Hybrid drives for military vehicles?

By Dr. Martin Krause //

Hybrid and electric vehicles are increasingly successful on the commercial market and are no longer a technical vision. But is this technology also usefully applicable in a military context? The voices calling for more environmentally friendly operations of the armed forces are getting louder worldwide and the list of ongoing research programmes on this topicis long. Does this mean that fuel will no longer be needed in the future and that everything will be fully electric and digital? Or is it more about an absurd scenario in which batteries have to be charged in the middle of the desert and standardised charging stations need to be installed in war zones?

The best way to evaluate the sense and nonsense of such concepts is to understand the advantages and disadvantages of hybrid drives, identify the relevant key technologies and then assess for which types of military vehicles hybrid drives may make sense.

Explanation Hybrid Electric Drive

A Hybrid Electric Drive (HED) system is generally understood to be a drive system in which combustion engines are combined with electric drive components such as batteries, generators, and electric motors. Depending on the driving mode, these then power the vehicle individually or together, with the combustion engine usually charging the batteries via a generator. The design comprises either two parallel powertrains, or the combustion engine only drives a generator, but no longer provides propulsion itself via a gearbox. Fuel is still used for refuelling.

This HED architecture differs from purely electric drives, where batteries have to be charged via external power sources. Synthetic fuels such as biodiesel also result in an improved CO₂ balance but are usually not considered HED drives.

Worldwide activities

There is a large number of military test programmes for HED prototypes around the world with the objective to better understand the technology, to develop meaningful concepts and to realistically test the vehicles. Many of these activities centre around the United States, the UK and France, such as for example the Bradley Hybrid Electric Vehicle Program in the US, which BAE Systems has been working on together with QinetiQ since 2020. The $32 million contract with the US Army’s Rapid Capabilities and Critical Technologies Office (RCCTO) involves the hybridisation of two Bradley A2 Infantry Fighting Vehicles (IFVs), and a possible extension to the Armored Multi-Purpose Vehicle (AMPV) and the Optionally Manned Fighting Vehicle (OMFV) programmes is considered. Also in the US, Oshkosh Defense has created the Electric Joint Light Tactical Vehicle (eJLTV), a hybrid version of the JLTV that operates significantly more fuel-efficient.

In France, Nexter is working with Arquus and Texelis on several HED vehicles from the Scorpion Programme: Griffon 6×6 and Serval 4×4, both multi-role armoured vehicles. In this context, the French Armed Forcesshow a clear interest in HED vehicles and Nexter considers HED propulsion systems a key area of its research and development. For example, Nexter intends to use the technology as well in the Franco-German Main Ground Combat System (MGCS) project. During the recent years, the French company Arquus developed two own HED vehicles, the Electer 6×6 armoured personnel carrier (APC) and the Scarabee 4×4 light armoured vehicle. The Scarabee was notably designed for hybrid propulsion from scratch. Texelis, also from France, focused on the HED elements of the powertrain and in 2021 formed a strategic partnership with QinetiQ to develop electric hub motors and methods for recuperation of braking energy.

The UK Armed Forces already started testing three different HED vehicles under mission-relevant conditions as part of the Technology Demonstrator 6 Programme (TD6) which began in 2020. The Jackal 2 4×4 is a highly mobile patrol vehicle of the UK company Supacat, the Foxhound 4×4 is a protected patrol vehicle of General Dynamics UK and the MAN HX60 4×4 is a tactical truck of the Anglo-German joint venture Rheinmetall BAE Systems Land. QinetiQ works on a variety of customer-funded development programmes, including programmes of the US-based Defense Advanced Research Projects Agency (DARPA), the Office of Naval Research (ONR), and the RCCTO. In the UK, QinetiQ works with the Mobility Test Rig (MTR) on a programme run by the UK Ministry of Defence’s Defence Science and Technology Laboratory (DSTL). This involves a one-third-scale model of an electrically powered armoured fighting vehicle.

Rheinmetall BAE Systems Land (RBSL) designs a concept to replace the diesel propulsion system of the Challenger 2 main battle tank (MBT) with a 1,000 kW HED system. Apparently, they manage to reduce the weight of the propulsion system by 25% and its volume by 15%.

In Switzerland, General Dynamics European Land Systems – Mowag (GDELS – Mowag) builds and tests a hybrid propulsion system for the EAGLE 4×4 patrol vehicle. A little more exotic is Sweden’s Splitterskyddad Enhetsplattform Programme (SEP), where BAE Systems Hägglunds hybridised wheeled and tracked armoured vehicles as a test platform back in the early 2000s to make them smaller and lighter for air transport.

Technological potential

The list of improvements that can be achieved, at least in theory, is quite long. First of all, an HED drive causes a reduced noise and heat signature, which opens up new operational possibilities (“silent watch” / “silent running”). Furthermore, the performance of the vehicle can be optimised since each wheel can be controlled and driven individually by using hub motors and drive-by-wire technology. Depending on whether the hybrid architecture connects the combustion engine and the electric motor in parallel or in series, both drives together can significantly increase the engine power for a short time if required (“burst mode”). And separate drive trains also increase the reliability of the overall system through their redundancy. Furthermore, in a hybrid structure it is possible to operate the combustion engine in a favourable speed rage, which positively influences its service life and fuel consumption.

A central advantage of HED concepts is that new vehicle architectures become possible, so that the components of the powertrain can be redistributed inside the vehicle. This also allows to realise better protection.

Furthermore, in the context of ongoing digitisation, the on-board electricity demand will continue to increase in the future due to energy-hungry systems: high-power radio, IED jammers, battle management systems, radars, cameras, remote-controlled weapon stations, charging docks for drones or infantry equipment, air-conditioning, laser weapons, etc. HED architectures can not only meet these energy needs more easily but can also be used flexibly as mobile generators for local power grids.

Last but not least, HED propulsion systems consume significantly less fuel when idling and can also be much more energy-efficient by means of braking energy recuperation. For example, main battle tanks spend most of their operational time in static idle mode and consume high quantities of fuel at the same time, especially if powered by a gas turbine. A reduced fuel consumption increases the vehicle’s range, simplifies overall fuel logistics and reduces the armed forces’ carbon footprint. This leads to “greener” operations.

Challenges and key technologies

However, all these advantages of an HED architecture are not for free, and one of the biggest problems in this context is the severely limited space inside the vehicles. Electric motors and batteries not only add further complexity, weight, and cost, but also require a lot of installation space, which is a scarce resource in all military vehicles. The total weight of armoured vehicles grows strongly out of proportion with the protected volume and therefore the solution is not to simply increase the vehicles’ size. For example, because the maximum weight is usually restricted.

From this it can be deduced that the energy density of the storage medium is a central key variable for the realisation of HED vehicles. The same applies to alternative approaches using liquid or gaseous hydrogen for fuel cells. Currently, only fossil fuels have an energy density large enough to operate heavily armoured vehicles in a meaningful way.

The key factors for successful HED realisation are an increase in the energy density of the batteries by about one order of magnitude, a very compact realisation of the HED drive unit, as well as a robust, reliable, and cost-efficient maturity of the technology. Here, for example, the danger of thermal overheating of the batteries should be mentioned, which can lead to battery fires with toxic gases in the vehicle interior.

So, for which vehicles can HED technology be usefully applied in the near future? Ideally, these vehicles should not have heavy armour, spend a lot of time idling, be equipped with a variety of energy-hungry systems, require a reduced noise as well as heat signature, and travel long distances at the same time. Based on these criteria, scout and patrol vehicles, for example, seem to be suitable first candidates for hybridisation.

When implementing HED architectures, it is fundamental to ensure that the technology represents a real improvement compared with a conventional drive system under realistic operating conditions. It should not be installed just because it appears to be modern and popular, and the CO₂ balance of specific vehicles is certainly not the most important aspect in a war.

A perspective for Germany

In the recent decades, German companies have set worldwide standards with conventional drive technology for tanks and other military vehicles, and this technology is still used very successfully on a global scale today. In the case of HED drives, however, the technical pioneering role currently tends to be abroad, while comparatively little is known about the activities of German companies. Exceptions to this are, for example, the concept studies done by Rheinmetall and the companies Magnet-Motor (RENK Group) and Vincorion (JENOPTIK Group). At Eurosatory 2022 in Paris, some national and international manufacturer will most likely surprise the industry and present new HED products.

The critical question with all these concept studies is to what extent the technology is ready for use or series production. This usually requires many years of experience in dealing with HED systems, and the British company QinetiQ, for example, has been working increasingly on such systems for the past 10 to 20 years.

While in the past decades cutting-edge military technology often resulted in commercial derivates in the civilian market, this relationship has already reversed. Today, commercial off-the-shelf (COTS) technologies often replace high-end solutions from various military niche markets, which clearly shows how much disruptive potential there is in HED drive units. Procurement cycles in this field last easily for 10 to 20 years, and in some cases significantly longer. Thus, if drive manufacturers miss a trend and cannot deliver these innovative systems in time for a tender, their drive components will consequently no longer be installed in the next product generation. This means that the respective markets would possibly be closed for decades, and it would remain unclear to what extent these markets could be opened up again at a later point.

An example for the use of COTS drive technology in military vehicles is the Lynx infantry fighting vehicle of Rheinmetall. It no longer has a highly specialised military engine but is powered by a Liebherr diesel engine of the type also used in the construction sector. These established engines are robust, durable, reliable, proved themselves over time and can be serviced worldwide. And in addition to that, they are also significantly more cost-effective and available in a timely manner simply because of their large production quantities. Now that Liebherr also has the first hybrid engines in its product portfolio, it is likely only a matter of time until hybridisation of the Lynx is considered. The Lynx is a possible candidate to succeed the Bradley IFV in the United States and it can be assumed that this successor programme will include at least some hybridisation.

In this respect, it becomes apparent how fundamental the threat of disruptive HED concepts is to the strategic position of established drive technology companies. It requires a technical vision to adapt the strategic orientation of these companies to the technology change and to acquire technology leadership in the new areas as well as to implement the technology worldwide. ACTRANS is a management consultancy with a focus on technology and innovation. Together with our network of experts, we support our clients in aligning their product portfolios, their processes as well as their technologies with growth and innovation.