On the night of July 16, 2023, the Kerch Bridge connecting occupied Crimea with Russia was severely damaged in a surprise attack. Ukrainian intelligence officials told Ukrainian state media agency Ukrinform that their Navy had carried out the attack with kamikaze drone boats, an account later repeated by Russian officials.

A curious feature of the attack is that bridge supports at water level were not damaged; instead, the blast displaced the roadway high above the water. This suggests Ukraine used an unusual weapon, a type not seen since a secret U.S. bridge-busting munition developed in the Vietnam War which was claimed to deliver the force of a nuclear bomb.

The Invulnerable “Dragon’s Jaw” Bridge

Air forces often undertake bridge-dropping missions, targeting vulnerable points in supply lines. The standard technique is to destroy the supporting columns and bring down one or more spans. The rub is that bridge supports are massive concrete or metal structures and must be solidly built, making them stubbornly resistant to attack. It takes a substantial bomb to do more than scratch the surface of a bridge support.

The Thanh Hoa bridge in North Vietnam was a road and rail bridge carrying vast quantities of supplies to the south. A concrete abutment at each end supported the bridge, with a pier in the middle—a layout which reminded some of a gaping mouth, giving it the nickname “Dragon’s Jaw.” Five whole anti-aircraft regiments with over 150 medium and heavy anti-aircraft guns defended the bridge. The U.S. Air Force flew over 800 sorties against the bridge, and succeeded in hitting it with 120 weapons. These were a mix of 250-pound “Bullpup” guided missiles and unguided 750-pound bombs, but they all failed to bring down the supports or close the bridge.

The Air Force needed something more powerful to destroy the robust Dragon’s Jaw bridge, a radical new type of weapon. So in 1965, the service’s Armaments Research Laboratory at Eglin Air Force Base set out to build it. The details of the plan, part of a top-secret mission called Caroline Moon, were revealed in an Air Force monograph published 20 years later.

Kiloton-Power Punch

The bomb was based on a new principle known as mass focus: rather than distributing its blast in all directions, it would concentrate all the force in one direction, focusing it like a lens. This is similar in concept to shaped charges used to penetrate armor, but with the goal of transferring as much blast as possible along a single axis. The mass-focusing effect was achieved by detonation from the circumference traveling to the center.

The weapon built at Eglin was a pancake-shaped bomb eight feet across and two feet thick, weighing around 5,000 pounds; the Air Force built ten of them. Too big for any attack aircraft, the bomb was to be carried by C-130 transports and parachuted into the water upriver. It would float downstream and detonate when it passed under the bridge.

The Carolina Moon weapons required a whole set of new technologies including parachute deployment, river transit, and fusing.

The device had two fuses. The first was a radar fuze borrowed from the CIM-10 BOMARC surface-to-air missile with a wide field of view; this would give warning that the weapon was approaching the bridge. The second was an infrared optical fuze with a narrow field of view to trigger the bomb when it was exactly underneath the span.

Air Force tests on concrete targets showed that the mass focus effect was extremely effective. Theoretical calculations indicated that 20 to 30 feet above the bomb, the blast was equivalent to a one-kiloton nuclear warhead detonated under the bridge. That would be more than enough to bring down the road and railway above, which were not built to withstand upward pressure.

Five of the floating bombs were dropped upstream of the bridge. Later reports suggested that four of them went off, but for unknown reasons they had not damaged the bridge. The most likely suggestion is that a fusing failure detonated the bombs either too early or too late, and the focused blast missed its target.

The Carolina Moon plan had to be secret because there was a simple countermeasure: nets upstream would prevent any bombs from reaching the bridge. The U.S. attempted a second attack, but the C-130 was shot down before it could complete its mission, and no further efforts were made with this method.

In 1968, the Air Force deployed another new type of weapon against the bridge: Paveway laser-guided 2,000-pound and 3,000-pound bombs. Eight F-4 Phantoms attacked the bridge in 1972 and dislodged it from one abutment, finally halting traffic.

Since then, 2,000-pound laser-guided bombs able to bring down supports have been the preferred weapon for bridge dropping. This has not been an option for Ukraine, though, as Russia dominates in the air and Ukraine lacks effective strike aircraft able to get through Russian air defenses.

Kamikaze Drone Boats

Uncrewed surface vessels, or USVs, represent another way of attacking bridges, which by their nature are accessible by water. While Ukraine has used this type of vessel to target Russian warships several times, it was Russia that sent a USV packed with explosives to hit the Zakota bridge connecting Ukraine with Moldova in the first attack of its type. News reports suggested that one of the concrete bridge supports was damaged, though the bridge was not closed.

This is a sign that targeting the toughest part of the bridge with a relatively small explosive charge is not the best approach.

The damage is very different from the Kerch Bridge, where videos show that the concrete supports appear to be intact, but one stretch of the roadway they support is severely damaged. Rather than hitting the supports, Ukraine may have used a floating bomb with its blast focused upward.

New video of the attack released by the Ukrainians does not reveal much new detail. However, the CCTV footage from the bridge does show two of the drone boats, each of which is at least partially underneath the bridge when it detonates, rather than striking a support beam.

The Ukrainian USVs seen so far only carry a charge of about 440 pounds, far smaller than the Carolina Moon weapons, but something bigger may have been used on this occasion. It is also possible that the design is more sophisticated than that of almost 50 years ago, and precise modeling can produce those kiloton-strength effects with something smaller.

Either way, the effects speak for themselves. The roadway is more vulnerable than the bridge supports, and the bridge has been partly closed. Another strike could close it completely, and the success proves this is a viable way of striking bridges.

Ukrainian designers will be looking very closely at the effects of the July 17 strike on the Kerch Bridge and tailoring their next attack accordingly.