Industry innovations aid electronics systems designers and integrators grappling with thermal management challenges in mil-aero environments.
BY Courtney E. Howard
Few issues plague military and aerospace systems designers, developers, and integrators as persistently as thermal management. Size, weight, and power (SWaP) constraints are challenging, but they also exacerbate the thermal management problem. At the same time, mil-aero electronics are employed in a vast array of environments, each with unique challenges–be they sand, snow, humidity, or temperature highs and lows. Today's mil-aero electronics must withstand the extremes of deserts and space, and virtually everything in between.
"Thermal management is absolutely critical in military and aerospace environments," says Curtis Reichenfeld, chief technical officer at Curtiss-Wright Controls Electronic Systems in Santa Clarita, Calif. "Functionality, reliability, and safety require maintaining electronics within qualified temperature limits."
"We all know that heat can adversely affect the performance of electronics," says Dr. Erich Buergel, general manager of the Mentor Graphics Mechanical Analysis Division (formerly Flomerics) in Frankfurt, Germany. "Minimizing weight and optimizing space are always key design goals, but an effective cooling solution is essential to reliability. Since field failure is not an option, thermal management is of utmost importance for mil/aero applications."
|The SprayCool Multi-platform enclosure, MPE-3U, from Parker Hannifin sports an integrated heat exchanger.|
Several trends are influencing thermal management in mil-aero environments. "As we automate more functions to minimize the need for human involvement, we are increasing our reliance on electronic devices," admits Buergel. "This, in turn, creates a need to design smaller electronic packages, as well as components that fit into tighter spaces.
"At the same time, chips and IC packages are becoming more powerful. Lastly, most devices used for mil-aero applications now must be self-contained/air-tight to ensure contaminants do not enter the device and adversely affect performance," Buergel adds. "The increased density that results from fitting more powerful components into smaller air-tight spaces creates heat dissipation problems."
On the business side, those serving the mil-aero community are "urgently pursuing cost-saving measures to deal with today's economic and market realities," Buergel says. "Due to the complexity of electronic products for mil-aero applications, plus the safety and regulatory issues that uniquely affect them, saving costs is not easy."
CFD and avionics
Component designers and systems architects are increasingly turning to electronic design automation (EDA) to reap cost saving and timesaving benefits. EDA software tools enable the design, simulation, analysis, and testing of electronic components and systems in the digital realm, rather than investing considerable time and money in crafting physical prototypes. "Simulation is proving to be the salvation of manufacturers working under pressure to deliver end products of proven quality at lower cost and in less time," Buergel says.
Computational fluid dynamics (CFD) software, in particular, saves a tremendous number of engineering man hours through the employ of algorithms to perform the millions of calculations required to analyze and solve problems that involve fluid flows–such as the movement of air throughout an electronics enclosure, Buergel says. "CFD simulation is one of the most reliable methods of understanding and managing thermal issues. Not surprisingly, CFD has become a cornerstone of aerospace mechanical design. With CFD simulation, manufacturers can greatly reduce the cost of getting a product out to market." For example, instead of testing multiple physical prototypes, design engineers can simulate and test multiple variations of the design in a fraction of the time and cost.
|Curtiss-Wright Controls Electronic Systems engineers designed a chassis to meet the requirements of the Northrop Grumman Advanced Mission Management System for the Broad Area Maritime Surveillance Unmanned Aircraft System program.|
Mentor Graphics in Wilsonville, Ore., has a strong pedigree in electronics cooling, Buergel says, given that the company's software simulation tools have been employed in thermal simulation applications for more than 22 years. Of late, mil-aero systems designers have increasingly adopted the Mentor Graphics FloEFD software with Concurrent CFD technology.
The tool embeds within mechanical computer-aided design (MCAD) platforms, bringing thermal analysis inside the digital design environment. FloEFD automatically prepares the design model in a host MCAD system for CFD analysis, and requires no special user training. "This process saves engineers time because they no longer need to outsource their designs to CFD specialists for evaluation. Mechanical designers can now perform accurate thermal analysis routinely without leaving their accustomed MCAD environment," Buergel explains.
All in all, thermal management is a major challenge from the very first design phase and it can be a significant design bottleneck, Buergel admits.
|Mentor Graphics' FloTherm plots airflow direction (shown with arrows) and temperature or velocity data (with color) for enclosures.|
Tecnobit, an industrial and defense electronics company in Madrid, Spain, designed a special chassis to house cockpit avionics in an enclosure whose maximum dimension was approximately 10 centimeters, or 4 inches. The system was designed to be completely sealed without any ventilation slots, requiring heat transfer through the outside surface by conduction, radiation, and natural convection. Tecnobit's preliminary design did not meet the design requirements; it was unacceptable from a thermal standpoint and conflicted with the trend toward higher power and heat dissipation in avionics systems.
Tecnobit's design team employed FloTHERM to evaluate avionics chassis design options in virtual form, with no need for expensive, time-consuming hardware prototypes. The simulations enabled the Tecnobit engineers to optimize the thermal design rapidly, while observing the effects of their design changes.
The company's engineers modified the internal chassis structure to increase heat conduction from the components to the chassis walls. At the same time, the Tecnobit design team added heat-dissipating fins to the enclosure's outer surface to transport heat away from the box. Sand-blasting treatment and electrostatic painting further enhanced convection and radiation exchange with the external ambient air. Tecnobit engineers used FloTHERM 3D thermal simulation to perform steady-state and transient thermo-fluid simulations, and predict system thermal behavior as various heat-conduction refinements were added. Ultimately, the team reduced component junction temperatures by 40 degrees Celsius compared with the initial design.
Mentor Graphics' FloTHERM spans applications ranging from evaluating sealed electronic modules to predicting airflow in server racks and blade enclosures. FloVENT tracks the flow of cooled or heated air through vehicles and buildings. The MicReD T3Ster (Trister) hardware measurement tool characterizes thermal impedances over an entire heat path–from the semiconductor junction that generates the heat to the outermost system housings and into the ambient–for the design of lighting and laser systems, printed circuit boards, and electronic enclosures.
|Parker Hannifin delivers thermal management solutions such as the above Liquid Flow Through (LFT) chassis and Heat Rejection Unit (HRU).|
The company's FloEFD Concurrent CFD tool set spans various mil-aero applications, including modeling the flow of liquids and gases, as well as evaluating aerodynamic surfaces. It has been used in the development of a micro aerial vehicle concept, in the evaluation of "nose pod" cooling and aerodynamics for a military reconnaissance aircraft, and in evaluating a nitrogen-injection feature for the fuel tanks in a Bell Helicopter military rotorcraft.
Engineers at Bell Helicopter, a Textron Company in Fort Worth, Texas, used the Mentor Graphics FloEFD for helicopter inlet temperature distortion modeling. "Flight testing revealed some marginal engine inlet air temperature distortion levels, so CFD was used to try to identify the culprit," says David H. Loe, principal engineer, Bell Helicopter Textron.
"It was assumed that either hot gas re-ingestion or inlet air heat transfer was the root cause, so we set up a helicopter model that simulated the problematic flight condition. Exhaust re-ingestion did not initially appear to be likely based on the CFD, so we added heat transfer to the model and applied some ballpark surface temperatures to the engine inlet," Loe explains. "We quickly learned that it was highly possible that surface heat transfer to the incoming fresh air could be taking place and modified the CFD model to simulate insulation on some of the inlet surfaces. The 'insulated' model showed improved inlet temperature distortion levels, so the flight test aircraft was ultimately outfitted with insulation blankets on critical surfaces identified in the CFD model." The software tool enabled the team to overcome time constraints associated with a test program and to model complex geometry in a short time.
FloEFD also aided Bell engineers in helicopter oil cooler airflow management modeling. "Space constraints forced a non-standard, blower-to-heat exchanger air duct, with rapid diffusion and some awkward, undesirable twisting of the duct," Loe describes. "The primary concerns were: excessive total pressure losses in the ducting that would adversely affect the cooling blower airflow rate and non-uniform air distribution at the cooler core inlet face that would result in poor cooler heat rejection characteristics.
"The CFD model helped identify the areas within the ductwork where the flow was separated from the surface, and to clarify the system total pressure losses," Loe continues. "Some changes to the duct detailing were recommended to improve the airflow characteristics and flow uniformity at the cooler inlet face." In the end, the simulation tool enabled the team to model complex internal airflow geometry, and to work an airflow system into an area with a high level of packaging constraints.
|TDI Power's LiquaCore power module "wraps" electronics in a cold plate through which cooling liquid is flowing.|
Effective thermal management at the chassis/system level requires detailed knowledge of the platform storage/operating environments, as well as the system power dissipation and available cooling options, Reichenfeld notes. Electronic assemblies are designed to maintain component junction and die temperatures within specified limits. The chassis/system design must consider cooling methods to dissipate the heat into the operational environment from electronics thermal convection/conduction paths, he adds.
Extreme cold- and hot-start requirements are the challenge with commercial off-the-shelf (COTS) components that are typically limited within -40 to 71 degrees C operation at the system level. "Careful consideration of the criticality of the system is necessary to determine the effects caused from a loss of heating/cooling on functions provided to the platform," Reichenfeld says. "In some cases, redundancy and loss of cooling analysis is necessary for safety and mission-critical elements to prevent loss of life."
A forced air chassis from Hybricon Products, part of Curtiss-Wright Controls Electronic Systems, houses conduction-cooled electronics in a current airborne application at 50,000 feet. The modified 1-ATR-tall chassis includes a 6U, 13-slot custom Hybrid VME64x-VXS-OpenVPX backplane with a 1400-watt, MIL-grade power supply and two MIL-grade fans. The application required a high-performance heat exchanger due to very high power dissipation.
|TDI Power's 30-kilowatt power system includes LiquaCore power modules and electrical and water connections.|
Unmanned systems, whether in the air or on the ground, are delivering much-needed information and soldier protection on the increasingly digital battlefield. Armed with sensitive electro-optics, such as sensors, unmanned vehicles often venture into and persistently survey harsh environments, gathering mission-critical data while helping to keep warfighters out of harm's way. Unmanned systems' thermal management needs are nearly as complex and intricate as their compact electronics, however.
Curtiss-Wright Controls Electronic Systems in Santa Clarita, Calif., is designing the next-generation chassis for Northrop Grumman's Advanced Mission Management System (AMMS) for the Broad Area Maritime Surveillance Unmanned Aircraft System (BAMS UAS) program. The BAMS UAS will provide the U.S. Navy with a persistent maritime intelligence, surveillance, and reconnaissance (ISR) system to protect the fleet and provide a capability to detect, track, classify, and identify maritime and littoral targets. The chassis has six integral cooling fans providing air flow for up to 800 watts power dissipation at 55 degrees C. AMMS operation is critical to the mission objectives and protection of U.S. Navy fleets during operations, Reichenfeld describes.
Dontech in Doylestown, Pa., has also recognized the need to maintain electronics at optimal operating temperatures. The company launched its Therma Klear transparent heaters to provide the warmth necessary to extend the operating temperature of liquid crystal displays (LCDs) in cold environments (from 0 degrees to below -40 degrees C) and for the anti-fog, anti-icing, and de-icing of optics and optical camera, sensor, and display assemblies.
A Therma Klear heater, composed of an electrically conductive thin-film coating on a transparent substrate, generates heat when current flows across the coating. Dontech heaters employ different types of crystalline materials (e.g., zinc sulfide or germanium), glass, acrylic, and polycarbonate substrates. Applications include avionics displays, vehicle displays, mobile computers, and handheld devices.
Thermal management of military and aerospace electronics continues to be one of the foremost challenges facing design teams, admits Ivan Straznicky, principal engineer and technical fellow at Curtiss-Wright Controls Embedded Computing (CWCEC) in Ottawa, Ontario. "The desire to use the latest generation, multi-core processors to meet the performance needs of ever more sophisticated applications are driving power dissipation levels and densities beyond what was once considered unachievable in mil-aero electronics.
"In order to make use of these high-performance processors in standard circuit card cooling configurations (e.g., conduction cooling), thermal engineers are painstakingly re-examining every element of thermal designs to eke out enough improvement for successful products," Straznicky adds. "Without such improvements, the use of these processors would be limited to lower speed grades and/or more benign environmental conditions (e.g., lower allowable card edge temperatures). Even with such improvements, the cooling ability of standard configurations, such as conduction and forced air over components, is not infinite and is quickly reaching limits of the governing physics (e.g., material properties, airflows)." Continued innovation at all levels of heat removal, in particular the chassis and system levels, is required to optimize their capabilities to efficiently remove heat to ambient environments, he says.
High-density processing solutions, such as those designed and developed by CWCEC, require highly efficient cooling solutions to meet challenging mil-aero environments. Most of the company's card-level products are designed to standard cooling configurations, such as forced air over components or conduction cooling; however, some of its cards can now dissipate approximately 4X the heat compared to a decade ago, with similar boundary conditions, Straznicky says. CWCEC engineers have also worked with customers and partners to develop and design products using such advanced cooling technologies as spray cooling, air flow through cooling, and liquid flow through (LFT) cooling. In fact, engineers at CWCEC and Parker Hannifin Corp. in Cleveland, Ohio, developed an LFT prototype capable of cooling over 650 watts of power using 55 degree C Polyalphaolefin (PAO) coolant.
Parker Hannifin has expanded its portfolio of liquid thermal solutions with the acquisition of SprayCool in Liberty Lake, Wash. The company's newest mil-aero applications center on liquid-cooled electronics enclosures and two-phase cold plate solutions.
"We continue to see more demand for both conduction-cooled enclosures that are side-wall cooled with single phase liquid, such as ethylene glycol and water (EGW) or PAO, coupled to a remote Heat Rejection Unit (HRU), and SprayCool enclosures where we take advantage of direct-spray and the evaporative cooling process on the electronics inside a sealed enclosure," explains Joe Baddeley, business development manager, Parker Aerospace, Thermal Management Systems Team.
"Sensor and image processing applications that utilize standard 6U board form factors are still leading the pack in terms of liquid cooled enclosures, but we are seeing a lot more proposals and trade studies for smaller 3U systems, especially with the release of VPX and the ability to push more power into these smaller modules," Baddeley notes.
Two-phase cold plate solutions are being employed in power electronics and radar applications. "In both applications, the trades and early development units are showing the potential for not only improved thermal performance and subsequent reliability gains over air or single phase liquid cooling at the electronics level (IGBTs, power amplifiers, transmitter modules), but also the ability of pumped two-phase systems to accomplish the task without the need for larger vapor compression systems (VCS) or chillers," Baddeley continues. "Thermal efficiency gains realized with two-phase cooling give the integrator the best chance at reducing or eliminating the need to chill down the coolant before it enters the electronics assembly."
Cooling and thermal management have become the biggest challenge for military electronics due to: significantly increased power levels, more severe environments, and condensed schedules with a desire for COTS, explains Mike Henderson, director, military and aerospace products at TDI Power in Hackettstown, N.J.
"Regardless of whether it's an aircraft, vehicle, or shipboard application, the power loads for new command, control, and communications (C3) systems, counter-weapon systems, and other applications being developed are driving the need for better thermal management," Henderson says. "More power means more heat, which has to be dissipated and managed efficiently. In addition, you have traditionally non-electric, belt driven systems being electrified to improve reliability and lower the overall weight of the particular platform. Very simply, without innovative thermal management techniques for power electronics, the military will not be able to field the new systems or hybrid platforms currently being developed."
Environmental conditions for electronics are getting worse; mil-aero systems are exposed to water egress, blowing talcum-powder-like sand, high levels of shock and vibration, and operating temperatures that range from Artic conditions as low as -46 degrees C to in excess of 149 degrees C near engines that are dissipating very high levels of heat. In the latter situation, "we actually have to insulate the power electronics to keep heat out," Henderson reveals. "In these conditions, simple convection cooling using blown air is just not an option, as the fans would have to be very large and noisy blowing contaminants that would quickly render them useless, thus disabling the very systems they were meant to cool."
TDI Power's DC-DC or DC-AC products, employed in military ground vehicles, are typically mounted below the fording plane, which means the power electronics must be completed sealed. Liquid cooling is the only feasible way to cool such high-power products that can be upwards of 30 kilowatts, Henderson says. The company's LiquaCore technology employs liquid cooling from the vehicle's coolant system to dissipate heat away from sensitive electronics in a modular, scalable architecture. A typical LiquaCore module "wraps" the electronics in a cold plate through which cooling liquid is flowing in multiple, parallel passageways. Power semiconductors, magnetics, and PC boards come in direct contact with the liquid-cooled metal housing for maximum cooling efficiency.
The ability to cool electronics with a water/glycol mix at a typical temperature of 80 degrees C enables TDI Power engineers to use materials already present on the vehicle and to avoid the addition of relatively unreliable fans or heavy, heat-dissipating plates, Henderson explains. "With LiquaCore, we can quickly configure a power solution using standard modules in a customized outer skin that meets the end users' available space claim. For example, the power conversion system can easily be mounted in the V-shaped undercarriage of vehicles like MRAPs (Mine Resistant Ambush Protected vehicles)." A power-conversion system configured with LiquaCore modules is also well suited for hybrid or all-electric vehicles in which tightly regulated DC power is needed for on-vehicle applications or AC power is required to meet off-vehicle needs.
Hot under the collar
Soldiers and soldier borne electronics, especially those deployed in desert environments, require innovative and exceedingly compact and efficient thermal management tactics. Engineers at RINI Technologies Inc. in Oviedo, Fla., have developed a miniature refrigeration product weighing less than 4 pounds.
The primary application for the refrigeration unit is personal cooling, yet it has been adapted to cool small lasers and electronic modules operating in hot military environments. "When a laser or electronics module is unable to operate in hot environments, our technology is increasingly one of the only solutions to provide the needed cooling without exceeding typically strict size and weight budgets," says Dr. Daniel P. Rini, president of RINI Technologies.
Frigid, final frontier
Space represents a very challenging environment for electronics, considering its extreme thermal conditions, radiation hazards, costly and remote maintenance, and so on. Technology firms continue to innovate in an effort to warm electronics in the absence of solar heat and to shield systems from the intense heat emitted from rocket motors.
Engineers at NASA's Jet Propulsion Laboratory (JPL) sought the most efficient and maintenance-free methods to dissipate heat from the Moon Mineralogy Mapper's (M3's) sensitive electronics, which enable the instrument to identify lunar minerals from orbit 100 kilometers above the moon's surface. Staff at the k Technology Division of Thermacore Inc. in Langhorne, Pa., helped JPL engineers not only develop thermal specifications for optimum rejection of high heat loads from the M3, but also design a system that would work without the need for maintenance or adjustments.
Thermacore also delivered six radiator panels, featuring the company's patented k-Core advanced, high-conduction composites to reject excess heat into space. Thermal straps, also fabricated by Thermacore's k Technology Division, were used as heat spreaders within the instrument. In addition, k Technology team members tested the completed components to ensure compliance with performance objectives, such as meeting the most rigorous specifications in the unique environment of outer space.
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 to cool 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 temperature control 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.
|RINI Technologies' compact, portable cooling unit connects to a soldier's vest to provide personal cooling in a variety of mil-aero environments.|
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. Liquid-cooled chassis provide the highest thermal density in the smallest package size; however, this needs to be balanced against the SWaP-C of pumps, reservoirs, and heat exchangers 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 cooling approaches."
Curtiss-Wright Controls Electronic Systems
Curtiss-Wright Controls Embedded Computing
Meggitt Defense Systems Inc.
RINI Technologies Inc.