[net.columbia] Shuttle hardware and software query

stevans (11/18/82)

Does anybody know exactly what sort of computers are on board shuttles,
and who programs them?
Mark Stevans
(allegra or seismo)!rochester!stevans

dave (11/20/82)

  (Of course, no guarantee, written or implied, about the accuracy of
any of the following, though I think it's correct.)

   There are five computers controlling the operation of the shuttle.
Four of them comprise the main system and the fifth is an independent
backup computer in case the first four completely fail.  All five
computers are IBM AP-101 aviation computers, developed in the
mid-1970's for military aircraft.

  The primary computer consists of four computers running the same
program.  Voting is done to determine the majority's result (sort of a
democracy).  A big problem is the synchronization of the computers to
make sure that they are in lock-step.  The backup computer is also
communicating with the primary computer at all times.  You might recall
that a timing problem between the primary and backup computers caused a
delay in the first flight in April 1981.

  IBM Federal Systems Division in Houston did the software for the
primary computer.  There are about half a million "lines of
instructions," whatever that means.  (.5M bytes of object?
.5M lines of assembler?  .5M lines of Fortran?)  Rockwell did the
software for the backup.

  By having a different company do the backup software, chances of a
common software error in both programs were reduced.  (One could argue
that the backup computer should also have been designed and
manufactured by a different company to reduce chances of a common
hardware fault in all five machines.)

  I believe the computers are (or at least they should be) physically
located in different places on board the shuttle to reduce the chances
of fire, explosion, collision, etc. from destroying all computers
simultaneously.  Similarly, the power supplies are probably as
independent as possible.

  The computers on the shuttle (unlike previous manned spacecraft) are
central to the safety of the craft.  For example, the pilots do not
have direct hydraulic control of the aerodynamic surfaces (flaps, etc.)
on the shuttle.  Rather, the pilots give their commands to the
computer; the computer interprets those commands and drives the
surfaces.  If the computers should fail, there would be no control of
the shuttle during reentry.  Hence, the concern for the reliability of
the computers is much greater for the shuttle than has been the case
for previous space flights.