The US Department of Defense (DOD), US Department of Energy (DOE), and NASA have been working together in the planning necessary to initiate a program to develop a nuclear propulsion system. President Bush's speech of July 20, 1989, outlining a goal to go back to the moon and then Mars initiated the Space Exploration Initiative (SEI). The overview and comparison will be complemented by a closer look at ongoing activities related to research and development in this area and by an outlook on what kind of systems might be employed to carry the first astronauts to Mars and beyond. This paper intends to provide an overview on the various fission- and fusion-based Nuclear Power and Propulsion system concepts and tries to compare these systems' different working principles and technical implementations with each other.
However, studies and analyses show that all of these future plans are hampered by today's available propulsion systems and by the problematic of solar power generation at distances at and beyond of Mars, a problem, however, that can readily be solved by the utilisation of space nuclear reactors and propulsion systems. Today, after more than three decades, manned exploration of the Solar System has become an issue again and so are missions to Moon and Mars. Unfortunately the 70's-paradigm shift in space policies did not only bring an end to the Apollo programme, but it also brought a complete halt to all of these technology programmes and confined the human presence in space to a tiny bubble including nothing more than the Earth's sphere and a mere shell of a few hundred kilometres of altitude, too small to even include the Moon. Nuclear space power and propulsion played an important role in these entire future scenarios, hence ambitious development programmes were undertaken to make these technologies available. Consequently ambitious research and development programmes were undertaken at that time to enable what seemed to be the next logical steps: the establishment of a permanent settled base on the Moon and the first manned mission to Mars in the 80's.
When mankind made its giant leap, the exploration of our cosmic neighbour was seen as the more » initial step for the manned exploration of the whole Solar System. Numerous spacecraft have left the Earth in this endeavour, but while unmanned spacecraft were already sent out on missions, where they would eventually reach the outer limits of the Solar System, manned exploration has always been confined to the tiny bubble of the Earth's gravitational well, stretching out at maximum to our closest celestial companion - the Moon - during the era of the Apollo programme in the late 60's and early 70's. Humanity has started to explore space more than 40 years ago. Using chemical propulsion for lunar missions and delaying NTR development will only result in higher costs and delayed or cancelled Mars missions. There is potential for orderly growth into nuclear concepts far beyond NERVA. Nuclear thermal rockets can take mankind to the moon, to Mars, and beyond, but development must begin now. Only 3-4% of the nuclear fuel would be used in a Mars mission, and an engine can be used until about 40% of the fuel is expended. The NTR is reusable, and can be developed into a complete space transportation system. Also, some of the current research undertaken to improve chemical rocket more » performance, such as aerobraking or schemes to minify payload, applies to an NTR as well, although it is not essential. Performance of an NTR depends on achievable materials temperatures, but materials has progressed significantly since the 1960's. The Nuclear Engine for Rocket Vehicle Application (NERVA) program of the 1960's built engines with Isp's of about 825 seconds. Advanced cryogenic chemical propulsion systems achieve a maximum specific impulse (Isp) of about 470 seconds. Preliminary review indicates that the propulsion system most likely to meet all mission requirements is the Nuclear Thermal Rocket (NTR). It is now possible to make general comparisons of candidate propulsion systems for human exploration of Mars.
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