Rabu, 10 Mei 2017
JPL's MSR spacecraft, which used the Mars Orbital Rendezvous mission mode, consisted of an orbiter for transporting a lander to Mars and returning the Mars samples to Earth, a rover for sample collection, a Mars Ascent Vehicle (MAV) for boosting the collected samples to Mars orbit for retrieval by the orbiter, and a lander for delivering the rover and MAV to Mars's surface.
In May 1998, JPL rover engineer Brian Wilcox proposed a possible solution: replace the MSR mission design's massive (about 500 kilograms) liquid-propellant MAV with a low-mass solid-propellant MicroMAV. The following month, JPL engineers Duncan MacPherson, Doug Bernard, and Bill Layman began a preliminary study to attempt to validate Wilcox's concept. As part of their effort, they held a "mini-workshop" at which they consulted with space industry propulsion engineers. By September, MacPherson was ready to present his group's findings to the second meeting of the NASA-appointed Mars Architecture Team (MAT).
Wilcox had envisioned an alternative MSR scenario in which a large rover would carry and launch his 20-kilogram MicroMAV. MacPherson, Bernard, and Layman proposed a MAV that burned solid propellants but had a more realistic estimated mass of 110 kilograms. This would, they found, require a return to a more traditional MSR scenario in which the MAV would lift off from a stationary lander. A rover would collect samples and deliver them to the MSR lander, which would load them into a container in the MAV's third stage.
Wilcox assumed that, during first-stage flight, airflow over four canted fins on his MicroMAV's first stage could spin it to provide gyroscopic stability. MacPherson, Bernard, and Layman judged, however, that martian air was not dense enough for canted fins to be effective. Prior to first stage ignition, thus, a spin table on the MSR lander would spin their MAV about its long axis 300 times per minute to provide gyroscopic stability. The first stage motor would then ignite and hurl the MAV skyward at from six to 10 gravities of acceleration.
Industry experts attending the mini-workshop had told MacPherson, Bernard, and Layman that metal-based solid propellant yields molten slag when it burns. In a rapidly spinning motor, the centrifugal force causes the slag to adhere to the nozzle, producing unpredictable mass imbalances. These could destablilize the ascending rocket, causing it to tumble out of control. A high spin rate could also cause uneven solid propellant burning. MacPherson told the MAT that metal-free solid propellant would eliminate both problems, though at the price of reduced motor performance.
After first stage burnout, a small despin motor would slow the MAV's rate of spin to 20 revolutions per minute. The MAV would then coast to an altitude of 90 kilometers. Wilcox assumed no active attitude control during the coast, but MacPherson, Bernard, and Layman invoked cold-gas attitude control thrusters to compensate for winds and to orient the MAV accurately for the second stage burn.
An inertial measurement unit and a sun sensor would provide data to the thruster guidance system and to a timer that would govern subsequent MAV operations. The spent first stage would detach one second after timer activation, then the second stage motor would ignite one second after that. Second stage acceleration would peak at 35 times the pull of Earth's gravity just before burnout. The second stage would boost the MAV's apoapsis (orbit high point) to 300 kilometers above Mars, then would separate two minutes after timer start.
Wilcox gave little attention to the MicroMAV's role in preventing biological contamination of Mars (forward contamination) or Earth (back contamination). MacPherson noted that the second-stage motor's trajectory after separation would take it back into Mars's atmosphere, thus eliminating it as a possible source of back contamination.
As in the Wilcox design, the MacPherson/Bernard/Layman third stage motor nozzle would point forward during first stage and second stage flight, ensuring that it would point aft when the gyro-stabilized MAV attained apoapsis halfway through its first orbit. The timer would ignite the third stage motor 50 minutes after timer start; if all had functioned as planned up to that point, this would coincide with apoapsis. The brief burn would raise the MAV's periapsis (orbit low point) out of the atmosphere to an altitude of at least 300 kilometers.
As its last act, the timer would fire a motor that would halt the MAV's spin so that the orbiter could more easily capture it. The waiting orbiter would then maneuver to retrieve the MAV third stage and the precious Mars samples it carried.
MacPherson, Bernard, and Layman found that minor guidance errors, motor performance variations, and the vagaries of Mars's atmosphere could affect the MAV's final orbital parameters and thus the magnitude of the maneuvers the orbiter would need to perform to rendezvous with it. Wilcox, always optimistic about his MicroMAV's capabilities, had calculated that compensating for orbital uncertainties would require that the orbiter carry only enough propellants to enable velocity changes totaling about 100 meters per second. MacPherson's team, by contrast, estimated a possible MAV periapsis range of 300-to-500 kilometers, an apoapsis range of 600-to-800 kilometers, and an orbital inclination range spanning one degree. In the worst case scenario, this would mean that the MSR orbiter might need to make velocity changes totaling about 260 meters per second.
The MacPherson group's results might have thrown cold water on the MicroMAV concept. A 110-kilogram MAV was, however, an improvement over one with a mass of 500 kilograms. Even before they finished their work, JPL adopted the small solid-propellant MAV as part of its baseline MSR mission design.
One of O'Neil's first initiatives in his new role was a pair of workshops aimed at generating fresh ideas for JPL's Mars Sample Return (MSR) mission, which had become mired in fiscal and engineering problems. In its April 1998 iteration, a single launch vehicle boosted the MSR spacecraft to Mars in late 2004. By the start of the first MSR workshop in July, a redesign effort begun in June had yielded a baseline MSR plan that split the orbiter and lander between two rockets launched in August and September 2005. The lander would carry the Mars Ascent Vehicle (MAV), which would launch the Mars sample to orbit for retrieval by the orbiter and return to Earth. The 512-kilogram liquid-propellant MAV was overweight, contributing to mass limitations which meant that only a small, short-range sample-collecting rover could be included in the mission.
In his presentation to the first MSR workshop, Brian Wilcox, a JPL rover engineer and former model rocketry enthusiast, described a possible alternative to the baseline mission's liquid-propellant MAV based on the U.S. Navy's 1958 PILOT microsatellite launcher design. His "MicroMAV" was a 20-kilogram solid-propellant rocket with no moving parts. Wilcox noted that, unlike liquid propellants, solid propellants would not freeze during the frigid martian night.
Wilcox envisioned an MSR similar to one he proposed in 1989, in which a rover with six wheels and a top-mounted solar array would carry the three-stage MicroMAV with it while exploring Mars. The MicroMAV would ride slung horizontally along one of the rover's sides. The rover would collect an unspecified quantity of rocks and dirt and load them into the sample canister in the MicroMAV's third stage, then would pivot the MicroMAV onto the top of the solar array and point its nose skyward. The MicroMAV would then ignite its first stage motor.
The first stage, which would loft the MicroMAV above most of Mars's atmosphere, would have a total mass at ignition of 9.75 kilograms, of which 7.8 kilograms would comprise solid propellant. It would include four fins and a horizon sensor. The fins would be canted slightly so that the thin martian air rushing past them during ascent would spin the MicroMAV about its long axis for gyroscopic stabilization.
After first stage burnout, the MicroMAV would coast upward, still spinning about its long axis. As it neared the top of its trajectory, its nose would begin to tip downward toward the horizon. The horizon sensor would alternately "see" the sky above and the ground below.
When the sensor tallied a pre-set number of rotations, it would trigger second stage ignition. This would also discard the first stage. The second stage, which would supply most of the MicroMAV's orbital velocity, would have a mass of 9.4 kilograms with 7.8 kilograms of propellant. After second stage burnout and separation, the MicroMAV third stage would be in Mars orbit; its periapsis (orbit low point) would, however, remain within Mars's atmosphere. Second stage burnout would thus trigger a timer designed to ignite the third stage motor.
The tiny 0.85-kilogram third stage would include 0.05 kilograms of propellant and the Mars sample. During first and second stage flight, its motor would point forward. Because it would be spinning like a gyroscope, it would remain pointed in the same direction following second stage separation. This would mean that, half a Mars orbit later, the motor would point away from its direction of motion. At that same moment, the MicroMAV would attain apoapsis (orbit high point) and the timer would reach zero. The third stage engine would then ignite to raise the MicroMAV's periapsis to a safe altitude.
Third stage ignition would also ignite a "pyrotechnic layer" that would heat its exterior "white-hot for an instant." This would destroy any martian microbes that might have hitched a ride on the third stage and would also solder shut the sample canister to prevent the escape of any contaminants inside.
The grapefruit-sized MicroMAV sample canister would be entirely passive, with neither a radio beacon nor a flashing light to aid the orbiter in locating it. The orbiter would begin looking for the MicroMAV from a position about 100 kilometers above its orbit. For 18% of its orbit, the canister would be sunlit but set against Mars's nightside as seen from the orbiter. At such times, the orbiter would point its wide-angle imager toward the MicroMAV's predicted position and image the area several times to enable controllers on Earth to determine the MicroMAV sample canister's orbit. Wilcox estimate that controllers using orbiter images would need no more than 31 hours to locate the MicroMAV.
The MicroMAV concept excited much interest among JPL engineers. Though further study revealed the MicroMAV MSR scenario to be unworkable in the form Wilcox described - for example, JPL quickly abandoned rover launch in favor of a more conventional launch from a fixed lander (image above) - the concept of a simplified solid-propellant MAV profoundly influenced subsequent JPL MSR planning.
In a paper presented at the Fifth International Astronautical Federation Congress in 1954, Stuhlinger pitched interplanetary travel using low-thrust ion (electric) propulsion. The spacecraft design he proposed comprised three major parts: the crew/payload compartment at the ship's center; a 146.4-ton multi-unit solar-electric power system; and a multi-chamber low-thrust ion drive system.
Stuhlinger provided no details about the layout of his ship's crew/payload compartment, other than that it would carry up to 50 tons of crew and cargo. He did, however, offer abundant details on his solar-electric power and ion drive systems.
The former would include two 350-meter-wide "wings," each comprising 19 independent electricity-generating "sub-units." A dish-shaped mirror 50 meters wide would form the largest component of each 4400-kilogram sub-unit. Stuhlinger wrote that his spacecraft would gain speed very slowly, accelerating at a rate equal only to about 1/1000th of Earth's surface gravity. At such a low rate of acceleration, a fork dropped in the ship's messroom would need more than five minutes to strike the floor. The low acceleration would mean that the mirrors would have no need of robust construction; they might comprise "thin aluminum foil with a very light supporting frame."
Each 450-kilogram, 2000-square-meter mirror would concentrate sunlight onto a boiler, causing a working fluid within it to turn to steam. The steam would drive a turbine, which would in turn drive a generator capable of producing 200 kilowatts of electricity. The steam, meanwhile, would enter a disk-shaped radiator cooler and condense back into fluid. Boiler, turbine/generator, and cooler would revolve together as a unit, completing one revolution every 10 seconds. This would generate acceleration that would cause the working fluid to flow to the cooler's outer rim, from which it would be pumped back to the boiler.
The multi-unit solar-electric power system would have built-in redundancy, Stuhlinger noted. Even if a large "meteor" hit the ship, he wrote, "the total loss of one or two sub-units would mean only a minor reduction of the capacity of the power plant."
Stuhlinger rejected an "atomic pile" as a heat source; in addition to having a mass of "hundreds of tons," a reactor would emit harmful radiation that would demand heavy shielding and make in-flight repair difficult. He added, however, that "an atomic pile will be a very promising power source for an electrically propelled space ship as soon as the mass problem, the shielding problem, and the maintenance problem have been solved satisfactorily."
The third major part of Stuhlinger's ship, the ion drive, would consist of many clustered thrust chambers. Within each, electricity from the solar-electric power system would ionize cesium or rubidium vapor using heated platinum grids and paired positive and negative electrodes. The cesium or rubidium ions would then depart the chamber through an opening at a large fraction of the speed of light to push the ship through space.
Stuhlinger wrote that cesium would be a more efficient propellant than rubidium. A cesium-fueled ship would need only 1833 thrust chambers to produce as much thrust as a rubidium-fueled ship with 2200 chambers. He noted, however, that cesium is "a rare element which might not be available in quantities as required for space ships."
Despite its large number of thrust chambers, Stuhlinger's ion drive would generate at most nine kilograms of thrust. This would, however, be applied continuously for long periods. Assuming no interference from planetary or solar gravity, Stuhlinger's ship could in a year travel 183 million kilometers in a straight line and reach a velocity of 12 kilometers per second.
Stuhlinger calculated that his ship would need just 18.6 tons of rubidium to accelerate continuously for one year. Even with its elaborate solar-electric power and ion drive systems, his ship's mass would total just 280 tons. To reach the same 12-kilometer-per-second velocity, a chemical-propulsion Mars spaceship would need a mass of about 820 tons, most of which would comprise propellants. For his calculations, Stuhlinger assumed that the chemical ship's rocket motors would burn nitric acid oxidizer and hydrazine fuel. He also assumed that both the ion and chemical Mars ships would be assembled in Earth orbit from components launched atop chemical-propulsion cargo rockets; his ship's lesser mass meant that it would need about a third as many cargo launches for assembly as would its chemical counterpart.
An ion drive spaceship would, of course, not travel between Earth and Mars in a straight line; it would instead gradually spiral out of Earth orbit into solar orbit, follow a curved course around the Sun to Mars, capture into a distant Mars orbit, spiral gradually down to a low Mars parking orbit, spiral out of Mars orbit, follow a curved course around the Sun back to Earth, capture into distant Earth orbit, and gradually spiral down to low Earth parking orbit. Halfway to Mars and again halfway to Earth the ship would turn end for end to face its thrust chambers forward and begin a slow deceleration. Stuhlinger determined, nonetheless, that his low-thrust solar-electric ion drive spaceship could travel from Earth orbit to Mars orbit and back in just two or three years; that is, in approximately the same period of time that a high-thrust chemical spaceship would need.
Stuhlinger did not call his spaceship the Cosmic Butterfly; that name originated with Frank Tinsley (1899-1965), an artist, cartoonist, and author famed for his futuristic technical illustrations. Tinsley used the term "gigantic butterfly" in reference to Stuhlinger's design in a 1956 article in Modern Mechanix magazine. The illustration at the top of this blog post, which Tinsley painted in 1959 for an American Bosch Arma Corporation advertisement titled "Cosmic Butterfly," depicts a ship little different from Stuhlinger's 1954 design.
Senin, 08 Mei 2017
At the same time, the U.S. had developed new priorities - for example, prosecuting the war in Indochina. This was reflected in changes in NASA's budget and workforce. The space agency's budget peaked in 1967 at nearly $6 billion, or about 0.9% of Gross National Product (GNP) - the largest fraction of GNP NASA ever attained. By the time of the triumphant first Apollo moon landing on July 20, 1969, NASA's budget had been pared to $4 billion. In 1965, the NASA and NASA contractor workforce totaled 420,000 people across the United States; by the end of Fiscal Year 1969, this had slumped to 220,000, triggering an “aerospace depression.” States like California and Florida, where space contractors were concentrated, bore the brunt of the cuts. With this background in mind, NASA’s report to the STG began with the following prescient words:
At the moment of its greatest triumph, the space program of the United States faces a crucial situation. Decisions made this year will affect the course of space activity for decades to come. . .NASA argued that the Nixon Administration had before it a unique opportunity for greatness. Nixon could, if he so chose, become the President known for launching America to the planets.
This Administration has a unique opportunity to determine the long-term future of the Nation's space progress. We recommend that the United States adopt as a continuing goal the exploration of the solar system. . .To focus our developments and integrate our programs, we recommend that the United States prepare for manned planetary expeditions in the 1980s.In an effort to stem its critics, who increasingly asserted that NASA's programs and goals were irrelevant to America's many pressing problems, the NASA report devoted considerable attention to Earth-centered benefits of spaceflight - for example, the potential public health benefits of medical experiments performed aboard Earth-orbiting space stations - and claimed that "the national civilian space effort has contributed $35 billion in goods and services to the U.S. economy." At the time, a large Earth-orbiting space station was NASA’s top priority as an immediate post-Apollo goal. Recognizing that the Soviet space threat no longer carried the weight that it had a decade earlier, and knowing the Nixon Administration’s own geopolitical preferences, NASA proposed spaceflight as a vehicle for international cooperation, not competition.
The report then asserted that NASA should receive sufficient resources in the 1970s to build on the capabilities it developed in the 1960s, a period during which
the American space program progressed from the 31-pound Explorer 1 in earth orbit to Apollo spacecraft weighing 50 tons sent out to the moon; [and] from manned flights of a few thousand miles and 15-minute duration to the 500,000 mile round-trip 8-day [Apollo 11] mission which landed men on the moon and returned them safely to earth.Continued manned lunar exploration after Apollo would, the report explained, "expand man's domain to include the moon." Large space stations and a space transportation system comprising reusable vehicles - a winged shuttle for delivering crews and supplies to the Earth-orbiting station, a nuclear-propulsion cislunar shuttle for transportation between Earth orbit and a lunar-orbiting space station, and a chemical-propulsion space tug that would do double-duty as a moon lander - would support the post-Apollo lunar program. This “integrated program” would lay the groundwork for the first manned Mars landing in the 1980s.
This vision, often identified with rocketry pioneer Wernher von Braun, director of NASA’s Marshall Space Flight Center in Huntsville, Alabama, is probably better ascribed to George Mueller, who became NASA’s Associate Administrator for Manned Space Flight in November 1963, NASA’s new Administrator, Thomas O. Paine, an STG member and chief author of NASA's report to the STG, and U.S. Vice-President and STG chair Spiro Agnew. More politically savvy and technically conservative STG participants – for example, U.S. Air Force Secretary Robert Seamans, a former NASA Deputy Administrator - did their best to rein in the breathless enthusiasm of Washington neophytes Paine and Agnew. Seamans, Office of Management and Budget chief Robert Mayo, and others understood that neither President Nixon nor the Congress would support a new Apollo-scale space program, let alone one several times larger and more costly.
NASA’s report proposed four possible "program rates" based on available funding. The “maximum rate” (that is, fully funded) program would begin in 1975, immediately following the last Apollo moon missions and the Skylab Program, with the launch on a two-stage Saturn V of the Earth-orbiting station and the maiden flight of the winged Earth-to-orbit shuttle. The following year, NASA would use Saturn Vs to launch a space station to lunar orbit and would debut the space tug/lunar lander. The year 1978 would see introduction of the nuclear cislunar shuttle and a lunar surface base established using space tug landers. By 1980, a 50-man Space Base would orbit the Earth. The next year, NASA would launch the first in a series of three-year Mars expeditions. The Space Base, meanwhile, would expand by 1985 - just one decade after program start - to support a crew of 100.
NASA’s Program I was only a little less ambitious than the maximum rate. It would start a year later, in 1976, with the Earth-orbiting space station and Earth-to-orbit space shuttle. The lunar-orbiting station and space tug/lunar lander would be postponed two years to 1978. The nuclear cislunar shuttle would, however, also debut in 1978, the same year as in the maximum rate program. The year 1980 would see both the lunar surface base and the 50-man Space Base brought into service. The first Mars expedition would be bumped to 1983, but the 100-man Space Base would be in place by 1985, as in the maximum rate plan.
Program II, the pacing option Agnew favored, would get off to a delayed start, with the Earth-orbiting space station and Earth-to-orbit shuttle both coming on line in 1977. The lunar-orbiting station, space tug/lunar lander, and nuclear cislunar shuttle would begin operations simultaneously in 1981, with the lunar surface base following two years later. The following year (1984), 50 men would orbit Earth in a Space Base. Men would walk on Mars for the first time in 1986, and the Space Base population would reach 100 in 1989.
NASA’s Program III, tacked on almost as an afterthought, was hardly an attempt at conservatism: it was identical to Program II, except that it set no date for the first Mars expedition. The Nixon Administration paid lip service to elements of Program III, declaring that the decisions it was making about NASA's future would make possible a manned Mars voyage before the end of the 20th century. At the same time, however, Nixon continued to cut NASA's budget until it touched bottom at about $3 billion in Fiscal Year 1971. This spelled the end for the technologies and hardware NASA had identified as necessary for humans on Mars, including nuclear propulsion, the Saturn V, and the large Earth-orbiting station. Mueller saw the handwriting on the wall and left his NASA post in December 1969; Paine resigned close to the first anniversary of NASA's report to the STG.
For his part, Mueller's departure did not signify that he had abandoned support for an integrated NASA program including space stations, a moonbase, and Mars expeditions. Almost as a parting shot, he published "An Integrated Space Program for the Next Generation," the cover article in the January 1970 issue of Astronautics & Aeronautics.
Significantly, the cover illustration for Mueller's article depicted a two-stage fully-reusable Space Shuttle (top image above). The Shuttle had already begun its rise from a mere utilitarian logistics spacecraft in Mueller and Paine's plan to the multipurpose centerpiece of Nixon's post-Apollo space program. On January 5 of the election year 1972, Nixon and new NASA Administrator James Fletcher unveiled a partially reusable Space Shuttle design in California, a state crucial for Nixon's reelection bid. They announced that the Shuttle would be assembled there, creating tens of thousands of aerospace jobs.
For more than a year before President John F. Kennedy's May 25, 1961 call for a man on the moon, Apollo had been seen primarily as an Earth-orbital spacecraft capable of both independent manned missions and crew ferry flights to Earth-orbiting space stations. A decade after Kennedy's call, NASA was preparing for Skylab A, its first Earth-orbiting space station, which would receive at least three three-man crews on board Apollo Command and Service Module (CSM) spacecraft (images above). The agency also studied independent CSM missions in Earth orbit and CSM missions to Earth-orbiting stations other than Skylab A.
The CSM, which measured a little more than 11 meters long, comprised the conical Command Module (CM) and the drum-shaped Service Module (SM). The CM's nose carried a probe docking unit, and at the aft end of the SM was mounted the Service Propulsion System main engine. The CM also included the pressurized crew compartment, flight controls, a bowl-shaped heat shield for Earth atmosphere reentry, and parachutes, while the SM included hydrogen-oxygen fuel cells for making electricity and water, propellant tanks, four attitude-control thruster quads, and room for a Scientific Instrument Module (SIM) Bay.
On August 27, 1971, Philip Culbertson, director of the Advanced Manned Missions Program at NASA Headquarters in Washington, DC, dispatched a letter to Rene Berglund, Manager of the Space Station Project Office at NASA's Manned Spacecraft Center (MSC) in Houston, Texas, in which he outlined five Earth-orbital CSM missions that were "still under active consideration" at NASA Headquarters. Culbertson explained that his letter was meant to "emphasize the importance" of statements he had made in a telephone conversation with Berglund on August 19.
Culbertson referred to an unspecified new contract MSC had awarded to North American, prime contractor for the CSM. He told Berglund that, in "the early stages of your contract. . .you should concentrate on defining the CSM modifications required to support each of the [five] missions and possibly more important defining the effort at North American which would hold open as many as possible of the [five] options until the end of the [Fiscal Year] 1973 budget cycle." U.S. Federal Fiscal Year 1973 would end on October 1, 1973.
The first and simplest of the five missions was an "independent CSM mission for earth observations." The mission would probably use a CSM with a SIM Bay fitted out with remote-sensing instruments and cameras. At the end of the mission, an astronaut would spacewalk to the SIM Bay to retrieve film for return to Earth in the CM.
The second mission on Culbertson's list was an Apollo space station flight unlike any envisioned in the year before Kennedy diverted Apollo to the moon. It would have seen a CSM dock in Earth orbit with a Soviet Salyut space station.
Salyut 1, the world's first space station, had reached Earth orbit on April 19, 1971. The 15.8-meter-long station remained aloft as Culbertson wrote his letter, but had not been manned since the Soyuz 11 crew of Georgi Dobrovolski, Viktor Patsayev, and Vladislav Volkov had undocked on June 29, 1971, after nearly 24 days in space (a new world record). The three cosmonauts had suffocated during reentry when their capsule lost pressure, so the Soviet Union had halted manned missions while the Soyuz spacecraft underwent a significant redesign.
The third Earth-orbital CSM mission on Culbertson's list combined the first two missions into a single mission. The CSM crew would turn SIM Bay instruments toward Earth before or after a visit to a Salyut.
Culbertson's fourth CSM mission would see the Skylab A backup CSM (CSM-119) with a crew of three dock first with a Salyut for a brief time, then with Skylab A. CSM-119's crew would remain on board 26-meter-long Skylab A for an unspecified period. NASA planned that, during the three missions to Skylab A in the basic Skylab Program, CSM-119 would stand by as a rescue vehicle capable of carrying five astronauts (Commander, Pilot, and the three rescued Skylab A crewmen). It would thus need to be refitted for the Salyut-Skylab A mission. Culbertson added that the Salyut-Skylab A mission would begin 18 months after Skylab A reached orbit.
The fifth and final Earth-orbital CSM mission was really two (or, possibly, three) CSM missions. A pair of "90 day" CSMs would dock with the Skylab B station while a rescue vehicle modified to carry five astronauts stood by. Beginning in 1969 (that is, at the same time it started Skylab A funding), NASA had funded assembly of Skylab B as a backup in case Skylab A failed. Culbertson gave no date for the Skylab B launch, which would have required one of the two Apollo Saturn V rockets made surplus by the September 1970 cancellation of the Apollo 15 and 19 missions (the Apollo 20 mission had been cancelled in January 1970 to make its Saturn V available to launch Skylab A).
Of the five missions Culbertson declared to be on the table in August 1971, not one flew. Skylab A, re-designated Skylab I (but more commonly called Skylab), reached orbit on May 14, 1973. It suffered damage during ascent, but NASA and its contractors pulled it back from the brink. In August 1973, with Skylab I functioning well in Earth-orbit, NASA began to mothball its backup. Several plans for putting Skylab B to use were floated in the 1973-1976 timeframe, but Space Shuttle development had funding priority, so NASA's second space station wound up in the National Air and Space Museum.
The three CSM missions to Skylab spanned May 25-June 22, 1973, July 28-September 25, 1973, and November 16, 1973-February 8, 1974, respectively. Leaks in attitude control thrusters on the second CSM to dock with Skylab caused NASA to ready CSM-119 for flight; the leaks stopped of their own accord, however, so the rescue CSM remained earthbound.
In early April 1972, shortly before finalizing its agreement with NASA to conduct a joint Apollo-Salyut mission, the Soviet Union declared the concept to be impractical and offered instead a docking with a Soyuz. At the superpower summit in Moscow on May 24, 1972, U.S. President Richard Nixon and Soviet Premier Alexei Kosygin signed the agreement creating the Apollo-Soyuz Test Project (ASTP).
Apollo CSM-111 was the ASTP prime spacecraft, while CSM-119 was refitted to serve as its backup. In the event, the backup was not needed. CSM-111, designated simply Apollo, docked with Soyuz 19 on July 17, 1975. The last CSM to fly undocked on July 19 and returned from Earth orbit on July 24, 1975.