The NTR utilizes a 'graphite' fuel form consisting of coated UC2 particles in a graphite substrate, and has a specific impulse capability of approximately 870 s, and an engine thrust-to-weight ratio of approximately 4.8. The NTR stage liquid hydrogen (LH2) tank has a 10 m diameter, 14.8 m length, and 68 t LH2 capacity. In terms of performance benefits, studies indicate that an expendable NTR stage powered by two 50 klbf engines can deliver approximately 96 metric tons (t) to trans-lunar injection (TLI) conditions for an initial mass in low earth orbit (IMLEO) of approximately 199 t compared to 250 t for a cryogenic chemical TLI stage. Lunar mission applications would also provide valuable operational experience and serve as a 'proving ground' for NTR engine and stage technologies. The use of an NTR-based lunar transfer stage, capable of evolving to Mars mission applications, could result in an accelerated schedule, reduced cost approach to moon/Mars exploration. At present, the Exploration Program Office (ExPO) is considering chemical propulsion for its 'First Lunar Outpost' (FLO) mission, and NTR propulsion for the more demanding Mars. Integrated systems and mission study results are presented which quantify the rationale and benefits for developing and using nuclear thermal rocket (NTR) technology for returning humans to the moon in the early 2000's. These include measurements of the heat transfer of gas filled insulation, evacuated insulation and during the transition in between. In this paper, we report on experimental tests and modeling that we have done on MLI used to insulate a cryogenic tank. One approach is to purge the MLI during ground hold with an inert gas which is then vented during launch ascent and on-orbit. However, the size and mass constraints of these propulsion systems will not allow a structural shell to be used to provide vacuum for the MLI during ground hold and launch. Multilayer insulation (MLI) will be critical to achieving the required thermal performance since it has much lower heat transfer than any. This will require the efficient long term, on-orbit storage of these cryogens. The NASA Exploration Program is currently planning to use liquid oxygen, methane and hydrogen for propulsion in future spacecraft for the human exploration of the Moon and Mars. Mission risk can be reduced by increasing the redundancy of each individual system and the architecture by using multiple copies of the same systems. By enabling larger margins in the design of exploration platforms and the ability to send multiple copies of atmospheric and surface probes, higher resolution spatial and temporal data can be collected in a single mission. The large throw mass of the Block 1B provides a game-changing capability for the exploration of other worlds. Only the SLS can deliver the 27.5 mt Orion Crew Vehicle to the Moon it delivers significantly more payload to LEO and BEO destinations than any other existing or planned launch systems. The later, 2030's era SLS Block 2 features a TLI capability of 53 mt. The Block-1 ICPS is a derivative of the Delta-IV upper stage the Block 1B EUS is in development now and its TLI capability will range between 39 and 43 mt. Payload capabilities to Trans-Lunar injection (TLI) are shown in Fig. 1) utilizes the Interim Cryogenic Propulsion Stage (ICPS) and the Block 1B features the large Exploration Upper Stage (EUS). The NASA Space Launch System SLS development consists of a series of increasingly capable launch vehicles to incrementally expand BEO exploration from cis-lunar space and then to Mars. In this report the Block 1 and 1B configurations and the EUS will be described along with some missions that take advantage of the larger, higher performing EUS upper stage. The EUS optimizes the powerful SLS Core and Booster Stages, and will provide the capability of achieving greater human exploration, operations and science objectives for 2020-2040 era Beyond Earth Orbit (BEO) missions, including crewed Cis-lunar missions in the mid-2020s, crewed Lunar Surface missions in the late 2020's and crewed Mars missions in the mid-2030s. In 2023, the new NASA Exploration Upper Stage (EUS) will evolve the SLS to a significantly higher performance level. The NASA Space Launch System (SLS) will provide a game-changing capability for the exploration of other worlds, beginning with the Block 1 configuration that utilizes the Interim Cryogenic Propulsion Stage (ICPS).
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