Alternative Energy and Propulsion Power for Today's U.S. Military

The U.S. military has the most sophisticated weapons systems in the world, but they are fueled and mechanically powered by old technologies. Given the mission of the U.S. military, it is without question the largest oil-using organization of its kind in the world. More than half of the Department of Defense's (DoD's) fuel budget is spent on fueling the U.S. Air Force. The Navy consumes about one-third of defense oil resources, and the Army uses about 12%. Twenty-five percent of military energy is used to power and heat buildings and facilities. The remaining 75% is consumed for mobility purposes.

Figure 1. The three technologies that have the potential to improve the DoD’s capabilities, with rough estimates of their operational gains and fuel savings.

Of the total U.S. government liquid fuel use, about 97% is consumed by the DoD, making that agency the world's single-largest fuel-burning entity. It is certainly understandable why the U.S. military uses such a great deal of fuel for mobility to conduct tactical operations. The Air Force is focused on airlift and platforms that can deliver strike packages from the air. The Navy and Marine Corps are focused on sealift and sea-delivered strike packages. The Army has a mission focused on maneuvering and fighting, and seizing and holding terrain. This is a simplification of the respective service missions, which are quite broad, complex, and very much interrelated, but it illustrates the need to explore alternative energy to satisfy the operational necessities of our military.

Alternative Energy Research

The Army is replacing the High Mobility Multipurpose Wheeled Vehicle (HMMWV), a battlefield vehicle that gets as few as 4 miles per gallon in city driving and 8 miles per gallon on the highway, and a great deal worse over cross-country terrain. The Army wants to see a HMMWV replacement that weighs 30 to 40% less and uses proportionately less fuel.

Since the Air Force uses the most petroleum-based fuel, it is leading the way in alternative fuel research. The Air Force is qualifying new types of fuel derived from both natural gas and coal. In 2006, a B-52 bomber flew with one engine mount using a newly produced liquid fuel derived entirely from natural gas. Due to the nature of the manufacturing process, the fuel contains virtually no sulfur and hardly any heavy metals, which is good for prolonging engine longevity when compared to jet fuel derived from refined petroleum. In ground-based testing, the engines that burned this fuel did not experience any measurable loss of performance and required less maintenance. All services could benefit from this research if synthetic fuel could be used in ground tactical vehicles, helicopters, and other support systems.

With the Navy being the second-largest DoD user of petroleum-based fuels, it is experimenting with ship designs and construction techniques that are anticipated to produce vessels ten to 100 times more efficient than in years past. Some novel ideas envision certain future classes of Navy ships using masts and sails, with the sails and the exterior of the hulls coated with photovoltaic cells, all with the goal of reducing the requirement for liquid mobility fuel.

Both the Navy and the Air Force are among the largest generators and consumers of "green energy," almost all of it derived from windmills. This helps to reduce the burden of procuring petroleum fuel for base operations, and allows them to focus resources toward the petroleum needs of their mobile tactical and operational systems.

The payoff to the DoD, in terms of mission effectiveness and human lives, is probably greater than for any other energy user in the world. More efficient platforms would reduce the burden of owning, employing, operating, and protecting the people and equipment needed to move and protect fuel from the point of commercial purchase to the point of use. Not only will there be direct savings in energy cost, but combat effectiveness will be increased and resources otherwise needed for resupply and protection redirected.

Alternative Fuels

When considering alternative fuels, it is important to understand that the primary consideration must be energy density. Fuels may either be derived directly from natural resources or by a method of storing energy in a more convenient form (e.g., alcohol from biomass or hydrogen from electrolysis of water). As such, the stored energy density is a useful metric for comparing various fuels. Since fuels may be solid, liquid, or gaseous, both gravimetric and volumetric energy densities are important. Other than uranium, liquid hydrocarbons offer the most attractive combination of volumetric and gravimetric energy densities. Alcohols offer approximately half of the energy density of the liquid hydrocarbons. Although all fuels require containment, the only fuels that sustain a significant impact on energy density due to containment are the hydrogen fuels. Liquid hydrogen requires cryogenic storage at -253 °C, which consumes energy equal to about 30% of the energy being stored.

Hydrogen-Powered Internal Combustion Engine

Figure 2. The Badenoch “blast-bucket” vehicle weighs less than half of an up-armored Humvee, has greater fuel efficiency, and improves protection against blasts and projectiles.

Despite the limiting physics of hydrogen when considering its energy density volumetric characteristics, much research continues to be done because of its natural abundance and environmentally friendly nature. Hydrogen must be contained under high pressures to allow for form-and-fit and suitable integration into an automobile-type configuration. This high compression also introduces a certain degree of hazard to personnel. These challenges do not make hydrogen entirely suitable for tactical vehicle systems yet. Consequently, most future research remains in liquid hydrocarbon fuels (JP-8, diesel, and gasoline) for tactical systems.

Automobile manufacturers have been leading progress with the advent of the hydrogen-powered internal combustion engine (ICE). They can run on pure hydrogen or a blend of hydrogen and compressed natural gas (CNG). They perform well under all weather conditions, require no warm-up, have no cold-start issues, and are up to 25% more fuel efficient than conventional spark-ignition engines. The U.S. Army Tank Automotive Research, Development, and Engineering Center (TARDEC) National Automotive Center (NAC), in collaboration with the Department of Energy and Chevron Oil, are conducting an assessment of a converted ICE hydrogen-fueled vehicle.

To offset the cost of imported petroleum for the military, Defense Advanced Research Projects Agency (DARPA) is conducting research to replace jet petroleum type 8 (JP-8) with biological-based fuels. JP-8 is used by the military in everything from tanks and aircraft to generators that power base camp operations. DARPA's initial biofuels research focused on converting agricultural crop oils (canola, jatropha, soy, palm oils, and others) to a JP-8 surrogate or biojet/biofuel. Currently, the most promising research has expanded to cellulosic and algal feedstocks to produce a second-generation biofuel that is noncompetitive with food sources.

Hybrid Electrics

Elements of the DoD have shown great interest in hybrid electric technology for some time. They have been actively engaged in a research, development, and engineering program aimed at developing and fielding combat and tactical hybrid electric vehicles. In the past, these DoD partners converted the mechanical drive systems to an electric drive configuration for a conventional M2 (Bradley fighting vehicle), M113 (armored personnel carrier), and HMMWV to investigate the viability of the hybrid electric drive technology. It was demonstrated that this technology had potential benefits and was feasible. The TARDEC NAC hopes to develop hybrid electric solutions to support ongoing vehicle programs such as the Future Tactical Truck System (FTTS), Joint Light Tactical Vehicle (JLTV) system, and Future Combat Systems (FCS) manned and unmanned ground vehicles.


The EPA's National Vehicle and Fuel Emissions Laboratory (NVFEL) in Ann Arbor, MI is engaged in engine, alternative fuels, and hydraulics research, including hybrid hydraulic technology. This technology uses a hydraulic energy storage and propulsion system in the vehicle. The hydraulic system captures and stores a large fraction of the energy normally wasted in vehicle braking, and uses this energy to help propel the vehicle during its next acceleration. Hydraulic hybrids draw from two sources of power to operate the vehicle — the diesel or gasoline engine and the hydraulic components. The primary hydraulic components are two hydraulic accumulator vessels and one or more hydraulic pump/motor units.

Hydraulic hybrids can quickly and efficiently store and release great amounts of energy due to a higher power density. Hydraulic hybrid technology cost effectively allows the engine speed or torque to be independent of vehicle speed, resulting in cleaner and more efficient engine operation. The current hydraulic hybrid technology has a parallel design that is integrated to compliment the vehicle's existing conventional drive train.

The growing electric power needs of modern combat systems have been driving the need for electrical storage capacity. The Army's TARDEC NAC has engaged in designing high-power, high-energy-density lithium-ion batteries for use in hybrid electric vehicle propulsion systems. This energy storage research is being considered for other critical applications including auxiliary power units, plug-in hybrids, silent-watch energy storage, pulsed power delivery applications for direct-energy weapons, and future hybridized power source designs for fuel-efficient vehicles. This is all being done principally to support emerging new operational requirements for tactical platforms to operate temporarily in a stealth mode, and to power electrical systems on future mobile combat systems.

Technologies to Improve Energy Efficiency

There are three technologies with the potential to fundamentally alter DoD capabilities and enable new concepts of operations (see Figure 1). These offer the potential of double-digit percentage improvements in energy efficiency over current technologies. The three technologies are:

  • Blended wing body for fixed-wing, heavy-lift aircraft;
  • Variable-speed tilt rotor for vertical-lift aircraft; and
  • Badenoch blast-bucket design concept for light-armor ground vehicles.

The blended wing body (BWB) design would fundamentally alter the design of heavy aircraft such as tankers, bombers, and transports. It offers the possibility of two times the gain in range and payload, and five to ten times the gain in system-level fuel efficiency.

Emerging vertical-lift technologies and new rotorcraft designs, specifically advanced tilt rotor designs exploiting variable speed rotors, hold promise of far greater range, speed, and operational flexibility, with substantially reduced fuel consumption. New technologies available in engines, structures, drives, flight controls, and subsystems make significant improvements possible in empty weight, propulsive efficiency, and overall fuel economy.

In Iraq, Army ground vehicles have proven highly vulnerable to improvised explosive devices (IEDs). To mitigate this problem, the Army has up-armored its vehicles. However, this has reduced fuel mileage from about 10 mpg for a standard HMMWV to about 4 mpg. The additional weight also puts the vehicle beyond the design limit for its suspension, brakes, and tires. The Badenoch vehicle, developed at the Georgia Tech Research Institute (GTRI) with funding from the Office of Naval Research, weighs less than half of an up-armored HMMWV, has much greater fuel efficiency, carries as many soldiers, provides better ability to fight from the vehicle, and improves protection against blast and projectiles (see Figure 2). It has a new lightweight armor, and is made from two layers of aluminum sandwiching and a combination of unique materials. This design is able to resist much larger, higher-velocity projectiles than existing, much heavier steel armor. The vehicle will have advanced power-generating capabilities — portable power – to provide up to a megawatt to power emerging battlefield concepts such as electrostatic armor and bunker-busting rail guns. Badenoch vehicle plans call for a hybrid engine that combines diesel and electric power plants.

Nuclear energy, with many advances made over the past 40 years, should be explored for its potential for military application other than Navy ships. A moderate degree of research effort could produce nuclear-based technologies that enable employment of multiple-use portable and small form-fit transportable systems on the battlefield, the production of synthetic fuels aboard reactor-designed ships to support ground forces in remote locations, applications that provide electrical propulsion energy to mobile combat systems, and combat systems powered by nuclear energy.

This article was written by U.S. Army Colonel Gregory M. Fields, Institute for Advanced Technology (IAT), University of Texas at Austin. For more information, Click Here .