The VT Fuze is the most important new development in the ammunition field,
since the introduction of high-explosive projectiles
(Gen Benjamin Lear)
I think when all armies get this shell
we will have to devise some new method of warfare
(Gen George S. Patton Jr)
Adm Lewis L. Strauss wrote : One of the most original and effective military developments in World War II was the proximity, or VT fuse. It was of incalculable value to both the Army and Navy, and it helped save London from obliteration. While no one invention won the war, the proximity fuse must be listed among the very small group of developments, such as radar, upon which victory very largely depended.
The proximity fuze was a dramatic improvement over previously used contact fuzes or timed fuzes, against both aircraft and land targets. Gen George S. Patton Jr called its effects devastating on the enemy, and said that the proximity fuze won the Battle of the Bulge for us.
One of the first practical proximity fuzes was codenamed the VT Fuze and nicknamed : Pozit ! Buck Rogers ! Special Influence ! Bonzo !, an acronym of Variable Time Fuze, as deliberate camouflage for its operating principle. The VT fuze concept in the context of artillery shells originated in the UK with British researchers (Samuel Curran and W. A. S. Butement, whose schematic design for a radar proximity fuze was used with only minor variations and was developed under the direction of physicist Merle A. Tuve at the Johns Hopkins University Applied Physics Lab. The Germans were supposedly also working on proximity fuses in the 1930s, based on capacitive effects rather than radar. Research and prototype work at Rheinmetall were halted in 1940 to devote available resources to projects deemed more necessary.
Before the fuze’s invention, detonation had to be induced either by direct contact, or a timer set at launch, or an altimeter. All of these have disadvantages. The probability of a direct hit with a relatively small moving target is low. To set a time-triggered or height-triggered fuze one must measure the height of the target (or even predict the height of the target at the time one will be able to get a shell or missile in its neighborhood). With a proximity fuze, all one has to worry about is getting a shell on a trajectory that, at some time, will pass close by the target. This is still not a trivial task, but it is much easier to execute than previous methods.
Use of timing to produce air bursts against ground targets requires observers to provide information for adjusting the timing. This is not practical in many situations and is slow in any event. Proximity fuzes fitted to such weapons as artillery and mortar shells solve this problem by having a range of preset burst heights (e.g. 2, 4 or 10 metres, or about 7, 13, or 33 feet) above ground, which can be selected by gun crews prior to firing.
A proximity fuze is a fuze that detonates an explosive device automatically when the distance to the target becomes smaller than a predetermined value, which can also take place when the fuze and the target pass by each other. Various kinds of proximity fuzes are designed for various targets such as planes, missiles, ships at sea and ground forces. They provide a more sophisticated trigger mechanism than the common contact fuze. The proximity fuze is considered one of the most important technological innovations of World War II. It was so important that it was a secret guarded to a similar level as the atom bomb project or D-Day invasion. It was also fiendishly tricky to manufacture.
- More Readings
Klein Maury (2013)
A Call to Arms : Mobilizing America for World War II, New York, NY : Bloomsbury Press
Thompson Harry C.; Mayo, Lida (1960)
The Ordnance Department : Procurement and Supply, Washington, D.C.
Woodbury, David (1948)
Battlefronts of Industry : Westinghouse in World War II, New York, NY
Parker, Dana T. (2013)
Building Victory : Aircraft Manufacturing in the Los Angeles Area in World War II, Cypress, California
In the late 1930s the UK was working on a variety of developments to increase air defence efficiency. Into this stepped W. A. S. Butement, designer of radar sets CD/CHL and GL, with a proposal on Oct 30 1939 for two kinds of radio fuze :
- (1) a radar set would track the projectile, and the operator would transmit a signal to a radio receiver in the fuze when the range, the difficult quantity for the gunners to determine, was the same as that of the target
- (2) a fuze would emit high-frequency radio waves that would interact with the target and produce, as a consequence of the high relative speed of target and projectile, a Doppler-frequency signal sensed in the oscillator.
Coincidentally, a German neon lamp tube and a design of a prototype proximity fuze based on capacitive effects was received by British Intelligence in mid November 1939. Butement, Edward S. Shire, and Amherst F.H. Thompson proposed the radio frequency proximity fuze concept in a memo to the British Air Defense Establishment in May 1940. A breadboard circuit was constructed by the inventors and the concept was tested in the laboratory by moving a sheet of tin at various distances. Early field testing connected the circuit to a thyratron trigger operating a tower-mounted camera which photographed passing aircraft to determine distance of fuze function. Prototype fuzes were then constructed in June 1940, and installed in unrotated projectiles (the British cover name for solid fueled rockets) fired at targets supported by balloons. During 1940-42 a private venture initiative by Pye Ltd., a leading British wireless manufacturer, worked on the development of a radio proximity fuze. Pye’s research was transferred to the United States as part of the technology package delivered by the Tizard Mission when the United States entered the war. It is unclear how this work relates to other British developments. The details of these experiments were passed to the United States Naval Research Laboratory and National Defense Research Committee (NDRC) by the Tizard Mission in September 1940, in accordance with an informal agreement between Winston Churchill and Franklin D. Roosevelt to exchange scientific information of potential military value.
Following receipt of details from the British, the experiments were successfully duplicated by Richard B. Roberts, Henry H. Porter, and Robert B. Brode under the direction of NDRC section T chairman Merle Tuve. Lloyd Berkner of Tuve’s staff devised an improved fuze using separate tubes (British English : thermionic valves or just “valves”) for transmission and reception. In December 1940, Tuve invited Harry Diamond and Wilbur S. Hinman, Jr, of the United States National Bureau of Standards (NBS) to investigate Berkner’s improved fuze. The NBS team built six fuzes which were placed in air-dropped bombs and successfully tested over water on 6 May 1941. While working for a defense contractor in the mid-1940s, Soviet spy Julius Rosenberg stole a working model of an American proximity fuze and delivered it to the KGB.
Parallel NDRC work focused on fuzes for use with anti-aircraft artillery. Major problems included micro-phonic difficulties and tube failures attributed to vibration and acceleration in gun projectiles. The T-3 fuze had a 52% success against a water target when tested in January, 1942. The US Navy accepted that failure rate, and batteries aboard cruiser USS Cleveland (CL-55) tested proximity-fuzed ammunition against drone aircraft targets over Chesapeake Bay in August 1942. The tests were so successful that all target drones were destroyed before testing was complete.
The German proximity fuze was developed in the Rheinmetall Borsig AG Plan. The program was halted in 1940, restarted in early 1944 and then terminated again when the factories were overrun by the Allies. The fuze had the following characteristics : the fuze was based on electrostatic principles; the nose of the shell was electrically insulated and isolated from the rest of the shell. Initial fuze testing demonstrated a sensitivity of 1–2 meters and a reliability of 80% when fired against a metal cable target. A circuit adjustment yielded an increase to 3–4 meters and a reliability of close to 95%. Further work showed a 10-15 meter sensitivity. This was with 88-MM cannon shells. The shell was for all intents and purposes ready for production.
In contrast, the Allied fuze was ready. Technically, the Allied fuze used constructive and destructive interference to detect its target. The design had four tubes.
- One tube was an oscillator connected to an antenna; it functioned as both a transmitter and a receiver:
- when the target was far away, it would reflect little of the oscillator’s energy back to the fuze and have almost no effect on the circuit;
- when a target was nearby, it would reflect a significant portion of the oscillator’s signal back to the fuze;
- the amplitude of the reflected signal indicates the closeness of the target;
- this reflected signal would affect the oscillator depending on the round trip distance from the fuze to the target;
- if the reflected signal were in phase, the oscillator amplitude would increase and the oscillator’s plate current would also increase;
- if the reflected signal were out of phase, then the plate current would decrease:
- the distance between the fuze and the target is not constant but rather constantly changing due to the high speed of the projectile (shell and fuze) and any motion of the target:
- when the distance between the fuze and the target changes rapidly, then the phase relationship also changes rapidly:
- the signals are in-phase one instant and out-of-phase a few hundred microseconds later;
- the result is a heterodyne beat frequency that indicates the velocity difference.
Viewed in another way, the received signal frequency is doppler shifted from the oscillator frequency by the relative motion of the fuze and target. Consequently :
- a low frequency signal corresponding to the frequency difference develops at the oscillator’s plate terminal;
- two additional amplifiers detected and filtered this low frequency signal;
- if the amplified beat frequency signal is large enough (indicating a nearby object), then it triggers the 4th tube (a gas-filled thyratron);
- the thyratron conducts a large current that sets off the electrical detonator.
There were many shock hardening techniques including planar electrodes and packing the components in wax and oil to equalize the stresses.
First large scale production of tubes for the new fuzes was made at a General Electric plant in Cleveland, Ohio formerly used for manufacture of Christmas-tree lamps. Fuze assembly was completed at General Electric plants in Schenectady, New York and Bridgeport, Connecticut.
By 1944 a large proportion of the American electronics industry concentrated on making the fuzes. Procurement contracts increased from $60 million in 1942, to $200 million in 1943, to $300 million in 1944 and were topped by $450 million in 1945. As volume increased, efficiency came into play and the cost per fuze fell from $732 in 1942 to $18 in 1945. This permitted the purchase of over 22 million fuzes for approximately one billion dollars (short scale). The main suppliers were Crosley, RCA, Eastman Kodak, McQuay-Norris and Sylvania. There were also over two thousand suppliers and subsuppliers, ranging from powder manufacturers to machine shops.
Vannevar Bush, head of the United States OSRD (Office of Scientific Research and Development) during the war, credited the proximity fuze with three significant effects :
- It was important in defense from Japanese Kamikaze attacks in the Pacific. Bush estimated a sevenfold increase in the effectiveness of 5-inch antiaircraft artillery with this innovation;
- It was an important part of the radar-controlled antiaircraft batteries that finally neutralized the German V-1 bomb attacks on England;
- It was used in Europe starting in the Battle of the Bulge where it was very effective against German divisions, and changed the tactics of land warfare.
At first the fuzes were only used in situations where they could not be captured by the Germans. They were used in land-based artillery in the South Pacific in 1944. They were incorporated into bombs dropped by the USAAF on Japan in 1945, and they were used to defend Britain against the V-1 attacks of 1944, achieving a kill ratio of about 79%. (They were ineffective against the much faster V-2 missiles.) There was no risk of a dud falling into enemy hands.
The Pentagon had decided it was too dangerous to have a fuze fall into German hands because they might reverse engineer it and create a weapon that would destroy the Allied bombers, or at least find a way to jam the radio signals. The Pentagon refused to allow the Allied field artillery use of the fuzes in 1944, although the US Navy fired VT-fuzed anti-aircraft shells during the July 1943 invasion of Sicily. After Gen Dwight D. Eisenhower demanded he be allowed to use the fuzes, the VT fuzes were used during the Battle of the Bulge (Dec 1944 – Jan 1945), when they made the Allied artillery far more devastating, as all the shells now exploded just before hitting the ground. It decimated German divisions caught in the open. The Germans felt safe from timed fire because they thought that the bad weather would prevent accurate observation. The effectiveness of the new VT fused shells exploding in mid-air, on exposed personnel, caused a minor mutiny when German soldiers started refusing orders to move out of their bunkers during an artillery attack. Gen George S. Patton said that the introduction of the proximity fuze required a full revision of the tactics of land warfare.