Talking with the Moon: Inside Apollo's premodulation processor | Hacker News Reader

Talking with the Moon: Inside Apollo's premodulation processor

righto.com

104 pointspicture25 comments

Author here if anyone has questions about obscure Apollo hardware...

Do you have a guess?

>The unusual feature of this module is the encapsulated module in the upper left. This module appears to contain three transistors and five capacitors. It's unclear why these components are encased in plastic. The block diagram for this module doesn't show any special circuitry that would motivate encapsulation. I hope to reverse-engineer this module to figure this out.

temperature consistency across all the parts, probably. It's the reason things like discrete current sources use dual transistor packages: important transistor parameters track strongly with temperature, and if you want them to match, the temperatures need to be the same.

I'd also say thermal coupling to ensure these components drift with some common average temperature. One thing where this would be done is e.g. an exponential converter or an antilog-amplifier, although there one would need only two transistors (one PNP and one NPN).

Fascinating as usual Ken, thank you. I am still absorbing the tech, but one thing that struck me in the very detailed photos (thanks for that) it does not appear that the components, resistors, capacitors etc. were soldered to the binding posts, it looks almost as if they were welded or fused?

I am wondering if that was for reliability due to vibrations of launch ?

oops never mind, i see your comment on one photo.

Yes, the components were spot-welded. This was common in the Apollo electronics. I think it was more reliable than soldering.

Right, it had to survive the vibration of the mighty Saturn V. But even so, that construction, the insulating tubes and hot glue, I am not sure I would risk my life on that!

I am sure it (the construction) was extensively tested.

It's unlikely the astronauts would trust their life to this processor alone. There would be at least one backup, and probably a plan in the event of total communication failure. System design!

Could be but just looking at the block diag Ken showed, that thing looked like a really critical component that all the comm ran through, although it looks like there may have been a way to bypass it for voice only comm in an emergency.

But regardless, I still think those guys were amazingly brave, they knew the thing they were riding in was pretty cutting edge and there were big risks. (Apollo 13 of course is an example).

I was interested to see how communication between and from the individual astronauts was handled via VHF and then relayed by the lunar lander.

Did the lunar rover used on Apollo 15-17 have the same equipment as the lunar lander, to transmit back to Earth?

The Lunar Communications Relay Unit (LCRU) on the rover provided S-band communication to Earth (MSFN) for voice, telemetry and TV.

What's your favorite bit of obscure Apollo hardware?

I like the traveling wave tube amplifier because it's such a wild way to amplify a signal. The digital ranging using a pseudorandom sequence is also pretty cool.

That article you did on the digital ranging was fantastic in part because once you read it, you think "Yea, that is a great way to do that, why didn't I think of that!". It was both novel and easy to appreciate.

Thank you for all of your articles, they make hardware engineering understandable for someone like me who is a software engineer and burns himself every time he touches a soldering iron :)

> The components are liberally covered in what looks like hot glue. I suspect that the hot glue was only used in equipment for ground testing, while modules for spaceflight were fully encapsulated to prevent short circuits.

This is making me wonder, where in the world are you getting this stuff from?

A collector loaned us the premodulation processor. I think he got it from RR Auction, which regularly has auctions with lots of amazing space hardware.

A great write-up, but for me the cherry on top is footnote #7 that describes exactly what "SCE to AUX" actually did and how it managed to reset the telemetry when Apollo 12 was struck by lightning. Very cool to finally understand what happened.

Was this featured on Curious Marc?

He's done a bunch of videos on the S-band transponder that is connected to the premodulation processor, but hasn't covered the premodulation processor yet.

This is really cool! I wonder if nowadays those collections of passives would be replaced not with a PCB, but with a single IC.

Yes, I expect you could put the whole premodulation processor on a single IC nowadays. Even in the late 1960s, an op amp IC could have replaced the amplifiers built from discrete components.

The amazing thing is now days an entire WIFI AP, RF section and all is on a single chip. https://en.wikipedia.org/wiki/ESP32

I was 14 when Neil Armstrong set foot on the Moon, it was one of the inspirations for me to become and engineer, and no doubt had that effect on generations.

I love seeing this old Apollo gear. I know the Apollo AGC has been posted here a million times but this RF stuff was really cool to get a look at!

How rad-hardened would this single-chip solution be compared to something made with discreet components?

I was wondering that as well: what a drop-in replacement would look like from 2022. I would like to think it would be the size of an Altoid tin.

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