Next in a set of primers on the major parts of an AFV powerpack, this primer covers the issue of cooling down an AFV, mainly talking about the engine and transmission, but with a short look at climate control at the end too.
For ease of access, I've split this primer into five parts:
Introduction to powerpacks.
The cooling system.
The engine.
The transmission.
Future AFV propulsion.
AFV cooling comes in a few forms. The main thrust of this primer is on the cooling of the engine and transmission as part of the integrated powerpack system, but also has some overviews of the additional challenges of cooling in AFVs - climatic cooling of the occupied compartments and crew, and the additional cooling required for the increasing volume and thermal burden of electrical systems.
Why is this series starting with cooling? Whilst the headline metrics that will always be taken from a powerpack are its hp output, maybe its torque levels, and sometimes a rather reductive comparison of gear numbers, one of the greatest challenges and a capability that can govern an engine's performance more than anything else is often cooling.
With the space available for a tank powerpack being constrained to a fairly typical 3 to 3.5 m³, the only way to get bigger and more powerful engines is to reduce the size of the cooling system. In a modern liquid cooled 1,500 hp powerpack the cooling system is generally around 1 m³, which is 33% to 40% of the total volume of the pack.
The cooling system itself is also a significant 'parasitic' burden on the engine, with the fans alone consuming as much as 10-15% of the engine's power as they battle to keep the engine and transmission oil temperatures at safe levels whilst buried in the middle of an armoured box with almost no inherent airflow.
The burden of the cooling system has been most recently seen in the UK's Challenger 3 programme, where a lack of capacity in the cooling system is the core roadblock for increasing the engine power output, not any limitation of the engine or transmission. I wrote a twitter thread on that topic which you can find here.
Anatomy of an AFV cooling system
A cooling system is quite simple in concept, if not in implementation. Air goes in, heat is transferred from the vehicle to the air, and this hot air is expelled from the vehicle.
Intake(s): openings on the outside of the vehicle, typically on the engine deck, that allow fresh air into the system.
Fans: very high capacity fans draw the air into the system through the intake(s).
Heat transfer mechanism / coolers: depending on whether air or liquid cooled, an interface needs to transfer heat from the vehicle to the airflow. Typically this is a liquid cooled system using radiators in a modern AFV engine, as well as oil coolers for transmission and hydraulic oils.
Ducting: pathways for the air to follow between the various major components above.
Exhaust: openings on the outside of the vehicle to expel the hot air. Some vehicles mix the cooling air with the engine exhaust through a single exit.
Whilst this is an outwardly simple concept, the reality of fitting an efficient airflow and heat transfer mechanism into the incredibly cramped confines of an AFV is an engineering nightmare, particularly due to the environment of cooling an AFV being pretty well the opposite of what any engineer would want for such a design task.
The (many) challenges of AFV cooling
Cooling any vehicle can be quite tough, but the peculiarities of AFV requirements and how they are used makes it a particularly hard challenge.
Airflow
The first challenge is the highly unhelpful profile of the cooling profile being demanded of an AFV system. Most automotive systems increase the demand for cooling broadly in line with an increase in speed. As a car runs the engine harder, it generally goes faster, and so the designer has available to them increasing volumes of high speed airflow to utilise for cooling.
The inverse is typically the case with AFVs, with maximum engine load and cooling demand being at very low or even stationary speed as the tank manoeuvres off road or accelerates from a standstill, and very little, if any, airflow is available to work with.
AFVs don't go fast at all really, and even if they do it makes no difference as their intakes are not aerodynamically placed because any prime location for an aerodynamic air intake would be a dangerous weakness in the armour of the vehicle that is unacceptable.
As a result, airflow has to be achieved by pure mechanical effort - drawing it in with fans and expelling it as quickly as possible. This is the reason essentially all AFVs have large grills over their engine bays - these are covers for very large high performance fans desperately drawing as much air as possible through the cooling system.
Shock
High impact loadings, high stress and load conditions are imparted on all components of an AFV, including vibration and shock impacts through a wide range of profiles. Shock forces when travelling off road are commonly up to or exceeding 8 g, and ballistic impacts and nearby blast effects can peak at 20-25 g, with vibration forces for each in the range of 5 g to as much as 140 g. All of these forces need to be accommodated without damaging or interrupting the fans and pumps.
Blast
The cooling system must be able to accommodate the effects of blasts on or near the vehicle. Whilst many of these are chaotic and challenging to plan for, at a minimum the system must be capable of accommodating the substantial pressure wave generated by firing the main gun at all angles of traverse and elevation, as well as any defensive systems which increasingly include blast and fragmentation-based hard kill active protection systems, all without interrupting airflow, stalling or damaging fans, or otherwise preventing efficient cooling to continue.
Blockages and damage
Terrain and physical obstacles are also a problem for the designers - AFV are self evidently used in rather adverse places. Cooling systems therefore have to be well protected against splash, brush, scrape and impact damage as well as general obstruction by dust, snow, mud, debris and other material that could fall onto the vehicle.
General dust, sand and airborne particles that risk blocking the system can be scrubbed out with cyclonic filters, but as cooling air is not used for combustion or human consumption, other small particulate filters are generally unnecessary and can be omitted so as to not further impede air flow.
The need to avoid contaminants entering the intakes wherever possible constrains their location and design, and requires them to be well protected, which inherently calls for the opposite of the desire to enhance their ability to efficiently draw the highest volume of air possible or expose a suitable surface area of radiator or fan to the air.
Given tanks are very much there to be shot at, the intakes and exhausts have to have good ballistic protection, and providing required protection whilst offering minimal restriction to air flow, as well as provide for the minimum entry of debris is hard. Intakes are often heavily louvred to direct anything (ballistic or otherwise) from directly entering the system.
Most AFV are also capable of sealing their intakes if required in order to protect the engine from external threats - for example if a firebomb were thrown onto the engine deck, but this too adds more complexity and obstructions to the air pathways that restricts efficient air movement.
Temperature & altitude
AFVs must be certified for operation in essentially all climatic conditions from -50° C to +50° C with high tolerance for snow, ice, mud, sand, dust and water without significant issue and with minimal or no preparation to mitigate them.
That ambient temperature is a significant factor, in fact temperature is the single greatest variable that impacts cooling system performance, followed by environmental pressure (altitude). Increasing temperature from 0° C to 40° C decreases capacity for dissipating heat by 70%. Even just a modest increase to a fairly typical global temperature of 20° C or 30° C decreases performance by 30% and 50% respectively.
Designs have to factor this into the overall capacity of the system to ensure that they can still provide sufficient cooling in the anticipated extreme of temperature and thermal load from the powerpack.
Altitude is a less significant risk factor than temperature for most users, especially Western ones, owing to the majority of historic usage scenarios having been at relatively low altitude.
Is that representative of the future, though? The bulk of central Africa is 1,100 to 1,600 m elevation. Afghanistan, where many Western forces operated for the majority of this century so far, has an average altitude of 1,884 m. The border zones of China, India and Pakistan all average between 3,000 m and 5,000 m altitude.
Why does that matter? A cooling system operating at 3,000 m altitude loses over 30% performance to one operating at sea level, this would be in addition to any temperature induced losses. At 5,000 m it is as much as 45%.
The engine also loses a lot of power at these low pressure altitudes, and those that want to operate AFV at these altitudes have to make specific modifications to their vehicles if they want to maintain useful performance. An example of this are Chile's Leopard 2A4CHL, which are fitted with a high altitude package that includes uprated superchargers and cooling systems to keep the tanks working effectively at the country's 1,871 m average altitude.
External surface of the vehicle
The temperature of the external surface of the vehicle is also a problem in hot climates. With high ambient temperatures and solar loading the surface of the turret and hull can exceed 70° C or even 80° C, hot enough to cause 1st degree burns from brief contact or second degree burns with as little as 20 seconds contact.
Thermal coverings like Saab's Barracuda MCS are extremely effective at limiting this, dropping surface temperatures by 10-25° C. Interestingly, the surface temperature of an AFV does not correlate to significant changes in internal temperature, with other factors making much bigger differences. More on this below.
The impact of heat
The obvious primary concern of excessive heat build up is that the coolant will boil and the engine, transmission or other automotive components will overheat and fail, immobilising the vehicle.
Beyond the simple consequences of a vehicle's automotive system overheating and failing, the impact of transmitted heat and solar loading on the vehicle and its components are also significant and generate their own need to manage temperature in the vehicle.
Heat results in differential expansion of materials, meaning that dissimilar metals, plastics and rubbers will variously change shape as they heat or cool, and this can result in problems where they interface with one another. Rubbers and plastics in particular can discolour, crack, bulge or craze under thermal loading. Gaskets and seals can soften and even melt, fouling moving parts or compromising seals.
Low temperature are just as problematic, making flexible parts stiffen or become brittle, and reducing viscosity of fluids, as well as causing differential materials to expand or shrink inconsistently.
Signatures
The power generation system is the greatest single persistent source of heat, only surpassed as a peak heat by the running gear when it has been in motion for some time or the gun barrel immediately after firing. Due to the constant need to expel cooling and engine exhaust air, the system is something that is nigh impossible to avoid.
To combat this, some AFV mix their cooling air with the exhaust gasses from the engine to cool the resultant mixture (though the cooling air is hot, it is not as hot as fresh combustion gas), and expel this in the most opportune way to mask the resultant heat plume from likely enemy observers - in practice this means locating it at the rear of the vehicle and ideally angled downwards to make it less observable from above (though this brings its own concerns, as downward forced airflow will generate dust, an issue observed in early Boxer 8x8s that was adjusted in the A2 Drive Module build standards).
However, the general (and considerable) heat rejection being passed into the structure of the vehicle is nigh-impossible to prevent, and after even a short period of running there will be a range of hot spots across the vehicle, especially around intakes and exhausts. Modern coverings like Saab's Barracuda MCS help, but can run the risk of insulating the vehicle and causing even greater adverse heat build up inside.
A primer on vehicle signatures and means to mask them will be coming up at a later date.
Practical considerations
The norm is to place the intakes on the engine deck, giving them the shortest path to the powerpack and keeping them up and away from dust, debris and gunfire that pervade the sides of the vehicle. However, many specialist variants cannot use this space, as the deck area is a valuable space for a range of equipment such as cranes, excavators and bridges on ARV/AEV/AVLB, as well as the storage and transport of powerpacks, fascines and other engineering bits and bobs.
On these vehicles, the intakes and exhausts are typically relocated to the sides and rear of the vehicle, as can be seen when comparing the rear of the Challenger 2 and its AEV variant, the Trojan.
Cooling the electronics
Cooling is also a bigger topic than just the powerpack, with the need for climatic cooling and cooling provision for electrical systems within the vehicle a huge demand in a modern tank, and one that only increases as more and more electrical systems are added to vehicles.
Though electronics can be cooled using the main cooling loop, this is rarely practical in a tank application. This is because the bulk of the electronics are in the turret of the tank, and routing coolant from the powerpack, through a rotary junction to allow the turret unrestricted and unlimited traverse, to the electronics and then back again, is not practical at all.
Second is that the heat profile in electronics is quite different to that in automotive systems, creating different requirements of the system to cool them. As mentioned earlier, most coolant is a ethylene glycol and water (EGW) mix, or sometimes to reduce toxicity a propylene glycol and water (PGW) mix. In both cases, it is water based, and these mixtures (at 50:50 concentration) boil at 107° C and 106° C respectively.
The issue is that where automotive systems can be held below this figure quite readily, most silicon power electronics have a full-power operating temperature limit of c.125° C and so leave a very fine margin between the steady state coolant inlet temperature (typically in the 65° C to 80° C range) to the maximum where dangerous consequences lie. This requires extremely efficient coolers to avoid the much more volatile heat fluxes of electronics from surging the system into a dangerous state.
Added to this is that traditional coolant is not actually very efficient compared to water for cooling. EGW/PGW are around 30% to 40% less efficient as transferring heat than a pure water system, owing to increased viscosity and the thermal load carrying properties of the chemicals. Add in the impact of other chemicals like antifoaming agents and a traditional coolant is unlikely to be the best solution for a high demand electronics system.
As a result, most electronics are managed by a secondary cooling loop that is tailored to the needs of electronic systems in design and location - typically manifesting as a separate cooling system in the turret bustle. There are some clever technologies that work very well for the unique requirements of electronics cooling, such as two-phase cooling systems, which are by no means new but much better tailored to the needs of electronics cooling than the automotive primary loop in the hull.
Two-phase systems could be their own primer, but to briefly summarise they use coolant (typically a sealed system of pure water) where the coolant exists in two states - liquid as it starts the cycle, before being turned into vapor by the heat transfer, then condensing back to liquid as the heat is transferred out of the circuit and into the outside air.
These systems work really well for electronics cooling, with better management of the higher temperatures and rapid temperature changes.
Cooling the crew
Even in mild temperate weather, the interior of an AFV can be an uncomfortably warm place, rising to dangerously hot as the climate warms. Any increase in heat has a negative cognitive and performance effect, with excessive heat quickly leading to operational impairment and rapidly turning into heat casualties.
Getting the crew cool is no easy task, and typically achieved in two ways - cooling the whole fighting compartment, or cooling the crew individually and directly.
Compartment cooling
With am ambient temperature of 44° C and with solar loading commensurate with that air temperature, the temperature in the fighting compartment can climb to 65° C or more (the worst location for heat is the commander, who is also subject to a lot of solar loading when hatches are open. The best is generally the driver, though this position can still regularly exceed 50° C).
The obvious means of cooling the crew compartment is via air conditioning, and most modern tanks carry some form of air conditioner on them. However, this can be a challenging ask of the systems as it is a sizeable space and when in operational usage is under constant attack from disruptive heat sources such as the main gun firing and hatches letting hot ambient air in.
This requires quite powerful air conditioners, with the best approach generally being to adopt a microclimate approach, that is to pump air at high volume directly to each crew station to cool the immediate area, and not seek to achieve the nigh-impossible task of cooling the entire compartment. This makes it credible to achieve at least a 10° C temperature reduction at each station.
Personal cooling
Though not that widely adopted, active cooling systems for crews are a very effective way to get cooling to the crew in spite of whatever conditions may be in the fighting compartment - with hatches opening and closing, weapons firing and other effects, a more general compartment air conditioning system may be unable to maintain the required climatic conditions.
A typical active cooling system is a vest worn below any body armour that distributes either conditioned air or liquid coolant throughout itself so that it cools the core of the crew member's torso via an umbilical connection at each crew posiiton.
These can be very effective (cold air-based systems typically are slightly more effective than liquid cooling, as well as being preferred by users as they also facilitate the body’s natural thermal control process - sweat evaporation), a typical system delivers around 150 W of cooling to each crew member, which can effectively double the time before a crewmember becomes impaired through heat strain, which in temperate summer of c.35° C with solar loading on the vehicle too, is an extension to c.3-5 hours.
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