Reasons for Final Choice

The design concepts have been explained, as have the criteria for the design. It is now shown which design concepts more closely meet the specific criteria. Each criteria is considered for all design concepts in Table 2.

Table 2: Design Concept Comparisons

Design Concept Number of Launches Structural Support Construction Reliability Maintenance
SMA 2 Little -Temperature Dependent Built on Earth Tethered in Space Fails at Lower Temperatures Intermittent Power to Stiffen Structure
Truss 11 Strong Massive Space Assembly Numerous Parts Increase Chance of Failure Virtually Maintenance Free
Centripetal 2 Rotational Forces Attachment of Solar Array in Space Loss of Balance can be Disastrous Rotation must be Maintained
Inflatable 1 Variable w/ PSI No Assembly Required in Space Subject to Punctures Seal Leaks Replenish Gases

Probably the most important criteria is the number of launches required to put the entire structure into a geostationary orbit. At $230 million per launch the cost of the total launches far exceeds the possible cost of the structure. An inflatable structure is the only one of the four design concepts to meet the "one launch" criteria. Shape memory alloy designs and centripetal designs fall just short of the criteria. They require a minimum of two launches. However, the truss design concept far exceeds the criteria for number of launches.

Structural support is typically not a huge concern in the weightless environment of space. However, such a large structure as the solar array does require more support to withstand rotational forces to keep the structure from folding onto itself. Designing the structure with shape memory alloys presented a possibly insurmountable problem. The stiffness of the structure is lost at lower space temperatures. The alloy offers substantial power to deploy the structure as heat is added to change the alloy from its deformable to is stiff, or parent state. After the alloy begins to cool, it returns to the deformable state and fails to support the structure. The truss design offered the most structural support of the four design concepts, but this strength is overshadowed by the eleven launches required to deliver the structure into space. The centripetal design concept uses rotational forces to support the structure. The amount of structural support varies with the speed of rotation. Therefore, rotation must be maintained. The solar array must be applied to both sides of the structure. Only minimal rotation can be accomplished on the other two axes due to the gyroscopic nature of rotation. Like the centripetal design, the structural support provided by the inflatable concept is variable. The support varies with the amount of PSI produced by the gas contained within the structure. The inflatable and centripetal concepts offer enough support for the structure.

The third criteria for the design is construction. Obviously, the design concept that can be built completely on Earth offers substantial benefits. A completely autonomous deployment requires a single launch. The inflatable design is the only concept that can be launched in one transport. The solar array will be attached to the structure when packed for launch. Unlike the centripetal concept there is no need to attach the array to the structure in space. The shape memory alloy concept requires the attachment of two structures. A robotic crew must be produced to construct a truss design. Astronauts can not work in geostationary orbit. Therefore, robots are required to assemble the trusses.

Finally, maintenance and reliability are also criteria for a structural design. These are the criteria in which the inflatable design concept appears not to meet. An inflatable structure is subject to punctures and may not maintain support. Punctures in space can be caused by micrometeorites traveling through space at high velocities. Some punctures may be prevented using Kevlar as a material for the inflatable. Coated with Neoprene, the Kevlar, a woven material, will become airtight. Future research may suggest the use of foams, self sealant, or stiffening reactions so the structure will not rely on the gas pressure to maintain support.

The problem of deflation from punctures can be reduced to be comparable with other design concepts' reliability. Shape memory designs offer no support at low temperatures. Maintenance of the structure will require power to be applied to the alloy at short intervals to stiffen the structure. This maintenance power will subtract from the power delivered to Earth and increase the problem of uneven heating. Truss structures are virtually maintenance free. However, the multitude of parts dramatically increase the chance of failure. A centripetal design may be the most unreliable. As the structure rotates it must remain balanced. Any unbalance will be amplified to the point of destroying the structure.