. Although ramjets are capable of producing speeds of up to about Mach 5, their optimal velocity–i.e. the velocity at which running the engine is not a complete and total waste–is close to Mach 3. The supersonic combustion ramjet engine (i.e. the scramjet, which I’ve mentioned previously) is currently under development. No one is sure what the top speed of an ideal scramjet would be, but I can tell you that current technology has managed to produce flight of Mach 10 for a few seconds, a far cry from Brown’s Mach 15 for an hour.

What kind of propulsion Brown’s Boeing X-33 uses is not particularly clear. According to the pilot, the plane has “HEDM engines” (high energy density matter?) that operate on “slush hydrogen”. Provided that the author actually meant ‘high energy density matter’ as the full version of HEDM, then his engine sounds to me like some nondescript liquid rocket engine. Chemical rocketry is more than capable of Mach 15 flight, but then, chemical rocketry is also incredibly expensive, even by the standards of CERN, which supposedly owns this X-33 Impossiplane.

Big Ba-da-boom

As I mentioned before, supersonic flight is, in general, not permitted over land except at extremely high altitudes. The reason for this is pretty clear: it’s loud. Normally when any object moves through a fluid–a ship going through the ocean, your car driving down the highway, etc.–the liquid or gas will move smoothly around the object. This is why we talk about streamlined shapes and such. The reason that the air normally moves smoothly around an object is because, when that object starts moving, it pushes the air particles in front of it and that vibration, or pressure wave, is sent ahead in the air. In effect, the air ‘knows’ that the plane is coming before the plane gets there and is thus able to get out of the way. These pressure waves are equivalent to the minute pressure waves and vibrations that humans interpret as sound. Move faster than sound and you move faster than the pressure waves. In other words, the air in front of you has no idea that you’re coming until you slam into it. This, incidentally, is where the concept of the sonic barrier comes from.

So what happens when you slam into a slab of unsuspecting air at supersonic speeds? Well, the air needs some way of getting out of the way and this is where the idea of a shock wave comes in. Any object traveling faster than sound will generate a pattern of shock waves that’s dependent on the shape of the object. This is all fine and dandy, except that airplanes don’t exist in an infinite fluid where the shock waves can go on uninterrupted forever. At some point, the ground gets in the way. Without going any further into the physics, I’ll just say this: going faster than the speed of sound means subjecting everyone you and your shock waves pass over to a horrible deafening sound. It’s like a gunshot (because, after all, the bullet travels faster than sound) but worse when the object is the size of an airplane.

This is why the Concorde was banned from traveling at supersonic speeds over land. It was deafening to anyone unfortunate enough to live near the flight path. It’s also why you couldn’t keep a supersonic aircraft that flies at Mach 15 very secret. But I digress…

Don’t Touch That, Honey

One of the other side effects of crossing a shock wave is that air experiences a tremendous jump in pressure, density, and temperature. How tremendous, you ask? Well, for a shock wave traveling at Mach 15, the air outside the aircraft would become 45 times hotter. This calculation does not take the friction between air and the surface of one’s plane into account. Continuing with that assumption, I found that, given normal temperatures at 60,000 ft (18,000 m), the temperature on the other side of a shock wave at Mach 15 would be just under 17,000 degrees Fahrenheit (about 9,400 degrees Celsius). Okay, so that sounds hot, but how hot is it? The surface of the sun is just under 10,000 degrees Fahrenheit (about 5,800 degrees Celsius). This begs an important question: how is this aircraft going to withstand that kind of heat?

Not to worry. Not to worry. The pilot informs Langdon that “the shell’s a titanium matrix with silicon carbide fibers”. Alrighty. I’ll interpret that to mean that it’s made of titanium silicon carbide. A quick jaunt around the Internet (known to Brown as the “Worldwide Web”) produces for me a scientific paper on this very material. It turns out that the substance is under investigation for use in jet engines, which are definitely hot places. Unfortunately, the first page of said paper also says that the high melting point of TSC is 3,200 degrees Celsius (about 5,800 degrees Fahrenheit). I’m sure that everyone will agree with me when I say that 17,000 is a bigger number than 5,800. This equals a problem. Actually, I guess it’s only a problem if you intend to have an airplane and passengers left when you get to the other side.

…

Interestingly enough, if they were to fly at, say, Mach 7 instead of Mach 15, life would become a whole lot less impossible. At that speed, one could still get to Geneva quickly without flying faster than sound over land. The propulsion becomes much more likely, and the temperature outside of the aircraft becomes a much more manageable 3,600 degrees Fahrenheit (2,000 degrees Celsius).

But Mach 7 just doesn’t sound as much like a gutsy thriller now, does it?