Date:   Sun Mar 28 1993  22:07:26
From:   Don Allen
Subj:   Ufo Problem: Sonic Booms (also: A "real" Ufo?)

 * Forwarded from "Sci.Skeptic"
 * Originally by Loren I. Petrich
 * Originally dated 19 Mar 1993, 14:40

From: lip@s1.gov (Loren I. Petrich)
Date: 18 Mar 93 19:20:28 GMT
Organization: LLNL
Message-ID: <1oai1s$b02@s1.gov>
Newsgroups: sci.skeptic

        One of the problems with the extraterrestrial spaceship
hypothesis is that many of them supposedly travel at supersonic speeds
through our atmosphere and if that is the case, then why don't they
produce sonic booms?

        Here, UFOlogy collides with a well-established result of the
hydrodynamics of compressible fluids.

        A practically universal result of supersonic flow is the
formation of shock fronts, which are what make sonic booms. This is
apparent from observations, experiments, computer simulations, and
simplified analytic developments. In effect:

        Direction of motion:

                /\
                ||
                ||


               /@@\
              / @@ \
             /  /\  \
            /  /  \  \
           /  /    \  \
             /      \
            /        \


        I did my thesis project on a computer simulation of accretion
of interstellar gas (or something similar) onto a black hole. Whenever
the hole moved faster than the gas's speed of sound, a shock front
formed. And this was without any direct contact; all that the gas
"saw" was the hole's gravitational field. Similar shocks appeared for
purely Newtonian calculations of accretion, onto (say) protoplanets or
white dwarfs.

        Analytic developments? Try linearizing the equations for fluid
behavior to get the behavior far away from the object, and take the
stationary (no time variation) case. For subsonic flow, the equations
are elliptic, like Poisson's equation, implying no shocks, while for
supersonic flow, the equations are hyperbolic, like a wave equation,
implying that a shock will propagate from the object.

        Thus, shockless supersonic travel represents an Extraordinary
Claim, which demands extraordinary evidence, which no UFO case to date
has provided, as far as I am aware. In every clear case so far, there
are alternate explanations at least more plausible (see the works of
Menzel, Klass, and Sheaffer).

        And if you want to consider "real" UFOlogy, consider the March
1993 issue of _Popular Science_, which discussed a supposed secret
Mach 6 airplane nicknamed "Aurora" that has produced several sonic
booms and that has even been sighted on occasion. The sonic booms are,
of course, what a supersonic aircraft is expected to produce. The
article also went into detail into what the aircraft would be expected
to be like. Like the SR-71 and the underside of the Space shuttle, it
would be black, to radiate away the heat generated by the friction of
its fast travels.  It would look much like the "Stealth" fighter, with
stubby wings, pointed nose, and a streamlined body. Air intakes would
be underneath, and the contours of the plane would help compress the
air and absorb some of the thrust from the exhaust.  Its fuel would be
liquid methane, whose boiling would absorb some of the heat, and the
engine would be a rocket/ramjet. At low speeds, rocket engines would
be lowered into the ramjet engines, and a mixture of methane and air
compressed by a turbocharger would be burned. Liquid oxygen would be
added for extra thrust. As the plane's speed increase beyond Mach 2.5,
first the liquid oxygen would be shut off, and then the rocket engines
would be pulled upward, letting the engines run as pure ramjets.

        There were discussions of other projects in that article, like
a subsonic delta-wing spy plane nicknamed the "Manta", a big airplane
with a flat space on top, presumably for piggybacking another airplane
(to get into orbit?), and an external-combustion hypersonic airplane
nicknamed the "Pumpkin Seed"; at a big enough Mach number, its
turbojets would be turned off and fuel supplied to be burned outside
(!). The flame would be kept next to the plane by the shock wave
produced by its travel, so it transmits some of its thrust to the
plane's body. It has supposedly been seen as a rapidly moving bright
light in the night sky.

        The article discusses such things as Pentagon budgeting for
secret projects, and mentions the curiosity of the Air Force retiring
its SR-71 fleet not too long ago, despite there being no necessity of
doing so.

        These aircraft might be called Unidentified Flying Objects,
but they are almost certainly not extraterrestrial spaceships.

        Have the advocates of the extraterrestrial spaceship
hypothesis shown this kind of rigor? Not that I'm aware of.

/Loren Petrich, the Master Blaster
/lip@s1.gov