Gregory's ServerThis photo shows the D-37C computer with advanced 10-layer circuit boards covered with Texas Instruments flat-pack integrated circuits. It used 22 custom integrated circuits; a flip-flop chip cost over $500 in current dollars. 11/N
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 A nearby nuclear blast could cause transient errors in the computer. To prevent this, a "transient current detector" (photo) sensed a radiation pulse and the computer was shut down until the pulse went away. The computer would then continue where it left off, extrapolating the trajectory for the time it was shut down. 13/N  The Missile knows where it is because it knows where it was ... 😉 The guidance system also contains a stack of amplifier boards, interfacing the computer to the rest of the missile. The amplifier includes high-current drivers for the various pyrotechnics on the missile such as igniting the rocket stages and retrorockets. 14/N Another block of electronics is the Missile Guidance Set Control, which has amplifiers, power supplies, and servo circuits for the gyroscopes and accelerometers. It has a modular construction with a removable module for each function. 15/N The missile is powered by an unusual battery. This silver/zinc holds the electrolyte in a separate tank so it has almost infinite shelf life. At launch, a gas generator is ignited, forcing the KOH electrolyte into the battery cells, powering up the battery. 16/N Another pyrotechnic component is the squib-activated switch, a switch that is activated by a tiny explosive squib. The missile is connected to ground equipment by an umbilical cable, which is disconnected at launch. The explosive switch severs critical signals just before, presumably to avoid any electrical noise when the umbilical is disconnected. Also note the window in the side of the missile for the alignment light beam. 17/N The guidance system runs 24 hours a day so the missile is ready for immediate launch. On the ground, the system is water cooled; ground support equipment provides a chilled sodium chromate solution through the umbilical connection. This photo shows the coolant path through the computer and other components. 18/N The missile is programmed with launch codes through the Permutation Plug, a plug that holds the 48-bit code. With great ritual, an armed officer plugged it into the socket below. The launch code also depended on values on disk, so each Launch Control Center had a unique code for each missile. 19/N A launch normally requires launch orders from two separate Launch Control Centers. But a single surviving Launch Control Center could launch the missiles, unless vetoed before a timeout. A complicated state machine managed the launch process. 20/N Although the Minuteman guidance system is interesting technologically, one has to keep in mind its purpose was to unleash nuclear devastation On the other hand, Minuteman has been successful as a peacekeeping deterrent (so far). In any case, it is morally ambiguous compared to, say, the Apollo Guidance Computer. There are currently 400 Minuteman missiles active, down from a peak of 1000. 21/N I wrote a blog post that goes into much more detail on the Minuteman guidance system and computer, so check it out: https://www.righto.com/2024/08/minuteman-guidance-computer.html  @kenshirriff Two "super stupid questions" .. actually one maybe a bit less stupid .. 1st question "Of course all that ( very expensive ) tech gets destroyed once the missile is launched ? I suppose when doing tests with no explosive they managed to recover it ? 2nd maybe less stupid have they ever had a "self destruct" thing to avoid such tech to accidentally end up in enemy hands if a missile launched failed to explode and could be recovered ? Maybe I am "too tinfoil hat territory" ? 😅 @gilesgoat Since the missile (and warhead) are crashing down at Mach 23 from hundreds of miles up, I don't think there would be anything left to recover. @kenshirriff Thought so, it breaks my heart to think all that fantastic tech destroyed in an instant .. 😢 @kenshirriff came for the electronics, stayed for the typefaces :D so 60s  @kenshirriff The technology is fascinating, and you've explained it very well, but promoting/sharing weapons of war doesn't feel good to me 😅  @kenshirriff This is a fantastic writeup, basically all of the ~1% of curious people in the space industry will end up reading it. As is often the case, the asides are many of my favorite parts; it feels very American to be specifically concerned about the environmental impact of your Global Thermonuclear War Machine’s cold gas thruster fuel. On footnote 8 there is a typo, “3000 km, even times the altitude”. @kenshirriff Absolutely fantastic post. Pure science and technology ( although much better to never see it in action :blobsweats: ). Many thanks for your fantastic posts !!!!  @kenshirriff @johnefrancis  @kenshirriff @johnefrancis the 1964 Minuteman incident is written up here: https://www.armytimes.com/news/2017/11/04/details-of-south-dakota-nuclear-missile-accident-released/ As for the Damascus incident, there's Eric Schlosser's whole book, Command & Control @scruss @johnefrancis I heartily recommend the book Command and Control. @kenshirriff @johnefrancis agreed. The whole thing starts with "You did *what* on top of two tanks of hypergolic fuel?" I think the moral of both stories is : go back to the damn truck to get the right tool  @kenshirriff was the guidance ever upgraded or are those 400 Minutemen still running on this vintage technology? @root42
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@Eupeptic I think the bar unscrewed and then served to hold the plug in place.
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@DeltaWye That makes sense. Weird things could happen right at launch.  @kenshirriff Document with a great example of a sneak circuit involving the cargo door and landing gear in an aircraft. There are some other examples out there. If the emergency door open switch and the normal door open switch were operated, the landing gear would deploy. (Not a sneak circuit but there was an incredible wiring error in a train and a design flaw where sounding a whistle on a locomotive caused the carriages to decouple.)  @kenshirriff @DeltaWye I'm wondering if this still makes sense with modern computers clocking away at GigHz – would pyrotechnic be "fast enough"? @kb9ens I don't know the internals of the power supply oscillators. It could be a quartz oscillator, but I wouldn't be surprised if it were just R-C, since the timing isn't particularly important. @kenshirriff Woah... sounds like that would also be resilient to transient currents and restoration of time after a transient induced shutdown. As ever clever clever...  @kenshirriff @kb9ens that is an interesting combination of design decisions. |
Minuteman III used the upgraded D-37D computer with 14,137 words of storage. It used flexible PCBs, advanced for the time, soldered with low-melting-point indium/tin solder. 12/N