Forced convection – Airborne application
Airborne applications may use a fan-based thermal scheme, or forced convection, although this depends significantly on the altitude and implementation of the system. For example, unmanned systems typically fly with unpressurized equipment bays at high altitudes and so forced convection is often inadequate for cooling modern high powered processors. Higher altitude, unpressurized systems perform with reduced air density and the resulting lack of sufficient airflow prohibits the effective use of fans; however some aircraft systems operate within more benign pressurized environments that are very effectively managed with forced convection. Using this cooling method, heat can be dissipated by moving air in direct contact with system boards and components. This assumes that space, cost and noise are not critical design issues and that SWaP parameters provide allowances for high altitude fans.
One such manned aircraft implementation used the Kontron FS-1290 forced air convection system, a 9U ruggedized VME/cPCI enclosure designed for 6U VME/cPCI boards with an optional peripheral carrier. The Kontron FS-1290 was integrated into the P-3C Orion Maritime Reconnaissance Aircraft, an anti-submarine and maritime surveillance aircraft with proven longevity and performance. Introduced in the 1960s, the P-3C is one of a handful of manned aircraft that has delivered decades of continuous military service based on ongoing upgrades to avionics systems and mission equipment. In a series of updates, the U.S. Navy implemented a number of major improvements to the P-3C including aircraft communication, navigation, acoustic, non-acoustic and ordnance/weapon systems – each intended to keep pace with new and evolving multi-mission requirements. The FS-1290 was implemented onboard the P-3C as STORES management, upgrading the critical system that controls automatic ordnance release.
The FS-1290 is a forced air 9U ruggedized VME/cPCI enclosure designed for 6U VME/cPCI boards with an optional peripheral carrier - available in 10 or 12 slots.
In determining the ideal thermal management approach, altitude for this manned aircraft was established to be within requirements for acceptable airflow over system components. The maximum ambient temperature inside the pressurized cabin storing the ordnance control system is 55°C; this represents the craft flying at a relatively low altitude on a hot day, in general, the normal ambient air temperature should be less than 45°C. Power dissipation for the ordnance release system needed to be in the range of 100 Watts per board.
The Kontron FS-1290 provides flexibility in a standardized system, as the peripheral carrier is removable and mounts in the card cage; the number of slots ranges from 10 up to 18 and allows designers to carefully evaluate bandwidth and performance against very specific SWaP requirements. FS-1290 meets MIL-S-901 in an isolated rack with front loading access, as well as MIL-STD-810 and MIL-STD-167 demands for shock and vibration specifications for severe environments such as those common onboard the Orion. Three internal fans cool the card cage, drawing air from front to rear through an inlet positioned in the front door of the chassis. Air is routed up through the VME/cPCI card cage, around the power supplies and ultimately exhausted through the upper rear of the enclosure. Fans are mounted on a removable tray for simplified maintenance, an advantage for systems integrated into the complex physical layout of an aircraft.
The average slot airflow achieved is 500 LFM (linear feet per minute) ensuring proper cooling by limiting the system to a 9.7 degree rise in temperature. Forced convection implemented into Orion’s ordnance control system presents an ideal choice over a liquid system due to lower weight, lower MTBF, easier maintenance and avoiding risk of leakage into sensitive, mission-critical electronics installed in the aircraft.