29 November 2017
Between 1959 and 1968, three X-15 rocket planes, two modified B-52 bombers, and a dozen pilots took part in joint U.S. Air Force/NASA X-15 research missions. Before the start of each mission, an X-15 was mounted on a pylon attached to the underside of a wing of a B-52 carrier aircraft at Edwards Air Force Base, California. Wearing a silver pressure suit, a single pilot boarded the 50-foot-long X-15 as it hung from the pylon, then the B-52 taxied and took off from a runway.
Early X-15 missions were "captive" flights, meaning that the rocket plane stayed attached to the B-52, or gliding flights, meaning that it carried no propellants and relied on its wings, which spanned only 22 feet, to make a controlled - though fast and steep - descent to a landing. Early powered flights used stand-in rocket engines taken from earlier X-planes. By late 1960, however, the X-15's throttleable 600,000-horsepower XLR99 rocket engine was ready. The engine was designed to burn the nine tons of anhydrous ammonia fuel and liquid oxygen oxidizer in the X-15's tanks in about 90 seconds at full throttle.
Most missions followed two basic profiles. "Speed" missions saw the rocket plane level off at about 101,000 feet and push for ever-higher Mach numbers. The X-15 reached its top speed - Mach 6.72, or about 4520 miles per hour - during the 188th flight of the series on 3 October 1967 with Air Force Major William "Pete" Knight at the controls.
Knight flew X-15A-2, the former X-15 No. 2, which had rolled over during an abort landing on 9 November 1962, seriously injuring its pilot, John McKay. When NASA and the Air Force rebuilt X-15 No. 2, they modified its design to enable faster flights. McKay resumed X-15 flights after his recovery, though injuries he sustained plagued him until his death in 1975 at age 52.
For "altitude" missions, the X-15 climbed steeply until it exhausted its propellants, then arced upward, unpowered. X-15 reached its peak altitude - 354,200 feet (almost 67 miles) above the Earth's surface - on 22 August 1963, with NASA pilot Joseph Walker in the cockpit.
During altitude missions, the pilot experienced several minutes of weightlessness as the X-15 climbed toward the high point of its trajectory, above 99% of the atmosphere, then fell back toward Earth. Aerodynamic control surfaces (for example, ailerons) could not work while the X-15 soared in near-vacuum, so the space plane included hydrogen peroxide-fueled attitude-control thrusters so that the pilot could orient it for reentry.
It was during an altitude mission that the X-15 program suffered its only pilot fatality. On 15 November 1967, Major Michael McAdams piloted X-15 No. 3 to 266,000 feet despite an electrical problem that made control difficult. During descent, McAdams lost control of the space plane, which went into a flat spin at Mach 5, then an upside-down dive at Mach 4.7. McAdams might have recovered control at that point, but then an "adaptive" flight control system malfunctioned, thwarting maneuvers that might have damped out excessive pitch oscillations and compensated for increasing atmospheric density. The X-15 broke apart at about 65,000 feet.
Flights of early rocket-powered X planes, such as the first aircraft to break the sound barrier, the Bell X-1, took place over Edwards Air Force Base, but the X-15 needed more room for its speed and altitude flights. In both powered X-15 mission profiles, the B-52 released the X-15 about 45,000 feet above northern Nevada with its nose pointed southwest toward its landing site on Edwards dry lake bed. Two radio relay stations and six emergency landing sites on dry lake beds were established along the X-15 flight path. McAdams might have landed on Cuddeback dry lake bed, 37 miles northeast of Edwards, had he regained control of X-15 No. 3.
NASA's Project Mercury, which began officially on 6 October 1958, opted for a different approach to aerodynamic heat management: a blunt, bowl-shaped, ablative heat shield (that is, one that charred and broke away during atmosphere reentry, carrying away heat). As piloted Mercury capsule flights commenced (5 May 1961) and President John F. Kennedy put NASA on course for the moon (25 May 1961), public attention shifted away from the X-15 and Edwards Air Force Base and toward Mercury, Apollo, and Cape Canaveral, Florida. X-15 research planes continued to fly, however, pushing the hypersonic flight envelope well past their original design limits.
In the same period, some within NASA planned Earth-orbiting space stations. Before Kennedy's moon speech, a space station was seen as the necessary first step toward more advanced space activities. It would serve as a laboratory for exploring the effects of space conditions on astronauts and equipment and as a jumping-off place for lunar and interplanetary voyages. Station supporters often envisioned that it would reach orbit atop a two-stage Saturn V rocket, and that reusable spacecraft for logistics resupply and crew rotation would make operating it affordable. After the moon speech, station proponents hoped that, once Kennedy's politically motivated moon goal was reached, piloted spaceflight could resume its "proper" course by shifting back to space station development.
In November 1966, James Love and William Young, engineers at the NASA Flight Research Center at Edwards Air Force Base, completed a brief report in which they noted that the reusable suborbital booster for a reusable orbital spacecraft would undergo pressures, heating rates, and accelerations very similar to those the X-15 experienced. They acknowledged that the X-15, with a fully fueled mass of just 17 tons, might weigh just one-fiftieth as much as a typical reusable booster. They nevertheless maintained that X-15 experience contained lessons applicable to reusable booster planning.
Love and Young wrote that some space station planners expected that a reusable booster could be launched, recovered, refurbished, and launched again in from three to seven days. The X-15, they argued, had shown that such estimates were wildly optimistic. The average X-15 refurbishment time was 30 days, a period which had, they noted, hardly changed in four years. Even with identifiable procedural and technological improvements, they doubted that an X-15 could be refurbished in fewer than 20 days.
At the same time, Love and Young argued that the X-15 program had demonstrated the benefits of reusability. They estimated that refurbishing an X-15 in 1964 had cost about $270,000 per mission. NASA and the Air Force had accomplished 27 successful X-15 flights in 1964. The cost of refurbishing the three X-15s had thus totaled $7.3 million.
Love and Young cited North American Aviation estimates when they placed the cost of a new X-15 at about $9 million. They then calculated that 27 missions using expendable X-15s would have cost a total of $243 million. This meant, they wrote, that the cost of the reusable X-15 program in 1964 had amounted to just three percent of the cost of building 27 X-15s and throwing each one away after a single flight.
When, in 1968, NASA Headquarters management first floated Space Station/Space Shuttle as the space agency's main post-Apollo piloted program, the Shuttle was conceived as a reusable piloted orbiter vehicle with a reusable piloted suborbital booster - that is, the design that Love and Young had assumed. By late 1971, however, funding limitations forced NASA to opt instead for a semi-reusable booster stack comprising an expendable External Tank and twin reusable solid-propellant Solid Rocket Boosters.
The space agency was also obliged to postpone its Space Station plans at least until after the Space Shuttle became operational. Saturn V was on the chopping block, so the semi-reusable Shuttle would be used to launch the Station as well as to resupply it and rotate its crews.
Shuttle Orbiter Columbia first reached Earth orbit on 12 April 1981, but no Orbiter visited a space station until Discovery rendezvoused with the Russian Mir station on 6 February 1995 during mission STS-63. The first Shuttle Orbiter to dock with a station - again, Russia's Mir - was Atlantis during mission STS-71 (27 June-7 July 1995).
Survey of Operation and Cost Experience of the X-15 Airplane as a Reusable Space Vehicle, NASA Technical Note D-3732, James Love and William Young, November 1966
"I Fly the X-15," Joseph Walker and Dean Conger, National Geographic, Volume 122, Number 3, September 1962, pp. 428-450
Hypersonics Before the Shuttle: A Concise History of the X-15 Research Airplane, Monographs in Aerospace History No. 18, Dennis R. Jenkins, NASA, June 2000
Space Station Resupply: The 1963 Plan to Turn the Apollo Spacecraft Into a Space Freighter
McDonnell Douglas Phase B Space Station (1970)
From Monolithic to Modular: NASA Establishes a Baseline Configuration for the Shuttle-Launched Space Station (1970)
An Alternate Station/Shuttle Evolution: The Spirit of '76 (1970)
Where to Launch and Land the Space Shuttle? (1971-1972)
25 November 2017
Oddly enough, though, they took me to see 2001: a Space Odyssey during its first theatrical run. (I think I had to seize hostages to induce them to take me - it's all hazy now.) I vividly remember building an Apollo LM model with my dad. I think that stands out because it was the only time he did something with me that was related to space.
The LM was great, but it was not enough for me. It was a display piece; I needed sturdy vessels with which I might conquer the Solar System.
I was eight or nine when I began to use materials I had at hand to make models of spacecraft of my own crude design. In the 1970-1975 period, in fact, I designed my own space program set in the then-distant year 2020. Arthur C. Clarke's Rendezvous with Rama and Earthlight were major sources of inspiration, as were the book and film 2001: A Space Odyssey. Bob McCall's first art compendium, Our World in Space, also influenced my vision. Some Star Trek influence was inevitable, though my space travelers didn't wander among the stars or tangle with aliens.
Foam cups, pins, dixie cups, pens, popsicle sticks, colored markers, pipe cleaners, tape, curtain weights, and rubber cement were my construction materials. The weights made excellent footpads; by far the heaviest parts of my spacecraft, the disk-shaped lead weights help them to stand upright in the face of stray breezes and casual sideswipes from affectionate cats.
Perhaps in keeping with Star Trek, my ships included two propulsion systems. Chemical rockets permitted proximity operations near space stations and facilities on asteroids and other vacuum worlds. A far more advanced "photonic" drive enabled high-gee acceleration with minimal propellant expenditure. Think the Epstein Drive from The Expanse series.
The first spacecraft I built was an all-purpose explorer/police vessel in the tradition of Endeavour from Rendezvous with Rama or Star Trek's Enterprise. I envisioned a fleet of such craft. It was not designed to land, though it carried a small sortie vehicle and a 2001-esque service pod. Sortie vehicle and pod could be combined to yield a beefier sortie vehicle and the sortie vehicle could be broken down to create a second service pod.
Much of the action in my space program centered on the Asteroid Belt between Mars and Jupiter and the asteroidal moons and trojan asteroids of Jupiter. Asteroid settlement was well under way in 2020. My explorer/police vessel could "dock" with low-g small- and middle-sized asteroids with compatible facilities. It could also dock with and push smaller vessels with higher-g landing capability, much as the Apollo Command and Service Module pushed the Lunar Module.
The second vessel I built was a long-range explorer with higher-g landing capability. It had a beefy photonic drive, powerful chemical verniers, and a small crew compartment - perhaps room enough only for four people. Basically, it was a big engine cluster with scientific instrument pallets standing in as skin, eight adjustable landing legs, and a crew module on top.
A more conventional vessel needed weeks to travel between worlds of the Inner System and the Asteroid Belt and months to travel between the Outer System worlds. The long-range explorer could reach Pluto from Ceres in five weeks. It could also descend through atmospheres: an optional disposable heat shield (a thick paper plate) enabled Titan landings. Mysterious Titan was a major focus of scientific exploration in my space program.
The third vessel I designed, the Vulpecula-class space tug/freighter, was a small ship capable of pushing a standardized cargo module between worlds. It could accompany a cargo module to its destination or simply boost it on its way, then dock with an incoming cargo module and return to port. It could operate with or without a crew and could land on higher-g vacuum worlds bearing a cargo module.
Though I only built a pair of cargo modules, I imagined that they would take many forms. They could, for example, serve as tankers for refueling spacecraft. Another module was decked out as a passenger pod. The influence of the Franz Joseph space freighter in the Star Trek Technical Manual is unmistakable.
I also built a fast courier. Like the explorer/police ship, vessels of the Pegasus class weren't meant to land on higher-g worlds. They had a photonic engine identical to the explorer/police ship's engine, but could accelerate harder because they included only a small crew module, no auxiliary vehicles, and minimal instrumentation. They were meant to move people rapidly between scattered ships and worlds. For example, if an isolated trojan asteroid colony urgently needed a surgeon, one could be dispatched in a fast courier.
Finally, I built a long-range explorer capable of really epic trips. An extended version of the explorer/police ship, I envisioned that only a few would be built. Most traveled in pairs to interesting worlds beyond Pluto. (I'm not sure if I knew of the then-hypothetical Kuiper Belt - probably I just assumed there would be more planets past Pluto.) Their large crews hibernated in shifts. They traded speed for on-site crew expertise.
I didn't spend much time on Earth-to-orbit transportation. I assumed that rockets larger than the Saturn V would exist. That's all I remember.
Individuals and companies could own Pegasus-class fast couriers, Vulpecula-class freighters, and cargo modules. Some fast couriers became the equivalent of private jets. Some Vulpecula-class ships pushed cargo modules outfitted for asteroid prospecting.
Though I often lamented never acquiring Major Matt Mason's big moon base, in retrospect I am glad that I was thrown back on my own devices. Missing out on ready-made 1960s space toys helped to turn me creative.
What became of my space fleet? After Star Wars came out, I switched to building kit-bashed hyperdrive starships. The foam-cups-and-popsicle-sticks fleet grew dusty on a closet shelf. One summer day, as I prepared to depart for my first semester of college, I ceremoniously set fire to that fleet. The foam, rubber cement, and paper burned rapidly, leaving behind in moments only blackened pins and curtain weights. At college, my spaceships mostly became built of words, and it has remained so ever since.
18 November 2017
|Pioneer Venus Orbiter (PVO) in Venus Orbit. The Pioneer Mars Orbiter (PMO) would have been based on this design. Image credit: NASA|
Pioneer 5 (March 1960), a unique design, was a pathfinder for future NASA interplanetary missions. Managed by NASA's Ames Research Center (ARC), it set a new record by transmitting until it was 36.2 million kilometers from Earth.
The Pioneer series seemed to draw to a close. In 1965, however, NASA ARC applied the name to its drum-shaped interplanetary "weather stations." The first, Pioneer 6, entered solar orbit between Earth and Venus in December 1965, where it monitored magnetic fields and radiation. Pioneers 7, 8, and 9 performed similarly prosaic (and generally little noticed) missions.
|Pioneer 5, intended originally as a Venus probe set for launch in June 1959, was launched instead in March 1960 as a pathfinder for subsequent NASA planetary missions. Image credit: NASA|
|Pioneers 6 through 9 were drum-shaped spacecraft that measured "space weather" conditions in interplanetary space near Earth's orbit. Image credit: NASA|
The final Pioneer launches occurred in 1978. The Pioneer Venus Multiprobe spacecraft dropped four instrumented capsules on Venus, while Pioneer Venus Orbiter (PVO) surveyed the planet until 1992. The latter was informally designated Pioneer 12 and the former Pioneer 13.
|Pioneer 10 and Pioneer 11, the only nuclear Pioneers, were Earth's first probes to traverse the Asteroid Belt and voyage through the outer Solar System. Image credit: NASA|
|Pioneer Venus Multiprobe deploys its one large and three small atmosphere probes. Against expectations, two probes survived landing and return data from the surface of Venus. No other U.S. spacecraft has landed intact on Venus. Image credit: NASA|
Hughes described the PVO upon which the PMO would be based as drum 2.5 meters in diameter and 1.2 meters tall with a 3.3-meter antenna mast on top and a solid-propellant Venus orbit insertion motor on the bottom. The company then cited differences between the PMO and PVO designs: for example, PMO's orbit insertion motor would need to be larger since it would arrive at Mars traveling faster than PVO would at Venus. In addition, PMO would operate in Mars orbit, about twice as far from the Sun as Venus, so solar cells would entirely cover its sides so that they could make enough electricity to operate the spacecraft's systems. PVO would operate in Venus orbit, so it would need to be only partly covered with solar cells.
The most obvious difference between the PVO and PMO designs were the Mars spacecraft's six 2.3-meter-long, 0.3-meter-diameter penetrator launch tubes. These would replace PVO's science instruments; apart from unspecified instruments in the penetrators, PMO would carry no science payload.
PMO, like PVO, would leave Earth on an two-stage Atlas-Centaur rocket. Because PMO would weigh more than PVO (1091 kilograms versus 523 kilograms), however, it would need a solid-propellant third stage to complete Earth escape. To make room for the third stage and penetrators, PMO's conical launch shroud would be 0.8 meters longer than its PVO counterpart.
PMO would need to reach Mars on 7 September 1980 so that its Mars orbit insertion motor could place it in its planned Mars orbit. To reach the planet on that date, PMO would need to depart Earth during one of 10 consecutive daily launch opportunities starting on 28 October 1979. 2 November 1979 would be the nominal launch date. The launch opportunities would only last from 10 to 15 minutes each.
The Centaur second stage would place PMO into a low-Earth orbit, then would ignite again 30 minutes later to begin pushing the spacecraft out of Earth orbit. The third-stage motor would then ignite to place PMO on course for Mars. PMO would weigh 1069 kilograms after third-stage separation. Launch on 2 November 1979 would yield a 310-day Earth-Mars transfer.
Following third-stage separation, PMO would use hydrazine thrusters to set itself spinning at 15 revolutions per minute (RPM) for stabilization. The antenna mast bearing the high-gain, low-gain, and two penetrator data reception antennas would revolve in the opposite direction at the same rate, so would appear to stand still. Controllers on Earth would use the thrusters to carefully target PMO so that it would not accidentally hit Mars and introduce terrestrial microbes. They would perform a final course correction 30 days before Mars arrival.
One day out from Mars, on 6 September 1980, PMO would orient itself for its Mars orbit insertion burn and increase its spin rate to 30 RPM. The spacecraft's high-gain antenna would not point at Earth during the insertion burn. Controllers on Earth could, however, send PMO commands through the low-gain antenna.
|Candidate PMO orbits. Image credit: Hughes Aircraft Company|
The PMO mission's Mars orbit phase would last one martian year (686 terrestrial days). During this mission phase, PMO would deploy its six 45-kilogram penetrators singly and in pairs using a penetrator deployment system based on the Hughes-built TOW missile launcher. Before Earth departure the penetrators would be sealed inside their launch tubes and heated to kill hitchhiking microbes.
After impact, the penetrator would extend its antenna and begin transmitting data from its science instruments. PMO would record the penetrator data for relay to Earth through its high-gain dish. Chemical batteries in the penetrators would enable each to collect and transmit data from Mars for about eight days.
For their weak signals to be received, the penetrators would need to impact the surface not far from PMO's periapsis point. The orbiter could maintain radio contact with a given penetrator for at least eight minutes at a time. A PMO in south polar orbit would initially place its penetrators between 63° and 87° south; a north-polar-orbiting PMO would place them between 56° and 80° north. Periapsis would gradually shift north or south, however, permitting placement at other latitudes. With all six penetrators deployed, PMO would have a mass of 412 kilograms.
Viking 1 and Viking 2, each of which comprised a three-legged lander and an orbiter bearing cameras, were designed with certain assumptions in mind; for example, that microbial life on Mars would be ubiquitous, so that a scoop of surface dust and a jury of three biological experiments would readily reveal its presence. Unfortunately for the proposed 1979 PMO mission and NASA Mars exploration planning in general, the Viking biology experiments yielded equivocal results that were generally interpreted as indicative of a lifeless world. This, combined with the loss of the Mars Observer spacecraft as it attempted capture into Mars orbit in 1993, helped to create a two-decade gap during which no new U.S. spacecraft would explore Mars.
Pioneer Mars Surface Penetrator Mission: Mission Analysis and Orbiter Design, Hughes Aircraft Company, August 1974
Pioneer Mars 1979 Mission Options, A. Friedlander, W. Hartmann, and J. Niehoff, Science Applications, Inc., 29 January 1974, pp. 61-99
Solar System Log, Andrew Wilson, Jane's, 1986, pp. 12-13, 16-17, 21
The Russians are Roving! The Russians are Roving! A 1970 JPL Plan for a 1979 Mars Rover
Prelude to Mars Sample Return: the Mars 1984 Mission (1977)
12 November 2017
|The Apollo 13 crew of Commander James Lovell (left), Command Module Pilot John "Jack" Swigert (center), and Lunar Module Pilot Fred Haise (right). Image credit: NASA|
In Apollo Program parlance, this was dubbed "bootstrap photography." It took advantage of the piloted CSM, which had to loiter in lunar orbit anyway to collect the lunar surface crew after they completed their mission, to collect data useful for planning future Apollo missions.
Duncan noted that previous Apollo lunar missions had followed a "free-return" path that would enable them to loop behind the moon and fall back to Earth if their CSM Service Propulsion System (SPS) main engine failed. The Apollo 13 CSM, on the other hand, would fire its engine during the voyage to the moon to leave the free-return trajectory. This was necessary so that the mission's Lunar Module (LM) could reach its target landing site at Fra Mauro.
|The Apollo 13 Saturn V rocket clears the tower. Image credit: NASA|
Apollo 13 would then become a "CSM-only lunar alternate photographic mission." The CSM would remain on a free-return path until it reached the moon, then its crew would perform a standard two-impulse lunar orbit insertion (LOI) maneuver; that is, they would fire the SPS to slow their CSM so that the moon's gravity could capture it into an elliptical lunar orbit, then would fire the engine again at perilune (the low point of its lunar orbit) to circularize its orbit.
Duncan noted that some "desirable photographic orbits with high inclinations. . .require a three-impulse LOI." He argued, however, that "since the crew has not been trained for this type of LOI. . .this type of profile [should] not be flown."
In Duncan's alternate mission, Apollo 13 would capture into a lunar orbit that would take it over the craters Censorinus and Mösting C. These were, respectively, ranked first and eleventh in priority on the Apollo 13 list of targets for lunar-orbital photography.
Censorinus was the leading landing site candidate for Apollo 15, which at the time Duncan wrote his memo was planned as an H-class mission similar to Apollo 13 (that is, its LM would not carry a Lunar Roving Vehicle and would remain on the moon for only about a day and a half). Apollo 12 in November 1969 had been the first H-class mission, so had been designated H-1; Apollo 13 was H-2, Apollo 14 would be H-3, and Apollo 15 would be H-4, the final H-class flight.
Duncan advocated delaying the crew's scheduled sleep period by two lunar revolutions to enable them to photograph Censorinus and Mösting C. The photographic program would begin during Revolution 3 with vertical stereo photography using window-mounted Hasselblad cameras. Revolution 4 would see the first high-resolution vertical Hycon LTC photography, then the astronauts would conduct high-resolution oblique (side-looking) LTC photography during Revolution 5. They would perform "landmark tracking" using the CSM's wide-field scanning telescope (a part of its navigation system) during Revolutions 6 and 7, then would begin their delayed sleep period.
The Apollo 13 crew would awaken during Revolution 12 and fire the SPS to change their spacecraft's orbital plane (that is, the angle at which its orbit crossed the moon's equator). They would do this so that, beginning with Revolution 14, they would pass over Descartes, a suspected volcanic site in the moon's light-colored central Highlands, and Davy Rille, a chain of small craters of suspected volcanic origin. The astronauts would repeat the five-revolution photography sequence they used to image Censorinus and Mösting C. Duncan noted that Descartes ranked second on the Apollo 13 list of photographic targets, while Davy was fourth.
Duncan briefly considered a scenario in which the Apollo 13 LM was incapable of landing yet had a working descent engine which the crew could use to perform plane-change maneuvers in lunar orbit. He noted that the LM would block some CSM windows while it was docked. The astronauts might undock the CSM from the LM for photography and dock again for additional plane changes, or they might discard the LM after only a single plane change. Duncan favored a simpler approach: jettison the LM as soon as it was judged to be incapable of landing whether its descent engine was functional or not and use only the CSM SPS.
The astronauts would perform "target of opportunity" photography during Revolutions 18 and 19, then would sleep. They would wake during Revolution 24 and perform a plane change during Revolution 25 so that they could fly over Alphonsus crater, Gassendi West, and Gassendi East beginning with Revolution 27 and again carry out the five-revolution photography sequence. Alphonsus, where surface color changes and luminescence have been reported, was ranked ninth on the Apollo 13 target list, while the two sites in dark-floored Gassendi crater were ranked thirteenth and fourteenth, respectively.
Duncan estimated that, by the time the astronauts finished photographing the Alphonsus and Gassendi crater candidate landing sites, Apollo 13's cameras would likely have run out of film. He recommended that the crew fire the SPS to leave lunar orbit and return to Earth during Revolution 32 or two revolutions after the film ran out, whichever came first.
Because the extent of the internal damage to the CSM was unknown, NASA wrote off Odyssey's SPS and looked to the LM for salvation. Astronauts James Lovell, Jack Swigert, and Fred Haise used Aquarius's descent engine to get back onto a free-return trajectory.
During their lunar flyby, the crew photographed the moon through Aquarius's windows using hand-held cameras. Odyssey blocked part of their field of view, but there was no thought of discarding it: the crew needed the conical Command Module (CM), with its bowl-shaped heat shield, to reenter Earth's atmosphere at the end of their voyage.
Apollo 14 (31 January-9 February 1971) became the first (and last) lunar mission to use the Hycon LTC. By the time it flew, NASA had cancelled Apollo 15 and 19 as part of its efforts to preserve its proposed Space Station/Space Shuttle Program. It had renumbered the remaining Apollo flights so that they ended with Apollo 17. Apollo 14, H-3, became the last H-class mission. The camera's chief target was Descartes, which had moved to the top spot among Apollo 16 landing site candidates. Apollo 16, planned as a J-class mission, would include a two-seat Lunar Roving Vehicle, an LM capable of remaining on the moon for three days, and a CSM with an ejectable subsatellite and a pallet of sophisticated sensors and cameras in its SM.
The Hycon camera captured 192 images, but malfunctioned while imaging the lunar surface about 70 kilometers east of Descartes. Though Apollo 14 returned no images of the site, Apollo 16 (J-2) landed at Descartes in April 1972.
Memorandum with attachment, FM5/Lunar Mission Analysis Branch to various, “Lunar alternate missions for Apollo 13 (Mission H-2),” Rocky Duncan, 13 February 1970
"Scientific Rationale Summaries for Apollo Candidate Lunar Exploration Landing Sites – case 340," J. Head, Bellcomm, Inc., 11 March 1970
"Significant Results from Apollo 14 Lunar Orbital Photography," F. El-Baz and S. Roosa, Proceedings of the 1972 Lunar Science Conference, Vol. 2, pp. 63-83, 1972
North American Aviation's 1965 Plan to Rescue Apollo Astronauts Stranded in Lunar Orbit
What If Apollo Astronauts Became Marooned in Lunar Orbit? (1968)
Apollo's End: NASA Cancels Apollo 15 & Apollo 19 to Save Station/Shuttle (1970)
29 October 2017
|Chrysler-built Saturn IB first stages in the final phase of assembly at NASA's Michoud Assembly Facility, November 1967. The clustered Redstone rocket bodies are most obvious on the stage at far right. Image credit: NASA|
That Chrysler built the Saturn IB first stage should not be surprising. The company's Missile Division built intermediate-range Redstone missiles for the U.S. Army starting in 1950; the first flew in 1953. An upgraded Redstone, the Jupiter, served both as a missile and a space launcher. A modified Jupiter launched Explorer 1, the first U.S. Earth satellite, on 31 January 1958. Safety-enhanced Redstones launched suborbital Mercury spacecraft containing Alan Shepard, the first American in space (5 May 1961), and Virgil Grissom (21 July 1961).
|The first Saturn I rocket lifts off, 27 October 1961. For this suborbital test of the Chrysler-built S-I first stage, the rocket carried a dummy S-IV second stage. Image credit: NASA|
The last three Saturn I rockets each launched a Pegasus meteoroid-detection satellite, the first active payloads launched on a Saturn rocket. Pegasus 1 reached orbit in February 1965, Pegasus 2 in May 1965, and Pegasus 3 in July 1965. The Pegasus series was crucial for understanding the threat micrometeoroids posed to spacecraft and astronauts.
Shortly after Pegasus 1 reached space, at the Second Space Congress in Cocoa Beach, Florida, Chrysler engineers R. Dutzmann and E. Dunford described a small free-flying capsule for performing work outside spacecraft and space stations. They presented their paper in April 1965 during the six-week period that separated humankind's first spacewalk by Alexei Leonov (Voskhod 2, 18 March 1965) from the first U.S. spacewalk by Ed White (Gemini IV, 3 May 1965).
Dutzmann and Dunford's capsule design arose from a perceived need for new methods of protecting spacewalking astronauts - methods that would enhance protection but not compromise the ability to perform work. The Chrysler engineers explained, for example, that planned nylon fabric space suits might shield an astronaut from meteoroids if a coverall made of woven aluminum wire were added; the coverall would, however, make astronaut movement difficult.
Meteoroids were only one possible hazard of walking in space. Dutzmann and Dunford noted that objects in space have no weight, but retain their mass. They feared that a massive object - for example, a rocket stage - inadvertently set in motion might catch an astronaut unawares and crush him against another massive object.
The Chrysler engineers wrote that NASA planned to use a pure oxygen atmosphere at a pressure of 3.5 pounds per square inch (psi) inside its spacecraft and space suits. Experiments had shown that, should a space suit develop a leak, the astronaut would experience oxygen starvation if the pressure fell by just 0.8 psi. Increasing the flow of oxygen into the suit might keep pressure above the 2.7-psi critical level long enough for him to reach safety, but only if the suit perforation were small.
In addition, they noted that a space-suited astronaut would have no place to keep his tools. Attaching tools to the astronaut would impede movement.
The Chrysler engineers offered a brief assessment of past proposals for heavy (up to 2.5-ton) "taxis" that would include "comfortized" pressurized cockpits and mechanical manipulator arms. Such "luxuries," they wrote, could not realistically play a role in space operations before the 1970s. The technological leap required to move from a basic fabric space suit to a complex work vehicle was too great; also, tests had shown that existing mechanical manipulators were up to four times less efficient for doing work than astronaut arms and hands.
The capsule hull would comprise two layers of aluminum, each a fraction of an inch thick, separated by an empty space a little less than an inch wide. Meteoroids would strike the outer layer, break apart and partly vaporize, then strike the inner layer. Testing showed that this design, based on the Whipple Bumper concept, could provide meteoroid protection equivalent to that offered by a solid aluminum hull four times as thick. The Whipple Bumper was named for its inventor, comet astronomer Fred Whipple.
Dutzmann and Dunford suggested that the empty space between the aluminum layers be filled with aluminum honeycomb to improve structural strength. The lightweight Whipple Bumper hull meant that the Chrysler capsule's structure would weigh just 88 pounds.
The capsule's dome-shaped ends would each contain a thruster group. A total of 12 catalyst-bed thrusters, not too different from the Mercury spacecraft attitude-control thrusters, would draw hydrogen peroxide from a tank located at the capsule center of gravity. Upon contact with the catalyst, the hydrogen peroxide would turn to high-temperature steam and vent from the thruster nozzle. Each thruster could produce up to 10 pounds of thrust.
The astronaut, who would stand within the capsule, would open a pair of sliding doors with windows and lean out through the open doorway to perform tasks using tools gripped in gloved hands. Two pairs of telescoping arms with sticky pads at the ends, arranged one above the other, would extend through the 27-inch-by-78-inch door opening on either side of the astronaut's legs to hold the capsule in place at the work site.
A unique feature of Chrysler's capsule was its "pressure seal curtain" system. In the event of a suit puncture or tear, a transparent plastic sheet sleeve would rise up a deployment "channel" from a donut-shaped storage area in the capsule floor to surround and enclose the astronaut.
Dutzmann and Dunford offered a timeline for pressure seal curtain activation. They assumed that the astronaut's fabric space suit would most likely become damaged while the capsule was attached to a work site since at all other times the astronaut would remain inside the capsule with the doors closed.
They expected that a "pressurization emergency" would be detected 10 seconds after suit damage occurred. Five seconds post-detection, the capsule would automatically detach from the four arms holding it at its work site. This would clear the way for its sliding doors to shut 10 seconds after detection. Fifteen seconds after detection, the seal curtain would rise up and attach itself to the capsule "ceiling." Simultaneously, a backup oxygen supply mounted behind the astronaut's shoulders would activate, increasing flow into the astronaut's damaged suit. Air leaking from the suit would begin to fill the seal curtain volume 30 seconds after leak detection.
Much like the fabric space suit, the seal curtain would vent oxygen overboard to prevent buildup of exhaled carbon dioxide. Dutzmann and Dunford assumed that, to avoid oxygen depletion and carbon dioxide buildup within his helmet, the astronaut would open his visor soon after pressure within the seal curtain exceeded 2.7 psi.
The astronaut would then pilot the capsule back to its docking structure on the home spacecraft. If the capsule remained within 1000 feet of the docking structure, as Dutzmann and Dunford recommended, the trip would last less than 10 minutes.
The three Saturn I-launched Pegasus satellites would reveal that the threat from meteoroids in space was less severe than expected, but other dangers lay in wait for 1960s spacewalkers. The Soviet Union would for years claim that Alexei Leonov's spacewalk was a complete success, when in fact he could not control his movements, overheated, and became stuck sideways in Voskhod 2's inflatable airlock.
Ed White's excursion outside the Gemini IV spacecraft, less than a month after the Chrysler engineers presented their capsule design, was nearly as successful as Leonov's was claimed to have been. More careful analysis would, however, have pointed to potential problems - White's suit expanded during his spacewalk, and he exceeded the cooling capacity of his air-cooled space suit while struggling to squeeze into his narrow seat and close his balky spacecraft hatch.
Not until humankind's perilous third spacewalk on 5 June 1966 would the inadequacies of air-cooled space suits become obvious. During an ambitious attempt to fly free of the Gemini IX spacecraft using a 168-pound hydrogen peroxide-fueled Astronaut Maneuvering Unit (AMU) backpack, pilot Eugene Cernan tore his suit's outer layers, overheated, and became blinded by perspiration as he struggled against his suit's internal pressure. Cernan's AMU flight was called off and NASA was forced to descope its planned series of complex Gemini Program spacewalks (see "More Information" below).
"Design Considerations for a Free Space Transportation and Work Station Capsule," R. Dutzmann and E. Dunford, Proceedings of the 2nd Space Congress, April 1965, pp. 403-430
Chrysler's Ballistic Missile and Space Activities: First 20 Years, Chrysler Corporation, 1972
Walking to Olympus: An EVA Chronology, Monographs in Aerospace History #7, David S. F. Portree and Robert C. Trevino, NASA History Office, October 1997, pp. 1-5, 11 - https://history.nasa.gov/monograph7.pdf (accessed 12 November 2017)
The Spacewalks That Never Were: Gemini Extravehicular Planning Group (1965) (how Soviet deception concerning Leonov's spacewalk led NASA to plan complex Gemini spacewalks)
Rocket Belts and Rocket Chairs: Lunar Flying Units (1960s plans for rocket-powered lunar surface transportation)
15 October 2017
An Earth-based observer cannot view the largest and oldest giant impact basin because it is out of view on the moon's hidden Farside. South Pole-Aitken (SPA) Basin is about 2500 kilometers wide, making it perhaps the largest impact scar in the Solar System. Lunar Orbiter data revealed its existence in the 1960s, though little was known of it until the 1990s, when the U.S. Clementine and Lunar Prospector polar orbiters mapped surface chemistry over the entire moon. Their data showed that the basin floor probably includes material excavated from the moon's lower crust and upper mantle. In the first decade of the 21st century, laser altimeters on the U.S. Lunar Reconnaissance Orbiter (LRO) and Japanese Kaguya spacecraft confirmed that SPA includes the lowest places on the moon.
In 2002, however, the National Research Council's planetary science Decadal Survey declared SPA sample return to be a high scientific priority and, at the same time, proposed a new class of competitively selected medium-cost missions. The latter marked the genesis of NASA's New Frontiers Program, which originally had a cost cap per mission of $700 million.
The New Horizons Pluto/Kuiper Belt Object flyby mission was already under development when NASA created the New Frontiers Program. NASA gave New Frontiers a highly visible first mission by adopting New Horizons into the program. Selection of the Pluto/KBO mission came to be regarded as the first New Frontiers proposal cycle, though it included no competition. NASA had taken a similar approach when it made Mars Pathfinder its first Discovery Program mission in 1992.
|Geologist Michael Duke in 2004. Image credit: NASA|
Duke argued that the SPA sample-return mission could collect ancient deep crust and mantle rocks without a costly rover. Clementine and Lunar Prospector had shown that at least half of the surface material in the central part of SPA was native to the basin, so stood a good chance of having originated deep within the moon.
Furthermore, Apollo demonstrated that any lunar site is likely to yield a wide assortment of samples because the moon's low gravity and surface vacuum enable asteroid impacts to widely scatter rock fragments. The Apollo 11 mission to Mare Tranquillitatis, for example, found and returned to Earth rocks blasted from the moon's light-hued Highlands. Duke proposed that the SPA sample-return lander sift about 100 kilograms of lunar dirt to gather a one-kilogram sample consisting of thousands of small rock fragments. These would have many origins, but a large percentage would be likely to have originated in the moon's deep crust and mantle.
A $700-million budget would also enable a relay satellite "more competent" than its bare-bones Discovery predecessor. It might be placed in a halo orbit around the Earth-moon L2 point, 64,500 kilometers behind the moon as viewed from Earth. From that position, the satellite would permit continuous radio contact between Earth and the landers. A satellite in lunar orbit could remain in line-of-sight contact with both the landers and Earth for only brief periods.
NASA had argued that a single day on the moon provided too little time to modify the SPA Discovery mission if it suffered difficulties. The SPA New Frontiers mission would, therefore, remain on the moon for longer. Duke noted, however, that stay-time would probably be limited to the length of the lunar daylight period (14 Earth days) because designing the twin landers to withstand the frigid lunar night would boost their cost.
Juno's selection did not end proposals for SPA Basin sample-return, though it did mark the end of Duke's involvement. In the third New Frontiers proposal cycle, which began in 2009, a Jet Propulsion Laboratory/Lockheed Martin/Washington University in St. Louis team proposed a SPA Basin mission called MoonRise. In 2011, the SPA sample-return mission was again selected as a New Frontiers finalist, but it lost out in the final selection to the OSIRIS-Rex asteroid sample-return mission.
"Sample Return from the Lunar South Pole-Aitken Basin," M. Duke, Advances in Space Research, Volume 31, Number 11, June 2003, pp. 2347-2352
"NASA Selects Two 'New Frontiers' Mission Concepts for Further Study," D. Savage, NASA Press Release 04-222, NASA Headquarters, 16 July 2004
NASA Facts: MoonRise - A Sample-Return Mission From the Moon's South Pole-Aitken Basin, NASA Facts, JPL 400-1408, June 2010
"MoonRise: Sample Return from the South Pole-Aitken Basin," L. Akalai, B. Jolliff, and D. Papanastassiou; presentation to the International Planetary Probe Workshop, Barcelona, Spain, 17 June 2010
Peeling Away the Layers of Mars (1966)
An Apollo Landing Near the Great Ray Crater Tycho (1969)
Catching Some Comet Dust: Giotto II (1985)
Lunar GAS (1987)
The Eighth Continent
27 September 2017
Next came Apollo. NASA conducted four piloted preparatory missions in 1968-1969 ahead of the first lunar landing attempt. Apollo 7 (October 1968), launched on a two-stage Saturn IB rocket, tested the Command and Service Module (CSM) in Earth orbit. The CSM comprised a drum-shaped Service Module (SM) and the conical Command Module (CM) bearing its three-man crew.
As in biological evolution, contingency played a role in spaceflight evolution: for example, Apollo 8, intended originally as a Saturn V rocket-launched high-Earth-orbit test of the CSM and the bug-like Lunar Module (LM) moon lander, became a CSM-only lunar-orbital mission after the LM was delayed and the Soviet Union appeared close to launching a cosmonaut around the moon.
The Apollo 8 astronauts reached lunar orbit on Christmas Eve 1968. In addition to forestalling Soviet attempts to upstage the first Apollo lunar landing mission, Apollo 8's 10 lunar orbits tested upgrades made to the Manned Space Flight Network, NASA's world-wide radio communications and tracking system, and gave console operators in Mission Control early experience in supporting a piloted lunar mission. The Apollo 8 CSM left lunar orbit on Christmas Day 1968, and splashed down in the Pacific Ocean on 27 December 1968.
Apollo 9 (March 1969) saw the first Earth-orbital test of the LM and CSM together. Apollo 10 (May 1969) was a dress-rehearsal in low-lunar orbit for Apollo 11 (July 1969), the first piloted lunar landing.
Apollo 11 is perhaps best understood as an engineering mission; it was a cautious end-to-end test of the Apollo system with a single two-and-a-half-hour moonwalk and only limited science objectives. Apollo 12 (November 1969) demonstrated the pin-point landing capability required for pre-mission geologic traverse planning by setting down near a known point on the moon: specifically, the Surveyor III automated soft-lander, which had landed in April 1967. It also saw a pair of moonwalks lasting nearly four hours each and deployment of the first Apollo Lunar Scientific Experiment Package (ALSEP).
Apollo 13 (April 1970), the first science-focused mission, suffered a crippling explosion midway to the moon, scrubbing its lunar landing, but its crew's safe return to Earth demonstrated the Apollo system's maturity and the Apollo team's experience. Apollo 14 (January-February 1971) included two moonwalks, each lasting more than four-and-a-half hours. They included a strenuous 1.3-kilometer trek through the hummocky ejecta blanket surrounding 300-meter-wide Cone Crater, a natural drill hole in the scientifically important Fra Mauro Formation.
Apollo 15 (July-August 1971), Apollo 16 (April 1972), and Apollo 17 (December 1972), designated "J" missions, featured a host of evolutionary improvements. Beefed-up LMs permitted surface stay times of up to three days at complex and challenging landing sites, heavier returned lunar samples, and more complex ALSEPs. Space suit improvements and Boeing's Lunar Roving Vehicle enabled geologic traverses ranging over kilometers of the lunar surface. Each "J" mission CSM included a suite of sensors which its pilot could turn toward the moon. Apollo 15 visited the Hadley Rille/Apennine Mountains area; Apollo 16 the central Nearside Lunar Highlands; and Apollo 17, Taurus-Littrow, on the edge of Mare Serenitatis.
One path would see moon missions continue more or less indefinitely, growing ever more capable and culminating in a permanent lunar base in the 1980s. Alternately, NASA might repurpose Apollo hardware to build, launch, and maintain an evolutionary series of space stations in Earth orbit.
The space station path appeared pedestrian compared to the lunar base path, yet it offered great potential for long-term future exploration. This was because it promised to prepare astronauts and spacecraft for long-duration missions in interplanetary space. In 1965-1966, NASA advance planners envisioned a series of Earth-orbiting space workshops based on the Apollo LM and the Saturn IB rocket S-IVB stage. Apollo CSMs would ferry up to six astronauts at a time to the workshops for progressively longer stays.
Some planners thought that NASA should jump straight from the early space workshops to piloted Mars landing missions using nuclear-thermal propulsion, but others called for a conservative continuation of the evolutionary approach. If the latter had won the day, the mid-1970s might have seen a new-design space station climb to Earth orbit atop an improved Saturn V rocket. Derived from Apollo hardware and new technology tested on board the orbiting workshops, the station would in fact have constituted a prototype interplanetary Mission Module. A crew might have lived on board without resupply or visitors for almost two years to help prepare NASA for its first piloted Mars voyage.
In keeping with the evolutionary approach, the first piloted voyage beyond the moon might have been a Mars flyby with no piloted Mars landing. The piloted Mars flyby spacecraft, which would have carried a cargo of robotic Mars probes, would have been built around the Mission Module tested in Earth orbit. The mission might have commenced as early as late 1975, when an opportunity to launch a minimum-energy Mars flyby was due to occur.
As they raced past Mars in early 1976, the four flyby astronauts would have released automated probes and turned a suite of sensors mounted on their spacecraft toward Mars and its irregularly shaped moons Phobos and Deimos. They would have reached their greatest distance from the Sun in the Asteroid Belt, so asteroid encounters would have been a possibility. As their Sun-centered elliptical orbit brought them back to Earth's vicinity in 1977, they would have separated in an Apollo CM-derived Earth-return capsule and reentered Earth's atmosphere.
In addition to observing Mars close up, the astronauts would have continued the effort, begun in earnest during Gemini flights and continued on board the Earth-orbiting workshops and prototype interplanetary Mission Module, to determine whether extended piloted missions were medically feasible. The flyby crew might have confirmed, for example, that artificial gravity is a must during years-long interplanetary voyages. Their results would have shaped the next interplanetary mission, which might have taken the form of a piloted Mars orbiter in the spirit of Apollo 8 and Apollo 10, or, if the space agency felt sufficiently confident in its abilities, a Mars orbital mission with a short piloted excursion to the martian surface in the spirit of Apollo 11.
|The Orbiter backs away from the Flyby Spacecraft. The two astronauts on the Flyby Spacecraft inspect the Orbiter's exterior and televise its departure to Mission Control on Earth.|
Titus explained that, in the "standard stopover mode," a label that encompassed Mars orbital and landing missions, all major maneuvers would involve the entire Mars spacecraft. This meant that the Mars spacecraft would need a large mass of propellants, which in turn meant that many expensive heavy-lift rockets would be required to launch the spacecraft, its propellants, and its Earth-orbit departure stages into Earth orbit for assembly.
Propellant mass required would vary greatly from one Earth-Mars transfer opportunity to the next over a roughly 15-year cycle because Mars has a decidedly elliptical orbit. Because of this, the Mars spacecraft and the sequence of launches needed to boost its components and propellants into Earth orbit would have to be redesigned for each standard stopover mode Mars mission.
The United Aircraft engineer added that errors or malfunctions during standard stopover mode "high-risk" Mars capture and escape maneuvers could result in "complete mission failure" because the entire spacecraft would be involved. Because the Mars spacecraft would be very massive already, it would be difficult and costly to include extra propellants that would enable a mission abort that could save the crew.
Titus noted that required propellant mass might be reduced and made to vary less over multiple Earth-Mars transfer opportunities if the spacecraft skimmed through Mars's upper atmosphere to slow down and capture into Mars orbit (that is, if it performed aerocapture). If, however, artificial gravity were found to be necessary for crew health, then stowing a spinning artificial-gravity system of sufficient radius behind an aerocapture heat shield would probably prove infeasible. The mission would then have to rely entirely on propulsive braking.
Titus explained that his FLEM concept, in addition to forming a natural evolutionary extension of piloted Mars flybys, would address many inherent problems of the standard stopover mode. One part of the FLEM spacecraft, the "parent" spacecraft, would not capture into Mars orbit. It could include a spinning artificial-gravity system. The other part, the "excursion module," would capture into Mars orbit using chemical rockets or, perhaps, by skimming through the martian atmosphere behind an aerocapture heat shield.
In the latter case, the excursion module would not need a large mass of propellants to capture into Mars orbit, making it the least massive of the two FLEM spacecraft. It would thus fire rockets to speed up and reach Mars ahead of the parent spacecraft.
Titus calculated that separation 60 days ahead of the Mars flyby would enable the excursion module to reach the planet 16 days ahead of the parent spacecraft; separation 30 days before flyby would enable it to reach Mars while the parent spacecraft was nine days behind it. While it awaited the arrival of its parent, the excursion module might remain in Mars orbit or all or part of it might land on Mars for a stay of several days.
Assuming that the mission took place as planned, the excursion module would ignite its rocket motors as the parent spacecraft passed Mars to depart Mars orbit and catch up with it. Following rendezvous, docking, and crew transfer, the excursion module would be cast off.
The maneuver would be optional in the sense that, if it could not occur, the FLEM spacecraft's Sun-centered orbit would return it to Earth, though after a longer than expected trip. During return to Earth after a powered flyby, the FLEM spacecraft would pass as close to the Sun as orbits the planet Mercury.
Titus determined that a powered-flyby maneuver in 1971 would have almost no effect on spacecraft mass at Earth-orbit departure - both the standard ballistic and powered-flyby FLEM spacecraft would have a mass of about 400,000 pounds - but would slash trip time from 510 to 430 days. The most dramatic improvement would occur in 1978, when the ballistic-flyby FLEM spacecraft's mass would total nearly two million pounds and its mission would last 540 days. The powered-flyby FLEM spacecraft would have a mass of just 800,000 pounds at the start of Earth-orbit departure and its mission would last only 455 days.
Titus's FLEM concept influenced NASA piloted flyby studies that took place under the auspices of the Planetary Joint Action Group (JAG). The NASA Headquarters-led Planetary JAG, which met between 1965 and 1968, included representatives from NASA Marshall Space Flight Center, NASA Kennedy Space Center, and the NASA Manned Spacecraft Center, as well as Washington, DC-based planning contractor Bellcomm. The Planetary JAG's work, including the Mars Surface Sample Return concept that FLEM apparently inspired, will be described in detail in subsequent posts.
http://william-black.deviantart.com/) and are used by special arrangement with the artist.
Manned Mars and.or Venus Flyby Vehicle Systems Study - Final Briefing Brochure, SID 65-761-6, North American Aviation, 18 June 1965
"FLEM - Flyby-Landing Excursion Mode," AIAA Paper 66-36, R. R. Titus; paper presented at the 3rd AIAA Aerospace Sciences Meeting in New York, New York, 24-26 January 1966
Planetary Exploration Utilizing a Manned Flight System, Planetary Joint Action Group, NASA Office of Manned Space Flight, 3 October 1966
"Manned Expeditions to Mars and Venus," E. Z. Gray and F. Dixon, Voyage to the Planets, Proceedings of the Fifth Goddard Memorial Symposium, 14-15 March 1967, pp. 107-135
Wonderful Life: The Burgess Shale and the Nature of History, Stephen Jay Gould, W. W. Norton & Co., 1990
After EMPIRE: Using Apollo Technology to Explore Mars and Venus (1965)
Relighting the FIRE: A 1966 Proposal for Piloted Interplanetary Mission Reentry Tests
To Mars by Way of Eros (1966)
Apollo Ends at Venus: A 1967 Proposal for Single-Launch Piloted Venus Flybys in 1972, 1973, and 1975
Triple-Flyby: Venus-Mars-Venus Piloted Missions in the Late 1970s/Early 1980 (1967)