Thomas Lee Elifritz wrote:
January 17, 2004

Charles Buckley wrote:


At some point, in any engineering endeavor, someone has to
run the numbers and pull the plug. The plug here is Shuttle.
By my count, there are about 20 more flights for Shuttle.
Which is damn close to a 50-50 chance of another Shuttle
accident under the current safety rating before it is retired.


Time to double check your Shuttle units. Let's see, the shuttle has a demonstrated
1 in 100 flight loss risk, and now we've made some improvements ... How many
shuttle flights before 2010? I'm so confused.


2 losses in 113 flights = 1.77% loss rate demonstrated. Projected
at about 1 loss in 50 flights as they are expecting reduced safety
as they are not going to be implementing periodic main out thru the
end of cycle.

Unless you are saying that either Columbia or Challenger did not
demonstrate a loss as they found the wreckage. Or maybe that they
only recovered half off each shuttle.

In any case, you are looking at a 40-50% chance of another loss
before EOL.

Any Shuttle mission now has a discernable risk.


As opposed to a previously indiscernible flight loss risk?


Yes. There does seem to have been a certain degree of
complacency in risk analysis over the past decade. Read
the CAIB.


Simple risk analysis. Is an incremental maint mission
of Hubble worth the 2% chance of a loss of an irreplaceable
Shuttle? Nope.


First you state the Shuttle will be retired, but now you call them irreplaceable.
Can you please make up your mind!



If you lose another, there is nothing to replace it for the ISS
missions. ISS will not be completed. The lost Shuttle will not be
replaced. Irreplaceable when completing a mission does not equate
to continued service after it's mission is complete. The day that the
last heavy element is lifted to ISS, Shuttle will have served it's
purpose and is then expendible. Hubble servicing needs to be done approx
two years prior to the end of ISS construction. So, a loss of a shuttle
there would, in fact, take ISS with it at 90% completion. It's an
unacceptable risk.