anthropometric spectrum (large and small), (2) the worst-case scenario of the resulting suit design not fitting the anthropometric range of the selected and trained crew population, or (3) the requirement for loads and forces of the suit to accommodate worst-case deconditioned crew members on the surface of the Moon or Mars, or on the ISS, then EVA operations, and therefore mission success, may be significantly compromised.

PRESERVING THE OPTION FOR NUCLEAR THERMAL PROPULSION

A member of NASA’s Mars Architecture Working Group study briefed the committee on those activities results during the committee’s visit to the Johnson Space Center (JSC). That briefing summarized the trade-offs in technologies that had been considered for a human mission to Mars. For propulsion, the propulsion system selected was nuclear thermal propulsion (NTP). This choice is consistent with advice given in previous Mars mission architecture studies.12,13

In 2005, an NRC committee reported on its examination of the potential benefits of a nuclear thermal rocket (NTR) to enhance uncrewed exploration of the outer solar system and for human exploration:

Finding: Nuclear propulsion technologies will likely be used initially for moving relatively large scientific payloads (~1,000s kg) to destinations in the outer solar system and beyond and extremely large payloads (~10,000s kg) in support of human exploration activities in the inner solar system. But it is necessary to investigate nuclear propulsion technologies more thoroughly to determine if they can provide fast, affordable access to the outer solar system and beyond and can move large payloads in the inner solar system cost-effectively and efficiently.14

The basic feasibility of the NTR was demonstrated in the Rover and the Nuclear Engine for Rocket Vehicle Applications (NERVA) programs in the 1960s, which tested an integrated engine/stage system to TRL 6. Because of its high performance, the NTR offers the potential of reduced mass in orbit (one-half to one-third that of chemically propelled systems), freedom from the need to develop aerobraking/aerocapture technologies for Mars, and the option of executing opposition-class missions with a stay on the surface that might extend to a few months. Total round-trip times of less than 500 days are possible for spacecraft that have an initial mass in low Earth orbit equivalent to those of chemically propelled missions lasting 900 days. Shorter trip times translate into reduced radiation doses from cosmic rays, microgravity effects, and psychological stresses associated with being confined in a spacecraft for months at a time.

The NERVA engines used fuels clad in graphite that had a tendency to crack, erode, and leak fission products into the exhaust. Such performance is not acceptable in today’s environment. However, one of the alternative fuel forms investigated in the 1960s, tungsten loaded with uranium dioxide, demonstrated the ability to retain radioactivity and did not lead to cracking or to erosion due to thermal loading under the hydrogen flow conditions. Thus, the major issue for fuel development is materials behavior, including cracking, erosion, and thermal expansion. Electrical heating of candidate fuel elements can be accomplished in university or government laboratories—no nuclear conditions need be considered in the early stages of research. Development and demonstration of improved fuel material behavior would be a first, modest-cost step.

According to one NASA Glenn Research Center estimate, the cost to develop a flight-ready NTR system is on the order of $3 billion (in 1996 dollars).15 The committee recognizes that constraints on the program may preclude

12

T.P. Stafford, America at the Threshold: Report of the Synthesis Group on America’s Space Exploration Initiative, U.S. Government Printing Office, Washington, D.C., 1991.

13

Space Task Group, “The Post-Apollo Space Program: Directions for the Future,” available in NASA Historical Reference Collection, History Office, NASA, Washington, D.C., September 1969.

14

National Research Council, Priorities in Space Science Enabled by Nuclear Power and Propulsion, The National Academies Press, Washington, D.C., 2006.

15

S.K. Borowski and L.A. Dudzinski, “High Leverage Space Transportation System Technologies for Human Exploration Missions to the Moon and Beyond,” Paper AIAA-96-2810 in 32nd Joint Propulsion Conference Proceedings, American Institute of Aeronautics and Astronautics, Reston, Va., 1996. Also published as NASA-TM-107295, available at http://trajectory.grc.nasa.gov/aboutus/papers/AIAA-96-2810.pdf.



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