Technology during World War II
|World War II|
|0-9 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z|
Technology played a crucial role in determining the outcome of World War II. Much of it was developed during the interwar years of the 1920s and 1930s, some were developed in response to valuable lessons learned during the war, and some were beginning to be developed as the war ended.
Effects on Warfare
Almost all types of technology were customized, although major developments were:
- Weaponry: ships, vehicles, aircraft, artillery, rocketry, small arms; and biological, chemical, and atomic weapons
- Logistical support: vehicles necessary for transporting soldiers and supplies, such as trains, trucks, ships, and aircraft
- Communications and intelligence: devices used for navigation, communication, remote sensing, and espionage
- Medicine: surgical innovations, chemical medicines, and techniques
- Industry: the technologies employed at factories and production/distribution centers
- Others which might be unknown.
This was perhaps the first war where military operations were aimed at the research efforts of the enemy. For example:
- The exfiltration of Niels Bohr from German-occupied Denmark to Britain in 1943
- The sabotage of Norwegian heavy water production
- The bombing of Peenemunde
Between the wars 
In Britain there was the Ten Year Rule (adapted in August 1919), which declared the government should not expect another war within ten years. Consequently they conducted very little military R & D. On the other hand, Germany and the Soviet Union were dissatisfied powers that for different reasons cooperated with each other on military R & D. The Soviets offered Weimar Germany facilities deep inside the USSR for building and testing arms and for military training, well away from Treaty inspectors' eyes. In return, the Soviets asked for access to German technical developments, and for assistance in creating a Red Army General Staff.
The great artillery manufacturer Krupp was soon active in the south of the USSR, near Rostov-on-Don. In 1925, a flying school was established at Vivupal, near Lipetsk, to train the first pilots for the future Luftwaffe. Since 1926, the Reichswehr had been able to use a tank school at Kazan (codenamed Kama) and a chemical weapons facility in Samara Oblast (codenamed Tomka). In turn, the Red Army gained access to these training facilities, as well as military technology and theory from Weimar Germany.
In the late 1920s, Germany helped Soviet industry begin to modernize, and to assist in the establishment of tank production facilities at the Leningrad Bolshevik Factory and the Kharkov Locomotive Factory. This cooperation would break down when Hitler rose to power in 1933. The failure of the World Disarmament Conference marked the beginnings of the arms race leading to war.
In France the lesson of World War I was translated into the Maginot Line which was supposed to hold a line at the border with Germany. The Maginot Line did achieve its political objective of ensuring that any German invasion had to go through Belgium ensuring that France would have Britain as a military ally. France and Russia had more, and much better, tanks than Germany as of the outbreak of their hostilities in 1940. As in World War I, the French generals expected that armour would mostly serve to help infantry break the static trench lines and storm machine gun nests. They thus spread the armour among their infantry divisions, ignoring the new German doctrine of blitzkrieg based on the fast movement using concentrated armour attacks, against which there was no effective defense but mobile anti-tank guns - infantry Antitank rifles not being effective against medium and heavy tanks.
Air power was a major concern of Germany and Britain between the wars. Trade in aircraft engines continued, with Britain selling hundreds of its best to German firms - which used them in a first generation of aircraft, and then improved on them much for use in German aircraft. These new inventions lead way to major success for the Germans in World War II. Germany had always been and has continued to be in the forefront of internal combustion engine development. Göttingen was the world center of aerodynamics and fluid dynamics in general, at least up to the time when the highly dogmatic NAZI party came to power. This contributed to the German development of jet aircraft and of submarines with improved under-water performance.
Induced nuclear fission was discovered in Germany in 1939 by Otto Hahn (and ex-patriot Jews in Sweden), but many of the scientists needed to develop nuclear power had already been lost, due to anti-Jewish and anti-intellectual policies.
Military weapons technology experienced rapid advances during World War II, and over six years there was a disorientating rate of change in combat in everything from aircraft to small arms. Indeed the war began with most armies utilizing technology that had changed little from World War I, and in some cases, had remained unchanged since the 19th century. For instance cavalry, trenches, and World War I-era battleships were normal in 1940, however within only six years, armies around the world had developed jet aircraft, ballistic missiles, and even atomic weapons in the case of the United States.
The best jet fighters at the end of the war easily outflew any of the leading aircraft of 1939, such as the Spitfire Mark I. The early war bombers that caused such carnage would almost all have been shot down in 1945, many with two shots, by radar-aimed, proximity fuse-detonated anti-aircraft fire, just as the 1941 "invincible fighter", the Zero, had by 1944 become the "turkey" of the "Marianas Turkey Shoot". The best late-war tanks, such as the Soviet JS-3 heavy tank or the German Panther medium tank, handily outclassed the best tanks of 1939 such as Panzer IIIs. In the navy the battleship, long seen as the dominant element of sea power, was displaced by the greater range and striking power of the aircraft carrier. The chaotic importance of amphibious landings stimulated the Western Allies to develop the Higgins boat, a primary troop landing craft; the DUKW, a six-wheel-drive amphibious truck, amphibious tanks to enable beach landing attacks and Landing Ship, Tanks to land tanks on beaches. Increased organization and coordination of amphibious assaults coupled with the resources necessary to sustain them caused the complexity of planning to increase by orders of magnitude, thus requiring formal systematization giving rise to what has become the modern management methodology of project management by which almost all modern engineering, construction and software developments are organized.
In the Western European Theatre of World War II, air power became crucial throughout the war, both in tactical and strategic operations (respectively, battlefield and long-range). Superior German aircraft, aided by ongoing introduction of design and technology innovations, allowed the German armies to overrun Western Europe with great speed in 1940, largely assisted by lack of Allied aircraft, which in any case lagged in design and technical development during the slump in research investment after the Great Depression. Since the end of World War I, the French Air Force had been badly neglected, as military leaders preferred to spend money on ground armies and static fortifications to fight another World War I-style war. As a result, by 1940, the French Air Force had only 1562 planes and was together with 1070 RAF planes facing 5,638 Luftwaffe fighters and fighter-bombers. Most French airfields were located in north-east France, and were quickly overrun in the early stages of the campaign. The Royal Air Force of the United Kingdom possessed some very advanced fighter planes, such as Spitfires and Hurricanes, but these were not useful for attacking ground troops on a battlefield, and the small number of planes dispatched to France with the British Expeditionary Force were destroyed fairly quickly. Subsequently, the Luftwaffe was able to achieve air superiority over France in 1940, giving the German military an immense advantage in terms of reconnaissance and intelligence.
German aircraft rapidly achieved air superiority over France in early 1940, allowing the Luftwaffe to begin a campaign of strategic bombing against British cities. With France es based near the English Channel were able to launch raids on London and other cities during the Blitz, with varying degrees of success.
After World War I, the concept of massed aerial bombing—"The bomber will always get through"—had become very popular with politicians and military leaders seeking an alternative to the carnage of trench warfare, and as a result, the air forces of Britain, France, and Germany had developed fleets of bomber planes to enable this (France's bomber wing was severely neglected, whilst Germany's bombers were developed in secret as they were explicitly forbidden by the Treaty of Versailles).
The bombing of Shanghai by the Imperial Japanese Navy on January 28, 1932, and August 1937 and the bombings during the Spanish Civil War (1936–1939), had demonstrated the power of strategic bombing, and so air forces in Europe and the United States came to view bomber aircraft as extremely powerful weapons which, in theory, could bomb an enemy nation into submission on their own. As a result, the fear of bombers triggered major developments in aircraft technology.
Nazi Germany had put only one large, long-range strategic bomber (the Heinkel He 177 Greif, with many delays and problems) into production, while the America Bomber concept resulted only in prototypes. The Spanish Civil War had proved that tactical dive-bombing using Stukas was a very efficient way of destroying enemy troops concentrations, and so resources and money had been devoted to the development of smaller bomber craft. As a result, the Luftwaffe was forced to attack London in 1940 with heavily overloaded Heinkel and Dornier medium bombers, and even with the unsuitable Junkers Ju 87. These bombers were painfully slow—Italian engineers had been unable to develop sufficiently large piston aircraft engines (those that were produced tended to explode through extreme overheating), and so the bombers used for the Battle of Britain were woefully undersized. As German bombers had not been designed for long-range strategic missions, they lacked sufficient defenses. The Messerschmitt Bf 109 fighter escorts had not been equipped to carry enough fuel to guard the bombers on both the outbound and return journeys, and the longer-range Bf 110s could be outmanoeuvred by the short-range British fighters. (A bizarre feature of the war was how long it took to conceive of the Drop tank.) The air defense was well organized and equipped with effective radar that survived the bombing. As a result, German bombers were shot down in large numbers, and were unable to inflict enough damage on cities and military-industrial targets to force Britain out of the war in 1940 or to prepare for the planned invasion.
British long-range bomber planes such as the Short Stirling had been designed before 1939 for strategic flights and given a large armament, but their technology still suffered from numerous flaws. The smaller and shorter ranged Bristol Blenheim, the RAF's most-used bomber, was defended by only one hydraulically operated machine-gun turret, and whilst this appeared sufficient, it was soon revealed that the turret was a pathetic defence against squadrons of German fighter planes. American bomber planes such as the B-17 Flying Fortress had been built before the war as the only adequate long-range bombers in the world, designed to patrol the long American coastlines. Defended by as many as six machine-gun turrets providing 360° cover, the B-17s were still vulnerable without fighter protection even when used in large formations.
Despite the abilities of Allied bombers, though, Germany was not quickly crippled by Allied air raids. At the start of the war the vast majority of bombs fell miles from their targets, as poor navigation technology ensured that Allied airmen frequently could not find their targets at night. The bombs used by the Allies were very high-tech devices, and mass production meant that the precision bombs were often made sloppily and so failed to explode. German industrial production actually rose continuously from 1940 to 1945, despite the best efforts of the Allied air forces to cripple industry.
Significantly, the bomber offensive kept the revolutionary Type XXI U-Boat from entering service during the war. Moreover, Allied air raids had a serious propaganda impact on the German government, all prompting Germany to begin serious development on air defence technology—in the form of fighter planes.
The jet aircraft age began during the war with the development of the Heinkel He 178, the first true turbojet. Late in the war the Germans brought in the first operational Jet fighter, the Messerschmitt Me 262. However, despite their technological edge, German jets were overwhelmed by Allied air superiority, frequently being destroyed on or near the airstrip. Other jet aircraft, such as the British Gloster Meteor, which flew missions but never saw combat, did not significantly distinguish themselves from top-line piston-driven aircraft.
Aircraft saw rapid and broad development during the war to meet the demands of aerial combat and address lessons learned from combat experience. From the open cockpit airplane to the sleek jet fighter, many different types were employed, often designed for very specific missions.
During the war the Germans produced various Glide bomb weapons, which were the first smart bombs; the V-1 flying bomb, which was the first cruise missile weapon; and the V-2 rocket, the first ballistic missile weapon. The last of these was the first step into the space age as its trajectory took it through the stratosphere, higher and faster than any aircraft. This later led to the development of the Intercontinental ballistic missile (ICBM). Wernher Von Braun led the V-2 development team and later emigrated to the United States where he contributed to the development of the Saturn V rocket, which took men to the moon in 1969.
Theoretical foundation 
|This section requires expansion. (June 2008)|
The laboratory of Ludwig Prandtl at Göttingen was the main center of theoretical and mathematical aerodynamics and fluid dynamics research from soon after 1904 to the end of World War II. Prandtl coined the term boundary layer and founded modern (mathematical) aerodynamics. The laboratory lost its dominance when the researchers were dispersed after the war.
The Axis countries had serious shortages of petroleum from which to make liquid fuel. The Allies had much more petroleum production. Germany, long before the war, developed a process to make synthetic fuel from coal. Synthesis factories were principal targets of the Oil Campaign of World War II.
The USA added tetra ethyl lead to its aviation fuel, with which it supplied Britain and other Allies. This octane enhancing additive allowed higher compression ratios, allowing higher efficiency, giving more speed and range to Allied Airplanes, and reducing the cooling load.
The Treaty of Versailles had imposed severe restrictions upon Germany constructing vehicles for military purposes, and so throughout the 1920s and 1930s, German arms manufacturers and the Wehrmacht had begun secretly developing tanks. As these vehicles were produced in secret, their technical specifications and battlefield potentials were largely unknown to the European Allies until the war actually began. When German troops invaded the Benelux nations and France in May 1940, German weapons technology proved to be immeasurably superior to that of the Allies.
The French Army suffered from serious technical deficiencies with its tanks. In 1918, the Renault FT-17 tanks of France had been the most advanced in the world, although small, capable of far outperforming their slow and clumsy British, German, or American counterparts. However, this superiority resulted in tank development stagnating after World War I. By 1939, French tanks were virtually unchanged from 1918.[dubious ] French and British Generals believed that a future war with Germany would be fought under very similar conditions as those of 1914–1918. Both invested in thickly armoured, heavily armed vehicles designed to cross shell-damaged ground and trenches under fire. At the same time the British also developed faster but lightly armoured Cruiser tanks to range behind the enemy lines.
In contrast, the Wehrmacht invested in fast, light tanks designed to overtake infantry. These vehicles would vastly outperform British and French tanks in mechanized battles. German tanks followed the design of France's 1918 Renault versions—a moderately armoured hull with a rotating turret on top mounting a cannon. This gave every German tank the potential to engage other armoured vehicles. In contrast, around 35% of French tanks were simply equipped with machine guns (again designed for trench warfare), meaning that when French and German met in battle, a third of the French assault vehicles would not be able to engage enemy tanks, their machine-gun fire only ricocheting off German armour plates. Only a handful of French tanks had radios, and these often broke as the tank lurched over uneven ground. German tanks were, on the contrary, all equipped with radios, allowing them to communicate with one another throughout battles, whilst French tank commanders could rarely contact other vehicles.
The Matilda Mk I tanks of the British Army were also designed for infantry support and were protected by thick armour. This was ideal for trench warfare,[dubious ] but made the tanks painfully slow in open battles. Their light cannons[dubious ] and machine-guns were usually unable to inflict serious damage on German vehicles. The exposed caterpillar tracks were easily broken by gunfire, and the Matilda tanks had a tendency to incinerate their crews if hit, as the petrol tanks were located on the top of the hull. By contrast the Infantry tank Matilda II fielded in lesser numbers was largely invulnerable to German gunfire and its gun was able to punch through the German tanks. However French and British tanks were at a disadvantage compared to the air supported German armoured assaults, and a lack of armoured support contributed significantly to the rapid Allied collapse in 1940.
World War II marked the first full-scale war where mechanization played a significant role. Most nations did not begin the war equipped for this. Even the vaunted German Panzer forces relied heavily on non-motorised support and flank units in large operations. While Germany recognized and demonstrated the value of concentrated use of mechanized forces, they never had these units in enough quantity to supplant traditional units. However, the British also saw the value in mechanization. For them it was a way to enhance an otherwise limited manpower reserve. America as well sought to create a mechanized army. For the United States, it was not so much a matter of limited troops, but instead a strong industrial base that could afford such equipment on a great scale.
The most visible vehicles of the war were the tanks, forming the armored spearhead of mechanized warfare. Their impressive firepower and armor made them the premier fighting machine of ground warfare. However, the large number of trucks and lighter vehicles that kept the infantry, artillery, and others moving were massive undertakings also.
Naval warfare changed dramatically during World War II, with the ascent of the aircraft carrier to the premier vessel of the fleet, and the impact of increasingly capable submarines on the course of the war. The development of new ships during the war was somewhat limited due to the protracted time period needed for production, but important developments were often retrofitted to older vessels. Advanced German submarine types came into service too late and after nearly all the experienced crews had been lost.
The German U-boats were used primarily for stopping/destroying the resources from the United States and Canada coming across the Atlantic. Submarines were critical in the Pacific Ocean as well as in the Atlantic Ocean. Japanese defenses against Allied submarines were ineffective. Much of the merchant fleet of the Empire of Japan, needed to supply its scattered forces and bring supplies such as petroleum and food back to the Japanese Archipelago, was sunk. This kept them from training adequate replacements for their lost aircrews and even forced the navy to be based near its oil supply. Among the warships sunk by submarines was the war's largest aircraft carrier, the Shinano.
The most important shipboard advances were in the field of anti-submarine warfare. Driven by the desperate necessity of keeping Britain supplied, technologies for the detection and destruction of submarines was advanced at high priority. The use of ASDIC (SONAR) became widespread and so did the installation of shipboard and airborne radar.
The actual weapons; the guns, mortars, artillery, bombs, and other devices, were as diverse as the participants and objectives. A bewildering array were developed during the war to meet specific needs that arose, but many traced their development to prior to World War II. Torpedoes began to use magnetic detonators; compass-directed, programmed and even acoustic guidance systems; and improved propulsion. Fire-control systems continued to develop for ships' guns and came into use for torpedoes and anti-aircraft fire. Human torpedoes and the Hedgehog were also developed.
- Armour weapons: The Tank destroyer, Specialist Tanks for Combat engineering including mine clearing Flail tanks, Flame tank, and amphibious designs
- Aircraft: Glide bombs - the first "smart bombs", such as the Fritz X anti-shipping missile, had wire or radio remote control; the world's first jet fighter (Messerschmitt 262) and jet bomber (Arado 234), the world's first operational military helicopters (Flettner Fl 282), the world's first rocket-powered fighter (Messerschmitt 163)
- Missiles: The Pulse jet-powered V-1 flying bomb was the world's first cruise missile, Rockets progressed enormously: V-2 rocket, Katyusha rocket artillery and air-launched rockets.
- HEAT, and HESH anti-armour warheads.
- Proximity fuze for shells, bombs and rockets. This fuze is designed to detonate an explosive automatically when close enough to the target to destroy it, so a direct hit is not required and time/place of closest approach does not need to be estimated. Magnetic torpedoes and mines also had a sort of proximity fuse.
- Guided weapons (by radio or trailing wires): glide bombs, crawling bombs, rockets.
- Self-guiding weapons: torpedoes (sound-seeking, compass-guided and looping), V1 missile (compass- and timer-guided)
- Aiming devices for bombs, torpedoes, artillery and machine guns, using special purpose mechanical and electronic analog and (perhaps) digital "computers". The mechanical analog Norden bomb sight is a well-known example.
- Napalm was developed, but did not see wide use until the Korean War
- Plastic explosives like Nobel 808, Hexoplast 75, Compositions C and C2
Small arms development 
New production methods for weapons such as stamping, riveting, and welding came into being to produce the number of arms needed. While this had been tried before, during World War I, it had resulted in quite possibly the worst firearm ever adopted by any military for use: the French Chauchat light machine gun. Design and production methods had advanced enough to manufacture weapons of reasonable reliability such as the PPSh-41, PPS-42, Sten, MP 40, M3 Grease Gun, Gewehr 43, Thompson submachine gun and the M1 Garand rifle. Other Weapons commonly found During World War II include the American, Browning Automatic Rifle (BAR), M1 Carbine Rifle, as well as the Colt M1911; The Japanese Type 100 submachine gun, the Type 99 machine gun, and the Arisaka bolt action rifle all were significant weapons used during the war.
World War II saw the establishment of the reliable semi-automatic rifle, such as the American M1 Garand and, more importantly, of the first widely used assault rifles, named after the German sturmgewehrs of the late war. Earlier renditions that hinted at this idea were that of the employment of the Browning Automatic Rifle and 1916 Fedorov Avtomat in a walking fire tactic in which men would advance on the enemy position showering it with a hail of lead. The Germans first developed the FG 42 for its paratroopers in the assault and later the Sturmgewehr 44 (StG 44), the world's first assault rifle, firing an intermediate cartridge; the FG 42's use of a full-powered rifle cartridge made it difficult to control.
Developments in machine gun technology culminated in the Maschinengewehr 42 (MG42) which was of an advanced design unmatched at the time. It spurred post-war development on both sides of the upcoming Cold War and is still used by some armies to this day including the German Bundeswehr's MG 3. The Heckler & Koch G3, and many other Heckler & Koch designs, came from its system of operation. The United States military meshed the operating system of the FG 42 with the belt feed system of the MG42 to create the M60 machine gun used in the Vietnam War.
Despite being overshadowed by self-loading/automatic rifles and sub-machine guns, bolt-action rifles remained the mainstay infantry weapon of many nations during World War II. When the United States entered World War II, there were not enough M1 Garand rifles available to American forces which forced the US to start producing more M1903 rifles in order to act as a "stop gap" measure until sufficient quantities of M1 Garands were produced.
During the conflict, many new models of bolt-action rifles were produced as a result of lessons learned from the First World War with the designs of a number of bolt-action infantry rifles being modified in order to speed up production as well as to make the rifles more compact and easier to handle. Examples of bolt-action rifles that were used during World War II include the German Mauser Kar98k, the British Lee-Enfield No.4, and the Springfield M1903A3. During the course of World War II, bolt-action rifles and carbines were modified even further to meet new forms of warfare the armies of certain nations faced e.g. urban warfare and jungle warfare. Examples include the Soviet Mosin-Nagant M1944 carbine, which were developed by the Soviets as a result of the Red Army's experiences with urban warfare e.g. the Battle of Stalingrad, and the British Lee-Enfield No.5 carbine, that were developed for British and Commonwealth forces fighting the Japanese in South-East Asia and the Pacific.
When World War II ended in 1945, the small arms that were used in the conflict still saw action in the hands of the armed forces of various nations and guerrilla movements during and after the Cold War era. Nations like the Soviet Union and the United States provided many surplus, World War II-era small arms to a number of nations and political movements during the Cold War era as a pretext to providing more modern infantry weapons. Besides seeing conflict long after World War II ended, the small arms of World War II are now considered collector's items with many civilian firearm owners and collectors around the world due to their historical nature, low cost (due to many of these firearms now appearing on the firearms market in large numbers over the past decade), and their durability.
The atomic bomb 
The massive research and development demands of the war included the Manhattan Project, the effort to quickly develop an atomic bomb, or nuclear fission warhead. It was perhaps the most profound military development of the war, and had a great impact on the scientific community, among other things creating a network of national laboratories in the United States.
Development was completed too late for use in the European Theater of World War II. Its invention meant that a single bomber aircraft could carry a weapon sufficiently powerful to devastate entire cities, making conventional warfare against a nation with an arsenal of them suicidal.
The strategic importance of the bomb, and its even more powerful fusion-based successors, did not become fully apparent until the United States lost its monopoly on the weapon in the post-war era. The Soviet Union developed and tested their first nuclear weapon in 1949, based partially on information obtained from Soviet espionage in the United States. Nuclear competition between the two superpowers played a large part in the development of the Cold War. The strategic implications of such a massively destructive weapon still reverberate in the 21st century.
There was also a German nuclear energy project, including talk of an atomic weapon. This failed for a variety of reasons, most notably German Antisemitism. Half of continental theoretical physicists including (Einstein, Bohr, Enrico Fermi, and Oppenheimer) who did much of their early study and research in Germany, were either Jewish or, in the case of Enrico Fermi, married to a Jew. Erwin Schrödinger had also left Germany for political reasons. When they left Germany, the only leading nuclear physicist left in Germany was Heisenberg, who apparently dragged his feet on the project, or at best lacked the high morale that characterized the Los Alamos work. He made some faulty calculations suggesting that the Germans would need significantly more heavy water than was necessary. Otto Hahn, the physical chemist who had the central part in the original discovery of fission, was another key figure in the project. The project was doomed due to insufficient resources.
Electronics, communications and intelligence 
Electronics rose to prominence quickly in World War II. While prior to the war few electronic devices were seen as important pieces of equipment, by the middle of the war such instruments as radar and ASDIC (sonar) had proven their value. Additionally, equipment designed for communications and the interception of those communications was becoming critical.
Theoretical physics, is closely related to radar development, because these are both largely scattering problems and field calculations. Half of the German theoretical physicists were Jewish and had emigrated or otherwise been lost to Germany long before WW II started. Germany started the war ahead in some aspects of radar, but lost ground to work in England and especially by physicists and engineers at the "Radiation Laboratory" of the Massachusetts Institute of Technology.
Industrial technology 
While the development of new equipment was rapid, it was also important to be able to produce these tools and get them to the troops in appropriate quantity. Those nations that were able to maximize their industrial capacity and mobilize it for the war effort were most successful at equipping their troops in a timely way with adequate material. An outstanding German innovation was the Jerrycan which carries by its name a tribute to its success.
One of the biggest developments was the ability to produce synthetic rubber. Natural rubber was mainly harvested in the South Pacific, and the Allies were cut off from a large quantity of it due to Japanese expansion. Thus the development of synthetic rubber allowed for the Allied war machine to continue growing, giving the US a significant technical edge as World War II continued.
See also 
- Military funding of science
- Military production during World War II
- Technology during World War I
- List of equipment used in World War II
- Secret and special weapons in Showa Japan
- American Military technology During WWII
- German Military Technology during World War II
- Anderson, J. (2005). Ludwig Prandtl's boundary layer. Physics Today.