Path: utzoo!attcan!uunet!lll-winken!ames!elroy!usc!venera.isi.edu!raveling
From: raveling@venera.isi.edu (Paul Raveling)
Newsgroups: sci.space.shuttle
Subject: Re: Atlantis is home!
Message-ID: <8465@venera.isi.edu>
Date: 23 May 89 17:12:12 GMT
References: <272@skipper.dfrf.nasa.gov> <9090001@hp-lsd.HP.COM> <4453@ttidca.TTI.COM> <6949@bunny.GTE.COM>
Reply-To: raveling@venera.isi.edu (Paul Raveling)
Organization: Information Sciences Institute, Univ. of So. California
Lines: 78

In article <6949@bunny.GTE.COM> hhd0@GTE.COM (Horace Dediu) writes:

>The booms come from two different shock waves generated by the shuttle.  At
>supersonic speeds the shuttle looks like:

	Approximately right, but there's a different aerodynamic reason.
	What I say next is subject to the disclaimer that it's been
	some years since I studied it, and my main aerodynamic interest
	was in sailplanes with a typical Vne of 135 knots.

	Both booms are caused by an abrupt pressure transition.  This
	is easy to understand for the leading boom, but the transition
	causing the trailing boom is triggered by the fact that the
	shuttle no longer occupies the volume of airspace it just passed
	through.  The trailing boom's shock wave forms at a different
	angle than the leading boom's shock wave because local airspeed
	is higher at that point than at the nose.

	This effect is visible in wind tunnel photographs of simpler
	forms, such as bullets, in supersonic flow.  To update Mr.
	Dediu's drawing, this is roughly what I believe the Shuttle's
	case would look like:

		      .                  			      .
                     .                           		     .
  		    .    <- shock wave 1            shock wave 2 ->.
		   .  	(in 3-space, these                        .
		  .  	 are cones)	      --.               .
		 .  			     /  |              .
		.  			    /   |            .
	       .			   /    |           .
	      .    _______________________/-----|__       .
             .   /   USA                 :      /  |     .
	    . /--                               ---|   .
	   . <                                  \__|  .
            . -------------------------------------    .
	     .  				         .
	      .						  .
	       .					    .
		.    			     		     .
		 .   			      		       .
		  .   		  yay!	       		        .
		   .  		 o				  .
		    .  		-|-		 		   .
		     .  BOOM	/ \		  		     .BOOM
_____________________________________________________ (observer) on ground


	The time interval between the two booms is due to the different
	spreading angle; if the shuttle goes by at, say Mach 1.1 & 50K
	feet, the separation increases from the shuttle's length to
	a few hundred feet.

	Also note that these shock waves form ahead of and behind the
	object.  An form with a relatively pointed nose, like an F-104,
	has a relatively large distance between its leading shock wave
	and the nose itself; this produces minimal drag due to feeding
	energy into the shock wave.  A relatively blunt nose, such as
	the shuttles, will generate its shock wave much closer to, or even
	even on, the nose, and will produce much higher drag.
	

>(Not to scale).  There are many shock waves, but these are the biggest.  The
>nose, and the bumps in the fuselage at the back which cover the engines
>cause the two shock waves.

	You bet.  Shock wave structure, particularly on airfoils, can
	be pretty complex.  The aerodynamicists from Lockheed who
	taught the class that I took mentioned that there are airfoils
	that can have local supersonic flow, initiating local shock
	waves, at airspeeds as low as .2 Mach.  Needless to say,
	design of things such as airliner wings gets lots trickier as
	the intended speed gets into the normal transonic regime.


----------------
Paul Raveling
Raveling@isi.edu