Jay Miller wrote a book called The X-planes: X-1 through X-31, and later revised it to include craft through X-45. (Presently, the number is up to X-56.) If you are any kind of a space aficionado, you need to look it up in your local library. The only thing wrong with the book is its title; not all X-craft were planes.

When I was growing up, I was in love with the F-104 Starfighter and the X-15 rocket plane, two aircraft with markedly similar outlines. The F-104 was designed, a few prototypes were built and tested, then it went on to become one of the most successful fighter jets in history. The X-15 was designed, three were built (and rebuilt after various incidents), and tested. There were never any more X-15s, nor had there ever been a plan to build more. The X-15 was never a prototype, because a prototype is a first iteration, built to be tested, perfected and turned into a series. That is true whether you are talking about fighter jets or can openers, but not true of X-craft.

X-craft were something different. They were flying laboratories – a much overused term, but still accurate, although technology demonstrator is preferred.

After World War II, advances in flight by Germans, British, and Americans, along with nascent cold war tensions, put us on the edge of an unknown frontier. The “sound barrier” loomed as the best known obstacle to further advances in aviation, but there were a hundred other unknowns that never made it into the popular press. Wind tunnels could only tell so much, computer modeling was decades in the future, and it made no sense to build a squadron of high-performance aircraft that might or might not fly.

A glance at the first X-plane, the Bell X-1, gives clues to what was known and what was not known. We already knew that turbulence off the wing would foul up the tail controls at high speed, so the horizontal stabilizer was attached high up on the vertical stabilizer, not on the body. It was not known what negative effect the cockpit bubble would have, so the windscreen was faired into the shape of the fuselage. It was shaped like a 50 caliber bullet – everybody says that without explaining. It is a reference to the machine guns carried on fighter planes. Fired from a ground rest, those bullets would have been sub-sonic. The planes they were mounted on flew at subsonic speeds, but bullets fired from a plane in flight had been going supersonic for a long time.

An aside here for the non-nerd. Planes flying slower than the speed of sound are subsonic. That includes all commercial aviation except the Concorde. Planes flying faster than the speed of sound are supersonic. The X-1 and its follow-ons proved that supersonic flight is not problem. The problem is the transition zone, the trans-sonic region. Slower than sound, the accumulated shock wave is out in front of you. Faster than sound, it is behind you. At the speed of sound, it is right in your lap, trying to tear your plane apart. No modern, supersonic plane lingers at that speed.

The X-1 broke the sound barrier (i.e., passed through the transonic region into the supersonic region) on October 14, 1947, with Chuck Yeager at the controls.

Since this is A Writing Life, I’ll add that I was born about two months after the sound barrier was broken. I was present on the planet for almost all of the early X-craft explorations, although far too young to notice. When I became aware of the X-craft, I fell gloriously in love with them and the infatuation never passed.

The X-2 was a more normal looking aircraft, with swept wings and a pilot’s bubble. It carried supersonic speeds to new heights, but killed its pilot in the process. I will tell that story next Wednesday.

The X-3 was an extreme aircraft, stretched out and incredibly streamlined right down to the tip of its needle nose. It looked faster than any plane before or since, but it wasn’t. All that streamlining couldn’t make up for the fact that the engine slated for the plane wouldn’t fit, and the one that did fit was underpowered. The fastest (looking) plane in the sky flew slowly.

The X-4 was tailless and not successful. The X-5 tested variable sweep wing technology. The X-6 was an aborted project testing out the possibility of a nuclear powered aircraft. The X-8 was a small, unmanned rocket designed for upper atmospheric research. The X-7, and X-9 through X-12 were test beds for missile research. X-13 and X-14 were early attempts at Vertical TakeOff and Landing (VTOL).

Then came the coolest aircraft/spacecraft in the history of mankind, the X-15, which will be the subject of tomorrow’s post.