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STCAEM SEP

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STSCAEM SEP Credit: © Mark Wade

Status: Study 1991. Specific impulse: 5,500 s.

It offered advantages of the lowest IMLEO of the four reference vehicles; a reusable, extremely high Isp (5,000 sec) system; a fully propulsive capture at Mars and Earth which avoided the need for high energy aerobraking; good mission flexibility (relative insensitivity to mission opportunity, capture orbit astrodynamics, or changes in payload mass); and low resupply mass (the argon propellant required amounted to roughly a third of total vehicle mass). Disadvantages included uncertainty of the cost of production of acres of solar arrays, and the need to deploy and control a relatively fragile vehicle, which was bigger than six football fields, in space.

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 solar electric mission is presented here.

Nominal Mission Outline

• The SEP vehicle was assembled and checked out in LEO
• TMI was a slow spiral out of Earth's gravity well
• Just prior to Earth escape, the crew transferred onboard using an LTV
• Thrust continued throughout the interplanetary transfer, first accelerating relative to Earth and then decelerating relative to Mars, except for a 45 - 60 day no thrust hiatus en route.
• MTV flew by Mars with low relative encounter velocity
• MEV separated from MTV for aeroentry
• MEV descended to surface, jettisoning aerobrake prior to landing. Surface operations ensued.
• MTV continued decelerating into loosely captured, highly elliptical orbit
• Ascent vehicle left descent stage and surface payload on surface
• MAV rendezvous occurred at MTV periapsis; docking and crew transfer
• MAV jettisoned in Mars orbit
• Reversal of interplanetary acceleration / coast / deceleration sequence
• Crew departed MTV for direct entry at Earth
• MTV spiraled back to LEO for refurbishment (optional loose capture at L2 was attractive, if refurbishment infrastructure was available there and if resupply trips from LEO use EP or beamed power propulsion for high efficiency)

Vehicle Systems

Primary vehicle systems were: power plant; main propulsion; vehicle bus; and crew systems.

The power plant consisted primarily of a field of solar arrays kept normal to the sun line at all times. The solar array area required to produce 10 MWe of power was 35,000 m2 and was maintained sufficiently rigid and in position by a deployable area truss (spaceframe) one bay deep. Details of deployment of the lightweight solar cell blankets across the structure were not worked out in the study.

The propulsion system included engine assembly, propellant storage subsystem, and plumbing components, split into two identical modules located at distal ends of the vehicle bus. Each engine assembly had five individual ion thrusters (the total of ten included two spares) in a 5 x 8 rectangular array. Each thruster was 1 m wide by 5 m long; beam neutralizers were located between the thrusters. The argon propellant was stored cryogenically in insulated, spherical tanks, mounted on the forward sides of the engine assemblies via structural and fluid quick-disconnects. Including tanks, the propellant storage system massed about 35% overall vehicle IMLEO. This relatively low propellant mass was a strong resupply advantage.

Thrust loads were extremely low for the electric propulsion (EP) system. Probable maximum loading was from impulses like attitude control system (ACS) firings, berthing operations, and construction and maintenance activity. The primary vehicle bus structure had two components: the area mass covered by the solar array field, and truss outriggers extending sufficiently far beyond the edge of the solar array that the ion engine plumes did not impinge on, and therefore erode, the power system. The crew systems were attached to the underbelly of the area truss (in the center for mass balance). Two communications satellites were also attached to the truss near the crew systems, to be deployed in Mars orbit for maintaining communication with Earth. Also mounted to the truss near the habitation system were thermal radiators for the power conditioning equipment.

The crew systems consisted of a long duration transit habitat and one or more MEVs (the reference design had one MEV). All habitable volumes were contiguous throughout each mission. Electric propulsion had the least sensitivity to increased payload mass, so an important option was provision for multiple MEVs. A multiple docking adapter would allow several MEVs to be used without altering the vehicle configuration (additional propellant tanks would be required).

SEP Mass Breakdown

Payload

• Descent aerobrake: 7.0 metric tons
• MEV Descent stage: 18.7 metric tons
• MEV ascent stage: 22.5 metric tons
• Surface equipment: 25.0 metric tons
• Transit hab module: 44.3 metric tons

Propulsion

• Thruster assembly: 8.0 metric tons
• Power management and distribution: 20.0 metric tons

Solar array blanket

• Photovoltaic cell: 840 mg/cell x 23,464,336 cells = 19.7 metric tons
• Reinforced SiO2:211 mg/cell x 23,464,336 cells = 5.0 metric tons
• Adhesive (fiber/cell): 60 mg/cell x 23,464,336 cells = 1.4 metric tons
• Kevlar support structure: 68 mg/cell x 23,464,336 cells = 1.6 metric tons

Structure

• Graphite epoxy struts (6,728): 7.7 metric tons
• Nodes (1,741): 2.4 metric tons
• Aluminum cladding (0.051 mm): 1.2 metric tons

Utilities

• Communications: 0.6 metric tons
• Attitude control: 5.7 metric tons
• Avionics: 2.5 metric tons
• Housekeeping power distribution: 0.5 metric tons
• FV/RFC power subsystem: 2.0 metric tons
• Robotics: 3.6 metric tons

Propellant system

• Tanks: 3.8 metric tons
• Feed lines: 0.1 metric tons
• Propellant: 190.0 metric tons

Subtotal: 393.3 metric tons

• 10% Growth (structure and array): 3.9 metric tons