Subsystem Overview

Any solar array used for an orbiting power collector must be fabricated, packed, launched, placed in orbit, and be deployed. A very large solar array will be difficult to deploy due to relatively small payload volumes and high cost per launch. The deployment mechanism needs to be lightweight, reliable, and able to support a 1km2 solar array. The solar array must be expanded from a small payload volume to create a large surface area. The deployment mechanism is responsible for this expansion.

Some assumptions are made for the solar cell array based on current thin film technology. These assumptions are worse case scenarios for expected future solar cell properties. Table 1 summarizes the assumed properties for the solar cell array.

Thickness 0.2 mm
Flexibility 0.4 mm radius of curvature
Density 2.33 g/cm3
Efficiency 17%
Durability subject to fractures

The volume in which the solar array can be packaged is dependent on these properties. A one km2 array that is 0.2 mm thick has a volume of 200 m3. The packed volume of the array will obviously be larger than the volume of the array due to its radius of curvature. The radius of curvature is defined as the smallest possible radius for a 1800 fold, as shown in Figure 2

Figure 2: Radius of Curvature

The volume of the payload is estimated at 300 m3.For a singledeployment, 100 m3 is allotted for the deployment mechanism and packing losses. If packing losses can be minimized to 10% of the payload, the structure can have a maximum volume of 70 m3. The subsystem diagram is shown in Figure 3.

Figure 2: Subsystem Diagram

The deployment mechanism may also be expected to support the solar array for the expected life of the system. The minimum expected life for this system is defined as fifty years. A 70 m3 volume does not allow for current satellite support structures to be used for a 1 km2 array. The challenge of the project is to deploy the large array using a highly compactable structure in one launch. If the deployment mechanism is to support the array it must be strong enough to withstand forces and moments resulting from rotation about two axes amongst the Sun and Earth.

The array must face the Sun continuously. Therefore, it must be rotated about two axes. The primary axes of rotation are parallel to the Earth's axis, (Figure 4). The solar array must rotate about the primary axis one revolution per 24 hrs as does the sun. On the secondary axis the array must rotate once every year as the Earth revolves around the Sun. The simultaneous revolution about both axes as it revolves about the Earth will ensure one side of the array will face the Sun at all times. The array will be in the Earth's shadow approximately one hour per day, as was seen in Figure 1.

Figure 2: Subsystem Diagram