Here's a question for all you old-timers.
I've been reading a few articles about the WWII vintage proximity (VT) fuse. Some describe it as using the change in radiation resistance caused by the proximity of a conductive object. Others say it worked on based on the Doppler effect.
My vote is for radiation resistance, but I've only (briefly) examined a poorly documented schematic of one version.
So, which is it?
-- Paul Hovnanian mailto:Paul@Hovnanian.com ------------------------------------------------------------------ A noble spirit embiggens the smallest man.
This topic came up on ABSE a few years ago when I scrounged a set of schematics for proximity fuze.
AIUI the most common type uses constructive interference from a wave reflected back by the target.
Usually this crosses a threshold to trigger a subminature thyratron.
My understanding is that it is the former, at least for the commonly used fuses. They basically built a directional metal detector that depended on the rotation of the missile to provide and AC component to the detection, then they looked for that wobble in the oscillator bias.
Certainly the detailed descriptions that I've read (there was one in the IEEE Spectrum several years ago) were consistent with the "metal detector" idea, and had schematics that matched.
There probably _were_ fuses that were designed to use Doppler, but I think the ones that actually worked in WWII didn't. Instead, I suspect that the "Doppler" effect was being cited by journalists who either didn't know what the heck the real "Doppler effect" is (and think that _any_ change in the signal due to motion must be "Doppler"), or were confused by later, more sophisticated fuses, or both.
-- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
"Ian Field" wrote in news:1qERq.1406$ snipped-for-privacy@newsfe03.ams:
Wiki has a nice article on VT fuses,but Wiki is off-line today.
-- Jim Yanik jyanik at localnet dot com
Disable javascript or press the escape key as soon as the page loads, to stop it from redirecting.
-- Reply in group, but if emailing add one more zero, and remove the last word.
"Paul Hovnanian P.E."
** A variety of VT fuses were developed during WW2 by the allies for different jobs.The first and main types were for fuses in AA shells and these relied on the Doppler effect.
The basic idea stems from the fact that an AA shell that misses its target has *zero relative speed* with that target at the point of closest approach. So when the Doppler shift frequency drops to near zero it fires.
Fuses fitted to mortar shells detect the proximity of the ground while in a steady vertical descent so variation Doppler shift cannot be used.
... Phil
The information is still there, if you look at the HTML source. The ESC = key=20 did not work for me. But I copy/pasted the body of the HTML into a blank =
document, and added the following so the images would show up:
Works a charm.
Paul=20
The few schematics that I've seen don't look to be that smart. The fire threshold seems to be some audio amplitude within some bandpass. That could get close to what you describe: large amplitude at low frequency is damned close to the doppler minimum. Those guys got a lot of functionality out of three tubes.
They also had to account for the firing delays, a few milliseconds, so they started the shot a little before the closest approach to the target.
That was probably just the amplitude of the doppler signal. The frequency would be pretty fixed on any given shot, just proportional to the vertical velocity of the shell.
John
"Paul Hovnanian P.E."
** Early proximity fuses for AA shells responded to the amplitude and frequency of the beats generated by the reflected wave summing with the transmitted one.The beat frequency is in the order of 100Hz to 500Hz as the AA shell is approaching its target and the amplitude steadily gets stronger - then as it nears the point of closest approach the frequency suddenly plummets to zero, rising again afterwards.
The detector in the fuse was made most sensitive to low audio frequencies so that triggering occurred at about 20 to 30 Hz allowing enough time for the shell to explode and avoiding late firing after the point of closest approach.
... Phil
Not really. The antenna dipole was symmetrical with the axis of the shell (between the nose cap and shell body). So it wasn't directional in a sense that it could sweep an off axis target to modulate the return.
Versions of VT fuses were deployed in non rotating projectiles (mortars, missiles and bombs).
At one point, some people thought its signal was being modulated by airplane propellers. But that was put to rest when VT fuses proved to work very well for detonating shells at preset altitudes above the ground. Its an interesting subject. Until around 1940, the two primary areas of investigation were photoelectric fuses and remotely (radio) triggered detonators. Around 1940, the emphasis was shifted to radio proximity fuses.
All of the from 'The Deadly Fuze' by Ralph Baldwin
I just finished reading 'Hedy's Folly' by Rhodes. There's a bit about Hedy Lamarr and George Antheil submitting a proposal for a proximity fused antiaircraft shell to the National Inventor's Council (the wartime group tasked with promoting R&D). That resulted in a temporary falling out between Hedy and George when he came back from a meeting in 1940 empty-handed. Hedy suspected him of trying to cut her out of the proceeds of that potential invention.
But given the secrecy of the VT fuse development at the time, that's exactly the reaction an outsider (a piano player/composer) would have gotten at the time. 'Run along now, buddy. That's a stupid idea.'
-- Paul Hovnanian mailto:Paul@Hovnanian.com ------------------------------------------------------------------ When I was in high school, I remember boys and girls slept together all the time. We called it algebra class. -- Jay Leno
"John Larkin" "Phil Allison"
** The simple, low pass filter in the audio amp stage converts the sudden drop in frequency to a sudden increase in amplitude - up to a level sufficient to fire the thyratron.When the AA shell passes far from the target, trigging ocurrs at a low audio frequency where the gain is maximum and when is passes closer or is headed for a direct impact at proportionally higher frequencies where the gain is less.
In each case, it produces close to the optimum result.
... Phil
There was a great 1-hour documentary made 15 years ago. I actually set a watch on this web page to alert me if it is repeated:
According to this it used only 2 tubes, and triggered on the strength of reflection. Doppler RADAR with 2 tubes would be a feat. Any Doppler RADAR that early in the history of RADAR would be a feat.
-- Reply in group, but if emailing add one more zero, and remove the last word.
SNIP
LOL. Not the ones I've seen recently.....
100 feet? With a 6" coil?
John
yes and no :)
"Ian Field" wrote in news:9LZRq.478$ snipped-for-privacy@newsfe09.ams:
don't forget that the shell had a battery or batteries in there to power the tubes!
-- Jim Yanik jyanik at localnet dot com
From what I've read so far, it seems they had dry-charged lead acid batteries and a glass ampule of acid that shattered when the shell was fired and soaked the plates.
There was also the generator type with a small turbine ring on the nose cone.
"Ian Field" wrote in news:eGfSq.67$ snipped-for-privacy@newsfe01.ams:
lead-acid batteries doesn't seem like enough voltage to run vacuum tubes.
2v per cell,and even the tiny VTs used probably needed 30V or more for the plate V. Hmm,ISTR vaguely that TEK used 12V on Nuvistor plates used in 400 series O'scopes,not 100% sure though.maybe they used the L-A cells for filaments,and the turbine for the plate V.
-- Jim Yanik jyanik at localnet dot com
For the plate draw of 3 subminature tubes to last - at most not much more than a minute, I'd have thought they could probably cut mass produced plates from lead foil.
All they'd have to do is chemically process each side of the foil strip to make one side +ve and the other -ve. Chop the foil into small disks and stack those with absorbent sheets of separator and you can make a tiny voltaic pile with a pretty decent terminal voltage.
There were types with primary cell battery, but they deteriorated rapidly in hot humid theaters of operations.
"John S"
** That giant clang you heard was the penny dropping ....Just to round out the explanation, consider the actual speeds involved.
AA shells leave the barrel at up to 1000m/s, giving a Doppler shift of
3.3ppm compared to a stationary object.VT fuses oscillated at VHF frequencies around 175MHz - making the shift frequency 578Hz.
The closing speed of the same shell to an aircraft flying at 25,000 feet would be more like 100m/s - making the shift frequency 58 Hz.
As the shell whizzes past, relative speed momentarily drops to zero and so does the shift frequency.
.... Phil
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