The Pentagon Deployed Scent Warfare in Vietnam
Electronic sniffers literally smelled for the enemy
by ADAM RAWNSLY
During the war in Vietnam, the U.S. military struggled with a basic requirement of any war—merely finding the enemy.
The rugged country’s thick forests proved impenetrable to many sensors. Making matters worse for the Americans, communist guerrillas dug miles and miles of underground tunnels and could disappear before U.S. troops’ eyes.
In its desperation, the Pentagon experimented with some truly exotic, even bizarre, technologies. Troops received new generations of starlight scopes in order to see in the dark.
Researchers also developed unattended ground sensors including the Acoustic Seismic Intrusion Detector, which the Navy deployed over land from sub-hunting surveillance planes.
But no device was stranger than the personnel detector—an electronic nose for literally sniffing out the bad guys.
Although the “sniffer” personnel detector certainly stands out as among the military’s Vietnam-vintage man-trackers, it wasn’t an entirely new concept.
Years before, the Navy had equipped surveillance planes such as the P-3 Orion with the General Electric ASR-3, which smelled for the fumes from a submerged submarine’s exhaust.
Building on the ASR-3, in 1965 General Electric also developed—for the Army Chemical Corps—a backpack version of the system that it dubbed the XM-2.
Instead of diesel, the personnel detectors smelled the air for ammonia particles and condensation nuclei from sweat and campfires—strong indicators of human presence.
The device looked something like a proton pack from the Ghostbusters movies. Troops wore the XM-2 on their backs and fed it air samples through a hose that attached to the barrel of an M-16 rifle.
In 1967, the Associated Press took notice of the 20-pound sniffer’s popularity among some troops in Vietnam.
But the device was hardly perfect. According to the AP, troops carrying the backpack version were quick to get false readings from the ammonia effluence of their own unit. Officers had to assign sniffing duty to whomever was on point and upwind.
The Army improved on the XM-2 with a larger iteration for helicopter use—the XM-3 Airborne Personnel Detector. Copters sampled the air while flying low above the treeline.
Flying at most 50 feet over the trees, the sniffing aircraft were prime targets for insurgents lurking below. The Army stipulated that that sniffer missions should include a minimum of three helicopters.
One to carry the detector. Another, a gunship, flying ahead as a navigator at an altitude of at least 500 feet. And another gunship chasing no more than 50 feet behind the sniffer to protect it.
Fortunately for the operators, humidity — a constant in the Vietnamese climate — actually improved the accuracy of readings, particularly after the rain washed away background scents.
But high winds, nearby civilian villages and the residue of recent firefights could made detection difficult.
Once a sniffer sensed large concentrations of ammonia or condensation nuclei or received “maximum readouts,” troops could call in air strikes, troops or artillery or deploy C.S. — more commonly known as “tear gas” — to force the enemy out of hiding.
Whether the personnel detector was worth it costs is a contentious question.
Troops who used the detector claimed that the device could get false readings from animal life. One story, from an Australian army aviation regiment veteran who fought in Vietnam, illustrates the skepticism some felt about the device.
In 2002, Peter Rogers told an interviewer about his experience flying over a tree containing “30 or 40” birds’ nests.
“We flew directly over it and as we went over it the operator said, ‘Mark 10,’ which is the maximum and I immediately radioed and said, ‘No, cancel that, we’ve just flown over a tree full of birds’ nests.’ Too late! It went into the books. The next day they put a B-52 strike on the tree.”
Others reported that the Vietcong were wise to the scent warfare and devised countermeasures. Communist forces were known to use soap, aftershave, garlic and chili peppers to throw off American tracking dogs. The same methods might have foiled the electronic sniffers.
There were reports the guerrillas hung buckets full of mud and ammonia-rich urine as decoys for the smelling devices.
Still, some soldiers swore by the detectors. In a controversial book revisiting the war, Lt. Gen. Julian Ewell, commander of the 9th Infantry Division during the war, and his then-chief of staff Maj. Gen. Ira Hunt praised the helicopter-borne scent finders.
The generals claimed that “over 33 percent of all significant readings were confirmed by operational contacts.”
The two officers also dismissed reports of false positives from animals, writing that “repeated structured tests have shown that most animals do not exude a scent which is detectable on the people-sniffer.”
Decoy urine buckets “not widely encountered,” they wrote.
Some units in Vietnam also lauded the detectors. A 1967 report from the 101st Airborne Division claimed the devices “effectively produced intelligence in areas of heavy vegetation where visual reconnaissance was ineffective.”
The 101st’s reports state that the detectors were useful for vetting human intelligence sources’ reports of insurgent activity.
That’s a sentiment that echoes throughout the literature on the Vietnam War’s strange sniffing machine. Ultimately, it appears the detector was most helpful when combined with other sources of intelligence.