# Blackbird Singing in the Dead of Night

The Ultimate Spy Plane

One nit:

Created as the ultimate spy plane, the SR-71, which first took to the air in December 1964, flew reconnaissance missions until 1990, capable of hurtling along at more than Mach 3, about 2,280 miles per hour—faster than a rifle bullet—at 85,000 feet, or 16 miles above the earth. It is the fastest jet-powered airplane ever built.

Mach 3 is about 2280 mph … at sea level. But it varies with density altitude, so at 85,000 feet, it’s about 2000 mph. The speed of sound, i.e. Mach 1, is not a constant of nature — it’s defined by the conditions (as opposed to the speed of light, which is c in a vacuum)

## 4 thoughts on “Blackbird Singing in the Dead of Night”

1. It’s difficult to know whether to be impressed or not once you find out that the temperatures and pressures on the hull change so wildly that on the ground the blackbird’s fuel tanks don’t seal. Until it heats up, it leaks so badly that it needs to be refuelled in the air immediately after takeoff.

2. Uncle Al knows a retired military air traffic controller. One can trivially look at shock cone angle vs. engine placement. An SR-71 doing Mach 5 was unremarkable. A speed run could launch it suborbital in principle, but nobody ever figured out how to restart the (cooled) engines at altitude – JP-7 was difficult to ignite at sea level. The SR-71 glided unpowered like a brick.

3. Hey pedantic man. The speed of sound varies with temperature, not density. Since pressure and density are directly related at a given temperature, any decrease in density is offset by a corresponding decrease in the ‘spring constant’ pressure represents when describing sound waves. Temperature is the proportionality constant that relates the two. The speed of sound goes with the square root of temperature (sqrt(gamma*R*T) to be complete).

4. True. In general, though, the speed of sound depends on density. In an ideal gas, there is a relation which ties the bulk modulus and density together such that the dependence is on temperature. OTOH, the speed of sound is not the same at 100 km as it is at sea level, even though the temperature is the same.