The major, and really only, effect is temperature. But temperature varies with altitude. (Aircraft and engine performance depends on pressure as well, but we are just talking the velocity corresponding to Mach 1 at a particular altitude/temperature).
On a standard day (59 degrees Fahrenheit (15 degrees Celsius at sea level, the speed of sound is 1116 fps or 761 mph. Using a "standard atmosphere" temperature profile. The following gives the ratio of the speed of sound at altitude to that at sea level.
Altitude in feet Speed of Sound ratio
Sea Level 1.00
5,000 ft 0.9827
10,000 ft 0.9650
15,000 ft 0.9470
20,000 ft 0.9287
25,000 ft 0.9100
30,000 ft 0.8909
35,000 ft 0.8714
40,000 ft 0.8671
50,000 ft 0.8671
60,000 ft 0.8671
(temperature is constant above 37,000 feet (-69.7 deg F, -56.4 deg C). The speed of sound is proportional to the square root of the absolute temperate (that add 460 deg to the F or 272 to the C temperate to get the value above absolute zero)
As you can see the speed of sound actually drops with altitude, so a particular Mach number represents a slower speed.
Thus the Mach 3.5 at 80,000 feet (bet they were really higher. :) ) would be 761 mph * 0.8671 or about 660 mph, or 3387 feet per second.
Consider the the muzzle of velocity of the standard M-16 cartridge (M855) is only 3,100 fps, there were really humming along. Considering also that at 300 meters that's down to 2,115 fps, and they were really really humming along.
Gato - Thanks for the lesson! I knew someone would come along with the science to prove this out.
El Gato es muy inteligente
Gato - Thanks for the lesson! I knew someone would come along with the science to prove this out.
El Gato es muy inteligente