30 October 2015

An Alternate Station/Shuttle Evolution: The Spirit of '76 (1970)

North American Rockwell concept of a fully resuable Space Shuttle. Image credit: NASA
NASA Administrator Thomas Paine was optimistic that national enthusiasm for Apollo 11, the first piloted lunar landing, would translate into national support for an expansive future NASA program. He viewed a 12-person Space Station served by a reusable Space Shuttle as the necessary first step toward such a future for the U.S. civilian space agency.

On 22 July 1969 - the day after Apollo 11 astronauts Neil Armstrong and Edwin Aldrin lifted off from the moon in the Ascent Stage of the Lunar Module Eagle - NASA awarded Phase B Space Station study contracts to McDonnell Douglas Aerospace Company (MDAC) and North American Rockwell (NAR). Each contractor led a team of subcontractors, so in all as many as 30 aerospace companies were involved in executing the Phase B studies. Marshall Space Flight Center in Huntsville, Alabama, directed the MDAC contract and the Manned Spacecraft Center in Houston, Texas, directed NAR.

I described the MDAC study in considerable detail in a March post (see the links at the bottom of this post). NAR's 12-person Space Station was, like the MDAC Station, meant to reach low-Earth orbit atop a two-stage Saturn V rocket in 1975 and operate for up to 10 years. The barrel-shaped 33-foot-diameter, 50-foot-tall Station would be ready for staffing as soon as it deployed automatically in 270-nautical-mile-high orbit.

Plan drawing of North American Rockwell's Phase B Space Station. Image credit: North American Rockwell/NASA/David S. F. Portree
The Phase B Station would comprise four pressurized living decks. Deck 1 would include a galley and a wardroom with recreation equipment and seating for 12, a sickbay with space medicine research equipment, two "neuter" docking units, and two observation portholes set into one of the round docking port hatches. Deck 2 would include individual staterooms for six astronauts (a large cabin with an office for the Station Commander and smaller cabins for crew members), a personal hygiene compartment including a full-body shower, and the Primary Control Center (PCC).

Deck 3 would resemble Deck 2, except that its large stateroom would be set aside for the Chief Science Investigator and a repair shop would replace the PCC. Deck 4, the laboratory deck, would include experiment equipment for eight major scientific disciplines, an airlock with an extendable boom for exposing experiments to space, a small backup control center, and two docking units.

Image credit: North American Rockwell/NASA
Image credit: North American Rockwell/NASA
Image credit: North American Rockwell/NASA
Image credit: North American Rockwell/NASA
To enhance crew safety, Decks 1/2 and Decks 3/4 would comprise a pair of independent, redundant living volumes. If a fire broke out and burned out of control on Deck 4, for example, the Station crew would evacuate to the Deck 1/2 volume through an "inter-volume airlock" adjacent to the repair shop on Deck 3. They would seal off the damaged volume and call for help from Earth.

Upper and lower equipment bays atop Deck 4 and below Deck 1, respectively, would each contain a conical "tunnel" airlock, a pressurized "torus" ring for storing supplies and spare parts, and an section open to vacuum containing spherical storage tanks for life-support gases and liquids. The equipment bays would also house propellant tanks supplying the Phase B Station's attitude-control thrusters. Spacewalking astronauts would leave the station through the lower tunnel. Rectangular openings in the Deck 1 floor and the Deck 4 ceiling would enable astronauts to enter the storage rings.

Image credit: North American Rockwell/NASA
Image credit: North American Rockwell/NASA
NASA envisioned that the Phase B Space Station design would, with minimal modifications, serve as a "building block" module for advanced space projects. Multiple modules might, for example, be stacked and clustered to form a 100-man Earth-orbiting Space Base. A single module - perhaps cut down to two decks - might serve as a lunar-orbital Space Station or Mars spacecraft crew module.

After considering Brayton-isotope and nuclear-reactor power systems, NAR settled on solar power for its Phase B station. The company explained its decision by noting that a Brayton-isotope power source would provide adequate electricity for the 12-person Station but could not be scaled up to serve a Space Base; by the same token, a reactor capable of supplying a Space Base would be too large and complex to efficiently power a 12-person Station.

A cylindrical "power boom" would carry four rolled-up advanced-design steerable solar arrays. A total of 10,000 square feet of solar cells would generate 25 kilowatts of electricity. The rotating boom would reach space attached to a port atop the upper equipment bay. The upper bay's conical tunnel would lead through a hatch into the hollow power boom.

The major technical challenge of the Phase B Station was, NAR explained, its anticipated long lifetime in orbit. The company invoked detailed on-board subsystems monitoring, subsystems designed for maintainability, easy subsystems accessibility, and a large on-board stockpile of spare parts as solutions to the Station lifetime problem.

Astronauts would reach NAR's orbiting Station on board fully reusable Space Shuttle Orbiters. The Shuttle's mission would be to economically change out Station crews, replenish supplies, and deliver scientific equipment and other cargo. Most of the many Shuttle designs under study by mid-1970 comprised a winged piloted Booster and a winged piloted Orbiter; the latter would include a cylindrical payload bay that could be opened to space.

The payload bay would be sized to transport standardized cylindrical modules. Most commonly carried would be the cargo/crew transfer module; other modules would arrive at the Station's five ports outfitted as specialized laboratories, instrument carriers, or free-flyers.

Upon achieving orbit, the Orbiter crew would open the payload bay doors and activate a mechanism that would pivot the module it carried onto a neuter docking port on the Orbiter crew cabin roof. The Orbiter would then dock with the Station using the neuter docking unit on the other end of the module. Astronauts would enter and depart the Station through the module. When time came to leave the Station, the Orbiter would undock from the module, leaving it attached to the Station, or would undock the module from the Station and pivot it back into the payload bay for return to Earth.

A Space Shuttle Orbiter docks with the NAR Phase B Space Station using a module deployed from its payload bay and linked to the docking port atop its crew cabin. Image credit: North American Rockwell
On 28 July 1970, a little more than a year after NASA awarded the Phase B Station contracts, Administrator Paine resigned effective 15 September. The next day (29 July), NASA instructed MDAC and NAR to examine Stations that could be launched in pieces in Space Shuttle Orbiter payload bays and assembled in space. This marked the continuation of a gradual shift toward a "phased" Station/Shuttle Program.

The phased approach, a response to deep cuts in NASA funding, would postpone Station development until late in the Shuttle development phase, when Shuttle development costs would wind down, or until after the first few Shuttle orbital flights. In either case, the Shuttle would begin operational flights before it began to launch Space Station modules into orbit.

Shortly after the 29 July directive, NAR engineers offered an alternative to the Shuttle-first phased approach. In a brief presentation titled "Spirit of '76," they proposed that NASA postpone Shuttle development and instead in 1976 launch a prototype Phase B Station on a two-stage Saturn V.

The Station-first phased approach was, they argued, superior to the Shuttle-first phased approach because the Shuttle would demand a much greater technological leap than would the Station. This meant that it might hit development roadblocks that would increase its estimated cost and delay its first launch (as indeed did happen). In addition, the Spirit of '76 Station could better address the emerging post-Apollo space priorities of President Richard Nixon. These included international space cooperation and direct benefits to people on Earth.

NAR's Spirit of '76 Station was outwardly very similar to NAR's Phase B Station. Differences included less advanced, smaller solar arrays capable of generating 20 kilowatts of electricity and docking ports of the Apollo passive drogue design. The Spirit of '76 Station would support a smaller crew complement (normally six astronauts - nine during crew rotation) and have a rated lifetime of 72 man-months instead of 10 calendar years. The latter attribute would largely eliminate the technical challenges of building for a long lifetime in orbit.

Like the Phase B Station, the Spirit of '76 Station would circle the Earth in an orbit inclined 55° relative to the equator, causing it to overfly nearly all inhabited regions. Earth observations, NAR claimed, would yield improved weather forecasts that would save the U.S. $2.5 billion per year (how this figure was calculated was not explained). Spirit of '76 crews would also "patrol" for storms, research "weather modification," seek geothermal energy sources and "new sources of dwindling resources," watch out for crop diseases and water pollution, predict earthquakes, and improve "sea food production."

Besides Earth observations, the Spirit of '76 astronauts would conduct "aerospace medicine" experiments. In keeping with the goal of benefits for people on Earth, many of these would aim to discover the healing potential of the "benign space environment" and seek new and improved "medical diagnostic and treatment techniques." Other experiments would assess and develop countermeasures for spaceflight effects on humans.

The Spirit of '76 presentation bears no date, but its projected 1970s NASA flight schedule indicates that it was prepared in August 1970 - that is, after NASA directed the Phase B contractors to study Shuttle-launched Stations (29 July) but before Paine canceled two Apollo missions (2 September). It has Apollo 18, the final piloted moon mission, leaving Earth in the second quarter of 1974. For reasons not immediately clear, Apollo 19 is not shown on the NAR schedule. As it turned out, Apollo 17 was the last piloted lunar flight; it flew in December 1972.

NAR expected that the Skylab Program, precursor to the Spirit of '76 Program, would take place between Apollo 17 and Apollo 18. The Orbital Workshop, a converted Saturn S-IVB rocket stage, would reach Earth orbit in the last quarter of 1972, and the last of its three crews would return to Earth in mid-1973. In reality, Skylab did not reach orbit until May 1973 and its last crew did not return to Earth until February 1974.

After no NASA piloted flights in 1975, the Spirit of '76 Station would reach Earth orbit early the following year. As its name implies, it would be staffed during the U.S. Bicentennial festivities on 4 July 1976. The orbiting Station would stand as a "source of national pride" as the United States celebrated its 200th birthday.

Unlike Skylab, which would operate unstaffed between crews, the Spirit of '76 Station would be staffed continuously after its first crew arrived early in the second quarter of 1976. Four consecutive three-person crews would launch to the Station for overlapping six-month stays.

Apollo 7 Saturn IB rocket lifts off. Visible below the silver-and-white Command and Service Module (CSM) is the tapered Spacecraft Launch Adapter (SLA). Image credit: NASA
This artist concept from 1966 shows a CSM turning end-over-end to dock with and extract an Apollo Lunar Module (LM) from the top of a spent S-IVB stage. Note the four partially open SLA segments; these would protect the LM during ascent through Earth's atmosphere. NAR envisioned that Spirit of '76 Cargo Modules would also ride to orbit within the SLA attached to the top of an S-IVB stage. Image credit: NASA
In the absence of a Shuttle Orbiter, NAR invoked two-stage Saturn IB rockets and modified Apollo Command and Service Module (CSM) spacecraft as its Spirit of '76 crew transports. Cargo would reach the Station inside modules carried within the tapered Saturn Launch Adapter (SLA) which linked the CSM to the top of the Saturn IB S-IVB second stage.

After CSM separation, the SLA's four petal-like segments would fold open, exposing a Cargo Module with an Apollo drogue docking unit on top. The CSM, which carried an active probe docking unit on its nose, would link up with the drogue unit and detach the Cargo Module from the top of the S-IVB stage. The crew would then ignite the CSM's Service Propulsion System main engine to begin maneuvering to Station altitude. They would dock with the Spirit of '76 Station using an Apollo probe unit on the bottom end of the Cargo Module. All four crews would arrive at the Spirit of '76 Station with a Cargo Module. At least one Cargo Module would be outfitted as a instrument carrier: after docking with the Station it would deploy cameras, a radar, and other sensors for Earth observations.

The second crew would arrive three months after the first crew arrived, increasing the Spirit of '76 crew complement to six. Three months later, at the end of their six-month stay in space, the first crew would depart and the third crew would replace them. The fourth crew would replace the second crew three months after that; then, having completed their six-month stint, the third crew would return to Earth three months later, leaving the three astronauts of the fourth crew to finish up the planned experiment program and mothball the Station. Late in the third quarter of 1977 they would undock in their CSM, reenter, and splash down, ending the Spirit of '76 Program.

NAR offered two funding models for the Spirit of '76 Station. Both would require a $2.3-billion Spirit of '76 Station, four Cargo Modules at a cost of $9 million each, and $220 million for experiments.

The first funding model, with a cost of $2.8 billion spread over six years, assumed use of re-purposed or leftover Apollo and Skylab rockets and spacecraft. It would see the CSMs built for Apollo 18 (designated 114) and Apollo 19 (115) diverted from the lunar program. Along with the Skylab backup/rescue CSM (119) and 115A, which was committed to no program, they would be converted into Spirit of '76 Station ferries. Ending Apollo with Apollo 17 would free up two Saturn V rockets (514 and 515, the last remaining of the original Apollo Program buy), one of which would launch the Spirit of '76 Station (the other, presumably, would launch Skylab). The four CSMs would reach Earth orbit on the last remaining Saturn IBs (designated 209, 210, 211, and 212).

NAR's other Spirit of '76 funding model, with a total cost of $3.1 billion, would see lunar missions continue through Apollo 19 in the fourth quarter of 1974. NASA would buy two new CSMs (120 and 121) and convert 119 and 115A for the Spirit of '76 program. To trim costs, 120 and 121, which would launch the third and fourth crews, might include the 119 and 115A Command Modules; NAR envisioned refurbishing them after they returned the first and second crews to Earth. A new two-stage Saturn V (516) for launching the Spirit of '76 Station would cost $260 million including launch operations.

Artist concept of Space Shuttle Orbiter with Saturn V S-IC first stage. Image credit: NASA
If all of its many technological challenges could be met successfully, the first Space Shuttle would soar into orbit in the first quarter of 1978. Early in its career, it would take the form of a reusable Orbiter launched atop an expendable Saturn V S-IC first stage. NAR suggested that the Spirit of '76 Station might be revived and become a destination for Shuttle Orbiters during this period. Through phased development, NASA would soon replace the S-IC stage with a reusable winged Booster, then a Shuttle-launched modular station would be assembled in Earth orbit sometime in the 1980s.

Ironically, Rockwell International - formerly North American Rockwell - became the Space Shuttle prime contractor. The company that argued that a Space Station should be developed first because Space Shuttle development would be fraught with technical challenges thus became responsible for tackling the challenges of building the Shuttle.


"Spirit of '76," North American Rockwell Space Division, undated presentation (August 1970)

Space Station Program, North American Rockwell Space DivisionBriefing to the European Space Research Organization on Space Station Plans and Programs in Paris, France, 3-5 June 1970

Astronautics and Aeronautics 1970, NASA SP-4015, pp. 193-194

Space Stations: A Policy History, J. Logsdon, George Washington University, NASA Contract NAS9-16461, NASA Johnson Space Center, no date (1980), pp. I-16, II-1-5, II-8-10, II-13-15, II-18-33

More Information

McDonnell Douglas Phase B Space Station (1970)

Where to Launch and Land the Space Shuttle? (1971-1972)

A Forgotten Rocket - The Saturn IB

Apollo's End: NASA Cancels Apollo 15 & Apollo 19 to Save Station/Shuttle (1970)

25 October 2015

Touring Titan By Blimp & Buoy (1983)

Image credit: NASA
The planet Saturn needs a little more than 29 years to circle the Sun once. At its mean orbital distance, 1.43 billion kilometers from our star's warming fires, it receives about 1% as much solar energy as does Earth. The planet was known to ancient peoples the world over, but its most distinctive feature – its bright and complex ring system – remained undiscovered until after the invention of the telescope.

Galileo Galilei, famous for his telescopic discovery of Jupiter's four largest moons, spotted Saturn's rings in 1609-1610. Though perhaps the most advanced in the world at the time, his telescope was too crude to enable him to determine their nature.

A half-century later, Christian Huygens announced that the "appendages" that had defied Galileo's analysis were in fact a ring that encircled the planet without touching it. Huygens also discovered Titan, Saturn's largest moon, and determined that it circles the ringed planet in about 16 days.

Little new was learned of Titan until 1944. In that year, planetary astronomer Gerard Kuiper discovered that it has an atmosphere containing methane.

Data from the Voyager 1 spacecraft, which flew past Titan at a distance of about 4000 kilometers on 12 November 1980, showed that 98% of its atmosphere is nitrogen, and that its surface atmospheric pressure is roughly half again as great as Earth's at sea-level. Titan's surface temperature averages about 94 Kelvin (-179° Celsius, -290° Fahrenheit) and the low-density moon's surface gravitational pull is just 14% of Earth’s. The 5150-kilometer-diameter satellite's surface remained mysterious; it lay hidden beneath a high-altitude haze layer and dense orange clouds.

Titan as observed by Voyager 1, November 1980. The haze layer above the dense orange cloud deck is just visible, as is the ephemeral polar hood. Image credit: NASA
In 1983, the NASA Advisory Council's Solar System Exploration Committee (SSEC) released the first part of its report Planetary Exploration Through the Year 2000. The SSEC, chartered in 1980 by NASA Administrator Robert Frosch at the recommendation of NASA Associate Administrator for Space Science Thomas Mutch, aimed to develop missions to carry out the scientific strategy put forward by the National Academy of Sciences Committee on Planetary and Lunar Exploration (COMPLEX).

The SSEC report described a "core program" of planetary missions for the remainder of the 20th century. The four "initial" missions of the core program were a Venus Radar Mapper, the Comet Rendezvous/Asteroid Flyby (CRAF) mission, a Mars Geoscience/Climatology Orbiter, and - reflecting the many questions the Voyager 1 flyby had raised - a Titan Probe/Radar Mapper.

This last would see a Saturn flyby or orbiter spacecraft drop a short-lived instrument capsule into Titan's dense atmosphere and probe its hidden surface using an imaging radar. The SSEC hoped that it would leave Earth between 1988 and 1992 and return data from Saturn and Titan between 1995 and 1997.

Even as the SSEC published its core program, it commenced work on a new report outlining an "augmented program" of planetary exploration; that is, a collection of candidate missions that might follow and expand upon its core program. As part of its new study, it convened a workshop in Snowmass, Colorado, in the summer of 1983. Science Applications Incorporated (SAI) briefed workshop participants on 2 August 1983 on a six-month study of advanced Titan missions it had completed a month earlier for NASA's Solar System Exploration Division.

SAI's presentation began with an overview of the scientific rationale underlying its mission proposals. The study team told the SSEC workshop that "the most important characteristic of Titan is the chemical evolution that has occurred and is still occurring in its atmosphere." For example, carbon monoxide and hydrogen cyanide found by Voyager 1 in trace amounts in Titan's atmosphere had the potential to evolve into nucleotide bases and amino acids, critical building blocks of terrestrial life.

Scientists suspected that Titan's atmospheric chemistry offered clues to the nature of its surface, though they split over what those clues meant. Some believed that Titan was awash in an ocean - or at least large lakes – of liquid ethane or methane. In that model, ethane or methane behaved on Titan much as water behaves on Earth.

Others believed that organic goop from the orange clouds drizzled down and accumulated to a depth of several kilometers on its solid ice surface. In places, perhaps, exotic ice volcanoes poked through the goop layer and belched methane into Titan's dense atmosphere, providing raw material for more chemical evolution.

SAI proposed eight spacecraft systems for its Titan missions. These were: the non-imaging Titan orbiter; the imaging Titan orbiter; the Titan flyby bus; the combined haze probe/penetrator probe; the sounding rocket; and the large and small buoyant stations. The orbiter and flyby bus would operate outside of Titan's atmosphere. The other systems would operate within it.

Whether imaging or non-imaging, an orbiter would be an essential element of all SAI's proposed Titan mission concepts. In addition to collecting valuable scientific data, it would provide the crucial radio-relay link between the Titan atmosphere/surface systems and mission controllers and scientists on Earth. Based on the proposed Saturn orbiter/Titan probe spacecraft design, the orbiter would circle Titan in a 1000-kilometer-high circular polar orbit requiring 3.93 hours to complete. This would enable it to link a system floating in Titan's atmosphere near its equator with controllers and scientists on Earth about half the time. The orbiter might reduce its required propellant load by employing aerocapture; that is, by skimming through Titan's upper atmosphere to slow down so that the cloudy moon's gravity could capture it into orbit.

Of SAI's eight Titan exploration systems, only the flyby bus would carry no scientific instruments. Based on Galileo Jupiter orbiter and Pioneer Venus hardware, the flyby bus would leave Earth about a year after the Titan orbiter. Its mission would end as it flew past Titan and released a cluster of atmosphere and surface probes.

The simplest system in SAI's Titan exploration arsenal was the combined haze/penetrator probe, the design of which was based on a proposed Mars penetrator. A solid-propellant rocket motor would blast the haze/penetrator probe from a launch tube on the orbiter and slow it so that it would fall into Titan's atmosphere. An umbrella-like fabric decelerator would then deploy, slowing the probe to a speed of Mach 1 by the time it fell to within 265 kilometers of Titan's surface. It would then begin to collect data on the hazy uppermost atmosphere.

The penetrator would then separate and descend to a hard landing (or a splashdown) on Titan's surface. The haze probe, meanwhile, would descend for 23 minutes to an altitude of 100 kilometers, at which point the orbiter would pass below its horizon. This would break the radio link with Earth and end the haze probe's mission.

The penetrator would be more long-lived; it would collect and store Titan surface data for transmission to the orbiter when it rose above the horizon again. If Titan's surface were confirmed to be covered by an exotic ocean before the orbiter left Earth, then the penetrator might be fitted out as a floating sonar buoy.

This image from the Huygens probe shows Titan's misty, icy surface from a height of five kilometers. Image credit: ESA/NASA
SAI's most novel and picturesque Titan exploration systems were its large and small buoyant stations. The small stations, instrument-laden gondolas suspended from balloons, would be delivered into Titan's atmosphere by the flyby bus packed into 1.25-meter-diameter aeroshells based on the Galileo Jupiter atmosphere probe design. The large stations, packed into aeroshells twice as large, would take the form of either larger balloons or powered blimps. The small buoyant stations would operate between 100 and 10 kilometers above Titan, while large buoyant stations would operate between 10 kilometers above Titan and Titan's surface.

SAI provided few details about its proposed sounding rocket, which it envisioned would explore the same level of Titan's atmosphere as the haze probe. During descent, at an altitude of about 100 kilometers, the solid-propellant rocket would detach from the a buoyant station, ignite its motor, and ascend into the haze layer.

The company looked at several methods for launching its Titan missions from Earth. These included an advanced Nuclear-Electric Propulsion (NEP) system, though most relied instead on one or more Centaur G' chemical rocket stages.

In keeping with U.S. space policy in 1983, all the Earth-departure methods assumed that the Titan mission spacecraft would reach Earth orbit packed into the payload bays of Space Shuttle Orbiters. Reliance on the Shuttle imposed severe penalties on the Titan missions, SAI found. These included minimal science payloads and trip times of up to eight years with multiple Venus, Earth, and Jupiter gravity-assist flybys.

SAI sought to circumvent these penalties by assuming that NASA would become capable of On-Orbit Assembly (OOA) and in-space liquid oxygen/liquid hydrogen refueling by the time its Titan missions were ready to depart Earth. These operations might take place at an Earth-orbiting space station, SAI suggested.

SAI then described five Titan exploration mission concepts which combined its eight systems in what it called "mix 'n match" fashion. Concept #1, a minimal mission, included only a Titan orbiter with a limited Titan atmosphere probe complement. The company explained that the 1978 Pioneer Venus mission - which included separately launched Orbiter and Multiprobe spacecraft - had inspired Concepts #2, #3, and #4, all of which included a Titan orbiter and a separate flyby bus. Concept #5 relied on NEP in place of chemical-propellant rocket stages.

The company described in some detail its Concept #4 mission; with 28 experiments, it was SAI's most ambitious in terms of science return. A Centaur G' stage loaded with propellants in Earth orbit coupled with a Star-48 solid-propellant rocket motor would boost Concept #4's 1885-kilogram imaging orbiter toward Saturn in July 1999, and a pair of Centaur G' stages filled in Earth orbit with liquid oxygen and liquid hydrogen would launch its 2730-kilogram flyby bus a year later. SAI calculated that these stage configurations combined with Titan aerocapture for the orbiter would permit direct Earth-to-Saturn flights with no planetary gravity-assists.

In January 2004, after a flight time of 4.5 years, the imaging orbiter would aerocapture into Titan orbit. Over the next eight months, it would deploy three haze probes without penetrators and bring to bear on Titan's mysteries an impressive array of cloud-penetrating sensors.

In September 2004, after a 4.2-year flight, the flyby bus would speed past Titan and dispense one large buoyant station (a blimp) and three small buoyant stations. The buoyant stations would enter Titan's atmosphere, decelerate, and deploy their gas envelopes as they slowly fell on parachutes. Kept aloft by heat from radioisotope thermal generators, they would each operate for at least two months. The large buoyant station might fly close enough to Titan's surface to lower an instrument package on a tether, permitting the first direct sampling of Titan's surface materials.

SAI placed the cost of its Concept #4 mission at $1.586 billion in 1984 dollars. This included a 30% contingency fund, but did not include launch costs. Adding in the cost of 2.5 $100-million Shuttle launches, three $45-million Centaur G' stages, one $5-million Star 48 motor, and OOA (the cost of which SAI optimistically placed at $10 million per Titan-bound spacecraft) yielded a total mission cost of $1.99 billion.

Artist's concept from 1988 of the Mariner Mark II Cassini Saturn orbiter releasing the Huygens probe above Titan's orange clouds. Image credit: NASA
In its 1986 final report, the SSEC ranked SAI's advanced Titan mission proposals below Mars sample return and comet nucleus sample return on its list of desirable augmentation missions. Meanwhile, the 1983 core program's Titan probe/Radar Mapper mission shifted emphasis to take in the entire Saturn system. This helped to move it closer to reality.

Reflecting this new broader focus, the Saturn orbiter/Titan probe mission was named for Giovanni Cassini, discoverer of Saturn's "second-tier" moons Iapetus, Rhea, Tethys, and Dione. In 1675, Cassini detected the broadest division in Saturn's rings, which is also named for him.

NASA and the European Space Agency (ESA) jointly studied Cassini, and ESA agreed to build the Titan probe, which was named Huygens. The U.S. Congress approved new-start funding for Cassini in 1989.

Initially Cassini was meant to be one of the first Mariner Mark II spacecraft, along with the Comet Rendezvous/Asteroid Flyby (CRAF) spacecraft. Mariner Mark II was intended to be a standardized (and thus inexpensive) spacecraft bus for advanced interplanetary missions. Congress scrapped CRAF in 1992 after it went over budget and diverted its remaining funds to Cassini, marking the end of the Mariner Mark II cost-cutting experiment.

Image credit: NASA/JPL
Following the January 1986 Challenger Shuttle disaster, NASA cancelled Centaur G-prime and moved planetary spacecraft off the Shuttle launch manifest. The bus-sized Cassini spacecraft left Earth on a Titan IVB/Centaur expendable rocket in October 1997 and, after gravity-assist flybys of Venus, Earth, and Jupiter, arrived in Saturn orbit in July 2004.

The Huygens probe entered Titan's dense atmosphere in January 2005 and floated on a parachute to a rough landing. Its six instruments included an imaging system, which revealed an enigmatic surface covered with rounded water ice "pebbles."

The following year, scientists using Cassini's radar discovered ethane lakes large and small in Titan's north polar region. By early 2008, several lines of evidence pointed to a global water ocean perhaps 100 kilometers beneath the water-ice crust of Titan.

Radar swaths from the Cassini Saturn orbiter reveal Titan's north polar "land of lakes" in this false-color image. The largest, Kraken Mare, is roughly the size of Earth's Persian Gulf. Image credit: NASA
In late 2005, scientists using Cassini's imaging system found evidence that another world orbiting Saturn besides Titan has biological potential: bright white Enceladus, which William Herschel discovered in 1789. They detected numerous geysers near the 500-kilometer-diameter moon's south pole. Driven by tidal flexing and possibly other processes that generate heat, these shoot water laced with salt, silica particles, and organic chemicals into space.

After Cassini flew past Enceladus 20 times at a distance of less than 5000 kilometers - eight of those flybys were within 100 kilometers - scientists in September 2015 announced that a global ocean up to 31 kilometers deep underlies its icy surface. During its last close Enceladus flyby on 28 October 2015, Cassini will fly past at a distance of 49 kilometers. Cassini's 22nd and last planned Enceladus flyby is scheduled for 19 December 2015 at a distance of 4999 kilometers.

In May 2008, Cassini completed its primary mission and began its first extended mission (the Equinox Mission). In February 2010, NASA agreed to extend Cassini's mission until September 2017 to enable it to observe Titan's north polar region at mid-summer. Assuming that the spacecraft survives to complete its new extended mission (the Solstice Mission), it will have carried out more than 125 Titan flybys since it reached Saturn orbit.

Cassini/Huygens was, of course, meant to be followed at some point by more intensive follow-on Titan exploration as described in the 1986 SSEC report. Many alternative missions have been proposed and studied, but to date none has gone very far. Partly this is because NASA has focused much of its robotic program on Mars, a world which is relatively easy and inexpensive to reach and which remains of high biological potential and considerable geological interest.

In addition, Titan now has competition in the Saturn system in the form of remarkable little Enceladus. Several proposed missions have focused on Titan and Enceladus together; others have focused on Enceladus alone or Titan alone in an effort to prevent sticker shock.

Titan and Enceladus might both have to wait, however, because NASA has begun the long-awaited development of a mission to survey Jupiter's moon Europa, the prototypical ice-covered global-ocean world. Assuming that current White House and Congressional support for the Europa multiple-flyby mission persists, it might leave Earth as early as 2022. Experience gained by exploring Europa through as many as 45 flybys will almost certainly be applicable to the in-depth exploration of other moons of the outer Solar System, including Titan and Enceladus.


Titan Exploration with Advanced Systems: A Study of Future Mission Concepts, Report No. SAI-83/1151, Science Applications Incorporated; presentation to the SSEC Summer Study in Snowmass, Colorado, 2 August 1983

Planetary Exploration Through Year 2000: A Core Program, Solar System Exploration Committee, NASA Advisory Council, 1983

Planetary Exploration Through Year 2000: An Augmented Program, Solar System Exploration Committee, NASA Advisory Council, 1986

More Information

The Challenge of the Planets, Part Two: High Energy

The Challenge of the Planets, Part Three: Gravity

What Shuttle Should Have Been: NASA's October 1977 Space Shuttle Flight Manifest

The Seventh Planet: A Gravity-Assist Tour of the Uranian System (2003)

23 October 2015

Apollo's End: NASA Cancels Apollo 15 & Apollo 19 to Save Station/Shuttle (1970)

Edwin Aldrin, Apollo 11 Lunar Module Pilot, stands by the first U.S. flag astronauts planted on the moon, 21 July 1969. Visible in the background is one of the Lunar Module (LM) Eagle's landing legs, the LM's long, dark shadow, and the flat, cratered-pocked Sea of Tranquility. Image credit: NASA
On 5 August and 13 August 1970, NASA Administrator Thomas Paine dispatched letters on the future of the U.S. lunar program to the Lunar and Planetary Missions Board (LPMB) and the Space Science Board (SSB) of the National Academy of Sciences National Research Council. In his letters, he outlined three options for curtailing Project Apollo.

Of these options, the first (Option I) would cancel one Apollo mission, while the others would nix two. The options he described were in part aimed at avoiding a delay in the Skylab Program, which constituted an important step toward Paine's favorite mid-1970s NASA goal: a 12-man Earth-orbiting Space Station that would be staffed and resupplied using a fully reusable Space Shuttle. Members of the LPMB and the SSB held an urgent two-day meeting (15-16 August 1970) in Woods Hole, Massachusetts, to develop a response to Paine's letters.

NASA Administrator Thomas Paine. Image credit: NASA
By the time the LPMB and SSB met, NASA had flown three manned lunar landing missions: Apollo 11 (16-24 July 1969), which landed off-target on Mare Tranquillitatis; Apollo 12 (14-24 November 1969), which landed close by the derelict Surveyor 3 automated lander on Oceanus Procellarum, thereby demonstrating the pinpoint landing capability essential for geologic traverse planning; and perilous Apollo 13 (11-17 April 1970), which suffered an oxygen tank explosion in its Command and Service Module (CSM) that scrubbed its planned landing at Fra Mauro. Of these, Apollo 11 and Apollo 12 were mainly engineering missions intended to prove the Apollo system, while Apollo 13 had been intended as the first science-focused mission.

Paine had already canceled one Apollo mission, Apollo 20, in January 1970, so that its Saturn V rocket could launch into low-Earth orbit Skylab A, a Saturn S-IVB stage converted into a temporary space station. That left six moon landings before the program concluded with Apollo 19.

The program meant to extend piloted lunar exploration deep into the 1970s, the Apollo Applications Program (AAP), had taken repeated funding hits since 1966, and so had abandoned its lunar ambitions. It became the strictly Earth-orbital Skylab Program in February 1970. Some concepts proposed for AAP lunar missions – for example, three-day lunar-surface stays and a manned roving vehicle – would find their way into Apollo before its end, but when Apollo ended, so would end piloted lunar exploration.

Space Science Board chair Charles Townes. Image credit: National
Academy of Sciences
With the goal of a man on the moon by 1970 successfully attained, pressure had begun to build to cancel some or all of the remaining Apollo lunar missions. In the aftermath of the Apollo 13 accident, some policy-makers - and even managers within NASA - questioned the wisdom of continuing to place astronauts at risk. Apollo 11 had humbled the Soviets on the technological prestige front of the Cold War; future landings could do little to enhance prestige, they argued, but a single lost crew could erase much of what the U.S. had gained by being first on the moon.

In addition, President Richard Nixon's Office of Management and Budget was eager to rein in Federal expenditures. By mid-1970, the United States was spending roughly the entire $25-billion cost of the Apollo Program every three months to wage war in Indochina. Public interest in U.S. spaceflight, only occasionally strong, had faded rapidly after Apollo 11. Though NASA's budget had decreased to only about $3.7 billion in Fiscal Year 1970 - down from a little over $5 billion in 1966 - the agency still constituted a highly visible and thus highly vulnerable target for new cuts.

This had become evident during Fiscal Year 1971 deliberations. Despite Paine's strident protests, the Nixon White House had on 2 February 1970 submitted to Congress a NASA funding request of only $3.33 billion, of which $110 million was devoted to Station/Shuttle. The U.S. House of Representatives added $80 million to Station/Shuttle in committee. An amendment debated on the House floor on 23 April 1970 then sought to cut Station/Shuttle entirely; the amendment's supporters argued that the program was a foot in the door for an expensive piloted Mars mission. The amendment failed (per House rules) in a tie vote of 53 to 53 - that is, by the narrowest possible margin.

The Senate trimmed Station/Shuttle funding back to $110 million in committee. Repeated amendments on the Senate floor sought to delete all Station/Shuttle funds. Though in the end Station/Shuttle kept its $110 million, NASA's budget suffered other cuts. In early July 1970, House and Senate conferees settled on a NASA budget of $3.27 billion for Fiscal Year 1971.

In their joint response to Paine, dated 24 August 1970, LPMB chair John Findlay and SSB chair (and Nobel Laureate) Charles Townes reminded Paine that past scientific advisory boards - including one Townes had chaired, which prepared a January 1969 report for then-President-elect Nixon - had advised that NASA should continue manned lunar exploration throughout the 1970s, and that from 10 to 15 manned moon landings should be flown. They cited this when they refused to consider cutting more than one Apollo mission. The SSB had, incidentally, in its January 1969 report expressly opposed Paine's large Earth-orbiting station.

Apollo, they told the NASA Administrator, was of the greatest scientific importance. They explained that "the Apollo missions do not simply represent the study of a specific small planet but rather form the keystone for a near term understanding of planetary evolution." They then wrote that
We respect the serious fiscal and programmatic constraints…. However, it should be recognized that any reduction in the number of missions will seriously threaten the ability of the total Apollo program to answer first-order scientific questions. We are on the very beginning of a learning curve, and it is clear that the loss of one mission will have much greater than a proportional effect on the instrumented experiments and, more critically, on the design and execution of the geology experiments involving the astronauts.
Findlay and Townes explained that at Woods Hole the LPMB and SSB had jointly considered their own trio of options for Apollo's future, all of which were different from Paine's. Option I was to fly missions 14, 15, 16, and 17 about six months apart, fly missions to the Skylab A Orbital Workshop over a period of about 20 months, and then carry out Apollo missions 18 and 19 six months apart.

Missions 14 and 15 would be H-class walking missions, as had been 12 and 13; 16 and subsequent would be J-class missions. The latter would include a Lunar Module (LM) capable of increased lunar surface stay time, a rover, improved lunar surface experiments, remote sensors on the CSM in lunar orbit, and a CSM-released lunar subsatellite. The long gap between Apollo 17 and 18 would permit lunar scientists to digest data from the previous missions and to design new experiments for the final mission pair. Findlay and Townes noted, however, that the gap might also make Apollo 18 and 19 vulnerable to budget cuts. Paine's Option I had cut Apollo 15 and flown all the remaining lunar missions before Skylab A.

The LPMB and SSB's Option II was to cut Apollo 15, fly 14, 16, 17, 18, and 19 about six months apart, and then fly the Skylab A missions. Their Option III was to cut Apollo 15, fly 14, 16, 17, 18, and 19 five months apart, and then fly Skylab A. Paine's Options II and III had both omitted 15 and 19.

Regions of the moon surveyed using instruments on board the Apollo 15, 16, and 17 Command and Service Modules in lunar orbit. Flying the J-class Apollo 18 and 19 missions would have nearly doubled surface coverage. Image credit: NASA
As might be expected, the LPMB and SSB favored their Option I, which cut no missions. If, on the other hand, "retreat from Option I proves unavoidable," they recommended their Option III. This would, they explained, sacrifice Apollo 15 to save Apollo 19, which, they explained, would include 20% of the Apollo program's moonwalk time and cover 25% of the total area to be included in Apollo traverses. In addition, by reducing the time between launches, they hoped to limit the costly delay in Skylab A's launch.

They conceded that most of the experiments planned for Apollo could be carried out even if both Apollo 15 and 19 were cut. However, an automated station in the passive seismic network would be lost, surface samples would not be obtained from two geologically significant locations, and several experiments would be flown only once, so would have no backup. They concluded by reiterating that the cuts Paine envisioned could prevent lunar scientists from answering first-order questions about the moon, and added that "the consequences of such failure for the future of [NASA] and, we believe, for large-scale science in this country are incalculable."

In his reply to Townes and Findlay, dated 1 September 1970, Paine announced that he had selected his Option II as originally proposed (that is, elimination of both Apollo 15 and 19). He explained that Option I was not feasible because earlier budget cuts had forced a change from four-month to six-month gaps between Apollo moon flights. This might be reduced to five months "at some added cost," he wrote. Even with the gaps between flights reduced, however, a delay of seven or eight months in the launch of Skylab A would occur, "requiring a high, non-productive expenditure to retain the [Skylab] teams beyond the scheduled launch date." Paine did not address the LPMB and SSB's suggestion that Apollos 18 and 19 fly after Skylab A.

Cutting 15 and 19, along with closing down Apollo operations in mid-1972 and terminating Saturn V after the Skylab A launch in late 1972 would, Paine explained, produce "substantial saving over the next four years." The immediate savings from cutting Apollo missions 15 and 19 would amount to only $40 million; if NASA opted to fly both, however, an additional $760 million would need to be spent by the time Apollo 19 returned to Earth.

Paine argued that his cuts placed NASA "in a better position to keep our total program costs down while still pressing forward with our future plans for scientific and application programs and an integrated, low cost space transportation system." Paine referred, of course, to the large Earth-orbiting Space Station and the reusable Space Shuttle he favored.

Paine invoked Apollo 13, then argued that selecting the minimum Apollo program option would enhance safety. Rather than arguing that fewer missions meant fewer chances for failure, he maintained that making cuts up front would preserve "momentum and morale," keeping the NASA/industry team focused and thus reducing risk to crews. He asserted that "rather than risk the integrity of the entire program by cutting out a mission at a time in response to budgetary constraints, we feel we must now take a stand on what constitutes the minimum viable program and then carry it out effectively."

The following day (2 September 1970), Paine held a press conference during which he announced his Apollo program cuts. It was one of Paine's final public acts as NASA Administrator; on 28 July he had tendered his resignation effective 15 September 1970. He denied that his decision to resign had anything to do with cuts in the Fiscal Year 1971 NASA budget.

Apollo 14 (31 January-9 February 1971), the last H-class mission, landed at Fra Mauro. Alan Shepard and Ed Mitchell pushed the limits of walking astronauts by attempting to climb to the rim of Cone Crater, where geologists hoped that they could sample ancient rocks from deep inside the moon.

Apollo 16, the first J-class flight, was renumbered Apollo 15 and launched on 26 July 1971. The Apollo 15 LM Falcon, bearing astronauts Dave Scott and James Irwin, landed at Hadley-Apennine, on the mountainous fringe of Mare Imbrium, on 30 July. They conducted three Lunar Roving Vehicle (LRV) traverses. Meanwhile, on board the CSM Endeavour in lunar orbit, Al Worden released a subsatellite and turned remote sensors and cameras toward the lunar surface. Apollo 15 splashed down in the Pacific Ocean on 7 August.

On Apollo 16 (16-27 April 1972), John Young and Charlie Duke landed at Descartes in the heavily cratered Lunar Highlands. As they deployed their LRV from the side of the LM Orion, Ken Mattingly ejected the panel covering sensors and cameras on board the orbiting CSM Casper.

Apollo 17 Commander Eugene Cernan salutes the sixth and last U.S. flag American astronauts planted on the moon. Visible behind him are the Lunar Module Challenger, the third and final Lunar Rover to reach the moon (directly behind Cernan), and mountains surrounding Apollo 17's complex Taurus-Littrow landing site. Image credit: NASA
The last Apollo lunar mission, Apollo 17 (7-19 December 1972), touched down at Taurus-Littrow, on the edge of Mare Serenitatis, nearly six months after Paine's mid-1972 Apollo end date. Eugene Cernan and Harrison Schmitt, the only professional geologist to reach the moon, used the LM Challenger as their surface exploration base while Ron Evans surveyed the moon from the orbiting CSM America.

The last Saturn V rocket to fly launched Skylab on 14 May 1973, again about six months after Paine's planned date. Three crews docked with and worked aboard Skylab between May 1973 and February 1974. A second Skylab, Skylab B, was built, but was not launched even though a Saturn V for launching it and Saturn IB rockets, Apollo CSMs, and astronauts for staffing it were available. Skylab B would become an exhibit in the National Air & Space Museum.

Nixon opted to replace Apollo and Skylab with the partially reusable Space Shuttle (but no Space Station). He had in fact never supported Paine's Station/Shuttle plans. Nixon liked to be seen with astronauts, a trait which by and large defined the extent of his interest in NASA; partly because of this, U.S. space policy drifted and was often confused and contradictory during much of his time in office.

Nixon postponed announcement of his Space Shuttle decision until the Presidential election year of 1972. By then, he had nominated and had confirmed James Fletcher as NASA's fourth Administrator. Fletcher read Nixon's Shuttle announcement to reporters on 5 January 1972, in the place where Shuttle Orbiters would be built: California, a state critical to Nixon's reelection bid. The Space Shuttle, Nixon promised, would generate thousands of aerospace jobs.

NASA Administrator James Fletcher (left) and Nixon pose with a model of an early version of the semi-reusable Space Shuttle stack, January 1972. Image credit: NASA


Letter, Charles Townes, Chairman, National Academy of Sciences Space Science Board, and John Findlay, Chairman, Lunar and Planetary Missions Board, to Thomas Paine, NASA Administrator, 24 August 1970

Letter, Thomas Paine, NASA Administrator, to John Findlay, Chairman, Lunar and Planetary Missions Board, 1 September 1970

Chapter 3, The Space Shuttle Decision, NASA SP-4221, Thomas A. Heppenheimer, NASA, 1999

More Information

What If Apollo Astronauts Became Marooned in Lunar Orbit? (1968)

McDonnell Douglas Phase B Space Station (1970)

Where to Launch and Land the Space Shuttle? (1971-1972)

Skylab-Salyut Space Laboratory (1972)

16 October 2015

The 1991 Plan to Turn Space Shuttle Columbia Into a Low-Cost Space Station

This NASA artwork from 1972 portrays the sheer volume of the Skylab Orbital Workshop, the first U.S. space station.
The first U.S. space station was Skylab, which NASA carefully dubbed an "Orbital Workshop" in order to distinguish it from the "real" space station it hoped to launch into low-Earth orbit by the mid-1970s. Skylab - a converted Saturn S-IVB rocket stage with a pressurized volume of more than 12,500 cubic feet - was launched on the last Saturn V heavy-lift rocket to fly. Three three-man crews lived and worked on board the 22-foot-diameter single-launch station for a total of 171 days between 26 May 1973 and 8 February 1974.

Nearly three years earlier, budget cuts had halted Saturn V production, so NASA had been forced to abandon plans for a single-launch, 33-foot-diameter core station. The Space Shuttle, originally intended as a cost-saving fully reusable space station crew and cargo transport, was subsequently tapped to serve also as the sole launch vehicle for a multi-modular space station built up over the course of many flights. This meant that the Shuttle Orbiter's payload bay dimensions (15 feet in diameter by 60 feet long) and maximum payload mass (in theory, up to 32.5 tons) would dictate the size and mass of station modules and other components.

NASA's single-launch core station (left) would throughout its life receive independently maneuverable add-on modules delivered by fully reusable Shuttle Orbiters. This 1970 illustration depicts one such module outfitted to transport astronauts and cargo from the Shuttle payload bay to the core station main docking port and back. The modules would also be outfitted as special-purpose labs that would link up with round ports scattered over the station's hull. Image credit: McDonnell Douglas
Launching the space station in the Shuttle payload bay meant also that NASA could not begin to assemble it until after Space Shuttle development and flight testing were completed. When the last crew left Skylab, the Shuttle's orbital maiden flight was officially set for early 1978. Operational flights were to start by 1980. Some hoped that an early Shuttle flight might boost Skylab to a higher orbit, postponing its eventual reentry and perhaps permitting it to be outfitted as a temporary interim space station in the early 1980s.

In the event, the first mission of the partially reusable Shuttle, STS-1, did not lift off until 12 April 1981, nearly two years after Skylab reentered and broke up over Australia (11 July 1979). The Orbiter Columbia remained aloft for two days before gliding to a landing on the dry lake-bed at Edwards Air Force Base (EAFB), California.

By then, engineers at NASA's Johnson Space Center had been at work for more than two years on a design for a Shuttle-launched station they dubbed the Space Operations Center (SOC). The SOC included a laboratory for experiments in microgravity, but was conceived mainly as a construction site for large structures, a servicing center for satellites, and a home port for a small fleet of space tugs. It was intended, in fact, to serve as a space shipyard, where would be assembled spacecraft for voyages beyond low-Earth orbit and large space structures such as Solar Power Satellites.

The 1982 Space Operations Center design became the point of departure for NASA station studies after the creation of the Space Station Task Force. Visible is a skeletal "false Shuttle payload bay" for satellite servicing and a hexagonal space tug hangar. Image credit: NASA
On 20 May 1982, a little more than a year after STS-1 and a little more than a month before STS-4 (27 June-4 July 1982), NASA Administrator James Beggs established the NASA-wide Space Station Task Force. President Ronald Reagan was on hand at EAFB Runway 22 that U.S. Independence Day to welcome home the STS-4 crew. Some within NASA hoped that he would use the occasion to declare his support for a permanent Earth-orbiting space station, "the next logical step" after the Shuttle. Instead, Reagan declared that STS-4 was the final Shuttle test flight. With its next flight, STS-5, the Space Shuttle would be considered operational.

Reagan withheld his support for a further 18 months, until the beginning of the 1984 election year, when endorsing a space station - which was bound to create thousands of jobs - could provide maximum political advantage. During his 25 January 1984 State of the Union Address, he echoed President John F. Kennedy's May 1961 "Urgent National Needs" speech by calling on the U.S. civilian space agency "to develop a permanently manned space station and to do it within a decade." Reagan made mention only of the station's role as a laboratory. The station would, he said, "permit quantum leaps in our research in science, communications and in metals and life-saving medicines that can only be manufactured in space."

The Reagan White House disdained a space shipyard for two reasons. First, it was a relatively complicated design that could not be built for $8 billion spent over 10 years, the maximum price Administration budget watchdogs were willing to pay for a space station. The second reason was related to the first: a shipyard in space implied that things would be built there, and that in turn implied a commitment to new expenditures in the future.

Despite this clear message, NASA did not abandon its plans for a shipyard in orbit. In August 1984, the space agency released a "reference configuration" intended to guide aerospace companies bidding on Space Station Program contracts. Called the "Power Tower," it included a 400-foot-long single main truss where SOC-type space construction equipment might eventually be mounted. In NASA artwork depicting the station, featureless boxes stood in for unspecified large user payloads and hoped-for shipyard elements.

The August 1984 Power Tower station configuration was the Space Operations Center with trusses added. The small spacecraft with twin solar arrays at upper right is a self-propelled free-flyer bearing experiments likely to be interfered with by other station activities (for example, astronaut movement). Image credit: NASA
NASA envisioned that spacewalking astronauts would bolt together the Power Tower truss in orbit piece by piece. During Shuttle mission STS-61B (26 November-3 December 1985), in fact, spacewalkers successfully tested two truss-assembly methods in the payload bay of the Orbiter Atlantis.

From the Power Tower evolved the "Dual Keel" in late 1985. In May 1986, NASA released its Space Station "Baseline Configuration," a Dual-Keel station measuring 503 feet wide and 361 feet tall. The new design included about twice as many truss elements as the Power Tower, providing ample room for both space-facing and Earth-facing user payloads and eventual addition of space construction facilities. Assembly in orbit was to begin in 1992 and to be completed by Reagan's 1994 deadline.

The Baseline Configuration was dead on arrival, however, because of the 28 January 1986 loss of the Shuttle Orbiter Challenger and its seven-member crew. By March 1986, NASA and its contractors had begun to scale back the station. At first it shrank but retained its Dual-Keel shape. After that, in the "revised baseline configuration" of 1987, it lost its keel trusses, becoming only a single truss with solar arrays at either end and laboratory and habitat modules at its center. NASA made sure, however, that the design included "hooks" and "scars" that would enable eventual expansion to the Dual-Keel design.

NASA's ambitious Dual-Keel Baseline Configuration of May 1986 was dead on arrival. Image credit: NASA
President Reagan christened the Space Station Freedom in 1988; a gesture which for some rang hollow (they had hoped he might support a moon and Mars program that would give Freedom a long-term direction). The following year, with the Station expected to be over-budget, over-weight, under-powered, and too demanding to build, NASA entirely abandoned the Dual Keel configuration. At the same time, planners proposed that NASA make plans to build an advanced "transportation node" space station in the early 21st century. This proposed separation of functions was an acknowledgment that the jolts and vibrations one could expect on board an orbital ship-yard would wreak havoc with microgravity laboratory experiments.

The year 1990 saw new problems. Persistent hydrogen fuel leaks grounded the three-orbiter Shuttle fleet for nearly half the year, renewing doubts about the Shuttle's ability to reliably launch, assemble, resupply, and staff Freedom. Against this background, news emerged of a dispute within NASA over estimates of the number of spacewalks required to build and maintain the Space Station. The row triggered congressional hearings in May 1990.

In a report released on 20 July 1990, former astronaut and spacewalker William Fisher and JSC robotics engineer Charles Price, co-chairs of the Space Station Freedom External Maintenance Task Team, declared that Freedom would need four two-man spacewalks per week during its assembly and 6,000 hours of maintenance spacewalks per year after its completion. This amounted to 75% more spacewalks than the official NASA estimate, which was already considered excessive. Fisher called the spacewalk requirement "the greatest challenge facing the Space Station."

In November 1990, with new budget cuts in the offing, NASA began yet another Freedom redesign. At about the same time, Space Industries Incorporated (SII), a small engineering firm for which Maxime Faget, co-designer of the Mercury capsule, worked as Technical Advisor, began to examine a radical new approach to solving Freedom's persistent problems. SII performed its Orbiter-Derived Station (ODS) study on contract to Rockwell International, prime contractor for the Shuttle Orbiter.

SII noted that the U.S. House of Representatives Committee on Science, Space, and Technology wanted a "permanently manned Space Station, that meets our International Agreements, retains a capability for evolution, and has minimum annual and aggregate cost." At the same time, it explained, scientists and engineers of the space technology development and microgravity and life sciences research communities wanted NASA to provide an orbiting laboratory "without spending the entire available budget on the laboratory rather than on the experiments."

To satisfy these needs, SII proposed to draw upon Space Shuttle design heritage and operational experience. Specifically, the company proposed that NASA launch in 1996 an unmanned "stripped-down" Orbiter – one without wings, tail, landing gear, body flap, forward reaction control thrusters, and reentry thermal protection – to serve as Freedom's largest single element.

Orbiter Derived Station in Man Tended Configuration after Mission Build-1. Image credit: SII/NASA
Removing systems with a total mass of 45,600 pounds would boost the Orbiter's payload capacity to 81,930 pounds, permitting it to transport a 56.5-foot-long pressurized module permanently mounted in its payload bay and four pairs of rolled-up 120-foot-long solar arrays under streamlined housings along its sides. The pressurized module would include a single docking port on top and a short tunnel linking it to the stripped-down Orbiter's two-deck crew compartment. In effect, SII's ODS launch approach would briefly restore the heavy-lift capability lost when the U.S. abandoned the Saturn V rocket.

What follows is a synthesis of information from two SII documents concerning the ODS. The first, a set of presentation slides, is not dated, though individual slides in the presentation carry July 1991 dates. The second document is SII's final report to Rockwell International dated September 1991. When the documents differ in significant ways, this is noted.

Copying NASA parlance, SII referred to the launch of the stripped-down Orbiter as Mission Build-1 (MB-1). Upon achieving a 220-nautical-mile-high orbit inclined 28.5° relative to Earth's equator, the ODS would turn its payload bay doors toward Earth, open them to expose the pressurized module and door-mounted radiators, and unroll its solar arrays to generate up to 120 kilowatts of electricity. At that point, the ODS would achieve Man-Tended Configuration (MTC). MTC meant that the station could be staffed while a Shuttle Orbiter was docked with it. According to SII, NASA's Freedom would not achieve MTC until MB-6, and its solar arrays would not generate 120 kilowatts of electrical power until MB-10.

Orbiter Derived Station (top) and Shuttle Orbital Maneuvering System propulsion pod design differences. Image credit: SII/NASA
During a normal Space Shuttle mission, the twin 6,000-pound-thrust Orbital Maneuvering System (OMS) engines would ignite twice to complete orbital insertion after the Orbiter's three Space Shuttle Main Engines (SSMEs) shut down and and its External Tank separated. The OMS-1 burn would put the Orbiter into an elliptical orbit; then, at apogee (the high point of its orbit about the Earth), the OMS-2 burn would raise its perigee (the low point in its Earth orbit) to make its orbit circular. Subsequently, the OMS engines would be used to perform major maneuvers and would slow the Orbiter at the end of its mission so that it would reenter the atmosphere. The OMS engines would burn hypergolic (ignite-on-contact) hydrazine/nitric acid propellants.

SII proposed changes to the stripped-down Orbiter's OMS pods to increase reliability and enable long-duration use. A hydrazine monopropellant system would replace the baseline Orbiter bi-propellant system. The SSMEs would insert the stripped-down Orbiter directly into its initial elliptical orbit, then two sets of four 500-pound-thrust OMS engines – one set per OMS pod – would each draw on a pair of propellant tanks to perform the OMS-2 orbit-circularization burn at apogee. The roughly 13,000 pounds of propellant remaining after the OMS-2 burn would be sufficient to resist atmospheric drag and supply OMS pod attitude-control thrusters for two years.

SII suggested that the OMS tanks be refilled in orbit after they exhausted their initial load of hydrazine, but provided no details as to how this might be accomplished. Alternately, the company suggested, a new propulsion module might be docked with the ODS after the modified OMS pods ran out of propellant.

With MB-1 complete, SII's ODS would provide 11,000 cubic feet of pressurized volume containing 58 standardized payload racks. NASA’s Freedom, by comparison, would have no pressurized volume at all until the addition of the U.S. Lab during MB-6, and would not exceed 10,000 cubic feet of pressurized volume until MB-13. The U.S. Hab and Lab modules would together hold only 48 racks.

In SII's July 1991 ODS design, the large module launched in the stripped-down Orbiter payload bay on MB-1 included only Hab module functions, and MB-2 in 1997 would see a piloted Shuttle Orbiter deliver the U.S. Lab module. In its September 1991 final report, SII combined Lab and Hab in the stripped-down Orbiter payload bay and substituted a 47.5-foot-long "core module" for the Lab on MB-2. The cylindrical core would include eight docking ports on its sides and one at either end.

One of the core module end ports would be docked permanently with the port on the Hab/Lab module. Visiting Shuttle Orbiters would dock with the Earth-facing port at the core module's other end. Addition of the core module would increase ODS pressurized volume to 15,000 cubic feet. NASA's Freedom station would not exceed 15,000 cubic feet of volume until MB-16.

SII envisioned that ODS assembly flights would be interspersed with utilization flights beginning immediately after MB-1. The first ODS utilization mission would occur in 1996, and three would take place in 1997.

In addition to permitting early research on board the ODS, some utilization flights after MB-2 would deliver supplies and equipment in a drum-shaped Logistics/Life Support Module (LLSM). Astronauts would dock the LLSM to a core module side port using the visiting Orbiter's Canada-built Remote Manipulator System (RMS). Spent LLSMs would be returned to Earth for refurbishment and reuse. SII placed the ODS toilet and shower in the LLSM, arguing that servicing waste and water systems on the ground would be preferable to doing so in orbit.

SII noted that its Station would need very few assembly and maintenance spacewalks. It would, nevertheless, include a modified Shuttle Orbiter airlock attached to one of its core module side ports. The airlock would reach the ODS during an unspecified utilization flight after MB-2. Because ODS assembly would be relatively simple and assembly spacewalks minimal, SII assumed that the Station could do without its own Canada-built RMS. The company did not address how deletion of the Station RMS would affect U.S.-Canada relations.

The second assembly mission of 1997, MB-3, would see arrival of an Orbiter bearing in its payload bay an eight-man Assured Crew Return Vehicle (ACRV), a space station lifeboat. With the docking of the ACRV at a core module side port, the ODS could be staffed by eight astronauts with no Orbiter present. NASA called the ability to maintain a full crew with no Orbiter present "Permanent Manned Configuration" (PMC). NASA's Freedom Station would not achieve PMC until MB-16.

Orbiter Derived Station in Assembly Complete configuration after Mission Build-6 in late 1998. The image displays an RMS robot arm, though SII stated that the stripped-down Orbiter would not carry one. Image credit: SII/NASA
The year 1998 would see three assembly flights, all international in character. In his January 1984 State of the Union speech, Reagan had invited U.S. allies to lend a hand in building NASA's space station in exchange for opportunities to reap its rewards. In addition to Canada, Japan and Europe had answered the call.

MB-4 would see an Orbiter deliver the pressurized part of the Japanese Experiment Module (JEM). Astronauts would use the Orbiter's RMS to dock it to a core module side port. During MB-5, astronauts would use the visiting Orbiter RMS to add the European Space Agency's Columbus laboratory module. With that, the SII's ODS would achieve its maximum pressurized volume: 24,000 cubic feet, or about 8,000 cubic feet more than planned for NASA's Freedom Station. MB-6 would add Logistics and unpressurized Exposure components to complete the JEM.

SII recommended that the core module's Earth-facing port be designed to rotate so that visiting Orbiters could optimally position themselves for assembly missions. During MB-5, for example, the visiting Orbiter's nose would face in the ODS's direction of flight so that its RMS could place the Columbus module at its designated core module side port. During MB-4 and MB-6, it would face in the opposite direction so that JEM components could be added.

MB-6, which would take place near the end of 1998, would mark the end of ODS assembly. By then, SII's station would have hosted seven utilization flights. For comparison, NASA's Freedom Space Station would host no utilization flights until 1998, when three would take place. Freedom would not reach "Assembly Complete" until 2000.

SII proposed ways that the baseline ODS might be upgraded. The company noted that, beginning with MB-10, NASA's Freedom would provide experimenters with more electricity (180 kilowatts) than would the ODS. If this power level were judged to be necessary for ODS operations, then a 60-kilowatt "power kit" could be added during a utilization flight. The company suggested that the kit's rolled solar arrays be attached to a special port installed in the stripped-down Orbiter's nose behind a streamlined faring.

The ODS included no provision for space-facing experiments; all of its modules were expected to be mounted on its Earth-facing payload bay side. This reflected the science and technology community's desire for a microgravity lab and the fact that highly capable automated space-facing satellites (for example, the Hubble Space Telescope) were available. If, however, space-facing experiments were desired on board the ODS, then it could be launched with a docking port on the Orbiter's space-facing belly. A tunnel through the ODS payload bay floor would link the port to the Hab/Lab module.

Probably the company's most controversial proposal was to accelerate ODS assembly by stripping down Columbia, NASA's oldest Orbiter. SII noted that Columbia was the heaviest Orbiter, so had the least payload capacity. It assumed that NASA would want to replace Columbia with a new, less heavy Orbiter, thus increasing the Shuttle fleet's overall lift capacity. SII called this "disposing of the worst and and replacing it with the best." Some components stripped from Columbia could, it suggested, be reused in the new Orbiter to save money.

By the time SII submitted its final report, NASA's latest Freedom configuration had been public for three months. The new design included truss segments launched pre-assembled, smaller U.S. modules, and other changes meant to reduce the number of spacewalks and assembly flights required to build and maintain it. The station would, however, lose yet more capability (notably in the area of electrical power, which was reduced to about 60 kilowatts at PMC). The April 1991 redesign set the stage for Freedom's near-cancellation in June 1993 (it survived by a single vote in the U.S. House of Representatives) and, beginning later that year, its revival as the International Space Station.

This rendition of Space Station Freedom in its 1991 configuration contains several interesting features. The overall station design is obscured by shadows, denoting the uncertainty surrounding the station's future form. Only the international pressurized modules - the JEM and Columbus labs - are visible. Beginning with the the May 1986 Dual Keel, these modules changed very little in NASA artwork because the International Partners insisted that the U.S. adhere to its agreements. The U.S. modules, in the meantime, decreased in number and shrank to a fraction of their planned former size. A Shuttle Orbiter is displayed, but not attached to Freedom; placing it too close to the station would show plainly the 1991 station's small size relative to earlier designs. The moon and Mars are visible above Freedom; in 1991, NASA still paid lip-service to carrying out President George H. W. Bush's abortive Space Exploration Initiative (SEI), which aimed to launch humans to those worlds. Freedom was meant to play a role in furthering SEI's goals, though the precise nature of that role was not clear. Image credit: NASA

The Space Shuttle at Work, NASA SP-432/EP-156, H. Allaway, NASA, 1979, pp. 64-72

Aboard the Space Shuttle, NASA EP-169, F. Steinberg, NASA, 1980

Space Station, NASA EP-211, D. Anderton, NASA, no date (1984)

Space Station: The Next Logical Step, NASA EP-213, W. Froehlich, NASA, no date (1985)

Space Station: Leadership for the Future, NASA PAM-509, F. Martin & T. Finn, NASA, August 1987

Space Station: A Step Into the Future, NASA PAM-510, A. Stofan, NASA, November 1987

Space Station Freedom Reference Guide, Boeing, 1988

Space Station Freedom: A Foothold on the Future, NASA NP-107, L. David, NASA, October 1988

"Freedom Spacewalks 'unacceptable': NASA," Flight International, 1-7 August 1990, p. 18

"Freedom failure threatens NASA's future," T. Furniss, Flight International, 29 May-4 June 1991, p. 34

"Operation Scale-Down," T. Furniss, Flight International, 29 May-4 June 1991, pp. 76-78

Shuttle Derived Space Station Freedom, Space Industries International, Inc./Rockwell International Space Systems Division, presentation materials, n.d. (July 1991)

Expanded Orbiter Missions Final Report: Orbiter Derived Space Station Freedom Concept, prepared by Space Industries, Inc. (SII), Webster, Texas, for Rockwell International, Inc., Downey, California, September 1991

"House Retains Space Station in a Close Vote," C. Krauss, International New York Times, 24 June 1993 (http://www.nytimes.com/1993/06/24/us/house-retains-space-station-in-a-close-vote.html - accessed 16 October 2015)

International Space Station, Boeing, May 1994

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