Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!tut.cis.ohio-state.edu!att!cbnews!military
From: ae219dp%prism@gatech.edu (Devon Prichard)
Newsgroups: sci.military
Subject: Re: Faster than a Falling bullet
Summary: pojectile spin lasts awhile
Keywords: spin, loss of
Message-ID: <13443@cbnews.ATT.COM>
Date: 24 Jan 90 03:34:47 GMT
References: <13422@cbnews.ATT.COM>
Sender: military@cbnews.ATT.COM
Organization: Georgia Institute of Technology
Lines: 33
Approved: military@att.att.com



From: ae219dp%prism@gatech.edu (Devon Prichard)

two anecdotes;

my roomate's father wasin the German army in WW II. he was in a
bomb crater when the Russians put a artillery shell in the crater,
right between him and his squad-mate. the fuse didn't blow
(otherwise I'd be in a one-bedroom apt. !!) but the shell lay
on the dirt spinning like mad _for several minutes_ !!

a friend of mine who is a cop, told a story to me about someone
firing an AK-47 in a concrete office building (I think it was
a stairwell). the bullet ricocheted around until it stopped
travelling, but lay on the ground spinning for awhile (making a
buzzing noise).

a dumb calculation; if the muzzle velocity of a hypothetical bullet
is 3000 ft/sec, and the barrel has 1 in 12 twist (1 turn in 12 inches
of travel), then, assuming that, for the last foot of travel along the 
barrel, the bullet is at 3000 fps, the spin rate out of the muzzle would be
15,000 rpm. don't ask me how long the aerodynamic boundary layer
would take to dissipate that much rotational energy, because it
would require an unsteady analysis, from the time the bullet left
the barrel to rest condition (i.e. from supersonic to transonic to
subsonic conditions). give me $150K and I'll do it !!

-- 
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 | Devon Prichard             making the world safe for helicopters ... |
 | ae219dp@prism.gatech.edu                                             |
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