
Inside a Titan missile guidance computer - mcrute
http://www.righto.com/2020/03/inside-titan-missile-guidance-computer.html
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dmix
This is another good read about using automated terrain mapping for cruise
missile guidance systems.

[https://en.wikipedia.org/wiki/TERCOM](https://en.wikipedia.org/wiki/TERCOM)

Notably how it fit on older computer systems with limited memory by only
sampling the terrain using a predefined path which a plane will travel over
with imaging sensors beforehand. For minutemen style systems with predefined
targets it would work well as the terrain won’t change significantly and there
will be plenty of sampling data to support a high error rate and/or a
relatively low resolution while moving at high speeds.

Of course it increased in quality and effectiveness over time (especially as
whole topographical maps could eventually be stored, not just the path in
question) but as an entirely ‘unguided’ rocket it seems to be relatively
accurate for its era.

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pvg
Isn't it an example of a highly guided missile?

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dmix
Yes that’s correct, I meant compared to more controlled guidance by wire,
radiation, satellite, etc. I phrased that improperly.

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pvg
Fun tangent - the most unguided missile ever.

[https://en.wikipedia.org/wiki/AIR-2_Genie](https://en.wikipedia.org/wiki/AIR-2_Genie)

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chrisseaton
> Lethal radius of the blast was estimated to be about 300 metres (980 ft).
> Once fired, the Genie's short flight-time [12 seconds] and large blast
> radius made it virtually impossible for a bomber to avoid destruction.

Why can’t you dive or climb just 150m in 12 seconds and be out of the blast
range?

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duxup
I suppose you have to be sure of the rockets detonation point to know if you
are moving away in the right direction. I'm not sure that is a thing you could
be sure of the moment the rocket is fired (or if you would know).

Also the target (at least initially) was large formations of Tupolev Tu-4s who
can only climb 15m/s. So that puts them right on the edge, but assuming more
than one rocket is fired or that they simply don't always have time to realize
it was fired quickly enough... that probably does it.

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rgovostes
I visited the Titan Missile Museum in Tucson, AZ, where you can see the
control room from which the missile could have been launched.

> At launch, orders from the National Command Authority would have specified
> one of three pre-programmed targets which, for security reasons, were
> unknown to the crew. ... Target 2, which is classified to this day but was
> assumed to be within the borders of the former Soviet Union, was designated
> as a ground burst, suggesting that the target was a hardened facility such
> as a Soviet missile base. [Wikipedia]

While explaining this, my tour guide showed some punched tape that could be
fed into a computer to program the targets. I wonder if the tape was original,
and if so, how difficult it would be to decode its coordinates.

~~~
kens
Yes, the Titan museum is very interesting and I recommend it.

I wondered the same thing about the punched tape, if I could extract the
coordinates from it. I read somewhere that they were extremely strict about
keeping the targets secret; before any maintenance on the computer, a special
team came in just to ensure that the coordinates were erased from memory. It
would have been a huge oversight if they didn't encrypt the punched tape, so
my guess is that it can't be decoded.

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beerandt
Y'all are overthinking it- true lat long "coordinates" would be meaningless to
this computer. What was fed into the computer (ahead of time) was a series of
firing instructions (per target for 3 targets), calculated to take into
account the launch site of each missile. (Core memory values persist long
term, even through power outages, hence the need to manually erase values
before maintenance.)

So even two missiles/warheads launched from different sites at the same target
would have different guidance instructions.

You'd need to do some major reverse engineering and geodetic calculations even
after decoding targeting punchcards to figure out what the actual targets
were.

The punchcards used to select the target during launch are simply instructions
to execute "target stored in memory 2" or something similar. The minutemen
would not have had the ability to change the set of targeting options
programmed in the missile, only select one of the pre-programmed options.

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kipchak
If you're interested in the history of the Titan missile program and US
nuclear program in general I recommend "Command and Control: Nuclear Weapons,
the Damascus Accident, and the Illusion of Safety".

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kens
I definitely agree with your recommendation. Much of that book describes the
Titan missile that blew up in 1980 after someone dropped a wrench socket that
punctured the rocket's fuel tank, throwing the 9 megaton warhead 100 feet.
(The warhead did not detonate.) What I didn't realize until I visited the
Titan museum near Tucson is that it wasn't a regular wrench socket, but a big,
fist-sized 8-pound socket. This made it much more understandable how the
socket put a hole in the missile.

~~~
blueatlas
American Experience (PBS) did one of their documentaries on this event. It is
very well done.

Prior to the socket being dropped, there was a change order issued to use a
new type of socket that would not dislodge from the wrench. As I recall, the
two technicians tasked with the repair didn't have the new type of socket, but
proceeded into the silo anyway.

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sytelus
The fascinating thing is that the Titan was completely inertially guided,
i.e., no GPS or ground-based navigation. The high precision IMU + gyro send
the computer acceleration and angular velocity and you integrate that over
time to figure out where you are now. I'd think this would be very challenging
and error-prone for inter-continental travel especially when the thing is
planning to detonate nuclear war head. I wonder what algorithms they used.
Typical code to do dead reckoning from IMU+gyro is fairly short but drifts
usually is huge killer.

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timdierks
"Inventing Accuracy" is a fascinating history of ICBM inertial guidance.
[https://mitpress.mit.edu/books/inventing-
accuracy](https://mitpress.mit.edu/books/inventing-accuracy)

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userbinator
GM/Delco were very diverse in manufacturing, so I'm not surprised that they
also made missile components at one point. At one time they owned Frigidaire
too.

Motorola made "MTTL II" which had 20xx part numbers for commercial temperature
range and 21xx for industrial/military, so these could be Signetics' variant
of that. In the late 60s through the 70s semiconductor companies made _lots_
of different logic families.

I've always found it a bit sad that the ultimate purpose of things like this
was for destruction.

~~~
kens
Thanks for the part number info; that's a good lead and I'll see if I can
track anything down.

As far as your last point, I basically agree. This computer, though, is not
quite as bad as it could be. It was a later, 1970s model originally designed
for the Titan IIIC rocket to put satellites into space, a more positive role.
It was also closely based on the Carousel IV commercial navigation system used
for aircraft such as the Boeing 747, which is a nice heartwarming application.

Edit: the Motorola 21xx parts have the wrong pinout, e.g. ground on pin 10 vs
11. And many of the numbers don't exist as Motorola parts. Unfortunately, I
don't think these are the right parts, although it seems so close.

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kator
I was just thinking, imagine trying to use a modern-day smartphone for this.
So much complexity how could you be sure the missile would even get off the
ground. There is a big advantage to systems that engineers can mostly "keep in
their heads". Very few systems are that way these days.

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djmips
A Teensy 4.0 just as an example which is much more like this. Has a bunch of
general purpose I/O; it's quite powerful; and you can essentially keep it in
your head.

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api
I find it fascinating how much those grids resemble RAM chip die photos. I
know the principle is different but in a sense we have just been shrinking
things for over 70 years.

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djmips
Of all the details, I particularly enjoyed the keying method with the half hex
nuts. I'll file that away for future projects.

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serf
I'm surprised they opted to use a passively cooled power supply.

I imagine it was much heavier than an equivalent active system.

I guess MPG doesn't matter much on a Titan missile.

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DuskStar
I'd guess the opposite, actually. Mass is extremely important for a rocket, so
there would be strong incentives to make the cooling system as light as
possible.

But a passive system has a few major advantages - it's maintenance free,
unlike an active system (imagine having to fill up the coolant tank before
launch), and perfectly reliable. Sure, a phase-change coolant might be more
efficient - but with flight times in the hours I'm not sure that the
difference would be enough to compensate for the complexity.

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ncke
Great read! Intriguing to find out that earlier computers were using rotating
drum memory systems in space flight and then delicate wiring in cores, I
really hadn't considered that. I wonder about the reliability of those systems
and whether there was software compensation for that issue.

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xornox
Some engineers designed this - used big part of their life for the work and
the system was never used.

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BostonFern
Sure it was used--as a credible deterrent.

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bobowzki
I love reading about avionics. New and old.

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julienreszka
Always scares me to see this displayed for the public. Enemies watching.

