Open Standard Takes Flight



VITA 48.8 is the open Air Flow Through cooling standard that will reduce weight in airborne embedded systems and could herald innovation with new materials.

The ANSI Standard ANSI/VITA 48.8-2017 Mechanical Standard for Electronic VPX Plug-in Modules Using Air-Flow-Through (AFT) Cooling was ratified in October 2017. It allows systems to increase significantly the amount of VPX modules in small aircraft, where weight is a prime consideration.

The standard is intended to enable Original Equipment Manufacturer (OEM) suppliers with a license-free, industry standard for cooling high-performance electronics in aerospace and defense. VITA 48.8 will support small form factor 3U and 6U VPX COTS modules to reduce weight.

Platform integration of latest generation electronics has pushed conduction cooling beyond the limits of ambient air cooling without active cooling. The resulting demands at the platform integration level have required introduction of liquid or refrigeration systems that can weigh more and draw more power than the electronics being installed.

It defines design requirements for platforms that need high-performance processing, graphics, or electronic warfare capabilities and retain VITA 46.0 connector interoperability. Initially, it is expected to be used in fixed wing (i.e., airplanes) and rotary wing (i.e., helicopters or drones) aircraft.

Figure 1: For agility, rotorcraft, such as Lockheed Martin’s S-92, need power but not at the cost of weight and space. (Courtesy Lockheed Martin)

David Vos, Lockheed Martin Fellow, Lockheed Martin Rotary and Mission Systems, sums up how the standard differs from the earlier AFT cooling standard, ANSI/VITA 48.5. “ANSI/VITA 48.8 includes both 3U and 6U modules, in contrast to 6U only,” he notes. “It uses jack screws instead of wedgelock retainers for cost and weight savings and consolidates thermal requirements to modules, thus optimizing chassis Size, Weight and Power—and Cost (SWaP-C) through the use of lighter weight structural materials,” he continues. “ANSI/VITA 48.8 has a cross sectional inlet area [that is] increased an order of magnitude to dramatically reduce pressure drop and enhance thermal performance at altitude,” he adds, making it particularly relevant for defense and aircraft systems.

Figure 2: The latest processor technology is taking its toll on thermal management in aircraft and UAV design. (Courtesy Lockheed Martin)

Power Densities Take Off
Conventional defense and airborne systems use conduction-cooling to meet the performance requirements of the selected processors. The technology was outgrowing the market’s demands, as Vos explains: “Platform integration of latest generation electronics has pushed conduction cooling beyond the limits of ambient air cooling without active cooling. The resulting demands at the platform integration level have required introduction of liquid or refrigeration systems that can weigh more and draw more power than the electronics being installed.”

The latest processors used in COTS 3U and 6U VPX modules, however, are delivering power densities of up to 200W per square cm. In rotorcraft and Unmanned Aerial Vehicles (UAVs), such as drones, these conduction-cooled systems are prohibitive in terms of space and weight. According to Lynn Bamford, Curtiss-Wright Senior Vice President and General Manager, Defense Solutions division: “VITA 48.8 provides a low-cost, effective means to cool the latest generation of components.”

Curtiss-Wright chaired the VITA 48.8 Working Group, which was sponsored by Lockheed Martin, Curtiss-Wright and Abaco.

The agility of rotorcraft and UAVs is highly prized, allowing them to access mountainous terrain, for example, or perform vertical lifts in rescue operations. The increased use of drones and other UAVs also calls for weight- and space-conscious design.

Increasing the electronic content of an aircraft can increase its functionality, but this has to be offset against power-performance parameters as well as the aircraft’s weight and space restrictions. Any reduction in weight will increase the aircraft’s range, payload or fuel economy. Alternatively, the original functionality can be maintained, but by using the latest processors to achieve this in a small form factor.

The performance and functionality of an aircraft’s system can be raised by increasing electronic content and functionality. However, using the latest processors to achieve the same functionality in a small form factor can help reduce aircraft weight and thereby increase range, payload, or fuel economy.

Replacing Components
The standard reduces weight and cost in high-density, high power dissipation VPX systems by replacing the wedgelocks and ejector/injector handles with lightweight jack screws.

At the Association of United States Army (AUSA) 2017 meeting, where Curtiss-Wright announced a collaboration with Lockheed Martin for the standard with COTS VPX modules on fixed wing and rotary wing aircraft, the company described VITA 48.8 as a means to preserve investment in existing electrical and software architectures and to protect electronic components from environmental contamination. “ANSI/VITA 48.8 compliant plug-in modules will provide government and industry customers significant cost savings and approximately 40 percent weight savings for avionics systems deployed in platforms such as future vertical lift aircraft,” the company announced.

Thermal Issues
The standard also supports alternative air-flows, with an air inlet at the edges of the card, in addition to the conventional top edge inlet. David Vos explains that ANSI/VITA 48.8 introduces “additional air flow arrangements (three side openings) that further reduce pressure drop and enable hybrid chassis implementations housing both A/V 48.2 conduction and A/V 48.8 AFT modules plugged into the same VPX backplane.”

The new standard addresses design challenges that have been encountered in AFT cooling, such as air cooling, air flow intake, heat exchanges, and exhaust paths. Using a finned heat exchanger in the central section, compliant modules can top-cool the primary circuit board and mezzanine board components. The thermal efficiency of ANSI/VITA 48.8, says Vos, enables embedded computing boards to operate at full clock speeds without thermal throttling or cumbersome platform liquid cooling infrastructures.

Figure 3: Qualitative system level power impacts (Courtesy of David Vos, Lockheed Martin).

“Liquid cooling or refrigeration systems are uncommon on rotary wing and UAV platforms due to the extraordinary weight demands for these platforms,” explains Vos. “For example, a high power (~1kW) half Air Transport Rack (ATR) avionics box would require an additional half ATR air-to-liquid heat exchanger unit, drawing another 1kW to provide cooling—thus doubling SWaP at the system level installation. De-clocking or thermal throttling means that multiple embedded computing boards must be added to the solution to meet the computational requirements while not exceeding the thermal cooling capabilities. Lockheed Martin implementations of ANSI/VITA 48.8 on rotary wing and UAV applications have resulted in an average of 40 percent weight savings compared to ANSI/VITA 48.2 conduction cooling, popularly used for the past 20 to 30 years. In addition, fan cooled ANSI/VITA 48.8 implementations avoid the condensation issues that plague active cooled systems operating below the dew point. Deployed systems have experienced up to a 200-pound weight increase from condensation in the field,” he adds.

 

This cooling strategy, in place of module-to-chassis conduction cooling, opens up possibilities for alternative materials to be used for the chassis, such as polymer, which is light in weight, or for composite materials to be considered.
The open standard also includes a VPX connector layout and is believed to be the first AFT cooling technology for small form factor 3U VPX modules, used in military and aerospace applications. The standard allows pitch increments of 1.0, 1.2 and 1.5-inches, rather than “1.52-inch maximum [in ANSI/VITA 48.5]” to improve system level SWaP, says Vos.

Figure 4: Comparison of top-cooled and air-flow-through cooling. (Courtesy of David Vos, Lockheed Martin).

An advantage of the open standard, he goes on to point out, is that there is no associated essential Intellectual Property (IP), which reduces cost and its barriers to use. Additionally, the open standard allows for reuse, leading to a cost-effective architecture. “A/V 48 (including A/V 48.8) is a mechanical architecture open standard. OpenVPX is an A/V 46 and A/V 65 electrical and software open system architecture. Coupling these standards together maximizes reuse, thus cost savings, for the [Department of Defense] DoD customer.”

The new standard is ready to take flight, with “numerous types of electronics flying on multiple fixed and rotary wing aircraft” from Lockheed Martin already in use, using AFT cooling similar to the ANSI/VITA 48.8, confirms Vos. Curtiss-Wright has ANSI/VITA 48.8 AFT-compliant offerings on each of its product types, he adds.


Caroline Hayes has been a journalist covering the electronics sector for more than 20 years. She has worked on several European titles, reporting on a variety of industries, including communications, broadcast and automotive.

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