Solar sail technology depends heavily on the total surface area of the sail. In other words, minimizing mass and volume of its support structure is the main objective, particularly when it comes to launch configuration, i.e. mass, volume constraints, etc. There is a need to develop a low-cost concept of a deployable support structure that can stow in the EELV Secondary Payload Adapter (ESPA) volume, and carries as much sail material as possible. This structure must then be able to deploy the sail material out, and provide the surface area needed.
A low-cost, deployable hexagonal truss was developed. This truss will serve dual functions: first, to stow compactly within the ESPA volume of 24 × 28 × 38", and second, to deploy to a 68" diameter using an actuator-driven mechanism. The actuators disengage from driving the truss and serve as a pitch mechanism of the sail material for blade attitude control and thrust vectoring. A separate actuator is implemented inside the sail material, generally compacted on a reel, and is used to control the roll-out rate of the sail blade for stability. When fully unrolled, each of the six sail blades rolls out to 0.75 × 220 m, providing roughly 1,000 square meters with all six blades.
The hexagonal truss is a simple assembly of six lightweight graphite tubes that are hinged together at the ends. Actuators are mounted in the middle of each tube, and are used to drive the deployment. The actuators drive two short members that are connected to a set of linkage in a four-bar linkage configuration. The rotation motion is transferred at the ratio 1:1 from the actuators to the tube hinges; a full deployment can be performed by a 90-degree rotation at the actuator output shaft. A synchronous deployment is achieved by simultaneously actuating all six actuators at the same rate.
At the nodes of this hexagonal truss, cable spokes are used for truss stiffening, and are attached back to the spacecraft structure. Near the end of truss deployment, the spokes will be preloaded with cable tension and provide truss stiffness similar to bicycle wheel spokes. When the truss is fully deployed and the spokes are tensioned, a mechanism inside the actuator assembly locks the truss in the deployed configuration and disengages the actuator from driving the truss. The actuators can now be used to precisely control the pitch angle of the blade reels.
The spacecraft will apply the initial spin to initiate the reel unrolling. However, as the sail material is rolling out, the pitch angle is changed to utilize solar pressure, and the centrifugal acceleration will cause the system to continue spinning. System stability can be obtained by controlling the sail blade unrolling rate using an actuator that can drive the reel from inside.
Currently there is not a design of such a truss that can fit in the ESPA volume. The concept takes advantage of utilizing the same actuators for truss deployment and blade reel pitching. There are also advantages in low mass, low power input requirements, and small packaging volume. Rideshare opportunities exist to make affordable flight demonstrations possible.