The Proximity Fuse, Fire Control, and a bit more on Naval Guns

We’ve discussed briefly the challenges of anti-aircraft gunnery facing designers of naval guns (and to some extent, the designers of land based anti-aircraft guns, as the challenge is very similar).

The use of gunfire directors was originally intended to increase the effective range of naval gunnery against moving surface targets. From the Spanish American War of 1898 to the First World War, naval gunnery went from simple open sights that were no more advanced than in Nelson’s day at Trafalgar, to complex analog mechanical computers that could predict the future position of an enemy ship, and compensate for factors such as the firing ship’s own pitch, roll, speed and course, as well as meteorological factors such as temperature and air pressure, gun barrel and powder temperature, and even the coreolis effect of the earth’s rotation. Effective gunfire range went from about 1 mile to as much as 20 miles in less than 20 years.

But the use of airplanes in war at sea presented added challenges to gunnery experts. At the shortest ranges, machine guns with their high rates of fire would suffice, but longer range gunnery against airplanes needed complex fire control to have any hope of even coming close to enemy aircraft.

Fire control systems would have to predict the future location of a target not just in two dimensions, but in three. Further, the much greater speed of airplanes over ships meant that fire control computers had to have much higher “processor” speeds than surface fire directors. In a surface gunfire duel, corrections to the inputs for the computer were generally limited to either range or bearing errors. Changing only one or two inputs meant the “dwell time” for the director to achieve a solution was relatively short- generally shorter than the time needed to reload the massive guns of a battleship. By the time the guns were ready to fire, the solution had been updated.

But airplane targets were a much more complicated matter. Range to the target was very difficult to determine. Further, the range to the target was rarely constant. Knowing the rate of change for the target was just as critical. The bearing to the target was generally simple to determine, but the critical factor was actually the rate of change for the bearing to the target.  Also, the altitude of the target was critical. The speed and maneuverability of airplanes meant that mechanical computers would rarely be able to correctly predict the future location of a target with any degree of precision. Even worse, any maneuver by the target after the gun fired meant that the solution went out the window.  Time of flight for a 3” shell to 3 miles could be as long as 12 seconds. You can move an airplane a fair bit in 12 seconds. If you couldn’t reasonably expect to hit the target, what could be done to destroy enemy airplanes?

The first technique was to use time fused projectiles. Even if the gunnery solution wasn’t perfect, gunfire directors could come close to determining the time of flight for a projectile to the expected future position of the target. At first, mechanical time fuses were set by hand just prior to loading the shell into the gun. Later, mechanical fuse setters, controlled by the gunfire director itself, set the mechanical fuse to the proper time just before the shell was loaded. Thus, the gunfire director was transmitting bearing, elevation and time data to the gun mount. The gun crew need only point and elevate the gun as indicated by needles on a gauge mounted the side of the gun. The system was known as “follow the pointer.” One gun would probably not be too effective, as errors in range, bearing and elevation estimation would generally tend to put the burst outside the lethal radius. But multiply the number of guns firing, and the odds of getting a burst close enough to an enemy plane increased dramatically. When you watch Memphis Belle, or 12 O’clock High, and see lots of puff of angry black flak, that’s mechanically time-fused 88mm guns in action.

Right about the time WWII began, the British had the bright idea to combine advances in radar technology with fuses. Radar wouldn’t just be used by gunfire directors to detect and track targets, but actually miniaturized to trigger the fuse of an anti-aircraft shell. Thus the idea for the proximity fuse was born. The name said it all- the fuse was triggered whenever it came within the proximity of its target. Radio waves transmitted from the fuse itself would bounce off the target. When the reflected signal was strong enough, the shell would detonate. The sensitivity of the fuse was set to coincide with the lethal bursting radius of the shell.

By eliminating the errors associated with setting mechanical time fuses, the proximity fuse made gunnery far more effective at destroying enemy airplanes. Shells that failed to pass within the lethal radius would self destruct at a given time, but those shells that did pass within range would detonate.


As a result of the US and British agreement to share technology in 1940, the US took the lead in developing the proximity fuse for production. By the end of the war, virtually all US 3” and 5” anti-aircraft fire used the “VT” fuse. The “VT” designation stood for “Variable Time” and was  a deception measure to prevent the Axis powers from developing their own reverse engineered proximity fuses.

And idea of the importance placed on the VT proximity fuse can be gleaned from the money spent buying them. During WWII, the US spent roughly a billion dollars on VT fuses alone. The entire Manhattan Project to build the atom bomb cost about $2.5 billion, the entire B-29 bomber project cost $3 billion. The technical limitations that prevented (then) fitting the VT fuse to the Bofors 40mm projectile was a major reason the US Navy decided to develop the rapid fire 3”/50 gun as a replacement for the Bofors. Today, improvements in technology mean that virtually all guns 40mm and above can be proximity fused.

And it turned out that proximity fuses weren’t just useful in anti-aircraft gunnery.  The ability to consistently detonate artillery shells at a uniform height above their target was devastating to troops in the open. Rather than shells sending most of their fragments into the dirt, or skyward, VT fused 105mm and 155mm shells would rain down a lethal shower of shrapnel onto exposed personnel. While mechanical time fuses had been used before to cause airbursts, the ease and speed of using VT vastly increased the effectiveness of airburst artillery. To this day, “VT in effect” is used in calling for artillery fire on exposed troops.  I’ll leave it to Craig to discuss the old school artillery and cutting fuses.

Finally, when we get around to writing about the 5”/38 and 3”/50  guns, you’ll probably hear me use the terms “single purpose” and “dual purpose” guns, or SP and DP.  Single purpose guns are those intended only to attack surface targets such as other ships.  Dual purpose guns were those intended to be used against both aircraft and surface targets. A 3”, 4”, or 5” single purpose gun could be a relatively simple weapon, little more than a tube on a stand-hand cranked for bearing and elevation, and loaded by hand. Dual purpose guns had to have increased elevation capability, were almost always power driven (in order to track fast moving targets) and usually included power ramming of the shells, to facilitate rapid loading of heavy shells at high gun elevations.

1 thought on “The Proximity Fuse, Fire Control, and a bit more on Naval Guns”

  1. Once upon a time, long long ago…
    I was at Ft. Sill in artillery school and we learned about VT-Fuzes. It was emphasized; “Don’t use these in heavy rain!”
    On the day we were to shoot them, it was raining, hard! The Co. Commander and his driver were in his Jeep, crossing about 150 yards in front of the guns when our battery fired. Some of the rounds detonated directly over his jeep., tearing holes in canvas and metal.
    To do CYA, they condemned the whole-lot of fuzes as “defective”, when in fact, it was the Captain and NCOs who were “defective”. Pretty damned stupid.

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