Proximity Fuses

When I was at university the sum total that they taught us about valves was treat them like a FET move the decimal point one place to the right for voltages and add a pilot light. Bit of a pity really.

The concept of a proximity fuse has had me baffled for some time. I understand that it worked on the concept of looking for the echo return of a radio signal but how did they actually do it and how did they get the electronics to survive being fired from a gun?

I was quite surprised when I studied the circuit diagram, I always wondered how the actual detonation took place but apparently a small electric current from a thyratron was all that was required, the use of two audio frequency amplifier stages surprised me but the use of a thyratron was predictable.

I guessed the operating frequency would be about 200MHz this was no surprise neither the use of centrifugally operated switches to prevent premature operations.

I was familiar with the Hivac mini tubes as I sometimes had to service deaf aids at the time.

I rather thought that the operation would be rather on the lines of a superegenerative receiver that was popular at this time but I think if I had been given the task of designing one in 1940 I would have come up with something similar, it is very surprising that Siemens or Nippon electric did not build them too.

Circuit diagrams from 1940 may seem very strange to modern eyes, if anyone would like any assistance to understand them please feel free to e.mail me.

I wonder whether or not using a thumping great transmitter on the correct frequency could get them to go off prematurely. After all if they are looking for a return echo at around 200 MHz so hitting them with 100 W at the right frequency should activate the circuit at a considerable distance. The concept of them finding your location from the radio transmitter wouldn't' be a problem as if they are firing AAA at you they already know where you are.

Yes indeed such a transmitter was built that was mechanically swept over a 20MHz range and tested by the fuse manufactures with complete success.

it is strange that this device was kept secret from Germans and Japanese who neither copied them or implemented these simple counter measures.

The carrying of such a transmitter by aircraft that are being targeted would have not endangered them as no homing missiles had been developed at this time

I find it fascinating to look back at the technological developments that came out of WWII. There are examples of thinking completely out of the box like the V2 etc yet at the same time restrictive thinking kept something as obvious as a radio transmitter to trigger a proximity fuse.

By the way do you know how they made the circuitry robust enough to be able to tolerate being shot out of a gun? That's some really serious acceleration it needs to tolerate.

One thing with valve technology is that people would make the circuit as simple as possible. There was not of this sticking half a billion semiconductors in a microprocessor just to tune to a particular frequency. I makes we wonder some time when people over complicate things by trying to convert everything to digital then back to analog in control circuits.

I've mentioned this before so if you have read it skip this bit. When I was a cadet engineer the engineer I worked with needed to design an interface for a computer system and lift control panel. The specifications were a clean contact closure in the lift panel, one for each floor, needed to be turned into a current sink binary coded decimal input for the computer. He designed this incredibly complex box using CMOS ICs that worked for approximately 30 seconds before the induced voltage on the interface cabling blew the crap out of it. You should have seen the colour of his face when I replaced his box with 4 bits of wire. You see the lift could only stop on the 1^st, 2^nd, 4^th and 8^th, or in other words binary, floors so no encoding was necessary at all. I got my own desk, phone, business cards and a new supervising engineer out of it.

I have not read your story of the lift position encoder before but it rings very true.

It reminds me of Harrison and his ship borne time keepers, he added layer upon layer of complexity to his weird devices until the job was handed over to professional clock makers who Incorporated his ideas into a compact instrument of superior performance.

I think the mini valves made for deaf aids were found very G resistant as long as they were not powered up until after the initial acceleration although of course development was done to improve them, batteries resistances and condensers as they were called then gave a lot of trouble, unless you tried to build electronics's 60 years ago you would not realise how poor these component's were!.

I think components were potted in some kind of resin, this was long before the days of printed circuits.

I suppose I should be thankful for the development of proximity fuses I lived in Maidstone during the war which was right in the path of the V1 missiles heading for London, initially these were attacked by fighter aircraft 'To bring them down harmlessly in the country side' which often included my home town.

Later AA guns with proximity fused shells were deployed near the coast and these were able to intercept most of them.

Here is an excellent picture of an actual fuse, I note the electronics do not seem to be encapsulated

Towards the end of the war the Germans captured 20,000 fuses so 200 jammers were hurriedly built.

They did not use or copy them misguidedly believing they could not withstand the acceleration involved in firing them.

http://en.wikipedia.org/wiki/Image:MK53_fuze.jpg

They look great design. I thought bombs and just full of explosives and to blow on impact with other objects. This proximity fuse concept was not at all in the mind. Was there a trigger mechanism for this mechanism also and then how all that worked. Do you have complete sequence of happening inside this explosive device? I am very much scared of even fire crackers.

Trying to hit an aircraft with a shell fired from a gun is pretty much an impossibility so a contact fuse is next to useless. Even more important is what happens to the shell after it has missed. You don't want thousands of unexploded shells falling form the sky at supersonic speeds and exploding when they come in contact whit whatever is unlucky enough to get in the way. You could easily end up causing more damage than the enemy aircraft.

Until the proximity fuses came about they used altitude fuses that were set to detonate the shell when it reached a set altitude and throw shrapnel in all directions. With shells like this all you needed to do was set them to explode at the same altitude as the aircraft and then fire in their general direction. Given that there were up to 1,000 aircraft in the sky, once you got the altitude right you stood a good chance of hitting one of them with shrapnel and damaging the aircraft or injuring the crew.

There was also a time delay fuses but these were not as effective because the altitude a shell reaches is controlled by not only the time in flight but also the angle of elevation so they tend to go off in a spherical pattern rather than a horizontal plane.

The good thing about the proximity fuses was there was no messing about setting the fuses before they needed to be fired. All you needed to do was point the gun in the rough direction of the aircraft and fire it. However, I believe you could adjust the sensitivity of them so they would go off at different ranges. They ware also not very effective against aircraft like the De Havilland Mosquito which were primarily wood but the Germans did not have proximity fuses in any thing like the numbers the allied forces had.

Like all forms of antiaircraft ammunition proximity fuses also needed some sort of mechanism that would detonate the shell if it missed the target completely.

I can understand it much better now. I have seen those anti air craft gun firing fire balls at rapid rate. Any stupid can see them and fly away. I think they were ineffective. Only heat missiles were some what effective. In 1971 India Pakistan small war, I was near the firing range when Pakistani Aircraft attacked zone near Tajmahal and Indian air force used those guns and finally heat missile/s to blow the aircraft. I could see the missile reaching the aircraft and then there was this white smoke cloud where the fighter aircraft was. I think the Pilot came looking at Tajmahal as it must have been visible from a distance.