Settle a bet: Mach speeds

On Sat, 15 May 2004 21:25:59 -0500, "tscottme"
wrote:

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.

[I'm sorry to be so late in replying, but life got weird and it took
me a little while to get back to this. MFS]

It's a definition thing. The Orbiter goes Mach 25, by definition,
until the dynamic pressure gets high enough to be sensed, which is at
(surprise!) Mach 25. If you use the real dynamic pressure, which is
zero, the Mach number goes to infinity, so we set it to Mach 25 by
definition to avoid that.

It isn't sea level Mach number for that airspeed. No one uses
"equivalent Mach number". That's because the aerodynamics and
aerothermodynamics are functions of Mach number, so it has to be
local.

This means you're right and someone else is wrong.

Perhaps that someone else is confused by equivalent airspeed, usually
presented by PAO as miles per hour. That is the sea-level airspeed
that the airplane would have to fly at to have the same Mach number at
sea level that it has at altitude.

I don't have my calculator or my standard atmosphere here, so the
numbers I'm about to give you will be only approximate. Flying Mach 3
at 80,000 ft is an indicated airspeed of about 300 knots (300 KIAS).
At sea level, the equivalent airspeed would be about 3,000 fps, which
you'll have to convert to knots (this will be KEAS).

I know to discount what reporters say, but I'm referring to engineers
and their official spokesmen.

You can check this in "Space Shuttle Hypersonic Aerodynamic and
Aerothermodynamic Flight Research and the Comparison to Ground Test
Results", Kenneth W. Iliff and Mary F. Shafer, NASA-TM-4499, June,
1993. If you go to http://www.dfrc.nasa.gov/DTRS/1993/Bib/H-1894.html
there's a link to a pdf version.

One of the early references explains the air data system, including
this Mach number definition.

Mary

--
Mary Shafer Retired aerospace research engineer