"Jeff Findley" wrote in
:

"Jim Oberg" wrote in message
...
see
http://www.usatoday.com/tech/science...uttle-debris_x
.htm

But the preliminary analysis dated April 26 placed the odds
that orbital debris could destroy the next shuttle at a range
from 1 in 54 to 1 in 113. That risk estimate stems from recent
tests showing that the space shuttle's heat shield is more
fragile than NASA had realized.

It's always hard to come up with estimates like this, but if these
numbers are in the ballpark, then this risk is a bit high.

Those numbers are almost certainly high for what the shuttle will actually
be facing post-return-to-flight. They don't account for changes in
shuttle/ISS stack attitude and the extra layer of insulation behind the RCC
to handle "sneak flow" from small holes/cracks. These steps only mitigate
the problem, not eliminate it; orbital debris will remain one of the major
threats to the shuttle.

How would you reduce such a risk for a next generation spacecraft?
Certainly better micrometeorite protection would help against smaller
objects, but how do you protect a heat shield on something as big as
the envisioned CEV?

First you'd recognize that the majority of the debris threat is in LEO and
optimize the design for that. LDEF data shows that the debris threat in LEO
is fairly directional, with the worst impacts along the velocity vector,
whereas the nadir surface is shielded by the Earth filling almost half the
sky.

Operationally, while in LEO, CEV should present the smallest frontal area
to the velocity vector and orient the most vulnerable surfaces to the nadir
and rearward directions. (For the shuttle, this aligns the tail along the
velocity vector and the payload bay to the Earth.)

This extends to ISS docked operations as well; the CEV docking port should
be oriented so as to allow CEV to fly a debris-optimal attitude while
docked. CEV thermal control system design should take this into account.

(The shuttle can't fly a debris-optimal attitude while docked to ISS, owing
to a complex combination of constraints on shuttle/ISS thermal control, ISS
solar power, shuttle/ISS attitude control, constraints on moving the
shuttle docking port, and RCS plume-impingement constraints. The new post-
Columbia docked attitude is a compromise that improves debris protection
without breaking any of the other constraints.)

--
JRF

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