Future combat vehicles will require unconventional weapons and armor systems such as electromagnetic (EM) or electrothermal chemical (ETC) guns, electromagnetic (EM) armor, and directed- energy weapons (DEWs). To meet these requirements, a hybrid electric power system has been identified as the best alternative to support the demand for propulsion, continuous auxiliary power demand, and pulsed power demand for weapons and armor.

Although the development of these weapons and armor technologies is progressing at a fast rate and can be demonstrated at a smaller scale today, the power supply needed in the vehicles to support these systems presents a great challenge to technology developers and vehicle integrators.

In a 20-ton combat hybrid vehicle platform, power supply will mainly consist of two sources of energy: a prime power source driving an AC generator such as a heat engine, and an energy storage system consisting of advanced batteries, ultra capacitors, and flywheels, or a combination of these three devices. Currently and in the near term, the prime power will be either a diesel engine or a turbine, and in the far term, fuel cells may become viable options.

The electrothermal chemical (ETC) gun utilizes electrical energy to augment/control the release of chemical energy from existing or new propellants for significant performance enhancements using existing projectile conventional guns.
The power supply has to meet the demands of mobility, lethality, and survivability. The demand for electric power becomes even more challenging during silent watch, when the power draw must be provided solely from energy storage for extended periods of times (4 to 8 hours). Power supply must be delivered in two forms: continuous and pulsed. For a vehicle weighing about 20 tons, the continuous power ranges from 400 to 500 kW, which is supplied from the main prime mover, supplemented by 25-30 kW-hr of energy from a storage system. Pulsed power, however, depends on the loads and repetition rate.

Since electric power is used for continuous loads such as mobility, and also for pulsed loads such as electric weapons, it would make sense to have one common power and energy management system onboard the vehicle to distribute electric power to various users according to a defined precedence. Thus, a Combat Hybrid Power System (CHPS) program was introduced to evaluate such a power management and distribution system.

Enabling Technologies

The CHPS program was initiated by the Defense Advanced Research Projects Agency (DARPA) and continued by the U.S. Army RDECOM – TARDEC (Tank Automotive Research, Development and Engineering Center). The major goal was to design, develop, and test a 15-ton notional hybrid electric combat vehicle, incorporating all the power demand onboard a vehicle system, and assess the feasibility of simultaneous power distribution to propulsion.

In the course of designing the components for the 15-ton combat vehicle, some critical and enabling technologies were identified. They included high-temperature power electronics, high-energy-density and high-power-density batteries (namely Li-Ion batteries), and high-torque-density traction motors. To the extent possible, all the components and auxiliary systems had to be integrated within the space available in a 15-ton combat vehicle. Two technical challenges appeared: the amount of power needed for all the loads, and the size and weight of the components. A first estimation revealed that using state-of-the-art technologies would require at least twice the space available within a combat vehicle.

The most aggressive goals were set for the power electronics, the motor and generator inverters and rectifier, and the DC-DC converters, and also for the thermal management. Another aggressive metric was set for the Pulse Forming Network (PFN), which had to be reduced by more than half of its current size in order to install it in the vehicle. Improvement in both the power converters and the PFN hinged on the maturation of wide bandgap (WBG) materials such as SiC. This material provides the capability to build converters that operate at high temperature, high frequency (50-100 kHz), and higher efficiency.

Vehicle Mobility

In a combat vehicle, there are three main users of continuous power: mobility, thermal management, and silent watch. Power is supplied to most of the mobility and thermal loads from the prime mover (the engine), whereas the silent watch is solely supplied from the energy storage (a battery bank), which is also recharged from the engine driven generator. For optimum performance, the power is split between engine and battery for either best fuel efficiency or burst power, according to the specified vehicle duty cycle.

Military vehicles must have the capacity to operate anywhere in the world, under extreme environmental conditions, from the frigid temperatures of the Arctic, to the intense heat of the deserts, and from hard rocky and paved roads, to hilly and soft soil. They must withstand the vibration, shock, and violent twisting experienced during crosscountry travel over rough terrain, and they must be able to operate for long periods of time with very little or no maintenance. However, there are additional requirements that are changing the whole philosophy of vehicle design. Future vehicles must be lighter, faster, and more deployable, but at the same time, more lethal and more survivable.

For a 20-ton vehicle, the power required to meet the acceleration, top speed, and gradeability requirements at 10 kph is about 400-500 kW. In a hybrid electric vehicle, the engine provides most of that power. The engine is normally programmed to operate within the band of optimum efficiency on its fuel map. Boost power for transient operation is supplemented by the stored energy from the battery pack. Thus, for propulsion, a relatively small energy storage system would be sufficient.

The Solid State Heat Capacity Laser (SSHCL) is a directed-energy weapon (DEW) integrated into a vehicle platform.
Requirements such as acceleration, top vehicle speed, steering at large radii, and cross-country speed depend on the available horsepower from the prime mover and the energy storage device (batteries) getting to the sprockets or wheels when needed for the various vehicle mobility conditions. For all vehicles, the power is transmitted from the prime mover to the wheels or sprockets according to a specific architecture, series, or parallel, depending on the application and duty cycle of the vehicle.

Thermal Management

In the current fleet, in addition to the power demand to operate the vehicle, 10 to 15% of the generated power from the prime mover is needed for the cooling system. For a hydraulic drive using hydrokinetic or hydromechanical transmission, all of the mobility requirements are manageable. For a hybrid electric system, the situation is more complex. The cooling system of a hybrid electric vehicle with the currently available technologies can conceivably be four to six times the size of its mechanical counterpart. Consequently, high-operating-temperature components must be developed to reduce the current hybrid electric cooling requirements.

Silent Watch

The power demand for silent watch is difficult to establish because the requirements are not well defined. However, using high-energy-density batteries such as Li-Ion, it is conceivable to have 25-30 kW-hr of energy onboard the vehicle. This amount of energy storage can support silent watch missions for a duration of two hours if the power requirements do not exceed 10 kW. The amount of energy supply must exceed the amount of energy demand by 50% to account for the system efficiency, degradation at temperature extremes, and cycle life.

High-voltage pulse power is required to power the ETC/ETI, the ETC gun, and the EMA envisioned for future hybrid electric vehicles. Intermediate energy storage for these pulse power loads is achieved through the use of high-energy-density thin-film capacitors, currently utilizing Biaxial-Oriented Polypropylene (BOPP) as the dielectric film. The total amount of energy storage required varies based on such factors as simply achieving ETI or full–blown temperature compensation (ETC), the type of propellant used, the specific armor design, the threats to be countered, and the desired safety factor. This results in intermediate energy storage from tens to hundreds of kilojoules.

The basic building block for these high-voltage pulse power systems is an extremely high-power-density (~4.5 kW/l) 150-kW pulse charger. Depending on factors such as the total amount of intermediate energy storage required, the number of rounds per minute the system is required to fire, and the required rate of recharge for the EMA, additional chargers can be added.

Directed Energy Weapons

The most likely DEW to be integrated into a vehicle platform is the Solid State Heat Capacity Laser (SSHCL), which will be integrated onto a hybrid electric ground combat vehicle in order to provide forward air defense against unmanned aerial vehicles (UAVs), helicopters, and other low-flying craft, as well as area-wide active protection against incoming threats including mortars and missiles. The system will generate a 100-kW laser output at ~10% electrical efficiency, meaning the hybrid electric vehicle will need to provide 1 MW for short durations (several seconds).

Although the power management and distribution to both types of loads is feasible within the hybrid architecture, the integration burden is extremely challenging in a 20-ton vehicle, and depends to a great extent on the pulsed load specifications. Currently, it is difficult to envision a hybrid electric combat vehicle without the development and maturation of the emerging technologies in power semiconductors and energy storage. However, in the last ten years, these technologies have progressed by more than one order of magnitude.

This article was written by Gus Khalil, Eugene Danielson, Edward Barshaw, and Michael Chait of the U.S. Army TARDEC in Warren, MI. For more information, click here .

Defense Tech Briefs Magazine

This article first appeared in the April, 2009 issue of Defense Tech Briefs Magazine.

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