Re: Ariane 5 - not an exception?

[stt@henning.camb.inmet.com (Tucker Taft)]

Nancy Mead (nrm@sei.cmu.edu) wrote:

: ...
: The Space Shuttle software has 4 computers running the same software, and
: a 5th running different software (same function, different development team).

This mode is only used during critical phases of a mission.  In other
phases, the "backup" computer may be used for independent activities.

: I'm not sure about the Apollo software, although I think there were some 
: calculations that could be done on-board as well on the primary computer.

: You may recall that one of the early shuttle launches was cancelled because
: of a timing difference between the 4 computers and the single computer.  

Not really.  It was a timing difference among the 4 computers running
identical software.  The timing difference was detected by the 5th ("backup")
computer, and it ordered the shut-down.

: ... This
: was indeed an intermittent software error that caused the problem, and the
: glitch resulted in cancellation of the launch in that particular case.  Of
: course, error recovery was a lot less sophisticated in those days, and
: it was probably impossible to isolate the cause of the discrepancy in real
: time and proceed with the launch.  

It was not trivial to determine the cause, and it is highly unlikely
it could have been determined in "real time," even using more "sophisticated"
error recovery.  The problem related to the fact that the computers were
started almost exactly on a clock tick, and 2 of the computers got
one time, and the other 2 got a time one tick later.

: I was not one of the developers, but I was at IBM Federal Systems HQ at the
: time, and IBM FSD was one of the development organizations.  I believe
: Rockwell (the prime contractor) developed the software that ran on the single
: computer, but it might have been another subcontractor.

The backup flight software was developed by Intermetrics, as a subcontractor
to Rockwell.

For what it is worth, the Space Shuttle software is developed in the
language Hal/S, which was also developed by Intermetrics ;-).

--
-Tucker Taft   stt@inmet.com   http://www.inmet.com/~stt/
Intermetrics, Inc.  Cambridge, MA  USA