Lower Thrust Engine Options Based on the Small Nuclear Rocket Engine Design

Abstract

Advancement of U.S. scientific, security, and economic interests through a robust space exploration program requires high performance propulsion systems to support a variety of robotic and crewed missions beyond low Earth orbit. Past studies, in particular those in support of both the Strategic Defense Initiative (SDI) and Space Exploration Initiative (SEI), have shown nuclear thermal propulsion systems provide superior performance for high mass high propulsive delta-V missions. An extensive nuclear thermal rocket technology development effort was conducted from 1955-1973 under the Rover/NERVA Program. The Small Nuclear Rocket Engine (SNRE) was the last engine design studied by the Los Alamos National Laboratory during the program. At the time, this engine was a state-of-the-art design incorporating lessons learned from the very successful technology development program. Past activities at the NASA Glenn Research Center have included upgrading and modernizing nuclear thermal propulsion system models and analysis methods. The SNRE had been adopted to serve as a computational benchmark for these activities. A highly detailed MCNP Monte Carlo transport model of the reactor core was developed and exercised along with a number of simpler MCNP models of the reactor core. The reactor core models, calculated results, and comparisons with available documentation for the SNRE were described in a previous (2007) Joint Propulsion Conference paper. The SNRE was a nominal 16,000-lbf thrust engine originally intended for unmanned applications with relatively short engine operations and the engine and stage design were constrained to fit within the payload volume of the then planned space shuttle. The recent NASA Design Reference Architecture (DRA) 5.0 Study re-examined mission, payload, and transportation system requirements for a human Mars landing mission in the post-2030 timeframe. Nuclear thermal propulsion was again identified as the preferred in-space transportation system. Future mission applications may require or benefit from moderately larger higher thrust engines. Options for engine designs in the 25,000-lbf thrust range were described in a recent (2009) Joint Propulsion Conference paper. Moderately lower thrust engines may also have important roles. In particular, lower thrust engine designs demonstrating the critical technologies that are directly extensible to other thrust levels are attractive from a ground testing perspective. Recent activities have included extending the SNRE engine design into the lower thrust range. Results are presented for not-yet optimized lower thrust SNREbased engine options.