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 3 of 5)
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.
That fin is basically just a wedge. Why a wedge? At low speed, an airfoil shape is very important. As you reach higher and higher speeds, it actually becomes less important. Here’s what that wedge does for you: It turns out that when you’re flying at high speed, you get a shock wave forming on either side of the wedge. It’s almost like a wake coming off the bow of a ship, just as you get a shock wave off the front of the aircraft. If you deflect that wedge—say you deflect it a degree; you gain a degree on one side, and lose a degree on the other—at high speed, the differential pressure caused by a change in angle of the shock waves is about three times greater than at low speeds. So that becomes an incredibly effective control surface. It’s not an effective control surface at low speed, but it’s extremely effective at high speed. Shock waves are inherently non-linear. That means you change the angle by a certain amount and the change in properties doesn’t scale in linear proportion.
Hallion: Put another way, if you were to use the conventional thickness ratio for that vertical fin, for the same degree of directional stability, you’d have to have a much larger fin. It would have to be huge. It would be so huge it would be structurally impractical. This in a way is fooling the air into thinking that the fin is much, much bigger than it actually is.
Lewis: It was brilliant aerodynamics.
Hallion: It really was. It was a very creative design.