This account will be featured in a new, non-fiction book out this fall by Stephen Coontz, author of The Flight of the Intruder and many other techno-thrillers.
The Vietnam War was not going well. The Dragon's Jaw was biting hard, devouring American planes and aircrew since 1965 and nobody knew how to pull its teeth. The Thanh Hoa bridge over the Song Ha River 3 miles (4.8 km) north east of Thanh Hoa, North Vietnam carried both rail and road traffic, a key link on the main route for supplies flowing south to the Viet Cong. Both sides in the war understood well the importance of the bridge as a possible choke point. The North Vietnamese stationed 3 air defense battalions near the bridge armed with AAA guns of all calibers. MiG-17 interceptors were on call in the area as well. By some estimates, 75 US aircraft, including a giant C-130 transport and its 13 man crew, were lost in operations trying to drop the bridge, earning it the nickname, "The Dragon's Jaw" from the Vietnamese.
Nothing seemed to work. Squadrons of F-105 Thunderchief fighter-bombers attacked with 750-pound iron bombs and Bullpup command-guided missiles. The US Navy tried Walleye TV-guided glide bombs. USAF C-130 four-engined transports dropped magnetic mines upstream, which floated down and detonated under the bridge. Scorched, bent, and scarred, the bridge endured, never out of service for more than a few hours. Along with its counterpart, the Paul Doumer Bridge over the Red River, linking Hanoi and Haiphong, the Thanh Hoa bridge was an obsession for US planners working the target list in North Vietnam considered politically acceptable by the White House. But, absent a way to sever the steel trusses spanning the river once and for all, additional mass raids, with the inevitable losses, seemed criminally futile and dangerous to the over-stretched aircrews.
At the US Navy's Naval Air Weapons Center, China Lake, CA, a group of engineers tried to come up with a plan. Accuracy wasn't the problem, a Walleye's TV guidance system could deliver ordinance to the bridge. The weapon's 250 pound linear shaped-charge warhead was successful in cutting steel beams and girders in the test arena. But, against real bridges like the Dragon's Jaw, not so much. The team urgently needed a new, more powerful warhead for the Walleye.
The weapons wizards at China Lake thought the Los Alamos National Laboratory, north of Albuquerque, NM, might hold the key to fracturing the Dragon's Jaw. Originally developed for the Davy Crockett recoilless rifle system, W72 nuclear warheads were available. The miniature device was the most tested nuke in the US inventory. Dozens of trials had been run, not to increase the explosive yield as with other warheads, but to decrease its power. A short-range nuke with the equivalent power of many kilotons of TNT would be like a hand grenade with a 100 foot kill radius and a 2 second delay, i.e. suicide to deliver from a jeep-mounted recoilless rifle. Eventually, the yield was dialed down to "only" 20 tons of TNT, .02 kilotons, 1/1000th of the size of the fireballs at Hiroshima and Nagasaki which ended WWII.
The USAF had adapted the tiny nuke warhead for the Falcon air-to-air missile, tasking it to blast whole formations of enemy bombers out of the sky at once. Warhead power was scaled back up to four hundred tons for the Falcon. The atom scientists in New Mexico were confident they could improve the yield to just over six hundred tons using the W72's salvaged from retired Falcons. The equivalent of six hundred tons of TNT would trash any bridge in the world with one weapon delivered by one fighter aircraft.
The cylinder-shaped W72 measured just 10 3/4 inches (27cm) in diameter with hemispherical ends 15 1/2 inches (39 cm) apart and weighed only 50 pounds (23 kilos). It would slip easily inside the Walleye's tubular body, replacing the heavy conventional warhead and extending the glide bomb's range. The China Lake team considered it a marriage made, not in heaven, but in the fires of nuclear hell.
The Walleye had been developed at China Lake and was the first American terminally guided munition, a "smart bomb," to actually perform well in combat. The term "smart bomb" came from the different paths bombs take once released from a aircraft. Dumb bombs only know where the ground is, smart bombs know where the target is.
The glide bomb's nose featured a flattened glass dome with a TV camera staring out through it. Resemblance to a giant fish's eye earned the Walleye its nickname. Why the China Lake boffins, stuck out in the middle of the Mojave Desert, named their creation after a cold water member of the pike family from Minnesota remains a mystery. Launched from altitude, the Walleye could glide over 30 miles with a spinning ram air turbine supplying electrical power, and would hit within a few feet of the aim point. Six hundred tons of explosive power vaporizing the center span of the Thanh Hoa bridge should break the Dragon's Jaw forever.
Employing the Nuke Walleye would be a piece of cake. As with the conventionally-armed weapon, a fighter pilot would set up a shallow dive from high altitude miles from the bridge, aiming his aircraft to put his gun sight on the bridge. The Walleye's TV camera was bore sighted to stare at the target under the projected gun sight. A radar screen in the launch aircraft doubled as a black and white TV set displaying the target area as seen by the weapon's sensor. Usually, only a slight adjustment in flight path was needed to superimpose the Walleye's aiming crosshairs on the scope over the target and to lock the camera on. Once the display had settled down after a second or two, a quick press of the bomb release button launched the Walleye. The TV guidance system would guide the bomb to a direct hit, homing in on optically contrasting details on the bridge.
After the conventional Walleye left the aircraft, the cockpit TV display went snow white, no further action by the aircrew was needed, or even possible, the bomb was on its own. The launch aircraft was free to get the hell out of the reach of the Dragon's breath defenses.
Nuke Walleye boasted one additional trick. After launch, the cockpit TV display wouldn't go blank. A video data link would transmit the TV camera's picture back to the aircraft until impact. Dropped with the warhead in a safe condition, the W72 had to be armed after the aircrew monitoring their TV display determined that the bomb was headed for the right target. A second button push would arm the weapon with a signal sent back along the data link from the jet to the bomb. A black stripe across the scope signified the arming command had been sent and that a mini-mushroom cloud would soon blossom. This added step was easier in a two-seat aircraft like the A-6 Intruder or F-4 Phantom. The second crewman, the Bomb/Nav (USN) or the Weapon Systems Operator (USAF), could confirm the aim point and arm the W72 while the pilot kept his attention out of the cockpit, scanning for MiG's or surface to air missiles.
What about the effects of a nuclear blast on the area around the bridge? The W72 was not a threat to a unprotected person standing only 3000 feet from ground zero. There would be some collateral damage, but this was judged to be worth the price for not losing any more aircraft, or the war.
The Nuke Walleye was designed, the components were tested, the aircraft were modified, and aircrews were trained in its use. I can attest to this as I trained the USAF crews. It is unclear from the records if any of the terminally-guided, nuclear-armed smart bombs were ever fully assembled, certainly none were used. Why not?
American leaders came to the conclusion that nuclear weapons were not just devices with bigger bangs, but represented an entirely different class of weapon, one reserved for existential national threats, not hard tactical problems. So, the Vietnam agony dragged on, war material still flowing south over the bridges. But, conventional terminally-guided weapons were improving rapidly. In 1972, 12 F-4's flying from Ubon, Thailand, dropped the Thanh Hoa bridge with 2000 pound Laser-Guided Bombs with laser illumination provided by Pave Knife targeting pods. The LGB's hit within 3 feet of their aim point. The Dragon's Jaw was shattered by the USAF Phantoms, without a single aircraft loss and without unleashing the nuclear genie.
The only surviving (unarmed) Nuke Walleye can be seen in the Atomic Weapons Museum on Kirtland AFB, Albuquerque, NM. To enter the museum, you need a military ID card and to not be pregnant.