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From: elt@astrovax.UUCP (Ed Turner)
Newsgroups: net.space
Subject: Re: Most Dangerous: Launch or Landing?
Message-ID: <739@astrovax.UUCP>
Date: Mon, 17-Feb-86 11:17:13 EST
Article-I.D.: astrovax.739
Posted: Mon Feb 17 11:17:13 1986
Date-Received: Tue, 18-Feb-86 04:04:01 EST
References: <8602131734.AA03845@s1-b.arpa>
Reply-To: elt@astrovax.UUCP (Ed Turner)
Organization: Princeton Univ. Astrophysics
Lines: 21


The claim that landings are more dangerous than launches may have been based
on several risk analyses carried out before the loss of Challenger.  I believe
that some or perhaps all of these concluded that the landing procedure was
quite hazardous.  One particularly controversial study (Rand Corp. maybe?)
concluded that the chance of destroying the Orbiter on landing is of order
1% per mission.  Also there is the empirical fact that there have been a
couple of "close calls" during shuttle landings.  In any case it is not hard
to see why a no power landing in a "plane" as massive as the Shuttle with
such poor low speed glide and handling characteristics could be quite tricky.

One unhappy possibility that must be considered is that the Shuttle has 
several critical failure modes, all of roughly the same small probability
but adding up to something in the few percent range.  Since identification
and evaluation of such failure modes and probabilities is always a difficult
and uncertain business, one might be left with finding them in the same
way that the O-ring/SRB problem (if that's what it was) was uncovered.
This sometimes happens with experimental aircraft.

Ed Turner
astrovax!elt