Bimodal NTR and LANTR propulsion for human missions to Mars/Phobos

The nuclear thermal rocket (NTR) is one of the leading propulsion options for future human missions to Mars due to its high specific impulse (Isp similar to 850-1000 s) and attractive engine thrust-to-weight ratio (similar to 3-10). Because only a miniscule amount of enriched uranium-235 fuel is consumed in a NTR during the primary propulsion maneuvers of a typical Mars mission, engines configured for both propulsive thrust and modest power generation (referred to as "bimodal" operation) provide the basis for a robust, "power-rich" stage enabling propulsive Mars capture and reuse capability. A family of modular "bimodal" NTR (BNTR) vehicles are described which utilize a common "core" stage powered by three 66.7 kN (similar to 15 klbf) BNTRs that produce 50 kWe of total electrical power for crew life support, an active refrigeration / reliquification system for long term, "zero-boiloff" liquid hydrogen (LH2) storage, and high data rate communications. Compared to other propulsion options, a Mars mission architecture using BNTR transfer vehicles requires fewer transportation system elements which reduces mission mass, cost and risk because of simplified space operations. For difficult Mars options, such as a Phobos rendezvous and sample return mission, volume (not mass) constraints limit the performance of the "all LH2" BNTR stage. The use of "LOX-augmented" NTR (LANTR) engines, operating at a modest oxygen-to-hydrogen (O/H) mixture ratio (MR) of 0.5, helps to increase "bulk" propellant density and total thrust during the trans-Mars injection (TMI) burn. On all subsequent burns, the bimodal LANTR engines operate on LH2 only (MR = 0) to maximize vehicle performance while staying within the mass limits of two similar to 80 t "Magnum" heavy lift launch vehicles (HLLVs).