Space Habitat, Assembly and Repair Center - Abstract
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Space Habitat, Assembly and Repair Center
Todd Colangelo, et. all
May 15, 1992
Integrated Space Systems (ISS) has taken on the task of designing a Space Habitat, Assembly and Repair Center (SHARC) in Low Earth Orbit to meet the future needs of the space program. Our goal is to meet the general requirements given by the 1991/1992 AIAA/LORAL Team Space Design competition with an emphasis on minimizing the costs of such a design. This semester, we have created a baseline structural configuration along with preliminary designs of the major subsystems.
Assumptions and Requirements
Our initial mission requirements, which were set by AIAA, were that the facility be able to:
- Support simultaneous assembly of three major vehicles
- Conduct assembly operations with minimal EVA
- Maintain orbit indefinitely
- Assemble components 30' long with a 10' diameter in a shirt sleeve environment
Our group also made several assumptions to further refine the mission parameters:
With these assumptions in mind, we began conceptual designs of SHARC's baseline configuration.
- "Three major vehicles" were defined as two lunar vehicles and one Mars vehicle. For relative sizes, see Table A.
- SHARC must begin limited operations after eight launches.
- No HLLV of Shuttle-C will be available.
- The maximum crew size is eight and the maximum work tour is 35 days.
- A garbage collection system will be available to deal with orbital debris.
Table A: Interplanetary Vehicle Sizes
Twelve different conceptual designs were reviewed (see Appendix B) using a decision matrix. The designs we looked at were versatile enough to accommodate most of the different subsystem concepts we considered. Our chosen design is called the Hammerhead II.
The Hammerhead II configuration, shown in Figure A, will be composed of two 35' x 200' double deployable trusses separated by four 35' erectable trusses. There are two smaller bays for lunar vehicles and one large bay for assembling the Phobos and Mars Transfer vehicles. A track system mounted by remote manipulator arms will encircle each bay allowing the arms to assist in vehicle assembly, hence minimizing EVA. There will be a total of seven robotic arms to help in vehicle assembly: one 30 ft arm for each lunar bay, two 30 ft arms for the Mars bay, one 30 ft arm for storage of parts, and two 60 ft arms located on the sides of the main deployable trusses for berthing and transporting payloads.
A general storage area is located in the 21'x50'x35' area between the two double fold deployable trusses, making it easily accessible to all assembly bays. An alternate storage area is located on the double fold deployable truss leading out to the solar arrays, which is accessible by a robotic arm. The spring-loaded 31'x14' diameter Phobos fuel tanks will be located near the Mars bay ready to be jettisoned for safety.
The emergency escape pod will be located in the center of the four habitation and control modules and will be accessible from two pressurize corridors for quick use. The modules are arranged in a racetrack configuration to provide dual egress in case of emergencies. The two control modules will contain windows which will overlook the lunar bays to help in vehicle assembly and payload berthing.
The eight sets of solar arrays and the battery system are located at the end of the double fold deployable truss. The 40x20 ft pressurized sleeve, which is attached to the airlock, can contain a 30x10 ft component and is accessible to the robotic arms. Finally, the shuttle will dock upside down to the remaining airlock. This provides plenty of clearance for docking, and the Shuttle can be rigidly connected to the double fold deployable truss through attachment points in the Shuttle payload bay.
Figure A Hammerhead II Configuration
Orbit and Altitude
We determined that the orbit of SHARC should be at an inclination of 28.5o and altitude of 380 km. This altitude is accessible to all current medium and heavy lift launch vehicles in use with only minor reductions in payload capacity. The inclination angle was chosen because it provides an ideal transportation node for future Mars and Lunar exploration missions. This inclination can also be reached by rockets from both the Kennedy Space Center and Kourou. We determined the Ballistic numbers of SHARC using a simplified model and then recalculated the results using a much more accurate model. We also considered utilizing the Space Station Freedom as a habitation depot for the workers at SHARC but calculations showed that the synodic period of the two facilities was 14.5 days.
Crew and Life Support
A work tour on SHARC will consist of a maximum crew of eight over a period of 35 days. The shuttle will stay docked at SHARC for the full duration of the mission. Each astronaut would work for 8 hours per day, 6 days a week. Life support supplies would be carried on the Shuttle, with any assembly materials carried on an unmanned vehicle which would be launched from 3 to 10 days after the Shuttle.
The Crew and Life Support group performed sizing estimates for a closed-loop life support system involving full air and water recycling. Further calculations were made involving specific supply requirements. Preliminary estimates reveal that 147 kg of nitrogen gas and 343 kg of food will be required for each work tour. 107 kg of methane and 183 kg of solid waste matter will be generated during the work tour and will have to be removed.
The amount of power required to run SHARC was determined by compiling the amount of power required by each subsystem, along with estimated values for special items such as exterior flood-lighting for bays, robotics, power tools, and EVA. This method resulted in a power requirement of 62 kilowatts. Assuming 10% line losses, the total power required was 68 kW.
Photovoltaic silicon solar arrays were chosen as the primary power system. From several calculations it was determined that a total area of 1854 m2 was required to provide the 68 kW of power. The arrays are arranged as eight pairs of fold-out panels which deploy along an erectable mast or boom for stability. The total mass of the arrays is 2267 kg and have a calculated lifetime of 10 years after which they will have experienced approximately 25% degradation in efficiency.
The storage system chosen to power SHARC during eclipse periods were 27 Nickel-Hydrogen (Ni-H2) individual pressure-vessel batteries connected in parallel for increased capacity and redundancy. The batteries are arranged together in groups and are placed in thermally controlled cases for optimum performance. The cases are placed between the two large sets of solar arrays. Each Ni-H2 battery has a capacity of 100 amp-hours, an energy density of 25 W-hr/kg, and a mass of 112 kg. The total mass of the battery system (not including wiring) is approximately 3024 kg. For a worst case scenario, the batteries have a lifetime of 2 years if they are required to generate continuous peak power. Using a more probable average power of 48 kW, the lifetime increases to 5 to 6 years. After this time, the batteries will experience significant degrading and must be replaced.
The construction and operation of SHARC will require the extensive use of robotics. The need for robotics stems from the hazardous nature that long-term EVA operations would present to astronauts and the need to relieve crew work loads. In addition, SHARC's main purpose of servicing space vehicles necessitates the use of robotics.
Two principal robotic systems were selected for use on SHARC: a remote manipulator system (RMS) and flight telerobotic servicer (FTS). These two systems are advanced versions of the ones to be used on Space Station Freedom. The use of robotic systems like these would reduce the uncertainties and costs in building SHARC.
On SHARC, the primary function of the RMS will be to capture and move large cargo and parts of spacecraft to be assembled around the service area. Then the FTS will attach itself, or be transported by the RMS, to the work site and proceed to work on light, precision assembly tasks. The FTS will also be able to examine the structural elements of SHARC for maintenance purposes.
The GNC/Reboost subsystem determined the propulsion requirements of SHARC during operation in space. Based upon our drag model, the propulsion system must be able to reboost SHARC from an altitude of 364 km to 380 km every two months. The total required DV was found to be 9.107 m/s. In addition, SHARC will be rotated 90 degrees during reboost periods, and there will be enough propellant stored to allow one additional reboost without re-supply. The location of the attitude thrusters and the reboost thrusters is shown in Figure A.
Propellants were compared on the basis of specific impulse and storage requirements. Hydrazine (N2H4) was selected for standard attitude control, while the reboost thrusters will use an OME/UR bipropellant (N2O4/MMH) rocket produced by Aerojet.
The communications subgroup used existing SSF information as a basis for choosing the communication system for SHARC. Communications will be separated into a local system and a space to ground system. The local system will consist of an optical network because of its low power requirements and higher efficiency. The maximum data rate for the local system is 10 Mbps (Mega-bits per second) with the option of using point to point fiber optics for a maximum data rate of 100 Mbps.
The space to ground system will consist of two virtual channels operating at a data rate of 150 Mbps. The frequency will be in the range of approximately two gigahertz to overcome any atmospheric or noise attenuation. The data will be transmitted to the Tracking and Data Relay Satellite System (TDRSS) and then to the Data Interface Facility which will allocate the data to the appropriate users. This link design will maintain continuous contact with the ground stations so that tracking and telemetry can be monitored.
The first objective of the thermal control group was to identify the different station elements that have specific temperature limits. After these temperature limits were determined, various passive thermal measures were studied to determine if they would be adequate by themselves. This proved true in the case of the cryogenic fuel tanks. For the rest of the station, we estimate that a peak load of 60 kW of waste heat must be dissipated. An active thermal control system was designed using Freon-12 as a working fluid. A radiator panel 35' x 20' was found to be adequate for our needs.
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