Lunar Rover Expedition: Technical Considerations Abstract In 1994 Carnegie Mellon University (CMU) and LunaCorp forged an alliance to launch the first private lunar mission in the year 2000. Over the two year mission, two self-powered teleoperated rovers will travel 625 miles (1000km), transmitting real-time high quality video images from the Moon to television, science centers, and one or more theme parks worldwide. The current study has determined that the H-IIA launch vehicle, in its 4 ton configuration, can be used for this mission. An existing lander, derived from the Phobos, fits within the shround and can deliver a payload of 600 kg to the lunar surface. Given technological and cultural constrains, the rovers are be solar powered and use isotope heating to survive the lunar night. Each rover has a mass of 240 kg and requires generation of 430 watts of electric power. Technical Issues The Moon is highly suitable for planetary mobile robotic technology. Its proximity to Earth makes remote operation and real time video transmission possible, and the fixed position of its near side allows a direct line of sight for continuous communication. The Moon's barren terrain is favorable to robotic perception and navigation, and its unfiltered atmosphere guarantees a rich supply of solar power. However, the lunar environment does present challenges for a mission of this duration. Temperature: The surface temperature during the lunar day (14 Earth days) reaches 130C and plummets to -180C during the lunar night (14 Earth days). Because of the drastic extremes in temperature, thermal insulation is critical to survival of onboard electronics. Batteries and electronics fail at -20C and -80C, respectively. For the designed payload, 117kWhrs of thermal power are needed to maintain survival temperatures throughout the lunar night, given an achievable 0.1 emmissivity. Radiation: Over the two year mission, the expected radiation dose accumulated is 25 kilorads. Incorporating a safety margin to account for unexpected cosmic activity, the components are tolerant to 100 kilorads. During the lunar day, the rover surface is exposed to the solar flux of 1385W/m^2 and infrared radiation from the lunar surface of 1136W/m^2, and so the rover body is highly reflective. Lunar Dust: Lunar dust is comprised of very fine, highly abrasive particles. The dust also carries an electrostatic charge, which enables it to cling to all non-grounded surfaces. All mechanisms are sealed and the rovers include design features and/or hardware to prevent the accumulation of lunar dust on optics and thermal radiators. Terrain: Analysis of known lunar terrain indicates that a long duration traverse will encounter 20 degree slopes and craters or obstacles 25cm in height. Due to the softness of the soil, the ground pressure of the rovers (with all weight on two of the four wheels) is limited to 6kPa. Payload Specifications As remotely deployed robotic vehicles, the rovers will have full onboard capabilities. These capabilities include: Imagery: A realistic telepresence of the rover's surroundings is obtained using fully-immersive panospheric imaging with resolution superior to current television standards. The digital imagery can be displayed using any broadcast standard. Real-time wavelet compression techniques utilize compression ratios in excess of 100:1 with virtually no visible loss of fidelity. Communication: The rovers use a phased array antenna to communicate with Earth ground stations. Each antenna is comprised of over 1600 elements and is 0.5m in diameter with an input power of approximately 100W. The 10MHz of allocated downlink frequency in X-band allows for up to 6Mb/sec of transmitted data (overhead of error-correction and encryption is already included). Computing: Custom boards using high speed radiation-tolerant DSP based microprocessors and a radiation-hard R3000 CPU are applicable to this mission. Including imagery compression, the entire computing components requires approximately 105W. Budgets: Image acquisition (described above), vehicle safeguarding and perception, and rover state monitoring is projected to require approximately 80W. Including safety margins, the total payload (imagery, communication, computing, and safeguarding) for each rover is approximately 70kg. The remaining mass (approximately 180kg per rover) is allotted for structure (including solar array panels), locomotion, thermal and power supply. Thermal Source and Power Generation Due to the lunar environment, solar based power generation has considerable advantages. However, the extended duration and low temperatures of the lunar night require a constant supply of heat. Storing the required energy in batteries to be trickled slowly throughout the night is unpractical. Therefore isotope based heating options with long halflife were considered. Using thermoelectric conversion, solar power generation can be avoided, but the amount of isotope required is presumed to be unacceptable in the current socio-political atmosphere. So, a compromise between solar power generation and isotope heating for night survival yields a technically feasible and culturally acceptible solution. Solar Only: While solar power is widely accepted as a benign power source, when incorporated in rover design it presents immediate technical and design issues. To provide the watts required for operating the vehicle, 15m^2 of array must be constantly exposed to sunlight (angle of incidence shouldnt exceed 30 degrees). In addition, operating time is restricted to the lunar day, and power for night survival (the rover "sleeps") must be generated from battery discharge. Since batteries are heavy, and the power required for locomotion is dependent upon the vehicle mass, a technically realistic solar vehicle requires 2148W of electrical power and a mass of 5513kg (batteries alone account for 2930kg). This immense mass makes launch from the H-IIA an impossibility, and means that only one rover would likely be possible. Further, this configuration precludes a scientific payload and diminishes the overall commercial return on investment generated by two rovers televising each other as they travel the lunar landscape. ThermoElectric Power: Of the three scenarios examined, the RTG (Radioactive Thermal Generator) powered rover presents the lightest, most efficient design configuration. Utilizing thermoelectric conversion from twelve 250Wthermal GPHS (General Purpose Heat Sources), this rover would have available 2800Wther for systems operation, require 420We, and have a mass of 205kg. With no need for sunlight, the rover would have a 100% duty cycle, operating both day and night, and be generating excess heat (eliminating the difficulty of night survival). However, while documented as a safe and reliable power source, the use of 5.1kg of plutonium 238 makes this technology socially and culturally unacceptable, and thus not feasible for this mission. Solar & RHU Compromise: The argument against solar power centers on the need for batteries for night survival and the resultant excessive mass. By combining solar power with RHU (Radioactive Heating Units) for generation of the thermal power necessary for night survival, a rational design solution evolves. As with RTGs, RHUs use the natural radioactive decay of plutonium 238. However, designed to be a source of thermal heat as opposed to a building block for thermo-electric conversion, RHUs are small (60mm x 40mm) and contain approximately 15g of isotope per unit. The desgined rover includes 39 RHUs, the aggregate providing 325Wther from 600g of radioisotope. Also included in design is 10kg of NiCad battery power to support 30 minutes of full rover functionality in shadow or grazing solar angles during the lunar day. Since RHUs generate heat at night, they generate heat during the day as well. This heat must be vented, which more than doubles the size of the required radiator for the electronics payload (from 1.45m2 to 3.5m2). An alternative solution is active conditioning, whereby actuators pull the RHU component away from the rover body during the day and return it to its component bed during the lunar night. The Solar/RHU Rover design with auxiliary batteries for more robust daytime navigation and operation, with the active RHU conditioning results in a configuration that requires an estimated 430We power and 240kg mass. The required solar array drops to about 3m^2 and unlike the scenario involving purely solar power, allows deployment of two rovers launched on a H-IIA rocket with around 15kg available for scientific payloads. Launch and Landing In assessing technologies for launch, factors such as economy, availability, large payload capacity (mass and volume), and compatibility with lander design are examined. The H-IIA launch vehicle meets these criteria. However, while the payload parameters of 4.6m (diameter) X 3.7m (height) and capacity of 600kg are acceptable, the proposed launch vehicle, delivering 3000kg to geosynchronous orbit (GEO), is of some concern and its feasibility is as yet undetermined. For lunar deployment, mass delivery of 4000kg provides a more desirable level of flexibility in rover design and allows for a larger scientific and commercial payload. Currently under investigation are two landers: Lockheed Martin's lander for the 1998 Mars expedition and the Russian Phobos lander. The Mars lander utilizes promising technology, and its potential for adaptation to this lunar mission is being examined. Of continuing interest is a derivative of the Phobos lander. When fully fueled, this lander has a mass of approximately 6700kg with a delivered payload of 600kg. It can land within 4.3km of the designated coordinates with a maximum velocity of 2m/s vertical and 1.2m/s horizontal. Target mass for both rovers is 500kg, with the additional 100kg reserved for landing ramps and contingencies. While deployment strategy and launch integration are still in development, a working scenario is in place. Within the lander, the two rovers will be secured by pyrotechnic bolts in a vertical position, separated by an insulation wall and encircled by a large flexible collar designed to prevent dust contamination during landing. The wheels of each rover will be attached to ramps that, at deployment, will discharge pyrotechnic bolts, pivot, and lower to the surface. With the flexible collar depressed, the rovers will drive down the ramps onto the lunar surface.