Azon Bomb

The AZON bomb was perhaps the world's first smart bomb.  AZON stood for Azimuth Only, which I'll explain. It was officially named the VB-1, the VB stood for Vertical Bomb. The whole idea of a smart bomb is that primitive explosives aren't very clever. You drop them you throw them... they detonate wherever they land when the impact, fuse or timer triggers the explosion. At some point some military minds began thinking about steering them. This is the fundamental idea behind a "smart" bomb. The  first steering mechanism of course was radio control. This was used mostly in 1944 and 1945. More here.

On this topic, I must give the hat tip to Tesla for inventing radio control in 1898. All later designs are derived from his ideas. This particular design was developed by by Major Henry J. Rand and Thomas J. O'Donnell. the word "design" there sounds like they designed a whole bomb, not so. The Azon Bomb was just a special tail fin unitbolted to a 1,000-pound GP bomb (General Purpose). While that's true, it understates what a big step this was.

With the addition of this unit, the bomb’s trajectory could be adjusted in flight via radio signals which moved the fins. But this didn't allow total control through three dimensional space. It only adjusted the yaw axis. So let's explain that. An airplane manners in 3 axis, yaw, pitch and roll. Roll is controlled by the flaps on the wings and literally could roll the airplane like a barrel. So imagine that rotation as one axis. Pitch is effectively up and down. The pilot pushes the stick forward, you dive, he pulls back you climb. It's controlled by the horizontal flaps (ailerons) on the tail. Yaw is the one you're trying to figure out. Yaw is only left/right movement controlled by the single vertical flap on the tail fin.  Here the Axon bomb is falling (a purely vertical movement) and the axon unit steers with fin adjustments. To aid steering a candela  flare was attached to the tail so a bombardier could see the bomb better for remote steering.

Four antennas wrapped around the fins and signal was tuned by a single on board radio. This was powered by a small battery with about 3 minutes of life. But three minutes was plenty. The transmitter ont eh aircraft operated at 25 watts and controlled 3 signals.  The first signal at 30-40 hertz, triggered the flare. Then two other signals controlled the flaps: one at 475 hertz for left deflection, one on 3,000 hertz for right deflection. these are extremely low frequencies.

Prior to this given good weather hitting large targets like factories was doable, but small targets like railroads, roads and bridges was difficult. But the axon could be steered onto much smaller targets.

Radio Proximity Fuses

In the late 1930s the proximity fuse was developed in the UK.  The Germans were also working in the same arena and had developed some models but the Brits beat them to it. They had been working on a variety of developments to increase what the military called "air defense efficiency."

Into this technological melee they dropped William Alan Stewart Butement. In 1931 he and P. E. Pollard, invented a shipboard radio device for the detection of ships. Today we call it radar. It operated at 600 MHz and using pulse modulation was able to detect ships100 yards away. The Navy wasn't big on it, but others saw potential. By 1938 Butement had ramped up his invention into large scale devices that could be used from land to protect the coast.

In 1939 Butement attempted to improve anti-aircraft guns. It's hard to hit a moving target. He had a better idea.  His plan was a very compact  Radio Direction Finder (RDF) unit placed on the projectile. It would then trigger the detonation when near the target. This was not simple. The RDF had to be small and also durabel enough to be fired out of a cannon. This circuit included glass vacuum tubes. Somehow just over a year the United States was manufacturing projectiles with a proximity fuse. This was also called a VT (variable-time) fuse.

This was not an induction trigger. It wasn't sensing a ferrous body. It was detecting the reflection of radio signals. A later improvement was The transmitter which used the shell body as an antenna and sent out a continuous wave at around 200 MHz. As the shell approaches a reflecting object, (a ship, a plane, the ground) the reflected signal created interference. That pattern changes with proximity. As the objects get closers the signal moves in and out of resonance as the reflected signal length changes; half a wave is resonant, so is a quarter, an eighth etc.

This causes a small oscillation of the radiated power and consequently the oscillator supply current of about 200–800 Hz, the Doppler frequency. This signal is sent through a band pass filter, amplified, and triggers the detonation when it exceeds a given amplitude. Later in life, Butement said that he considered the proximity fuse as his most significant accomplishment.