opening. It is important to note that a flight harness will need to pass over the hinge of the door in order to  power the radiation cover heater. Testing with an appropriate mockup of this harness in place is planned.  The testing also revealed a few lessons learned about working with the FD04 Frangibolt. These lessons  include the importance of the thermal isolation of the FD04, the need for additional telemetry to signal actuation and cut power to the FD04, and the need for careful housing design to avoid cosmetic damage to the actuator jacket. Actuation Time and Thermal Path As expected, actuation time increases at lower test temperatures. This trend is likely a combination of two  phenomena. First, it naturally takes the actuator heater longer to heat the shape-memory alloy to the necessary temperature when starting at colder temperatures. Secondly, at colder temperatures and once the Frangibolt starts heating, there is a greater temperature difference between the Frangibolt and its surroundings. As the heat transfer rate due to conductance is based on the temperature gradient between two surfaces, heat is lost from the Frangibolt to its surroundings at a higher rate when at colder temperatures. This increased heat transfer rate and demand for heat directly correlates to an extended amount of time power must be applied to achieve the necessary stroke on the actuator to fracture the bolt. Therefore, the specific structural/thermal path within the Frangibolt mechanism directly impacts this temperature vs. actuation time relationship. Testing has shown that the use of G10 washers and the application of 10V both significantly reduce the actuation time at low temperature. The G10 washers better thermally isolate the actuator, allowing for quicker actuation when compared to the Ti washers andthe increase in input power allows the heater to come to temperature more quickly. The use of G10 washers raises several questions and potential issues, however. The heater installed on  the radiation cover is meant to ensure the Frangibolt mechanism stays above survival temperatures during its cruise to Bennu. A trade exists between power allocation and actuation time. On the one hand, good thermally isolating washers such as G10 will enable a quick and efficient actuation. On the other, they make it more difficult for the heater to keep the Frangibolt warm during cruise and therefore demand a larger power allocation from the spacecraft. There was an additional concern that G10 washers may reduce the preload of the Frangibolt at actuation. As G10 is a much softer material than Ti, it is unknown whether compression under long periods of time will cause the washer material to creep and cause the bolt to lose its desired installation preload. However, due to the large stroke on FD04, there is a significant margin of safety against bolt preload, such that the Frangibolt should be able to actuate even with zero installation preload. A test was performed successfully with G10 washers and significantly reduced preload (test annotated in Figure 4) to verify the insensitivity of the FD04 to preload.   Application of Power Post-Actuation Once the bolt breaks, there is no longer a thermal path or heat sink to absorbs heat from the actuator. Therefore, if power is applied to the FD04 actuator post actuation, the shape-memory alloy can overheat and outgas. With strict outgassing and contamination control requirements in place, REXIS must ensure  that its operations do not contaminate the regolith sample collected by OSIRIS-REx. Unlike other  Frangibolt models, the FD04 does not have an internal temperature sensor. For this reason, it is not possible for REXIS to prevent overheating by monitoring the Frangibolt temperature and control power  based on this temperature. REXIS must incorporate additional hardware and software modifications as a means to determine successful actuation. One solution is to incorporate a limit switch that is compressed  when the door is closed and released when the cover opens. This switch is constantly monitored by the  instrument’s main electronics board and allows for immediate power cutoff upon Frangibolt actuation.  There is some coupling between the actuation time and the concern of power post-actuation. While it is  not necessary for REXIS to experience short actuation times, it has been found that longer actuation  times result in a greater uncertainty of actuation time. There is more spread in the data at colder  temperatures because small differences between tests amplify over time. The faster the Frangibolt can be actuated, the greater certainty there is that it will do so closer to an expected time. Since failure to cut power to the actuator within several seconds after actuation can potentially cause outgassing issues, it is crucial that the actuation time for the Frangibolt be characterized and fully understood. 150