Ever since we learned to fight we have striven to protect our most important places but with modern weapons is there a safe place anymore?
The most secure places we could build, hardened secure and even nuclear bunkers are now vulnerable to bunker-busting weapons that can go clean through 20 feet or 6 meters of reinforced concrete. They can even count the number of floors they punch through to make sure they detonate in exactly the right place.
But like every war, it’s a battle between two sides and in this case, it’s a battle between the construction of the bomb and the bunker and when one takes the lead it’s often not long before the other catches up but now new advances in concrete technology may well put an end to even the biggest bunker busters.
So in this video, we’ll look at how do you punch through 6 meters of reinforced concrete and can bunker busters remain an effective weapon.
The war between weapons and defensive armour has been going on for thousands of years and in a way it’s a war between toughness and durability, the toughness of the weapon to break through and the durability of the defences to absorb the blows.
This is the way armour and defence have worked since ancient times when kings & emperors would build forts of mud and stone walls to keep attackers at bay.
Soon siege weapons such as trebuchets or catapults were developed that threw large rocks and boulders to break the walls. Thicker walls gave greater protection until the invention of the canon and iron cannon balls capable of breaking even the thickest stone walls.
Over time sunken walls and Earthen mounds blunted the cannon balls effect until rifled barrels gave artillery much greater precision to take out any exposed positions.
The advent of steel reinforced concrete before the first world war made defensive positions much harder if not impossible to destroy with conventional bombs and shells and this is how it remained until the mid part of WW2.
However, 300-odd years ago Isaac Newton developed an approximation for the impact depth for projectiles at high velocities based only on momentum.
Simply put, the more dense the impactor is relative to the target the greater the depth of the impactor will be.
Using this simple theory a 1-meter-long uranium rod with a density of 19gm per cubic cm would be able to punch it way through 6 meters of rock with a density of 3gm per cubic cm.
However, there are many caveats to this, such as the hardness and durability of the materials used and what happens to the kinetic energy released as the impactor slows down and it also doesn’t allow for the impactor moving faster than the speed of sound and the shock waves created.
Take for example a lead bullet and the Kevlar vest. In theory, the lead bullet is much denser and should just push its way through but the Kevlar is durable enough to absorb the bullet’s kinetic energy slowing it down and causing it to deform and stop.
But put a hard steel case around the bullet and the kevlar will give way because the bullet is now hard enough to overcome the Kevlar. If we put hardened armour plates made from boron carbide in the Kevlar, the steel jackets around the bullets aren’t hard enough and will break up.
Putting a tungsten tip on the bullet will cause the ceramic plates will break because although the plates are hard they are not durable enough to withstand the Impact Toughness of tungsten and they shatter due to brittle failure.
A similar thing happens with bombs and bunkers.
Conventional bombs falling onto reinforced bunkers would cause little damage due to most of their energy being dissipated into the surrounding atmosphere. In order to destroy the bunker, they would have to break through the concrete and explode inside.
Although concrete may be hard and can withstand high compression loads such as when it supports a building, it is weak when stretched which makes it brittle.
When you can hit the concrete hard enough, the impact will cause the concrete to shatter and crumble around the impact zone. Although it breaks, it has absorbed some of the kinetic energy from the projectile to slow it down, if the concrete is thick enough, all of the energy of the projectile will be absorbed and it will be stopped. This is why hardened bunkers have roofs and walls that are 5-6 meters thick.
However, if you can hit the concrete hard enough with very dense, hard, high-speed mass it will shatter and crumble to such an extent that just the rebar is left which allows the impactor to continue through.
To do this the projectile and its contents have to withstand the immense forces involved.
The first real breakthrough came in ww2 when the British engineer and inventor Barns Wallis came up with first the 5-ton Tallboy and later the 10-ton Grandslam earthquake bombs.
These weren’t designed to hit the target directly, they were meant to land close to the target and using their mass and terminal speed when dropped from altitude and reaching 750 mph or 1250km/h to bury themselves deep in the ground before detonating with a timed fuse.
This would create large shockwaves, most of which would travel through the ground to destroy or damage beyond-repair traditional brick-built buildings up to 150 meters away. It would also create a camouflet or underground cavern which would collapse causing buildings of any construction nearby to fall into the resultant crater.
However, they were also used against some of the biggest and toughest buildings the germans ever made like the German U-Boat pens on the French Atlantic coast with layered roofs of up to 8 meters thick using reinforced concrete, steel and granite. Nothing short of a direct hit with a massive bomb would suffice and the Grandslam would be the biggest there was available.
In order to survive hitting the ground at near supersonic speeds, the cases and nose cones of the bombs were made from a steel alloy of chrome-molybdenum and just over half the weight came from the case alone.
Using their streamlined design, mass and speed when they were dropped from upto 22,000 ft, the 10-ton Grandslam could punch through up to 6 meters of reinforced concrete and then explode inside.
Although some did achieve this many broke up on impact which is not surprising as the shape of the bombs was designed for hitting the ground and was not ideal for punching through reinforced concrete.
Going back to Newton’s approximation for impact depth and the empirical design equation known as Young’s equation, the deepest impact depth could be achieved by a projectile that is long, thin and dense and strikes at very high velocity.
Whilst the Grandslam wasn’t ideal, the Disney bomb was.
This was created by British Royal Navy Captain Edward Terrell after he saw a Disney War propaganda film which showed a rocket-powered bomb, which also provided its name.
He wondered if it could be done in real life, if it could, it could accelerate to a much higher terminal velocity than the free-falling Tallboy and Grandslam. This very high speed coupled with a long thin dense construction could punch through hardened bunkers better even though it was smaller and had less mass.
Using the features of Newton’s and Young’s equations the bomb was 5 meters long and 28cm wide with thick walls and contained just 230kg of explosives yet the total weight was 2000kg or 2 tons. The long thin design concentrated all its kinetic energy onto one small point like an ice pick rather than a hammer.
19 x 3 inch solid rocket tubes were attached to the rear of the bomb which was timed to fire after 30 seconds from drop which would accelerate it to 990mph, 1590 km/h MACH 1.29.
Although it was designed by the British it was only ever used by the Americans, dropping 158 Disney bombs by B-17s in the last few months of the war.
In tests after the war, they were able to penetrate 4.47 meters at the Valentin U-boat bunker with one bomb not only going through the roof but also the 1-meter concrete floor below and ending up in the sand beneath.
Being much lighter, many more of the Disney bombs could be carried by a single aircraft but one of the biggest problems came from the accuracy required to hit the targets from 22,000 ft, there was no guidance system so they were effectively unguided missiles that relied on being dropped at precise height, distance and speed.
Although bunker busters as they became known were developed in the decades after WW2 by the US and other countries, advances like laser guidance enabled the precision to hit single buildings that was not available before.
However, when it came to the gulf war with Iraq in 1991, the US was concerned about the series of new command bunkers around Baghdad built deep underground and protected by several feet of reinforced concrete which the USAF estimated to be invulnerable to its existing 2,000lb or 907kg bunker busters. What they needed was something like the size of the Disney bomb.
Without the time to develop a new bomb casing, they looked to the industry for a solution, a long thin, heavy and very tough projectile and found one in used 8-inch howitzer gun barrels.
These barrels were modified at research laboratories including the Air Force Research Laboratory Munitions Directorate located at Eglin AFB, Florida and the Watervliet Armory in New York.
At 19 ft or 5.7 meters long and weighing at 5000lbs or 2200Kg they were filled with 285kg of explosive, fitted with a hardened nose cone, laser guidance and controlling fins from the GBU-27 LGB kits and given the designation GBU-28 or Guided Bomb Unit 28.
In sled tests, it could penetrate 20ft or 6m meters of concrete and in in-flight tests it reached a depth of 100 ft or 30 meters in the earth.
Only two were dropped in operation desert storm from F-111 bombers. One missed the target and the other scored a direct hit destroying a command bunker. Two days later the Iraqis surrendered when they realised that their bunkers were no longer a safe hiding place.
However, after the 2003 second gulf war, analysis of the bunkers hit by bunker busters showed that they hadn’t performed well or caused the required amount of destruction.
Over time concrete technology has come on with the development of UHPC or Ultra High-Performance Concrete first used by the U.S. Army Corps of Engineers in the 1980s, though it didn’t become commercially available in the US until 2000.
One of the leaders in this technology is Iran. Iran is an earthquake zone so it has a natural interest in making earthquake-proof buildings and that same technology can be used to make stronger bunkers.
UHPC uses normal cement but much purer quartz flour and fine sand plus other additives but also included is the addition of upto 1% fibres of high carbon steel, PVA, Glass, Carbon or a mixture of them. These fibres help stop cracks from propagating. The finer, purer quartz and sand produces a smoother, denser concrete that holds together better and combined with the fibres, the new UHPC is much less brittle and has more fracture energy, meaning it takes more energy to split it open.
Compared to the normal high-strength concrete which has a yield strength of 5 – 10,000 psi, UHPC has a yield strength of 40,000 psi or more.
In the late 2000s, word leaked out that a bunker-busting bomb had failed to penetrate an Iranian bunker and instead was embedded in the concrete without detonating, a worrying development for the US.
Since the early 2000s, the US after seeing the reduced impact of the standard bunker busters, worked on creating a super-large bomb even bigger than the MOAB or Mother Of All Bombs.
This would culminate in the 2011, GBU-57 or MOP Massive Ordinance Penetrator, a 6-meter-long, 30,000lb or 14-ton bunker buster with 2.4 tons of explosives that could penetrate 200ft of earth.
The GBU-57 is constructed using Eglin steel, a high-strength, high-performance, low-alloy, low-cost steel that was made for a new generation of bunker busters.
It has a tensile strength before deforming of 193,000 psi compared to construction grade steel at 30,000 psi and ordnance steel for gun barrels at 75,000 psi.
Although Eglin is the gold standard, it has now been superseded by USAF-96 steel, which has similar properties but is easier to produce and work with.
The MOP also has the same problems that the Tallboy and Grandslam had, in that their size and weight means only the biggest bombers can carry them on missions, currently the B2 stealth bomber and there is only so high that they can be dropped from before they reach their terminal velocity.
But even the MOP has been upgraded now four times because of the threat that newer stronger types of concrete represent.
While the MOP will go through 60 meters of normal 5000 psi concrete that drops to only 8 meters of 10,000 psi concrete and could be as low as 2 meters of 30,000 psi UHPC.
As far back as 1995 studies showed that with additional polymer fibres, although the compressive strength of concrete was only slightly incresed, its impact resistance improved sevenfold
In 2007, the University of Tehran made several concrete cubes capable of between 50,000 to 60,000 psi.
Micro-reinforced UHPC is the next generation of UHPC and is characterized by extreme ductility, energy absorption and resistance to chemicals, water and temperature. It was even used in the construction of the new World Trade Center in New York.
So while UHPC can greatly affect the performance of existing bunker busters, new types of hybrid construction can go even further, and one of those is “Functionally Graded Cementitious Composite”, or FGCC.
This is made by layering different types of UHPC. First an outer layer of very hard UHPC, then a thick layer of hybrid fibre-reinforced UHPC on top of a thick layer of tough steel fibre-reinforced UHPC.
The effect of combining these layers is to create a type of concrete that is tougher than any single type.
According to Chinese research published in June 2022, this layered approach of FGCC resisted penetration and explosion far better than UHPC and of course, the US and other countries are developing their own versions.
So where is the US going next, well back to the Disney bomb idea of using rocket assistance of smaller bombs to get the terminal velocity up and hence its ability to punch through.
But we are reaching the limits of what can be done without resorting to nuclear bunker busters. Hypersonic missiles could be used without an explosive warhead just relying on the huge amount of kinetic energy they have but at the multiple MACH speeds, they run the risk of breaking up or vaporising when they hit the target and does not guarantee that it would work against the new FGCC composite concrete.
In the end, maybe the best strategy would be to destroy the entrance and exits and communications links, because if no one can get in or out or communicate to the outside world, even if they are alive inside, then they might as well be just a crater in the ground.
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