Newsgroups: sci.space.shuttle
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Interface 9099
Date: Sat, 5 Aug 2000 20:49:22 GMT

In article <398c8e65.391639373@news.seanet.com>,
Derek Lyons <elde@hurricane.net> wrote:
>>(And *that* being said, it's also worth noting that the 60s NASA sometimes
>>did things better -- it was more cautious and put more emphasis on keeping
>>things simple and manageable.  There were only about 100 wires between the
>>Apollo spacecraft and the Saturn V, and only 36 between CSM and LM.  There
>>are over 1000 between the shuttle orbiter and the ET.)
>
>That's an utterly mislead comparison Henry.  The CSM was largely a
>passenger with limited emergency capabilities, vice the orbiter which
>is an active participant. (And some of those wires go to the SRB's.)

Agreed that it is a somewhat unfair comparison, although I would point out
that it came from a NASA paper on management practices.  (Morris on
manned-flight management, in NASA SP-6101, vol. 5, 1992.)  However, the
CSM had more of a role than you might at first think -- for example, it
could take over ascent guidance if necessary -- and a good bit of this
does reflect differences in policy rather than differences in requirements.

>A far more accurate comparison would be, how many wires connected the
>IU to the remainder of the Saturn V?

I can't find a definitive number on a quick search, but I think you might
be surprised.  For example, most commands were issued to the stages using
a digital bus of about 30 wires.  A diagram listing all major signals
going across stage boundaries in the Saturn V fits on one page, although
it's a little vague in spots and definitely isn't listing things down to
the individual wires.

This didn't happen because the job was simpler.  Strenuous efforts were
made to *make* it simpler.  The stages were made as independent as
possible, even when this added weight and complexity, to keep interfaces
between them severely simple and easy to understand.  For example, each
stage had its own telemetry transmitters.

George Low, in a 1970 retrospective, commented that one man could
understand the *entire* interface between Apollo and the Saturn V, and
assess *all* the effects of a change on either side.  Ten times as many
wires would require "a hundred or a thousand" men to do the same job,
because it takes the complexity beyond what one man can keep in memory.
--
Microsoft shouldn't be broken up.       |  Henry Spencer   henry@spsystems.net
It should be shut down.  -- Phil Agre   |      (aka henry@zoo.toronto.edu)