How Embedded Systems Are Coping with the Heat of Today's Avionics

With faster processors, more functions, and higher bandwidth than ever before, today’s avionics are pushing the cooling technology envelope to the breaking point – and beyond.

“The result of this trend is increasing heat flux at both the component and circuit board levels with heat flux levels in excess of 100W/cm2 for commercial electronics and over 1000W/cm2 for selected military high-power electronics,” Nelson J. Gernert, VP of engineering and technology at Boyd Corporation’s Aavid thermal division, told Avionics International. “There is also a growing demand for more sophisticated and capable electronics used in harsh aerospace and defense environment applications.”

Embedded systems companies and others in this area are tackling the cooling conundrum with a number of innovative solutions. Here is how they are turning down the heat.

Helping Conduction Cold Plates Do More

Conduction is the most basic form of avionics cooling. The heat generated by the circuit boards is thermally conducted into a metal ‘cold plate’, where it is passively radiated away from the electronics into ambient space.

Boyd Corporation has bolstered the passive cold plate’s ability to conduct heat away from circuit boards (and components) by supplementing its line of solid aluminum cold plates with ‘aluminum k-Core®’ cold plates. These cold plates are made of aluminum encapsulated APG (Annealed Paralytic Graphite), which “combines the mechanical strength and CTE of aluminum with the increased thermal conductivity and reduced weight of graphite,” said Gernert.

Boyd also offers two-phase heat pipe enhanced cold plates, where a fluid within pipes embedded in the cold plate is evaporated to draw heat away from working electronics, and then condensed in another location to release that heat. This approach is the most effective way to enhance cold plate heat transference, Gernert said.

Advanced Cooling Technologies (ACT) is a thermal solutions provider to the aerospace market.

“Essentially, we’re here to help when you have something that will get too hot and you can’t figure out how to cool it on your own,” said Jens Weyant, ACT’s manager of defense and aerospace products.

A Heat Pipe Embedded conduction card (HiK™ card) with ICE-Loks™ installed, to provide extra cooling to avionics. ACT

In cases where passive conduction through aluminum or copper isn’t sufficient, ACT endorses the use of APG and evaporation through two-phase heat pipes.

ACT’s HiK ‘heat spreader’ magnifies the conductivity of heat pipes by embedding more of them into each aluminum cold plate. Meanwhile, the company’s isothermal card edge wedge lock or ICE-Lok™ aluminum card retainer pulls more heat from the circuit board being held, simply because it provides more metal to move the heat through.

“Traditional retainers only expand in one direction to clamp the card,” said Weyant. “The ICE-Lok expands in two directions, one to clamp the card and the other to thermally connect the retainer to the chassis groove. By doing this we are able to maintain sufficient clamping force while doubling the conduction contact area.”

Cooling Without Fans

General Micro Systems (GMS) builds small form factor servers, switches, and routers for military applications; including avionics. Although it also builds fan-cooled avionics systems, the company recently unveiled Titan, which it bills as ‘the industry’s first sealed, fan-less, conduction-cooled rackmount servers with artificial intelligence (AI) and mil-circular (38999) connectors for superior ruggedness in the most demanding defense and aerospace applications.’

Given that the Titan uses up to four of Intel’s latest second generation Scalable Xeon processors in either a 1U or 2U chassis, how does GMS get rid of the heat?

“We use the air frame itself to provide plenum cooling to this server,” replied Chris Ciufo, the company’s chief commercial officer. “Custom internal heat sinks conduct heat to the plenum, and air is either forced through the server or evacuated out the back in a vacuum-like fashion. This allows for a 100% silent server, which makes it ideal for use in operator spaces.”

General Micro Systems bills its new Titan as “the industry’s first sealed, fanless, conduction-cooled rackmount server.” General Micro Systems

Blowing air across circuit boards is a time-honored cooling method. The approach embodied by the VITA 48.8 open standard makes air cooling more efficient by spacing the circuit cards within the chassis to allow enhanced air flow. Cool air is bought in from numerous directions at the bottom of the cards, flowing upwards across them all and venting through the top sideways. This eliminates dead spots in the airflow, and harnesses the natural power of convection to move the air through.

“VITA 48.8 allows you to blow air directly through the cards, which creates a much shorter thermal path for removing heat,” said Brian Hoden, principal mechanical engineer at Abaco Systems.

Not only does VITA 48.8 deliver much better cooling than conventional air-based approaches, but it is less complicated than admittedly efficient liquid cooling. “With liquid cooling, you have to incorporate a liquid-to-air heat exchanger than takes up additional space; along with the piping and pumps,” Hoden said. “There is also the risk of leaks when components are switched out, or when a connection fails; which is not something you want to happen in an avionics bay.”

Liquid Cooling Is Superior

Along with Abaco and Lockheed Martin, Curtiss-Wright sponsored the VITA 48.8 Working Group that brought this standard to the industry. In fact, Curtiss-Wright chaired this working group, which speaks to the company’s belief in the value of VITA 48.8 air cooling.

This said, VITA 48.8 is one only of the thermal solutions Curtiss-Wright is willing to use for avionics cooling, said Ivan Straznicky; a technical fellow at Curtiss-Wright Defense Solutions. Unlike Abaco’s Hoden, Straznicky believes that liquid cooling (in line with the VITA 48.4 liquid flow-through standard) is safe enough for use in high-end avionics systems; due to its many benefits.

“The fact is that liquid is a much better coolant than air is,” said Straznicky. “It conducts and removes heat far more effectively; however, the use of piping, pumps and heat exchangers does complicate the implementation. You can use various liquids in the system; such as a dielectric fluid like Polyalphaolefin (PAO) that does not conduct electricity if it spills. All in all, liquid cooling is worth serious consideration.”

Northrup Grumman shares Curtiss-Wright’s opinion and approach. “The majority of our modern avionic systems utilize liquid cooling technologies as we have found it allows higher heat transfer,” said Greg Simer, the company’s VP of Air Dominance and Strike. “However, we also utilize air-cooling technology as well, depending on the performance requirements and power density of the system.”

According to John Eunice, chief engineer for Sikorsky’s maritime and mission systems, Lockheed Martin was one of the companies that initiated the ANSI/VITA 48.8 standard, for air flow through cooling to allow greater processing performance across more active cores and higher clock rates. The type of cooling provided by ANSI/VITA 48.8 is ideal for avionics systems because it allows for heat generated by the processor to be quickly transferred to air and blown out of the chassis, according to Eunice.

“Lockheed Martin is using AFT cooling on multiple aircraft including the S-97 Raider, S-B>1 Defiant, and HH-60W Combat Rescue Helicopter, with significant savings in weight compared to alternatives. S-97 RAIDER has experienced solid box reliability during the initial four and a half years of flight testing. HH-60W has successfully completed box qualification, aircraft flight tests, and has entered low rate initial production,” said Eunice.

Eunice outlines the company’s avionics cooling strategy as focusing on meeting the “comprehensive aircraft environment, infrastructure, and processing requirements, while adopting advances in manufacturing techniques such as 3D printing and engineered materials.”

Elma Electronic has recently added VITA 48.4 liquid cooling in an OpenVPX chassis that can hold up to six 6U VPX modules. “Especially for avionics applications, liquid cooling is the way to go,” said Walter Schindler, Elma Electronic’s engineering manager. “Liquid cooling is nine times more efficient than air cooling, and when you’re at high altitudes around 35,000 feet, the air has no mass; so there it doesn’t have much cooling potential.”

Siemen’s Mentor Graphics generates simulations of avionics cooling pathways in aircraft.Siemens

As mind-bending as today’s avionics cooling dilemmas can be, they are child’s play compared with cooling challenges to come. In the near future, “Commercial aircraft will be built with hybrid electrical propulsion systems to reduce their ecological footprints,” said Thierry Olbrechts, Siemens’ director of Aerospace Industry Solutions (Simcenter Simulation and Test Portfolio). “Sixth generation fighters will have to integrate energy weapons systems, which will create higher demand on electrical power onboard. These fighters will have to reject three to five times more heat during the mission than the previous generation.”

Some current aircraft are already seeing their avionics’ CPUs being throttled down when their heat exceeds safe margins and the cooling systems can’t keep up. Until new solutions are found (and embedded systems companies are working hard to find them), the limits of avionics cooling could hamper the development and full usage of next generation aircraft!

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