As CubeSats take on increased functionality, including larger solar arrays for increased power demands and large antennas for science and communications needs, the requirements for launch tie-down and release mechanisms are evolving. In the past, some large CubeSat-deployable structures (solar arrays) relied on the confining walls of the CubeSat canister to act as the restraint mechanism. However, this practice is largely eliminated now, with most CubeSat specifications requiring a minimum amount of dwell time (after the CubeSat has been ejected from its parent canister) before the deployable structure can be released and deployed on orbit. Thus, a reliable restraint and release mechanism that does not depend on the geometry of the canister walls must be implemented.
The proposed restraint/release mechanism has three main components: 1) tie-down cable, 2) tension mechanism, and 3) burn wire actuation mechanism. The tension mechanism allows the Vectran tie-down cable to be easily tensioned without having to tie off a knot under tension (which can be difficult, especially in tight spaces). The integrated spring in the mechanism ensures that the assembly stays stowed and under tension, even if the Vectran tie-off cable changes length slightly due to thermal loads or launch vibrations. Lastly, the tie-off cable is released using two redundant burn wire mechanisms. These mechanisms are based on an earlier design conceived at the Naval Research Laboratory (NRL) using a moving Nichrome hot-wire that thermally cuts through the Vectran tie-off cable.
The tie-down cable is a special space-rated Vectran material. The Vectran cable is a braided construction with eight carriers, each having a denier weight of 400. The cable has a breaking strength of 140 lbf and an approximate outer diameter of 0.036". The tension mechanism uses a 5/16-24 thread that has a linear travel of about 0.24". Thus, the tie-off cable can have a little bit of slack when it is tied off in the stowed position. The tension mechanism then uses some of the 0.24" of travel in order to fully tighten the tie-off line. The compression spring inside the tension mechanism has a force of 10 lbf in the fully stowed position. Under operation, when tightened, the nut of the tension mechanism compresses the spring to the fully closed position. The nut is fully tightened, but then backed off slightly to ensure the spring is not jammed. Thus, under launch vibration, if the acceleration force exceeds the 10 lbf force of the spring, the tie-off cable does not travel forward because it hits the hardstop of the nut. However, if the tie-off cable length increases slightly due to thermal displacements, the tension on the tie-off cable remains (at 10 lbf).
The Vectran tie-down cable is thermally cut using two redundant burn wire mechanisms. These burn wires are based on the original design developed by the NRL. However, in the updated design, the mechanism underwent several significant modifications to optimize and enhance its capabilities. In order to leverage the heritage of the NRL design, the burn wire mechanism uses the same 30 awg (.010" diameter) Nichrome wire. Using the same Nichrome wire geometry allowed a decrease in the number of verification tests.
In operation, a 1.6-amp current is applied to the Nichrome wire for approximately 5-10 seconds as the Nichrome wire heats up and thermally cuts the Vectran tie-off cable. The burn wire mechanism was demonstrated to work in ambient lab conditions, as well as at vacuum conditions at -60 ºC and +125 ºC. Most of the other elements of the burn wire design were modified and enhanced for this application. The electrical leads are constrained by a highly insulated ULTEM tip.
The Nichrome wire is preloaded using a single spring and 1/8" diameter shaft. The single shaft simplifies the design and allows for the ULTEM tip to freely rotate in order to minimize built-up mechanical stresses at its interface with the Vectran cable. It also allows for multiple configurations of the burn wire mechanism, depending on the orientation of the Vectran tie-down cable. For example, the Nichrome wire can be mounted in the horizontal or vertical configuration without changing the design. The Nichrome wire is preloaded using a stock compression spring with a spring constant of 2.5 lb/in, and a maximum suggested load capacity off 0.74 lbf.