user: The invention of proximity fuse technology in anti-air artillery shells was one of America's best kept secrets during World War II, and arguably one of the most impactful advantages of the United States. Explain what a proximity fuse is, why it was so desirable compared to existing techniques, and why it was so challenging to achieve.

assistant: A proximity fuze is a fuze that detonates an explosive device automatically when the distance to the target becomes smaller than a predetermined value. The VT fuse developed by the US has a miniature radio transmitter and receiver with an amplifier inside. The transmitter sent out a continuous radio signal that bounced back from the moving target, and the receiver detected the return signal, which increased in intensity the closer the shell approached its target. Once the signal level passed a certain threshold, it activated the thyratron trigger, releasing an electrical charge stored in a capacitor. That charge tripped an electrical detonator that in turn set off the main explosive.
Before its invention, it was estimated that it took as many as 20000 rounds to shoot down a plane according to the experiences of the UK at the start of The Blitz, and the lowest statistic is also up to around 1200. Meanwhile, the Empire of Japan began to apply kamikaze which was more accurate than conventional attacks. Therefore an increase in the accuracy of anti-aircraft weapons was quite necessary.
The fuzes for use with anti-aircraft artillery need to work under extreme conditions. The challenges include:
-To create an optical, magnetic or electronic feedback mechanism that signaled “detonate” to an artillery shell moving at an initial speed of 2,600 feet per second at the moment it neared an enemy aircraft traveling at 300 miles per hour.
-To fit a radarlike radio (transmitter and receiver) device into a 5-inch projectile only 20¾ inches long and already packed with 7 pounds of high explosive.
-To miniaturize such an electronic device to fit the volume of a pint milk bottle in an era when no one had imagined transistors or integrated circuits.
-To develop a battery of sufficient power and durability to operate the fuze and function in temperatures ranging from 100 degrees to minus 50 degrees.
-To develop a fuze able to withstand the impact of being fired from a 5-inch gun—a shock of acceleration equal to 20,000 times the force of gravity—as well as the spinning projectile’s centrifugal force of 500 revolutions per second.
-To incorporate safety features to enable safe handling and prevent accidental detonation.
-To craft a fuze adaptable for use in a range of U.S. and British weapons.
-To engineer a design for rapid, high-quality mass production—well into millions of units, with components sourced from a multitude of American factories.

user: You said prior to the invention of the proximity fuze, it took 1200-20000 rounds to down an aircraft. What are the statistics for anti-air accuracy after it was implemented?

assistant: The proximity fuze was a significant improvement in anti-aircraft accuracy during World War II. According to the US Navy Bureau of Ordnance, the proximity fuze increased the hit probability against aircraft by a factor of 10 compared to the earlier contact fuzes. The proximity fuze allowed anti-aircraft guns to detonate their shells at a set distance from the target, increasing the likelihood of hitting and damaging the aircraft.

Specific statistics on the effectiveness of the proximity fuze vary depending on the source and the circumstances of each engagement, but generally, it is believed that the proximity fuze greatly improved the effectiveness of anti-aircraft fire during World War II. In some cases, it is reported that the proximity fuze reduced the number of rounds required to down an aircraft to as few as 5-10 rounds, although this figure could vary widely depending on the type of aircraft and the tactical situation.