LUNAR MICRO ROVER SOLAR SHROUD. Daniel Hong Department of Mechanical Engineering University of Hawai`i at Mānoa Honolulu, HI ABSTRACT

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1 LUNAR MICRO ROVER SOLAR SHROUD Daniel Hong Department of Mechanical Engineering University of Hawai`i at Mānoa Honolulu, HI ABSTRACT The objectives of my internship at NASA Ames Research Center were to determine feasibility in a solar shroud and creating a physical mock-up of the design. Many ideas were created and modeled in SolidWorks. Matlab was used to model the behavior of heat transfer with and without the shade. After weighing the pros and cons, the three lift designed was chosen as the first prototype. The solar shroud was proven a feasible option to shade the rover and collect solar energy from the sun to extend mission life. INTRODUCTION The Robotics Academy at NASA Ames consists of For Inspiration and Recognition of Science and Technology (FIRST) students and alumni working on a micro rover that is designed for the moon. This program was started by Mark Leon who works for Dave Lavery, a leader in FIRST organization. It is the fruit of their labor; yet another way to inspire young minds to pursue careers in Science, Technology, Engineering, and Math (STEM) fields, as well as to evaluate past members on applications of skills learned during the course of their involvement with the program. I worked with Valeriu Tocitu, a graduate of Georgia Institute of Technology s Aerospace Engineering program. The leader of the thermal and structural was Jacques Dolan, a three year member of the academy. The solar shroud is a type of covering that shields the rover from 1366 kw/m 2 of solar radiation. Since the moon has no atmosphere, the intensity of the radiation from the sun does not decrease when it reaches the surface of the moon. This means that the temperature range on the moon s surface (173K to 400K) is more extreme than the Earth s (243K to 333K). Operational temperature for most of the electronics ranges from 263K to 323K, and so the moon s environment would easily destroy the electrical components and cause mission failure. The solar shroud would also act as an energy gatherer from the use of photovoltaic cells (PV cells). PV cells that we worked with are a second generation type. This means that the PV cells are thin pieces of a metal alloy that has electrical properties thus generating a current by moving electrons from the metallic bonds of the silicon family atoms when infrared light hits the surface of the alloy. The minimum requirement for the micro rover is to survive two hours in the lunar environment, long enough to finish its tasks. Since half a million dollars is a hefty price for a two hour mission, the objective is to be able to fully recharge within less than a day, allowing the rover to do its routine for the rest of the time. This will extend the life of the rover, thus making it a more worthwhile mission.

2 METHODS The basic requirements for the solar shroud included size and weight limitations, simplicity, cost, and largest packing area for PV cells. The designs were modeled in SolidWorks which showed how effective each idea was. These designs also helped us to visualize how the shroud would fit into the construction of the rover. Many suggested ideas on the rover s solar shroud were deliberated, based on their pros and cons to find the optimal design. The preliminary design was a basic top and bottom covering with three small supporting columns to suspend the shade above the rover while having a minimal path of heat to the rover. Although it was a simple design that allowed the rover to recharge and shade parts of the rover, it was not able to gather enough sunlight to recharge the rover in the allotted time. At the lunar poles, the angle of incidence is about thirteen degrees from the horizon, thus, the panels that are parallel to the surface would be ineffective because the strength of the light depends on the angle of incidence; the more orthogonal the light is to the surface of the cells, the more energy is captured. A similar design was to cover every large flat surface of the rover like a shell. It would shade more areas of the rover and have more area receiving sunlight. However, this design was deemed inefficient due to the fact that the sun would only shine on a portion of the rover, while the rest of shading is unutilized. Another suggested design was to use an idea similar to the Hubble Space Telescope, which has a scroll-like mechanism to capture light energy. It starts as a roll then it elongates to become a rectangular shape. It was compact and portable but required too many mechanisms that outweighed the benefits. Figure 1: The chosen three-lift shroud design Figure by: Daniel Hong The chosen design, in Figure 1 shown above, is a large overhead panel with three actuators to support the structure and allow 360 degrees of rotation to track the sun's movements, thus maintaining the optimal sunlight for the PV cells and shading for the rover. The actuators change of height allows a large range of tilts to become orthogonal with the sun s light. A generalized model of the rover with the mounted PV cells was made with the assumption of the rover being one solid object using Matlab. Another assumption was that the

3 rover was covered with the highest obtainable emissivity paint, which reflects sunlight and has the lowest heat absorption factor. The variables in our model were the area and material of the solar shroud, as well as the efficiency of the Ultra Triple Junction (UTJ) PV cells that were rated at 28.3% efficiency. Aluminum and titanium were modeled to find the most efficient material usable in space for the solar shroud's purposes. Geometry and calculus were used to find the optimal area available to cover the rover with minimum weight possible. They were also used to find the best actuator location to provide stability of the rover when the solar shroud was moving. Below is the equation that we used to calculate these things. The equation that was mainly used was Where: = Heat flow (sun s radiation) A = Surface area of rover = Stefan-Boltzmann constant = Emissivity of the surface area of the rover = Temperature of the Rover = Temperature of where the radiation is emitting to Since the soil on earth is composed of different compounds that the lunar regolith (lunar soil), it reacts differently. It is hazardous when inhaled and conducts heat differently than the soil on Earth because of its composition and the lack atmosphere on the Moon; therefore, it heats at a much faster rate than it would on Earth. The lunar regolith was also modeled to view the heat transfer effects on the rover due to radiation of the sun. The general solution that was used to model the lunar radiation to the rover was: Figure 2: UTJ Cell Photo by: Daniel Hong

4 After that, a working model was made to determine proper mounting of the PV cells. It gave an outlook on how it well would it work, as well as the flaws that were not accounted for in the simulation. The model was made with sheets of metal and polycarbonate as well as a wooden version of the rover. The size that was chosen to test was eight inch by eight inch base and eight inch by six inch flaps. There was room for about 28 Ultra Triple Junction (UTJ) 28.3% efficiency bare solar cells made by Spectrolab (Figure 2). Bare solar cells do not have the protective covering and the interconnecting wires that connect cells together. The voltage needed to recharge the rover was 24 volts with a nine amp current. This was achieved by first connecting two strings of ten cells in series, then connecting the two strings in parallel. RESULTS Figure 3 shows how the rover is affected by the moon s radiation. The temperature differences are small enough, compared to the sun s radiation, to where the radiation from the moon to the rover can be neglected. The graphs below (Figure 4) shows that with the solar shade, the rover will get slightly hotter than if the shade were not there. However, the conditions are better than if the PV cells were attached directly onto the rover. The deployment mechanism needs more research to determine the best way to allow the three degrees of freedom and a steeper angle for lower angles of incidents of the sun. Also, the hinges that attached the flaps to the base of the shroud needs more research to optimize the area of the solar panels with the limited space on the shroud. Figure 3: Effects of Lunar Radiation to the Rover Graph by Valeriu Tocitu and Daniel Hong

5 Figure 4: Effects on Rover due to various configurations Graph by Valeriu Tocitu and Daniel Hong DISCUSSION This portion of the project was purely conceptual. Many parameters were undetermined due to fact that this was subsection was new. With a two hour mission, a solar shroud was unnecessary; however with the goal of an extended mission life, a solar shroud is currently an essential part of this rover. This section of the project is still in the research and development phase. CONCLUSION The solar shroud is a feasible solution for shielding the rover from the intense conditions on the lunar surface while collecting solar energy via PV cells. It proved to be more effective than mounting the solar cells directly onto the rover which would provide a large heat path than the shroud provides. More research needs to be done to finalize the design of the thermal shroud. ACKNOWLEDGEMENTS I would like thank the NASA Ames Robotics Academy for the wonderful experience. Also, I would like to thank Mark Leon for giving the idea of a solar shroud for the rover and providing the opportunity to work on such a project. I would also like to thank my team leader, Jacques Dolan for all the guidance and help he had given throughout the internship. I would also like to thank Art Kimura and the Hawaii Space Grant Consortium for giving me the opportunity to participate in the internship.

6 REFERENCES Gilmore, David G. ed. Spacecraft Thermal Control Handbook. El Segundo, CA: The Aerospace Press, Holman, J. P. Heat Transfer: J. P. Holman Heat Transfer (9th edition). New York: McGraw-Hill College, Fortescue, Peter, Stark John, and Swinerd Graham. Spacecraft Systems Engineering, Third Edition. West Sussex, England: John Wiley and Sons Ltd, "Spectrolab". Spectrolab. June 2009 < FIGURES Figure 4: Top and Bottom Configuration Drawn by: NASA Ames Robotics Academy Figure 5: Shell Configuration of Solar Shroud Drawn by: NASA Ames Robotics Academy

7 Figure 7: Umbrella concept for the shroud. Drawn By: Valeriu Tocitu Figure 8: Scroll idea similar to Hubble Telescope. Drawn By: Valeriu Tocitu Figure 6: Prototype of solar shroud. Photo By: Daniel Hong