Introduction:The fundamental capability of Nuclear Thermal Propulsion (NTP) is game changing for space exploration. A first generation Nuclear Cryogenic Propulsion Stage (NCPS) based on NTP could provide high thrust at a specific impulse above 900 s, roughly double that of state of the art chemical engines. Characteristics of fission and NTP indicate that useful first generation systems will provide a foundation for future systems with extremely high performance. The role of the NCPS in the development of advanced nuclear propulsion systems could be analogous to the role of the DC-3 in the development of advanced aviation. Progress made under the NCPS project could help enable both advanced NTP and advanced NEP.The Nuclear Cryogenic Propulsion Stage Project: The Nuclear Cryogenic Propulsion Stage (NCPS) project was initiated in October, 2011, with the goal of assessing the affordability and viability of an NCPS. Key elements of the project include 1) Preconceptual design of the NCPS and architecture integration; 2) Development of a High Power (~1 MW input) Nuclear Thermal Rocket Element Environmental Simulator (NTREES); 3) NCPS Fuel Design and Testing; 4) NCPS Fuels Testing in NTREES; 5) Affordable NCPS Development and Qualification Strategy; and 6) Second Generation NCPS Concepts. The NCPS project involves a large (~50 person) NASA/DOE team supplemented by a small amount of procurement funding for hardware and experiments. In addition to evaluating fundamental technologies, the team will be assessing many aspects of the integrated NCPS, and its applicability to NASA architectures of interest.Pre-Conceptual Design of the NCPS and Architecture Integration: The NCPS will be designed to integrate with the Space Launch System (SLS), and to leverage technologies and configurations being developed for the SLS. The NCPS design will focus on ensuring maximum benefit to human Mars mission, although the stage will have numerous other applications as well. Two leading fuel candidates for the NCPS are tungsten cermets and composite fuels, both with an extensive development history. The sensitivity of stage performance to specific impulse and engine thrust-toweight ratio will also be assessed under this element. Both propulsion only and "bimodal" (propulsion and power) systems will be assessed under the NCPS.

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The Heatpipe Power System (HPS) is a potential, near-term, low-cost space fission power sy Q&Z I Heatpipe Bimodal System (HBS) is a potential, near-term, lowcost space fission power and/or propulsion system. Both systems will be composed of independent modules, and all components operate within the existing database. The HPS and HBS have relatively few system integration issues; thus, the successful development of a module is a significant step toward verifying system feasibility and performance estimates. A prototypic HPS module is being fabricated, and testing is scheduled to begin in November 1996. A successful test will provide high confidence that the HPS can achieve its predicted performance.

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This paper discusses the nuclear and thermal analysis of two fission-powered concepts: (1) the Heatpipe Power System (HPS), which provides power only, and (2) the Heatpipe Bimodal System (HBS), which provides both power and thermal propulsion. The HPS and HBS systems can provide substantial levels of power and propulsion at low mass with a high degree of safety and reliability. The systems have been designed to utilize existing technology and facilities, which will make the development cost relatively low.