Summary and Conclusions

In NASA’s recent DRA 5.0 study (NASA/SP—2009-566), the fast conjunction long surface stay mission was identified as the preferred approach for exploring Mars. Plenty of time for scientific investigation and study, coupled with short ~6-month transit times to reduce crew exposure to the 0-gE and radiation environments of space were key reasons for this selection. Similarly, NTP was identified as the preferred propulsion option because of its high performance (100 percent higher Isp than LOX/LH2
chemical propulsion), its demonstrated technology record, plus the strong synergy that exists between it and chemical propulsion systems and stages. Recent measurements of the energetic particle radiation environment inside MSL spacecraft during it journey out to Mars indicate that astronauts could receive a radiation dose of ~0.66 Sv (~66 rem)—the limiting value established by NASA—during a 1-year journey out to Mars and back. With the potential for the crew to receive additional dose during the exploration phase of the mission, several questions immediately arise: Can NTP’s performance capability be used to reduce transit times further and by how much? Also, what are the engine operating requirements and vehicle design impacts? The answer to the first question is yes, and analysis presented here indicates transit time reductions as much as 50 percent are possible. The key requirements are more propellant and reduced U-235 loading in the engine’s reactor fuel elements to allow higher operating temperatures. For the 2033 mission opportunity baselined in this paper, Copernicus can achieve 1-way transit times of ~150 days when filled to its maximum propellant capacity of ~190 t. To shorten transit times further, the Copernicus spacecraft can be reconfigured in incremental steps that allow it to carry larger quantities of propellant needed for these more demanding missions.

To reduce the 1-way transit times from 150 to 130 days, an in-line tank is inserted between Copernicus’ NTPS and saddle truss/drop tank assembly that increases the vehicle’ propellant capacity by ~42 percent to ~270 t. A fourth engine is also added to the NTPS to reduce the total engine operating time and minimize g-losses. To achieve transit times in the range of 120 to 100 days, Copernicus’ saddle truss/drop tank assembly is replaced by a 4-sided star truss to which paired sets of LH2 drop tanks are attached as needed. For 120day transits to Mars and back, twin drop tanks—each carrying ~80 t of propellant—are used. Further reductions in transit times from 120 to 100 days requires an ~42 percent increase in total mission V— from ~8.83 to ~12.5 km/s. To compensate, the crew size is reduced to four, composite propellant tanks are utilized and the engines are operated at their maximum temperature capability to achieve a specific impulse of ~940 s. Four drop tanks—each carrying ~82 t of propellant—are also used. The “all up” vehicles for 120- and 100-day cases require 5 and 7 HLVs respectively, as well as on-orbit assembly. The engine operational requirements for the 100-day case also exceed those demonstrated in the NERVA program. A “split crewed mission architecture” is outlined that uses the Copernicus-2 spacecraft configuration plus an ERV. It can reduce vehicle size and complexity, eliminate the need for orbital assembly and an extra engine, and can achieve transit times as low as 90 days. Finally, it is important to note that both the total and longest single engine burn times required for the split mission, while substantial, are still below the capabilities demonstrated by the NRX-XE (~2 hr) and NRX-A6 engines (~62 min).