re: Windshield damage

Philip Chien (kc4yer@amsat.org)
Mon, 7 Oct 1996 22:19:17 -0400

Jim Varney <jvarney@mail2.quiknet.com> said:
>Subject: Columbia Delayed: Debris Damage
>
>Press release from NASA. Though they don't say directly, the
>windshield damage is due to orbital debris.

Not necessarily.  In the LEO environment the concerns are both artifical
junk (e.g. orbital debris) and natural junk (micrometeoroids).  Depending
on the shuttle's attitude one or the other may dominate at shuttle
altitudes.

Whenever another attitude isn't required the shuttle flies with the cargo
bay to Earth and engines in the flight direction.  This gives the best
protection to the shuttle's windows, which face 'away' from the flight
direction.

What very few outsiders know, especially dumb-ass reporters or editors who
choose to ignore, is that the shuttle's three-layer window design provides
_MORE_ than adequate protection.  The outer layer (the 'thermal pane') is
only required for ascent because of friction heating.  On orbit and during
descent it's a redudant piece of protection.

According to the flight rules if some one-in-a-thousand-year meteroid was
to completely destroy the themal pane, the mission would be able to
continue normally - without any changes since the inner two layers would
still provide enough redundancy.  Source - lead flight director Al
Pennington, JSC MOD.

The shuttle's windows were designed with the micrometeorid environment in
mind, and certainly the orbital debris environment was not anticipated.

About the only legitimate complaint about the shuttle's windows and debris
problem is that, on the average, one window is replaced after each shuttle
mission.  Which is an additional cost which wasn't originally anticipated.

However several factors (more 'space friendly launch vehicles', lower
launch rates for both Russia and the U.S., and no tests of 'killer
satellite' systems in the past several years) have combined to reduce the
chances of damage by almost an order of magnitude at shuttle, space
station, Mir altitudes.  At higher altitudes it's a different story though.

And the REX-II Pegasus upper stage has certainly added its fair share to
the space junk equation.



Philip Chien, Earth News - space writer and consultant  PCHIEN@IDS.NET
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