Figure 7.22. Fuel element structure and assembly inside a MITEE reactor (MITEE http:// www.newworlds.com/mitee.html ).
Outwardly similar to conventional NTR (see Figure 7.21) , MITEE designs use fuel elements where hydrogen propellant flows radially inwards, crossing the metal matrix composite encapsulating the fissioning fuel, as shown in Figure 7.22. This flow topology produces a compact NTR. While most initial MITEE designs used only 235U as fuel, recent MITEE proposals include also 233U and 242mAm, since these materials produce even more compact engines (242mAm has a critical mass about a hundred times less than that of uranium). Published estimates of engine size and mass are surprising: total engine mass (using 235U) 200 kg for a 75-MW NTR, with Isp = 1,000 to 1,250 s for the combined cycle described below (and assuming realistically that nozzle expansion is frozen) and a thrust of order 1.4 x 104N. The engine mass is estimated to drop to 100 kg replacing 235U with the much scarcer 242mAm metastable isotope [Powell et al., 1998, 1999, 2004; Maise et al., 2000]. A recent MITEE NTR design is claimed to be capable of Isp about 1,000 s (based upon a hydrogen exit temperature 3,000 K), overall weight 140 kg, total one-time burn of several hours, with engine diameter 50 cm corresponding to a power density of order 10 MW/liter. Figure 7.23 shows a comparison between a hypothetical MITEE-class nuclear rocket and some typical chemical rockets for interplanetary missions already proposed or considered.