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)

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We turned our back on that as well. When you had then the fulfillment of Apollo in 1969 coming after the shut down of the X-15 program [which ended in 1968], and you had the cancellation of any plans to extend the X-15 program to use a delta-wing configuration and to make use of an experimental scramjet propulsion system or at least to test an experimental scramjet propulsion system—you had the stage set for the plateauing of hypersonic research.

In the 1970s, we had two opportunities to reinvigorate our hypersonics research. One was a program called the National Hypersonic Flight Research facility, which would have built on lifting-body experience to give us a manned hypersonic demonstrator up to about Mach 7. And the second was in support of the space shuttle. That was a proposal by a group of engineers within NASA at the Dryden Flight Research Center to develop a subscale shuttle that could have been flown out to Mach 5 to 6 to collect reentry data in support of the shuttle program. That would also have given us the ability to use that vehicle at some point for plain old hypersonic research, to evaluate materials, systems, propulsion concepts—things of that sort. But when we turned our back on that, and the bills started to come due to attempt to meet the anticipated launch rate for the space shuttle, we simply didn’t have the money. And so the next big step, the NASP, had its own challenges.

A&S: Was the X-15 data used in that X-30 program, the National Aerospace Plane?

Lewis: It’s easy to bash the X-30 program. They spent a ton of money, and there’s been a bit of revisionist history, of people looking back at the X-30 and saying "Oh we got all this great stuff. We got new materials, we got this, we got that." The reality is the X-30 was very much the antithesis, in my mind, of the X-15. It was "Let’s not do a logical, reasoned science effort. Let’s jump to Mach 25 the first time out of the barn. We’re going to build it, we’re going to hop in, we’re going to fly it, it’s going to work. We’re not going to do any ground testing because we have all the computers we need to simulate anything we’d ever need to simulate." And they were completely wrong. It’s still wrong to this day.

Hallion: There was over-enthusiasm about computational fluid dynamics. It was going to replace all ground test facilities and much of flight test facilities; you could do it all by crunching numbers.

Lewis: We had people saying that the X-30 marked the end of the wind tunnel. We no longer needed wind tunnels because we could simulate everything on a computer. And you can contrast it with work today. We’ve gotten into this mode where we don’t really do envelope expansion. We keep shooting for the Next Greatest Thing.

So look at where we are with today’s programs. I have a personal favorite: the Air Force’s X-51 [a small-scale hypersonic vehicle]. It draws as much from the X-15 as from any other program in terms of the materials that are being used and our understanding of the basic physics.

It’s an Inconel structure [as was the X-15]. It uses some space shuttle materials, but our understanding, for instance, of the airflow around a very sharp leading edge—that didn’t come out of the Shuttle program; it came out of the X-15.

A&S: And how did that information, that data, from the X-15 program get into the hands of the engineers working on the X-51?

Lewis: It comes out of the scientific literature. I’ll give you an example—in terms of fluid mechanics. The fluid physics associated with the fin on the X-15 prove that the people who designed the aircraft knew exactly what they were doing.

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