By Courtney E. Howard
Engineers at Aggreko plc, headquartered in Scotland with a support center in Houston, Texas, are working with Alliant Techsystems (ATK) staff to test Development Motor-2 (DM-2), NASA’s second Ares five-segment solid rocket motor. Aggreko’s low-temperature chillers were used to execute the DM-2 “cold motor” test, supporting NASA’s specification for electronics cooling in the motor to 40 degrees F to measure solid rocket motor performance at low temperature and verify design requirements of new materials.
“This project was unique due to its many special requirements,” says Steven Bukoski, project manager for Aggreko Process Services, a process engineering group within Aggreko. “Aggreko’s specialized, large-capacity portable equipment and skilled technicians were critical factors in successfully achieving freezing temperatures under challenging environmental conditions, such as hot summer temperatures, cooling 1.6 million pounds of propellant, and working with a movable structure.”
Aggreko process engineers and thermal management experts used specialized temporary utility equipment to cool the structure to target temperatures of 20 degrees F. Aggreko’s engineered solution for the cold motor test consisted of temporary generators to power a system of low-temperature chillers, specially designed low-temperature air handlers, a customized air-conditioning duct system, and a suite of temperature control and electrical distribution equipment.
Aggreko designed a first-of-its-kind low temperature air handler configuration to manage climate control for the mobile building: three stacks of two air handler units with a custom-made defrost unit. One of the air handlers drew air from inside the building, cooled it to 20°F, then recycled into the building while the remaining unit was on standby or defrost mode, enabling continuous cooling of air. A seventh air handler was installed to provide fresh air and positively pressurize the mobile building to eliminate infiltration of warm, moist air.
The approaches to cooling mil-aero electronics vary widely and depend on many factors: platform type and location of electronics, operating environments, environmental control system (ECS) capacity, SWaP considerations, and power/heat density, for example, says Straznicky. “That said, the established trend of rising power levels per unit area (heat density) of circuit cards, driven by higher power/density processors and component miniaturization, has led to the increased use of advanced cooling approaches like air flow through and liquid flow through. In parallel though, innovations in standard COTS cooling approaches, like conduction cooling, continue to meet the challenge of this heat/density increase, and thereby reduce the risks associated with implementing some of the advanced approaches. This is important as there are trade-offs required for the use of advanced cooling approaches.”
“The need for more power, lower weight, better reliability, and cost containment require very innovative solutions as these factors typically work in conflict with each other,” Henderson admits. “Liquid cooling provides a huge advantage over fan-cooling or pure conduction cooling.”
“In relatively lower-power applications, air cooling will continue to dominate as it is low cost and simple to implement,” Baddeley admits, “but in electronics applications where power densities are high, environmental conditions are extreme, and platforms are constrained by size, weight, and power, we will continue to see more mil-aero users rolling out both single- and two-phase thermal management solutions.
“Mil-aero customers still want and expect integrators to do their best to keep the electronics cooling solution simple, and liquid cooling will always add complexity over air cooling,” Baddeley adds. “The difference now is that as cost and complexity of liquid-cooling solutions come down and the size, weight, and power benefits gained by reducing the need to chill down massive amounts of air required to otherwise cool the electronics becomes more obvious, end users are increasingly becoming more accepting of advanced cooling solutions. That is, we are seeing more platforms and programs recognizing that the SWaP benefits are too good to ignore in relationship to the complexity factor.”
Today's mil-aero end users maintain their focus on size, weight, power, and cost (SWaP-C), says Reichenfeld, “so thermal management considerations are optimized to minimize these parameters, while maintaining operational performance in severe thermal environments. The thermal aspects of a system are considered at a platform level and impacts are determined in regards to cooling methods and burdens placed on the environmental control system contribution. For example, liquid-cooled chassis provide the highest thermal density in the smallest package size; however, this needs to be balanced against the SWaP-C of the pumps, reservoirs, heat exchangers, etc. at a vehicle level.”
No matter one’s thermal management preference, “industry will continue to innovate and provide high-performance, thermal management solutions to deal with high power dissipation with reduced size and weight,” Reichenfeld enthuses.
Thermal management of mil-aero applications continues to meet the challenges of higher power/density electronics; however, more innovation is required if this is to last, Straznicky says. “Fortunately, many developments are underway that are likely, in combination, to allow the continued use of high-performance electronics in mil-aero applications and environments. These developments span a number of areas, including new materials, optimization of legacy cooling methods, and maturation of advanced/exotic cooling approaches.”