Among the first reports related to rovers since the announcement of the VSE was Expanding Frontiers with Standard Radioisotope Power Systems, published in January 2005. Written primarily by scientists and engineers at the Jet Propulsion Laboratory, with contribution from NASA Headquarters, this document looked at the power requirements of lunar and Martian rovers for the future. The original Lunar Roving Vehicle employed rather conventional batteries, which were adequate for the length of the Apollo 15, 16 and 17 missions. The power requirements of future lunar and Martian rovers would be much more demanding. Radioisotope Power Systems (RPS) had been used by the United States for space exploration since 1961. RPS generate electrical power by converting the heat released from the nuclear decay of radioactive isotopes into electricity through any one of a number of conversion processes. These RPS were known for their long life, ruggedness, compact size and reliability. The standard RPS unit used in such successful interplanetary probes as Galileo, Ulysses and Cassini is the General Purpose Heat Source (GPHS) - Radioisotope Thermoelectric Generator (RTG). A new generation of RPS are currently under development, called the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) and the Stirling Radioisotope Generator (SRG). These new RPS will have the capability of operating both in the vacuum of space and within a number of different planetary environments. It is these RPS that the report proposed to power future rovers and other surface equipment. Among the first planetary robotic rovers to use this will be the Mars Science Laboratory.
Among the mission vehicle modes considered for use of the MMRTG is the Dual Mode Lunar Roving Vehicle (DMLRV). This was a concept explored during early rover design studies for Apollo by Grumman, Bendix and GM's Defense Research Laboratories. These vehicles were conceived to be operated by astronauts, but with the capability of being remotely and robotically directed. It should be mentioned that the fourth Lunar Roving Vehicle, planned for the eventually-cancelled Apollo 18 mission, was being studied for this conversion from manned to robotic operation. With the return to the Moon, longer lunar missions make the DMLRV a prime candidate as a rover design. The rovers that will operate on the Moon will have to perform for months and even years. They will not be disposable, as the first Lunar Roving Vehicles were, but they will be critical for crews, who cannot go further from their base than the projected walk back constraints - a practically unavoidable astronaut EVA mandate. In the manned mode, the DMLRV would serve as the primary means of transportation for the crew of two or more astronauts during their exploratory EVAs and performance of scientific experiments. In its robotic teleoperated mode, the DMLRV would be capable of long-range exploration, sending live TV images back to the lunar base for viewing by the astronauts. It would also conduct scientific experiments and take readings, much like the Martian Exploration Rovers have done. The stated mission goal for the DMLRV is "to provide a multipurpose infrastructure element and remote science platform for the exploration of the Moon. The DMLRV would be essential for extending the productivity of human exploration crews, and would provide a unique capability for diverse long-range, long-duration science exploration between human visits. An additional goal of the DMLRV would be to provide a reconfigurable vehicle system capable of conducting surveying and a range of site preparation activities in support of the establishment of permanent human presence on the Moon.''
The DMLRV and its systems, like most of the space transportation equipment to be employed in the Vision for Space Exploration, will be designed for years of operational use. The nominal mission length for the DMLRV would be five years. The vehicle would be made up of the four- or six-wheeled rover itself and a two-wheeled trailer containing the MMRTG and a suite of scientific instruments, as well as other equipment. The rover would be powered by rechargeable lithium-ion batteries, with renewable energy provided by the MMRTGs. However, the exposure of the crew and equipment to limited amounts of radiation from the MMRTGs was considered in trade studies. This option of long-term power generation is just one approach for the DMRLV.
The other means of power generation would employ the use of the SRG. These emit lower doses of radiation and have the added benefits of lower heat output and lower mass. The SRG poses less risk to the crew and the equipment and may be the preferred power configuration for the manned DMLRV. Without question, however, is the need for long-term power generation for this vehicle and the use of RPS to meet that mission requirement.
In January 2005, scientists and engineers from NASA's Glenn Research Center presented Exploration Rover Concepts and Development Challenges at the first Space Exploration Conference in Orlando, Florida, sponsored by the American Institute of Aeronautics and Astronautics. More than a dozen different rover designs, both unpressurized and pressurized, that had been proposed during the previous ten to twelve years (and some designs that were quite recent) were evaluated. These rovers were conceived to be used on the Moon and Mars. Rovers that had actually been deployed were discussed first, including the Apollo Lunar Roving Vehicle, the Soviet Lunokhod robotic rovers, the Mars Sojourner and the Mars Exploration Rovers, Spirit and Opportunity. The Apollo LRV, of course, was the only manned rover ever to be driven on the Moon. In many ways, the report echoed similar studies produced decades before. Several of the designs were the product of universities. Boeing revisited the rover concept in 1990 with its Advanced Civil Space Systems Piloted Rover Technology Assessment Study. In this study, Boeing proposed an unpressur-ized Light Utility Rover, practically identical to the LRV it built for Apollo 15, 16
and 17, with the addition of a two-wheeled trailer. It also proposed a pressurized rover.
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