Class: Manned. Type: Mars Expedition. Destination: Mars. Nation: USA. Agency: NASA. Manufacturer: Boeing. The STCAEM nuclear thermal rocket (NTR) concept offered advantages of higher Isp than cryogenic concepts, fully propulsive capture at Mars and Earth to avoid high energy aerobraking, and the potential for recovery and re-use of the expensive transfer habitation system. NTR represented a proven technology; early versions were extensively tested in the 1960s and early 1970s. STCAEM (Space Transfer Concepts and Analyses for Exploration Missions) was a major NASA funded study produced by Boeing in 1991. It provided an exhaustive trade analysis of mission profiles and trajectories for manned Mars missions using four different propulsion technologies (cryogenic chemical with aerobraking, nuclear thermal, nuclear electric, and solar electric). Within each study alternate mission profiles using split/sprint missions, flyby rendezvous, and additional aerobraking were examined. Only the baseline for the nuclear thermal mission is presented here. The Nominal Mission Outline was as follows:
- The vehicle was assembled, checked out, and boarded in LEO
- The TMI burn occurs, and two empty LH2 tanks are jettisoned (opposition case)
- The MTV coasts to Mars
- MOI burns capture the MTV into Mars orbit
- Two LH22 tanks are jettisoned
- The MEV was checked out, separates from the MTV and descends
- The MEV aerobrake was jettisoned prior to final approach
- The MEV touches down, and surface operations ensue
- The MAV ascends for rendezvous with the MTV, leaving the descent stage, surface habitat, and science equipment
- The MAV was jettisoned in Mars orbit after crew transfer
- The TEl burn occurs, and the MTV coasts back to Earth
- In the expendable scenario, crew return was accomplished with modified ACRV (MCRV), MTV was jettisoned at Earth
- In the re-usable scenario, MTV captures propulsively into high parking orbit (500 km by 24 hr) for 30 day cool-down period
- Crew returns to SSF using LEV-class taxi
- Post-cool down, MTV was refurbished in Space Station Freedom orbit
Crew Systems
The crew portion of the vehicle consisted of a transfer habitat (common with other concepts), deployable PV power plant, and an MEV (common with other concepts). All habitable volumes were contiguously connected, and located at the opposite end of the vehicle from the reactors. The ends of the vehicle were separated by a lightweight truss spine.
Propulsion System
The reactor/engine was a technology-upgrade from the NERVA reactor of the 1970s. A composite shadow shield limited both direct and secondary-particle-scattered dosage to the crew and sensitive electronics. LH2 propellant was used. Four cryogenic storage drop-tanks were located on the truss. Another, in-line propellant tank was for TEI and EOI. It remained full for most of the mission provided extra radiation protection to the crew systems.
All propellant from the drop-tanks was flowed through the in-line tank, so that its supply remained relatively un-irradiated throughout the mission.
The total space vehicle mass in low earth orbit was 673,475 kg with the mass breakdown was as follows:
- Habitat module, 34,939 kg, consisting of empty mass, 28,531 kg; 5,408 kg consumables and 1,000 kg of experimental equipment
- MEV 73,118 kg
- MTV spaceframe, NTR engine systems, and radiation shield: 12,086 kg
- Trans-Mars injection propellant: 262,100 kg
- Trans-Mars injection tanks: 39,973 kg
- Mars orbit capture propellant: 138,800 kg
- Mars orbit capture tanks: 24,296 kg
- Trans-Earth injection propellant: 51,727 kg
- Earth orbit capture propellant: 24,296 kg
- EOC/TEI common tank: 23,962 kg
STCAEM NTR Mission Summary:
- Summary: Major NASA funded study produced by Boeing in 1991; focus on in-space propulsion
- Propulsion: Nuclear thermal
- Braking at Mars: propulsive
- Mission Type: opposition
- Split or All-Up: split
- ISRU: no ISRU
- Launch Year: 2016
- Crew: 4
- Mars Surface payload-metric tons: 35
- Outbound time-days: 150
- Mars Stay Time-days: 30
- Return Time-days: 240
- Total Mission Time-days: 420
- Total Payload Required in Low Earth Orbit-metric tons: 800
- Total Propellant Required-metric tons: 290
- Propellant Fraction: 0.36
- Mass per crew-metric tons: 200
- Launch Vehicle Payload to LEO-metric tons: 140
- Number of Launches Required to Assemble Payload in Low Earth Orbit: 9
- Launch Vehicle: Shuttle Z
Main Engine Thrust: 333.000 kN (74,861 lbf). Main Engine Propellants: Nuclear/LH2. Main Engine Isp: 1,050 sec. - STCAEM MEV. Class: Manned. Type: Mars Lander. Destination: Mars. Nation: USA. Agency: NASA. Manufacturer: Boeing.
The reference Mars Excursion vehicle (MEV) was a manned lander that could transport a crew of four to the surface. It consisted of a surface-stay habitat module (roughly Space Station Freedom-module size), an airlock, 5 metric tons of surface-science payload, a cryogenic ascent propulsion system with four engines and bus structure, and the ascent vehicle (MAV). The MAV ascent vehicle consisted of a short-duration crew cab and a cryogenic ascent propulsion system with two engines. All propellant tanks were mass-balanced around their maneuver CMs so that no lateral CM shifting occurs. The entire MEV was packaged in a rigid, truncated-hyperboloidal aerobrake with L/D = 0.5, to which it was attached at eight points (four bus-frame corners and four landing-gear footpads). The aerobrake was fitted with doors, which opened to allow the descent engines to extend and ignite prior to aerobrake separation (allowing full benefit of the brake's drag). The brake was then jettisoned as the landing gear extends prior to terminal approach and hovering touchdown.
Dominant configuration constraints for the MEV were as follows:
- Payload manifesting
- Surface access
- Contiguous crew volumes
- Short vehicle stack
- Engine-out capabilities
- On-orbit assembly
Payload manifesting was mainly a proximity and mass balance issue. The surface habitat and airlock, which were the bulk (80%) of the payload, required access to the ascent crew cab and the surface, as well as being mass balanced for proper flight. The science payload required surface access for ease of unloading. Docking was facilitated by placing the crew cab high in the vehicle stack. The flight deck window was located to provide viewing of the surface for landing as well as to the upper hatch for docking. Keeping crew volumes contiguous allowed access during flight for check-out procedures and simulation training. The vehicle stuck was kept as short as possible for aerobrake wake protection, which tended to conflict with having the centre of mass (CM) as high as possible, desirable for a small engine gimbal-angle to provide minimal steering loss in an engine-out scenario. A high CM within a short stack was accomplished by placing the dense ascent LOX high in the configuration. Finally, although the dominant constraints for the MEV derived from its performance at Mars, consideration had been given to its earth-to-orbit launch. It was configured to be launched in a few, large, pre-integrated systems for on-orbit assembly. For example, the ascent vehicle could be launched intact in a 10 m diameter shroud, while the descent structure could be launched in 2 sections for fairly simple on-orbit assembly and integration.
The MEV for the cryogenic/aerobrake mission had a total mass of 84,349 kg with the breakdown was as follows:
- Mars capture and descent aerobrake, 15,138 kg
- MEV Ascent stage, 22,754 kg
- MEV Descent stage, 21,457 kg
- Surface cargo, 25,000 kg
The MEV for the NTR, NEP, and SEP missions (no aerobraking into Mars orbit required) had a total mass of 73,118 kg with the breakdown was as follows:
- Mars descent aerobrake, 7,000 kg
- MEV Ascent stage, 22,464 kg
- MEV Descent stage, 18,659 kg
- Surface cargo, 25,000 kg
Bibliography and Further Reading
- Boeing Aerospace and Electronics, Space Transfer Concepts and Analyses for Exploration Missions, NASA Contract NAS8-37857.
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