Randolph Ware (1), Phil Culbertson (2)
Just as Skylab was derived from parts of the Saturn rocket, a capable cost-effective space station can be constructed from the Space Transportation System (STS). A year ago NASA was actively seeking lower cost, alternative space station designs. This paper describes an interim space station concept proposed to NASA at that time.
Modified Orbiter and ET
The STS-Lab consists of two major elements: a modified Orbiter (called Orbiter-II) with a long, permanently installed laboratory in the payload bay, and a standard External Tank, in which the Intertank volume has been outfitted with docking ports and pressurized access tunnels connecting to Orbiter-II.
STS-Lab is placed in orbit with a single unmanned launch. Orbiter-II remains permanently attached to the External Tank (ET), and the combined system is launched into a circular orbit of 350 to 400 km. Propellants will be utilized as fully as possible to reduce orbital drag and minimize altitude reboost requirements. Orbiter-II Orbital Maneuvering System (OMS) or Reaction Control System (RCS) engines will be used for reboost and station keeping. Orbiter-II has been modified to remove systems required only for manned ascent and for reentry and landing, such as wings, tail, body flap, thermal protection tiles, landing gear, some avionics, some crew related controls, displays, and hardware. Weight reductions of 30,000 to 40,000 pounds (out of a standard Orbiter weight of roughly 160,000 pounds) will result from these deletions, and will be available for increased effective payload and performance.
The Orbiter-II Cargo Bay will contain a laboratory module such as Spacelab, an elongated version of SpaceHab, or one of the lab modules currently under development for Space Station Freedom (SSF). All equipment is fully integrated, installed and checked out on the ground before launch. The lab module can be heavier and longer than is possible in a standard Shuttle Spacelab, since a center-of-gravity limitation at landing does not apply to the STS-Lab. The lab module can be outfitted for microgravity, life sciences, or other research. Also in the Cargo Bay is a pallet-mounted Solar Powered Extended Duration Orbiter (SPEDO) package, a proprietary design of Teledyne-Brown Engineering and AEC-Able Engineering. SPEDO will provide about 18 kW of continuous power to an Orbiter or to STS-Lab.
Many other existing systems and capabilities of the Shuttle are fully utilized in Orbiter II. The solid amine carbon dioxide removal system and the environmental control system can be retained or modified to improve operational efficiency. Most of the electrical power distribution and the data management systems can be used. The Shuttle airlock is retained as either the prime or back-up mode of performing EVA, along with all of the necessary pressure suit and communication functions. The TDRS antenna is used for wide-band communication and the radiators mounted on the Cargo Bay doors provide heat rejection capability. The existing Remote Manipulator System (RMS) will be employed in berthing other modules and pallets brought up by the visiting Orbiter, and for maintenance and repair tasks.
Modified ET Intertank
The basic ET Intertank contains about 5,000 cubic feet of unused volume and is vented to the atmosphere, reaching near vacuum as the Shuttle reaches orbital altitude. For STS-Lab, it is intended that the Intertank volume be outfitted with three docking ports interconnected by pressure tunnels. This tunnel system also provides access to the Orbiter-II crew compartment. The oxygen and hydrogen tank domes extend into the central region of the Intertank and are not designed to accept an inward differential pressure. Therefore, the pressure tunnels will be designed to allow the dome surfaces and outer cylindrical hull to remain unpressurized. During launch the pressure tunnels will be vented to avoid accidental back pressure to the existing tank surfaces. After reaching orbit, the vents are closed and the tunnels repressurized.
A very important function of the Intertank is to provide docking ports for a visiting Orbiter (as shown in the figure), carrying crew, supplies, and perhaps additional laboratory or habitation modules or unpressurized payload pallets to be attached to Orbiter-II or its ET. Payloads would be transferred by the visiting Orbiter RMS to attach points on STS-Lab or handed off to the Orbiter-II RMS. Docking ports on the Intertank could accept either, or both, the Japanese Experiment Module (JEM) or ESA’s Attached Pressurized Module (APM). At another port, a Crew Emergency Return Vehicle (CERV) or Soyuz crew return vehicle could be berthed.
SPEDO provides electrical power for STS-Lab and augments that required for the visiting Orbiter. It is a folded photovoltaic array, mounted in the Orbiter-II Cargo Bay. Upon reaching orbit, the Cargo Bay doors are opened just as they are on Orbiter flights. Booms are extended along the y-y axis for the distance necessary to clear the Orbiter-II Cargo Bay doors and the ET. The panels are then unfolded in a direction normal to the y-y axis. The arrays are maintained in an attitude normal to the sun while the STS-Lab is maintained in a “local vertical” attitude. The panels are sized for 30 kW peak power, which is adequate with battery storage to permit an average of 18 kW to be delivered to STS-Lab. The pallet also provides for voltage regulation to the Orbiter-II power system. Heat rejection is provided either passively or with interconnection to the Orbiter-II radiators. A fixed axis (y-y) variable speed momentum wheel is used to control vehicle pitch attitude without use of the Orbiter Vernier Reaction Control System (VRCS). The SPEDO pallet is designed for emergency separation from the Orbiter-II. It can, therefore, be readily replaced in case of failure or degradation with time on orbit when used with STS-Lab.
The STS-Lab is designed to fly optimally in an attitude in which its coordinates remain fixed with respect to the local zenith direction (the same as SSF). This allows the laboratory to fly with the long axis nearly horizontal and not far above or below the STS- Lab center of mass. This is nearly optimum for the conduct of microgravity experiments. In addition, the solar panel booms will rotate at the orbital rate (about 0.06 degrees per second) and tilt to either side to face the sun directly (again, similar to SSF). However, in the SPEDO design, flexible power cables are used instead of the continuously rotating “alpha- and beta- joints” which are used on SSF. The boom rotation is reversed on the night side of each orbit to prevent the alpha rotation from winding up tightly the flexible power cables. For example, the panels remain solar inertial during orbital travel in sunlight, and are then counter-rotated with respect to the Orbiter during orbital travel in the earth’s shadow.
To offset the torque associated with this panel rotation, the speed of the y-y axis momentum wheel mounted on the pallet is altered. The wheel angular acceleration is driven to precisely counter the torque provided to the rotating solar panel boom, leaving the Spacelab microgravity environment unperturbed. In addition, the wheel torque can be used to maintain the desired pitch attitude. This attitude will be adjusted slightly from the horizontal to make the average external torques exerted on the STS-Lab by atmospheric drag and gravity gradient forces equal to zero, thereby eliminating momentum accumulation in the wheel. However, if the wheel speed ever reaches its maximum permitted value it can be “desaturated” by firing the Orbiter-II Vernier Reaction Control System (VRCS).
The STS-Lab configuration, with its large laboratory, Orbiter-II crew compartment and with provisions for attachment of the ESA APM and JEM modules to the ET, provides habitable living and working volume comparable with that of the presently planned SSF. During man-tended operations another SPED0 pallet carried in the visiting Orbiter could double the available electrical power. More extensive modification of the ET could be made before launch to allow the hydrogen and oxygen vessels to be repressurized following arrival on orbit. These 20,000 and 50,000 cu. ft. vessels could be used for additional work and living space. These large volumes could be appropriate, for example, for life sciences research, medical research on the aging, on-orbit closed life support systems research, development related to human exploration of space, and commercial enterprise. The single unmanned launch required to place the entire system in orbit, with little or no EVA required for assembly, would minimize early operational costs, scheduling problems, and risk.
Cost and Schedule
Consideration has been given to the use of the existing Shuttle fleet elements to construct STS-Lab. Columbia, the oldest and least capable Orbiter, could be converted into Orbiter-II, using Space Shuttle Main Engines (SSMEs) with significant flight time. One of the existing Spacelabs could be used for the lab module. An existing ET should be outfitted with pressure tunnels and docking ports as described. Testing for man-rated launch will not be required since STS-Lab will be placed in orbit in one unmanned launch. STS-Lab will be integrated and checked out on the ground as a complete facility before launch, and again in orbit with manned support provided by the visiting Orbiter.
Since STS-Lab evolves from existing STS hardware, it is possible to estimate development costs and schedule with reasonable confidence. Our best estimate for the total hardware development is about $3 billion, assuming the conversion of Columbia, an existing ET and an existing Spacelab module. Since STS-Lab is based on existing hardware technology, the associated development is low-risk, and it might be procured through a fixed-price contract to one or more aerospace companies. Contract management, cost of capital, profit and other costs of business would increase the dollar amount of a fixed-price contract above $3 billion. The single launch cost and experimentation hardware are not included. We estimate that STS-Lab could be operational in orbit within 4 years of award of contract.
|STS-LAB Cost Estimate||($B)|
|Orbiter-II conversion from Columbia||1.5|
|Redesign and performance verification||0.3|
It is technically feasible to create a large, capable, and cost-effective space station by straightforward modification of proven STS hardware. STS-Lab might be built under a fixed price contract to one or more aerospace companies and could be operational within 4 years of the award of contract. The estimated development cost of $3 billion is low, the technology is well known, the on-orbit capability is considerable, and the development time is short.
Acknowledgements.- Thomas F. Rogers, an External Tanks Corporation (ETCO) Director, conceived the basic design for STS-Lab. Owen K Garriott, a Vice President of Teledyne -Brown Engineering and former Scientist-Astronaut on both Skylab and Spacelab, contributed to STS-Lab design. Also contributing were John L. McLucas, ETCO Chairman and former Chairman of the NASA Advisory Council; and the late James C. Fletcher, former NASA Administrator and ETCO Director.
1 Randolph H. Ware is President of the External Tanks Corporation (ETCO) and an Adjunct Professor of Aerospace Engineering at the University of Colorado.
*Philip E. Culbertson is ETCO Senior Vice President. He directed NASA’s Space Station Program for two years before serving as NASA General Manager.
Consider Alternatives to Freedom
(Space News, July 15-28, 1991, pp 15)
Thomas F. Rogers
Several groups have proposed innovative alternatives to the international space station program led by NASA. These proposals were prepared and presented to NASA or administration officials earlier this year and then, evidently, roundly ignored in the congressional debate over continued funding of the current space station. Why?
Any proposal that would result in a viable space station costing billions of dollars less than space station Freedom deserves the careful consideration of legislators. The alternative in times of tightening federal budgets, as demonstrated recently in that House of Representatives could be to curtail other valuable NASA programs in order to continue funding Freedom.
The cost of space station Freedom is estimated to be $30 billion to $50 billion, but its true cost could amount to as much as $80 billion before operations aboard the station begin. This figure includes the cost of borrowing federal funds, costs of deferred space station components, true costs of using the space shuttle as a launch vehicle and the inevitable cost overruns.
Furthermore, the hazards of depending upon tens of Shuttle trips to assemble the station in low Earth orbit, and upon many hundreds of hours of spacewalks to maintain it, have become painfully apparent. And Freedom’s great cost and reduced capability have lost. it much scientific support.
The Report of the Committee on the Future of the US Space Program recommended that “…steps should be taken to reduce the station’s size and complexity, permit greater end-to-end testing prior to launch, reduce transportation requirements reduce extra-vehicular assembly and maintenance, and, where it can be done without affecting safety, reduce costs.”
In response, several groups conceived of new approaches to space station design. One group, which included the author, was composed of professionals with at least as much space station experience as now can he found in the government.
In this proposal, a space shuttle orbiter, stripped of all return-to-Earth capabilities, would he launched without a crew and carry its external fuel tank to orbit. The orbiter’s cargo bay would carry a pressurized laboratory currently planned for space station Freedom. Easy access to the now-empty external tank’s spacious intertank could be had to allow transfer of people and equipment through pressurized passageways. Docking ports would allow crews to enter the station and would; accommodate additional modules and a crew return vehicle.
The hazards inherent. in assembling a structure in orbit would be avoided by launching a fully assembled and tested station in one space shuttle flight, and routine spacewalks are inherently minimized. If desired, a cargo carrier at the aft end of the external tank could accommodate a centrifuge large enough for testing humans.
All of this could be accomplished in less than five years and for a total pre-operational acquisition cost of considerably less than $ 10 billion.
Such an alternative to space station Freedom could save U.S. taxpayers tens of billions of dollars. There would be no need for cuts throughout the civil space program to accommodate the current space station program within a reduced NASA budget.
Although this proposal has been widely circulated among Washington decision-makers in recent months, it. has been largely ignored along with other alternatives to Freedom.
Congress need not sacrifice the US goal of a space station if it votes against Freedom. Rather, our government leaders can and should see Freedom replaced soon by a less costly, larger, and safer space station.
Tomas F. Rogers is president of the Sophron Foundation of McLean, Va., and a director of the External Tanks Corporation, Boulder, Colo.