Heliostat with Adjustable Flatness Control

Last crawled date: 1 year, 11 months ago

This mobile heliostat uses a spring-tempered stainless-steel sheet for its mirror.

In the stored state, the mirror is rolled into a large diameter coil. This puts potential energy into the material. Cables connected to brake-motors hold this position. The generous large curvature of the coil allows the mirror and its structural “tape springs” to stay coiled while remaining well above the material yield points. Said another way, the "spring" free state of the mirror (which is extended and slightly concave) is maintained, and the amount of stress relaxation occurring during storage time is managed.

The deployment of the mirror is a slow and controlled “un-furling” towards the mirror’s free state. Cables connected to rotary dampers, are let out by brake motors to provide this controlled motion. The free, or natural, state of the mirror is slightly concave, which ensures the cables are always in tension during and after deployment.

When deployment is complete, the cable system provides control and fine-tuning of the mirror's "shape". The mirror can be made perfectly flat, or, it can be made slightly concave or convex which would allow focusing or dispersion of the reflected light if desired. After shape adjustments are made, spring-loaded brakes "lock in" the position of the cables and the brake-motors are powered off. This is for the sake of energy efficiency, ensuring that energy is no longer consumed after tuning is complete.

This system provides many benefits over hinged panel and inflatable designs because it is simple, robust, and allows active control over the flatness of the entire mirror. Fabrication tolerances, stress relaxation, creep, thermal expansion, damage, etc., in hinged joint could render a heliostat system inoperable, because all panels would not be perfectly flat to each other. Similarly, inflatable designs risk losing gas pressure due to permeation through bulk material, imperfect seals, degradation caused by solar radiation, or damage from micrometeorites. Pressure loss in an inflatable structure would cause the system to “sag” and lose its shape which would render the entire system non-functional – patching and recharging with new compressed air is difficult in space. The proposed design manages environmental risks well because it is robust and adaptable, it can physically stand up to the large variety of hazards and abuse which it is likely to encounter in space.

The structural support for the mirror is provided by side bends and “tape springs” along the edges and back of mirror. “Tape springs” are thin strips of steel with curved cross sections, they can be coiled up, and they also provide stiffness when extended (they are a common design used in tape measure blades, however note that the “tape springs” in this design are much stronger than a tape measure, also there is no recoil spring.) Additional structure is provided by square tubes mounted across the top and bottom of the mirror. These tubes provide the (horizontal) stiffness and flatness to the mirror.

The presented design fits inside the allowed envelope when retracted and provides over 10 m^2 of flat reflecting surface when deployed. However, without these constraints the concept is easily scalable to larger or smaller sizes, or to different aspect ratios (wider instead of taller for example). For increased mirror sizes, additional horizontal support can be added in the form of battens spot welded or riveted across the back of the mirror at incrementing heights. Additional cables systems could be added to these battens if even more shape control is desired. Similarly, vertical stiffness can be increased by adding additional or increasing thickness of the “tape springs”.

Pitch and yaw tracking motion is produced by a two-axis slew drive, which is connected to control system including a sun sensor. Slew motors are robust and simple devices already proven to work well for heliostats here on earth. A separate benefit to using these is that they allow the active motors and controls to be located near each other. This allows for simplified thermal management as the entire area can be covered by insulative blankets. Waste heat from the motors can be trapped and used to help maintain temperatures as opposed to being lost via radiation into space.

The overall system has low mass and a low rotational inertia with respect to the rover, due to the mirror in large part “supporting itself”. This makes for reduced rocket fuel required for transport, and allows for movement across the lunar surface while deployed.

The heliostat includes 1.12 m^2 of solar panels below the mirror, and battery energy storage. This provides the power to the motors, controls, and heat management systems.

Supplemental benefits of this design:

- The energy of the springs, which is released as the mirror unfurls, can optionally be used to deploy leg support mechanisms. This would be useful for stand-alone heliostats not mounted to rovers.

- In addition to being flat, the mirror can be adjusted to be slightly convex or slightly concave. As mentioned previously, this grants the ability to increase or decrease the reflected light intensity if needed for different applications.

See the attached PDF files for more engineering details of this concept. Also, I welcome and would be very happy to answer any questions.