New design overcomes intense lunar heat

The third prototype for Astrobotic Technology’s lunar robot has innovations that will enable it to survive the blistering heat at the Moon’s equator, which is the robot’s destination in May 2011 when it will visit the Apollo 11 site.
Noon at the equator is hotter than boiling water: 270 degrees F. The robot beats the heat by keeping a cool side aimed away from the Sun to radiate heat off to the black sky. It travels toward or away from the sun (generally east or west) without turning its radiator into the light. Only the solar cells on the hot side ever face the sun. The robot can travel north and south by tacking like a sailboat.
The Apollo 11 crew landed shortly after local dawn and left by mid-morning, so Neil Armstrong and Buzz Aldrin never encountered the noontime extreme. (Each day on the Moon is two weeks of sun followed by two weeks of darkness and extreme cold.)
Led by Dr. William “Red” Whittaker, famed Carnegie Mellon roboticist, Astrobotic intends to win up to $25 million in the Google Lunar X Prize with this robot. It will be a rolling TV studio and Internet node, sending back high-definition video of its adventures. As it examines the Apollo 11 site, it will discover how various materials used in Apollo 11 have weathered under 40 years of radiation and micrometeorite bombardment – information that will help NASA and other space agencies improve the designs for their upcoming human lunar expeditions.
The fundamental innovation developed at Carnegie Mellon is the rover’s asymmetrical shape. On the cold side, there’s a flat radiator angled up to the black lunar sky as well as a vertical panel for the logos of the corporations sponsoring the expedition. On the hot side, a half-cone of solar cells generates ample electrical power to power the wheels, run the computers and energize the transmitter beaming back stereo HD video to Earth.
The following photographs show the main body of the rover, prior to adding the camera system and antenna at the top.

Third prototype front view

Third prototype top view

Third prototype side view

The following illustration shows how the rover will appear when the two stereo HD cameras, the telephoto HD camera and antenna dome are added to the top of the robot.

Third prototype with camera and antenna dome

Another innovation places is a lunar-specific drive train. Unlike Mars rovers that have motors in the hub of each wheel, the Astrobotic lunar rover tucks two motors inside the body of the robot where they are safeguarded both from heat and the abrasive lunar dust. Each motor drives one side of the robot’s wheels using a chain drive, like a bicycle. The chain drive mechanism has been tested in a Carnegie Mellon vacuum chamber to ensure that is does not experience “cold welding” – a process where materials sometimes merge or weld to each other when touching in a hard vacuum.

Two interior motors will connect to gold hubs to drive each side of wheels

Composite parts and drive train tested in vacuum chamber

Key to the design are tailored composite structures made from carbon fiber tape and resin. Many aerospace designs use composite materials to achieve high strength at low weight; composite parts being shaped for the lunar machine have the added ability to transmit heat from the hot to the cold side with more efficiency than copper or other metals. The team has fabricated several of the most complicated and high-stress components and subjected them to both thermal and stress tests. The thermal tests instrumented keys pieces with thermocouples to measure the heat flow from hot end to cold end. The flexure test documented how much pressure a piece can withstand before buckling.

Composite material heat transfer test monitored by thermocouples

Sloping composite iBeam for radiator support

The challenge of surviving the lunar night, where the minus 240 degree F temperature is almost as cold as liquid nitrogen, is still to be solved. Surviving the night is not required to win the main Google Lunar X PRIZE but would generate a bonus. The team is examining how to package lithium ion batteries so they function again after the two weeks of cryogenic cold.
The team also must develop a landing platform to descend from lunar orbit to the surface; launch from Earth and transit to the Moon will be accomplished via already-established commercial launch vehicles and kick motors.