Some environmental constraints for deployed military computing systems are operational temperature, humidity, sand, dust, vibration, shock/basic, leakage (immersion), steam and water jet cleaning, rapid decompression, contamination by fluids (e.g. oil, fuel, cleaning), nuclear hardness, and input voltage. In addition, the type of platform or vehicle on which the electronics are to be placed will have its own typical operating environments and related ruggedization requirements. For example, tracked combat vehicles, such as tanks, will have quite different requirements compared to wheeled tactical vehicles, so immersion, while not typically a requirement for tactical vehicles, but will be a requirement for tactical combat vehicles because they will need to be able to ford through water.
The ruggedization requirements for different environments are well defined. For example, ground combat vehicle environments are covered by the US Army CCDC GVSC Automotive Tank Purchase Description: Interface Standard – Environmental Conditions for the Heavy Brigade Combat Team Tracked Vehicle Standard (ATPD-2404 21-OCT-2011) and newer revisions. This relatively straightforward and short document pulls together a number of standards (e.g. MIL-STD-810) by reference to provide operational environment constraints that must be considered for any materiel solutions intended for use within ground combat platforms. In short, any equipment within such a platform will suffer wide temperature ranges, high shock, lots of vibration, and a dirty environment, among the many other environmental conditions rarely if ever considered for commercial electronic systems – most specifically computing systems – that were originally designed for operation in commercial benign environments (e.g. home, office, server room, data center).
Today, system integrators have a wide range of commercial-off-the-shelf (COTS) electronics solutions from which to consider when designing their deployed electronics solutions for military vehicles. COTS electronics offered in the marketplace can range from pure commercial products, such as those one might find available at a so-called “big box store,” to rugged systems designed for use in industrial environments, and at the far end of the spectrum from commercial products, those fully rugged solutions designed from the ground up specifically to operate in harsh military environments.
The VPX (VITA 65) profile provides a robust and popular design for open standard-based, rugged electronics for defense applications. The backplane-based systems that use these removable modules (also known as plug-in cards or PICs) are housed in chassis that meet a range of environmental conditions addressed by MIL-STD-810. The VPX standard is a foundational element of Modular Open Systems Approach (MOSA)-supporting standards such as the Army’s CMOSS (C5ISR/EW Modular Open Suite of Standards) and the Sensor Open Systems Architecture (SOSA) technical standard currently being defined.
Leading military COTS electronics vendors actively contribute to the definition and advancement of the open standards included in CMOSS and those being defined by the SOSA Consortium. In addition, many of these vendors participate in the VITA Standards Organization (VSO) that oversees the definition of the OpenVPX, PMC, XMC, and FMC form factor standards called out by the CMOSS and SOSA technical standards.
Electronic systems designed specifically for defense applications also come in a range of ruggedization levels, depending on the type of platform and the environment in which they are intended to perform. For example, a mission computer deployed on a helicopter, where the ability to operate optimally when exposed to intense vibration conditions, will necessarily be more rugged than a mission computer with the same compute performance intended for use in a mobile ground command center.
Another issue is size, weight, and power (SWaP). Theoretically, while commercial equipment could be enclosed in a radiation-shielded, sealed, environmentally controlled case with shock and vibration isolation, this approach would be impractical and prohibitive for ground vehicles from a SWaP perspective. The enclosure would need to be too large and inefficient with regard to space, and it would require significant provisions for thermal control. One class of rugged electronics designed for SWaP-constrained platforms are ultra-compact subsystems for mission computing or network switching, designed to MIL-STD-810 standards. These sealable systems support an LRU – line replaceable unit – approach, so that rather than replace an individual module, the entire compact box can be readily removed and replaced.
Cost is also an important factor in selecting the correct ruggedization levels for deployed electronics. For example, equipment that is designed to operate on tactical vehicles may be technically suitable in terms of performance for a platform that operates in less harsh conditions, but that equipment, because of its high-level of ruggedization, may prove too expensive to consider for deployment in more benign environments. On the other hand, the significantly lower cost of commercial or industrial-grade electronics can tempt system designers to consider deploying solutions that would be unable to survive in the harsh environments typically experienced by tactical vehicles. For all these reasons, it’s critical that system integrators have a complete understanding of army environmental and operating requirements and the ability of the various types of electronics systems to meet, survive, and perform optimally in accordance with those requirements.
While ground combat platforms provide examples of some of the most intense and harsh environments in which military electronics must operate, they highlight the wide disparity in performance attributes between solutions that are designed specifically for deployment and commercial products. Under typical operational requirements, the required operational temperature for combat vehicles is well below 0°C to over 50°C. The electronics must even withstand up to 71°C for extended periods. Typical commercial computing equipment can only handle 10°C to 35°C. To get the commercial equipment to survive in military conditions would require a complete redesign to meet the necessary temperature ranges or the creation of an environmentally controlled chamber to house the electronics.
While different techniques for thermal management have benefits and drawbacks in specific operational environments, including reliability, performance, maintenance concepts, and other constraints, electronics designed to meet military ruggedization requirements are able to meet all cooling approach options, including liquid cooled, forced air, cold plate, and natural convection. Liquid cooling can provide the absolute highest thermal management performance; however, it presents a significant challenge regarding platform integration, maintenance, reliability, and overall integration requirements (tubing, radiators, reservoirs, etc.). Forced air-cooling with fans provides a lot of thermal management capability, and in the right environment, fans are a good solution. However, fans introduce numerous problems in a ground combat environment, precluding compliance to the typical requirements for sand, dust, and fluid contamination, immersion, and wash-down, reliability due to MTBF (requires scheduled maintenance) and noise. For example, one type of fan frequently used on military systems is designed to mitigate issues such as sand and dust, but its noise level is 82 dBA at a moderate static pressure. Two of those fans would result in 85 dbA – the limit in typical requirements.
The cold plate cooling approach requires the unit to be bolted directly to a large (relatively) cold mass (e.g., directly to the platform’s hull) with a thermal interface material between the electronics chassis and the vehicle hull. Although this may initially appear to be a good approach to the electronics designer, this method poses significant challenges in terms of integration, requires a suitable surface for the cold plate to interface with, and necessitates that the temperature range of that surface will actually provide the appropriate ΔT needed to be useful across temperature ranges. The designer must also consider what impact there will be at the platform level if heat is rejected through that surface and mass. For example, will this impact the platform’s IR signature? While cold plates have the advantage of being sealed enclosures (versus forced air), they preclude the use of any sort of shock / vibration mounting.
Natural convection is generally the easiest cooling approach for integration and reliability, especially since it uses no moving parts. Unfortunately, it also typically provides the lowest performance. It relies on radiating heat fins to increase surface area, through which heat transfers to the (assumed static) surrounding ambient air. Fundamentally, this approach conducts heat through a mass out to a surface, and the limiting factor is heat/area, which it then scales with the Δ T between the surface and the air. Natural convection does have the advantages of being fully sealed and allowing for shock/vibration mounting if required. Aside from the temperature challenges, natural convection provides system designers with the easiest approach for typical requirements compliance.
For almost all typical requirements, commercial-grade electronics will simply not survive in the extremes of military environments. Electronics are required to survive humidity conditions of up to 100% (non-condensing). Commercial electronics typically only meet 20% to 80% (non-condensing). Military electronics on platforms must survive conditions such as 4.7 Grms 10-500 Hz continuous vibration, but commercial electronics are typically rated only to 0.26 Grms 5-350 Hz for 15 minutes. Because commercial electronics are not sealed, they are unable to meet environmental requirements for leakage and immersion (1 meter for 2 hours), steam & water jet cleaning (172.2 to 241.3 kPa [25-35 psi]), and contamination by fluids.
Leading COTS vendors understand the complete continuum of environmental requirements and offer ranges of electronics solutions designed to meet the particular needs of specific platforms and battlefield conditions. COTS products for deployed defense applications, unlike commercial electronics systems, are designed from the ground up to address the unique needs of deployed military applications and deliver the latest technologies to our warfighters.
This article was written by Ivan Straznicky, CTO Advanced Packaging, Curtiss-Wright Defense Solutions (Ashburn, VA). For more information, visit here .