arrangement. The stacked tolerances on the multi-interface of DMC-CFESAT means that after craft destack and re-stack operation, the hinges and HDRM need realigning carefully, whereas DMC-UK2 and EXACTVIEW-1 can remain as a separate single subassembly, saving time and reducing variability.  HDRM Evolution and Actuator Change Hold Down Actuator Change Following on from the successful in-orbit deployment of EXACTVIEW-1 solar panel, there were  substantial lessons learned regarding the heritage hold down actuator choice. Firstly, the load capacity proved more restricting than previously thought to the extent where a launcher change caused the hold  down to have negative margins on gapping and slippage. Secondly, the attraction of being able to perform in-house refurbishment via frangible bolt change after hard deployments was offset by problems experienced during the setup whereby the exact preload achieved was questionable. This latter issue could be resolved for future use by better methodology and with additional instrumentation – for example  by monitoring the deflection of the hold down cup body. However, the decision was taken that a much higher load capacity was needed in order to avoid future  problems similar to EXACTVIEW-1, such that in terms of HDRM loading, the launcher selection could largely be ignored and the need for detailed analysis would almost disappear. Therefore, upon signing the  contract for the SST-US mission Orbital Test Bed, the decision was taken to change the hold down actuator. SSTL could have al ternatively changed other design feat ures of the HDRM, such as the cupcone angle, however, the actuator change and experience with a viable alternative proved favorable. Another actuator market study was conducted, but revealed that the availability was very constrained  based on the recurring cost budget permitted. Due to the criticality of the application, only established  actuators were considered and the possibility of new developments in-house or external was ignored.This market study identified two possible solutions:  either select a larger size in the frangible bolt and SMA family from the existing selection (1/4”) or change to a non-explosive separation nut. The larger  frangible-bolt was rejected since the corresponding SMA body would impart a significant increase in  volume of the HDRM unit. Thus the non-explosive separation nut (see www.neaelectronics.com) was chosen. This was also based on good experience gained with the separation nut on the SSTL NigeriaSat2 mission which used these to support a deployable optical payload bench.  The separation nut actuator works on the basis that a spigot shaft is constrained by a multi-piece nut,  which is held radially by a wrapped spring and fuse wire. When the fuse wire burns, the released spring allows the nut to separate and release the spigot. The separation nut housing is fastened to the cup  portion of the HDRM in place of the previous SMA and the mating cone is fastened by screwing a stud  into the separation nut’s spigot. Therefore, the spigot shaft is merely released in contrast to the fracturing  nature of the frangible bolt in the previous system.  For a similar size separation nut to the previous SMA, the load capacity increased threefold, with a rated  release load of >33 kN. This allows a Sep-Nut-HDRM to be assembled efficiently without any  instrumentation by torqueing alone. A nominal preload of 22 kN is sufficient to ensure that the achieved  preload always exceeds the 12-kN minimum requirement and is well within the actuator’s load capacity, even factoring in ±30% torque scatter. The significant load capacity allows the Sep-Nut-HDRM to remain  a generic solution for many more applications supporting larger panels/payloads over higher launch loads. The requirements for the HDRM development are summarized in Table 1.  Due to the hinged deployment nature of the solar arrays, once released, the spigot tip’s movement  describes an arc and thus the spigot tip would catch and snag on the bore of the cup. This was not a problem on the existing SSTL separation nut application on the deployable optical payload bench, since the bench was released parallel to the static panel, as opposed to being hinged from one edge. Any  snagging of the spigot tip on the cup could result in partial deployment and corresponding mission degradation via loss of power. 430