Why We Miss the X-15
Not only was it the fastest. It may have been the best flight research program ever.
- By Linda Shiner
- AirSpaceMag.com, November 01, 2007
(Page 4 of 5)
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?
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.