Encyclopedia Astronautica
STCAEM NTR

STCAEM NTR
Credit: © Mark Wade

STCAEM NTR
Credit: NASA

STCAEM NTR
Credit: NASA

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

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

• 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

Thrust: 333.00 kN (74,861 lbf).
Specific impulse: 1,050 s.

• USA USA More...

• STCAEM MEV American manned Mars lander. Study 1991. The reference Mars Excursion vehicle (MEV) was a manned lander that could transport a crew of four to the surface. More...

• Manned Category of spacecraft. More...

• Mars Expeditions Since Wernher von Braun first sketched out his Marsprojekt in 1946, a succession of designs and mission profiles were seriously studied in the United States and the Soviet Union. By the late 1960's Von Braun had come to favour nuclear thermal rocket powered expeditions, while his Soviet counterpart Korolev decided that nuclear electric propulsion was the way to go. All such work stopped in both countries in the 1970's, after the cancellation of the Apollo program in the United States and the N1 booster in the Soviet Union. More...

• Mars expedition Category of spacecraft. More...

• Mars Category of spacecraft. More...

• NASA American agency overseeing development of rockets and spacecraft. National Aeronautics and Space Administration, USA, USA. More...

• Boeing American manufacturer of rockets, spacecraft, and rocket engines. Boeing Aerospace, Seattle, USA. More...

• Nuclear/LH2 Nuclear thermal engines use the heat of a nuclear reactor to heat a propellant. Although early Russian designs used ammonia or alcohol as propellant, the ideal working fluid for space applications is the liquid form of the lightest element, hydrogen. Nuclear engines would have twice the performance of conventional chemical rocket engines. Although successfully ground-tested in both Russia and America, they have never been flown due primarily to environmental and safety concerns. Liquid hydrogen was identified by all the leading rocket visionaries as the theoretically ideal rocket fuel. It had big drawbacks, however - it was highly cryogenic, and it had a very low density, making for large tanks. The United States mastered hydrogen technology for the highly classified Lockheed CL-400 Suntan reconnaissance aircraft in the mid-1950's. The technology was transferred to the Centaur rocket stage program, and by the mid-1960's the United States was flying the Centaur and Saturn upper stages using the fuel. It was adopted for the core of the space shuttle, and Centaur stages still fly today. More...

• Boeing Aerospace and Electronics, Space Transfer Concepts and Analyses for Exploration Missions, NASA Contract NAS8-37857.

• NASA Report, STCAEM Trade Study, Web Address when accessed: here.

• NASA Report, STCAEM Nuclear Thermal Rocket Variant, Web Address when accessed: here.