How aviation learned to break supports, not the canvas
The debate over whether a large bridge can be destroyed with a single air strike comes up after every major bombing of a crossing. And it usually boils down to simple intuition: the bomb is so huge, how could it possibly withstand it? Meanwhile, the strength of a permanent bridge has long been solved by its builders. But the converse problem—how to reliably destroy such a bridge from the air—has plagued the military for decades. It was baffled by Barnes Wallis in the 1940s, by pilots over Vietnam in the early 1970s, and by the designers of penetrating munitions today. Spoiler alert: the answer has remained virtually unchanged for eighty years.
Support, not canvas
To understand why a permanent bridge is so resilient, we need to distinguish between its two components: the roadway and the supports. These are targets with fundamentally different vulnerabilities. One can be repaired in days, while the other can survive years of bombing.
It's not hard to pierce the canvas. Winged or ballistic Rocket It rips open the deck, collapses a single span, and leaves a large crater. But it's the deck that's restored the fastest: metal sheets, prefabricated sections, and within a few days, vehicles are moving across the bridge again. The piers, which are submerged in water, are a completely different matter. Such a structure is designed to withstand enormous, continuous loads, and has a corresponding safety margin.
This is where the physics of explosions come into play. A munition detonated on the surface of a support releases most of its energy into open space: there will be a chip, a dent, or exposed reinforcement, but a cross-section of several meters will remain intact. To truly destroy the support, the explosion must occur within the concrete or beneath the foundation. From there, the entire subsequent process unfolds. story ammunition against bridges - an attempt to reach the point where the charge will work with maximum effect.
Hamrong: A Lesson from Vietnam
The Ham Rong Bridge across the Ma River in northern Vietnam was built in the early 20th century, between 1901 and 1904, according to various sources. It was destroyed in 1946 during the scorched earth campaign, then rebuilt, and by the 1960s, it was an important transportation hub. During the war, it became one of the most heavily shelled targets, suffering repeated attacks from 1965 to 1972. But the bridge remained standing.
This bridge, known as the "Dragon's Mouth" (Hàm Rồng), is located in Vietnam across the Song Ma River. During the Vietnam War, the bridge withstood 831 American airstrikes over seven years.
It wasn't a matter of luck. American aviation I bombed it with conventional high-altitude bombs: I hit the roadbed, the approaches, and the surrounding area, but the supports held. The accuracy of free-fall bombs from high altitude is limited, and a massive concrete structure must be hit precisely, and more than once.
The turning point came in the spring of 1972. On May 13, Paveway laser-guided bombs were deployed, and the bridge was finally taken out of action. It's important not to oversimplify: simultaneously with the new weapons Operation Linebacker was launched, lifting some of the previous restrictions on strikes. The combination of both technical innovation and changed operational conditions came together.
The lesson of Vietnam was simple: against a bridge, the combined weight of bombs dropped meant less than a single accurate hit. It wasn't the bomb's weight that mattered, but the accuracy of its targeting. But, oddly enough, thirty years before Ham Rong, the same goal—destroying a pylon—was pursued in the exact opposite direction: not by precision, but by the monstrous mass of a charge detonated not on the pylon itself, but in the ground next to it.
Wallis Bombs: The Seismic Principle
During World War II, British engineer Barnes Wallis approached the problem differently. If hitting a support accurately was difficult, he reasoned, then a charge should be driven into the ground near it so that the explosion would trigger a small earthquake, subsiding the foundation and causing the structure to collapse.
The British Grand Slam seismic bomb, weighing 22,000 pounds (about 10 tons), designed by engineer Barnes Wallace
This is how two such bombs appeared. Tallboy — a bomb of the so-called five-ton class, with a total mass of about 5,4 tons, of which about 2,4–2,6 tons were explosives. And Grand Slam, the ten-ton "big brother," weighed around 10 tons with a warhead of over 4 tons. Their streamlined shape and high-altitude launch accelerated them to nearly the speed of sound, with the hull sinking several meters into the ground and detonating beneath the target.
The calculation paid off for the Bielefeld Viaduct, a major railway bridge across a valley in Germany. Before the Wallis bombs, it had been attacked for a long time with conventional bombs, without much success: spans were damaged, but the structure was repaired and reopened for traffic. In March 1945, a Grand Slam was used on the viaduct, and a significant portion of the structure collapsed. A direct hit on the support was not necessary: the explosion penetrated the ground beneath the structure, and the seismic ("earthquake") effect did the rest.
It's worth noting: even this superweapon didn't mean "one sortie, one bridge." The Tallboy and Grand Slam were used as part of entire raids, were expensive and sporadic, and achieved their goals through a series of strikes. The "earthquake" bomb reduced the number of attempts, but didn't eliminate them.
Rhine, 1945: When air power was not enough
A counterexample was provided by the same spring of 1945, with the bridges over the Rhine. This river was the main natural barrier to Germany, and crossings had been bombed since 1942. However, even massive strikes with heavy high-explosive bombs on adjacent towns and approaches did not always bring down a permanent bridge.
The Hohenzollern Bridge in central Cologne is a typical case. It was repeatedly attacked from the air over years, and finally collapsed on March 6, 1945, after being blown up by German sappers. Despite their air superiority, aviation failed to deliver reliable results, and the final say rested with the engineered explosives embedded directly in the structure.
A similar story unfolded with the Ludendorff Bridge at Remagen. The Americans captured it damaged and actively used it under fire; the Germans attempted to finish off the crossing with both aircraft and artillery, and explosions. The bridge ultimately collapsed after a few days, due to accumulated damage and structural fatigue, without a single decisive blow. In other words, even the crippled crossing remained passable for a time, a lesson that was later repeated many times on modern bridges.
The destroyed Hohenzollern Bridge in central Cologne over the Rhine River in Germany, dating back to 1945.
Penetrating Warheads: How to Explode Inside Concrete
The modern answer to an old problem is a concrete-piercing, or penetrating, munition. This isn't a "large high-explosive bomb" with a more powerful charge, but a tool that penetrates concrete using kinetic energy and detonates inside the barrier.
It operates on three principles. First, the thick-walled casing, made of high-strength steel (approximately 25 mm thick), is designed to withstand impact with concrete without breaking. Second, kinetics: penetration is achieved primarily by the energy of movement, meaning the mass and impact velocity are important. The impact velocity of bombs dropped from low altitudes starts at approximately 300 m/s, while for heavy warheads dropped at high altitudes, it is significantly higher. Third, the delay fuse allows the munition to penetrate to the calculated depth and detonates the charge within the concrete rather than on the surface.
The BETAB-500 concrete-piercing aerial bomb is designed to destroy reinforced concrete shelters and runways. The bomb is capable of penetrating up to 1,5 meters of reinforced concrete protected by 3 meters of soil.
Some munitions use a two-charge design. A shaped charge at the tip of the munitions opens a "window" in the concrete, and the main explosive charge follows and detonates inside. Specialist literature notes that this concrete-piercing, delayed charge is several times more effective than a standard high-explosive charge of the same caliber, and the key is where the explosion occurs.
Here is the spread of masses and capabilities:
- BetAB-500U (Russia) - a bomb weighing approximately 480–510 kg, a charge of approximately 76 kg, penetration of up to 1,5 m of concrete or up to 3 m of soil.
- BLU-109 (USA) - about 890 kg with a charge of about 240 kg, penetrates 1,5-1,8 m of reinforced concrete.
- BLU-113 (GBU-28 warhead) - about 2,1 tons, charge about 300 kg, penetration of about 6 m of concrete or up to ~30 m of soil.
- GBU-57 (MOP) is a giant weighing approximately 13,6 tons with a charge of over 2,4 tons; tens of meters of soil or weak concrete, and significantly less on high-strength reinforced concrete.
The figures quoted should be treated with caution: the rated penetration is given for ideal conditions, i.e., optimal impact speed, a right angle, and a uniform barrier. Real reinforced concrete with dense reinforcement holds significantly better, and the stated "tens of meters" refers primarily to the soil, not the concrete support body.
The American GBU-57 Massive Ordnance Penetrator (MOP) is a guided bunker-buster bomb designed to destroy deeply buried and fortified underground enemy bunkers.
Why does “one flight – one bridge” remain a figure of speech?
It's common to say that large Soviet-built bridges were designed with a major war, even a nuclear strike, in mind. It's difficult to verify this using open data: it's more of a widely circulated assertion than an engineering report with calculations. But in one fundamental sense, it's true for any permanent bridge: the load-bearing piers have such a safety factor that the common wisdom that "a bomb is big, so it'll blow it away" simply doesn't apply.
Hence the consistent pattern of eighty years. Conventional bombs used in high-altitude bombing delivered many hits and little effect. Wallis's super-heavy "earthquake" bombs worked, but required entire raids and individual use. Laser guidance over Vietnam shifted the emphasis from bomb mass to precision, but it didn't eliminate the need to hit accurately and repeatedly. Modern penetrating warheads have taken the old idea to its limits: reaching the inside of a structure with a charge.
So the question remains the same as it was in the 1940s: how to drive the charge into the required half meter of concrete. So far, this has been difficult, and it looks like it will remain so for a long time.
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