Date of Award

2007

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Aerospace, Physics, and Space Sciences

First Advisor

Daniel R. Kirk

Second Advisor

Paavo Sepri

Third Advisor

Hamid K. Rassoul

Abstract

The most likely candidate for human missions to Mars and beyond will be spacecraft using Nuclear Thermal Propulsion (NTP). NTP provides for high thrust-to-weight ratio and significantly improved specific impulse over even the best chemical rockets. The improved specific impulse reduces required trip times, thereby minimizing the exposure time of the crew to space radiation. Using the PHOEBUS 2A 5 GW reactor, developed in the 1960’s with a thrust of 1,113 N and a specific impulse of 820 s, as a basis for comparison this thesis focuses on efforts to improve the specific impulse and reactor longevity. Hydrogen is the best mono-propellant choice for NTP because of its low molecular mass, which gives the maximum specific impulse. Studies on hydrogen properties have shown that significant dissociation occurs for high temperature and low pressure. Dissociation decreases the molecular mass, which could potentially be used to further increase the specific impulse. The temperature of the hydrogen gas at the exit of the PHOEBUS 2A reactor is around 2,500 K, and although larger frictional pressure drops occurs over the reactor passages, no significant levels of dissociation take place. The idea of tailoring the cross-sectional area of the cooling channels to achieve supersonic speeds, with an associated pressure drop to obtain significant levels of dissociation may result in further gains in the specific impulse. A second drawback of the 1960’s reactor designs was the large thermal gradients within the core material that led to cracking of the fuel elements. A new grooved-ring high performance nuclear rocket fuel element design may provide significant performance and manufacturing advantages over present Nuclear Thermal Rocket (NTR) fuel element designs. The fuel rings can be readily manufactured out of hard to form high temperature uranium tricarbide or other nuclear fuel materials. In addition, the fuel element configuration is such that high heat transfer rates are achievable leading to potentially light and compact reactor designs. Thrust to weight ratios close to that projected for particle bed reactors should be possible with this design.

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