Chaff is a radar countermeasure used by military aircraft and ships to confuse enemy radar systems. It consists of millions of tiny metallic fibers released into the air, creating a cloud that appears on radar as a large, false target. This cloud can mask the real position of an aircraft or ship, disrupting radar-guided missiles and tracking systems long enough for the vehicle to escape.
How Chaff Works
Radar works by sending out microwave energy and listening for it to bounce back off objects. Chaff exploits this by scattering huge numbers of thin, highly conductive fibers that are cut to very specific lengths, each roughly half the wavelength of the radar frequency they’re designed to defeat. At that length, each fiber acts as a dipole antenna, efficiently reflecting radar energy back toward the source. A single cartridge releases millions of these fibers at once, and the resulting cloud produces a radar return large and bright enough to look like a real target.
The cloud doesn’t just create a blip. It produces a large, diffuse false target that lingers in the air as the fibers slowly flutter toward the ground. A radar operator or a missile’s guidance system suddenly sees what looks like a massive object where nothing existed a moment before. Meanwhile, the aircraft or ship that deployed the chaff has moved away from that spot. Radar-guided missiles can lock onto the chaff cloud instead of their intended target, and ground-based radar stations lose track of the real vehicle in the clutter.
What Chaff Is Made Of
Modern chaff is made from silica glass fibers coated in a very thin layer of high-purity aluminum (over 99.9% pure). Each fiber is up to 2 inches long and about 1 mil in diameter, which is roughly 25 micrometers, thinner than a human hair. The glass core provides structural strength while the aluminum coating gives the fiber its radar-reflective properties.
The fibers are also coated with stearic acid, a common fatty acid, which acts as a slip coating to prevent the fibers from clumping together when they’re ejected. Without it, the fibers would stick to each other and fail to spread into an effective cloud. Stearic acid breaks down naturally in the environment.
This wasn’t always the design. The original chaff used during World War II consisted of strips of solid aluminum foil. The switch to aluminum-coated glass fibers happened in the 1980s, making chaff lighter, cheaper to produce, and easier to package in large quantities.
A World War II Innovation
Chaff was first used operationally on July 23, 1943, during Allied bombing raids over Germany. The British called it “Window.” Scientists on both sides of the war had independently discovered that dropping strips of metal foil could overwhelm radar screens, and early experiments produced, as one account described, “quite spectacular” results, with radar displays completely disrupted. But both sides hesitated to use it. British leadership recognized that once metal strips were recovered from the ground, German scientists would immediately understand the technique and turn it against Allied forces. There was no known defense against it.
The Germans faced the same dilemma. Hermann Goering, head of the Luftwaffe, suppressed research into the technique and prohibited its use for fear the Allies would adopt it. Winston Churchill eventually authorized Window’s deployment, and it proved devastatingly effective. It was widely used for the remainder of the war.
How Aircraft Deploy Chaff
Modern military aircraft carry chaff in small cartridges loaded into automated dispensing systems. The most widely used system in the U.S. military is the AN/ALE-47 Countermeasures Dispenser System, a computer-controlled unit that can release chaff (and other decoys like infrared flares) based on pre-programmed threat responses or manual commands from the aircrew. The system is reprogrammable, meaning it can be tailored to specific mission profiles and threat environments.
Standard U.S. chaff cartridges include the RR-188 and RR-170. Both contain about 95 grams (roughly 3.35 ounces) of chaff material and are the same physical size, but they differ in which radar bands their fibers are cut to defeat. The RR-188, commonly used by A-10 aircraft during training, has certain frequency bands removed to avoid interfering with FAA civilian radar. The RR-170 retains a fuller range of dipole lengths to create a more realistic radar signature during threat-avoidance training. When fired, the cartridge ejects its payload and the fibers rapidly disperse into a cloud behind the aircraft.
Chaff on Navy Ships
Ships face the same radar-guided missile threats as aircraft, and they deploy chaff using deck-mounted launchers rather than internal dispensers. The primary U.S. Navy system is the MK 36 Super Rapid Bloom Offboard Countermeasures (SRBOC) system, a mortar-type launcher that fires chaff cartridges into the air at specific heights and distances from the ship. Each MK 137 launcher has six fixed tubes arranged in two rows, set at 45-degree and 60-degree angles, allowing the ship to place chaff clouds at different positions around itself.
The goal is slightly different from airborne chaff. A ship can’t outrun a missile, so the chaff cloud needs to form quickly (“rapid bloom”) at a distance from the ship, creating a false target that draws the incoming missile away. The system can also launch infrared decoys and, in its upgraded MK 53 configuration, supports the Nulka active decoy, a rocket-propelled hovering device that actively broadcasts a false radar signal.
Why Chaff Still Works
Despite being nearly 80 years old as a concept, chaff remains effective because it exploits a fundamental limitation of radar. Any system that detects objects by bouncing radio waves off them can be confused by a dense cloud of reflective material tuned to the right frequency. Modern radar systems have developed ways to partially filter chaff, such as tracking the Doppler shift of a moving aircraft versus a slowly drifting chaff cloud. But chaff continues to evolve in response. Fibers are cut to cover multiple frequency bands simultaneously, and dispensing systems are programmed to release chaff in patterns optimized against specific missile types.
The aerodynamic behavior of the cloud itself also plays a role. As fibers fall, they tumble in complex patterns, some flattening out and others spiraling in helical motions. These varying orientations change how the cloud reflects radar energy over time, making it harder for radar processors to simply dismiss the return as chaff. Combined with other countermeasures like flares, electronic jamming, and towed decoys, chaff remains a core layer of military self-defense.