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Today’s military applications routinely push the limits of size, weight and power (SWaP) restrictions while at the same time performing with the utmost reliability in intense environmental conditions. These extreme applications also are advancing to feature ever-increasing computational performance and communication bandwidths that, in turn, have driven significant power densities at the board, chassis and platform levels.
The evolution of military system design makes it a necessity that military designers be sharply focused on innovative thermal management techniques to ensure fault-free performance. COTS board and system-level solutions are increasingly providing new subsystem capabilities and are enabling new designs that meet or exceed current mil/aero thermal requirements. However, it is essential that military designers have thermal methodology expertise that includes a thorough understanding of the environmental influences that cause systems to generate heat and how design choices efficiently and reliably dissipate that heat.
Size, Weight, Power and Cooling
The challenges for military system design with regard to cooling techniques have increased along with higher performing processors, smaller system footprints and the evolution of extremely rugged environments. Battlefield settings today include severe temperatures, shock and vibration, explosive decompression, immersion or exposure to sand and dust—just a handful of the potential variables up for consideration by designers building rugged, high-performance military systems. As a result, the SWaP protocol has transitioned into SWaP-C (Size, Weight, Power and Cooling) as a priority focus for packaging engineers solving thermal challenges of these next-generation designs.
While thermal management is an acquired and practiced skill, there are a set of general guidelines (Table 1) that are useful in helping military designers determine limitations and attributes of cooling methodologies. The optimal thermal solution for a particular application, conversely, is much better defined by its unique set of design characteristics and requirements. Each thermal equation has its own parameters and data points that must be fully evaluated before committing to an embedded computing platform or systems packaging solution. A critical starting point is defining the operational environment—airborne, naval, ground command or mobile, or all of the above—that will guide packaging engineers in determining thermal options.
Shown here are approximate power dissipation capabilities for various cooling schemes. Environmental conditions, including the three primary factors of ambient temperature, altitude and generated power, may create notable variation in these approximate values.
Key Initial Data Points
The key initial data points include ambient temperature, altitude and the amount of generated power to be dissipated—each an essential detail in initiating development of optimal thermal techniques. Ambient temperature seems an obvious key consideration—an aircraft system sitting on the Baghdad tarmac in ambient 110°F has dramatically different thermal requirements and capabilities than one in a UAV airborne at 60,000 feet (Figure 1). Further, higher altitudes mean decreased air density, which directly affects a system’s ability to conduct heat via transfer of air based on fewer available air molecules.
Global Hawk UAV lands at Edwards Air Force Base, CA after an eight-hour mission. Its onboard systems need to withstand both the conditions at altitudes of over 60,000 feet and sometimes hot temperatures on a tarmac.
This applies to forced or passive convection solutions as there is simply less physical air surrounding the chassis. The question of altitude does not necessarily apply only to designs in development for UAV applications, and is valid for the range of military airborne settings. Systems performing in pressurized cabins, for example a cockpit application designed for human interface at 10,000 feet, must manage air density differently than a system placed into an unpressurized cabin on board an extended flight, high altitude UAV.
Total power generated within a system presents a more complex challenge. For example, a five-slot chassis may produce only 50W of power to be dissipated that would lead to a fairly straightforward thermal design solution. However, today’s military systems show a prevalence of high-speed signaling buses such as those supported by VPX-based platforms, and as such frequently demand much higher levels of power (up to 150W per board) dissipation. The thermal design options would be vastly different between these two types of systems. Using the 50W system as an example, a passive convection system with no fan may provide sufficient thermal performance based on the environment, altitude and ambient temperature. However, when compared to a 500W system, one or more fans may be required on board the chassis in order to dissipate heat effectively.
Asking the Right Questions
Defining the three major data points of ambient temperature, altitude and power dissipation provides the majority of information necessary to begin evaluating thermal options. Other pertinent data may not relate directly to thermal considerations but is still considered essential in this portion of the design phase. For example, a customer may have initially desired a liquid-cooled chassis for a 550W system implemented in a ground mobile application. However, deeper evaluation of key data points and environmental issues may indicate that a forced air conduction-cooled chassis would meet requirements while saving development time and money.
Questions such as “Does the system use an AC power supply or DC input?” or “What type of EMI filtering does the application demand?” are important. So too are shock and vibration demands. Does the environment in an extremely rugged and demanding helicopter system warrant mounting the computing boards in an alternative layout? Performance and reliability of moving parts such as fans may be affected in this type of setting; plus the chassis itself must be able to physically withstand the conditions. Size and weight requirements may also be very limited, restricting design choices once they are taken into consideration along with performance demands and existing environmental conditions.
Heat, dust and exposure to other airborne contaminants should also be evaluated; these elements can further constrict system airflow and may demand customized attention to thermal management. For example, a shipboard system may be exposed to salt spray or other corrosive environments. This may demand a specific element-resistant coating on the chassis, which in turn may influence its thermal efficiency.
Once a design layout is planned, thermal modeling tools such as Flotherm (Figure 2) are useful in confirming thermal performance. Through advanced CFD (computational fluid dynamic) evaluation techniques, military designers can accurately predict airflow, temperature distribution and heat transfer in components, boards and ultimately, complete systems. Packaging engineers can create and test modifications easily before any equipment prototypes go into production, by viewing and understanding flow paths and the impact of heat exchanger design and fan selection.
Using Computational Fluid Dynamics (CFD) analysis, engineers can optimize the pressure and temperature characteristics of each design. Thermal modeling protocols consider thermal, electrical and environmental stresses and illustrate careful consideration to heat exchanger design, flow paths and fan selection.
Engineering Partnerships Essential
However, taking a military design from concept to development, and then from prototype to production, requires extensive experience with military programs and specifications—going well beyond thermal expertise. Rigorous military requirements must be priority factors through custom design, thermal modeling and product development processes, and this is frequently where manufacturer support can drive a design to successful development. One manufacturer-recommended design approach is to implement all required system functionality in a chassis that has already been certified for ruggedized operation, rather than being simply listed as “designed to meet.”
Providing the shortest delivery lead time and lowest cost in a proven solution is an inherent benefit of using a custom variant of an off-the-shelf chassis enclosure. For instance, incorporating a chassis manufactured to meet the standards of MIL-E-5400 Class 1 thermal performance, MIL-901D shock and MIL-167-1 vibration, assures that it can withstand specified extremes of temperature, vibration, shock, salt spray, sand and chemical exposure—all while maintaining a sealed and temperature-controlled environment ideal for its interior computing elements and electronics.
Meeting Application Needs
The technical approach for a particular enclosure may be based on many proven and validated legacy enclosures—for example, conduction-cooled, VPX, VME and cPCI-based systems that are currently in field use for both ground and airborne applications. For example, the internal arrangement of heat generating VPX cards (upstream to the cooling airflow) and power supplies (downstream to the cooling airflow) may reduce the effective heat loading on the critical VPX cards. Further, mission requirements may demand the chassis is cooled and mounted in a specific manner. Beyond mounting systems within standard racks or into ARINC style equipment trays, ideal options include custom hard mounting or perhaps shock mounting within mobile platforms. From a cooling standpoint, some applications are able to use forced air or forced convection (using internal or external fans); however because of space, weight and environmental constraints, many UAV applications must incorporate conduction-cooling methodologies (with or without fan assist).
Ultimately, selecting the optimal thermal method or deciding on a custom solution hinges on the packaging designer’s comfort level in making thermal calculations and his overall knowledge of thermal methodologies. A proven and recommended approach is to cultivate a manufacturing partnership, a means by which many OEMs have gained significant competitive advantage. This approach streamlines the design process and speeds time-to-market by combining all critical levels of expertise into a single engineering resource for military OEMs. But more importantly, while much technical science and expertise goes into solving thermal challenges, speculation based on past experience is absolutely part of the process and a critical asset from a manufacturing partnership. As a result, it is essential for packaging engineers to understand cooling and mounting options at the outset, work with manufacturer resources for greater detail on advantages and limitations of specific solutions and then build optimized systems upon a proven foundation.
Tips, Myths and Realities
All cooling methodologies benefit from the always-present effects of cooling by radiation. It is important to note that the positive effects from radiative cooling are typically ignored unless the system’s overall power dissipation is low. In these lower power scenarios, the fractional contribution of radiation can be significant and should be included as part of the thermal equation. In fact, a common myth in systems packaging is that radiation plays a marginal role in cooling military electronics.
While this may hold true for higher power systems with greater power dissipation requirements, engineers should pay attention to the effects of radiative cooling in passively cooled convection systems that operate at low power. For example, a generic metallic box with dimensions of 8- x 12- x 7-inch can dissipate more than 27W by radiation effects alone in an environment of 100°F and 0 air pressure (a perfect vacuum). With the evolving range of military applications—particularly UAVs flying at extremely high altitudes with reduced air density and an absence of infrastructure to support liquid or air-cooled systems—these small, lower power systems (Figure 3) are carving a design niche driven by their efficient use of radiation as an applicable cooling methodology.
The COBALT (Computer Brick Alternative) uses a passive convection approach to cooling, which provides excellent power dissipation in a fanless, sealed system. It handles operating temperatures ranging from -20° to +55°C (with Core2Duo CPU) and -40° to +71°C (with Atom CPU).
Liquid Cooling Tradeoffs
Liquid cooling is frequently assumed to be required for higher performance systems dissipating power in the range of 500W to 1 kW. This is another thermal myth and there are alternative options for systems in this range. Liquid-cooled systems introduce a greater number of challenges into the system; systems are very heavy, costly and their mechanical complexity reduces overall reliability. This is driving designers to evaluate data points carefully and consider additional cooling options that meet or exceed the specific thermal requirements.
Many higher power systems can instead be cooled using conduction paired with forced air; however, they require multiple high-performance fans and skilled design modeling. The absence of a heat exchanger—which requires a costly and heavy infrastructure for support—is a design advantage and further avoids the challenges of liquid-based solutions. This is an especially important consideration for airborne or ground mobile combat applications where reliability and reduced weight is a premium design requirement.
Lessons in Thermal Design
It’s clear that effective thermal management is a significant challenge in today’s increasingly complex military systems. Considerations for power dissipations, design layouts, paths for air flow and overall thermal performance must be weighed and clarified early in order to develop the rugged systems suitable for mission-critical military situations. Packaging engineers benefit upfront with a clear understanding of each of the thermal alternatives, strong manufacturer relationships and a focus on ambient temperature, altitude and power dissipation as initial key data points.
Rugged military applications frequently require higher performance (with higher speed and density components), in tandem with smaller form factor boards for reduced system footprints. As thermal management options continue to evolve, next-generation applications will likely require further enhanced cooling solutions to meet new specifications standards or improved ruggedness capabilities. Extensive knowledge of thermal technologies and supporting design choices—whether semi- or full-custom solutions—is paramount to managing the complex cooling issues related to many of today’s military systems that must operate in unique or extreme environments.