http://www.aviationnow.com/awin/awin_awst/awin_awst_story.jsp?issueDate=2004 -05-31&story=xml/awst_xml/2004/05/31/AW_05_31_2004_p62-68-02.xml Space Technology Prometheus Nuclear Technology Eyed for Moon, Mars Exploration Aviation Week & Space Technology 05/31/2004, page 66 Frank Morring, Jr. Washington Prometheus technology is being eyed in plans for Moon, Mars exploration Space Nuclear Power The space-based nuclear reactors that NASA managers see as a key enabling technology for landing humans on Mars are starting to generate requirements for lots of chemical rocket lift capability to get them off Earth's surface. While nuclear power solves many problems--reducing a crew's exposure to radiation, for example--even the new lightweight reactors the Energy Dept. is starting to develop for NASA will be a heavy load to get into space. NASA exploration planners are working those figures, inherited from the Project Prometheus space nuclear power initiative, into the calculations for meeting President Bush's Jan. 14 call for human exploration of the Moon and Mars in the generation ahead. Estimates of the weight of xenon fuel for the nuclear-electric thrusters on the Jupiter Icy Moons Orbiter (JIMO) range from 10,000-13,000 kg. (22,000-28,600 lb.). JIMO is to be both an advanced scientific surveyor and a robotic testbed for exploration technology. "JIMO itself is fairly big," says Michael Lembeck, head of the requirements division in the new exploration systems office at NASA headquarters. "There's a lot of radiator mass; you have to dissipate a lot of the heat from the reactors, so we're looking at different ways of packaging it to get it into orbit." Lembeck presides over a handpicked team of engineers, astronauts and managers working in a warren of carrels and whiteboards dubbed "the Swamp" on the fifth floor of NASA headquarters. He sees the nuclear-reactor requirements issues for exploration as "a Goldilocks problem" of designing a reactor that is not too big, not too small. The medium-sized reactor built for JIMO probably can be derated to meet the power requirements for a base on Mars and perhaps also on the surface of the Moon, which would be used as a testbed for Mars exploration techniques. It could also push the sort of high-power, high-data-rate robotic exploration planned for Jupiter on to Saturn and beyond (AW&ST May 3, p. 37). But to drive a human crew to Mars or other points beyond Earth orbit would require a much larger reactor, and related hardware. Although firm requirements haven't been set, a JIMO-type space reactor would need to deliver a few hundred kilowatts for electric propulsion to provide continuous thrust and cut travel time, according to Bret Drake, a member of Lembeck's staff who has studied exploration architectures at Johnson Space Center for the past 15 years. To keep humans alive and scientifically productive on planetary surfaces would require about 30 kw. for life support and instrument power. Water-based in situ resource utilization, which could save considerably on launch weight from Earth by breaking water into hydrogen and oxygen, would add another 30-60 kw. to the power load, Drake says. "WHEN YOU talk human transportation, you need megawatt level," Drake says of nuclear-electric propulsion that could get a crew through the radiation perils en route to Mars relatively quickly. "So this Goldilocks problem is, for a space reactor for science missions, what is the best design to go after first, and can you make it extensible to human megawatt reactor needs or derate it for surface power." Derating seems to be the easier task, Lembeck said. A notional timeline on the wall of the Swamp left over from early Red Team/Blue Team studies of deep-space exploration requirements sets 2017 as the date for starting development of a Mars surface reactor (AW&ST Mar. 15, p. 30). The reactor would be ready on the surface by 2029 under what Lembeck stressed was a very preliminary schedule used only to give a first-cut idea of the work ahead. NASA has entered a preliminary agreement that would see the Energy Dept.'s Naval Reactors program develop space reactors for the exploration program. Although NASA will fund the work, and classified information will be protected, the two agencies are still working out details for exactly how the open NASA exploration effort will mesh with the secret naval work at the Energy Dept. However, NASA has conducted joint research with Energy Dept. laboratories in the past, including an ongoing project between Marshall Space Flight Center and the Los Alamos National Laboratory that has built unfueled components for space reactors rated as high as 500 kw. David Poston, head of the space fission power team at Los Alamos, says the three principal types of reactors under consideration are those cooled by a pumped liquid metal like sodium or lithium, by sodium or lithium liquid metal heat pipes, and by inert helium or helium-xenon gas. Regardless of which one ultimately is chosen, it would allow scientists to conduct the same kind of orbital research at distant Jupiter that is now only possible in Earth orbit. JIMO plans call for a direct nuclear-electric push to Jupiter that would take 5-7 years, without the gravity assists that sling chemically propelled space probes around other planets toward their final targets, says Raynor L. Taylor, JIMO program executive at NASA headquarters. "At some point in the thrust vector, the spacecraft is turned around and decelerated to enter Jupiter's orbit," Taylor says. "It would continue to thrust to continue to decelerate and fall into the gravity well of Jupiter." Preliminary plans call for a launch in 2015, a delay from an earlier date of no-earlier-than 2011 that Taylor says will allow more time for technology development. Once it reaches the Jovian system, JIMO would spend 4-6 years exploring, including at least 60 days in orbit around Callisto, at least 120 days orbiting Ganymede and at least 30 days around Europa, considered the most likely of the three icy moons to harbor a liquid-water ocean and perhaps even life beneath its frozen surface. Provided the JIMO mission succeeds, "we can move on to the systems needed for even more ambitious space exploration, such as multi-megawatt nuclear electric propulsion or nuclear thermal rockets," Los Alamos' Poston told a Feb. 9 space technology conference in Albuquerque, N.M. Those missions are likely to require even heavier lift to get off Earth than its robotic precursor. Taylor says the total launch weight of JIMO would be "upwards of 20,000 kg." Lembeck's requirements team is still figuring out how much other exploration payloads would weigh and working tradeoffs on how best to get them to Earth orbit. The work is part of a larger effort that should produce level-one requirements in December that will, for example, reduce the standard crew size from the present range of 2-6, he says. "We've got to move a certain amount of mass," Lembeck says. "That's part of our design reference architecture we'll look at, identifying how much mass. The next thing we'll do is ask them to look at what does it mean to chunk that mass into one, two, three, four pieces. What infrastructure do you need, what other technologies for automated rendezvous and docking, that sort of thing. So we're not presuming any solutions. We're trying to do this in a rather disciplined top-down approach to start with."
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