Sunday, May 25, 2008

The Fuze that Saved A Battle

In my last post, I mentioned the VT fuze. Before I get to the VT fuze and its importance, a short discussion of fuses is in order.

The earliest cannon projectiles were round shot. They were solid balls, first of stone, then of iron. Somewhere along the way, some enterprising person wondered if there was a way to combine the heavy throwing capability of a cannon with the blast effects of a hand grenade. Hand grenades, back then, were little more than containers packed with gunpowder. The grenadier would light a fuze on the grenade and throw it towards the enemy.

Obviously, stuffing a projectile with a lit fuze into a cannon barrel that has gunpowder at the other end is not a good idea. The early fuzes were little more than just that, the fuze would be lit by the combustion of the propellant charge, with the time to detonation determined by the length of the fuze. This had its drawbacks, so the impact fuze was developed. Depending on whether the shell was fuzed at the nose or at the base and how impact-resistant the fuze was, a projectile could be fuzed so that it detonated on impact or afterwards, to allow for penetration. Clockwork fuzes (also known as "mechanical timed fuzes") were also used to set projectiles to go off prior to impact, in order to spray shrapnel over a wide area. (In a throwback term to earlier days, setting a MT fuze is called "cutting the fuse").

This was about the state of fuze technology during the First World War. Still, fuzing was unreliable, I have seen estimates that a third of the shells fired on the Western Front did not detonate. As a result, the "Iron Harvest" continues to this day. Old shells, even from as far back as the Civil War, can still kill, as their fuzes have deteriorated and can be very unstable.

Timed fuzes were used against aircraft; the gunners would set the timers to go off at a set altitude. If the setting was wrong, the fuze would detonate the shell too high or too low. The Germans addressed this problem by having an airplane fly parallel to the Allied bomber formations and radio back the altitudes.

Timed fuzes were fine against level bombing attacks carried out at medium and high altitude, but they were useless against dive bombers. For a mechanical timed fuze to work. the gunner would have to estimate at what altitude the dive bomber would be when the shell reached the dive bomber, a near impossible task.

The answer was what was known as a "variable time fuze" or "VT fuze," which was a code name that did not reveal the true nature of the fuze, as every nation with antiaircraft artillery had timed fuzes. The VT fuze was a proximity fuze.

The VT fuze was a miniaturized radar set. That may sound like not so much of a big deal, but this was in the days before transistors had been invented. It was a huge advance to that point to have a radar set that was small enough to be installed in an airplane the size of a bomber. What the Navy's scientists and engineers had to do was develop a vacuum-tube range-only radar set that could not only fit in the fuze of a 5" shell, it would survive the massive G-force of being shot out of a cannon. And then, having solved all of those problems, they had to mass-produce them.

The only project that was deemed to be more critical than the development and production of the VT fuze was the development of the atomic bomb (a version of the VT fuze was used in the atomic bomb).

VT fuzes were used to shoot down Kamikazes and V-1 missiles. Without the VT fuze, the American death toll from Kamikazes would have been far higher. As it was, during the Battle of Okinawa, 34 ships were sunk, most by Kamikazes. The Navy had 5,000 sailors killed (the Army and Marines lost 8,000).

Whether the Kamikazes would have been able to cripple the Okinawa and Iwo Jima landing forces if the Navy did not have the VT fuze is a debate best left to the alternate history folks. But there is no doubt that VT fuzes saved thousands of American lives.


physioprof said...

Cool! Thanks for the details! Was the proximity at which the VT fuse detonates preset in the factory, or could it be set by the artilleryman in the field?

The Earth Bound Misfit said...

I think it could be set, but I don't remember for sure.

Jimh. said...

I seem to remember reading an article that said a version was used in Europe by our artillery guys and that the shell would burst at an altitude that decimated ground troops...but I don't recall which magazine it was I read this in.

Steve Sky said...

Here is a book about the history of the VT fuse, and it's quite an interesting read.

From the book, from memory, fuses were set to explode 30' above the ground, and 70' from an airplane. The distance was fixed by a resistor soldered into the circuit, and couldn't be changed.