Mary Shafer wrote in message . ..
On Sat, 19 Jul 2003 15:12:30 +0100, Alan Dicey
wrote:

As my news server doesn't let me post to a.b.p.aviation, this is the
next best place I can think of for this reply. In answer to a question
about pictures of the SR-71 showing a shock wave, several people have
responded saying that the SR-71 is not supersonic at low level. Looking
for confirmation, I found this URL

http://www.wvi.com/~lelandh/exec12.jpg

- a page from "The SR-71 Reconnaissance System Executive Handbook",
written by Bill Majors (Lockheed ADP), which gives the performance
envelope of the SR-71. And yes, it is subsonic to about 20,000 feet.

I hate to tell you this, but 20,000 ft isn't "low level". Low level
is a couple of hundred feet off the deck. I suppose you could stretch
it to 10,000 ft or so, but the SR is still subsonic at that altitude.

Mary

Is there a physical reason why? Or is it because of rules/regulations?
David.