The Insane Engineering of the M1 Abrams
The M1 Abrams entered service in 1980. A fast, heavily armored tank with the very latest technologies to give it every advantage on the battlefield. First seeing action in Operation Desert Storm. The plain arid camouflaged tank gained a reputation quickly in the barren deserts of Saudi Arabia and Kuwait. Powered by a high speed turbine engine, the M1 raced across this difficult terrain to liberate Kuwait from Iraqi occupation. The german designed 120 mm smoothbore cannon had longer range, more accurate fire and more advanced ammunition.
It easily dispatched the lower tech soviet supplied tanks of the Iraqi Army Sometime this year it’s expected that the US will deliver a battalion of 31 M1 Abram tanks to Ukraine to once again face off against the Soviet tanks it was designed to battle. The Ukrainian Army will need training to maintain and operate these new tanks, because it’s unlike any other tank. And although this tank are over 40 years old it's still more than capable of holding its own on the battlefield, thanks to continual modernisation and forward thinking design. This is the insane engineering of the M1 Abrams. The M1 Abrams, upon engine start up, sounds more like a military aircraft getting ready to take off. *minor pause 1-3 seconds to allow sound to be heard* A terrifying increasing pitch as the engine revs up to speed.
The whirring sound emanating from the rotating turbine blades hidden inside. The turbine engine, typically used for jet aircraft, is an engine designed for high speed operation with minimal weight, a counter intuitive choice for a heavy battle tank. So why was it chosen? The turbine engine would give the M1 two huge advantages. They are much lighter than an equivalent diesel engine, weighing just 1.1 tonnes, developing 1500 brake horsepower. The V12 diesel engine of the Challenger 2 produces just 1200 brake horsepower with 2 tonnes of metal.
This compactness and extreme high power to weight ratio allows the M1 to stack on layers of armor without sacrificing acceleration or top speed. We can plot torque vs shaft speed of the HoneyWell AGT1500 turbine engine of the M1 against the V12 MTU 883 of the Challenger 2E, and we can see the turbine has a massive torque advantage at lower shaft speeds. In its lightest configuration M1 weighs 61.8 tonnes. A typical toyota corolla weighs 1.1 tonne.
That’s equivalent to 56 Toyota Corollas, yet this thing can still accelerate from 0-32 kilometers per hour in just 7 seconds with a top speed of 72 kilometers per hour. The M1 Abrams primarily uses diesel fuel, as diesel can act as an additional layer of armor. Liquids are extremely good at absorbing energy from explosions and kinetic energy weapons, they are most effective against shaped charges, and tanks carry a lot of liquid in the form of fuel. It may seem counterintuitive to use a fuel as protection from an explosion, but diesel fuel is not very flammable. In fact if you throw a lit match into a puddle of diesel it will put out the match. Diesel engines require immense pressure and a sustained flame to ignite.
Because of this diesel can actually be used with relative safety as armor. However the turbine engine can also operate on most fuel types. This is a huge advantage in military logistics. Making joint operations with NATO vastly easier.
In the years following World War 2. The Soviets favored diesel compression ignition engines. The western Allies used a mixture of both gasoline spark ignition and diesel compression ignition.
Transporting fuel is one of the largest logistic challenges in war. Needing several types of fuel makes it all the more difficult. The M1 can run on marine diesel, gasoline or even kerosene if needed.
Which not only makes it easier to procure fuel in a warzone, but comes with the added bonus of helping the M1 Abram operate in hot or cold weather. Dealing with the wild swings in temperature that Ukraine can expect throughout the year with ease. Where diesel fuels can crystallize at low temperatures, kerosene can be used instead. The turbine engine of the M1 works similarly to aircraft jet engines, with some important differences. Air enters the engine here, where it is compressed by the low pressure compressor, and then the high pressure compressor, each individually driven by separate turbine stages. These turbine stages are driven by the combustor, which works a little differently to typical aircraft combustors.
It is mounted perpendicularly to the engine, and protrudes out of the engine. This makes maintenance access to the combustor easier, with only a simple bolted cover needing to be removed. There are two drive shafts in this engine. A secondary drive shaft, driven by the high pressure turbine, which runs forward to an accessory gearbox that runs things like compressors, electronics and hydraulics. The main drive shaft, driven by a dedicated power turbine, which is not connected to the compressors, runs rearward to the tanks drive sprockets.
This primary driveshaft and its reduction gearbox is surrounded by something called a recuperator. This is the biggest difference between this turbine engine and a typical aircraft jet engine. We don’t want the hot exhaust of the engine spewing out like a jet engine, the heat signature this would create would be a giant beacon for heat guided missiles and give the tanks location to any enemy using thermal vision. We also don’t want to waste all that heat energy. The recuperator is essentially a giant heat exchanger.
Air coming from the compressor stage is passed through it, where it is heated by the exhaust before entering the combustion chamber. This lowers the heat signature of the exhaust and increases fuel efficiency, transferring more heat energy back into the engine instead of losing it to the atmosphere. But the M1 Abrams is still a thirsty machine, even by tank standards. The turbine engine uses twice the fuel as a comparable diesel engine per kilometer. Where the M1A1 consumes around while cruising at 40 kilometers per hour, the Leopard 2 consumes 2.2 liters per kilometer.
[REF]  But the US military deemed this issue a cost worth paying for the capabilities the turbine engine provided. Allowing the M1 to carry an obscene amount of armor, while keeping its acceleration high. The armor installed on the M1 Abrams has evolved and changed over the last 4 decades. The exact details of its thickness, location, materials and layering is classified for obvious reasons, but there is a great deal known about the science and nature of its armor. It is well publicized that early M1 variants used a type of composite ceramic armor called Chobham.
Which derives much of its ballistic resistance from an extremely hard and light ceramic layer. Hardness in material science is a measure of a material's ability to resist localized deformation, like a scratch. Diamonds are extremely hard, and because of this industrial diamonds are coated onto cutting tools to help them cut through materials, without being eroded themselves. A hard material can scratch and erode a softer material.
Hardness can be measured with a vickers hardness test, which pushes a pyramid shaped diamond into the material. The hardness is then calculated by dividing the force applied by the resulting surface area of the indentation. The rolled homogeneous armor steel of world war 2, has a Vickers Hardness of 380, a high carbon hard steel is about 550, while a ceramic like silicone carbide offers a hardness 5 times greater, up to 2500  While being much lighter than steel.
Making it an excellent candidate for armor. But, anyone that has dropped a dinner plate knows that ceramics are extremely brittle. They shatter into a million pieces with little force, but this can be used to the tanks advantage when combined with a tougher metal backing plating. In this configuration the ceramic is placed on the outside of the metal plating, acting like an extremely hard outer shell. When a round strikes the armor the compressive strength and hardness of the ceramic coating causes the round to fracture and break apart, at the same time the ceramic coating begins to fracture and fragment, spreading the energy of impact across a larger area which is then absorbed by the tougher metal plate backing the ceramic. Tough in material science meaning it can absorb a lot of energy without fracturing, the opposite of brittle.
Further research and experimentation found that ceramic armor performed even better when placed under compression. This can be achieved by simply adding a face plate and bolting the two pieces together. This changes the dynamics of an impact significantly, and helps immensely with resisting attack from long-rod kinetic energy projectiles. [REF] These are rounds specifically designed as anti-armor weapons. They are thin, long dart-like projectiles that require a sabot to launch out of tank barrels. They are typically manufactured from high density materials like Tungsten.
With a thin aerodynamic shape and high density, these weapons have an extremely high ballistic coefficient, allowing them to ram into targets at a distance at high velocities. They can obliterate rows of concrete walls with ease. Embedded compressed ceramic armor can defeat these kinetic energy weapons. [REF] When the long rod projectile strikes the face plate it sends a pressure wave through the ceramic that pulverizes it and increases its volume.
This creates what is essentially an abrasive maze of extremely hard and sharp particles the penetrator has to push through, gradually grinding it away. [REF] Computer simulations of this effect show that the harder the material the better. This is the magic of ceramic composite armor.
[REF]  On top of these metal and ceramic layers there is typically a very dense inner liner called a spall lining. Projectiles don’t necessarily need to penetrate every layer of armor to be deadly. If they hit with enough force the kinetic energy can simply transfer through the material as a wave and cause material on the inside of the tank to splitter and turn into deadly shrapnel inside the crew compartment. This is called spall. Some munitions are specifically designed to cause this.
High explosive squash heads are made from soft plastic explosives that spread out over the armor's surface. With the increased surface area and direct contact with the armor, the explosion transfers a great deal of energy through the material and blows out the armor's backing. This works by sending a compressive shock wave through the material and reflects and rebounds inside the material, creating regions of intense stress that fractures the armor. The spall liner is a ductile and dense material that limits spalling. For early M1s this layer was typically composed of lead, but beginning in 1988 certain M1A1s, began to be upgraded with depleted uranium spall liners, which are even denser than lead.
And all new M1A2s were assembled with depleted uranium liners. However, modern composite armor makes it difficult for this shock wave to transmit through the material and a spaced layer with an air gap can defeat this squash head munition completely. Additional reactive armor tiles can be added to the outside of the M1 too.
Reactive armor is particularly effective at dealing with shaped charges. Shaped charges consist of a charge shaped with a hollow indentation, lined with a ductile metal liner. When the charge is detonated a pressure wave forms behind this metal liner, deforming it and accelerating the metal into a lance stream of particles. The shaped charge effectively creates a hypersonic projectile at point black range.
[REF] It’s highly effective at cutting through armor. Reactive armor works by placing an explosive charge between two metal plates. When a jet from a shaped charge strikes the upper plate it detonates the inner explosive. You may think this could damage the tank, but tank’s lower armor is more than capable of dealing with the relatively blunt pressure formed by the reactive armor detonation.
The outer plate then flies outwards to disrupt the incoming jet while the shockwave formed by the detonation also breaks up the stream of metal approaching the tank. Ofcourse, the best defense is to not be hit at all, and the M1 can create a smoke screen for itself when needed. The M1 has two systems for generating smoke to conceal itself in an engagement. The first involves simply spraying fuel into the engine exhaust, which vaporises the fuel and creates a large opaque cloud behind the tank. However it’s extremely important that the driver remembers which fuel the tank is running on. This works for diesel fuel, but if gasoline or kerosene is used it won’t conceal the M1, it will set it on fire.
The second system uses these grenade launchers mounted on the outside of the turret. There are two versions that the M1 uses. The 8 canister M257 typically used with US Marine Corps M1s, and the more widely used 6 can M250.
These launchers are controlled from the tank commander's seat here. Pressing 1 button launches 6 grenades, 3 from the left and right side. Pressing both buttons launches all 12 of the grenades. Launching them about 30 meters from the tank and providing a shrouding curtain of smoke to hide it’s movements. The engineers of the M1 Abrams did everything in their power to make the M1 as survivable as possible, protecting the crew inside. And the highly trained crew are the most important part of this machine.
The M1 Abrams does not have an automatic loader, like many modern tanks. It has a dedicated crew member, the loader, to load rounds into the breach. Autoloaders are a feature of modern Russian tanks, however the USA has shunned them in their tanks, seeing an autoloader as an unnecessarily complex mechanism that would impact the M1s reliability. Seeing a forth crew member, capable of keeping watch, maintaining the vehicle and taking over responsibilities in an emergency as an advantage, not a disadvantage. The loader enters the tank through the turret hatch, sitting to the left of the main gun with access to the ammunition box behind them . The gunner to their right is in charge of
aiming at targets using these day and thermal night vision sights, along with a laser range finder to input target distances into the ballistics computer. When instructed by the tank commander, sitting behind the gunner, the loader will press a switch with their knee to open a hydraulically actuated armored door behind them. The tank commander will specify the round needed and the loader will take it out and load it into the breach, close the breach and move the safe handle into the armed position. This process will be repeated until a ceasefire is called by the tank commander.
The armored ammunition access door is only open when loading, and this is crucial for the survivability of the M1 Abrams. If a round penetrates the ammunition box it can result in a lethal detonation of the ammunition stored inside. This armored door is capable of withstanding this blast, and panels on top of the tank, called blow out panels, are designed to break and allow the pressure and heat to be directed upwards and away from the crew. [REF] These rounds are fired out of the M1s 120 mm cannon.
Early M1s were fitted with a 105 mm cannon. 105 referring to the bore diameter of the gun. This decision was made primarily to allow a sharing of ammunition and parts between the 105 mm on the previous generation M60 tank. With the development of depleted uranium rounds it was viewed that this gun was more than adequate to deal with any soviet armor.
However US allies like Germany did not want to use depleted uranium round due to ethical implications, and were moving towards 120 mm cannons with the British Chieftan tank using the Royal Ordanance L11 and the German Leopard 2 using the Rheinmetall RH-120. This posed a problem for NATO’s goals of standardizing wherever possible to optimize logistics. With ammunition factories across NATO countries producing the same ammunition, this ensured ammunition could not only be shared, but manufactured as close to the frontline as possible.
The 120 mm guns of the M1A1 and M1A2 are in fact the German Rheinmetall RH-120 manufactured under license in the US. A 5.3 metre long, 3.3 tonne smooth bore cannon. This large bulge in the middle of the cannon is designed to help evacuate the barrel of propellant gasses after each firing. Once the round leaves the barrel atmospheric pressure can prevent the gasses from leaving, and once the breach is opened the potentially harmful and explosive gas can enter the crew compartment. The bore evacuator is a pretty simple solution, with holes that allow gasses to enter the evacuator as the round passes by. This acts like a pressure reservoir.
When the round leaves the barrel with this attachment, the pressure is released from forward facing holes at the far end of the evacuator that pushes the remaining gas outwards. The two primary rounds used by the M1 are the M829 Depleted Uranium round, which is a saboted kinetic energy round, and the M830A1 HEAT Round, which, despite its name, does not use heat as part of its offensive. It uses a kinetic energy shaped charge. We have been piling on weight in this vehicle. The armor, cannon, engine and ammunition makes this an extremely heavy vehicle.
Weighing between 52 and 68 tonnes depending on the generation and configuration. The M1 Abrams needed a track and suspension system capable of bearing that weight. For a vehicle this heavy typical helical springs have some problems. First, their maximum travel is limited. At a particular load the layers of a helical spring will meet and no further travel is possible, bottoming out and providing an extremely uncomfortable ride for the crew. To deal with a heavier vehicle we need to increase the spring coil diameter, taking up an increasing amount of space.
This was an issue for world war 2 era tanks, with the M4 Sherman opting to use a volute spring, which is a conical spring that is capable of compressing to a much smaller size as the sheets of spring steel overlap. However most modern tanks utilize the torsion springs, which take up very little space inside the tank, though they do raise the tanks height considerably. The torsion springs of the M1 are located here, and because they are inside the hull they raise the hull height by 150 millimeters. The road wheel is attached to a lever that can travel up and down, this lever arm is attached to the torsion bar so that it twists when the lever arm moves up and down.
The spring force is derived from the bars resistance to torsion. Older tanks incorporating torsion bars had limited travel. The M1’s predecessor, the M60, had a wheel travel of just 20 centimeters. During development of the M1 the two tanks were pitted against each other, with the M1 benefiting from 20 years of material science progress, its max travel was nearly double that of the M60 at 38 centimeters.
When tested on a track with a series of 30 centimeter bumps at 32 kilometers per hour. The M60 managed to break its front front wheels, lost control, and broke the drivers arm. The torsion spring of the M1 runs along the width of the hull.
Making them quite heavy. We need the torsion spring to be as long as possible because the travel distance is a function of the torsion bar's length. Longer torsion bars can twist more before breaking. An interesting quirk of the torsion spring system is that typical designs, like the M1’s, mean that the road wheels cannot be aligned.
We cannot simply connect the lever arms of opposite road wheels to the same torsion spring, as they would frequently travel together, creating no torsion. The far end of the bar needs to be rigidly mounted to the body of the tank in order to develop a spring force. So two torsion bars need to be mounted alongside each other, creating an offset in distance between the two sides of the tracks. The two sides of the M1 are not symmetrical. This comes with some drawbacks. The leading road wheel will hit bumps first, meaning it will absorb more force than the trailing road wheel.
Increasing the wear on the leading torsion spring, resulting in more frequent replacement. Torsion bars are very easy to swap out when they aren’t broken, but if bent or shattered, or if the body of the tank is bent, it can be difficult to pull the long torsion springs out. Modern tanks are increasingly moving towards hydrogas suspensions.
Which look like this. The road wheel has an attached axle pivot arm, which in turn turns a crank and con-rod piston. This piston connects to the hydragas suspension cylinder. The first chamber is filled with a fluid, typically oil. The oil is relatively incompressible, and so provides marginal spring force, but this chamber has a damper valve that serves to restrict the transfer of fluid between this chamber and the next, which contains a floating piston with a compressed nitrogen gas on the other side. This provides resistance to ensure the road wheels stay in contact with the ground, but also dampens out vibrations.
[REF]  This system is much smaller, lighter and easier to service than a torsion bar, while lowering the height of the tank, lowering its profile and making it harder to hit. Not only that, but each individual suspension can be remotely adjusted by changing the gas pressure inside the cylinder. Allowing the tank to take a crouched position if needed. Hydrogas suspensions were considered for the M1 during development, but it was a relatively new technology at the time, and so the torsion spring was chosen. [REF] The M1 Abrams has had incremental improvements added over the past 4 decades. With three primary variants the M1, M1A1 and M1A2, with even more specialized iterations in between.
With updated sensors and controls to bring the tank into the 21st century. Like the Crows II on the M1A2 SEP. SEP standing for systems enhancement package. Which added additional thermal sights for the tank commander, an auxiliary power unit to run it’s electronics without running the fuel hungry engine, and the CROWS II controller.
A remote control sight that allows the tank commander to operate the gun from the safety of the turret. [REF] We are currently on the M1A2 SEP V4 variant, and the army is considering the future of the M1 program, whether that’s an M1A2 SEP V5, or a completely new platform, but make no mistake, despite the M1 being one of longest serving tanks in a modern military, over 4 decades old it’s still highly capable and it will be a massive asset to the troops in the ground in Ukraine. The next episode of Real Engineering is about the incredible physics behind magnetic resonance imaging. It will be out on YouTube in 2 weeks, but maybe you want to prepare in advance to better understand what we talk about.
MRI machines use a pretty incredible property innate to our body tissues, the quantum spin of hydrogen atoms and how they interact in different tissue types. It took us a long time to write this script. Understanding quantum mechanics is hard, and quantum physics is becoming increasingly relevant in everyday engineering. From magnetic resonance imaging to quantum computing, Brilliant has 3 courses to get you started with understanding this scientific concept.
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