Why We Miss the X-15

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

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)

To commemorate the 40th anniversary of the fastest flight by an airplane—the X-15’s rocket-powered, Mach 6.7 dash on October 3, 1967—U.S. Air Force Chief Scientist Mark Lewis and former Air Force historian Richard Hallion recently met with Air & Space editor Linda Shiner at the National Air and Space Museum to talk about the X-15 program and its legacy.

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A&S: It’s been 40 years since the X-15 flew 4,520 mph, a record that’s never been broken. Why did we stop there? Why didn’t we continue to build faster and faster research aircraft?

Lewis: Embedded within that question is the question of whether we should have continued the X-15 or X-15-like programs. And to that the answer is clearly "yes." It’s one of the great tragedies in aerospace that we didn’t. Look at the record of the X-15 program: 199 flights—200, if you count the time an X-15 blew in half during a rocket engine ground test and shot Scott Crossfield forward a few feet. A very clever, well-conceived series of experiments and series of measurements. It was true science. The X-15 produced over 700 technical papers.

The science and the engineering that came out of the X-15 program were absolutely phenomenal. It was bold, it was daring, but it was very, very well grounded in real science, and that’s a very fine line to walk and it’s something I deal with almost every day. We want to do bold, great things, but we shouldn’t attempt to violate the laws of physics.

The X-15 was a culmination of our notion of flight test, the experiments in the air, pushing the envelope, but doing it in a logical, evolving manner.

Hallion: It was a tremendous focal point for activity. It was where you could actually take theory, and ground test, and simulation, and you could match all those against a real world flying system. Where the measurements you got were real world measurements. You didn’t have to figure out what are the scaling factors. You didn’t have to calculate from approximate data. You were getting real data.

A&S: And this data was used in the design of the space shuttle.

Hallion: The X-15 gave us the exact same kind of mission profile coming back in, once it got to Mach 6 and was headed downhill. There was a difference because the X-15 was not a delta [wing design] so you have differences in the actual approach and landing. But the low lift-to-drag ratio approach was identical, the workload on the pilot was identical. It gave us an exact indication of what would be involved in the way of cues, flight path management, energy management—everything came out of the X-15.

But back to the original question, I think where we got off track in aeronautics was the cancellation in 1963 of the Manned Orbiting Laboratory program, we began a pattern of fits and starts that has continued to the present day. We replaced DynaSoar with Manned Orbiting Laboratory, then we didn’t build the Manned Orbiting Laboratory. Had we fulfilled Manned Orbiting Laboratory, several things would have happened. First of all, we would have achieved a space station at a much earlier date, much like the way the Soviet Union achieved a space station. The second thing is you would have had to have had a launch vehicle to get a team of astronauts up to that space station, and we actually had that system: It was called Gemini.

Beyond Gemini and the Titan launch vehicle, there were plans to take a lifting-body shape, the so-called SV-5 shape, which was an Air Force-Martin program. Then NASA came on board as well, and NASA had plans themselves to look at a lifting-body shape that they called the M-2. Northrop was willing for a price of $200 million to build a piloted, demonstration orbiting vehicle. This would have led to a routinely operating hypersonic reentry vehicle that we could have used to support operations of the Manned Orbiting Laboratory.

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