torsion springs that are wrapped around the hinge. To ensure the door will open, the springs are sized to provide sufficient torque margin to overcome expected resistive torques such as friction in the hinge and  the resistance from the flight harness. Throughout cruise, a heater on the radiation cover will keep the  Frangibolt assembly and hinge components well above their survival cold temperatures.   The implementation of the FD04 Frangibolt actuator on REXIS includes a custom housing, stiff joint  design, and appropriate thermal path. In Figure 1b, the housing is hidden from view to show theFrangibolt stack-up including the bolt, thermal isolation washers, actuator, and locknut. It is important to thermally isolate the FD04 Frangibolt actuator from the surrounding structure in order for the cylinder to reach actuation temperature upon application of power. Custom low-conductivity washers are placed between the FD04 Frangibolt and its surroundings to achieve the required thermal isolation. For the FD04Mini Frangibolt actuator to operate properly, careful attention must be paid to the fastener and the joint  design. A stiff joint design is necessary to ensure that there are no gaps, the joint fits inside the grip  length of the fastener, and the fastener is properly torque loaded in order to achieve the required preload.The housing, shown in Figure 1a, is necessary to capture the parts that become free after the bolt fractures. The main Frangibolt housing mounts to the lower tab and contains the Frangibolt actuator, two thermal isolation washers, the lower portion of the bolt, and a locknut that holds everything in place. The  top portion, the bolt head cap, fastens to the top tab and retains the head of the fractured bolt as well as one thermal isolation washer.  Test Effort Prototypes of the REXIS radiation cover were developed,  designed, and tested as part of the RE XIS instrument engineering test unit (ETU) effort during the summer and fall of 2013. The main objectives of the REXIS prototype radiation cover testing were to validate the design of the REXIS bolted joint, establish confidence in the reliability of the Frangibolt actuator to operate successfully at predicted temperatures, determine an expected range for  actuation times at a given temperature, and gain  experience with installation and use of the FD04 actuator.Two prototypes were tested: a smaller ETU-1 model, and  a larger ETU-2 model (Figure 2). The ETU-1 design  represents an earlier concept that was later updated to a larger design (ETU-2) to accommodate moving the cover from the CCD assembly to the coded-aperture mask. Actuation and deployment of the cover was tested in thermal vacuum (T-Vac) chambers at temperatures  as low as -48°C (shown in Figure 3). Each test specimen was mounted to an interface that was  conductively cooled with liquid nitrogen. The mechanism was instrumented with resistance temperature  detectors (RTDs) to track the temperature history of critical features on the mechanism during testing.  Tests were performed with both Ti and G10 fiberglass washers to determine the impact of the washers on the thermal path. Electrical lines were fed through the chamber outside to a workstation, power supply, and multimeter as required. Tests were also performed at both 9 and 10 VDC to assess the impact of  input power on actuation time. Power was applied and manually controlled from an external source during  testing. In each test, power was applied to the Frangibolt and the actuation times and temperature  histories were recorded.  Figure 2. ETU-1 and ETU-2 prototypes 147