Bridge Dropping 101

Bridges are natural chokepoints, making them obvious targets for military action. But they are robustly built and most weapons have little effect. Blast and shrapnel effects that are lethal to personnel and soft-skinned vehicles barely scratch stone and concrete, and direct hits with large amounts of explosive are needed. Even heavy artillery generally fails to do the job, which usually requires 250 pounds or more of explosive in one hit.

During WWII, the U.S. military carried out extensive research on bombing road and rail bridges. They found it took an average of 190 sorties by medium bombers dropping a phenomenal 350 tons of bombs, to take out a bridge, These were a mix of 500-pound and 1,000-pound weapons.

The number of bombs needed decreased sharply with bombing accuracy, the report stating “It is worth noting that if bombing accuracy could be Improved to something approaching training school accuracy….The number of sorties required would be reduced from 190 to 33.”

These days the USAF uses precision-guided 2,000-pound bombs for dropping bridges, as these have the requisite weight of explosive and are accurate enough that only one or two weapons are needed.

FPV drones provide even better accuracy with a much smaller payload, typically a repurposed warhead from an RPG anti-tank rocket launcher weighing around five pounds. This warhead is a shaped charge which produces a narrow jet of high-speed metal capable of punching through a foot of steel armor or more than three feet of concrete.

This is not an obvious choice of munition for attacking bridge supports. The columns are reinforced concrete and the warhead is unlikely to damage the steel rebar. But the Russian drone operators knew what they were doing.

Termite Tactics

The video reportedly shows an attack by Kontora group of the Zapad unit, posted on the Telegram channel "Military correspondents of the Russian spring." The bridge is described as crossing the Nitrus river near the village of Andriivka in the Kherson region. Several successive FPVs strike each of the bridge’s five supporting columns. They claim that 43 FPVs were used, the video shows about 37 of them.

The first couple of hits on each column just chip the concrete, but after that the column cracks up and larger fragments break off. After multiple hits a section of concrete is stripped away, exposing the bare skeleton of steel reinforcing bars.

The key to the attack is the how reinforced concrete works. Adding steels bars to concrete makes it less brittle and greatly improves the tensile strength. Unlike plain concrete, a reinforced concrete beam can withstand lateral forces as well as compression without breaking. This makes a bridge resilient to lateral movement from earth tremors or other forces. But the compressive strength is still provided by the concrete. Blast that brittle material away and the beam loses perhaps 80% of its loadbearing capacity. (The actual proportion depends very much in the exact design).

Bridges are designed with a huge safety margin, and a reinforced concrete bridge can generally take at least twice what it is ever expected to carry. However, without the concrete in those support beams the bridge cannot carry its own weight and slumps, as we see in the final few frames of the video, the rebar starts to crumple and the bridge slumps.

It turns out you do not need to cut through the rebar if you chip away enough concrete. The entire two-lane road bridge, carrying the highway for something like 100 feet over a 15-foot drop, has been brought down by small quadcopters.

Future Bridge Busters

This tactic may already have been used widely by both sides but not shared for reasons of operational security. The way the drones head unerringly for aim points suggest well-drilled operators who may have done this before. It is unlikely to come as any surprise to the Ukrainians. But it does suggest that at the very least protective netting may be need on vulnerable bridges.

Precision makes this an extremely efficient type of attack: under 250 pounds of munitions, compared to the 350 tons of bombs dropped on each bridge in WWII.

It is also a remarkably low-cost operation. 43 FPVs probably cost less than $25,000, or perhaps half as much as a single U.S. precision-guided bomb, and without needing for a jet aircraft to deliver it. You could carry out forty such attacks for the cost of one ATACMS missile.

The attack seems to have used standard FPVs. We know Ukraine has FPV warheads with linear shaped charges which can cut through rebar and other structural materials, because these were used against Russian aircraft in Operation Spiderweb. Such weapons would destroy bridge supports with far fewer hits.

The changing daylight in the video suggests that the FPVs attacked over a period of hours. Systems like Ukraine’s Pasika which allows a single operator to control a large number of FPVs, keeping a number orbiting nearby and calling them down in succession, would allow such an attack to be completed in minutes.

The bridge supports are static targets, and easy to pick out visually. This would make programming a machine vision system to attack them relatively straightforward, removing the need for an operator to control the drones.

FPVs are sometimes seen as short-range weapons. But with add-on wings they can hit targets some 60 miles away. And FPVs transported by carrier drones can hit targets hundreds of miles away, a tactic which is used increasingly by both sides.

Previously FPVs have destroyed bridges already mined for demolition, by setting off the explosive charges; there have been several examples of this. But we now know that it only takes a few dozen drones to take down even a seemingly sturdy structure.

The next question is just how large a structure FPVs could bring down. There is no obvious theoretical limit; it is more a question of how long it takes to marshal and direct the required number of drones. Certainly larger road and rail bridges start to look like targets. FPVs can also attack traffic on the bridge, lay mines on it, and disrupt repair operations. Skyscrapers and other large buildings may also be at risk.

Small weapons can have big effects. We just do not know how big yet.