. 12), shorter (0.89 m/35 in.) FEs were used so additional TTs were included in the reactor to increase core reactivity. With the “SNRE” FE—TT pattern each FE has three adjacent TTs and three adjacent FEs surrounding it (shown in Fig. 4) and the FE to TT ratio is ~2 to 1. An important feature common to both the sparse and SNRE FE—TT patterns is that each tie tube provides mechanical support for six adjacent fuel elements.

[Figure: Figure 4.—Coated particle and composite SNRE fuel element and tie tube arrangement.]

Recent Monte Carlo N-Particle (MCNP) transport modeling of engine reactor cores by Schnitzler et al., (Refs. 11, 13, and 14) has shown that the SNRE design can be scaled down to even lower thrust levels (~5.3 to 7.4 klbf) or up to the 25 klbf Pewee-class engine used in Mars DRA 5.0. For low thrust engines,
short length elements (~0.89 m) and a “dense” FE—TT pattern are used consisting of parallel rows of FEs and TTs. Each FE has four adjacent TTs and two adjacent FEs surrounding it and the FE to TT ratio is ~1 to 1. By using the SNRE FE—TT pattern and increasing the FE length from 0.89 to 1.32 m (the same length used in the Rover program’s Pewee engine), the U-235 fuel loading in the core can be lowered from ~0.60 to 0.25 grams/cm3 allowing the FEs to operate at higher peak fuel temperatures (Tpeak) while
still staying safely below the melt temperature for composite fuel of ~3050 K. It also allows higher hydrogen exhaust/higher chamber inlet temperatures thereby increasing the engine’s Isp capability. Higher Isp operation can help stretch the available LH2 propellant loading to meet mission requirements, or in the
case of an emergency to allow a safe return of the crew. Engine and reactor performance characteristics for small and Pewee-class engines using composite fuel are summarized in Table 1. The reference NTR engine used in DRA 5.0 and this paper is a 25 klbf Pewee-class engine that is based on an “axial-growth” version of the SNRE. It utilizes an expander cycle and has the following nominal performance parameters: Tex ~2790 K, chamber pressure ~1000 psia, nozzle area ratio (NAR) ~300:1, and Isp~906 s. The LH2 flow rate is ~12.5 kg/s and the engine thrust-to-weight ratio is ~3.50. The
overall engine length is ~7.01 m, which includes an ~2.16 m long, retractable radiation-cooled nozzle skirt extension. The corresponding nozzle exit diameter is ~1.87 m. The higher chamber pressure helps to maintain reasonable nozzle dimensions at the assumed NAR. The engine’s reactor core contains 564 FEs and 241 TTs that are 1.32 m long like those used in Pewee. The core power level and fuel matrix power density are ~560 and ~3.44 MWt/liter, respectively. The U-235 fuel loading used in the reactor FEs is ~0.25 grams/cm3 and the inventory of 93 percent enriched U-235 in the core is just under 37 kg. At this fuel loading the engine is able to operate at a Tpeak 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]