Retrotechtacular: A Closer Look At The VT Proximity Fuze

Here at Hackaday, our aim is to bring you only the freshest of hacks, which carries the burden of being Johnny-on-the-spot with our source material. So if something of obvious interest to our readers goes viral, we might just choose to skip covering it ourselves, figuring you all have probably seen it already. But, if we can dig a little deeper and bring extra value over and above what the viral content provides — well then that's another story.

That's pretty much the story behind the excellent video recently released by [Real Engineering] about "The Secret Weapon That Changed World War 2." It concerns the VT series of proximity fuzes — it's a legitimate alternate spelling of "fuse" if a somewhat archaic one — that were used for artillery shells and spin-stabilized rockets in World War II. The video gives an excellent overview of the development of the VT, which was used primarily in anti-aircraft artillery (AAA). The details about the development of the American VT fuze are excellent, although curiously there's no mention that British experiments with a radio proximity fuze were part of the goldmine of information brought to America at great risk by the Tizard mission in 1940. While there has been plenty of contention about the exact role the British work played, it's fair to say that it at least informed the development and fielding of the American VT fuze.

As for how the fuze worked, the video goes into a slightly less than satisfying degree of detail. That's not a complaint, as too much detail in that department would likely not appeal to a broad audience. But for the Hackaday set, more detail is better, so I set about learning what made the VT fuze tick. As a radio proximity fuze, the basic concept is obvious — generate a strong RF pulse and listen for any signals that may have been reflected by a nearby solid object, such as an airplane. But the devil's in the details, as they say, and the challenges involved with getting this to work under battlefield conditions were immense.

So what kind of circuits were used to make all this work? And how did circa-1940 electronics survive the 20,000-g loads experienced by the fuze as the artillery shell is fired? Although a lot of details aren't available, the 1946 Bureau of Ordinance VT fuze manual reveals a lot of details, including schematics. Unfortunately, there's no overall schematic given, so I had to stitch together one from the fragments given in the manual:

The heart of the VT fuze is a one-tube transmitter-receiver, shown in the upper left of the schematic. It's described in the manual as a "grounded-grid Hartley oscillator," although with notes that Navy versions of the VT used a modified Colpitts oscillator. The single triode acted as both oscillator to transmit the RF signal and a detector for the reflected signal. No component values are given in the manual, so it's hard to estimate the frequency the VTs used, but since these were mostly designed to detect airplanes, we’d guess a minimum wavelength of above the size of a plane, perhaps 5 to 10 meters, or between 28 to 60 MHz or so. This seems like a reasonable frequency for the circuits shown, and for the antenna, which is described as a "standard dipole."

At the end of the day, though, the actual transmitter frequency isn't of much concern, because detection is based on the difference in frequency between the transmitted frequency and the reflected signal, and the beat frequency generated thanks to the fact that the range between the shell and the target is rapidly changing, The video does a great job of explaining this, and the manual details how the amplifier circuit (top center in the schematic above) accomplishes this. I’ve never had much luck understanding vacuum tube circuits, so I’ll leave it to better minds than mine to detail the theory here, but suffice it to say that proper selection of components in the amplifier circuit, particularly the screen and plate bypass capacitors, creates a filter whose center frequency corresponds to that expected when the target is within the effective blast radius of the shell. When that happens, the gas in the thyratron in the firing circuit (lower right) is ionized, allowing the fully charged firing capacitor to discharge through a squib and detonate the shell.

For naval use, a wave-suppression circuit (upper right) was added. "Wave" referred to ocean waves, not radio waves, which could reflect the transmitted signal and cause early detonation of a shell when fired at a low angle of elevation, as when an enemy torpedo bomber approached. The WSF essentially decreases the sensitivity of the amplifier in the presence of a steady signal, which can occur as the shell travels over the waves.

As with most military inventions, the VT fuze was fiendishly clever. That 1940s technology was able to come up with a circuit that was small enough to fit on an artillery shell, durable enough to survive the brief trip to the target, and cheap enough to be manufactured in bulk — the cost per unit had dropped as low as $18 by 1945 — is a testament to the ingenuity of the engineers involved.