* After the introduction of the first centimetric radar sets, radar technology matured rapidly. By 1944, new radar technologies were available that made those available only a few years before completely obsolete. In some cases, the new technologies offered capabilities that would have been regarded as complete science fiction before the war.
* H2S was much needed, but it was by no means perfect. In August 1943, RAF Bomber Command conducted a raid on Berlin that proved a fiasco. Five percent of the attacking force was lost and little damage was done to the city.
The Rad Lab was working on targeting radars in parallel with the British, but American progress was slow. Luis Alvarez was working on an advanced concept named "Eagle", of which more is said later, but it was a relatively long-term effort. In June 1942, the Rad Lab had begun a more conservative project named "NAB (Navigation And Bombing)" that was modeled after H2S, but the small NAB team could not match the efforts of the TRE.
One of the Rad Lab researchers, George Valley, who had been working on gun-laying radar, had witnessed a bomb raid while visiting London in the fall of 1942. Valley had noticed how ineffectual British anti-aircraft fire had been, and felt the best way to fight back against the Germans was not try to shoot down their bombers, but to bomb Germany. When he returned to America, he left the gun-laying project and took over the NAB group. Valley was smart, aggressive, abrasive, and determined to build a targeting radar that worked.
In early 1943, pressure on the Rad Lab to build a targeting radar increased. The USAAF had sent the Eighth Air Force to England to pursue precision daylight bombing, but that concept was almost a joke. While the USAAF played up the Norden optical bombsight as a miracle gadget, capable of putting a "bomb in a pickle barrel", it wasn't that good, and the region was cloudy much of the year anyway. The Sperry company had actually built a better bombsight, but the Norden company had some really good salesmen -- and in fact, much of what was said about the Norden bombsight was just Norden marketing "hype" that had gone into wide circulation.
Valley had managed to refine the NAB S-band targeting radar to the point where it worked as well as H2S, but that wasn't good enough. The Rad Lab's efforts to build a 3-centimeter (10 GHz) ASV had reached the advanced test stage, however, and Valley felt that an X-band targeting radar might just be the solution. Unfortunately, everyone else was focused on the advanced Eagle radar for the targeting job. It promised much greater accuracy, and at the time the USAAF did not like the idea of carpet-bombing cities -- though that would change. Still, nobody was exactly sure when Eagle would be ready for combat. Rad Lab officials knew that USAAF policy could change, and so Valley was allowed to quietly continue work on X-band targeting radar as a backup plan.
Valley called the X-band system "H2X", in imitation of the British H2S. Valley still feared that H2X wouldn't be accurate enough to do the job, until he realized that Army Air Force bombers flew in huge formations, with the aircraft staggered horizontally and vertically to provide overlapping fields of fire for their defensive armament. The bomb pattern from such an extended formation was not going to be precise, and in principle H2X provided as much accuracy as was useful.
While Eagle remained bogged down in technical difficulties, progress on H2X was rapid. Working from the S-band ASV being developed by Norman Ramsey's group, Valley's team redesigned the antenna system so that, instead of performing a 360-degree search, it scanned down and in front of the aircraft. Team members added precision ranging capabilities, and Valley himself built an analog bombsight computing system. Using it was simple: the bombsight included a rotating drum that was calibrated with altitude settings, and a bombardier could rotate the drum to determine how far to lead the target at a specific altitude. The drum's position was fed back to the PPI display used by the H2X system, bringing up a circle on the display. When the circle was on target, the bombardier released the bombs.
* In April 1943, there was a "radar summit" in England between officials of the Rad Lab, the TRE, and senior brass, and Lee DuBridge proposed that the Rad Lab take over targeting radar. According to DuBridge, the future should be H2X, not H2S, for both the RAF and the USAAF.
The suggestion shocked the British. The USAAF had traditionally been solidly against area bombing, and now the Americans had reversed themselves. The British had other reasons to be annoyed with the Yanks, since several Rad Lab researchers had a low opinion of H2S, the blunt Valley calling it "bucket of crap", even while the NAB effort was in worse shape. In reality, by this time the American effort was beginning to outstrip and dominate British work. Britain had been at war over two years longer than the US, and the British were becoming exhausted. One Yank engineer visiting his counterparts at the TRE was impressed by their expertise, but observed that they had been working seven-day weeks for a number of years and were worn to a frazzle.
On the other side of the coin, British visiting their American cousins were envious of the level of staffing, including large numbers of skilled machinists and draftsmen. As Churchill had understood, the Americans simply had far more resources to throw at radar. Bernard Lovell had begun an investigation into X-band radar, but he had only one staffer to spare on it, and though British X-band H2S would be available by late 1943, it would not be widely deployed.
The Rad Lab, in the meantime, was already beginning research on 1-centimeter (30 GHz) "K-band" radar. The two groups decided to continue on their own three-centimeter targeting systems, more or less independently, though in the fall the Rad Lab would establish a liaison office at Malvern to help improve communications with the TRE.
Incidentally, the Rad Lab's K-band radars didn't work out, at least not as planned. Although much was expected from the new "H2K" sets that the work was intended to produce and tests in early 1944 seemed to work fine, when spring came the K-band radar went blind. It turned out that 1 centimeter (30 GHz) radiation was absorbed by water vapor, and in the spring the high humidity blocked the transmissions. This discovery would have significant implications for scientific research after the war.
* By June 1943, the American H2X effort was in high gear. The USAAF desperately needed targeting radars, Eagle was clearly not going to be ready any time soon, and the British hardly had enough working H2S sets to fit their own Pathfinders, much less the USAAF's. Besides, operational experience had shown that H2S was only a minor improvement over sheer blind bombing.
The radar reflectivity of different terrain varies greatly and not very predictably, making interpretation of the radar image of the landscape below an aircraft on a PPI display difficult at best. Radio waves do bounce neatly away when striking reasonably smooth bodies of water such as lakes and rivers and so provide little or no return, meaning that they would show up on a PPI as distinctive dark areas. That in principle allowed identification of targets with distinctive nearby bodies of water, but even that was limited by the fact that shorelines could be obscured by vegetation and structures.
Hamburg had been an easy target, marked by the confluence of two rivers that were easily visible on radar. Berlin was much more troublesome, with no distinctive large bodies of water to clearly identify the target. In principle, X-band radar could provide more detail, revealing prominent buildings and other structures, and H2X became the Rad Lab's top-priority project.
Western Electric was selected to manufacture the USAAF version of H2X, while Philco was selected to build the US Navy version. By September 1943, the USAAF Eighth Air Force had a dozen B-17 Pathfinders equipped with H2X, which was also referred to as "BTO (Bombing Through Overcast)". H2X saw its first major use in combat on 3 November 1943, when the Pathfinders led a B-17 formation against the Wilhelmshaven docks. Despite cloudy conditions, the attack was regarded as effective.
In reality, a study released in September 1944 showed that H2X wasn't much more accurate than H2S, but the USAAF, like Bomber Command before them, had lost their illusions about precision bombing and becoming more resigned to area bombing. H2X, like most new technologies, was also temperamental and not particularly reliable. The cold, thin air of high-altitude operation encouraged electrical arcing, and soon the H2X sets were enclosed in pressurized boxes. The bombardier had to pump up the box every now and then with a bicycle tire pump. H2X was popularly referred to as "Mickey" by crews, it seems in reflection of the derisive name of "Mickey Mouse".
The Philco "AN/APS-15", the production version of H2X with the worst bugs worked out, wasn't available until February 1944. However, by the end of 1943, the USAAF was using H2X Pathfinders on 90% of their raids, and it had clearly eclipsed H2S. The weather was poor in the days leading up to the landing in Normandy on D-Day, 6 June 1944, but with the help of H2S and H2X Allied air forces were able to hit radar stations, bridges, communications centers, and other targets. Incidentally, the AN/APS-15 was also used as an ASV radar, with the cosecant-squared antenna replaced by a surface-search antenna, and saw widespread use in the ocean-patrol role.
The RAF continued to use S-band H2S, though since they had learned that German fighters could home in on its emissions, they were cautious not to turn it on until they were near the target and turned it off again after they dropped their bombs.
By the end of 1944, the USAAF had large numbers of Pathfinders equipped with H2X, including Lockheed P-38 fighters with long "Mickey" noses. The sets and operator training had been refined. A trick known as "offset bombing" was devised, in which a target that didn't have a good radar signature was located relative to nearby landmarks that did.
H2X also went to war in the Pacific, combined with a Bell airborne search system in a package with the designation "AN/APQ-13". Japanese cities were generally on coastlines, which made them excellent radar targets. Emulating RAF Bomber Command tactics, Boeing B-29 Superfortress bombers roared in over Japanese targets in streams, hitting them with incendiaries with devastating effect.
* Although H2X was inaccurate, a variation on the theme for anti-ship attack, known as "Low Altitude Bombing (LAB)" proved able to drop bombs precisely on target. It was, after all, an easier task, since a large vessel in normal seas presents a much more distinct radar target than a city.
Bell Labs began work on LAB in July 1942. In itself LAB, which was formally designated the "AN/APQ-5", was a control system that could be integrated with various types of centimetric radars. LAB was integrated with the aircraft's autopilot and bomb-release system. On the attack run, the bombardier kept the radar on target and synchronized with the aircraft's flight speed by turning knobs. A horizontal line moved up the B-scope display as the aircraft moved toward the target, and the bombs were automatically released when the line met the target centered in the display.
The first aircraft carrying pre-production LABs, modified Consolidated Liberators known as "Snoopers", reached service in the South Pacific in August 1943. They used LAB with the new SCR-717 radar, and the system proved extremely effective, with bombers roaring in on Japanese ships in the night at a few hundred meters and blowing them out of the water. Said one Snooper pilot: "We flew in the dark most of the time and we'd attack at about eight hundred or a thousand feet, and you couldn't miss at that altitude, you know."
* While the Americans built improved AI and ASV radars, they also built improved radar altimeters. The early SCR-518 led to the "SCR-718C", which was lighter and more reliable, as well as the "AN/APN-1", which was a low-altitude radio altimeter.
Since low-altitude operation requires a high pulse repetition rate and so an increased tendency to pick up ghost echoes, the AN/APN-1 was a continuous-wave radar. It could determine range because it generated a "frequency modulated" signal which sent out a low frequency and ramped up to a high frequency, over and over again in a cycle, allowing the interval of the echo to be determined.
The US also built a series of tail-warning radars, most notably the "AN/APS-13" for fighters. This was a low-power pulse radar set, operating at 67 centimeters (450 MHz), that flashed a red light and rang a bell when an enemy aircraft was approaching from the rear. Similar sets, the "AN/APS-16" and "AN/APS-17", were built for bombers.
BACK_TO_TOP* Although the Allied switch from the defensive to the offensive in air combat made centimetric AI less important, AI developed along with other Allied radar technologies.
Philip Dee's team at the TRE continued to develop AIS while the main focus of the organization remained on H2S. AIS was ready for service in early 1942, leading to the manufacture of a hundred production "AI Mark VII" in the spring of 1942, which were promptly installed on Beaufighters and Mosquitoes. AI Mark VII was a major improvement over AI Mark IV. Instead of using lobe switching, AI Mark VII used "conical scanning", rotating the antenna at a small angle around the boresight to pin down the target's location. If the radar was precisely on target, the return echo strength remained the same as the antenna went through its circular motion. If the target moved, the echo was stronger at one part of the antenna's circular motion and weaker at another, and the antenna would then be shifted to compensate. The Mark VII was quickly followed by 1,500 improved "AI Mark VIII" sets, which cleaned up a few bugs and added an IFF system.
* The Americans did not have the same need for night-fighting capabilities as the British, mostly because the Japanese didn't have any serious capability for night air combat. However, the Americans also recognized the necessity for the technology.
The first dedicated US night-fighter was the Douglas "P-70", a modification of the A-20 Havoc bomber with a belly tray carrying four 20-millimeter cannon and a copy of British AI.IV longwave radar in the nose. The P-70 lacked the performance to be an adequate night fighter, but it provided the USAAF with useful experience. The USAAF's real solution was a big twin-engine aircraft, the Northrop "P-61 Black Widow", which was designed from the ground as a night fighter and was to carry the definitive Western Electric "SCR-720B" centimetric AI, a lighter and improved version of the SCR-520.
The P-61 was an excellent aircraft, surprisingly fast and agile for its large size; however, it did not reach operational status until early in 1944, and had little effect on the war. The US Navy also built a night-fighter version of their twin-engine Grumman "F7F Tigercat" with the SCR-720B, but the Tigercat saw little or no combat.
The SCR-720B was still another major advance in AI, including features such as the ability to cut through jamming. The British had been working on a comparable improved centimetric AI, the "Mark IX", but had run into development troubles, to put it mildly. On 23 December 1942, a rookie Canadian Spitfire pilot "bounced" the Bristol Beaufighter being used to test the radar and shot it down, killing A.C. Downing, the chief engineer on the project. Since the SCR-720B was at least as capable as the AI.IX was meant to be, and delays in the P-61 program meant that it was available, the British adopted the SCR-720B as the "AI Mark X", fitting it to Mosquitoes. AI.IX was actually completed after the war, but it proved an exercise in futility, since it was no more capable than AI.X and substantially heavier. AI.X remained in first-line service with the RAF well into the 1950s.
Due to shortages of the AI.X, a less sophisticated set, the "ASH" or "AN/APS-4", was rushed into service as the "AI Mark XV", and also used to equip Mosquitoes. AN/APS-4 was actually designed as an X-band ASV, but could be used for AI. The Royal Navy used it with the Fairey Firefly two-seat naval fighter, naval versions of the Mosquito, and other aircraft.
A simplified display system was fitted to the AN/APS-4 to produce the "AN/APS-6", also known as the "AIA-1". The new display, which was only about 5 centimeters (2 inches) in diameter, effectively made the AN/APS-6 a "radar gunsight", allowing a fighter pilot to use the radar himself without need for a radar operator. The pilot wore red-tinted goggles to allow him to retain night vision. The AN/APS-6 was used with Grumman F6F Hellcat and Vought F4U Corsair single-seat night fighters.
BACK_TO_TOP* The development of centimetric radars by the Allies also meant that aircraft needed improved IFF units, since the older British IFF Mark II could not be modified to cover the shorter wavelengths. Even as the Rad Lab was ramping up, the British were moving towards manufacture of an improved "IFF Mark III" that was compatible with the new radars.
In fact, IFF Mark III was compatible with any radar, since it didn't pay any attention to radars to begin with. Although its predecessors responded to radar signals at specific wavelength ranges, this approach was inflexible and also too easy for the Germans to work out. IFF Mark III took a different approach, with a radar site or aircraft instead using an additional piece of gear, an "IFF interrogator", to send out a signal sweeping through the range of 1.9 to 1.6 meters (158 to 188 MHz). The IFF module then replied with a pulse train of specified length at its interrogating wavelength, with a long train indicating an emergency.
IFF Mark III also eliminated the troublesome adjustments required by its predecessors. Production began at Ferranti in Britain in early 1941, and by mid-1942, Hazeltine in the US was producing it as the "SCR-515". IFF Mark III was built in great quantity.
The Americans weren't entirely happy with IFF Mark III and pushed for their own IFF design, while the British defended the technology -- both sides expressing concerns over issues that didn't turn out to be all that important. The British had the upper hand in the bargaining, because they were already putting Mark III into production. Since IFF had to be standardized to be effective, the Americans had no choice but to grudgingly go along, holding their own technology, which became known as "IFF Mark IV", in reserve. A decision was made that IFF Mark III would be the standard for the aircraft and ships of the Western Allies by the spring of 1943.
IFF Mark III proved successful, though not entirely so:
Then there were the simple difficulties of making sure people used the IFF system properly. Figuring out if an IFF unit was working was trickier than checking, say, a radio, and so malfunctions could be overlooked. Aircrews were trained to turn off IFF when over hostile territory, so the enemy couldn't use it against them, and sometimes they would forget to turn it back on when they came home -- to get a very nasty reception. There were so many incidents due to bungled use of IFF that the US military developed a 40-minute training film in 1944 to pound it into people's heads how to do it right, and to emphasize what sad things might happen to them if they didn't.
* Along with IFF, the Allies built improved radar beacons. They were similar to IFF in that they could be interrogated and would respond, but were more selective, only responding to a specific wavelength. RAF bases set them up to allow night-fighters to find their way home. Extensive networks of radar beacons were set up, and by the end of the war a pilot could fly up the Pacific Coast from San Diego to the Aleutians without being out of touch with one.
Radar beacons were also used tactically. The British developed a battery-powered beacon that could be set up by pioneer teams to guide the way for paratroop drops. The interrogator, which was derived from ASV.II, was named "Rebecca", since she "called for her children", and the beacon was named "Eureka", for "I have found it!". The Americans also built these items, giving Rebecca the designation "AN/APN-2" and Eureka the designation "AN/PPN-1". Another tactical use of radar beacons was marking landing beaches for invasion forces, with landing craft marked by other beacons.
* After the war, radar systems became the backbone of commercial air traffic networks as well as military networks. IFF became useful for airliners and other commercial aircraft, and the idea was extended to allow IFF to provide data such as an aircraft's altitude, identification, and remaining fuel. The International Civil Aviation Organization adopted a universal standard for civilian IFF in 1958.
BACK_TO_TOP* While radar went into increasing use in aircraft, it was also employed by ground defenses. Traditionally, heavy anti-aircraft guns had been directed with optical sights in conjunction with analog mechanical computers. A timer in the shells was set before firing so they would burst at a predetermined altitude, with the timer setting determined by the mechanical computer.
As discussed earlier, the British Army had built the GL long-wavelength gun-laying radar, but it was clearly inadequate. The US SCR-268 was a much better piece of gear. During operations in New Georgia in July 1943, a US Marine battery of 90-millimeter guns used the SCR-268 to shoot down 12 out of 16 Japanese attackers using only 88 rounds, with the other four finished off by fighters. A month later, on 4 August, the Luftwaffe conducted a night raid on Palermo in Sicily and US Army 90-millimeter guns directed by the SCR-268 shot down seven out of the 29 attackers.
However, the SCR-268 was obviously not the last word in gun-laying radars, and the Germans would soon learn how to jam it anyway. An improved gun-laying radar system was one of the Rad Lab's three initial objectives suggested by the Tizard Mission. Both the Canadians and the British Army's radar development team were working on centimetric gun-laying radars as well.
The Rad Lab team working on gun-laying radar was led by Louis Ridenour, a University of Pennsylvania physicist, and included two brilliant engineers, Ivan Getting and Lee Davenport. The team was very ambitious. The British and Canadian gun-laying radar programs were simply intended to develop a radar to track enemy aircraft, with the operator telling the anti-aircraft guns where to shoot. The Rad Lab group, in contrast, wanted the radar to actually steer the gun so that it stayed locked on the target.
Most people would have thought such an idea sounded like science fiction at the time and neither the Tizard mission nor US military brass had suggested it, but MIT had considerable expertise in feedback systems. The team was able to quickly develop a system that integrated centimetric radar technology with feedback-controlled servomechanism systems to track aircraft.
To acquire a target in the first place, the antenna pivoted in a helical pattern, a scheme naturally known as "helical scanning". Once the radar acquired a target, the antenna remained automatically locked on to the target, performing conical scanning to precisely locate it. In the case of the SCR-584, the antenna remained focused on the boresight while the radio feed was offset and spun around at 4 RPM.
Determining range was trickier, since the distance to the target had to be fixed to within a few meters to permit a timed shell to burst at the appropriate instant. The automatic gun-laying system also had to discriminate between target echoes and false echo returns from nearby objects.
The false echo problem was particularly nasty, but the fact that most of the top Rad Lab researchers were physicists worked to advantage with the gun-laying system. Before the war, physicists had studied the mysterious "cosmic rays" that fell from space, and set up detectors that would be triggered by the spray of particles created by cosmic rays when they hit the atmosphere. These detectors were very often also set off by particles emitted by natural radioactivity in the soil, and even if multiple detectors were used the probability that both would be activated by natural radioactivity was still too high.
The answer was to use at least two detectors in series and build a "coincidence" circuit that only indicated a hit if both detectors went off sequentially. The same approach could be used for the gun-laying radar. The Rad Lab engineers designed a circuit based on a precise electronic quartz clock, driving switches known as "range gates". The transmitting pulse was synchronized to the clock, and when the pulse was sent out, the gates stayed shut for a very short instant. That meant that any return echoes from nearby objects would be ignored. After opening, the range gates then closed again after an interval, ensuring that echoes from beyond the maximum range were ignored as well.
The SCR-584 featured three displays. A PPI was used for wide-area search, while two small "J-scopes" were used for determining precise range. The J-scopes used radial sweeps synchronized to the transmit pulse. The sweep time for one of the J-scopes was about 5 milliseconds, which allowed it to register a return pulse as a spike whose angle gave the distance to the target at ranges of up to 29 kilometers (18 miles). This was a coarse indication of the distance to the target. The other J-scope had a sweep speed sixteen times faster, and what appeared as a spike on the first J-scope was a broad pulse on the second. A cursor line could be moved on the display to the center of the pulse using a knob, nailing down the precise distance to the target to within about three meters (ten feet).
* To test the scheme, the researchers obtained a prototype electrically-driven B-29 machine gun turret from General Electric and set it up on top of MIT Building 6. Instead of machine guns, the turret aimed a 16-millimeter movie camera with a telephoto lens. One of the group hired a friend who owned a private plane to fly around for ten dollars an hour so tests could be conducted, and after work and tests, on 31 May 1941, the radar-guided turret tracked the aircraft accurately.
Once the team got the system to work predictably, they showed it off to interested parties. Alfred Loomis brought in the Army Signal Corps' Roger Colton, now a brigadier general, for a demonstration. He immediately recognized the potential of the gear and put the Signal Corps behind its further development. As insurance, Colton also set Bell Labs to work on a parallel centimetric gun-laying radar effort. Improvements came rapidly, and by December 1941 the team had a truck-mounted radar designated the "Experimental Truck 1 (XT-1)" undergoing evaluation at Fort Hancock in New York state.
Ridenour's group worked closely with Bell Labs. A Bell engineer named David Parkinson had designed a strip-chart recorder, which was a machine that moved a pen across a scrolling piece of paper to track changing voltages over time. One night in May 1940, Parkinson had a dream in which he vividly saw his invention modified to control an anti-aircraft gun. When he woke up, he realized that controlling the movement of a pen was not so different from controlling the movement of an anti-aircraft gun. His management liked the idea, and Parkinson and his colleagues quickly outlined the design of an electronic analog computer, based on the stripchart recorder, to help direct an anti-aircraft gun.
As mentioned, mechanical analog computers had been in service for that purpose for some time -- but they were slow and heavy, and an electronic system would be far superior, at least in principle. The Bell Labs researchers worked on the project independently for some time, but eventually Ridenour heard about the work, and by early 1942 the Rad Lab XT-1 gun-laying radar and the Bell Labs analog computer project were working together. The analog computer was "programmed" by turning wheels that set potentiometer resistances. It accepted electrical inputs from the XT-1 radar, and sent electrical outputs to the anti-aircraft guns to direct them accordingly.
The entire system was ready for demonstration to the Army on 1 April 1942, and performed impressively. Maximum range was about 64 kilometers (40 miles) against a large target, with automatic tracking operating at about half that range. Accuracy was about a thousandth of a degree. The Army ordered 1,256 of the systems the next day. The production radar was designated the "SCR-584", and the production analog computer was designated the "M-9 Gun Predictor". One radar and M-9 could control a battery of four guns. Deliveries were delayed by bureaucratic confusion, but troops in the field began to get their hands on the new gear in early 1944.
90-millimeter guns controlled by the SCR-584 / M-9 were deployed to the Anzio beachhead in Italy in February 1944 and proved deadly accurate. They also were introduced to combat in the Pacific about the same time, though deficiencies in training left them ineffectual for many months.
* The Canadians and the British Army did complete their own centimetric gun-laying radars, which were designated the "GL Mark 3C", where "C" meant "Canadian", and "GL Mark 3B", where "B" meant "British". They were simply not in the same league with the "whizzbang" SCR-584, having separate transmit and receive antennas and no automatic tracking, and the British ended up obtaining the SCR-584 as the "GL Mark 3A", where "A" meant "American". The Soviets were provided with the SCR-584 under Lend-Lease and were also appropriately impressed by it, putting it into production themselves as the "SON-4".
Bell Labs also completed their "alternate" centimetric gun-laying radar, which went into production as the "SCR-545". This system included 1.5 meter (200 MHz) radar for wider target search along with the 10 centimeter (3 GHz) radar, and had automatic tracking for both radars. It was still not quite in the same league with the SCR-584.
The automatic tracking scheme pioneered with the SCR-584 was so useful that the Navy incorporated the scheme with their Mark 12 fire-control radar, discussed earlier. Radar gun-laying systems were also developed for aircraft. For example, the tail guns of some B-29 Superfortress bombers were directed by an "AN/APG-15" radar system, operating at 12 centimeters (2.5 GHz) and incorporating conical scanning.
BACK_TO_TOP* Originally, the M-9 gave the gun crews the timer settings for the shell fuzes, but in a few months a new electronic miracle arrived: the proximity-fuzed shell, which included an electronic device that allowed it to explode when it came within a certain distance of a target. Unlike the SCR-584, the proximity fuze had little or nothing to do with the Rad Lab.
The British had begun research on proximity fuzes for anti-aircraft shells in 1937. The initial British work envisioned photoelectric or acoustic fuzes, but these ideas didn't work out. In the fall on 1939, William Alan Stewart Butement, a New Zealander who was one of the lead engineers in the British Army radar effort, suggested that a shell with a remote-control fuze, activated from the ground over a radio link, or a shell containing its own radio-wave sensor, might do the job. The second option quickly proved more practical and a circuit was designed, but Butement, by all evidence a very good engineer whose work was badly hobbled by the British Army neglect, had no time to pursue development.
Even before the formation of the Rad Lab, the Americans had picked up hints that the British were working on a proximity fuze. In August 1940, Carnegie Institution physicist Merle Tuve -- head of the Carnegie Department of Terrestrial Magnetism (DTM) in Washington DC, and well-known to the NRL through various collaborations -- spoke with Vannevar Bush of the NDRC about proximity fuzes for shells, bombs, and rockets. Bush then helped organize NDRC Section T (for "Tuve") at the DTM to work on a proximity fuze. A section T engineer named Richard Roberts began by performing a series of increasingly rigorous experiments to show that a vacuum tube could survive thousands of gees of acceleration, as would be required if an electronic circuit were to be shot out of a gun.
Section T had been considering some of the same options for a proximity fuzing mechanism as the British had, and was coming to the conclusion that radio sensing was the best option when the Tizard mission arrived in September. Butement's research notes were included in the "Black Box" and were made available to the Americans. Butement's electronic circuit design was both elegant and practical, and threw the American proximity fuze effort well forward. Section T had a lab prototype of the circuit operating within days.
The circuit only required four vacuum tubes, including one for an oscillator, two for an amplifier, plus a gas-filled tube called a "thyratron" that operated as a switch. The oscillator was wired to an antenna, which in an operational fuze would actually be the shell casing itself, and if an object came within a few wavelengths of the antenna, its proximity affected the loading and operation of the oscillator circuit. The oscillator output was fed through an amplifier, whose output in turn was connected to the input of the thyratron. At a certain input level, the gas in the thyratron ionized and passed a large current pulse from a capacitor to trigger the shell. Calling such a simple device a "radar" was a stretch; it was really just a proximity detector.
* Although the focus of the effort was on developing proximity fuzes for anti-aircraft gun shells, Section T also conducted tests with small bombs fitted with radio or optical proximity fuzes in October 1940. Tuve decided that designing a proximity fuze for a spinning shell was not quite the same job as designing such a device for a bomb or rocket, since a shell endured much higher gee forces and required different arming mechanisms. Proximity fuzes for bombs or rockets were passed on to a group at the US National Bureau of Standards (NBS) under Harry Diamond that ended up working on both radio and photoelectric proximity fuzes. Section T focused on the radio proximity fuze for shells, which was judged the highest priority.
Proximity fuze work was regarded as urgent, and Section T grew rapidly. By early 1941, components and circuit elements were being shot out of guns to determine how well they would operate, with test shots of complete fuzes beginning in the spring. By summer, tests were being conducted to calibrate the sensitivity, and so the triggering radius of the fuze, and initial service tests were conducted on the brand-new cruiser USS CLEVELAND in Chesapeake Bay in mid-August 1942. The results were astonishing, with two target drones promptly shot down by the ship's 127 millimeter (5 inch) guns. The CLEVELAND then left port for overseas duty, making no stateside stops to ensure that the crew didn't have a chance to talk about what they had seen.
By September 1942, Eastman Kodak and component subcontractors like Sylvania were ramping up production. A batch of 5,000 early-production fuzes was sent to the Pacific theater in November 1942. The fuze was introduced to combat on 6 January 1943, when the cruiser USS HELENA used one to shoot down a Japanese aircraft.
* About 22 million proximity fuzes would be built to the end of the war. The British had been conducting their own proximity fuze development efforts at a low priority in parallel with the American work, but with fuzes pouring out of American factories, the British effort was given up.
Section T was transferred from the Carnegie Institution to control of Johns Hopkins University, where the group was given the vague name of "Applied Physics Laboratory (APL)". APL is still in existence, with a history of a wide range of different government projects, most significantly satellites and space probes.
The production proximity fuze was originally designated the "T3G Device" and then the "VT", which was suggested by the British, since it misleadingly implied "variable time" or "velocity triggered", which told snoops nothing. The fuze sent out a continuous radio signal in the range of 1.67 to 1.36 meters (180 to 220 MHz), and detonated when the shell got within a few wavelengths of a target. A backup self-destruct timer fuze destroyed the shell before it fell back to earth if it missed the target.
Building practical circuitry that could fit into an anti-aircraft shell and survive being shot out of a gun, with accelerations of thousands of gees and spinning at hundreds of revolutions per second, was a major engineering accomplishment, particularly in the days before solid-state electronics. A miniature ruggedized vacuum tube, the "T3", was developed by the Sylvania company and put into massive production. A particularly tricky issue was powering the proximity fuze, since conventional dry cells would drain away in storage.
The answer was to develop a battery that was inert until the shell was fired. The shock of firing broke a glass ampoule, flooding the electrodes with an acid electrolyte, which powered up the battery and activated the fuze. The battery only worked for two minutes, but that was well longer than the lifetime of the shell after firing. The fact that the battery wasn't active before firing also provided an arming mechanism, since the shell wouldn't be fully powered up until a tenth of a second after it was in flight, by which time it would be hundreds of meters away. The device was called a "reserve battery" even though it was the only battery in the fuze, the derivation apparently being from the fact that it was held in reserve until firing.
However, Tuve didn't rely on this feature as the primary arming mechanism since it wasn't entirely predictable. The fuze also featured an ingenious arming mechanism activated by the shell's spin. Instead of a complicated and bulky centrifugal clutch system, the fuze was fitted with a porous cylinder offset from the center, with the core of the cylinder filled with mercury. Under normal conditions, the mercury provided a conductive path that shorted out the fuze circuitry, but when the shell was set to spinning rapidly, the mercury leaked out through the porous material, opening the circuit. The charge time of the capacitors in the circuitry also provided a safety delay.
The proximity fuze was longer than the older timed fuzes and protruded into the interior of the shell, but the greater accuracy more than compensated for the reduction in explosive charge. Although proximity-fuzed shells tended to have a high rate of misfires, for example sometimes being set off by entering heavy cloud, they were still much more effective than timed shells. They were built in a wide range of mark numbers for different types of American and British guns -- the "Mark 53", for example, was for US Navy 127-millimeter (5 inch) guns.
* The proximity fuze project was top secret, with shipments protected by armed guards and the fuzes stored under lock and key. Even when the fuzes were deployed, they were at first restricted to naval forces in the Pacific, where it was unlikely that a dud shell would be recovered by the enemy. There were worries not only that the Axis might be able to duplicate the fuze, but could even generate countermeasures against it, running a sweep of radio waves through the fuze frequency range to set the shells off prematurely. This actually happened by accident in a few cases, when the fuzes were triggered by longwave radars that happened to be on their frequency.
In the summer of 1944, the Germans began firing their "V-1 flying bombs" at London. The V-1 was a small winged missile powered by a pulsejet engine that gave it a distinctive buzzing sound in flight. It flew at high speed on a straight and level trajectory, held on course by a gyroscopic guidance system. The flying bombs did terrible damage to London at first, but fighter and ground defenses were refined and slowly managed to pick off more and more of the bombs. The Americans released SCR-584 radars and M-9 fire control systems intended for the Continent to British 94-millimeter (3.7-inch) anti-aircraft gun batteries, and also set up similar batteries with their own 90-millimeter anti-aircraft guns. The batteries were set up in a screen along the English coast.
Churchill pleaded for the proximity fuze, pointing out that the shells would only fall in the English Channel or on English soil. He got his wish, and the combination of SCR-584, M-9 director, and proximity fuze proved to be the most effective countermeasure against the flying bombs. The straight and level path of the intruders made them relatively easy targets, and after a learning curve, fewer and fewer of the V-1s got through to London. In the end, statistics showed that it took 156 proximity-fuzed shells to kill a flying bomb, which may not sound good -- except in comparison with the 2,800 conventional anti-aircraft shells required to accomplish the same job. Incidentally, the proximity fuze had been designed to engage larger flying machines than the V-1, and so the fuzes supplies to the defenders were "recalibrated", following tests against a static V-1 model back in the US.
The TRE also developed a rangefinder to help fighters shoot down the flying bombs at night. It was a simple but clever device, developed on short notice. All it did was optically split the image of the bomb's orange jet exhaust and focus the images so they came together at a range of 180 meters (600 feet), providing the pilot with a glowing indicator that indicated he was in firing range.
The V-1 attacks ended as the Allies overran the launch sites in northern France. Although the Germans tried to continue attacks by air-launching the buzz bombs from Heinkel He 111 bombers, the effort proved expensive in men and airplanes; it was abandoned. Unfortunately, as the V-1 flying bomb threat faded out, the Germans began launching V-2 rockets that came hurtling down from space at over 4,800 KPH (3,000 MPH). The rockets came in fast, giving little warning, and were impossible to intercept.
* While the proximity fuze had been developed for anti-aircraft shells, of course Tuve and his people had always known it could be used for conventional artillery as well. A proximity fuze attached to ground bombardment artillery would allow the shells to burst in the air just before impact, showering the target area with fragments and leaving few places for victims to hide. Demonstrations of howitzer shells with proximity fuzes were performed to Army brass in September 1943. Although the demonstrations were characterized by a good deal of bungling, the Army was still impressed and wanted to get the proximity fuze into the hands of the field artillery as soon as possible.
Most of the field artillery used howitzers, which often used high-angle fire trajectories. That led to a problem in that sometimes small powder charges were used, resulting in low acceleration and spin that defeated the fuze arming mechanisms. Gun crews were told to use heavier powder charges with proximity-fuzed shells.
There was also the worry about the fuzes falling into enemy hands. In fact, the Germans were working on proximity fuzes themselves, mostly for rockets. One issue was that the V-2 missile tended to bury itself before detonating, reducing its effectiveness, and the Germans were also working on anti-aircraft missiles. They experimented with acoustic, optical, and radar proximity fuzes, but the effort was unfocused and went nowhere. An intact VT fuze might have helped them a great deal, but by late 1944 the Reich was obviously on its last legs. There was little chance that the Germans would have the resources or the time to duplicate proximity fuzes if they fell into their hands, and after strong lobbying by fuze advocates, their use was greatly expanded.
During the Battle of the Bulge in December 1944, proximity fuzes were installed on artillery shells for ground bombardment. A backup impact fuze detonated the shell if the proximity fuze failed. Proximity fuzed shells proved devastatingly effective, and shell-shocked German soldiers surrendered in large numbers. Allied brass worried enough about the possibility of the Germans copying the VT fuze to order the development of jamming systems that would cause proximity-fuzed shells to detonate prematurely. The jammers were built, but they were not needed. The Germans simply didn't have the time left to copy the fuze.
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