Russian social media has been circulating images of a Ukrainian drone bomb which is square with a metal point like the ground spike for a fence post. This is a penetrating weapon, as a sticker on the bomb proclaims, and it has Russian military bloggers worried. Now nowhere is safe.

FPVs and drone bombing have driven Russian forces underground into bunkers and forced them to cover roads and facilities with nets, wire cages and steel mesh. A few weeks ago Russian reports started circulating of a new drone bomb punching through such defenses. The new images suggest that this weapon is now being produced at scale, and a Ukrainian video shows it in action.

Ukraine operates a fleet of heavy bomber multicopters of various types, notably the Vampire, but also Kazhan (“Bat”), Heavy Shot and Nemesis. The Russians call them by the generic term of Baba Yaga. Vampire can deliver a 33-pound bombload to a target ten miles away.

Multicopter bombers typically drop modified mortar bombs or anti-tank mines and similar repurposed munitions, depending on the target. But there are some purpose-built munitions for specific tasks, like this one.

Penetrating bombs date back to WWII when they were developed to hit heavily protected targets like battleships and concrete fortifications. The U.S. revived the concept 1980’s with the 2,000-pound BLU-109 “Bunker Buster” first of a series of ever more powerful up to the mighty 30,000-pound Massive Ordnance Penetrator.

These bombs differ from standard or “general purpose” bombs in three ways: they have a streamlined penetrating nose, reinforced to withstand impact; they tend to be long and narrow to concentrate force in a small area and reduce resistance; and they feature a delayed action fuse, so the bomb goes off after it had broken through protection. The fence post bomb, primitive as it looks, follows the same pattern.

The nose is designed to withstand the impact force of being driven into the ground. Since fence post anchors already exist, there was no need to reinvent them. Comparison with other items in the image, notably the Tolsen 300 mm hacksaw, suggests that the design is based on spike for a standard 100mm fence post. Commercial fence spikes are stamped from sheet metal and machine welded whereas this example seems to be hand-welded, and the metal appears to be thicker than a normal fence spike. If this started as a product from a hardware store it has been heavily modified in a garage workshop.

A rough calculation suggests that, assuming a 25-pound munition dopped from a few hundred feet, the penetrator will go through a foot of hard-packed dirt or four feet of soft soil.

Bunkers are typically constructed with a log roof (more resilient than concrete), covered with a foot or more of dirt to absorb blast and shrapnel. The penetrator will go through the earth covering and may find a gap in the log roofing.

The other key feature is the delayed action fuse. Most drone bombs are impact fused and will be set off by hitting the ground, or can be prematurely detonated when they strike a metal cage or netting. A delayed fuse means the bomb does not go off until it has penetrated.

A Ukrainaian video shows one of the mini bunker busters in action, as bomb is clearly visible with the only obvious difference from the version seen on Russian media being the color of the tail fins. This is dropped beside a building, and the delay seems to be more than a second after it hits the ground. When it goes off, the underground blast is enough to collapse one side of the building. This is miniature version of the earthquake bomb concept developed by British engineer Barnes Wallis in WWII and successfully used to destroy viaducts and other structures.

The new bomb is significant because it poses a major threat to existing counter-drone protection. Before, drone bombers could break into bunkers, but only by bombing the same sport repeatedly , which often took several sorties. This gave the occupants time to get away before their protection was destroyed. The new weapon gives no warning: the first bomb could destroy the bunker.

Heavier dugouts may currently be resistant to small drone-dropped bombs. That now changes. The precision of multicopter bombing means that multiple penetrating munitions can be dropped to successively deepen a crater and blast a way into the best-protected dugout.

Meanwhile, hundreds of miles of Russian (and Ukrainian) roads are covered in counter-drone net tunnels, and buildings are protected with wire mesh screens . These are good enough protection against FPVs and most drone weapons but are likely to be useless against the fence post bombs.

The bombers have smart bomb-aiming software and can hit targets including moving trucks and so will likely hit targets straight through the netting.

This is a particular concern for the Russians because Ukraine has a decisive advantage in heavy bombers. According to some sources they may inflict more losses than the FPVs which get all the media attention. For whatever reason, Russia has not yet succeeded in fielding its own heavy bomber drones at scale and so has no answer to the Ukrainian development.

The fence post bomb looks crude compared to slick factory-made munitions, with an equally crude jokey name. But the design appears sound and it gives the Russians a real and deadly problem. Every Russian dugout and road is now under threat from this bargain basement bunker buster, and there is no longer anywhere to hide.