The electromechanical angle computer inside the B-52 bomber's star tracker

https://www.righto.com/2026/04/B-52-star-tracker-angle-computer.html

97•NelsonMinar•2h ago

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kens•2h ago

Author here if you have questions about this analog computer...

sebmellen•1h ago

Was the star tracked manually by the navigator (as in, did they have to manually “look for” and keep track of it)? Fascinating article, but I’m not grokking how it was used in practice.

kens•1h ago

The device has a spiral search mechanism to find the star. Then it locked onto the star and automatically tracked it. So this was unlike the Apollo star tracker where the astronaut has to manually aim at the star.

roger_•1h ago

Thanks, I was looking through the article for exactly that. Does it lock on to a configuration of stars?

Really curious how they did this mechanically.

kens•15m ago

I'll probably write another article on the star tracker itself. But I can give you a quick summary of the spiral search mechanism. It was electromechanical: a motor turned a resolver, a device with coils to generate sine and cosine from the shaft angle. This gives the X and Y deflections for a circle. These signals went through potentiometers that were also turned by the motor to produce constantly growing magnitudes, so you get a spiral. But you need to slow down the motor as you spiral outwards since you're covering a much larger linear region. So the motor also turns a stepping switch that progressively reduces its speed.

Once the system finds a star, a complicated feedback mechanism keeps it locked onto the star. There is a spinning slotted disk in front of the photomultiplier tube. If the star is off center, the output will peak when the slot lines up with the star. Thus there is an error signal with phase that indicates the direction to the star. This signal is demodulated to produce X and Y signals that change the aim to move towards the star.

palm-tree•3m ago

Am I right in thinking it didn't matter which star it locked onto, and it didn't need to know which star it was? Would it be a problem if it locked onto another celestial body (e.g. Venus)?

srean•1h ago

Reads like a labour of love. Thanks for sharing.

kens•1h ago

We couldn't find a wiring diagram so I had to trace out every wire.

t0mas88•1h ago

> The Angle Computer is one piece of the Astro Compass, a system that locked onto a star and produced a highly accurate heading (i.e., compass direction), accurate to a tenth of a degree.

I think it provides ground track information not just heading? Which is far more valuable for aircraft navigation, because the main issue is unpredictable wind drift.

kens•1h ago

No, it did not provide ground track. You could manually produce a ground track using the line of position technique described in the article.

TMWNN•1h ago

It's amazing, the things that can be done without what we would consider modern technology.

The 8-bit Guy recently released a video asking "What if everything still ran out vacuum tubes?" <https://www.youtube.com/watch?v=mEpnRM97ACQ>. Conclusion: A surprising amount of things we take for granted today would still be possible.

po1nt•1h ago

Everytime I read articles like that, I envy the engineers that worked in development of such tools. First microprocessors in jet fighters, electromechanical celestial navigation...

And here I am fighting gitlab pipelines.

SlightlyLeftPad•58m ago

I’m with you. The complexity yet simplicity of these mechanical devices is fascinating.

echelon•55m ago

Nothing is stopping us.

One life to experience the universe. Save up for a sabbatical. Find new engineering pastures.

It's always rose colored looking back. Not everybody got to work on this. Some people were storming the beaches...

therobots927•38m ago

And some people, specifically Vietnamese and Cambodian civilians, were on the receiving end of your fun little brain teaser.

And other people, like Henry Kissinger, drew random dots on a map to tell it where to drop the bombs. https://en.wikipedia.org/wiki/Operation_Menu

kingleopold•30m ago

another real fact: "Between 1964 and 1973, the United States conducted a covert "Secret War" in Laos, dropping over two million tons of ordnance during 580,000+ bombing missions, "

therobots927•21m ago

I’m about to read King Leopold’s Ghost. Great choice in username.

I must say it’s a little disappointing that things like “secret bombing campaigns” getting declassified don’t lead to much public response.

echelon•5m ago

> your fun little brain teaser.

I was upvoted before this dig. Now I'm negative.

To make it ABUNDANTLY CLEAR, I was referring to celestial navigation.

I guess we have to blame people who weren't alive at the time for wars we didn't participate in?

My wife is Vietnamese btw.

kens•31m ago

> First microprocessors in jet fighters

Don't get me started on that...

chiph•45m ago

> The Atro Tracker also has declination limits of +90° and -47° and a lower altitude limit of -6°. The latitude is limited to the range between -2° and +90°; the system automatically switches hemispheres so both the North and South latitudes are usable.

Why would the system need to have a much greater range of declination (celestial sphere) than latitude (Earth spheroid)? Because the Astro Tracker and Angle Computer could flip over to the Southern hemisphere (was this automatic or was there a switch?) having that much declination range seems unnecessary. Perhaps to allow for pitch of the aircraft in flight?

BTW, being able to operate in both the Northern & Southern hemispheres was an important capability for the B-52. Previous bombers (B-36 mostly) had the range but not the reliability or in-flight refueling for global reach.

Sadly, I didn't get the chance to look at the B-52 at the Museum of Flight when I was there. If you ever meet Charles Simonyi, please thank him for his support of the museum.

kens•31m ago

If you're flying in low latitudes, nearly half the stars that you want to use are going to have negative declination, so negative declinations are important. As for the hemisphere switching, this happened automatically.

chiph•9m ago

Once in the Southern Hemisphere, they'll need to pick a new set of stars. So their declination would still be expressed negatively?

Or is it that they considered the need to navigate below the lower fourth of Argentina a distant possibility?

black6•44m ago

> Each knob on the Master Control Panel has a different geometrical shape, allowing the user to distinguish the knobs by feel.

Auto manufacturers should take a clue here.

pests•42m ago

Read every word. i liked this detail in the footnotes:

> The Astro Compass needed to know approximately where in the sky to find the star, in order to point its sensor in the right direction. The direction didn't need to be exact because the Astro Compass performed a spiral search pattern to find the star. This search pattern covered ±4° in bearing and ±2.5° in altitude. In comparison, the Moon is 0.5° wide, so it's a fairly large target area. ↩

DarenWatson•24m ago

Honestly that footnote really stood out to me too! the spiral search detail makes the whole system feel a lot more alive than I expected like it’s actively hunting for the star rather than just pointing and hoping.

lb1lf•6m ago

In a very similar vein, Ars Technica did a very interesting story on the electromechanical targeting computers on WW2 battle ships a few years ago; the instructional videos embedded in the story are gold.

https://arstechnica.com/information-technology/2020/05/gears...

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