In article ,
"tscottme" writes:
When knowledgeable people speak of a vehicle going "X Mach" they are
referencing the speed of the vehicle compared to the speed of sound at
local conditions, right?

I ask because I've heard NASA folks mentioning that the Shuttle, at some
point in it's descent, is going Mach 25. I claim that means the vehicle
is going 25 times faster than Mach at that particular pressure and temp
where the Shuttle is, while someone else claims NASA means 25 times the
sea level value of Mach, even though the Shuttle is in the very upper
atmosphere at the time.

Upper atmosphere conditions vary quite a bit. The upper atmosphere
absorbs quite a bit of Solar Radiation, and gets relatively warm in
the process. So - the speed associated with Mach 1 varies.
To keep things in the FLight Control System (and the Public Releations
Office) as uncomplicated as possible, but still accurate enough, NASA
uses a sylized value of 1,000 ft/sec as Mach 1 at the very high (Peak
Heating Region) altitudes. The true value for the speed of sound
wanders back and forth across 1,000 ft/sec at those heights, but not
by much.
The Speed of Sound at Sea Leve, on a Standard (29.92"Hg Pressure, 59
Deg F - 1013 mBar/15C) is about 1115 ft/sec.

So, while the number is a little bit arbitrary, it's still related to
local conditions, and not Sea Level conditions.

--
Pete Stickney
A strong conviction that something must be done is the parent of many
bad measures. -- Daniel Webster