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In page Nuclear thermal rocket:

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Between 1987 and 1991, an advanced engine design was studied under Project Timberwind, under the Strategic Defense Initiative, which was later expanded into a larger design in the Space Thermal Nuclear Propulsion (STNP) program. Advances in high-temperature metals, computer modeling, and nuclear engineering, in general, resulted in dramatically improved performance. While the NERVA engine was projected to weigh about 6,803 kilograms (14,998 lb), the final STNP offered just over 1/3 the thrust from an engine of only 1,650 kilograms (3,640 lb) by improving the Isp to between 930 and 1000 seconds.[citation needed]