Case study: Direct spray maximizes environmental isolation and flexibility on airborne platforms
StoryMarch 20, 2009
Andy Finch
Parker Aerospace
Military embedded systems experience harsh and rugged environments. In recent years, the power levels of the electronics deployed in these systems have been rising rapidly. The combination of the harsh environments with the rising heat loads of the electronics has forced vendors to develop more effective packaging technology. Meanwhile, direct spray cooling is proving itself an acceptable alternative to air and conduction cooling, providing exceptional environmental isolation and enabling lower lifetime cost of ownership. A case study is presented on Northrop Grumman's (NG's) Airborne Signals Intelligence Payload (ASIP), which includes a configuration of the direct spray cooled Multi-Platform Enclosure (MPE) and appears on NG's manned U-2 high-altitude reconnaissance plane and the Global Hawk UAV.
Existing and emerging applications running on military embedded systems are demanding ever-increasing computational power and communications bandwidths. For example, UAVs running signal intelligence systems or executing real-time image processing are driving impressive power densities at the board, chassis, and platform levels. Even more impressive are the demands created by radar processing systems, with system-level power consumption in the tens of kilowatts. At the same time, platform integrators are requiring smaller, lighter, and more efficient board and chassis-level products. Thus, platform integrators are forced to look for alternative cooling solutions.
A number of liquid-based cooling approaches exist. Most familiar to end users are conduction-cooled systems. In addition, the Liquid Flow Through (LFT) cooling approach is emerging, which typically uses PolyAlphaOlefin (PAO) as the coolant but differs from conduction cooling by delivering coolant to liquid-cooled cold plates mounted to components on the electronics[1]. Increasingly, direct spray is being viewed as an acceptable alternative to air and conduction cooling, providing excellent environmental isolation (Table 1) and enabling lower lifetime cost of ownership. These principles are illustrated in the following case study on the Airborne Signals Intelligence Payload (ASIP), to be installed on all Block 30 Global Hawk aircraft.
Table 1: Temperature requirements and cooling capacity comparison – Increasingly, direct spray is being viewed as an acceptable alternative to air and conduction cooling, providing excellent environmental isolation.
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USAF Airborne Signals Intelligence Payload (ASIP)
ASIP, developed by Northrop Grumman (NG) for the U.S. Air Force, is the next-generation signals intelligence sensor. The system detects, identifies, and locates radar and other types of electronic and modern communication signals[2]. The sensor first flew on Global Hawk in 2005 and recently completed extensive qualification flights on the U-2 high-altitude reconnaissance plane.
Sidebar 1
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Environmental isolation
Figure 2 shows the ASIP's enclosure, a configuration of the direct spray cooled MPE (see sidebar) designed to environmentally isolate the sensitive electronics and provide necessary thermal management. Flying on the U-2 or UAV, the payload housed in the unpressurized areas in the aircraft is subjected to extreme environmental conditions: temperatures as high as +70 °C at sea level and -65 °C at 70,000 feet.
Figure 2: The ASIP's enclosure, a configuration of the direct spray cooled MPE, is designed to environmentally isolate sensitive electronics and provide necessary thermal management.
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The electronics packaging enabled by the direct spray system allows NG to field commercial grade electronics. Even in harsh environments, the commercial grade electronics successfully survive the rigors of the airborne platform. Typical ratings for fully rugged boards are in excess of 10 Grms and can be as high as 20 Grms. The vibration profile for Global Hawk is 2.5 Grms. Many of the electronics were only rated for 2.7 and 5.1 Grms sine profile from 15-2,000 Hz for operational and endurance profiles, respectively. Depending on the vehicle profile, stiffening ribs may be added to commercial cards. With the cost of fully rugged boards often double that of commercial grade electronics, significant life-cycle cost savings can be gleaned with less rugged cards.
Another intrinsic flexibility of direct spray enclosures is acceptance of varying heat densities. In ASIP, different card sets are used for applications ranging from 600 to 1,700 W in enclosure power, yet the enclosure is identical between all use cases. For diverse heat loads, an external heat exchanger easily scales with power densities. Heat rejection options include fuel, PAO, EGW, RAM air, ambient air, or skin/hull. Dependence on platform infrastructure for cooling options is eliminated as the direct spray enclosure and heat exchanger act as a self-contained system in pressurized or unpressurized environments.
Several ASIP cards are RF receiver cards tuned to between 20 and 3,000 MHz, while other cards range up to 18 GHz. With air-cooled systems, the electronics closest to the air supply are coolest while those near the air exhaust operate hotter. Conduction cards also create a thermal gradient from the middle of the enclosure to the first and last slots. Due to temperature sensitivity of the RF electronics, compensation is usually required between cards in the enclosure. NG took advantage of the innate ability of direct spray to limit the temperature gradient to less than 2 °C across a 20-slot system with power densities ranging from 30 to 60 W per board. In the ASIP enclosure, the electronics are maintained between +30 °C and +50 °C while the ambient temperature ranges from -65 °C to +71 °C.
Also, heat fluxes up to 50 W/cm2 are found on bare die processors such as the 1.8 GHz Pentium M and are similar to those used in ASIP. With 70 °C inlet fluid, direct spray is able to maintain die temperatures at or below 90 °C. This is at least 10 °C lower than conduction-cooled enclosures. For electronics, it is widely accepted that for every 10 °C in temperature rise, reliability is reduced by 50 percent. The reliability impact of pumps, valves, and controllers is easily overcome by doubling the reliability of twenty 6U cards.
Efficient technology refresh means lower ownership costs
Integrators face the challenge of integrating air- and conduction-cooled boards on the same aircraft. Traditionally, it is not possible to mix electronics in the same enclosure due to constraints in both board and enclosure designs. For the ASIP sensor, NG was able to do just that by combining proprietary boards, commercial grade air-cooled SBCs, and other commercial Radio Frequency (RF) receiver cards into the enclosure. The enclosure consists of 20 user slots with inherent flexibility to mix and match electronics in unconditioned space and pressurized locations on the platform.
This electronics packaging technology enables smaller, lighter, power-efficient systems that can go through a technology refresh in a matter of months as opposed to years, providing for significantly lower total cost of ownership over the lifetime of the program.
Direct spray enclosures meet today's demands
The direct spray enclosures have been thoroughly evaluated for performance, reliability, and maintainability in harsh environments. The ability to protect sensitive electronics under extreme altitude, temperature, and vibration has been proven on platforms like U-2 and Global Hawk. The inherent flexibility in the selection of electronics enhances the utility of direct spray enclosures on legacy platforms and developmental aircraft, resulting in lower ownership costs.
References:
1. Harvey, R., and Odar, A., 2007, "Liquid Cooling Facilitates Tomorrow's Embedded Systems," Military Embedded Systems, Vol. 3, No. 6, Nov/Dec 2007.
2. Avionics Magazine, 2007, "Northrop Grumman Completes ASIP Testing," May 17, 2007 edition (see also www.aviationtoday.com/av/topstories/11724.html).
Andy Finch is a product manager for SprayCool. Since joining SprayCool in 2000, he has held engineering and management positions supporting program management, product development, and systems engineering. Prior to SprayCool, he developed control systems and automated equipment for the food processing, agricultural, and HVAC industries. Andy earned a BSME from the University of Idaho, an MS in Engineering from Purdue University, and an MBA from Indiana University. He can be contacted at [email protected].
Jeff Weiler is a technical director for SprayCool. Since joining the company in 1992, he has held a variety of engineering and management positions, with responsibilities including technical oversight into product development, concept development, sales and business development, and customer interface. Jeff earned a BSME from Washington State University. He can be contacted at [email protected].
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