I'm not sure the X-15 was so much a secret, it was a test aircraft and I am sure it's technologies were guarded secrets but the aircraft wasnt used for military purposes, they were used for testing speed and height of aircraft, this is indeed a rare photo though, only 3 X-15's were made. Read this extract and then look at the link, it shows two other "in flight" photos of the exact same aircraft, can you imagine travelling at 6630 feet per second?
There were a total of 3 aircraft built, known by their tail numbers of 66670, 66671, and 66672.
At first the aircraft were powered by a pair of XLR11 rocket engines, which were the same engines used in the Bell X-1 due to the much more powerful XLR99 engine being behind schedule.
It was the first aircraft to routinely exceed 100,000 feet, and towards the end of the program, it routinely flew at 200,000~250,000 feet.
The record altitude for the X-15 was 354,200 feet, a record that stood for FORTY ONE years, until Brian Binnie in SpaceShipOne exceeded in 2004 by achieving and altitude of 367,441 feet.
In terms of speed, the X-15 was the first aircraft to exceed Mach 4, Mach 5, and Mach 6, with its fastest flight being Mach 6.72, or 4520 MPH.
The Mach 6.72 flight was flown by the X-15A-2, a modified version of the #2 aircraft, shown above, wearing it's ablative coating.
This is 6630 feet-per-second, nearly THREE times as "Fast as a speeding bullet". The aircraft was damaged by local heating, suffering several burnt-through areas on its lower (ventral) stabilizer. Although North American rebuilt the aircraft, it never flew again, and the program was terminated after 199 flights.
A typical flight would have the X-15 carried aloft by the NB-52 to 45,000 feet and Mach .85, and then released. The pilot would light the engine, and then fly a very precise profile for either a high-speed, or high-altitude flight. High-altitude flights typically released the X-15 in the vicinity of Wendover, Utah, while high-speed flights would release the X-15 in Northern Nevada. The ground track was 300~400 nautical miles, depending on flight profile, and the aircraft landed, dead-stick, approximately 8 to 12 minutes after release.
That's right, only eight to twelve minutes for the entire flight, from release to landing!
And the landing was unpowered, "Dead Stick", with a descent rate of 12,000 feet-per-minute, and a glide angle of 12*~20*, at 200~300 knots, depending on where the aircraft was in the landing pattern.
http://every-blade-of-grass.blogspot...years-ago.html
Dennis