A special tool will be required to release and support the main engine during removal and installation activities. This tool should be adaptable for either robotic or EVA operation. Main engine replacement can be accomplished in approximately 5. This projected time is supported by data received from Rocketdyne and Pratt and Whitney regarding the anticipated removal and replacement of their engines on-orbit. In comparison, EVA operations to perform this activity would require approximately 13 man-hours to accomplish.
If it is determined that the on-orbit removal of the turbopumps is cost effective and desirable during engine replacement, then an additional 4. This will result in an expenditure of approximately hours two turbopumps to complete the entire operation. On the opposite side of the interface plate to the probes are mounted the engine gimbal and its two gimbal actuators. This enables the engine to be installed just like a plug-in module. When fully engaged, both poppets fully open and the nmha fnH i? The probe side structurally attaches to the engine, tank, or aerobrake ACS system.
The configuration shown would only be for propellant tanks as the engine 2 EM!
Multiple engines provide increased reliability and improve man rating potential. No aerobrake doors are required. Expendable vs. Number of Elements In System 2. Built In Test vs. Aerobrake Assembly vs Deployable 2. A Level II directive BBA has been recently issued, changing the previous requirement of 10 pg in the laboratory modules.
The two dimensional constraints are in the Y or latitudinal dimension and the Z or radial dimension of the Station configuration. The boom dimensions are governed by the physical space available on the main truss structure as well as constraints in station controllability which govern the extent to which the truss can grow downward. As depicted on the figure, the maximum dimension the enclosure can grow along the Y axis is 35 meters. Thus the maximum STV diameter within the enclosure will be meters, depending on safety factors. In the Z dimension, the limit, as shown, has two components.
Forward of the lower keel truss structure plane, the maximum enclosure growth limit is Aft of the truss structure plane, the limit is relaxed to This configuration is from the November NASA day study on Human Exploration, which recommended the addition of a lower keel to support lunar operations. The addition of a lower keel will significantly improve the pitch attitude. Required momentum storage capacity is a function of many variables, including specific configuration and momentum management scheme during flight. Analysis using a momentum-management simulation indicates that increased STV mass will have low impact oh Station control.
Required momentum storage capacity initially increases, then is reduced for higher-mass STVs, when the aerodynamic torque effects are offset by the large gravity gradient torque gains. The maximum momentum storage requirements can most likely be met by the addition of two or three CMGs over the range of STV mass to be supported on a lower keel. This chart compares the propellant required for a low-mass STV based on the main truss as an attached payload with a large-mass STV supported on a lower keel. The addition of the lower keel and servicing enclosure increases Station propellant use by about lb Hydrazine.
After this initial increase, the entire range of STV mass will not require more than one additional propulsion module lb Hydrazine for the low solar cycle year.
Yearly required reboost Hydrazine is shown for both low and high solar cycle years over the range of STV mass on a lower keel. The high solar cycle year is the worst-case for reboost requirements and will require up to two additional propulsion modules over the STV mass range.
As the frontal area of the enclosure grows, the drag coefficient increases, and extra propellant must be provided to the Space Station for altitude maintenance. The Space Station Freedom reboost propulsion system is based on a monopropellant hydrazine system that is resupplied by propellant modules which contain lb each. Four of these pallets per year are planned for delivery to the Station. As can be seen on the left hand chart, even when the enclosure reaches its maximum size of 35x35 m, less than one additional propellant module would be needed in a high solar cycle yea.
This is when reboost requirements are at a maximum due to atmospheric expansion. As the enclosure size grows, added drag and mass cause the Station center of gravity and microgravity ellipses to move lower relative to the experiment module section.
This movement, less than three meters from minimum to maximum enclosure size, can be considered of a minimum impact. This cantilever effect has implications to Station flight dynamics and control which cannot be predicted at this time.
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There will be an uncertainty in STS position as it moves along its approach path which may lower the Z dimension growth limit below Additionally, there is a safety requirement for STS rendezvous which requires that all potential impact points be visible to the STS crew. Any size STV enclosure will violate this requirement, so operational procedures will have to be addressed. This sequence may have unforeseen effects due to plume impingement, and resulting overpressure, on the STV enclosure walls.
This will undoubtedly be dependent on STV enclosure size. Finally, contingency departure paths for a shuttle whose Station docking maneuver has been aborted have not been determined, but will be restricted by enclosure size growth. Two final issues involve Space Station payload operations.
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Downward viewing payloads on the horizontal truss will have their field of view blocked by the presence of the enclosure. Relocating them to the truss structure below the STV enclosure is one solution, but many operational issues still remain. A payload element to be supplied by the European Space Station partners is a man-tended free flyer which will be serviced at the Station on a regular basis to be determined Its approach path, and its docking point have yet to be determined, but lower node locations are the preferred option for this operation, and this may impact Z dimension growth limits.
These devices will have to be defined more clearly so that their functions and operational complexity may be better determined. With regards to current SSF mechanical devices that can be adapted to the STV program such as the space station remote manipulator system SSRMS , the STS docking adapter, and the SSF capture latches, more analysis will have to be performed to determine the degree to which these satisfy the STV mission without modification, and what modifications would have to be made to completely satisfy STV operations.
Additional power must be supplied to perform these servicing operations, and to operate STV systems during checkout.
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Additional thermal control will have to be provided for this extra power, and as is seen earlier, the provision for this growth still has to be incorporated into the Space Station design. The majority of servicing operations, such as aerobrake assembly, STV component connection and propellant tank handling will be growth impacts on the Assembly Complete Space Station. However, once the impacts are incorporated into the Station, the growth systems show little sensitivity to variations in the STV systems. Station flight control attitude remains within baseline requirements.
The original Station microgravity requirement of 10 pg is satisfied for all foreseen STV masses, while the new 1 pg requirement is never satisfied with a lower keel enclosure. Thus there is no benefit of STV mass targets. Size growth can be accommodated for all projected STV configurations, and altitude reboost logistics has only minor changes with STV size growth. The current array of Station mechanical devices will be usable for STV components, especially the Mobile Servicing Center, which is the key to Space Station operational flexibility. Finally, additional power must be provided to service the STV, but all foreseen power levels can be incorporated by adding photovoltaic fli solar dynamic arrays.
Additionally, procedures, tools and techniques could be developed and evaluated and demonstrations performed of propellant transfer and storage, adequacy of meteoroid and debris shielding, traffic control, communications, and STV utilization.
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Areas Key technologies were identified which require development for eight major STV systems. Six of the enabling technology areas are common to the eight systems and are shown in the center of the figure. All eight systems require enabling technologies that affect performance, however, technologies affecting performance are generally different for each system. Five of the STV systems also have enabling technologies which affect materials and structure, while all eight have two or more technology areas that are unique to that particular system and are listed under the individual technology heading.
The mission that was used for reference is concept 4E-2B which is similar to the 90 Day Study baseline. While some of the other architectural concepts may reduce this listing somewhat, this listing is believed to be more representative of those technologies that will cover almost all of the concepts that may be selected. The technologies are divided into groups which support each mission phase, with some duplication occurring where a single technology such as propellant settling spans multiple phases.
The SEI Option 5 program milestones are defined at the top of the schedule. BY: H. Several of the items are not truly separate but are different aspects of the overall vehicle-engine system. For example, space basing requires efficient vehicle turn around operations to accomplish mission goals at reasonable cost. The task at hand Is to convert these guidelines into mission, vehicle and engine requirements.
Safe return of the crew after any two failures has been interpreted as a requirement on the total vehicle which may result in unconventional approaches to engine interfaces and fault isolation. Trade studies must be conducted in parallel with evolution of the vehicle configuration to establish the approach to be used. For example, containment of a failed turbopump could be accomplished by the engine hardware or protective barriers could be provided between adjacent engines.
Special tests should be conducted to validate safety related redundancies, fault Isolation and containment of fragmented components. Testing with the engine mated to a simulated vehicle propellant system Is required to explore engine system dynamics and and interactions.
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The flight test program will evaluate engine start and autogenous tank pressurization In the same low acceleration space environment as the fully operational manned missions. The two main issues for space basing are materials compatibility and design of the engine and vehicle Interfaces for minimum maintenance. The high costs emphasize the need for highly reliable systems which will require little or no maintenance over the life of the vehicle.
The reliability of the functional hardware must be supported by comprehensive instrumentation to verify the status and confirm that reliability has not been degraded over the life of the vehicle. Redundant instrumentation with additional verification by cross referencing related measurements will be required to assure that health of the hardware is correctly diagnosed.
It is likely that each engine will have a health monitoring capability as part of the electronic engine controller. The same data used by the engines will be evaluated and stored by the vehicle health management computer and data storage system. The vehicle system will have complete historical data records for each engine to support diagnostic functions and develop recommended engine operating strategies to satisfy vehicle propulsion requirements. Vehicle health management system recommendations will be provided to the flight controls computer where they may be overridden by the pilot if necessary during critical maneuvers.
The health management system will use vehicle propellant system data and thrust commands as well as the engines components data to evaluate the engines status and ability to continue to function.