the large V requirements for the primary propulsion maneuvers—trans-Mars injection (TMI), Mars orbit capture (MOC), and trans-Earth injection (TEI)—smaller V maneuvers are needed for rendezvous and docking (R&D) of vehicle components during the LEO assembly phase, for spacecraft attitude control during inspace transit, and for “split crewed mission” orbital operations at Mars.]

[Table: TABLE 4.—MISSION AND PAYLOAD GROUND RULES AND ASSUMPTIONS
TABLE 5.—NTR TRANSPORTATION SYSTEM GROUND RULES AND ASSUMPTIONS]

For the crewed mission, the outbound payload mass varies with the crew size and mission duration. For long surface stay Mars missions, the crewed MTV carries contingency consumables equivalent to that found on the habitat lander. This allows the crew MTV to function as an orbital “safe haven” in the event of a major failure of a key surface system. For the nominal surface mission, the contingency consumables are jettisoned prior to the TEI maneuver. Fixed mass payload elements include the Orion Multi-Purpose Crew Vehicle (MPCV). For Mars DRA 5.0 and in this analysis it is assumed that ~250 to 500 kg of samples are returned. Table 5 lists the key transportation system ground rules and assumptions. The NTR engine and fuel type, operating characteristics, and thrust levels examined are summarized first. All engines use composite fuel with a U-235 fuel loading of 0.25 g/cm3. With Tex ~2790 K and NAR~300:1, the Isp is ~906 s with higher Isp values achievable by increasing the fuel operating temperature. The total mission LH2 propellant loading consists of the usable propellant plus performance reserve, post-burn engine cooldown, and tank-trapped residuals. For the smaller auxiliary maneuvers, a storable bipropellant RCS system is used. All transfer vehicle configurations utilize a “split RCS” with 16 of 32 AMBR thrusters and approximately half of the bipropellant mass located on the rear propulsion stage and the forward most saddle truss adaptor ring just behind the TransHab module. The LH2 propellant carried by the various vehicles is stored in the same “state-of-the-art” Al/Li LH2
propellant tank being developed for the SLS/HLV that will support future human exploration missions. For this analysis, tank sizing assumes a 30 psi ullage pressure, 5 gE axial/2.5 gE lateral launch loads, and a
safety factor of 1.5. A 3 percent ullage factor is also assumed. All tanks use a combination foam/ multilayer insulation (MLI) system for passive thermal protection. A zero boil-off (ZBO) “reverse turbo-Brayton” cryocooler system is used on the core NTPS and “in-line” LH2 tanks (where required) to eliminate boil-off during LEO assembly and the remainder of the mission which can be as long as 950 days in duration. The propellant tank heat load is largest in LEO and sizes the ZBO cryocooler system. Solar photovoltaic arrays are baselined for supplying the primary electrical power needed for all key transfer vehicle subsystems. Because of the decreased solar radiation at Mars (~486 W/m2), array areas can become quite large (~10 m2/kWe) necessitating multiple arrays for human Mars missions.

[Table: Table 5 also includes the “dry weight contingency” (DWC) factors, along with the lift and payload envelope requirements for the “upgraded” SLS/HLV. A 30 percent DWC is used on the NTR system and advanced composite structures (e.g., stage adaptors, trusses) and 15 percent on heritage systems (e.g., Al/Li tanks, RCS, etc.). The NTPS and crewed payload element (PL) determine the required lift and PL envelope. The optimum tank diameter and mass for a Mars NTPS are 10 m and ~140 t (Ref. 9). The crewed PL element includes the “packaged” TransHab module with PVA power system, the short saddle truss, consumables container and transfer tunnel with secondary docking module (DM), and the Orion MPCV (shown Figs. 2 and 6). The PL envelope’s diameter is ~11 m (the saddle truss outer dimension) and its length can be up to ~33.8 m (the DRA 5.0 PL length including the Orion MPCV).]