A third deployment technique for the large solar array is the retractable truss structure method. A retractable truss method beam should stow efficiently with alternating clockwise and counter-clockwise folding of the longer beams toward the packaged stack. The diagonals must be hinged in the center to be folded in midspan to allow packaging. The general packaging of the truss beam will approximately reduce to 2.75% of its deployed value.
There is no net rotation during the deployment or retraction of the beam because of the alternating direction of the diagonals. All hinges must have a single degree of freedom and will be designed for low compliance and minimum reduction in beam stiffness. The single degree-of-freedom hinges also will provide control over the kinematics of the structure during the transition from its retracted form to its deployed form.9
The deployable truss beam is an idea which has been considered by many. Trusses are some of the most widely used forms strengthening large structures. However, a truss of this magnitude yields many challenges. A truss structure would provide ample strength and stiffness to the solar power satellite. Due to the non-compactability of the truss materials, this design requires about twelve times as many launches as some of the other designs methods. The additional number of launches greatly increases the price of the structure. Each additional launches also requires further in-space construction of the structure. The construction must be performed by remote control or by robots which would be time and money consuming.
The deployment/retraction mechanism for the truss would use three synchronized lead screws which engage threaded open-nut fittings at the longeron cluster assemblies. The deployer itself is stowed and deployed using the same lead screw mechanism to minimize the height of the package. Bays can be deployed one at a time with the assistance of a torque motor for stack rotation or two bays at a time with no necessity for stack rotation. This allows for the deployment and retraction to be continuous. A typical collapsible truss structure is shown in Figure 6.
Figure 6: Deployable Truss
The truss structure would provide an extremely rigid and strong support. The truss would also be strong enough to withstand the extreme temperature differences and micrometeorite collisions associated with the space environment. This resistance would result in a virtually maintenance free structure.10