Looking back at NASA's X-15 aircraft

Back in the "Right Stuff" era, even before there were astronauts, there were test pilots. It was the test pilots who flew the early generations of high-powered aircraft such as the X-1 that opened the way to space programs from Mercury to the space shuttle.

In this slideshow, we'll take a look at the X-15 rocket plane, which first flew 50 years ago this week and which NASA calls "the most successful research aircraft ever built." This image shows the X-15-1 aircraft in 1958, at which point the XLR-99 rocket engine intended for it was not ready, so it was fitted with a pair of older XLR-11 engines, like the one used in the earlier X-1.