Fred Whipple Figured Out How to Hide Airplanes From Enemy Radar

How chaff was invented in World War II. 

In the summer of 2004, I met one of the great astronomers of the 20th century at his Boston home. I was there to help him clean out his front hall closet.

Let me explain. 

Fred Whipple has long been recognized as a major contributor to our understanding of space and spaceflight—with contributions ranging from determining the makeup of comets to developing the particulate shielding for spacecraft that still bears his name (the Whipple shield). It was, however, Whipple’s contributions to the Allied cause in World War II that had brought me to his house. One of his—largely unheralded—inventions is credited with saving the lives of an estimated 600 heavy bomber crews in Europe and 200 Boeing B-29 crews in the Pacific (a total of 8,000 airmen). Whipple, you see, was central to the invention of radar-jamming chaff at Harvard University’s Radio Research Laboratory, which had been tasked with creating electronic countermeasures that could disrupt enemy radar and direction-finding equipment.

In early 1942, noted electrical engineer Fred Terman established the laboratory and visited Great Britain, then the world leader in radar technology. Terman learned that British scientists had been experimenting with dropping paper-backed aluminum strips—called “window”—to reflect and scatter radar signals that could then conceal the radar returns of a large force of bombers. Window could also be used to trick enemy radar operators into believing that there were more bombers than there were. Terman placed Lan Jen Chu, an engineering professor at the Massachusetts Institute of Technology, in charge of developing a theory to optimize various window shapes. Terman also brought Harvard astronomer Whipple—who had a creative mind and a degree in mathematics—on board to refine the technology and make it practical for military operations. 

Whipple, however, had received a competing job offer. J. Robert Oppenheimer had invited him to his New Mexico ranch to offer him leadership of the Manhattan Project’s computing section. Whipple rejected the offer since he favored living in Boston—and because he considered the radar countermeasures work as equally significant to the war effort.

In developing what would become known as “chaff,” one of Whipple’s key realizations was that the more strips that could be dispersed, the more human radar operators would be confused by the flurry of radar returns. Dropping thousands of strips of chaff, instead of just hundreds of pieces of British window, was a markedly more effective countermeasure. 

Allied air crews now needed chaff, and fast. But Whipple was concerned that if commercial aluminum foil manufacturers undertook the production of chaff, the enemy could figure out which frequencies the Allies were intending to jam. A scientist working under Whipple created a prototype cutter that could cut the aluminum to the right length so that it could be shipped to forward bases. Whipple then set about packaging the strands so that bomber crews could quickly deploy them. Testing occurred at Eglin Field in Florida. Whipple then went to Europe to brief leaders and mission planners on the use of the new technology, all while wearing a U.S. Army uniform with no insignia.

One challenge for Whipple was that German and Japanese radars used markedly different wavelengths. For the German radars, Whipple created cardboard packets of chaff containing 1,000 aluminum strands (later 3,000) about 12 inches long. Just one of the packets could generate a radar return the size of a B-17, which is why the chaff packets became known as “dehydrated bombers.”  A B-17 radio, or radar, operator might release dozens of such packets through a special slot cut into the bomber’s fuselage.

For Japan’s radars, Whipple developed 400-foot by ¼-inch aluminum reels, designated “rope.” The rope unspooled from packets via tiny parachutes. B-29 crews deployed packets of rope from the camera bay in the unpressurized section between the bomber’s pressurized aft crew compartment and tail gunner position.

Initially, the Allies were reluctant to elevate the secret countermeasures to operational status for fear that Axis powers could copy the technology, but as Allied air superiority increased, the benefits for their use won out. Britain began deploying their window in July 1943 and the U.S. soon followed with chaff. Overall, the U.S. produced nearly 13 million pounds of chaff and more than eight million pounds of aluminum rope. 

Around the time of my visit to Whipple’s house, I was the National Air and Space Museum’s new curator of instruments and avionics. I realized that the lack of chaff and rope samples hindered the Museum’s ability to tell the story of offensive air operations in World War II. So I set out to find good examples. A consultation with Michael Hanz, a Museum volunteer and expert on electronic warfare systems, ultimately yielded a phone number for Whipple, who was 97 at the time.

Whipple enthusiastically supported my goal, inviting me to come up to Boston to root through his front hall closet, where he had stored many examples of his chaff development. I did just that, and the trip exceeded all expectations. Whipple had not only retained the original prototype chaff cutter, but also examples of everything he had worked on, providing a nearly complete and exceptionally well-preserved progression of successful operational types of chaff and rope. He had even saved experimental samples that had turned out to be dead-ends, as well as an advanced form made in anticipation of German engineers matching Allied advances in microwave radar (fortunately, they were unable to do so).

I packed up the collection and shipped it to the Museum. Sadly, Whipple passed away only weeks later. Some of his collection is currently on display, including the prototype cutter, in the military avionics case at the Museum’s Steven F. Udvar-Hazy Center in Chantilly, Virginia. Whipple’s donation is, without a doubt, the world’s best collection of World War II chaff—and proof that even a simple technology can save lives in combat.   

Roger Connor is the National Air and Space Museum’s curator of vertical flight. He also oversees the Museum’s collection of drones, aircraft instruments, avionics, bombsights, and gunsights. Connor is a commercial fixed-wing pilot.

This article, originally titled "Dr. Whipple's Chaff,"  is from the Fall 2025 issue of Air & Space Quarterly, the National Air and Space Museum's signature magazine that explores topics in aviation and space, from the earliest moments of flight to today. Explore the full issue.

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