Bonding heat-exchange structures in a space-borne platform

Application

Launched aboard Space Shuttle Endeavour on Mission STS-134 in May 2011, the Alpha Magnetic Spectrometer 2 (AMS-2) is a second-generation particle physics detector. Mounted on the S3 Starboard Truss Segment of the International Space Station (ISS), AMS-2 integrates a sophisticated tracker able to precisely measure energy, velocity, and direction of origin of charged particles such as protons, electrons and antimatter particles such as positrons. Comprising silicon microstrips, front-end electronics, and mechanical structures, the AMS-2 Tracker makes 25,000 detections per second, generating 7 gigabytes of data per second – and 144 W of heat. To move this excess heat, engineers created a CO₂ cooling system able to survive the stress of launch and maintain operation in space. Within this cooling system, Master Bond EP21TDC-2LO plays a critical role in maintaining a stable bond between the critical components required to ensure reliable heat exchange.

Key Parameters and Requirements

The AMS-2 Tracker Thermal Control System (TTCS) is a sophisticated thermal management system built to manage the thermal load imposed by AMS-2 Tracker electronics, direct solar energy, and even indirect solar energy reflected by the Earth. Within the TTCS, CO₂ passes through cooling-loop condensers to dump the thermal load into a pair of radiators designed to radiate the heat into space. Although CO₂ offers advantages over other coolants, a power shutdown of the AMS-2 can cause the condensers to freeze when temperatures drop below the CO₂ freezing point (-55°C). As the condenser begins to heat up due to solar energy or power restoration, CO₂ thawing can induce pressures as high as 3000 bar (about 43,500 psi) within the condenser tubes. Although the TTCS cooling loops includes heaters to mitigate this problem, freezing simply cannot be avoided in the case of power outage. Without the use of suitable materials, a rupture in the TTCS cooling loop could occur. In turn, the increasing thermal load in the AMS-2 Tracker itself could impact detector performance, damage sensitive electronics, and create an unstable thermal event in a two-billion-dollar experiment attached to the ISS itself.

Results

AMS-2 scientists determined through experiment that Inconel 718 tubes with inner diameter of 1mm and outer diameter of 3mm could withstand the pressures encountered during CO₂ thermal cycling. By embedding Inconel tubes between two aluminum plates, scientists determined they could create a suitable condenser. The problem remained one of securing the Inconel tubes to the aluminum base plates. For this task, the team required an adhesive able to reliably bond the dissimilar materials, provide high thermal conductivity, and exhibit exceptional thermal and mechanical stability needed to withstand the harsh conditions of launch and ongoing operations in space. To meet these requirements, AMS-2 scientists selected Master Bond EP21TDC-2LO, citing not only its stability and performance characteristics but also its ability to match the differences in coefficient of thermal expansion (CTE) between Inconel and aluminum. To test the design, developers mounted the manufactured condenser on a cold plate that simulated a TTCS radiator and applied repeated cycles of CO₂ freezing and thawing. The structure bonded with Master Bond EP21TDC-2LO fully passed the tests and was accordingly built into the AMS-2 platform launched on Endeavour in 2011.

Conclusion

The cooling system integral to the success of the multi-billion-dollar AMS-2 experimental package required a bonding agent able to meet demanding requirements for adhesion, thermal performance, and long-term stability. Using Master Bond EP21TDC-2LO, scientists created a heat-exchange condenser design able to survive intense internal pressure associated with CO₂ thermal cycling while exhibiting required thermal conductivity and bond strength between dissimilar materials exposed to the extremes of space.