The Risks Digest

The RISKS Digest

Forum on Risks to the Public in Computers and Related Systems

ACM Committee on Computers and Public Policy, Peter G. Neumann, moderator

Volume 2 Issue 04

Sunday, 2 Jan 1986


o Solid propellants
Mike McLaughlin
o Plutonium
Jim McGrath
o SRB Self-Destruct Mechanisms
Clive Dawson
o Details on the 1981 Quebec election -- a program bug
Jean-Francois Lamy
o Info on RISKS (comp.risks)

Solid propellants

Mike McLaughlin <mikemcl@nrl-csr>
Sun, 2 Feb 86 14:08:17 est
Odd topic for a computer centered forum - but worth discussing a bit.  The
computer hook relates to what could have been monitored, detected, and reac-
ted to in computer time; but not in human time.  I base this discussion on
long-ago experience in writing about solid propellant rockets, plus Sunday's
TV & radio news.

1.  Solid propellants burn at a surface.  If they are designed to burn at one
end, they are called "cigarette burning."  If they are designed to burn
through a hole in the middle, they are not.  The prepared hunk of propellant
is called a "grain."

2.  Cigarette burning produces roughly constant propelling force throughout
the burn.  Chunks of loose propellant (cylinders, spheres, etc.) produce more
thrust at the beginning, less at the end, as the surface area of the grains is

3.  The hole in the center of a grain can be tailored in shape to affect burn
characteristics just about any way the engineer wants.  In addition, "inhibi-
tors" can be put on the grain to further control its burn characteristics.

4.  In most boosters the grain fills the container, except for the hole in the
center, and a space near the nozzle.  An ignitor (actually, another small
rocket) is usually at the end opposite the nozzle.

5.  Remember, the grain burns at the surface.  A crack in the grain provides
another surface to burn.  If the grain separates from the casing, the
exterior of the grain provides another burning surface.  If the grain is
sectional, i.e. too large to build as one unit, the ends of the sections can
provide burning surfaces.  Naturally, it is the engineer's job to control and
prevent these undesirable burning surfaces, and to produce the thrust profile
required for the task at hand.

(tutorial ends, speculation begins)

It is my understanding that "SRBs" were built in 6 sections, and assembled
on-site. Nose, 4 grain sections (not necessarily identical, the hole can be
tapered), and tail.  I also understand that the casing sections were "bolted"
together (probably a fairly complex bolting system); and were considered to
be quite safe & reliable.  The casings were recovered after a launch, refurb-
ished, reloaded, & re-used.

Recently released film, computer-enhanced offline, after the accident, show
that the right hand SRB had a plume coming out the side, in a location that
appeared to me to be about where the joint between the 3rd and 4th grain/
casing sections would be - but, depending on the actual design, could have
been further aft, near the end of the grain, towards the nozzle.  If this
was a casing/grain burn-through, the mildest result would be assymetric
thrust.  *This should have been immediately detectable by the guidance system's
reaction in attempting to maintain the desired trajectory.*  If similar per-
terbations occurred in wind shears, etc., it might not be recognizable as

Another result could be that the errant jet impinged on the main fuel tank,
heating, penetrating, and igniting the fuel load. (It might be able to ignite
it without penetrating the tank structure.)  *This should be quickly detec-
table by excursions in tank pressure.*  Reaction times, even of computers,
might not be fast enough to make any difference in the outcome.

I believe that both of the above could have been detected with instrumentation
that was certainly on board.  Additional (or existing?) instrumentation could
detect temperature changes in SRB and fuel tank skins, torques on SRB mounts,
abnormal "seismic" vibrations within the SRB structure, abnormal "plumes",

It is so easy to second-guess.  I am sure the engineers concerned are casti-
gating themselves for what they failed to forsee, for what they concluded was
trivial, for what now seems eminently clear to them.  I wish they would quit it.
The whole program is so full of checks and balances that only a Higher Power
could add more.  From "MTM's" description of the safety system, it seems a
miracle that it was possible to destroy the SRBs under normal circumstances,
much less in the middle of disaster.  The astronauts participated in the design
and manufacturing process - they were ready to go.

We have lost seven of our best and brightest.  But perhaps we are seven closer
to whatever is out there in space, waiting for us to get on with it, get out
there, fulfill our dreams.
Peter: this is too long, but I had to write it, tell someone.  I went into
space in the '50s, with Heinlein and Bonestell.  The Challenger Seven must
not be regarded as sacrifices on the altar of science - they were just seven
of us who went a little closer to the edge of knowledge than the rest of us
dare.  The human/computer symbiosis will get us out there eventually, and
the Challenger Seven will have helped every one who follows them. - Mike


Sat 1 Feb 86 19:20:51-EST
First, I assume that everyone knows that no atomic explosion would
occur under any circumstances.  Nor any fallout.

That only leaves the actual radioactive fuel itself.  Plutonium's
danger, for a constant mass, depends upon the size of the particles.
The worse thing that can happen is for dust size particles to be
inhaled.  Large chunks would be a local danger, but one easily
handled.  Note that if the launch was from the Cape, then it would
eventually settle into the ocean.  This would aid considerably in
dispersing it to extremely low concentrations.  Finally, remember that
the Soviets lost a satellite powered by radioactives over Canada.
While the Canadians were not happy, and took clean up measures, the
real problem was getting the Soviets to pony up for the cleanup costs.

    .... Therefore, aside from several hundred pounds of plutonium ...

Are you sure about your numbers?  Hundreds of pounds of pure
plutonium?  The cost would be outrageous.  Moreover, this implies a
total mass would be thousands of pounds, if not tons (since the
plutonium would be diluted to a lower concentration and sufficient
shielding for the electronics would have to be provided).  Maybe you
mean a fuel assembly massing hundreds of pounds?  If so, then the
actual mass of Plutonium would be a small fraction of the total mass.


SRB Self-Destruct Mechanisms

Clive Dawson <AI.CLIVE@MCC.ARPA>
Fri 31 Jan 86 13:29:44-CST
One aspect of the SRB self-destruct mechanism which has bothered me the
most is the fact that a single action will destroy BOTH SRB's (and perhaps
the external tank as well?).  It is clear that recovery of the intact
casings would have been invaluable in the NASA investigation.  News reports
tell us that one of the SRB's was headed on a dangerous course toward
popluated areas and had to be destroyed.  Fair enough.  But why destroy
the other one unless and until it was also proved necessary??

Thinking about this further reveals it may not be that simple.  First of
all, I can imagine scenarios in which both SRB's would need to be destroyed
as quickly as possible, especially in the early phases of the launch.  You
would certainly want to have a mechanism for doing this as exists now.
On the other hand, last Tuesday's events show that it would be very
valuable to be able to destroy them individually as well.  This would imply
modifying the hardware/software such that each SRB responded to two sets
of tones: a common set for both and an individual set.  Perhaps a simpler
scheme would be to simply have two different frequencies which could be
used simultaneously or separately.

Those of us discussing this were momentarily satsified until somebody
asked, "Yes, but how do you tell which SRB is which??!"  In this case, it
was reasonably easy to answer that question when they emerged from the
fireball, but this might not always be the case.  Furthermore, it's not
clear that the task would be any easier when watching them on a radar
screen.  (What does the Range Safety Officer use?)  This difficulty
can presumably be overcome by electronic equipment on each SRB that would
tag its radar image in some fashion.

I'm wondering if this is a case of "good hindsight", or if there are
other considerations we didn't think of.


Details on the 1981 Quebec election -- a program bug (RISKS-2.1)

Jean-Francois Lamy <lamy%utai%toronto.csnet@CSNET-RELAY.ARPA>
02 Feb 86 09:40:43 EST (Sun)
> - Quebec election prediction gave loser big win [1981] (SEN 10 2, p. 25-26)

Election monitoring software for two television networks was faulty: votes
were being attributed to the wrong candidates.  Names were being kept
in alphabetical order while votes were kept in decreasing order.
This is a language related bug: the contractor was IP Sharp and the
software was programmed in APL -- the informations ended up in distinct
vectors, with one being mistakenly kept sorted.

  Jean-Francois Lamy
  Department of Computer Science, University of Toronto,
  Departement d'informatique et de recherche operationnelle, U. de Montreal.

  CSNet:      lamy@toronto.csnet
  UUCP:       {utzoo,ihnp4,decwrl,uw-beaver}!utcsri!utai!lamy
  CDN:        lamy@iro.udem.cdn (lamy%iro.udem.cdn@ubc.csnet)

     Engineering Notes, from a review by PGN of John Shore's "The
     Sachertorte Algorithm and Other Antidotes to Computer Anxiety",
     vol 10 no 2, pp. 25-26, April 1985.]

  The chapter on Myths of Correctness brings us the tale of the 1981
  provincial election in Quebec, Canada.  One station's computer had been
  misprogrammed, and it announced that the overwhelming underdog Union
  Nationale had won 19 out of 49 races.  Their announcers somehow even managed
  to come up with erudite analyses explaining why this amazing upset had
  occurred.  It was not until twenty minutes after the other station had
  declared that the Parti Quebecois and the Liberal Party had totally
  dominated the election that the first station realized that there had been a
  colossal mistake somewhere!                       [PGN]

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