Preliminary Comparison Between Nuclear‐Electric and Solar‐Electric Propulsion Systems for Future Mars Missions

Recent US and European initiatives in Nuclear Propulsion lend themselves naturally to raising the question of comparing various options and particularly Nuclear Electric Propulsion (NEP) with Solar Electric Propulsion (SEP). SEP is in fact mentioned in one of the latest versions of the NASA Mars Manned Mission as a possible candidate. The purpose of this paper is to compare NEP, for instance, using high power MPD, Ion or Plasma thrusters, with SEP systems. The same payload is assumed in both cases. The task remains to find the final mass ratios and cost estimates and to determine the particular features of each technology. Each technology has its own virtues and vices: NEP implies orbiting a sizeable nuclear reactor and a power generation system capable of converting thermal into electric power, with minimum mass and volumes compatible with Ariane 5 or the Space Shuttle bay. Issues of safety and launch risks are especially important to public opinion, which is a factor to be reckoned with. Power conversion in space, including thermal cycle efficiency and radiators, is a technical issue in need of attention if power is large, i.e., of order 0.1 MW and above, and so is power conditioning and other ancillary systems. Type of mission, I_sp and thrust will ultimately determine a large fraction of the mass to be orbited, as they drive propellant mass. For manned missions, the trade‐off also involves consumables and travel time because of exposure to Solar wind and cosmic radiation. Future manned NEP missions will probably need superconducting coils, entailing cryostat technology. The on‐board presence of cryogenic propellant (e.g., LH₂) may reassure the feasibility of this technology, implying, however, a trade‐off between propellant volume to be orbited and reduced thruster mass. SEP is attractive right now in the mind of the public, but also of scientists involved in Solar system exploration. Some of the appeal derives from the hope of reducing propellant mass because of the perceived high I_sp of ion engines or future MPD. The comparison, in fact, will show whether the two systems could have the same type of thruster or not, for automatic or for manned missions. The main drawback of SEP is due to photovoltaics and the total solar cell area required, driving spacecraft mass and orbiting costs up. In addition, the question of using superconducting coils holds also for SEP, while no space radiator is, in principle, needed. These and other factors will be considered in this comparison. The goal is to provide preliminary guidelines in evaluating SEP and NEP that may be useful to suggest closer scrutiny of promising concepts, or even potential solutions.