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Comment by kens | Hacker News Reader

Author here if anyone has questions on Apollo communication...

Do you know anything about the heart rate telemetry they did?

I'm blown away by the description of the helix that slowed down light by 90% so it could interact with the electron beam. Who figured that out? Brilliant.

The biomedical telemetry was a bit complicated (as was everything in Apollo). When astronauts were outside, there were 7 channels of analog telemetry transmitted by FM to the Lunar Module. The Lunar Module converted these channels to an AM signal which was transmitted over VHF to the Command Module. The Command Module converted these to PCM data, which was then sent to Earth over the unified S-band system.

For details, see "Unified S-Band Telecommunications Techniques for Apollo" page 11. https://core.ac.uk/reader/80678138

> Who figured that out? [traveling-wave tube amplifier]

It was Andrew Haeff at RCA in 1933: https://patents.google.com/patent/US2064469

Haeff was something of a prodigy, looking at his other patents. E.g., a 3D volumetric laser scanner from 1964: https://patents.google.com/patent/US3636250A

Dude didn't let any grass grow under his feet, tech-wise.

Hmm, I wonder if that's where late 60s Alan Watts got this idea:

"with the aid of laser beams, we can translate a physical object, let's take a complicated one, let's take a dandelion flower gone to seed a dandelion clock. You take a dandelion clock about so big, it can be turned into a formula passed through channels, enlarged to any size, say this big, and with laser beams, cut in solid plastic in a matter of moments. You can get this reproduction of a three-dimensional object. But the transition between the two was handled simply in terms of pulses."

"Rudolf Kompfner in 1942 —notably after his public announcement in 1946— when he was secretly working on microwave vacuum tubes for the British Admiralty at the University of Birmingham during World War II. But the history of this device is more complex because the traveling-wave tube was, consecutively, discovered thrice independently"

https://hal.archives-ouvertes.fr/hal-01754885/document

Always enjoy reading your blog. Very informative and entertaining. Thank you so much!

That epoxy-like plastic the components, posts and wires on the boards in https://static.righto.com/images/sband-twt/transformers.jpg are covered in - looks like it might be a way to perform vibration-proofing? Seems like it would be very robust.

Know anything more about this method?

The Juno II rocket launch in 1959 spectacularly failed when the rocket did a U-turn after launch. The cause was two diodes touching due to vibration. After that, rocket components were often potted in plastic to prevent similar problems.

Video of launch: https://www.youtube.com/watch?v=885as-VE50g&t=1446s Discussion of diodes: https://youtu.be/885as-VE50g?t=1688

Same for plenty of vehicles, early ECUs were often potted and the ones you see today sometimes still have a resin coating on the component side to harden the device against vibration.

How does one knew at that time that is the issue … must be a great detective work!

62 years ago. Just letting that sink in. (Thanks for the video links!)

My dad said he referred to those potted assemblies as “integrated circuits” when working on TV bandwidth compression for the phone company in early 1960s australia.

More generally, curious about what your plans are for a 400Hz power supply. There are so many interesting avionics, aerospace, etc. equipment which would be interesting to experiment with. Short of spinning a generator, have you found a VFD or something else OTS that can supply suitable power? Tnx!

CuriousMarc got a VFD box that he thinks will power this, as well as some gyroscopes. I don't know the particular model.

Love your articles Ken! Interesting content, straightforward but not dry writing, helpful diagrams and images and footnotes.

Do you have any I-V plots of the wave tube?

I'm not sure that an I-V plot makes sense for a traveling-wave tube. It's not like a regular triode tube where you're controlling via a grid. Instead, you put in an RF signal and the signal comes out amplified.

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