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Inconel X, a ferociously strong nickel alloy, gives the X-15 its gun-metal black color. Inconel was chosen for the airplane's skin because it retained its strength up to 1,200 degrees Fahrenheit, a temperature the X-15 would routinely experience at high speeds. (Eric Long/NASM)

Why We Miss the X-15

Not only was it the fastest. It may have been the best flight research program ever.

That fin is basically just a wedge. Why a wedge? At low speed, an airfoil shape is very important. As you reach higher and higher speeds, it actually becomes less important. Here’s what that wedge does for you: It turns out that when you’re flying at high speed, you get a shock wave forming on either side of the wedge. It’s almost like a wake coming off the bow of a ship, just as you get a shock wave off the front of the aircraft. If you deflect that wedge—say you deflect it a degree; you gain a degree on one side, and lose a degree on the other—at high speed, the differential pressure caused by a change in angle of the shock waves is about three times greater than at low speeds. So that becomes an incredibly effective control surface. It’s not an effective control surface at low speed, but it’s extremely effective at high speed. Shock waves are inherently non-linear. That means you change the angle by a certain amount and the change in properties doesn’t scale in linear proportion.

Hallion: Put another way, if you were to use the conventional thickness ratio for that vertical fin, for the same degree of directional stability, you’d have to have a much larger fin. It would have to be huge. It would be so huge it would be structurally impractical. This in a way is fooling the air into thinking that the fin is much, much bigger than it actually is.

Lewis: It was brilliant aerodynamics.

Hallion: It really was. It was a very creative design.

Lewis: I have to say, as much as I love the space shuttle, the space shuttle is a horrible aerodynamic design. The space shuttle was designed so it wouldn’t burn up on reentry, and you get these incredible aerodynamic compromises in its design.

Hallion: It’s a railroad car. You can make a model of the space shuttle by taking a box car and putting on a nose cap, a tail cap with the engines, delta wings, vertical fin, and the Orbital Maneuvering System pods. And you’ve got yourself a space shuttle, because the core of it is this big box, a 65,000-pound payload bay. But it doesn’t really have design elegance.

Lewis: There was another phenomenon on the X-15. There was a famous flight of an X-15 when they were testing the airflow around a dummy air-breathing engine, and the engine burned off. And we understand in gory detail now why it burned off. It was a shock interaction that at the time we really didn’t understand very well.

Hallion: It’s called a shock-shock.

Lewis: Basically, two shocks intersect. One shock wave hits another shock wave, and when they interact, there is a very, very hot jet of gas. And this is why it’s important: We now worry about that when we design a scramjet engine [a Supersonic Combustion ramjet]. In the flight of a scramjet, there will be a shock wave coming off the nose of the aircraft and another shock wave formed from its own lip, or inlet. And when those two shock waves meet, if we’re not careful, we could get the same style of interaction. So one of the very first design principles in selecting the inlet for a scramjet or a high-speed ramjet is "no shocks interact."

A&S: Another of the X-15’s innovations was the all-moving tail. Why was that important at high speeds?

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