Military Tanks

A tank is a mobile, armored fighting vehicle designed for front line combat combining operational mobility, tactical offensive and defensive capabilities. The tank has come a long way since it was first produced. They are becoming faster, more powerful, better protected and more efficient because of evolving technology. Tanks have traditionally been powered by gasoline and diesel piston-cylinder engines, however, gas turbines have also been employed as a prime mover.

Abrams Tank
Abrams tank [10].

History

During WWI, there was talk about making steam powered armored vehicles, as well as small gasoline powered vehicles. However, this seemed unfeasible due to the difficult terrain that the vehicles would encounter. In 1914/1915, several land ships were designed by the British Army. They were unsuccessful until a large vehicle was designed with tracks installed around the outside of the armored body. The tank was specifically designed to handle unpredictable terrain like having to climb mounds of 4.6 feet or cross trenches of 5 feet. This is the vehicle that began the development of tanks.

In 1940, after Germany's Blitzkrieg tactics, the United States rushed into developing tanks of their own. This led to the development of the M3 tank. The M3 tank was powered by an aircraft air-cooled gasoline engine. The M3 had one 75mm gun, with some models equipped with a 37mm power driven turret [63]. But the US didn't stop there; they improved the M3 tank and developed the M4 Sherman tank. The tank upgrades included a rotating turret with a 75mm gun, and a gasoline engine that nearly tripled the horsepower of the M3 [64]. The tank was extremely reliable but was no match for the German Tiger I whose 88mm cannon and thick armor allowed it to engage its opponent from much farther away than the M4 [65].

After World War II, the U.S. Army developed the M26 Pershing. The M26 was equipped with a 90mm main gun to counter the German Tiger I; the tank was also given heavier armor and better suspension than its predecessor, the M4 Sherman [66]. In 1948, work began to upgrade the M26 with a Continental AV1790-3 29.4L V-12 gasoline engine and a bore evacuator for the main gun. The variant was later designated as the M46 General Patton [67].

In 1951, the U.S. Army initiated the design of a new tank that featured a hemispherical turret, a redesigned hull, and improved suspension. The tank was dubbed the M48 Patton [68]. Less than a decade later, the U.S. military had plans for a larger tank with a 105mm main gun and redesigned hull for better protection. The tank was also given a twin-turbo diesel variant of its AV-1790-3 called the AVDS-1790-2. The M60 became the main battle tank for years to come in the United States [69].

Soldiers Working on an AGT1500
Soldiers working on an AGT1500 [23].
By the mid 1960's, it was apparent that the Russian Army had tanks with better firepower and protection than the fielded NATO forces. The United States and Federal Republic of Germany formed a joint venture to begin the development of the most modern tank using the latest technology. The tank was originally designed with a gas turbine engine, however, air filtration issues drove development to use diesel engines. The American version of the MBT 70 tank, contained a Continental Variable Compression Ratio diesel Engine [70] and a German Renk transmission [71]. It was capable of firing both missiles and projectiles and had variable height suspension. It was also the first Western tank that was able to use a three person crew rather than the standard four. This was possible due to the autoloader installed in the tank. It also had a separate compartment for the driver in the turret [72].

By 1969, the MBT-70 cost five times what it was projected. With a cost of $1 million a unit, Germany backed out of the project, however, both countries continued development leading to the German Leopard 2 which ran a turbocharged MTU MB 873 47.6L twelve cylinder diesel engine a [73] and the American M1 Abrams which ran a Honeywell AGT 1500 multi-fuel turbine, both engines rated at 1500 hp [74].

AGT 1500 Gas Turbine

The US Army felt that diesel technology was approaching its limits of development potential. They wanted greater engine power density, reliability and a chance to translate the aviation successes of gas turbines into ground vehicular applications. This led to the development of the AGT 1500 turbine engine. The United States was not the first country to use gas turbine this technology in ground vehicles. The Swedish S-tanks had been using turbines to boost power since the 1960's [75]. The US Army came to realize that the turbine engine would greatly improve the power density and reliability compared to the diesel engine.

Specifications

Change of gas turbine for tank M1 Abrams in field conditions
Change of gas turbine for tank M1 Abrams in field
conditions [21].

The AGT 1500 turbine engine has a length, width and height of 67, 39 and 32 inches, respectively and weighs 2500 lbs. The engine has a 1500 hp and can produce 3000 rpm output shaft speed, giving the tank power, speed and quick acceleration. The AGT 1500 turbine engine has five primary fuels: diesel fuels 1, 2, and arctic grade, and jet propellant fuels 4 and 5. In case of an emergency, the engine is also capable of using combat gasoline and marine diesel. The engine has a simple modular design, consisting of three major modules: forward, rear, and accessory gearbox module. This is an important feature for the reason that, if the engine is damaged, each module can be replaced or repaired in the field keeping more vehicles operating [76].

Recuperator

The recuperator is one of the most important elements in the AGT 1500 gas turbine engine. The purpose of the recuperator is to transfer some of the waste heat from the exhaust gas to the compressor discharge air. This preheats the air entering the combustion chamber allowing greater efficiency in the turbine. By utilizing a recuperator, less fuel can be burned to achieve a desired turbine inlet temperature. The recuperator is massive in size, almost as large as the rest of the engine (it is the largest component). This allows for a large surface area and for maximum heat transfer [77]. The recuperator is made using a stacked plate core that is cylidrical in shape. The air from the compressors is forced through the core where it is heated up by the exhaust which flows perpendicular the airflow through the core. The air is then returned back through the recouperator on its way to the engine.

The AGT 1500 turbine engine has become the primary engine of the Abrams family. Its compact design, cold-starting, instant power, multi-fuel capabilities, and stealthy operation made this engine the world standard for tank durability and survivability [76].

Gas Turbines vs. Diesel Engines

When comparing gas turbines and diesel engines, there are a few things to consider: performance, weight, signature, reliability/all-weather capability, fuel consumption, cost, growth potential and logistics.

Performance

With a twin shaft gas turbine having the ability to deliver its maximum torque to the sprocket at zero to low vehicle speed, a diesel engine just can't compete. In order to match the same power density as a gas turbine, the diesel engine would need to be heavily turbocharged. Highly boosted diesel engines, achieved through turbocharging, can experience poor transient response to sudden load conditions. This "Turbocharger lag" requires the diesel engine to utilize multiple devices in order to minimize the lag, which in turn increases the cost, complexity, fuel consumption, weight and under armor volume.

Weight

Diesel engines cannot manage to deliver the equivalent power density as the gas turbine unless it is heavily turbocharged. In other words, this requires multiple devices which significantly increase the weight of the engine and the tank. The AGT 1500 is about 50% lighter than a diesel engine of equivalent output. Pound for pound, the gas turbine will give you more power [122]. The weight of the engine is crucial when designed for a tank. If the engine is light enough, more armor protection can be added for the same vehicle weight and performance.

Signature

Gas turbines are known for their lack of noise and visible smoke while running the engine. These are important signatures to note since noise and smoke can give away a tanks position. On a quiet day, the rumbling of diesel engine can be heard from quite a distance. So having a low noise, like the gas turbine, can be very helpful. The heat signature on the other hand is a different story. Gas turbines run much hotter than diesel engines and makes infantry support next to impossible. With thermal optic technology becoming more prevalent, a discrete heat signature is imperative.

Reliability and Weather Capability

Maintenance being performed on an M1 Abrams tank turbine
Maintenance being performed on an M1 Abrams
tank turbine [22].

Reliability is one, if not, the most important measures of a tank. Knowing that the engine will start and continue running is something to appreciate. Gas turbines have less moving parts than a diesel engine. So, to make things simple, the gas turbine has fewer parts that can breakdown. In a diesel engine, the constant sliding of metal on metal will shorten the engines life span considerably, especially during the cold winter days when the oil is too thick to lubricate the engine. Often times, a pre-heater is needed to melt the oil in a diesel engine while it is not running. This way the engine doesn't encounter as many problems during a cold start. With that being said, a gas turbine does not have the metal to metal, frictional wear and tear and does not have a problem starting in extreme cold weather conditions.

Fuel Consumption

One advantage to a diesel engine is its fuel consumption. Compared to the gas turbine, the diesel engine uses less fuel. Even though there have been improvements on fuel consumption for a gas turbine, by improving cycle efficiency, material, digital electronic control systems, and recuperator design, which provide 30-40% improvements in fuel use over older turbines. The diesel engine still has the upper hand on fuel consumption.

Cost

Diesel engines and gas turbines both have their own distinctive advantages. Gas turbines have fewer parts than diesel engines, however, the parts are more intricate and expensive to manufacture. Gas turbines also require a superlative filtration device, due to the massive amounts of air they consume, to prevent performance and reliability degradation such as: erosion, corrosion, fouling, and passage plugging.

Growth Potential

The U.S. Army felt that diesel technology was approaching its limits of development potential in the late 60s and early 70s. This is why the Army adopted the gas turbine for use in their modern tanks. The aviation sector has proven that the gas turbine has potential to double its initial design power within the original installation envelope. This is critical because the engines won't need more parts, resulting in less weight and lower cost.

Logistics

Diesel engines are very sensitive when it comes to their type of fuel. They are designed to operate on diesel and are able to run JP-8 jet fuel. Gas turbines on the other hand, have broader multi-fuel capability but are voracious consumers. This means, if the engine needs to, it can use diesel fuel, jet fuel, gasoline, and leaded fuel without any modifications or problems.

LV 100-5 Gas Turbine

The LV 100-5 gas turbine engine has been designed to replace to AGT 1500 turbine. In its time, the AGT 1500 turbine was the best out there. Currently, they are far too expensive to use and maintain. Production of the AGT 1500 turbine ended in 1992 due to those reasons.

The LV 100-5 gas turbine was designed in 1990’s, with production beginning in 2003. The idea was to make a more cost efficient, reliable, and lighter engine, while still offering 1500 hp.

AGT 1500 vs. LV 100-5

Loading a Honeywell AGT1500 gas turbine engine back into an M1A1 Abrams
Loading a Honeywell AGT1500 gas turbine engine back into an M1A1
Abrams [11].
The two gas turbine engines are, to a certain extent, similar. They both perform at the same level and both have the same logistics as well as growth potential and signature. According to Honeywell, the LV 100-5 has 43% fewer parts which help increase the mean time between failures by four times. Honeywell also claims that their LV 100-5 has 50% better fuel economy at idle and 25% better fuel economy during movement [78]. The efficiency improvement was attributed to:

  • Full-authority digital engine control unit
  • Reduce inlet pressure loss
  • Increased compressor flow and airfoil design
  • Minimized leakage flows
  • Elimination of second state Variable Area Turbine Nozzle
  • Added bypass capabilities in the Engine Recovery Unit
  • Enhanced aero designs and lower inter-turbine duct losses in the High Pressure/Low Pressure Turbines

  • Components

    Components on Engaged that are included in Tanks:
    Gas Turbine
    Diesel Engine

    Table comparing different fuel types.
    Line drawing of an AGT 1500 engine showing the use of a recouperator.
    Dyno sheet comparing power and torque between a gas turbine engine and a diesel engine.
    This video shows how intricately gas turbines are made.