Apollo Guidance Computer switching power supply works after 50 years - <AUTHOR> http://www.righto.com/2019/08/reliable-after-50-years-apollo-guidance.html <comment><USER2> _We found that the capacitors were all in good shape with the proper capacitances. This is in contrast to modern capacitors, which often leak or fail after a few years. NASA used expensive aerospace-grade capacitors and X-rayed each one to test for faults, and this made a large difference._

That's also because these are hermetically sealed _wet tantalum_ caps, not the dry type that's notorious for shorting out and catching fire. "Wet tants" are _very_ expensive and you can still buy them today:

[https://www.mouser.com/Vishay/Passive- Components/Capacitors/...](https://www.mouser.com/Vishay/Passive- Components/Capacitors/Tantalum-Capacitors/Tantalum-Capacitors-Wet/HE3-Series/)

 _We were somewhat surprised that both power supplies worked flawlessly after 50 years._

I'm not all that surprised, actually --- but perhaps that's because I've seen plenty of videos on YouTube of more mundane equipment, like vehicles and appliances, coming back to life after several decades of storage or exposure to the elements with minimal repairs, so for a clearly high-reliability component like this AGC PSU to work is almost expected.

<comment><USER3> Working link: [https://www.mouser.com/Vishay/Passive- Components/Capacitors/...](https://www.mouser.com/Vishay/Passive- Components/Capacitors/Tantalum-Capacitors/Tantalum-Capacitors- Wet/_/N-75hr5?P=1z0zls5)

<comment><USER4> $/F record? [https://www.mouser.com/ProductDetail/Vishay/XTV157T270P0L?qs...](https://www.mouser.com/ProductDetail/Vishay/XTV157T270P0L?qs=sGAEpiMZZMuEN2agSAc2ptl%252BlXX0IRpaER3axfKJ7FA%3D)

</comment></comment></comment><comment><USER5> That is beautiful work, the _welded_ connections are really impressive. Obviously they had to take into account massive forces, vibration and possibly impact damage, but still, to see components welded in place is something I've never seen before, not even in very high end HF power electronics.

> The AGC that we're restoring belongs to a private owner who picked it up at > a scrapyard in the 1970s after NASA scrapped it.

That is one lucky find. Imagine that it had _not_ been found and scrapped for its metal value. The mind boggles at the potential destruction of such a historical artifact.

> the cordwood components are mounted differently from the other cordwood > modules.

For those from cities who have never seen cordwood:

Cordwood is a stack of wood of short length seen from the endgrain, typically used when referring to firewood but also sometimes used in construction.

<comment><USER6> > ... Obviously they had to take into account massive forces, vibration and > possibly impact damage ...

That's why they also glued them in epoxy. If you don't want you electronics to move (and thus break connections), put them tight in epoxy. And I mean completely glued in. No air left. This is as valid for modern solder joints as well as welded.

<comment><USER7> The AGC we restored was used for ground testing, so most of the modules weren't encased in epoxy. This made things much more convenient for us. We did have to dig through encapsulation to fix one module, though.

</comment></comment></comment><comment><USER8> I was shocked to see the title because I thought forward converters were invented in the 70s but it turns out they were invented in the 50s with some early examples in the 20s, if one gets loose enough with the definition of "converter".

[https://pdfs.semanticscholar.org/0911/34a5dc72c5a363ec937e40...](https://pdfs.semanticscholar.org/0911/34a5dc72c5a363ec937e40c0aa3706f1cd99.pdf)

<comment><USER7> Yes, these converters go way back, although the AGC used a buck converter not a forward converter. They became more practical in the 1970s as transistor technology improved. I wrote a history of power supplies in the IEEE Spectrum recently: [https://spectrum.ieee.org/computing/hardware/a-half- century-...](https://spectrum.ieee.org/computing/hardware/a-half-century-ago- better-transistors-and-switching-regulators-revolutionized-the-design-of- computer-power-supplies)

<comment><USER8> Nice article thanks for sharing it. What definition are you using for forward converter? The definition I always use, which I thought I got from Erickson's Fundamentals of Power Electrons, is any topology where the inductance is charged in series (as opposed to parallel like a boost or fly back) with the load. I've always liked this definition because it still conveys critical meaning even when engineers do trivial topology changes from the "standard" forward converter topology.

<comment><USER7> I've seen a forward converter described as buck-derived, or buck with a transformer, but buck converters and forward converters are generally viewed as two separate things. I'm not too attached to definitions but I consider a forward converter as using a transformer and transferring energy while the switch is on. This is the definition used by the article you linked to, and by Wikipedia for Forward Converter. I also like the TI poster of power topologies: [https://www.ti.com/lit/sg/sluw001f/sluw001f.pdf](https://www.ti.com/lit/sg/sluw001f/sluw001f.pdf)

<comment><USER8> I think the article I linked to didn't require the transformer. Only that there be transfer with the switch on which implies the series connection. :) Wikipedia certainly does though and honestly, especially in power conversions, there's not a lot of standardization in terms which is why I asked. I'm not too hung up on the definition either. I'm probably just over sensitive because all the marketing material I'm asked to review...

</comment></comment></comment></comment></comment><comment><USER9> Huh? They got the whole computer working, not just the power supply.

Anybody who hasn't seen the two dozen or so videos made by CuriousMarc is in for a geekout of historic proportions. Some truly amazing work, with more ups and downs than the Apollo program itself. I can't easily come up with a link on mobile but you'll find it by looking up CuriousMarc on YT. Watch the whole playlist, it's awesome.

<comment><USER7> Yes, the whole AGC works; this article discusses the power supplies in particular. The video playlist is at: [https://www.youtube.com/playlist?list=PL-_93BVApb59FWrLZfdli...](https://www.youtube.com/playlist?list=PL-_93BVApb59FWrLZfdlisi_x7-Ut_-w7)

<comment><USER10> Were you able to see who made the tantalum capacitors? Or did the cordwood construction prevent that?

I ask because my dad was Group Product Manager at Kemet during this time period. Growing up, he mentioned their products going into several aerospace systems (various ICBMs and military projects, etc) as well as the IBM System 360. But he never mentioned involvement with Apollo and I'm curious if he might have had a hand in the AGC.

BTW, your link to the NASA contract drawing is incorrect - here's the one you probably wanted.

[http://www.ibiblio.org/apollo/SCDs/scd_1006755t.pdf](http://www.ibiblio.org/apollo/SCDs/scd_1006755t.pdf)

<comment><USER7> The (corrected) drawing says Sprague made these capacitors: [http://www.ibiblio.org/apollo/SCDs/scd_1006755-.pdf](http://www.ibiblio.org/apollo/SCDs/scd_1006755-.pdf) (I don't know if Kemet made other capacitors used in Apollo.)

<comment><USER11> To the contrary! In our AGC, all of the capacitors not tucked into cordwood holes sport a KEMET logo and part number: [https://photos.app.goo.gl/oGsrgXBpNMccmkWV7](https://photos.app.goo.gl/oGsrgXBpNMccmkWV7)

I think the Computer History Museum's might have Sprague caps in it, though.

<comment><USER9> Yep, that's the capacitor I was referring to below. At least some of them were definitely Kemet parts. I don't remember seeing any Sprague logos but I imagine there were some of those around, too.

</comment></comment></comment></comment></comment></comment><comment><USER12> It's always impressive to see somebody try to build something that lasts, and succeed. It's even rarer to see such a thing in consumer electronics.

I'm currently sitting in the same room as a Bryston amplifier that, according to it's date code, was manufactured in late 1998. That means it's almost 21 years old and just 1 year off warranty. It's been switched on for most of those 21 years but still works great and has never been serviced. Even more surprisingly, it's not obsolete. It's currently hooked up to a 2018 receiver.

I'd love to see somebody do tear-downs of devices like this and explain how their construction takes longevity into consideration without resorting to the same extremes as NASA (e.g. X-raying caps).

<comment><USER13> I run my Carver amplifier, bought new in 1981, every day all day. That's 38 years of near constant use!

<comment><USER12> Damn! Take that NES!

</comment></comment></comment><comment><USER14> It's sad that there is a dichotomy between building a good product and building too good of a product that you don't get enough return customers.

<comment><USER15> The art would seem to be to build the durable product with room for extension, so that compelling upgrades are possible.

As seen with people who collect Apple products and just keep adding to the fruit basket.

<comment><USER16> Apple products are pretty much the opposite of good products, at least in the meaning of "good" implied in the context. Leaving aside the failure rate of these, most of their products are sealed with propriety battery. This gives them a pretty short service life which guarantees returning costumers.

<comment><USER15> Was that true of the old Mac computers, though?

I'm not talking about the post-Jobs stuff.

<comment><USER16> The old Mac computers were not as bad as the first iPod, but they weren't heavy-duty or anything of the sort. Nothing exceptional in that dimension.

</comment></comment></comment></comment></comment><comment><USER17> Slightly off topic but I understand as the Apollo 11 LEM descended, there were several "computer overload errors".

Is this out of memory or skipped input or dropped processing requests or too much power drain? What exactly is an "overload error" on such an unusual machine?

<comment><USER18> If you google for "apollo 1201 or 1202 alarm" you'll find several good magazine-length articles on the design of the landing guidance computer systems.

The TLDR is: a switch misconfiguration was tasking the computer to process some rendezvous tracking information which was not needed. This exhausted a rather limited set of storage locations and triggered the alarm.

The computer was properly programmed to treat the condition as a secondary failure and continue with its primary task.

The thing that was remarkable to me was the control room engineer's response to the 1201 right before touchdown - they hadn't seen one of those, and it was not the same as the 1202's they had seen earlier.

In the videos I've seen you hear the audio - "1201 alarm" and within less than a second the 28-year-old (average age, just guessing) responds "1201 go". He just gave instant clearance to proceed past a malfunction a few hundred feet above the moon's surface, in real time. Talk about being in the zone.

<comment><USER11> Sadly most of what you read by googling these problems is misinformation. It was actually an incredibly sinister systems integration bug, that wasn't well described even at the time.

It wasn't a switch misconfiguration; the Apollo 11 astronauts were flying to the checklist, and did as they had simulated. The Rendezvous Radar switch has three settings -- LGC, AUTO TRACK, and SLEW. In LGC mode, the AGC controls the positioning of the antenna; in AUTO TRACK, the radar automatically tracks the CSM based on return strength; and in SLEW it is automatically positioned.

The trouble came from how the trunnion and shaft angles of the antenna were measured. They used "resolvers", which are sort of like variable transfomers. Resolvers look like motors, and attached to the shaft there are two windings positioned 90 degrees apart from each other. An AC "reference voltage" is applied to an outer winding in the case, and that voltage couples onto the two inner windings with a magnitude proportional to the angle on the shaft. One winding (the "sine" winding) produces an output equal to Vref _sin(theta) and the other ( "cosine") winding produces an output equal to Vref_cos(theta), where Vref is the reference voltage and theta is the angle of the shaft. The voltage and phase of both windings can be used to determine exactly what the theta was that produced them.

The circuitry to do this is a bit involved though and lived outside the computer, in a device called the CDU, or Coupling Data Unit. The CDU constantly maintained its own idea of what the angle ("psi") in a digital register. It translated the incoming sine and cosine voltages into a digital representation by mechanizing the equation +-sin(theta-psi) = +-sin(theta) _cos(psi) -+ cos(theta)_ sin(psi). It did so by using the bits of its digital register containing psi to switch on and off resistor dividers that effected cos(psi) and sin(psi) onto the incoming signals, which were then added together with a summing amplifier. The goal of the CDU is to zero this sum; to accomplish this, it "counts" the angle register up or down to reduce the magnitude of the sum. As it counts, switches are changed, which switch out resistors in the circuit, which in turn change cos(psi) and sin(psi) in the above equation. And also, with every other increment, a pulse is transmitted to the AGC to indicate that the angle has changed slightly.

The problem comes in because in addition to the above, the CDU also, for many angles, added to the sum some fraction of the reference voltage directly. This is fine when the switch is in the LGC position; the resolvers are supplied with the same 28V, 800Hz reference voltage that is used inside the CDU. However, when the switch is put in either of the other two positions, the reference voltage for the RR resolvers is switched to an unrelated 15V rail. Critically, this 15V reference has no defined phase relationship with the CDU's 28V 800Hz reference. The phasing is locked in by the exact millisecond at which you power up your subsystems.

So when the switch is changed, the sine and cosine outputs from the resolver are suddenly derived from the 15V reference -- they are much lower before and at a random phase. The CDU doesn't know that this has happened, and still tries to perform the summing as before. However, for many theta/phase relationships, it becomes impossible for the CDU to actually null the sum. In these cases, the CDU becomes "manic", and starts seeking back and forth, frantically changing switches to try to figure out what the angle is, but never succeeding.

This causes a huge flurry of +1 and -1 pulses to the AGC. In order to minimize circuitry, the AGC implemented what was called "unprogrammed" or "cycle- stealing" instructions. The computer only contains a single adder, and adding or subtracting 1 from the current angle requires use of that adder and a memory cycle. Rather than generating a full interrupt, which would require many memory cycles and instructions to handle, the computer simply transparently inserts a single-cycle instruction in between two "programmed" instructions that performs the addition or subtraction. This is totally transparent to software, normally. But with a manic CDU that is incessantly seeking on both RR angles, the AGC receives something close to 12,800 pulses per second, which translates into something around 15% of its total computational time. The landing software had only been designed with a margin of 10% or so.

The 1202s were also a lot less benign than is often reported. They occurred because of the fixed two-second guidance cycle in the landing software. That is, once every two seconds, a job called the SERVICER would start. SERVICER had many tasks during the landing. In order: navigation, guidance, commanding throttle, commanding attitude, and updating displays. With an excessive load as caused by the CDU, new SERVICERs were starting before old ones could finish. Eventually there would be two many old SERVICERs hanging around, and when the time came to start a new one, there would be no slots for new jobs available. When this happened, the EXECUTIVE (job scheduler) would issue a 1201 or 1202 alarm and cause a soft restart of the computer. Every job and task was flushed, and the computer started up fresh, resuming from its last checkpoint. It was essentially a full-on crash and restart, rather than a graceful cancellation of a few jobs. And unlike is often said, the computer wasn't dropping low-priority things; it was failing to complete the most critical job of the landing, the SERVICER.

Luckily, the load was light enough that of the SERVICER's duties, the old SERVICER was usually in the final display updating code when it got preempted by a new SERVICER. This caused times in the descent when the display stopped updating entirely, but the flight proceeded mostly as usual. However, with slightly more load, it was fully possible that the SERVICER could have been preempted in the attitude control portion of the code, or worse yet, the throttle control portion. Since each SERVICER shared the same memory location as the last one (since there was only ever supposed to be one running at a time), this could lead to violent attitude or throttle excursions, which would have certainly called for an abort. Luckily, this didn't happen -- and the flight controllers didn't abort the mission not because 1202s were always safe, but because they didn't understand just how bad it could be, were the load just a tiny bit higher.

<comment><USER19> Could I ask how you know so much about this, or where I can read something more detailed than the usual story that's reported? Thanks.

<comment><USER11> Many years now of research and simulation of the system (I led the restoration of the computer mentioned in the article). There's not a single place where you can read everything, unfortunately, aside from the comment above. We're planning on making a video on it in the future. But I can cite sources:

CDU theory of operation (starting PDF page 15): [http://www.ibiblio.org/apollo/Documents/HSI-208435-003.pdf](http://www.ibiblio.org/apollo/Documents/HSI-208435-003.pdf)

CDU coarse module schematic: [https://archive.org/stream/apertureCardBox462NARASW_images#p...](https://archive.org/stream/apertureCardBox462NARASW_images#page/n1507/mode/1up)

Grumman memo (from 1968!) describing the problem, and mentioning it is due to the reference switching to a 15V 800Hz source: [https://www.ibiblio.org/apollo/Documents/Memo- GAEC_LMO_541_1...](https://www.ibiblio.org/apollo/Documents/Memo- GAEC_LMO_541_108_text.pdf)

Excerpt from the LM-8 Systems Handbook showing the reference voltage RR switch wiring: [https://i.imgur.com/fMsQ7RI.png](https://i.imgur.com/fMsQ7RI.png)

Don Eyles describes the software side best in his book Sunburst and Luminary (which I highly recommend) but he also talks about it in some detail on his website: [https://doneyles.com/LM/Tales.html](https://doneyles.com/LM/Tales.html)

</comment></comment></comment></comment></comment><comment><USER20> Now I wonder if fuel elements powering this PSU can be restored to a workable state? Then moving forward to restoration of the whole spacecraft. Shouldn't be too hard :) given that some of most complex units are already working...

</comment><comment><USER21> Buck converters, as any fan of Big Clive videos knows, are still widely used in low cost power supplies for LED light bulbs and cheap USB chargers, due to the few components needed and wide tolerances the circuits can handle.

<comment><USER22> Buck converters are still widely used in absolutely everything.

</comment></comment><comment><USER23> I wonder if there were any issues with the STBY button being located where it is. It seems really risky place to have a off key right between other frequently used button.

<comment><USER11> Nah, they put a couple of precautions in to make sure you couldn't put the computer into standby accidentally. First, software has to set a bit to enable standby mode to be entered. For the astronauts, this was keying in VERB 37 ENTER 06 ENTER, which started P06, the pre-standby program. They then had to press and hold the button for a period of time between 1.28 and 2.56 seconds (exactly how long it takes depends on where the clock divider chain was at when they first press in the button). To bring it out of standby, you have to push the button in for a similar duration.</comment></comment>


Taking the Gaia Hypothesis Seriously - <AUTHOR> http://nautil.us/blog/its-time-to-take-the-gaia-hypothesis-seriously <comment><USER2> If Earth is a living organism, then we -- with our understanding of rocketry and biospherics -- would be its reproductive organs. Which means that the Gaia is just beginning to reach sexual maturity.

Organisms hitting puberty are notoriously moody, weird, and confused, alternating between bouts of idealism and self-destructiveness. Looking at the state of the world today, I'd say the Gaia hypothesis, at least in this respect, is holding up.

<comment><USER3> If a dog is a living organism, then fleas -- as they gain the ability to jump to other dogs -- would be it's reproductive organs. Which means that a dog with fleas jumping off is just beginning to reach sexual maturity.

Organisms hitting puberty are notoriously moody, weird, and confused, alternating between bouts of idealism and self-destructiveness. Looking at the state of dogs crawling with fleas, I'd say the flea-bitten-dog hypothesis, at least in this respect, is holding up.

<comment><USER2> Touché -- Provided that, as part of the jumping-off process, the fleas are figuring out how to grow new dogs. (Analogy: terraforming.)

<comment><USER3> Flea larvae eat the feces of adult fleas containing undigested blood harvested from the host. The adult flea's efforts have improved conditions for future generations to thrive.

</comment></comment></comment></comment><comment><USER4> This article is garbage in that it provides no scientific claims, nor is the scientific community taking this claim seriously. "Exhibiting homeostasis of some sort" is not a very high bar for evidence of life, otherwise every corporation is biologically alive (just please don't ask our Supreme Court).

Honestly this was strongly uploaded in part because the community here is weaker in biology -- a similarly garbage engineering article would never have seen the front page.

<comment><USER5> > "Exhibiting homeostasis of some sort" is not a very high bar for evidence of > life, otherwise every corporation is biologically alive

But that doesn't disqualify homeostasis as evidence of life, as corporations don't emerge except when living creatures create them.

Anyway, as I understood it, homeostasis is by definition a quality of living beings. Many things have a tendency towards an equilibrium, and of course that doesn't have to have anything to do with life.

Am I misunderstanding the claim?

<comment><USER4> A cough may be evidence for a tuberculosis infection, but it would be foolish to take anyone who is coughing and say "hey! we should test for TB". Homeostasis is a necessary but far from sufficient test for biological life.

</comment></comment></comment><comment><USER6> If there is no hypothesis, the idea cannot be proved or disproved. In that case the approach equates with faith and it cannot (scientifically) be taken seriously.

The article has a strong title, yet at least to me presents no arguments to support it's claim.

<comment><USER7> I just wanted to add a counter point that there exists lots of serious(ly- taken) science that cannot be based merely on hypotheses and on attempts of proving or disproving those.

Each practical research project starts by defining the context and bounds of research. It seems to me that he is proposing the adaptation of a more general strategy when defining the context. He does not make any claims other than that the limited context apparently present in some astrobiological, geological, etc., studies might cause you to miss crucial things. A very hard claim to prove or disprove a priori but this does not render the viewpoint useless.

<comment><USER6> I do not think the viewpoint itself is useless. I was just trying to point out that you cannot even attempt to prove it, since it is not a hypothesis. And the title of that article strongly suggests it is a prove to convince the reader.

</comment></comment></comment><comment><USER8> You can sort of explain how life is in beautiful symbiosis with the planet it resides on, but mostly this sort of perspectives cause harm. It's not science and encourages magical thinking.

\- "Gaia" makes it easy to think we are bound to Earth, and that nature protects Earth. There is no magical force that ties us to Earth, or a magical force that keeps it liveable, it's just a lucky coincidence and a complex set of systems. Maybe we stop global warming, and then a supervolcano erupts.

\- "Balance of ecosystems" creates the illusion that even the tiniest change in nature will hurt it. We spend more time worrying about tigers going extinct than an asteroid wiping out all life.

\- "Biodiversity" makes us think that more variety is always better. No universal law rules that a single super-crop couldn't keep us happy and fed in an environment without biodiversity. If we bring two of each species to a space station, it won't be because of economic reasons. Wild animals and plants do their best to destroy biodiversity constantly anyways.

\- "Nature knows" causes us to think that naturally occurring solutions are better. For example the genetically engineered Golden Rice, which could save millions of lives yearly, is a victim of this belief.

Similar flaws in thinking even cause most of the medical profession to only see medicine as a way to treat disease instead of improving wellbeing. Ask doctors and they'll tell you: all medicine has side-effects! If you gave doctors a pill that eliminates the need for sleep without side effects, they would only give it to people who are dying from insomnia.

</comment><comment><USER9>
      The truth is, despite its widespread moniker, Gaia is not
      really a hypothesis. It's a perspective

Yes it is. However, the article does not explain, at least to me, what it means to take that perspective 'seriously'. Everything that is described as something that can be seen from that perspective are also reported by people that would not describe themselves as looking from the Gaia perspective.

</comment><comment><USER10> I don't see how they relate biological homeostasis to climate. If homeostatic systems are functioning, like in blood sugar or oxygen control, their values are kept in a very tight range near the setpoint. In contrast, temperature has varied greatly over the millennia, as have ocean acidity and other variables, thereby showing Earth is very much not like an organism.

</comment><comment><USER11> Wow. I have read a bunch of really good articles on nautil.us that I discovered through HN. I have been thinking of getting the 2 year digital/print subscription to support the type of journalism I want to see. But um... this article is kinda making me reconsider.

</comment><comment><USER12> Why stop there when by the same reasoning we can claim entire solar systems and galaxies are living? At what point does that exercise become pointless?

<comment><USER13> Should I consider a glass of buffers a living thing?

[1] [https://chem.libretexts.org/Core/Physical_and_Theoretical_Ch...](https://chem.libretexts.org/Core/Physical_and_Theoretical_Chemistry/Acids_and_Bases/Buffers)

</comment></comment><comment><USER14> The Gaia "Hypothesis" makes no testable claims and can therefore be safely dismissed immediately.

</comment><comment><USER15> From the description, it doesn't seem like a hypothesis - more like a way of looking at things.

<comment><USER16> Exactly, the idea of mystical living Gaia does not have any scientific value.

The biosphere of Earth is indeed a huge and very complex system, but it's pointless to "animate" it, it lives according to its own laws that should be studied objectively, not according to some fuzzy analogies rooted in our bias and perceptions (an example of a similar bias: [https://wiki.lesswrong.com/wiki/Anthropomorphism](https://wiki.lesswrong.com/wiki/Anthropomorphism)).

</comment></comment><comment><USER17> Isn't reproduction one of the necessary conditions for life? How would that apply to a planet?

<comment><USER18> It created us didn't it?

<comment><USER19> For extremely wide definition of "created", yes. But that definition is not very useful, since it implies pretty much no useful causal links, just the mere fact "here's a bunch of matter, and here we are located in this place on it". Yep, but so what?</comment></comment></comment>