How a Secret Invention Rescued D-Day

By Norman Fine

On the eve of Tuesday, June 6, 1944, General Dwight D. Eisenhower, Supreme Commander of Allied Expeditionary Forces in Europe, made one of the most difficult decisions of his life. If wrong, the consequences would be catastrophic.

Originally scheduled for June 5, Eisenhower had already postponed the D-Day invasion of mainland Europe for one day because of the beastly weather. Troops couldn’t be safely transferred over the sides of the transport ships to the landing craft in the tossing seas.

Relying on his meteorologist’s forecast for improving weather that night and weighing the potential consequences of another delay — losing the moon’s phase, the tide, and the critical element of surprise — he made the decision to send 150,000 armed men to the beaches of Normandy, France. In so doing, he initiated the largest land invasion by sea in world history.

It was, in fact, that same beastly European weather that had too often grounded the Allied bombers in their mission to destroy Nazi Germany’s war-making infrastructure and neutralize its potent air force in preparation for an Allied landing. The statistics through 1943 reveal, that over the years of the bombing campaign, seventy to eighty percent of all planned bombing missions had been scrubbed or recalled because of the weather. Bombing missions were completed only seven days a month, on average. Not enough. As 1943 drew to its close, Allied Air

Force leaders recognized that they were failing to achieve the prerequisites for a successful DDay invasion. Something different was needed if D-Day were to be launched as planned. But what?

American Air Force leaders listened reluctantly to the scientists and a handful of forwardthinking airmen. The Brits had already done so, but the entrenched American military establishment was stubbornly resisting. What could a lab full of civilian scientists at M.I.T. possibly have to offer the United States Army Air Force? After all, they had the B-17 Flying Fortresses and the Norden Bombsights that could “drop a bomb into a pickle barrel from an altitude of 20,000 feet!” But wait. What good was an optical bombsight when the targets couldn’t be seen through the persistent overcast?

This is why, in January 1944, just five months before D-Day, the USAAF completely changed its bombing protocols. The new bombing plan was resented and resisted by many air force leaders and their own experienced airmen in Europe. Nor was it ever mentioned by historians writing for a popular readership.

Starting in January 1944, all bomber wing formations were to be led by one bomber in which a new top-secret radar had been installed. The radar-equipped plane, called a Pathfinder, would lead the entire formation, navigate its way to the target no matter the weather or the cloud cover, “see” the target through the overcast, and drop the first bombs and marker flares upon which the following planes would simply toggle their bombs to fall on the Pathfinder’s markers. Blind Bombing!

Through that first month of January 1944, there were fewer than twenty heavy bombers equipped with the new radars to lead the formations. These radars were hand-built prototypes from the lab at M.I.T. The next combat-ready B-17 to arrive at the U.S. Eighth Air Force in England was fitted with the first production model of the new radar fresh off Philo’s production line. This, on January 29, only four months before D-Day. By D-Day, there were probably not even a hundred radar-equipped bombers in the entire theater to lead the 3,000 heavy bombers of the Eighth Air Force to their targets.

The time was short, and the assets were limited. The new offensive strategy was hardly tested before men who had never even heard of radar had to implement it in combat with a minimum of training. There were no operating or servicing manuals. Civilian physicists, engineers, and technicians from the M.I.T. laboratory Stateside were in England at the airbases, still making design changes and repairing equipment failures.

Yet just five months after the new bombing protocols were introduced, the Allies put 150,000 armed men on the Normandy beaches on Tuesday, June 6, 1944, and there was scarcely an enemy plane in the sky to oppose them. Those enemy planes that were still airworthy had no fuel to run them, no spare parts to repair them, and few experienced pilots to fly them.

They’d been shot down. The relentless bombing campaign, undeterred by the weather, with a Pathfinder plane leading every bomber wing, had crippled the Nazi warmaking infrastructure.

How the Allies overcame the U-Boats during the Battle of the Atlantic and rescued the D-Day invasion in just five months is the subject of Blind Bombing: How Microwave Radar Brought the Allies to D-Day and Victory in WWII. Blind Bombing received the silver medal for World History in the 2020 IPPY awards, a national prize sponsored by the Independent Book Publishers Association. Kirkus Reviews called it “A riveting addition to the literature on scientific innovation during the Second World War.” Published by Potomac Books, an imprint of the University of Nebraska Press, it is available in hardcover, digital, and audio formats. Blind Bombing is available from bookstores and popular online retailers.

A Secret Invention

We know from the history books that all the warring nations had radar: Britain, Germany, Japan, the U.S. What wasn’t made clear from the history books, however, was that radar was primitive compared to what the Allies were bringing to the battles. This primitive radar operated at low radio frequencies, which required massive antennas and cumbersome, power-hungry electronic equipment. Immobile, it was limited to fixed installations and was useful for defensive purposes in detecting a group of approaching airplanes or large ships on the seas. But it could not detect small objects or be built small enough to be usefully installed in trucks or planes.

Radar scientists and engineers around the world all wanted to operate radar at higher frequencies. They knew the advantages would be enormous: smaller antennas and electronic packages, the detection of smaller objects (like a single U-boat on the vast sea), and more detail in the radar signal returns. However, there was no known gadget capable of transmitting high-frequency radio waves at any useful power (distance).

Unknown to the rest of the world, however, a top-secret gadget (that scarcely anyone has ever heard of) was invented by two British physicists (whom scarcely anyone has ever heard of) on the very eve of the war. That invention allowed radar to operate at much higher frequencies than the earlier, primitive radar.

The secret gadget was called a resonant cavity magnetron. It was invented by John Randall and Harry Boot, and it was the key that unlocked radar’s enormous potential. The new high frequency radars overcame the two major obstacles to D-Day, the U-Boats and the beastly European weather. And only the Allies had it.

The resonant cavity magnetron turned out to be the single new invention most influential in the winning of the war. Without it, D-Day might not have been attempted on June 6, 1944, or even that year.

And if D-Day had been launched prematurely, the results would have been far different.

Yes, the atomic bomb was an earth-shaking new invention, but it didn’t win the war. The war was already won when the atomic bombs were dropped on Hiroshima and Nagasaki. The two bombs ended the war abruptly and, in so doing, saved thousands of American lives from having to invade the Japanese mainland. And, yes, the breaking of Germany’s Enigma code was a brilliant mathemati- cal achievement but hardly an invention. The resonant cavity magnetron, unknown to the rest of the world, was ultimately used by every branch of every Allied military force by the end of the war.

The resonant cavity magnetron enabled the Allies to design radar equipment that operated at frequencies more than two orders of magnitude higher than the radars known to the rest of the developed world. That improvement translates to more than a hundred times improvement in radar’s ability to detect smaller objects in greater detail from further distances. Plus, the antennas and electronic packages were that much smaller. Small enough to be mounted in planes and trucks for mobile operations.

A handful of civilian scientists in Britain and in the newly established Radiation Lab at M.I.T. in Cambridge, Massachusetts, used the cavity magnetron to design new radars that just months earlier hadn’t existed. For the first time, radar could be designed for effective use as a mobile, offensive weapon of war.

The enemy was soon baffled by their losses. Admiral Karl Doenitz, commander of the German U-Boat fleet, asked his scientists, “Is it possible the Allies have a new form of radar?” “No,” the scientists assured him, Radar is incapable of detecting the level of detail presented by a lone submarine.