peak of ~2860 K providing a temperature margin to fuel melt of ~190 K. During enhanced operation Tpeak is raised to ~3010 K (a margin-to-melt temperature of ~40 K) and Tex to ~2940 K resulting in an ~35 s increase in Isp to ~940 s if needed. Table 1 also shows a
“radial-growth” version of a Pewee-class engine using shorter FEs and a sparse FE—TT pattern for comparison.

[Table: TABLE 1.—PERFORMANCE CHARACTERISTICS FOR SMALL AND FULL SIZE “COMPOSITE” FUEL NTR ENGINES]

High temperature UC-ZrC in graphite “composite” fuel with ZrC coating was selected as the primary
fuel form in this analysis. Composite FEs were first tested in the “Nuclear Furnace” element test reactor
(Ref. 8) and withstood peak power densities of ~4500 to 5000 MWt/m3. They also demonstrated better
corrosion resistance than the standard coated particle graphite matrix fuel element used in the previous Rover/NERVA reactor tests. Composite fuel’s improved corrosion resistance is attributed to its higher coefficient of thermal expansion that more closely matches that of the protective ZrC coating, thereby helping to reduce coating cracking. Electrical-heated composite fuel elements were also tested by Westinghouse in hot hydrogen at 2700 K for ~600 min—equivalent to ten 1-hr cycles. At the end of Rover/NERVA program, composite fuel performance projections (Ref. 15) were estimated at ~2 to 6 hr at full power for hydrogen exhaust temperatures of ~2500 to 2800 K. In addition to these carbide-based fuels, a ceramic-metallic or “cermet” fuel consisting of uranium dioxide (UO2) in a tungsten (W) metal matrix was developed in the GE-710 and ANL nuclear rocket programs (Refs. 16 and 17) as a backup to the Rover/NERVA fuel. While no integrated reactor/engine tests were conducted, a large number of fuel specimens were produced and exposed to non-nuclear hot H2 and irradiation testing with promising results. Both fuel options are under development today in NASA’s Advanced Exploration Systems (AES) program and the Nuclear Cryogenic Propulsion Stage (NCPS) project (Ref. 18) which began in FY’12. This 3-year project is a collaboration between NASA and DOE involving five key task activities that include state-of-the-art engine modeling, mission design and requirements definition, fuel element fabrication and non-nuclear testing, and analysis of affordable options for nuclear ground testing and engine development. If successfully completed by the end of FY’14, a NCPS-Phase II effort would begin that could culminate in ground testing a small NTR engine at the Nevada Test Site (NTS) in the early 2020’s, followed by a flight demonstration mission several years later (Ref. 19). Finally, the motivation for selecting NTP as the propulsion system of choice for Mars is simple—it is a proven technology with a specific impulse that is twice that of today’s best chemical rockets. During the Rover/NERVA programs (1955 to 1972), a technology readiness level (TRL ~5 and 6) was achieved (Ref. 8). Twenty rocket reactors were designed, built and ground tested in integrated reactor/engine tests that demonstrated: (1) a wide range of thrust levels (~25, 50, 75, and 250 klbf); (2) high temperature carbide-based nuclear fuels that provided hydrogen exhaust temperatures up to 2550 K (achieved in Pewee); (3) sustained engine operation (over 62 min for a single burn achieved in the NRX-A6); as well as (4) accumulated lifetime at full-power; and (5) restart capability (>2 hr with 28 startup and shutdown cycles achieved in the NRX-XE experimental engine)—all the requirements needed for short transit time Mars missions. Just as important, NTP requires no large scale-ups in size or performance that are required with other advanced propulsion options. A cluster of three to four Pewee-class engines are more than adequate.
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