# An Overview of Cube-Satellite Propulsion Technologies and Trends
## 10 Solar Sails
### Design Considerations and Technologies  
A solar sail mainly comprises of four components: central bay, offset boom, masts and sail [130]. The central bay houses the control boom and the sail, while the masts (stowed) are arranged symmetrically around the central bay. Masts form the supportive structure for the sail that is the thin film exchanging momentum with incident photons. The deployment operations of a solar sail would usually begin after unfolding the solar arrays. First, the offset boom is deployed, followed by the deployment of the masts and the sail. The offset boom is rotated by means of a motor (after the sail is deployed) to provide attitude control [130].  
Solar sail performance (acceleration) is proportional to the ratio of its area and mass. As sails get larger, they also become heavier; however, in order to lower launch mass, they are designed to be as thin and light as possible. As a result, the rigid-body assumption in the sail dynamics becomes less valid, and there arises a need to incorporate flexibility effects [131]. A major challenge in determining the flexibility of a sail in a land based laboratory setting is its inability to be deployed and tested in vacuum and zero gravity conditions. Multiple methodologies have been undertaken to model the flexibility characteristics of solar sails [131]. One such research involves parametric studies of square sails (150 m length) using Finite Element Analysis (FEA). In this study, the effects of sail size, stiffness, and sail membrane parameters on static sail deflection and natural frequencies are examined [131].  
A sail-craft (spacecraft bearing solar sail) usually requires a desired attitude to obtain the maximum momentum transfer from solar radiation [132]. To achieve this, torque from on-board attitude control mechanism (such as reaction wheels) is required, enforcing limitations on the design (mass, power, volume) of the sail-craft. However, studies have shown that the spacecraft can take advantage of environmental torques due to solar radiation, gravity gradient or atmospheric drag in order to reduce the involvement of the attitude control mechanism [132].  
As solar sails have a large surface area, it becomes paramount to control their thrust vector. There are two methods proposed that can stabilize solar sails: first, spin stabilization; second, three-axis stabilization. Both these methods rely on the inherent rigidity of a support structure (mast). During spin stabilization, sails are stiffened by spinning about a central hub resulting in high propulsive efficiencies without strong compressive mast loads. Circular-disk-shaped sail and Heliogyro concepts were introduced based on spin stabilization [133]. In three-axis stabilization, sails are supported using long booms similar to a kite. It is possible to couple the booms with masts, stays, and cables to reduce the load on the booms and the weight of the structure at the expense of a greater design complexity [133].  
Due to significant losses incurred with the absorption of incident sun light on reflective solar sails, there is a need to develop alternate materials that can raise the absorption rate. Diffractive materials can be used in place of reflective sails, as they provide advantages with non-mechanical navigation (without having to vary the sail-craft attitude), photon recycling, and higher efficiencies [134]. When a 2-body quasi-synchronous simulation of a spacecraft traveling from Earth to Mars using diffractive and reflective sails was conducted, the former was found to have reached Mars 51 days earlier than the latter [134]. This new technology could potentially allow CubeSats being propelled by miniaturized solar sails in interplanetary missions.