Airborne Anti Submarine Warfare
From the First World War to the Present Day
This week’s excerpt comes from a new study (2022) of the history and practice of airborne Anti Submarine Warfare (ASW). Michael E Glynn is a veteran US Navy ASW pilot and instructor - he set out to write a guide to the ‘simple, enduring fundamentals of airborne ASW flying’ that would serve as an introduction to those new to the field. In doing so he traces the history and evolution of ASW technologies and tactics.
This is an excellent introduction for the general reader. The author covers many technological aspects including electronic sensors, oceanography and ‘sound propagation analysis’ but with a highly readable approach. The seeds of many of todays ASW techniques were sown during World War II - for the full context of how these developed into a specialised branch of modern warfare there can be no better guide.
The following excerpt is part of a general overview of developments in World War II:
The entry of the United States into the war following the attacks on Pearl Harbor opened the Western Atlantic to submarine operations. The Germans immediately dispatched long-range submarines to the Gulf of Mexico, the Caribbean, and the waters offthe eastern seaboard of the United States.
The U.S. Navy and Army Air Corps were completely unprepared for ASW operations. There were very few surface escorts, less than twenty ASW aircraft, and no convoy system for merchant ships in coastal waters. In addition, convoluted C2 [Command and Control], a complete lack of standard ASW tactics, and non existent civil defense measures allowed the U-boats to enjoy a very permissive tactical environment. German crews turned the American littoral into a killing field, as hundreds of ships were sunk in the first few months of 1942.
Alarmed at the extremely high number of sinkings off the eastern seaboard, British ASW experts traveled to the United States in the spring of 1942 to meet with their American counterparts. While initially met with skepticism and a measure ofhostility, the British officials were able to convince their allies to adopt a coastal convoy system and overhaul their C2.
These efforts resulted in the U.S. Navy standing up the ASW Operations Group (ASWORG), which mirrored the highly effective operations analysis research work on ASW being done by the British.
The ASWORG recruited extensively from academia and conducted pioneering work in the discipline ofoperations analysis.
As American ASW efforts improved in the Western Atlantic, German submarines withdrew to the waters east of the Canadian Maritime Provinces and south of Iceland, exploiting the remaining gaps in Allied air coverage. During this period, the Allies remained focused on the strategic bombing campaign in Europe. As a result, the vast majority of large aircraft that rolled off production lines were assigned to strategic bombing units, rather than the maritime patrol squadrons that desperately needed long-range ASW aircraft to provide air coverage at all points along the convoy routes.
This portion of the war witnessed important advances in sensor and weapons technology. The first airborne radar sets flew successfully, allowing ASW aircraft to detect surfaced submarines beyond the visual range of observers, at night, and in poor weather. Coastal Command fielded a powerful searchlight known as the “Leigh Light” that could be slewed to radar contacts and allow night attacks against U-boats.
Magnetic anomaly detection (MAD) equipment was introduced aboard ASW aircraft and airships. To help airships discriminate between naturally-occurring magnetic disturbances and actual submarines, engineers developed expendable hydrophones that could float in the ocean and transmit signals to an aircraft overhead. These first sonobuoys were eventually introduced in fixed-wing aircraft as well. Last, American scientists developed and deployed the first airborne LWT, which could be dropped from an aircraft and independently home on a submerged submarine using passive sonar.
‘The tactical environment had become so deadly for the German submarine force that more submarines were being sunk than were merchant ships.’
The introduction of airborne ASW radars launched a technical race between the Allied and Nazi scientists. German commanders were initially unsure of what was causing an increase in the number of nighttime attacks on U-boats until a British ASW aircraft equipped with radar crashed in France. This clue spurred the Germans to quickly design and deploy a radar warning receiver (RWR). By 1942, these RWR sets were deployed aboard many U-boats, allowing submarine crews to detect an approaching aircraft and submerge before the aircraft could close the range and attack.
Frustrated that airborne radars were becoming less effective and that aircrews were recording fewer sinkings, the Allies deployed new radar equipment that operated at a higher frequency and with a shorter wavelength. This in turn drove the Germans to deploy new RWRs tuned to detect the higher frequency signals. The Allies also began operating dedicated escort carriers, which were equipped with air wings specialized for ASW patrol flights.
In addition to convoy escorts, these carriers were employed very effectively in offensive ASW operations, hunting down and destroying high-value targets such as German tanker submarines.
These operations marked the first time a concerted effort was made to rapidly disseminate SIGINT, electronic intelligence (ELINT), and cryptographic information to accomplish a tactical ASW task.
By 1943, sufficient numbers of long-range maritime patrol aircraft existed to allow Allied forces to carry out widespread offensive ASW operations. Planners focused their efforts on the Bay of Biscay, which the majority of German U-boats passed through during their transit from bases in occupied France to patrol areas in the Western Approaches, North Atlantic, and the eastern seaboard of the United States. The Bay offered the highest density of submarine targets other than the waters near the convoys themselves.
The planners intended to maximize detection opportunities, since higher numbers of detections would result in a great number of attacks. A larger number of attacks would mean more submarines sunk or forced to return to port to repair battle damage. Even if the boats were not damaged, higher numbers of attacks would force U-boats to submerge more often, slowing their transit to their patrol boxes. U-boats might also be forced to take longer routes to their operating areas. Taken together, these outcomes would result in the average boat spending more time in transit and less time hunting, which would ultimately mean fewer merchant ships sunk.
Allied operations analysts identified a “ribbon” of water that was the most likely area on average for patrol aircraft to sight a U-boat. The zone was patrolled day and night. Aircrews had modest results at first but eventually sighting and sinking rates steadily increased.
The success of the Bay of Biscay offensive eventually forced U-boats to operate primarily underwater, surfacing only when required to recharge their batteries. It also drove the Germans to adopt the snorkel, an apparatus that extended from the conning tower and above the surface to allow the boat to run its diesel engine and ventilate while remaining submerged.
By 1944, the industrial might of the Allies had tipped the scales decisively in their favor. Large numbers of well-equipped and well-trained surface escorts and ASW aircraft savaged the U-boat fleet during the final months of the Battle of the Atlantic.
The tactical environment had become so deadly for the German submarine force that more submarines were being sunk than were merchant ships.
In the final months of the war, the Germans attempted to introduce radical new submarine technology. A new, highly advanced submarine design known as the Type XXI went to sea in 1943. The Type XXI featured capabilities that define the modern SSK [Diesel Electric Attack Submarine]. It had a streamlined hull optimized for underwater speed and maneuverability rather than prolonged surface operations. It had a high underwater speed and excellent submerged endurance. It had large-capacity batteries, a snorkel, a tactically-effective passive sonar suite, and automated torpedo-handling equipment.
The Germans had also developed radio technology that was poised to thwart the highly effective Allied HF/DF intercept system. The Nazis had perfected a burst transmission system known as “Kurier” that compressed a normal 20-second transmission into a signal that lasted less than a second. Such a short signal could not be intercepted and properly plotted by Allied SIGINT equipment.
Kurier was poised to deny the Allies the highly effective cueing that alerted ASW forces to the presence ofU-boats and provided an initial search location. Widespread introduction of the snorkel would drastically degrade airborne radar, which had become the primary search sensor for Allied ASW aircraft.
Between these two advances, the Allies faced the prospect of losing their primary methods of ASW cueing and search. Such a loss could cripple Allied ASW forces and deliver the submarine a measure of the invulnerability it enjoyed during World War I.
Luckily for the Allies, Germany was unable to field the Type XXI in sufficient numbers to undermine the effectiveness of ASW operations.
This excerpt from Airborne Anti-submarine Warfare: From the First World War to the Present Day appears by kind permission of Pen & Sword Books Ltd. Copyright remains with the author. NB the above image is not from this volume.
Sign up to get the free World War II Today newsletter every Sunday, with an exclusive excerpt from a new title every week. Or become a paid subscriber and follow the war day by day.