According to historians Andrew Dunar and Stephen Waring, writing in their 1999 book Power to Explor**e: A History of Marshall Space Flight Center, in the 1970s two lines of thought emerged within NASA concerning manned spaceflight's course after the Space Shuttle became operational. On the one hand, there was the "revolutionary" line taken by Johnson Space Center (JSC) in Houston, Texas. On the other was the "evolutionary" line of NASA Marshall Space Flight Center (MSFC) in Huntsville, Alabama.
At JSC, many managers assumed that, as soon as the Shuttle became operational, NASA would get a green light to assemble a large, new-design, multipurpose space station in low-Earth orbit (LEO). They envisioned that a future President would make a speech much like President John F. Kennedy's 25 May 1961 "moon speech." Visionary goal thus proclaimed, the funding floodgates would open.
At MSFC, by contrast, many managers expected that NASA budgets would remain tight for the foreseeable future, so any space technology development that took place would need to be incremental; that is, it would have to begin with existing space hardware and occur in small steps. MSFC's work on Skylab, a temporary LEO space station launched in May 1973 on the last Saturn V rocket to fly, probably helped to shape their outlook. The 169,950-pound Skylab "cluster," which comprised the Multiple Docking Adapter, the Apollo Telescope Mount (ATM), and the Orbital Workshop, had been conceived originally as an element of the Apollo Applications Program (AAP). As its name implies, AAP was meant to apply hardware developed for the Apollo lunar program to new tasks.
When MSFC engineers looked at the Space Transportation System (STS), as NASA called the Space Shuttle and its stable of expendable upper stages and European-built Spacelab components, they saw not the promise of a big new space station, but rather a system which, once operational, could benefit from evolutionary development. In particular, they noted that Spacelab, which MSFC was assigned to integrate with the Shuttle, could not reach its potential as an orbiting laboratory while the Shuttle Orbiter's planned maximum time in space was only seven days. The Orbiter and its payloads would rely for electricity on the former's fuel cells, which meant that the quantity of fuel-cell reactants the Orbiter could carry would determine their endurance.
In early 1977, with the first STS flight test officially planned for March 1979, MSFC proposed "the first step beyond the baseline STS" - a Power Module (PM) capable of supplying 25 kilowatts of electricity continuously. The solar-powered PM was meant to be deployed into LEO from a Shuttle Orbiter payload bay and left in space for up to five years. A succession of Orbiters bearing Spacelab modules and pallets in their payload bays would dock with the PM and use its electricity to remain in orbit for up to 30 days at a stretch.
Alternately, a Shuttle Orbiter could attach a "freeflyer" payload to the orbiting PM and leave it to operate on its own. This appealed to materials scientists, who worried that astronauts' movements on board the Shuttle Orbiter and Spacelab would rattle and ruin their microgravity experiments. Orbiters would periodically dock with the materials science freeflyer/PM combination to remove experiment products - for example, large flawless crystals - and to replenish raw materials.
In addition to electricity, the PM "building block" would provide thermal and attitude control. The latter would permit a docked Orbiter to conserve its Reaction Control System propellants. Freeflyer payloads meant to be docked with the PM could be built without thermal and attitude control systems, reducing their cost.
MSFC engineers planned at first to base the PM on the Skylab ATM design. They quickly found, however, that modifying the ATM to meet stringent Orbiter payload bay safety requirements would cost more than a new design. They retained the ATM's octagonal cross-section, however, because they found that it made efficient use of the Orbiter's cylindrical payload bay volume while providing flat surfaces upon which to mount subsystems.
Although it nixed the ATM, MSFC still aimed to lower the PM's cost by using subsystems developed for Skylab, Spacelab, Shuttle, and other programs. These included three Skylab Control Moment Gyros for attitude control and four curved Shuttle payload bay door radiators for thermal control. MSFC planned to update and improve Skylab systems used in the PM based on Skylab flight experience. All major PM subsystems would be redesigned for easy replacement by spacewalking astronauts.
The 31,000-pound PM would measure 55 feet long from the framework holding its aft- and side-facing international docking ports to the forward ends of its stowed twin solar arrays. The PM would fill most of the Shuttle Orbiter's 15-by-60-foot payload bay, leaving room only for a docking tunnel with an international docking port at the front of the bay, attached to the aft wall of the Orbiter crew compartment.
Upon arrival in LEO, the astronauts would open the Shuttle Orbiter's payload bay doors and release the five pins that secured the PM in the bay. They would then use the Orbiter's robot arm to lift the PM from the bay and berth its side-facing docking port on the Orbiter port. This would position the module so that it extended out over the crew compartment.
The astronauts would next extend the PM's twin solar arrays. Fully extended, each wing-like array would measure 131 feet long by 30 feet wide. They would together span a little more than 276 feet. MSFC sized the arrays to generate a total of 59 kilowatts of electricity; that is, 34 kilowatts more than the PM would supply to Spacelab-carrying Orbiters and freeflyers. A portion of this excess would power PM systems, but the majority would charge batteries in the PM so that it could supply a constant 25 kilowatts throughout its roughly 90-minute orbital day-night cycle.
MSFC acknowledged that the big solar arrays would degrade over time; its engineers estimated that over five years they would lose 5% of their generating capacity. Similarly, the PM's batteries would gradually lose their ability to charge and discharge. After five years, a Shuttle Orbiter might be sent up to recover the PM and return it to Earth for refurbishment. Another Orbiter would then launch it back to LEO to continue its duties.
MSFC engineers presented the PM concept to scientists at an MSFC-sponsored solar-terrestrial physics workshop in October 1977. They found broad support for the new capabilities the PM would give to the baseline STS.
They also proposed that the PM become part of plans to reuse Skylab. MSFC contractor McDonnell Douglas had "interrogated" the abandoned space station's data handling system and found that, nearly four years after its last crew had returned to Earth, reactivation remained feasible. The first step toward Skylab reuse would be for a Space Shuttle to rendezvous with it late in 1979 and boost it to a longer-lived orbit.
The PM would be a late addition to the revitalized Skylab cluster; MSFC did not expect that the new STS element would reach LEO for the first time until 1983, by which time several Shuttle Orbiters would already have visited Skylab. Once added to Skylab, however, the PM would enable the revitalized station to support as many as six astronauts without a Shuttle Orbiter present. They would perform experiments with large-scale space construction and early space industrialization.
MSFC engineers hoped that the PM would also contribute toward NASA's quest for Skylab's successor. They envisioned that PMs attached to Shuttle Orbiters, freeflyers, and Skylab might lead to PMs attached to Spacelab-derived habitat and laboratory modules during the 1980s.
In 1978, the Huntsville center contracted with Lockheed Missiles & Space Company to study PM evolution. MSFC expected that PM development might lead to simultaneous operation of several small specialized "space platforms," each with at least one PM attached. The platforms would not need to be staffed continuously. MSFC argued that several small platforms would best serve scientific and engineering disciplines with conflicting needs, and might cost less than a single large station besides.
In early 1979, NASA Headquarters authorized MSFC to spend $90 million on PM hardware development. The Huntsville center created a PM Project Office in March 1979. At about the same time, however, the space agency abandoned plans to reuse Skylab because the Space Shuttle would not be ready in time to prevent its uncontrolled reentry. Skylab reentered Earth's atmosphere over Australia on 11 July 1979.
JSC, meanwhile, pitched a new-design Space Operations Center (SOC). The space station would include hangars for reusable auxiliary spacecraft and satellite repair, robot arms, habitat and laboratory modules, and truss-mounted solar arrays spanning more than 400 feet.
STS-1, the maiden flight of Columbia, the first Space Shuttle Orbiter, took place in April 1981. James Beggs, President Ronald Reagan's choice for NASA Administrator, was confirmed two months later. Beggs soon sought presidential approval for a space station. This move seemed to favor JSC's revolutionary vision. At the same time, however, Beggs informed MSFC that he wanted to buy the new station "by the yard" - that is, as money became available. This approach seemed more in line with MSFC thinking.
In November 1981, NASA Headquarters halted PM, SOC, and other station-related work at MSFC and JSC. According to Dunar and Waring, it did this to take charge of station development and to end MSFC-JSC rivalry. Following Reagan's January 1984 State of the Union Address, in which he called upon NASA to build a space station by 1994, JSC's revolutionary vision seemed to win out. JSC was designated "lead center" for space station in early February 1984.
Although Reagan authorized NASA to spend only the $8 billion Beggs had told him the station would cost and had specifically called for a space laboratory in his State of the Union Address, the agency's first baseline station design, the "Dual Keel," was an elaborate combination of lab, Earth/space observatory, and shipyard measuring more than 500 feet wide. Like the SOC, the Dual Keel included hangars, robotics, and a small fleet of reusable auxiliary vehicles.
The Dual Keel's complex multipurpose design immediately came in for criticism. Scientists, for example, complained that space construction and the comings and goings of auxiliary spacecraft were bound to spoil the station's microgravity environment. Congress, meanwhile, accused NASA of low-balling its cost estimate to gain the project's approval.
Congressional cost containment, combined with the Challenger accident, concern over the number of assembly and maintenance spacewalks the station would need, and a rapidly expanding U.S.-Russian space partnership (one which would have been unthinkable when Reagan delivered his January 1984 speech), led to a decade-long series of station redesigns. The station shrank and lost many of its proposed capabilities. This untidy evolution yielded the International Space Station (ISS), a U.S.-Russian hybrid with Japanese and European labs and Canadian robotics.
Ironically, the first ISS element launched into space amounted to a PM. The Russian-built, U.S.-funded FGB provided the second ISS element to reach space, U.S. Node 1, with electricity and attitude control from December 1998 to July 2000, when they were joined by what amounted to a habitat module - the Russian Service Module. At that point, ISS became capable of supporting long-duration crews.
References:
Guntersville Workshop on Solar-Terrestrial Studies, NASA Conference Publication 2037, "summary papers from a University of Alabama in Huntsville/NASA Workshop conducted 13-17 October 1977, at Lake Guntersville State Park Convention Center, Guntersville, Alabama," NASA George C. Marshall Space Flight Center, 1978.
"The 25 kW Power Module - First step beyond the baseline STS," G, Mordan; paper presented at the American Institute of Aeronautics and Astronautics Conference on Large Space Platforms: Future Needs and Capabilities held in Los Angeles, California, 27-29 September 1978.
25 kW Power Module Updated Baseline System, NASA TM-78212, NASA George C. Marshall Space Flight Center, Alabama, December 1978.
Power to Explore: a History of Marshall Space Flight Center, 1960-1990, NASA-SP-4313, Andrew J. Dunar and Stephen P. Waring, NASA History Office, 1999.
