Double Bubble Lunar Heliostat

Last crawled date: 2 years ago

This design is an inflatable heliostat with a very simple, robust deployment mechanism.

It is inspired by the fact that two soap bubbles stuck together with equal pressure have a perfectly flat wall between them.

The rover features a photovoltaic-coated enclosure on top that opens to reveal an uninflated clear plastic membrane in the shape of two soap bubbles stuck together, with the wall between them made of reflective mylar, or a similar flexible, highly reflective material.

When the two "bubbles" inflate to the same pressure, the reflective surface between them is naturally stretched to be a highly flat 10 sq. m surface capable of reflecting light long distances into a crater.

Inside one of the bubbles is an inner "carriage" with ball bearings and magnets that align with a similar lower carriage affixed to the top of the rover. Magnets hold the inner and lower carriage bearings together, and therefore holds one of the bubbles firmly to the rover. On the lower carriage, one of the bearings is a drive wheel that can rotate, allowing the double bubble to be arbitrarily rotated 360 degrees about the vertical axis, and 180 degrees about a horizontal axis.

A sun sensor affixed to the center of the reflective mirror (with a power/data cable integrated into the mirror, leading down to the inner carriage) gives the rover feedback about sun position, allowing for accurate sun tracking. The back of the sun sensor features a high visibility pattern that can be tracked by a wide-angle camera on the inner carriage, which allows tracking of bubble orientation. Together with target direction information from remote operators, the rover has enough information to accurately orient the bubbles.

A unique extra capability of this design is that by merely altering the relative pressure in the two bubbles, the reflective surface can easily be made slightly concave or convex, allowing the rover to concentrate or disperse the light over an arbitrary area in the crater if desired.

Measurements, notes, and other considerations:
- Each of the two partial bubbles will have a diameter of 4.3 meters.
- The mirror will have a diameter of 3.7 meters.
- We estimate for a bubble material thickness of 0.4 mm, the bubble/mirror will weigh approximately 50 kg. The rover will need to weigh more than 80 kg to prevent the bubbles from unbalancing the rover.
- In order to inflate the bubbles to 0.5 PSI twice over (to allow for extra gas to replace leakage), we need a 6" diameter x 24" long gas cylinder, which easily fits on the rover.
- The inner carriage may require power to transmit sun sensor data to the main rover, and to heat the bearings - this can easily be provided by an integrated inductive charging mechanism.
Dust settling on the front bubble may reduce the light transmitted into the crater. It is possible that a simple vibration motor could intermittently vibrate the entire double bubble membrane, shaking dust off the surface. Thin embedded wires within the plastic membranes might also be used to dissipate electric charge and reduce dust accumulation.

Credit: The base rover for this design is a modified version of this rover: https://grabcad.com/library/mars-rover-prototype-1

Thanks to Mark Avara, Lissa Riley, and Aaron Bloomfield for helping me think through the design.