Tuesday, January 31, 2012

Real Men Don't Fly Pink Airplanes

A slightly haggard-looking William "Pete" Knight walks away from X-15A-2
on October 3, 1967, after setting a world record speed of 4,534 mph. 
The visual image that most folks have of the pioneering and record-setting North American X-15 is of a black airplane, mainly for the reason that the skin was largely made of Inconel, which has a natural black color. There were three X-15s built, and were designated A-1 through A-3. The middle ship, A-2, made 53 powered flights, two of which (in addition to four captive-carry flights) were flown with a distinctive white coating, rather the regular bare black skin. Today's featured image is an Associated Press wirephoto dated October 3, 1967, which shows A-2 after one of those historic flights.

The official caption which accompanies the image reads: "EDWARDS AIR FORCE BASE, Calif., Oct. 3 -- END OF A RECORD FLIGHT -- Air Force Maj. William J. Knight, left, walks away from the X15 at Edwards Air Force Base in California today after flying the plane at a record speed of 4,534 miles per hour. The new mark for wingee [sic] craft, almost seven times the speed of sound, was 284 miles per hour faster than the 4,250 Knight reached in the X15 last Nov. 18. (AP Wirephoto) (see AP wire story)."

Even though the X-15's Inconel skin is, by nature, quite heat tolerant, the aerodynamic heating that was expected at the speeds that the record flights were to be flown far exceeded even this exotic metal. To deal with this, the engineers decided to cover the rocket plane's skin with an ablative, or heat-resistant coating.

This rubbery, pencil eraser-like material had the unfortunate property of also being bright pink.

An official photo of A-2 from the NASA Dryden website.
Officially, the NASA explanation for the white coating, which was applied over the pink, was that the actual ablative material was not fuel resistant, and thus needed additonal protection against damage that could be caused by fuel spills.

The urban legend, told in the hallways at Dryden, however, is that Pete Knight walked into the coatings hangar after the ablative had been applied, and said, "Like hell, I'm gonna fly a pink airplane!" And shortly thereafter, the white coating was added.


  1. Fantastic photos and a great story! One correction, it's not friction that causes fast objects to heat up. It's compression of gas. Much like an air conditioner in reverse, when a gas is compressed, all of the heat that was present in the gas before compression is now concentrated into a small area. Love the X-15! One of the best looking machines ever made. Thank you for the post!

  2. Dale...cool! I learned something new today. http://en.wikipedia.org/wiki/Aerodynamic_heating gave more details...