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A big problem of NTRs, exposed by GWJ in a precedent topic, is the long duration of start-up and cool-down, which makes them unfit for low delta-V short burns like course correction maneuvers. For that reason the designed-but-never-built NTR spaceship Copernicus was forced to store tons of NTO-MMH for RCS and course corrections.
A potential solution of this problem is the NTR-based OMS system exposed in this study:
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The NTR is a low enriched uranium NTR derived from the old NERVA. The orbital maneuvering system is tied to the nuclear rocket and uses the same hydrogen propellant, which enters into the core via an auxiliary circuit and is expelled through a secondary nozzle (in a bimodal NTR the core is maintained hot for all the mission to produce electric power, so the OMS system is always ready for use).
This kind of OMS thermal rocket has an exhaust velocity of 5000 m/s, less than the main rocket, which is about 9000 m/s, but far above a NTO-MMH OMS like that of the Space Shuttle, and also has the advantage to use the same propellant of the main engines.
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