Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!henry
From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: sci.space.shuttle
Subject: Re: Throttling the Challenger
Message-ID: <8796@utzoo.UUCP>
Date: Mon, 19-Oct-87 16:43:15 EDT
Article-I.D.: utzoo.8796
Posted: Mon Oct 19 16:43:15 1987
Date-Received: Mon, 19-Oct-87 16:43:15 EDT
References: <340@ablnc.ATT.COM> <3143@ames.arpa>, <6563@ut-ngp.UUCP>
Organization: U of Toronto Zoology
Lines: 31
Keywords: Challenger Throttle

> 1.  Is it *necessary* to launch at 105% power? If not, it seems that such
> 	settings create an unnecessary risk...

Well, no, it's not *necessary*, it's just a matter of how much payload you
want to get into orbit.  Just off the top of my head, you could probably
run the shuttle engines at 90% and still reach orbit... with a payload
consisting of two toothbrushes and a comb.  The engines would probably be
more reliable and need less maintenance at 90%, too.  Of course, you couldn't
do much with the shuttle that way.

Engines, like lightbulbs, generally do not have a single specific setting
for which they are designed, in the sense that all other settings work
poorly or not at all.  It's a continuous tradeoff:  the harder you push
it, the more you get out of it, and the shorter its lifetime.  Where you
set the normal operating point is a judgement call, not a clear-cut and
obvious triviality.  The "100%" level of the shuttle engines is an arbitrary
number, not a point beyond which reliability deteriorates sharply.

NASA PR guano aside, there is no such thing as perfect safety:  it's just a
matter of how much risk you are willing to take for a given end result.
How "unnecessary" the risk is depends on how much you care about the result.

> 2.  How closely related is rate of burn-through (assuming it occurs, which it
> 	did) to power level? ...

Since the burn-through was in the SRBs, not the main engines, there is not
much correlation, especially given that the problem was a seal failure, not
a normal phenomenon to which a "rate" could be assigned.
-- 
"Mir" means "peace", as in           |  Henry Spencer @ U of Toronto Zoology
"the war is over; we've won".        | {allegra,ihnp4,decvax,utai}!utzoo!henry