Currently, trade-offs are being made among the various propulsion systems being considered for the Space Exploration Initiative (SEI) missions. It is necessary to investigate the reliability aspects as well as the efficiency, mass savings, and experience characteristics of the various configurations. Reliability is a very important factor for the SEI missions because of the long duration and because problems will be fixed onboard. The propulsion options that were reviewed consist of nuclear thermal propulsion (NTP), nuclear electric propulsion (NEP) and various configurations of each system. There were four configurations developed for comparison with the NTP as baselined in the Synthesis (1991): (1) NEP, (2) hybrid NEP/NTP, (3) hybrid with power beaming, and (4) NTP upper stage on the heavy lift launch vehicle (HLLV). The comparisons were based more or less on a qualitative review of complexity, stress levels and operations for each of the four configurations. Each configuration included a pressurized NEP and an NTP ascent stage propulsion system for the Mars mission.

Comparing Nuclear Propulsion Technologies for Crewed Missions to Mars Current chemical propulsion systems are limited in their ability to support crewed missions to Mars due to their low propellant efficiency and long mission times. Nuclear propulsion technologies offer a potential solution to these challenges, as they are capable of producing higher thrust and propellant efficiency than chemical rockets. This paper compares three nuclear propulsion technologies for crewed missions to Mars: nuclear electric propulsion (NEP), nuclear thermal propulsion (NTP), and nuclear fusion propulsion (NFP). Each technology has its own advantages and disadvantages, and the best choice for a crewed Mars mission will depend on a number of factors, including mission requirements, cost, and risk. NEP systems are the most mature of the three technologies, and they have already been used successfully in a number of space missions. NEP systems use a nuclear reactor to generate electricity, which is then used to power an electric thruster. NEP systems are very efficient, but they produce relatively low thrust. As a result, NEP systems require longer mission times than other nuclear propulsion technologies. NTP systems are less mature than NEP systems, but they offer the potential for higher thrust and shorter mission times. NTP systems use a nuclear reactor to heat a propellant gas, which is then expelled through a nozzle to produce thrust. NTP systems are more efficient than chemical rockets, but they are also more complex and expensive to develop. NFP systems are the most advanced of the three technologies, but they are also the most speculative. NFP systems would use the fusion of atomic nuclei to generate energy, which could then be used to power a variety of propulsion systems. NFP systems have the potential to offer even higher thrust and shorter mission times than NTP systems, but they are still in the early stages of development.

It is now possible to make general comparisons of candidate propulsion systems for human exploration of Mars. Preliminary review indicates that the propulsion system most likely to meet all mission requirements is the Nuclear Thermal Rocket (NTR). Advanced cryogenic chemical propulsion systems achieve a maximum specific impulse (Isp) of about 470 seconds. The Nuclear Engine for Rocket Vehicle Application (NERVA) program of the 1960's built engines with Isp's of about 825 seconds. Performance of an NTR depends on achievable materials temperatures, but materials has progressed significantly since the 1960's. Also, some of the current research undertaken to improve chemical rocket performance, such as aerobraking or schemes to minify payload, applies to an NTR as well, although it is not essential. The NTR is reusable, and can be developed into a complete space transportation system. 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. Nuclear thermal rockets can take mankind to the moon, to Mars, and beyond, but development must begin now. There is potential for orderly growth into nuclear concepts far beyond NERVA. Using chemical propulsion for lunar missions and delaying NTR development will only result in higher costs and delayed or cancelled Mars missions.

In 2003, NASA began an R&D effort to develop nuclear power and propulsion systems for solar system exploration. This activity, renamed Project Prometheus in 2004, was initiated because of the inherent limitations in photovoltaic and chemical propulsion systems in reaching many solar system objectives. To help determine appropriate missions for a nuclear power and propulsion capability, NASA asked the NRC for an independent assessment of potentially highly meritorious missions that may be enabled if space nuclear systems became operational. This report provides a series of space science objectives and missions that could be so enabled in the period beyond 2015 in the areas of astronomy and astrophysics, solar system exploration, and solar and space physics. It is based on but does not reprioritize the findings of previous NRC decadal surveys in those three areas.

A new impetus to manned Mars exploration was introduced by President Bush in his Space Exploration Initiative. This has led, in turn, to a renewed interest in high-thrust nuclear thermal rocket propulsion (NTP). The purpose of this report is to give a brief tutorial introduction to NTP and provide a basic understanding of some of the technical issues in the realization of an operational NTP engine. Fundamental physical principles are outlined from which a variety of qualitative advantages of NTP over chemical propulsion systems derive, and quantitative performance comparisons are presented for illustrative Mars missions. Key technologies are described for a representative solid-core heat-exchanger class of engine, based on the extensive development work in the Rover and NERVA nuclear rocket programs (1955 to 1973). The most driving technology, fuel development, is discussed in some detail for these systems. Essential highlights are presented for the 19 full-scale reactor and engine tests performed in these programs. On the basis of these tests, the practicality of graphite-based nuclear rocket engines was established. Finally, several higher-performance advanced concepts are discussed. These have received considerable attention, but have not, as yet, developed enough credibility to receive large-scale development.