Although aircraft were used to good effect in the First World War, their function was largely limited to roles of reconnaissance and dog-fighting. The use of aerial bombing was extremely limited. But after the war, Britain saw the potential of the bomber, and recognized itself to be an unfortunately ideal target; an aerial attack seemed tailor-made to an island nation with densely packed population and industry.
The devastating potential of aerial bombing became all the more clear in 1937, during the Spanish Civil War. German forces, in support of Franco’s attempt to overthrow Spain’s new Republican government, staged a large-scale air attack on the town of Guernica. Hundreds were killed, and the town nearly leveled, by waves of German bomber aircraft. The era of air warfare had begun. Long before the Guernica devastation, and convinced that “the bomber will always get through,” Britain had realized by the early 1930s that its existing aerial defenses, consisting of air patrols and ground-based visual observers, would provide little if any warning of an attack from Germany. A change was needed.
Radar Development Programs, and Chain Home
Throughout the 1930s, there were numerous national research programs concentrating on the development of systems employing the reflection of radio waves to detect distant objects, such as ships and aircraft. Such systems are what we now know as radar (the term itself was first coined around 1940), but was initially termed RDF, for Radio Direction Finding. In addition to Britain, other nations, including the United States, Germany, France, The Netherlands, Italy, Russia, and Japan, were active in this field of research, with varying levels of success.
Development progressed rapidly in Britain, under the auspices of a team of top ranking scientists, led by Sir Henry Tizard. Formed in late 1934 as the Committee for the Scientific Study of Air Defence (CSSAD), Tizard’s team immediately began investigating the work of a scientist at the National Physical Laboratory, named Robert Watson-Watt (pictured). Watson-Watt had several years’ of meteorological experience researching pulsed radio waves, particularly their ability to be reflected by the ionosphere, one of the upper layers of the atmosphere. By the end of February, 1935, Watson-Watt and Tizard’s committee had used a pulsed echo radio system to detect and locate a test airplane flying some eight miles distant from their test site, a BBC commercial radio antenna. By December of 1935, convinced that radar was vital to its defense, Britain committed funds to build the first five radar stations in the “Chain Home” project. By the end of 1940, Chain Home would have more than fifty stations in service (see picture), covering virtually the entire British coastline, with an aircraft detection range of nearly 100 miles.
The Need for Airborne Radar
With the first test radar station up and running in 1936, Britain conducted a series of training exercises using RAF Hawker airplanes acting as “enemy” bombers. Upon detecting and locating the incoming threat, personnel at the radar station would provide direction to scrambling fighter defense planes, leading them, in real time, to the approaching bombers. With practice and refinement of their techniques, the radar operators were able to direct the fighters to their targets, even when the invading bombers took evasive action by repeatedly changing direction and altitude. Radar-controlled air defense was born.
But the system was not without its shortcomings. The land-based radar stations required antenna towers reaching two to three-hundred feet high; attractive bombing targets should the German planes make it through the air defense screen. More importantly, the system still relied on visual contact with the target. Although the defending fighter planes could be directed to the incoming bombers, actually engaging the enemy to shoot them down still required visual contact by the fighter pilots. In short, although the ground-based radar worked day and night, in all extremes of weather, effective fighter defense still required daylight and clear skies. In addition to the ground-based system, what Britain needed was airborne radar, so the fighters themselves could find the bombers, regardless of the visual conditions. Fortunately, Tizard recognized the problem early on and assigned it to a brilliant scientist and engineer, Edward “Taffy” Bowen.
Bowen’s task seemed insurmountable. The ground-based radars used enormous antennas with transmitters that weighed several tons, consumed tremendous amounts of electrical power, and required a team of men to operate. How could such a system be miniaturized, installed in an airplane, and operated by pilot who was already very busy just flying the plane?
The Cavity Magnetron: Microwaves
Of the challenges faced by Bowen, the requirements of physical size and power seemed the most onerous. To fit the transmitting and receiving antennas into the small nose of a fighter aircraft, he determined the radar would need to generate high-energy radio signals at an operating wavelength of just ten centimeters—microwaves. To that point in time, the best that Bowen had achieved was a wavelength that was twelve and a half times too large. Even worse, any breakthrough in terms of shorter wavelength seemed to only reduce the power available for the radio signal, reducing the radar’s effective range. It wasn’t until early 1940, after Britain was at war with Nazi Germany, that two Chain Home scientists, John Randall and Henry Boot, created a device that would meet the demands of Bowen’s calculations: the cavity magnetron. This device, roughly the size of a dinner plate, could generate radio waves at up to ten kilowatts of power in the ten centimeter wavelength—microwave radar, at hundreds of times the power previously seen, but in a tiny fraction of the size and weight of earlier devices. The breakthrough came none too soon. By July of that year, the Battle of Britain had begun.
The Tizard Mission
With France lost and Britain reeling under nightly Blitz bombing raids by the Luftwaffe, President Roosevelt was under increasing pressure to both provide assistance to Britain, and to keep political peace at home by keeping out of the war. Britain badly needed American help, in any form that could be offered. Although the cavity magnetron was a profound technological breakthrough, Britain was already consumed by war-related production; she had neither the funds nor the excess manufacturing capacity needed to produce the magnetrons in the numbers needed. Tizard proposed to the new Prime Minister, Winston Churchill (pictured), that Britain offer a technology exchange, presenting America with the new magnetron, in return for mass-production of the device. Over the course of the summer months, delegates from London and Washington negotiated the terms of a mission to exchange technology and collaborate on new research efforts. Tizard was chosen to lead the British delegation that would travel to America. Churchill had serious reservations about freely giving away such valued technologies—not only the magnetron, which itself was considered one of Britain’s most valuable war secrets, but also research information relating to gyroscopic gun sights, jet propulsion, and the potential of atomic bombs. Nonetheless, he was so desperate for American help that he approved the plan.
Tizard’s team, including his radar expert Bowen, traveled to America with a box of documents, and the cavity magnetron. Through a series of meetings in Washington, New York, and at MIT, Bowen demonstrated the capabilities of the magnetron to the astonishment of the skeptical American scientists and engineers, who then offered their own ideas, primarily on radar receiver technologies that had been developed by American researchers. Convinced of the tremendous value in the new magnetron, the newly-created National Defense Research Committee (NDRC), the official government overseer of the technology exchange, immediately awarded a contract for thirty magnetrons to the research lab of the Bell Telephone Company. NDRC also helped MIT create its Radiation Laboratory (Rad Lab), which then launched a crash program of further research and development of radar systems.
Within a month, the first thirty magnetrons had been delivered. In less than a year, airborne radar was being used to locate enemy aircraft, and to a far greater extent, to hunt U-boats in the Atlantic. By the end of the war, Bell Labs had produced over one million magnetrons for use by the Allies.
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