Understanding air density

The North American X-15 research aircraft has flown faster and higher than any other airplane in history. On August 22nd, 1963, it reached an altitude of 354,200 feet or 67 miles above the Earth’s surface on the very edge of space.

To reach such heights, the plane’s designers had to understand how air density affects its ability to fly. The density of air passing over a wing affects the amount of lift it can produce. Less dense air generates less lift. An issue when flying high in the atmosphere where air density is low.

The density of air depends on pressure and temperature. At higher altitudes there are fewer air molecules pressing down from above, so the air pressure decreases. And because the pressure decreases at higher altitudes, the air molecules spread out and the temperature decreases. Decreased air pressure and temperature means decreased air density.

Most planes have a maximum altitude they can fly at before the air becomes too thin to produce further lift. To fly higher, airplanes need powerful engines to produce more speed to increase the air flowing over the wing to make up for the reduced air density. 

To generate enough speed at these high altitudes, the X-15 needed a powerful engine. It was fitted with a single rocket engine that produced an incredible 57,000 lb of thrust, more than 9 times the power of the first plane to break the sound barrier 20 years earlier, the Bell X-1.

The X-15 needed so much fuel to power its engine it could provide thrust for only 120 seconds of a flight. But, this was enough for it to fly 6 times faster than the speed of sound and past the edge of the Earth’s atmosphere into space.

At these high altitudes, the low density of air meant traditional airplane controls didn’t work. The X-15 needed special "thruster" control rockets located in the nose and wings of the aircraft to enable the pilot to maintain control when flying on the fringes of space.

The X-15’s 199 high altitude flights  contributed to the development of the U.S. space program. It helped develop the navigation controls and thermal protection of the Saturn rockets along with the pressurized spacesuits worn by the first astronauts.