Ed. Note: Mr. Messner qualified in DIODON (SS349) and served from 1954-1957. He subsequently spent
30 years as a microwave engineer in the defense industry with companies such as Litton Industries and Boeing
May 1943, often referred to as Black May, is recognized by most historians as the turning point in the Battle of
the Atlantic. In fact, author Michael Gannon has written a book entitled Black Mav in which he documents events of that month and clearly shows the role of the U-boat changing from being that of the hunter to being that of the hunted. Records indicate 41 U-boats were lost in May. That’s over twice the previous monthly high of 18. Additionally, 37 more U-boats were damaged and had to return to base. Losses for the month exceeded the Gennan shipyards build rate and continued to do so. Clearly, the tide had turned and the U-boat no longer ruled supreme. Why? The answer to this simple question is complex. One has to look at a multitude of items as no single event, thing or happening can be cited as being responsible for the dramatic turn of events. To begin, the organizational commands were changing and maturing. The Tenth Fleet was officially created in May 1943 by CNO Admiral Ernest J. King- their mission- Anti Submarine Warfare or simply ASW. The Antisubmarine Warfare Operational
Research Group (ASWORG) had been established to enlist top civilians and scientists to do a think tank analysis of ASW techniques employing theory of probability, past data, strategic and tactical procedures. Also the Bay of Biscay offensive action plan, code named Operation Derange, was underway. New ASW platfonns were rapidly being deployed in the fleet. Among them were task groups with escort carriers (CVEs) and their air squadrons in concert with, new to the fleet, destroyer escorts (DEs). Their mission – protect the convoys and keep the Uboats at bay, i.e., sink ’em. Also modified B-24 Liberators (VLRs) for extra range to further close the Atlamic Air Gap were becoming available. New weaponry and operational techniques were introduced. Among them were the hedgehog forward launched hand grenade, straddle bombing of surfaced U-boats, deeper settings for depth charges (ash cans), and acoustic torpedoes that chase sound.
Acquisition systems continued to be introduced and improved. Centimetric radar was made possible by the invention of the resonant cavity magnetron. The Leigh Light and its 400 kilowatt light source took darkness out of the equation. High Frequency Direction Finding (HF/OF commonly called Huff Duff) continued to be a gift for the convoys as the U-boat Wolfpacks, under micro management of Admiral Donitz and Rear Admiral Godt, continued to ignore radio silence and expose their locations.Sonar, Asdic as the British called it, was standard equipment on most convoy duty ships by this time, and Ultra top secret message intercepts, of which there were hundreds, routinely took 2 days to decrypt but still kept convoys informed as to the whereabouts of the enemy wolfpacks. Both, long established anti U-boat stalwarts, they continued to play an integral role in the demise of the U-boat.
As Samuel Eliot Morison, author of the 15 volume edition entitled Hist01y of United States Naval Operations in World War II summarizes the situation in Volume X, Tlte Atlantic Battle Won, “Donitz, as a naval commander, had been overpowered by his enemies’ anti submarine forces, overwhelmed by their superior seamanship and tactics, out-improved by their new devices.” It can safely be stated that all of the above were not autonomous unto themselves. They each contributed to the mission of “protect the convoy- sink the U-boats” in their own special way,
but when used in conjunction with other advanced systems, they were far more powerful. The synergy of the complementary systems made the combination significantly greater than the sum of the parts.
A prime example of synergy is that of the new escort carrier (CVE) task groups formed within the newly created Tenth Fleet which had autonomous control over all ASW missions. For the first time a centralized command with the authority to set priorities and staff the missions with equipment and personnel appropriately. The convoy protection afforded by the CVE task groups with their squadrons of F4F Wildcat fighters and TBFffBM Avenger torpedo bombers, operating with a squadron of destroyer escorts (DEs), a new class of ship specifically designed for this mission, armed with forward launching hedgehogs was formidable. The ASW missions of task groups formed around CVEs BOGUE, CARD, CORE, CROATAN, BLOCK ISLAND and SANTEE speak for themselves in naval history.
A second example, which this paper will explore in detail in keeping with the theme of the title, is the combination of IO centimeter radar, the Leigh light, straddle bombing and the B-24 Liberator VLR aircraft. A look at each individually and then in combination follows.
Microwave Radar / Centimetric Radar/ 10 Centimeter Radar:
The acronym Radar is derived from its definition, Radio Detection ~nd Ranging. The term Microwave Radar simply identifies the approximate frequency range at which the radar is operating, and Centimetric Radar was a tenn coined by the British in WWII to differentiate a new short wavelength, top secret radar from those available earlier in the war. But before discussing the merits of this new short wavelength radar and its effect on hunting U-boats, a brief tutorial on some technical terms will be helpful.
Wavelength and Frequency
Wavelength and frequency of a radar signal are not mutually exclusive. In fact, they are directly related, albeit in an inverse manner. Simply stated, as one gets larger the other gets smaller and vice versa. The mathematical relationship is shown by the following:
A. = v I f where A. = wavelength of radar signal (meters)
v = velocity of light (or the radar signal) in free space
(300 x I 06 meters I second)
f = frequency of radar signal (Hertz)
and an abbreviated conversion table shows:
|Frequency (MHz)||Wavelength (centimeters)|
The operating frequency, and thus the wavelength of the signal, is an important design consideration of any radar system. Low frequency radar signals with tonger wavelengths tend to bend with the curvature of the earth thus providing an over the horizon capability. Higher frequency radar signals with shorter wavelengths give better resolution, e.g., fire control radar, but are more line of sight transmissions and are lost in the ionosphere more quickly, i.e., shorter range.
For the purpose of this paper, low frequency radars are in the frequency range between l 00 and 1000 MHz (300 cm to 30 cm). Within this range, a curious transition of how the physical characteristics of component electronic parts is realized occurs. Whereas below approximately I 00 MHz standard resistors, capacitors and inductors, or lumped components including vacuum tubes can be used. (Note: it wasn’t until the early 1960s that transistors started to replace the vacuum tube.) Above 1000 MHz these components are physically realized in a distributed fonn due to the effect of component stray capacitances and inductances, and wavelength now becomes a major design consideration. Between 100 and 1000 MHz is the transition zone and presents the circuit designer with a significant technical challenge. This is mentioned lest the reader feel that extending the design of a radar to higher frequencies was a straight forward process. lt wasn’t. Remembering that frequency and wavelength are inversely proportional, the microwave engineer uses this to advantage for certain system components, the antenna being the most obvious. Antenna design for transmitter antennae has always been wavelength related, e.g., quarter wave, half wave, etc. It is quite critical and requires tuning to match the transmitters output to the antenna. Receiver antennae are more forgiving but follow the same general rules.
The first radars were in the VHF range (30 MHz to 300 MHz) and their antennae were huge because the wavelengths were between I 0 meters at 30 MHz and 1 meter at 300 MHz – not convenient for aircraft mounted radar. Centimetric radars are much higher in frequency, in the 3000 to 10000 MHz range and their corresponding wavelengths are 10 cm and 3 cm respectively. One can readily see that airborne centimetric radars take advantage of the shorter wavelengths for smaller, easier to mount antennae.
Pulse width (PW) is the duration, measured in time, of a single pulse emitted from a radar. This is where the transmitted power is packed, and until the invention of the cavity magnetron, radars were restricted to frequencies below 300 MHz as electron tubes couldn’t handle the necessary power to achieve an effective radar range at higher frequencies.
Short pulses provide better resolution as long pulses tend to smear the target. However infinitely short pulses can’t store the peak transmitted power so a compromise must be made. Pulse widths in the order of I or 2 µsec (microseconds) are common. A time domain analysis of the pulse would show a signal within the pulse resonating at the transmitted frequency.
Pulse Repetition Frequency
Pulse repetition frequency (PRF) is the number of pulses transmitted per second. A low PRF is necessary for long range radars, i.e., adequate time must be allowed for the pulse to travel to the target and return before another pulse is transmitted. High PRFs are for short range radars such as fire control radars. PRFs of 500 are common for normal search radars.
Theoretical maximum range as determined by the PRF is only a number and seldom a design criteria. The actual or maximum working range or useful range is determined by the radar’s power output, its frequency or wavelength, the curvature of the earth, the height of the transmitter’s antenna, the size and altitude of the target and lastly atmospheric conditions which in themselves arc not always predictable, e.g., solar activity, atmospheric attenuation and rain squalls.
Peak Power & Average Power
As previously mentioned, transmitted power is the power packed into the transmitted pulse which has a finite width, e.g. 1 or 2 µseconds. Peak power is simply the power generated and transmitted during the time of the pulse. Average power is the peak power multiplied by the ratio of pulse on time to pulse off time. For example, if the radar has a PW of 1 microsecond and a PRF of 500, the on time is 1 micro second and the off time is the time between pulses as determined by the PRF. In this case the off time is 2 milliseconds, i.e., every 2 milliseconds a pulse is transmitted which equates to 500 pulses per second, the PRF. In this example the ratio is 1/2000. This figure, at best, is a figure of merit number as it is driven by other specifications. Prior to the invention of the cavity magnetron, peak powers in the kilowatt range necessary for radar could only be generated at frequencies below 300MHz (100 centimeters) due to the limitations of the electron tubes available. In essence, the cavity magnetron replaced the high power vacuum tubes and allowed kilowatts of power to be generated in a pulse at centimeter wavelengths, a quantum leap in technology, allowing for centimetric radars.
British Airborne Radars:
Radar technology was co-invented in 1934-35 by British and American engineers. Robert Watson Watt, often called the father of radar, was British and was attached to the National Physical Laboratory in Berkshire, and three American engineers, Leo Young, known personally by the author, Robert Morris Page and Albert Taylor who were attached to the Naval Research Laboratory at Anacostia, Washington D.C. are generally given this credit.
But for the purpose of this mission, i.e., meeting the U-boat threat, the British contribution is more significant as will be shown. The first British airborne radar was flown on 17 August 193 7 in an Avro Anson aircraft. It generated 100 Watts of power at a wavelength of 1.25 meters (240 MHz) and, although crude, it demonstrated in sea trials with the aircraft carrier HMS Courageous and battleship HMS Rodney it was capable of tracking targets in adverse weather conditions. Crude is synonymous with prototype or breadboard- sometimes called a laboratory curiosity. But in this text, crude is an adequate description of the most challenging part of the radar, the antenna system.
ASV Mk I
For the next 3 years, improvements were made, and by the end of 1940 it was nomenclatured as the ASV (Air to Surface Vessel) Mark I radar and installed on a couple dozen Hudson light bombers and a like number of Sunderland amphibious patrol bombers. Although not designed specifically to hunt submarines, early tests showed a submarine could be picked up at 3 to 6 miles depending on the altitude of the aircraft, e.g., 1000 to 6000 feet. Further modifications, including a new antenna array, improved the range to 10 to 15 miles.
ASV Mk II
The second generation airborne radar, designated ASV Mk II, was a re-engineered Mk I designed for mass production. It was first flown in August 1940, but not until March 1941 was it flown for ASW missions as Bomber Command had higher priority. It operated on 1. 7 meters ( 176 MHz), had a peak output power of 7 .5 KW with a 2.5 microsecond PW and a PRF of 400. It had an effective U-boat detection range of up to 36 miles, but could pick up bigger targets at twice the distance. Two versions of the Mark II were manufactured, a forward looking and a side looking version, the difference being the antenna system. The side looking version proved best for anti-submarine warfare, and several thousand of these were manufactured and installed on various Coastal Command aircraft including Wellingtons, Sunderlands, Hudsons, Whitleys, Catalinas and, the real work horse, B-24 Liberators.
By mid 1941, the ASV Mk II was accounting for a marked increase in attacks on surfaced U-boats. The typical approach would be a radar run to within a mile or two and then visual for the bombing run (straddle bombing was soon found to be very effective). This proved quite effective for the daylight hours, but night runs were a problem because radar clutter, or sea return as it is often called, made the target obscure at ranges under a mile. The
cause of clutter is seldom discussed, but it is a natural phenomenon in any radar. It occurs during the PW transmit time when the receiver theoretically is desensitized or blanked. Because the desensitization or blanking process is not perfect, some of the transmitted pulse leaks into the receiver causing the perceived
clutter on the screen. On a PPI (Planned Position Indicator) scope, it looks like one huge, solid contact 360 degrees in azimuth stretching out for a mile or more. Something other than electronics would have to be found to solve this problem, and solve it they did. The installation of the Leigh Light, a topic to be covered in a following section, would overcome the problem by June 1942.
The next generation airborne radar was nomenclatured as the ASV Mk III. It was made possible by a quantum leap in
technology. Some WWII historians rate this as the most significant technological advance during all of WWII just short of the development of the atomic bomb, and this leap was made possible by the invention of the resonant cavity magnetron by two British physicists from the University of Birmingham, John Randall and Henry Boot. This invention allowed the technologists to move the radar transmit frequency from the 200 MHZ range (wavelength in
meters) to the 3000 MHZ range (wavelength in centimeters). Thus the coined word centimetric radar- S band as it was known in the U.S. Other benefits of the resonant cavity magnetron were, unlike the klystron, its ability to produce high power in a fairly narrow beam width which in itself reduced the close in clutter on a PPI display and increased the useful minimum or close in range. Also, a major advantage for the airborne version of centimetric radar was the relatively small size of the parabolic antenna in comparison with the 1.5 meter radar’s clumsy antenna. This made for a comparative easy installation on the aircraft.
The first naval radar employing the use of the cavity magnetron valve, as the British called it, was the shipboard Type 271 radar. The 271 operated at 9. 7 cm (3100 MHz), had a peak power of 70KW, a 1.5 µsec PW, and a PRF of 500. This equated to a range of 25 Km at sea level but in reality for acquisition and tracking of U-boats it was in the 3 to 5 Km range. It worked outstandingly well in the fog and at night- a real plus for the ASW team. By May of 1942, the Type 271 was on over 200 Royal Navy ships of all kinds. A further plus for the Allies ASW teams
was that the German radar warning receivers (RWR) were blind to centimetric radar until November 1943 when the 3rd generation RWR, Naxos, was configured on the U-boats. However, it wasn’t until February 1943 that centimetric radar
was adapted for ASW airborne use. The main reason for this delay wasn’t so much technological problems, but more one of priority. RAF Coastal Command, responsible for ASW, took second priority to RAF Bomber Command who lobbied intensively for the resources to manufacture the l 0 centimeter H2S (sometimes written as the chemical symbol for Hydrogen Sulfide, H2S) terrain mapping radar. Bomber Command received the top resources mainly because the H2S was close to production and 24 bombers, Halifaxes and Stirlings, were outfitted with the H2S by the end of 1942.
The new airborne centimetric radar was nomenclatured as the ASV Mk Ill and had the following characteristics: wavelength I frequency, 10 cm I 3000 MHz; peak power, 50 KW; PW, l µsecond; PRF, 750; effective maximum range, 160 Km depending on altitude; and effective range for U-boat detection, 10 to 16 Km. It was configured on reconnaissance bombers such as Wellingtons, Catalinas, Halifaxes, Sunderlands and 8-24 Liberators.
German Radar Warning Receivers:
To fully understand the impact radar had on the demise of the U-boat, one must be aware of the electronic counter measures the U-boat fleet had at its disposal. In today’s parlance, they would be ECM/ESM systems. From late 1942 through the end of the war, the U-boats had some form of passive radio/radar detector on board. Three generations are discussed below, the stories of which are equally as fascinating as that of centimetric radar.
I generation radar warning set – Metox
The need for some type of radar warning system was established as early as February 1942 by U-331 ‘s commander while operating in the Mediterranean. U-boat loses to aircraft were escalating and the German’s rightly suspected that radar equipped aircraft were the cause. Not only were the British using radar, but the Leigh Light was deployed on a squadron of Wellingtons in April 1942 to aid night, radar-guided attacks.
In August 1942, U-boats U-69, U-107 & U-214 were outfitted with a prototype radar warning system which, when a signal was intercepted within its frequency range, sounded an audible tone, the louder the tone, the closer the contact. Except for the performance of a clumsy, make-shift antenna, satisfactory results were reported. Donitz then ordered all U-boats to be outfitted with this I st generation radar warning system, FuMB-1 (Funk Mess –
Beobachtung- gerat which translates as a passive radio/radar detector system). This was essentially accomplished by year’s end. The receiver, the Metox R-600, was designed and produced by a French firm of the same name. It was gratuitously offered to Admiral Donitz by the French Admiral Darlan and was used to receive signals in the 1.25 to 2.5 meter band ( 120 – 240 MHz). The design was made possible by back engineering a captured ASV Mark 11 radar set which was recovered from a downed aircraft in Tunisia. It was capable of detecting many of the Allied radars which operated in the 1.4 to 1.5 meter band ( 200 – 215 MHz) as well the British airborne ASV Mark II air to surface radar operating at 176 MHZ and the British ship borne type 286 radar operating at 214 MHz. It had an effective range between 10 and 50 Km depending on the altitude of the radar, e.g., surface ship or aircraft.
The clumsy antenna for the Metox system was dubbed Biscay Cross (Biskayakreuz), named as such as the system was used primarily when the U-boat was traversing the Bay of Biscay en route or returning from patrol (see more about the Bay of Biscay in the following section titled Leigh Light). The antenna was not a factory design but rather a jury rigged fleet design. It was rushed into service as an intermediate fix until a more permanent design and installation could be installed by the shipyards. This reflected the urgent need as too many U-boats were being caught on the surface without warning while crossing the bay. (Note: There is some confusion about the Biscay
Cross. Some credible authors identify it as the U-boat ‘s J” generation radar Warning receiver in its OWIJ right.
This is in error as ii is simply the antenna which is cabled to the Metox receiver to complete the system.) The Biscay Cross antenna consisted of two pieces of lumber shaped like a cross to support the antenna wires. The transmission line came up through the open conning tower hatch which didn’t please anyone. It literally had to be brought topside in a disassem-bled state, assembled and then rotated by hand. In the event of a crash dive it was quickly disassembled and tossed down the hatch.
Because of the time required to disassemble it, and the cable running through the open hatch, many U-boat commanders disassembled it shortly after Metox gave the first alarm of a contact as more often than not, the contact was an aircraft and the commander didn’t want to jeopardize his boat for the sake of a cable preventing the upper hatch from closing.
Padfield in his book Donitz, the Last Fiihrer relates a story about a captured English pilot who told his interrogators that the RAF hardly ever used their radar in ASW work since Metox radiated spurious signals which could be detected up to 90 miles, and they simply homed in on the beacon. The German’s realized the story could be a deliberate deception, but they couldn’t risk the chance it was true, and in August 1943 Donitz ordered the use of Metox discontinued, one year after its introduction. Further, this reason seemed to be a logical explanation for many of the uncanny mysteries such as missed convoys and the escalating rate of losing U-boats since February 1943. In reality this was true, but the ruse was used to hide the fact that the Allies were using centimetric radar ( 10 cm band) which Metox could not detect.
In any event, it wasn’t long before U-boat commanders suspected that the Allies had radar outside the Metox frequency range. They were experiencing far too many surprises by Allied aircraft. This was confirmed when on 2 February 1943 an RAF Stirling bomber was shot down near Rotterdam. It was equipped with the 9.7 cm radar which German technicians reconstructed and discovered the magnetron valve which made centimetric radar possible.
Centimetric radar along with the Metox radiation problem and the Biscay Cross antenna deficiencies led to the 2nd generation radar warning system. 2nd generation radar warning set – Wantz (Wanze) or Hagenuk Officially nomenclatured as FuMB-9, the 2nd generation radar warning receiver used on U-boats overcame two of the three short comings of Metox, self radiation and the clumsy antenna.
Reception of centimetric radar would not be addressed until the 3nl generation. The Wantz system, introduced around August 1943 at the same time Metox was discontinued, was designed to receive signals in the 1.2 to 1.8 meter band (166 to 250 MHZ). Although a more narrow band than Metox, it still could intercept the Allied radars which operated in the 1.4 to 1.5 band and the British airborne ASV Mark II radar. The design of a system to capture centimetric radar, which was being widely used, was still in the R&D stage. Logically it can be assumed that Wantz was only intended to be a stop gap measure until a design capable of receiving centimetric signals would be available.
Developed by Hagenuk, a Gennan electronics company, Wantz did solve the antenna problem, and it improved, but didn’t eliminate, the self radiation phenomenon. The antenna called Runddipol was a round dipole type pennanently mounted to the superstructure with cable assemblies running through the pressure hull so it didn’t have to be disassembled prior to diving. A drawback to dipole antennas is their directivity which translates as their inability to accurately report bearing information unless rotated. The round dipole is a crude antenna array used to circumvent this problem. Very little in the literature discusses the effectiveness of this antenna as it wasn’t in use for more than a few months.
The second problem, that of self radiation, technically is in theory easy to solve but not completely eliminate. It is reasonable to assume that Metox was a regenerative receiver. This is based on how quickly the unit was produced and sent to the fleet and some of its performance characteristics, namely poor sensitivity which translates as short range- less than ten miles or about line of sight. The regenerative receiver is a simple design, more complex than the simple crystal set, but offering some selectivity (tuning range) and amplification using a minimum of parts. Hence, it could be built and tested rapidly. The down side, although not considered serious at first, was that its internal heterodyne oscillator signal radiated in the reverse direction through the antenna. The simple receiver of this type provides almost no reverse attenuation of this signal thereby becoming a small transmitter in its own right.
The Wantz most probably was a superheterodyne receiver for two reasons- it took longer to design and put into production as it is more complex, and its self radiation was significantly less than its predecessor. The superheterodyne receiver accomplishes the reduced self radiation by adding a tuned RF (radio frequency) front end section to the receiver. This front end, or first stage of the receiver, consists typically of an RF amplifier stage and a tuned circuit. The combination allows reception of the desired signals, with some amplification (gain), and provides greater than 20 dB reverse attenuation to the internal local oscillator signal – the culprit signal. In other words, in comparison, the superheterodyne’ s radiated signal is minus 20 dB or I/ 100 that of the Metox signal- a real significant improvement.
Because of the RF gain, Wantz had greater sensitivity than Metox which resulted in greater range – approx 50 – 100 Km all else being equal. However, Wantz was still ineffective as a warning receiver for IO cm (centimetric) radar which the British were using, and after too many Wantz configured U-boats were caught on the surface by Allied aircraft, its use was discontinued in November 1943 in favor of the next generation radar warning receiver.
3rd generation radar warning set – Naxos
There were attempts to improve Wantz, namely Wantz G2 and Borkum, but neither of these sets were capable of receiving 10 centimeter (cm) radar and their active duty time was just a stop gap measure. The true 3n1 generation radar warning receiver had to be capable of intercepting 10 cm radar.
As previously mentioned, an RAF bomber, a Stirling- four engine heavy bomber, was outfitted with a 9.7 cm radar, the British airborne H2S. It was sent on a covert mission over occupied Rotterdam on 2 February 1943 to determine whether the radar could clearly differentiate the city from the surrounding landscape. As fate would have it, the Stirling was shot down by the Germans and was not damaged sufficiently to effectively destroy the radar. The salvaged equipment was recognized as non standard equipment by the Germans and was dubbed the Rotterdam Gerat (apparatus). It was sent to the laboratory for evaluation where, through reconstruction, the Germans discovered it was a cavity magnetron radar and thus confirmed suspicions that the British had a radar outside the frequency limits of their current radar warning system, i.e., Metox.
As a result of this discovery, AEG Telefunken was tasked with the challenge to design a receiver capable of intercepting 10 cm signals. This was no simple task as vacuum tubes of the time couldn’t amplify 3000 MHz signals, and a method of heterodyning the incoming signal to a lower frequency where amplification could be achieved had to be used. Telefunken’s solution was to use a germanium point contact diode in the front end of the receiver to perform this heterodyning function. Design wise, at the time, it was probably the only viable solution, but every design solution has its compromises, and this one had two serious drawbacks. A germanium point contact diode is very fragile and strong signals will blow the diode much like a fuse-thus rendering the equipment useless. Also, without any preamplification, the ambient noise threshold of the diode is high resulting in poor sensitivity, i.e., detection range. The system also had growing pains with its new antennae much like the Metox system. Be that as it may, the resultant was a system capable of detecting 8 to 12 cm (2500 – 3750 MHz) signals with a detection range of 5 – 8 kilometers and was nomenclatured as FuMB-7, Naxos.
In spite of the importance of developing a I 0 cm radar warning system for the U-boats, first priority for the Naxos system was the Luftwaffe (I guess General Goring hollered louder than Admiral Donitz, as it was no secret the two didn’t see eye to eye). The Luftwaffe started flight tests with Naxos in September 1943 with the U-boat following the cancellation of Wantz later that year. In early 1944, Naxos was being installed on U-boats and the German Admiralty must have thought that all was well. The irony here is that as soon as the Germans could intercept 10 cm radar, the Allies deployed 3 cm (10,000 MHz) radar and held the advantage through the end of the war in 1945.
(Note: US submarines had the SV radar which was a 3 cm radar followed by the SS/ST radar which was a 3 cm radar.)
The Leigh Light was a British invention installed on Wellington bombers outfitted for ASW night missions during the Battle of the Atlantic in WWII. It was created out of necessity to improve the ratio of kills to sightings of U-boats during these night missions. It solved the close in radar interference problem of surface clutter allowing the pilot to switch from a radar guided approach to a visual approach for the final run.
It was well known that the U-boat Command established five submarine bases on French soil shortly after the fall of Paris and France in June 1940. These bases, located at Lorient, Saint Nazaire, Bordeaux, Brest and La Rochelle/La Pallice, were all on the Bay of Biscay which substantially forms the western most coast of France with direct access to the Atlantic. From these bases, the U-boats had to traverse the bay both departing for and returning from patrol, a distance of between I 00 to 400 mites depending on the base. It provided a much shorter run to the patrol area compared to leaving Kiel and exiting into the North Atlantic via the North Sea- thus maximizing time on station. Four of the bases were operational by the end of 1941 with the last, Bordeaux, following a year later.
This, then, was a natural place for Allied planes to stalk the enemy, and so they did. However, shortly after the stalking began, Donitz ordered the U-boats to cross the bay at night on the surface and submerge during daylight hours seeking the protection of darkness from the predators. This scheme worked for awhile, but soon Allied planes were equipped with radar and they began to harass the U-boats with various degrees of success. Radar worked well in the daylight because the final run was visual and the pilot could time his drop accordingly. But it worked only to a degree at night as the contact was lost in the radar’s clutter during the closein final approach. Unfortunately, all radars experience this clutter inconvenience in some form as previously discussed under ASV Mk II airborne radars.
Various schemes of lighting up the air were tried. First it was flares, but flares only illuminated an area in close proximity to the aircraft. Multiple flares were tried in a succession of drops until the U-boat was sighted, and then a fly around was executed for the line-up and bomb run. Often by this time the U-boat had submerged and the bomb run was ineffective. Time delayed flares were tried. These flares were fired from a buoy previously released from an aircraft which by now had circled around and was lining up for the kill. Again the U-boat often had sufficient time to pull the plug and avoid danger. A better solution was still needed.
Enter Squadron Commander Humphrey De Verde Leigh, a WWI RAF pilot. Aware of the problem, he designed and built the prototype model of what was to become the Leigh Light. It was a huge 24 inch (610 centimeter) diameter carbon-arc spot light which was rigged to fit in the under belly of a Vickers Wellington medium range bomber. Its lumination was rated at 22 million candela (see note below), powered by rechargeable batteries and controlled from the front turret of the aircraft. It was rotatable in azimuth and elevation meaning the aircraft didn’t have to bore sight on the target – a real advantage. Another advantage was that it didn’t exhibit back glare or dazzle to the benefit of the air crew.
In April 1942, RAF Squadron 172 flying Wellington VIIIs was outfitted with the Leigh Light. They became operational, literally, on 4 June 1942 when a target, the Italian submarine Luigi Torelli, was detected as it was crossing the Bay of Biscay having sailed from La Pallice out-bound on patrol. The Leigh Light was switched on and the Wellington dropped two braces of bombs seriously damaging the sub but not sinking it. The Leigh Light had operated as advertised and now the die had been cast. The following month on 5 July, U-502 was to become the first confirmed kill of a U-boat using the Leigh Light in the Bay of Biscay. From then on the German’s referred to the Leigh Light as das verdammte Licht.
Leigh Lights were not successfully fitted to the Halifax heavy bomber due to mechanical interference of the bomb bay doors and were not considered for the Sunderland amphibious patrol bomber.
However, they were successfully fitted under the wing of the Consolidated B-24 Liberator long range bombers as well as Wellingtons and Catalinas. They were in service to the end of the war.
(Note: 22 million candela is the most agreed upon published number. Other ratings are 80 million candlepower and 50 million candles. A candela is a standard unit for measuring light adopted in I 948. It equates to approximately 18. 4 milliwatts per steradian, a spherical measurement which bajjles most of us (It is analogous to antenna theory which most engineers don ‘t understand either). My best translation, and I am an electrical engineer, is that it is (22 x 106 ) x (18.4 x /0’ 1 ) Watts or approximately 400KW radiating from a point inside a sphere equally in all directions. Now put a reflector behind it to focus it and radiate in one direction and the result is one hell of a blinding, bright light.)
Straddle bombing of U-boats was reported as early as 27 August 1941 when an RAF Hudson, while on routine patrol about 80 miles south of Iceland, sighted a U-boat surfacing about 1200 yards distant. It immediately dropped to an altitude of I 00 feet, commenced an attack and released 4 – 250 pound depth charges set at 50 feet. The stick of four straddled the submarine while in the act of diving, and the resultant explosions caused sufficient damage to its water tight integrity forcing it to surface. Shortly the crew waved a white flag and the Hudson called for naval surface patrol craft support. When support arrived, the sea state was such that a boarding party could not be launched until the following afternoon. At such time U-570 was boarded and towed into port.
Score: RAF 1 – U-boat 0.
By Black May, straddle bombing had become the preferred technique for attacking surfaced U-boats. A preferred scenario after sighting the U-boat would be for the pilot to line up with the U-boat’s track, either up or down, drop to an altitude of about 50 feet off the deck and engage the intervalometer, an electromechanical device that enabled a stick of depth charges (or bombs) to be dropped at specified intervals or spacings-typically 40 to 60 feet. The depth charges were mounted, port and starboard, under the wings of the aircraft with their fuses set to ignite at a shallow depth of about 25 feet. The intervalometer was activated at the optimum release point, the intention being at least one depth-charge fell near enough to cause catastrophic damage. Terence Bulloch, the most decorated pilot in Coastal Command, preferred to line up at an angle 20° off of track, but depending on circumstances, attacks have been reported from all angles of the compass.
A text book example of a well laid pattern of 250 pound depth charges or bombs would consist of a brace dropped on each of the port and starboard sides set to explode at 25 feet. The explosions would crush the outer tanks of the submarine destroying the saddle tanks or literally blow the U-boat out of the water causing the keel to fracture. Both scenarios rendered the submarine incapable of diving making it an easy prey for a followup attack. Often the damage was serious enough that a second attack was not necessary as the initial damage caused the sub to surrender on the surface or seek the depths of Davy Jones’ Locker – forever.
B-24 VLR Liberator:
How apt the B-24 was given the name Liberator. Most of the first production run of B-24s went to Britain’s RAF (ca 1941) who nomenclatured it as the Liberator. The name stuck and was adopted by subsequent users including the US Army Air Corp, RCAF (Canadian), RAAF (Australian) and the US Navy which officially called it a PB4Y-1 instead of a B-24 as the Navy had their own nomenclature system and weren’t about to adopt the Army’s. And liberate it did. It was the venerable workhorse of WWII in all theaters. Its primary design mission may have been that of a 4 engine multipurpose heavy bomber, but as a multipurpose aircraft, it was assigned a plethora of missions including maritime patrol, anti-submarine patrol, reconnaissance, tanker, cargo hauler,
and personnel transport.
The beginnings of the B-24 date back to 1938 when Consolidated Aircraft was requested by the US Army Air Corp to produce B-l 7s under license to Boeing Aircraft. This was part of a government program to expand American industrial capacity for production of critical items as the hand writing was on the wall with regard to war in Europe and war in the Pacific. Of interest to submariners, a similar program was established between Manitowoc Shipbuilding Company and Electric Boat whereby Manitowoc would, under license to EB, use EB’s design for the modem Gato class fleet boat and provide submarines to the US Navy. (Note: Under 2 colllracts, Manitowoc delivered 28 of the finest Gato and, later, Balao class boats to the USN in an extremely successful program.) However, unlike Manitowoc, Consolidated decided not to build B-17s under license but instead to submit a more modem design of its own. The Army Air Corp then asked Consolidated to submit a design study for an aircraft with greater range, higher speed and greater ceiling than the B-17.
Thus, the beginning of the B-24. The contract for a prototype was awarded in March 1939 and the aircraft was delivered before the end of the year. Flight tests were successful and 7 more development aircraft flew in 1940.
Consolidated then began ramping up for production with orders from Army Air Corp (36), RAF (164) and France (120). Most of the early deliveries, including the 120 for France, who by this time had capitulated to the Germans, went to the RAF who immediately assigned a portion of them to Coastal Command for use on antisubmarine patrols in the Battle of the Atlantic – a fortuitous move. Bomber Command and BOAC, a passenger/transport company, received the balance.
An early variant of the B-24, known as the VLR or Very Long Range Liberator, became available early in March 1941. Much of its thick armor plating and some heavy turrets were removed to reduce weight and allow for extra fuel tanks thereby extending its range. This was a costly mistake for those used for bombing runs over Germany but a great benefit for Coastal Command’s use as reconnaissance aircraft. The British nomenclatured them as Liberators GR-Is. A later version, Liberator Ils, available late in 1941, introduced self sealing fuel tanks and powered gun turrets.
Prior to mid 1941, air cover for convoys in the mid Atlantic was very limited, but by July of that year, the air cover improved as VLR Liberators were assigned in to Coastal Command’s Squadron 120 based in Iceland for use on ASW patrols. Prior to this time, air coverage by land based aircraft varied from 400 to 700 miles vectoring east of Newfoundland, south of Iceland and west of Ireland – distances were a function of assigned aircraft and time on station. This left a 300 to 400 mile gap in the mid-Atlantic outside the range of land based aircraft where U-boats could and did roam at will. This gap became known as the Atlantic Air Gap or simply the Gap. The VLR Liberators mission was to close this gap.
For over a year, the VLR Liberators of Squadron 120 did a yeoman’s job protecting convoys transiting the gap as they were the only aircraft with the range to close the gap. The major problem was there were too few of them. It wasn’t until the Atlantic Conference held in Washington DC in March 1943 relief was provided by supplying the RCAF with additional VLR Liberators- a direct result of the number one priority of the conference being the defeat of the U-boat. By May, Black May as stated previously, these Liberators were on assignment and the gap was essentially closed. Shortly thereafter Donitz withdrew his Uboats from the mid Atlantic and sent them to greener pastures in the Indian Ocean and South Atlantic. Donitz’ strategy of just sinking enemy ships, regardless of the type, still was foremost in his mind.
Black May indeed was the beginning of the end of the Uboat’s reign in the Battle of the Atlantic and elsewhere. The VLR B-24 had succeeded in closing the Atlantic Air Gap. Outfitted with 10 centimeter radar, the Leigh Light and employing the straddle bombing technique, as discussed above, it became a most fonnidable ASW weapon system.
Another equally fonnidable weapon system was the escort carrier (CVE) task groups anned with their air wings and the new Mark XXIV acoustic airborne homing torpedo, Additionally the task groups squadron of destroyer escorts (DE), anned with hedge hogs proven to be 50% more efficient than standard depth charges, became the nemesis of
the U-boat. Advances in sonar, sonobuoy and MAD gear technology along with maturation of the Huff Duff triangulation system also came to fruition in May 1943. Factor in the success of breaking the Enigma code (Ultra) and using the information discreetly only added fuel to the fire to make life untenable for the U-boat.
The loss rate of U-boats throughout the rest of the war averaged over 20 per month. In 1943 the losses were 238 U-boats, and in 1944 the number was 245 Jost boats followed by 160 additional in 1945. The best documented numbers available show that 1160 U-boats were built and delivered. Of these 796 were sunk between 1939 and 1945. Additionally 203 were scuttled at wars end and 161 surrendered.
Grand Admiral Donitz knew that he was sending his submarine crews on suicide missions where maybe, at best, 20% would return. This must have pained “Onkel Karl”, as Donitz was affectionately called, to no end, but Germany had made a conscious decision to keep the U-boats at sea thereby tying down Allied resources such as the task groups and VLR squadrons lest they be used elsewhere.
Finally, not enough can be said about “that mysterious group of civilian scientists and university professors” called ASWORG. Founded in the spring of 1942, it was an Operations Research group chartered to do think tank analyses of ASW situations and submit recommendations on how to be more efficient and productive. Also, they recommended new/improved weapon systems, and the university laboratories helped develop them. Among the most famous was the Radiation Laboratories (Rad Lab) out of Massachusetts Institute of Technology where they
perfected radar. When Germany realized they needed a scientific organization such as ASWORG, it was two years later, and to their dismay, they realized most of their scientists and engineers were Jewish and had been interned in concentration camps- Hitler had shot himself in the foot again, but that is another equally fascinating story.