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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.

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Hallion: The X-15 had a rolling tail. It not only had a pivoting surface that was movable in pitch; it also had differential movements so you could use it as an aileron [to roll the aircraft]. And that’s why it was called a "rolling tail." On the wing, the inset aileron you might think would be used for roll control were actually flaps that would dramatically increase the lift on final approach.

The best configuration for [hypersonic] vehicles tend to be delta-wing blended-body configurations and we were headed down that road. Had we not lost the Number 3 [X-15] airplane [in a November 1967 crash that also took the life of pilot Michael Adams], there was every expectation that it would have been modified as a delta-wing aircraft.

Lewis: There’s an interesting comparison with the Orbital Sciences Antonio Elias, drew from the X-15 dimensions. The wingspan was set by what the B-52 could carry. That’s another example of the X-15 legacy.

Hallion: One of the things the program engineers discovered when the X-15 returned from a mission is that occasionally you’d get dimpling on the structure. Aircraft experience structural loads because they’re generating lift and experiencing other forces. But when you heat the structure, it experiences a whole different kind of structural load. These thermal loads would occasionally buckle the skin near rivets or where there were plates joining. And this little buckle would act like an inlet. At hypersonic speeds, air would ram through that inlet, and damage could occur downstream. So if you look at the skin of the X-15, you’ll see some interesting things. On the leading edge of the wing, you’ll see things that look like little cap strips. Those cap strips were used to prevent hypersonic leakage into the structure, based on what people discovered from actual flight test results.

Looking at the X-15 here from the second floor [of the National Air and Space Museum] under the lights, you can see a sort of beer canning effect. You see that this vehicle really has been through it; it’s withstood that thermal hypersonic environment, and it has that gun-blued look to it. When people make models of the X-15, they make a neat, flat black model, but it’s really not. It’s a heat-treated metal.

You take a look at the gaps and voids [in the skin] of the X-15, which were designed to accommodate the anticipated thermal expansion of the material. Where the X-15 wing joins the fuselage, there’s a significant gap that starts at the leading edge of the wing and reaches back almost to the quarter chord. That’s an open slot all the way back to the spar. Now this is a very robust wing; it’s hard to relate this to the conventional rib and spar construction because the whole thing is almost solid. But the point that I’d make is we have gotten away from the notion of test as test. We use this terrible term "technology demonstrator." It is a term we should walk away from. Technology demonstration is what I do when I go into a high school science class; it’s where you have repeatable experiments through which nothing new is learned. We’re standing in front of the X-15, which is a vehicle of actual flight test. In flight test, you go out and you explore unknowns. You’re not demonstrating anything; you’re learning things. And if you’re going to learn things, you need to be able to study what you’ve accomplished. We have tantalizing bits of data from some of these programs where we don’t actually know what happened in the last seconds of a flight or the last seconds of an experiment. We don’t have the complete understanding that we would only gain if we were actually recovering something on the ground. We need to look at that artifact and study and figure out where we go next.

Lewis: Several years ago Donald Rumsfield got some bad press for using the term "unknown unknown," but engineers use that term all the time. There are unknown unknowns and known unknowns.

Engineers working on the X-15 program faced both types of unknowns. Towards the end of the program, there was experimentation with a variety of high-temperature ablative coatings. And there was a question, "What happens if the ablative coatings out-gas during the reentry and fog up the windows? How’s the pilot going to land?" They didn’t know how bad the effect would be. But the program was characterized by very clever engineering, and they found a very elegant solution. They simply put an iris over one of the windows. And the pilot flew with one window; when it fogged up, he opened the iris and looked out the other window—and that’s how he landed.

Hallion: And that’s another way that the X-15 made a contribution to future programs. The X-15 became a test bed for other systems and concepts. We learned with that spray-on ablator that that was not a good way to configure a reentry vehicle for a human presence in space. Now there may be some future time when we find that we can do that, but this countered what people thought at the time. There was the idea that you could build a really cheap, lightweight, conventional-material vehicle, maybe even out of aluminum, and you could spray it with this ablator, and you could fly it, and you could refurbish the ablator, and sha-zam! you’d be right back in business again. This demonstrated that you weren’t able to do that.

The X-15 so quickly demonstrated its utility that people started using it for things that it was not intended for. They used it to collect micrometeorites, to collect imaging from a near-space environment. You see this by looking at the pods, which were not original to the basic design but were added later in the flight test program.

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