From: bowers@rigel.dfrf.nasa.gov (Al Bowers)
Subject: Re: Mach 1 is also density dependent
Organization: NASA Dryden, Edwards, Cal.
	<1992Dec16.174912.17981@eagle.lerc.nasa.gov>
Date: Wed, 16 Dec 1992 23:28:20 GMT

In article <1992Dec16.174912.17981@eagle.lerc.nasa.gov> totran@glider.lerc.nasa.gov (le tran) writes:

>In article <1gnia0INNnev@rave.larc.nasa.gov> turnock@eagle.larc.nasa.gov (D. L. TURNOCK) writes:

>>In article <1gn95vINNq3s@usenet.INS.CWRU.Edu>, br105@cleveland.Freenet.Edu (Jeffrey A. Del Col) writes:

>>> If I am not mistaken, Mach 1 is also dependent on the density of
>>> the transmitting medium. Mach 1 is faster in water than in air, and it
>>> is higher at lower altitudes than at higher altitudes largely because of
>>> the difference in air density. 

>>The speed of sound in any medium is

>>a  = SQRT(gamma*R*T)

>>a = speed of sound
>>gamma = ratio of specific heats - cp/cv
>>R = gas constant for particular gas
>>T = temperature (absolute)

>>This should work for any gas

>    Not true. The formula 
>    a  = SQRT(gamma*R*T) 
>    is applied for PERFECT GAS only.

I can think of only one aircraft where this assumption does not work.
And that aircraft is the Shuttle.  Assuming for a moment that a Mach 8
aircraft did exist, it would have to account for some of these
effects, both aerodynamically and in the propulsion system.  Allow me
to explain.

Below Mach 3 there is little effect due to aerothermodynaics on
airframes.  Air behaves essentially the same at Mach = 0 as it does at
Mach = 3.  By the time you get to Mach 3 you are beginning to excite
the vibrational modes of the air molecules (about 10%).  By the time
you get to Mach 8 you excite about 90% of all the available air
molecule modes.  Starting in the Mach 8 to 10 range you start to
dissociate the oxygen molecules (and here is where it starts to get
sticky). By the time you get to the Mach 12 to 16 range you have
dissociated 90% of the oxygen molecules (it is dependant on pressure
and temperature).  Starting between Mach 16 to 20 and on up to Mach 28
or so, you get the region of nitrogen dissociation.

The problem comes in when you start to dissociate the molecules.
Because you can no longer depend on the combustion and reactions to
occur in a neat well understood manner.  Free oxygen starts to strip
off the skin of the aircraft, this contaminates the boundary layer and
changes the viscosity, changing the boundary layer transisiton, which
ultimately affects drag and even lift.  In the propulsion system
things are even worse.  You are trying to react a fule with an
oxidizer that is no longer what you can create easily in a lab, the
enthalpy is absurdly high and the Mach number is impossible to create
in ground based tunnels (Mach numbers of higher than 8 with air are
not possible with _air_, higher Mach numbers can be made using
helium).  The fule reacts with the dissociated nitrogen, and the free
oxygen, and then because this is not stable it reacts again creating
all sorts of weird and wonderful byproducts.  NO, O2, O, OH, H2O, CO2,
N, N2, and any other imaginable combination.  And there is the
reaction time constants, which are very important.  A 100 foot long
vehicle traveling at 8000 ft/s (roughly) is not going to be in contact
with any one air molecule for any extended length of time.  :-) Even
less for the propulsion system, so there are minimum lengths that such
vehicles can be made.  Such large vehicles require large structures,
and larger fuel supplies, and on and on.

But before everyone starts in on Aurora, look at the following NASA
reports.NASA TP1252, NASA TP 1552, NASA TN D-8065, and NASA TM X-3475.
They all deal with a Hypersonic Research Airplane concept that Jim
Penland worked on at Langley Resravch Center in the late '70s.  I
think you'll find the Jane's drawings very revealing after seeing these
old 1975-1979 reports...

I'm still a sceptic.  I hope the optimists prove me wrong...

--
Al Bowers                       bowers@rigel.dfrf.nasa.gov
NASA F-18 High Alpha Research Vehicle, Lead Aero
NASA, Dryden Flight Research Facility, Edwards, California
Newsgroups: sci.aeronautics