The armament downed more American planes and helicopters than all other air defense weapons combined

American airmen flying over North Vietnam faced one of the most intensive and highly developed air defense systems in history. Although the North’s fighter planes and its surface-to-air missiles (SAMs) got the headlines, it was the light anti-aircraft guns and anti-aircraft artillery (AAA) that inflicted the heaviest losses. North Vietnam deployed more than 8,000 of these weapons around key targets throughout the country, with calibers ranging from 12.7mm machine guns to 57mm automatic cannons. These weapons inflicted more than 77 percent of the combat losses suffered by the Air Force and 52 percent of the Navy’s. They covered every major target in North Vietnam and in the countryside. After 1969, AAA guns also began to appear in key areas of Laos and some areas of South Vietnam. By 1972 they were deployed in and around North Vietnamese supply depots, artillery sites and staging areas in western and northern South Vietnam. Every aviator who flew over that country speaks of the ubiquitous AAA menace.

The largest of these weapons was the 57mm S-60 gun. Based on the German 55mm automatic flak cannon that was entering service just as World War II ended, the S-60 was a deadly close-in point defense system when employed by a well-trained crew. The gun fired a six-pound shell with a proximity fuze, at a maximum rate of 120 rounds per minute (rpm). The shells were loaded manually via four-round clips weighing 36 pounds. As a result, loader fatigue and other factors limited the gun’s sustained rate of fire to about 70 rpm. Maximum effective altitude was 28,000 feet and horizontal range was 12,000 meters, but the gun was used almost entirely for low-altitude engagements. Most batteries opened fire at ranges of 4,000 to 6,000 meters and fired on targets operating below 5,000 feet.

Each battery of six S-60s was equipped with a single “Flap Wheel” or “Fire Can” fire control radar and concentrated on a single target. Units employing radar fire control had a greater effective range than those using visual fire control. Successful American jamming and deception of the gun’s fire control radars eventually led the Vietnamese to rely increasingly on visual fire control. The American ALQ-71 and -72 Electronic Countermeasures (ECM) pods proved very effective when they were first introduced, and the later ALQ-134 ECM systems maintained American electronic superiority over the later versions of the Flap Wheel radars sent in from the Soviet Union.

The S-60 gun also was mounted in open twin turrets atop a modified T-55 tank chassis. Designated the ZSU-57-2, it was intended to accompany mobile units. It could not, however, use radar fire control or be integrated directly into the air defense network. Introduced into Soviet service in 1957, only limited numbers of ZSU-57-2s saw service in Vietnam.

The Vietnamese acquired more than 1,000 S-60s from both the Soviet Union and China. Those in Cambodia and South Vietnam later were committed primarily against helicopters. There are reports of several U.S. Navy A-1E Skyraiders surviving such hits, but the Skyraider was a tough plane to bring down. The S-60 only averaged one hit for about every 8,500 rounds fired, but the volume of fire from the one to four batteries protecting a North Vietnamese target amounted to 420-720 rpm. Moreover, the S-60 was rarely the only anti-aircraft gun protecting those facilities.

North Vietnam acquired more than 2,000 37mm M38/39 anti-aircraft guns from the Soviet Union and another 300 to 500 of the Chinese variant. Introduced in 1938, the M38/39 was the Soviet Union’s primary AAA weapon in World War II. Unlike the S-60, the M38/39 was fired over an open gun sight and didn’t have radar fire control. A manual range predictor determined the target range and altitude and when to open fire. In most engagements the entire six-gun battery massed its fire against the lead aircraft. In some circumstances, each gun crew chose individual targets and engaged independently.

The M38/39 fired 180 rpm for short bursts and had a sustained rate of fire of 80 rpm. Maximum horizontal range was 8,000 meters and theoretically, it could reach a target at an altitude of 19,000 feet. Its actual maximum effective range against an aerial target, however, was 3,000 meters and most gun crews didn’t open fire until the target was half that distance away. It fired a 1.5-pound shell loaded manually from 5-round clips. Although it had a good rate of fire and traversed very quickly, the M38/39 had difficulty engaging a high-speed crossing target. The guns were generally positioned along approach azimuths or in large numbers all around the target with very narrow engagement lanes.

The famous ZSU-23 came in both towed and self-propelled versions. The towed version consisted of two 23mm cannons on a twin-wheel carriage. A single gunner aimed at the target over an open sight. The loader fed the weapon from 50-round belts. Generally, each towed ZSU-23 had a sustained rate of fire of 200 rpm. Theoretically it had a maximum rate of 1,000 rpm per barrel, but few engagements lasted more than 15 seconds. The gun barrels overheated if anyone tried to put 1,000 rounds through them in a minute’s time. Maximum and normal target altitudes were 16,000 and 1,500 feet, respectively. Normally, a four-gun battery opened fire at a range of about 1,000 meters.

The self-propelled variant, the ZSU-23-4, was the deadlier of the two weapons systems. Equipped with an independent fire control using the “Gun Dish” radar, the ZSU-23-4 could start tracking its target from 20 kilometers out. The combination of four 23mm cannons and a jamming/deception-resistant fire control radar proved very effective, particularly against helicopters. The system was first delivered to North Vietnam in late 1971. The North Vietnamese deployed ZSU-23-4s into Laos shortly thereafter. They initially appeared in Cambodia in 1972 and there were reports of them west of Khe Sanh during North Vietnam’s “Year of the Rat Offensive” that year. Radar fire control increased the guns’ effective range to more than 3,000 meters and raised the effective engagement altitude to 4,000 feet. The ZSU-23-4s were deployed in four-vehicle companies that accompanied tactical units in the field. They rarely were encountered around fixed facilities in North Vietnam. Helicopter pilots and forward air controllers developed great respect for the ZSU-23-4, which was equally effective at engaging vehicles and other ground targets.

The weapon that helicopter pilots and forward air observers feared most was the 14.5mm ZPU-4, based on the 14.5mm KPV heavy machine gun first developed during World War II. Consisting of four machine guns, each with a cyclical rate of fire of 600 rpm, mounted on a four-wheel carriage, the ZPU-4 put out 2,400 rpm. Ammunition carried in drums contained a belt of 150 rounds. The loader’s job was to keep the weapons fed with fresh drums. The system’s two-ton carriage was less than 6 feet wide and it could be disassembled easily for transport or concealment. Pilots and aircrew rarely spotted it before they were engaged.

The ZPU-4 used the ZAPP-4 mechanical computing sight, but the gunners generally fired it via optical sights. Effective range against a target flying below 1,500 feet and 200 knots was about 1,000 meters. Although ineffective against crossing high speed targets or those maneuvering radically, it was deadly to aircraft flying directly into the gun-target line. Most often encountered in Laos and South Vietnam, the ZPU-4 served in large numbers throughout Indochina. Reportedly, the North Vietnamese acquired some 2,000 between 1958 and 1968.

Late in the war, the ZPU-4 was augmented by two-man teams carrying SA-7 Grail missiles. The ZPU-4 also was deployed in a single mounting, as were captured U.S. .50- caliber M-2 machine guns and Soviet 12.7mm machine guns. Although not as dangerous to fliers as the ZPU-4 and heavier antiaircraft weapons, machine guns could take out or severely damage helicopters at low altitude, especially if the pilot was caught by surprise. Scattered around the countryside, the machine guns ensured that no pilot could relax at low altitude anywhere in Indochina.

North Vietnamese air defense tactics relied on defense-in-depth, both horizontally and vertically. Surface-to-air missiles were the first line of defense, forcing the Americans to dedicate a growing percentage of their air assets to the SAM suppression mission. Though the SAMs didn’t down that many aircraft, particularly after the United States started employing chaff and ECM, they did force tactical aircraft down to lower altitudes where they were engaged by AAA. Heavy AAA provided medium altitude coverage around key facilities. Light AAA and small arms (14.5mm, 12.7mm and smaller automatic weapons) were scattered throughout the country and fired at whatever passed overhead. Most of the light AAA was positioned in rings around key facilities and scattered along the Ho Chi Minh Trail.

North Vietnam reportedly deployed more than 8,000 anti-aircraft guns and another 1,000 to 2,000 heavy machine guns for air defense during the war. Unlike the heavy AAA, North Vietnam’s light AAA units were widely deployed outside the country, including Laos, Cambodia and South Vietnam. They downed more American planes and helicopters than all other North Vietnamese weapons systems combined.

More significant than the losses the light AAA inflicted was the effect those weapons had on bombing accuracy in those days before standoff munitions and smart bombs. Any pilot flying into a target over North Vietnam had to dodge SAMs, fly through heavy AAA and then survive a hail of light AAA and small-arms fire, both as he made his final approach to the target and as he pulled away. Most of North Vietnam’s fighter pilots then tried to ambush the pilot just as he pulled out of AAA range, when his speed was reduced and his attention focused on the AAA. No pilots had faced such a deadly gantlet before, nor have any since.

Originally published in the October 2007 issue of Vietnam Magazine. To subscribe, click here