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From: tcliftonr@cc.curtin.edu.au
Newsgroups: rec.skydiving
Subject: Skydiving at supersonic airspeeds
Message-ID: <1991Jun24.135815.8781@cc.curtin.edu.au>
Date: 24 Jun 91 05:58:15 GMT
Organization: Curtin University of Technology
Lines: 83

In the discussions arising from
GRAPH OF OSC'N DUE TO HIGH WINDFORCE

Walter@cayman.AMD.COM (Walter Butler) writes:


>It is possible that the jumper ( at 750 mph and 1.6 gees of
>drag after a 120 000 ft exit) will have stability problems
>from the supersonic airflow.

Because of the soft impact with the atmosphere there will be
an extended period of 30 seconds or so of deceleration, where
much more wind force is available to induce "rocking" in a
normally utterly stable skydiver.  (A non skydiver is likely
to go further, into a tumble, and a spin is also likely)


>I am no expert, but believe that the shockwaves can cause
>very localized areas of high stress and drag.

Yes!  The sound waves going down cannot escape downwards and
so bank up as a flat cone of compressed air around each
extremity.  This would mean for a normally exposed skydiver
that chin, knees etc would take more of the wind force than
usual.  But how much more?  Will it make you sneeze or will it
make your eyes water?  Will it be enough to sense under all
that protective clothing?

The total force is still only 16 N/kg or so averaged over the
whole body.  It doesn't seem that local concentrations of
force would be intolerable, but perhaps enough to provide for
some rare phenomena to be first experienced by a brave
pioneer.

Certainly the position and attitude of the chin etc will
affect the amount of build up.  How long does it take to build
up to equilibrium anyway?  The compression can't accumulate
indefinitely,  it must bleed off sideways.  If it takes a
second to build up, the jumper will experience a tendency to
nod in free fall.  Hardly a terrifying danger.


>A simulation of the jump performed in this manner would
>be interesting.

It sure would.  Perhaps some of your NASA colleagues - the re-
entry people - might know enough of the physics.  Perhaps they
already have a re-entry simulator.  As it stands, this
(subsonic) simulator is extrapolated out of its proven valid
range.

In particular, we need to know how the drag coefficient (for
us brick-shapes) changes for low supersonic flight.  Then we
need to know the effect of temperature on the drag
coefficient.  Certainly the stratospheric cold of -56 degrees
affects viscosity and reduces drag.

For vibrations and oscillations, we need to know the effect of
the compression cones on the extremities and how long they
take to build up.


>The diver might use a style tuck while supersonic.

It would one way to get respite from some irritation due to
vibration or suchlike.  But it would be more unstable than an
arch and the moment of inertia would reduce, so increasing the
likelihood of a fast rotation or tumble in the high force
wind.


>This would also have the added "benefit" of increasing
>his top speed.

How gung-ho do they have to be?  What a story that would
make!   "First supersonic skydiver in history strains for a
morsel more of the unknown..."

Let's salute their first exploration of impact.  Whatever
they find out for us, it is going to be interesting.


Roger Clifton   Curtin University  West Australia.  6:91