Speed of Sound

U2

Some time ago I was reading about the U2 spy plane and I came across a statement that intrigued me. Seems that the U2 had a very narrow range of flying speed when operating at altitude. It was constrained on the low end by its' stall speed, below which it would fall out of the sky, and by the speed of sound on the upper end. The U2 was not a supersonic aircraft and bad things happen to planes that try to go faster than the speed of sound when they are not specifically designed for it: they fall out of the sky. So it had a range of maybe 10 or 20 knots (or 4 knots, depending on where you heard the story). In any case, there was a very narrow range of speed that the plane could operate in.

So I started looking into this the other day, and I found something very surprising: the speed of sound is not dependent on altitude. The air pressure and the density both affect the speed, but they do it in an opposite manner such that they cancel each other out. This was a big surprise to me. Apparently it is not an uncommon misconception. You learn in school that denser stuff, like steel and water conduct sound faster, so you might assume that less dense air would conduct sound slower. This is not the case.

US Standard Atmosphere Model

Now air temperature does affect the speed of sound, and at high altitudes it gets very cold. The U2 has typically operates upwards of 70,000 feet, which translates to about 21 kilometers, so the temperature has dropped a bit, and so has the speed of sound. The speed of sound at sea level is about 760 MPH, and at the U2's cruising altitude it is a about 100 MPH less, or 660 MPH. The U-2's maximum speed was only 500 MPH, so it was in no danger or breaking the sound barrier.

However, the stall speed at that altitude was 490 MPH, so the pilot was hemmed in by these two numbers. It just wasn't the speed of sound.

Of course all this talk about speed and density and air makes me wonder what is the speed of sound in a vacuum? Well, that may be overstating it a bit, but how low does air pressure have to go before talking about sound becomes pointless?

U2

Update December 2016 replaced missing pictures.