16 May 2017

Venus As Proving Ground: A 1967 Proposal for a Piloted Venus Orbiter

Mariner II during its final days on Earth, July-August 1962. Image credit: NASA
NASA won a significant prestige victory over the Soviet Union on 14 December 1962, when Mariner II flew past Venus at a distance of 22,000 miles. The 203.6-kilogram spacecraft, the first successful interplanetary probe in history, left Cape Canaveral, Florida, on 27 August 1962. Controllers and scientists breathed a sigh of relief as it separated from its Atlas-Agena B launch vehicle; failure of an identical rocket had doomed its predecessor, Mariner I, on 22 July 1962.

Astronomers knew that Venus was nearly as large as Earth, but little else was known of it, for its surface is cloaked in dense white clouds. Many supposed that, because it is a near neighbor and similar in size to our planet, Venus would be Earth's twin. As late as 1962, some still hoped that astronauts might one day walk on Venus under overcast skies and perhaps find water and life.

Data from Mariner II effectively crossed Venus off the list of worlds where astronauts might one day land. As had been suspected since 1956, when radio astronomers first detected a surprising abundance of three-centimeter microwave radiation coming from the planet, Venus's surface temperature was well above the boiling point of water. Mariner II data indicated a temperature of at least 800° Fahrenheit over the entire planet. Cornell University astronomer Carl Sagan explained the intense heat: Venus has a dense carbon dioxide atmosphere that behaves like glass in a greenhouse.

Venus's role in piloted spaceflight thus shifted from a destination in its own right to a kind of "coaling station" for spacecraft traveling to and from Mars. Mission planners proposed ways that a piloted Mars spacecraft might use Venus's gravity to alter its course, slow down, or speed up without expending rocket propellants.

Some also began to view Venus as a proving ground for incremental space technology development. In 1967, NASA Lewis Research Center (LeRC) engineer Edward Willis proposed a manned Venus orbiter based on an "Apollo level of propulsion technology" for the period immediately after the Apollo moon missions.

Willis rejected piloted Mars and Venus flyby missions, which were under consideration as a post-Apollo NASA goal at the time he wrote his paper, in large part because he believed that they would not provide enough exploration time near the target planet. Though he sought a piloted Venus orbiter, Willis questioned the wisdom of launching an equivalent mission to Mars. "It is generally felt," he explained, "that the. . .objective of a manned Mars flight should be a manned landing and surface exploration," not merely a stint in Mars orbit.

The NASA LeRC engineer calculated that the mass of propellants needed for a piloted Venus orbiter would be considerably less than for a piloted Mars orbiter even in the most energetically demanding Earth-Venus minimum-energy transfer opportunity. This meant that a piloted Mars orbiter would always need more costly heavy-lift rocket launches to boost its propellants and components into low-Earth orbit than would a piloted Venus orbiter.

A piloted Mars landing mission, for its part, would be "still heavier than the [Mars] orbiting mission," so probably would "best be done using nuclear propulsion." Whereas chemical rockets generally need two propellants - fuel plus oxidizer to "burn" the fuel - nuclear-thermal rockets need only one working fluid. Liquid hydrogen is most often cited, though liquid methane is also mentioned.

Because they need to lug around the Solar System only one propellant, nuclear-thermal rockets are inherently more efficient than chemical rockets. Nuclear-thermal propulsion would, however, need more development and testing before it could propel humans to Mars. Nuclear-thermal propulsion was unlikely to be ready by the time Apollo ended; therefore, Willis wrote, "in terms of [technological] difficulty and timing, the Venus orbiting mission has a place ahead of the Mars orbiting and landing missions."

The key to a Venus orbiter with the lowest possible propellant mass, Willis explained, was selection of an appropriate Venus orbit. Entering and departing a highly elliptical orbit about Venus would need considerably less energy (hence, propellants) than would entering and departing a close circular Venus orbit. He thus proposed a Venus orbit with a periapsis (low point) of 13,310 kilometers (1.1 Venus radii) and a apoapsis (high point) of 252,890 kilometers (20.9 Venus radii).

The 129,250-pound (dry weight) Earth-departure stage (A in the cutaway drawing above) and the Venus orbiter spacecraft would be launched into Earth orbit separately. After the stage was loaded with 942,500 pounds of propellants in orbit, it would link up with the spacecraft. The stage would expend 930,000 pounds of propellants to increase the spacecraft's speed by 2.8 miles per second, launching it out of Earth orbit toward Venus. It would stay attached to the spacecraft until after a course-correction burn halfway to Venus that would expend an additional 12,500 pounds of propellants. The 332,000-pound Venus orbiter spacecraft, which could reach Earth orbit atop a single uprated Saturn V rocket, would comprise 10,000 pounds of Venus atmosphere probes (B), the 103,000-pound Venus arrival rocket stage (C), a 30,000-pound Venus scientific remote sensor payload (D), the 95,120-pound Venus departure rocket stage (E), the 4,000-pound Venus-Earth course-correction stage (F denotes tanks; engines are too small to be seen at this scale), the Command Module (G) for housing the crew, and the Earth atmosphere entry system (H), a 15,250-pound lifting-body with twin winglets for returning the crew to Earth's surface at the end of the mission. Of the Command Module's 66,000-pound mass, food, water, and other expendable supplies would account for 27,000 pounds. Image credit: NASA
Willis calculated that a Venus orbiter based on Apollo-level technology, departing from a 400-mile-high circular Earth orbit, staying for 40 days in his proposed Venus orbit, and with a total mission duration of 565 days, would have a mass of 1.412 million pounds just prior to Earth-orbit departure in the energetically demanding 1980 Earth-Venus transfer opportunity. An equivalent Mars orbiter launched in 1986, the least demanding Earth-Mars transfer opportunity of any Willis considered, would have a mass in Earth orbit about 70% greater - about 2.4 million pounds.

As the spacecraft approached Venus, its crew would turn it so that the Venus arrival stage faced forward, then would ignite the stage as it passed closest to Venus to slow the spacecraft by 0.64 miles per second. This would enable Venus's gravity to capture the spacecraft into its elliptical operational orbit. The maneuver would expend 91,950 pounds of propellants. The spent arrival stage would remain attached to the spacecraft at least until the Venus atmosphere entry probes were released.

The spacecraft would complete two orbits of Venus during its 40-day stay. Time within 26,300 kilometers (three Venus radii) of the planet would total two days; that is, several times longer than a piloted Venus flyby could spend near the planet (Willis's Venus orbiter would, however, not pass as close to Venus as would a Venus flyby spacecraft). Throughout their stay in orbit, the crew would turn remote sensors toward Venus. During the two periapsis passes, the astronauts would use radar to explore the mysterious terrain hidden beneath the Venusian clouds.

Farther out from the planet, near apoapsis, they would deploy the Venus atmosphere entry probes. Their spacecraft's distant apoapsis, combined with Venus's slow rotation rate (once per 243 Earth days), would enable them to remain in direct radio contact with their probes for days - unlike a piloted Venus flyby spacecraft, which could at best remain in contact with its probes for a few hours.

At the end of their stay in Venus orbit, the crew would cast off the Venus scientific payload and ignite the Venus departure stage at periapsis, expending 86,970 pounds of propellants and adding 1.14 miles per second to their speed. During the trip home, which would take them beyond Earth's orbit, they would discard the Venus departure stage and perform a course correction, if one were needed, using the small course correction stage attached to the Command Module.

Near Earth, the crew would separate from the Command Module in the Earth atmosphere entry lifting-body and enter the atmosphere at a speed of 48,000 feet per second. After banking and turning to shed speed, they would glide to a land landing, bringing to a triumphant conclusion humankind's historic first piloted voyage beyond the moon.


Manned Venus Orbiting Mission, NASA TM X-52311, E. Willis, 1967

More Information

Centaurs, Soviets, and Seltzer Seas: Mariner II's Venusian Adventure (1962)

After EMPIRE: Using Apollo Technology to Explore Mars and Venus (1965)

Triple-Flyby: Venus-Mars-Venus Piloted Missions in the Late 1970s/Early 1980s (1967)

Floaters, Armored Landers, Radar Orbiters, and Drop Sondes: Automated Probes for Piloted Venus Flybys (1967) 

Things to Do During a Venus-Mars-Venus Piloted Flyby Mission (1968)

Two for the Price of One: 1980s Piloted Mars-Venus Missions With Stopovers at Mars and Venus (1969)

10 May 2017

"Still Under Active Consideration": Five Proposed Earth-Orbital Apollo Missions for the 1970s (1971)

The Skylab 2 Apollo CSM and Saturn IB launcher stand ready atop the "milk stool" on Pad 39B at NASA's Kennedy Space Center, May 1973. Image credit: NASA
From its conception in 1959 until President John F. Kennedy's 25 May 1961 call to put a man on the moon, Apollo was seen mainly as an Earth-orbital spacecraft. NASA intended to use Apollo in the second and third phases of its planned 1960s piloted space program. The first phase, characterized by suborbital flights lasting minutes and sorties into Earth orbit lasting at most a few days, would be accomplished by brave pioneers in missile-launched single-seater Mercury capsules.

In the second phase, three astronauts would live and work on board Apollo spacecraft for ever-longer periods. They would use a pressurized Mission Module (MM) launched attached to their spacecraft as a small space station. The third phase would see Apollo spacecraft transport crews to and from an Earth-orbiting space station. Cargo bound for the station would ride in the MM. An Apollo circumlunar mission - a flight around the moon without capture into lunar orbit - was an option, but was considered unlikely before 1970.

Simplified cutaway of the General Electric D-2 Apollo, perhaps the most widely known of the pre-Apollo lunar landing program Apollo designs. Colored lines represent separation planes: orange is the spacecraft/launch vehicle separation plane; red is the abort separation plane (two "pusher" solid-propellant abort rockets are visible on the outside of the Service Module); green is the shroud/Service Module separation plane; and blue is the Mission Module/Command Module separation plane. In the event of a launch abort, the part of the Service Module to the right of the red line would remain attached to the launch vehicle. During reentry, the spacecraft would first split along the green line, then the Command Module would separate from the Mission Module along the blue line. The shroud would remain with the Mission Module. The Command Module would lower on parachutes and perform a land landing while the Service Module and Mission Module/shroud would burn up. Image credit: General Electric/DSFPortree 
Following studies that lasted six months, in mid-May 1961 General Electric (GE), The Martin Company, and Convair submitted Apollo spacecraft designs suited to NASA's three-phase plan. In the event, none of the designs left the drawing board; after Apollo became NASA's lunar landing mission spacecraft, the agency funded new studies and selected North American Aviation (NAA) as its Apollo spacecraft contractor.

Initially, NASA intended to land NAA's Apollo on the moon atop a descent stage with landing legs. In July 1962, however, after more than a year of sometimes acrimonious discussion and study, the space agency selected Lunar Orbit Rendezvous (LOR) as its lunar landing mission mode. NAA's Command and Service Module (CSM) spacecraft became the LOR mission's moon-orbiting mother ship, and to Grumman's bug-like Lunar Module (LM) went the honor of landing on the moon.

Before the Lunar Module. Image credit: NASA
As flown, 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 MM of the May 1961 GE, Martin, and Convair designs was judged unnecessary for lunar landing missions. In fact, at first some sources perceived the LM as the MM's.

The CM's nose carried a probe docking unit, and at the aft end of the SM was mounted the Service Propulsion System (SPS) main engine. The SPS remained sized for CSM launches from the lunar surface, which meant that it was more powerful than necessary for CSM insertion into and escape from lunar orbit.

Technical details of the Apollo Command and Service Module (CSM) spacecraft configured for lunar missions. Image credit: NASA
The CM also included a pressurized crew compartment, crew couches, flight controls, a compact guidance computer, rendezvous aids, a bowl-shaped heat shield for Earth atmosphere reentry, reentry batteries, and parachutes for descent to a gentle splashdown at sea.

The SM, which was discarded before atmosphere reentry, included propellant tanks, fuel cells for making electricity and water, fuel cell reactants (liquid oxygen and liquid hydrogen), four attitude-control thruster quads, radiators for discarding excess heat generated by on board systems, a high-gain radio antenna, and room for a Scientific Instrument Module (SIM) Bay. An umbilical housing linked CM and SM.

Almost all piloted Apollo Earth-orbital missions were launched atop two-stage Saturn IB rockets. The sole exception was Apollo 9 (3-13 May 1969), which used NASA's fourth Saturn V. All Apollo lunar missions left Earth on three-stage Saturn V rockets.

Apollo 7 and Apollo 9 were test flights, so their CSMs operated exclusively in low-Earth orbit. This image shows the CM of the Apollo 9 CSM Gumdrop as viewed from the LM Spider in May 1969. Apart from thruster quads and antennas, very little of Gumdrop's SM is visible. No other Apollo spacecraft would operate only in low-Earth orbit until the Skylab 2 CSM flew in May 1973. Image credit: NASA
The United States began to abandon the technology of piloted lunar exploration by late 1967, nearly a year before the first astronauts reached Earth orbit in an Apollo CSM (Apollo 7, 11-22 October 1968). Abandonment of the moon began with deep cuts in the Apollo Applications Program (AAP), the planned successor to the Apollo lunar program. Ambitious two-week stays on the moon were among the first AAP missions to feel the budget-cutters' blades.

In early 1970, NASA brought together the parts of AAP that survived - several space station-related Earth-orbital missions - to form the Skylab Program, which was expected to include at least one and possibly two temporary Skylab Orbital Workshops. The first, Skylab A, was meant to receive at least three Apollo CSMs, each bearing a three-man crew, over a period of about nine months.

By late 1970, with just two Apollo moon landings (Apollo 11 and Apollo 12) and the Apollo 13 accident under its belt, NASA cancelled three lunar landing missions. Apollo 20, the planned final Apollo lunar mission, was cancelled in early 1970 to free up its Saturn V rocket to launch Skylab A. Apollo 15, the planned fourth and last walking mission, was cancelled in September 1970, as was Apollo 19. NASA Administrator Thomas Paine dropped the missions at least partly in an attempt to curry favor for a permanent Space Station from President Richard Nixon. The space agency renumbered the surviving missions so that Apollo lunar exploration would end with Apollo 17.

On 27 August 1971, Philip Culbertson, director of the Advanced Manned Missions Program Office 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 it, he outlined five Earth-orbital CSM missions for the 1970s 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 19 August. Based on his letter, Culbertson had phoned Berglund in an effort to impress on him the seriousness of NASA's budget situation.

Space Base: a large permanent Space Station, c. 1980. The nuclear-powered station would have had a crew of from 50 to 100 persons. Image credit: NASA
Berglund and his predecessor at MSC, Edward Olling, had throughout the 1960s remained staunch advocates of a large permanent Earth-orbiting space station. MSC Director Robert Gilruth was also a station supporter. They regarded AAP as at best a not-too-necessary rehearsal for a Space Station; they saw it at worst as a waste of time and money. They anticipated that before the mid-1970s AAP would draw to a close, freeing up funds for a real Space Station.

By mid-1971, however, it was increasingly obvious that a permanent Space Station was of interest neither to Nixon's White House nor the Congress. In fact, a reusable Space Station logistics resupply and crew rotation vehicle - a Space Shuttle - was by then emerging as the preferred post-Apollo program. The Space Station - if it were built at all - would have to wait until the Shuttle could launch its modules and bring them together in Earth orbit.

Culbertson referred to an unspecified new contract MSC had awarded CSM contractor North American. 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 options until the end of the [Fiscal Year] 1973 budget cycle." Fiscal Year 1973 would conclude on 1 October 1973.

Culbertson's five missions were all to some degree Space Station-related. The first and simplest was an "independent CSM mission for earth observations." Earth observation by astronauts was often mentioned as a Space Station justification. The mission's CSM would probably include 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.

A SIM Bay was part of the final three Apollo lunar CSMs. The image above shows the Apollo 15 CSM Endeavour in lunar orbit with its rectangular SIM Bay (upper center) open to space. Image credit: NASA
The second mission on Culbertson's list was an Apollo space station flight that would have been almost unimaginable at the time Kennedy diverted Apollo to the moon. It would see 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 19 April 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 Patseyev, and Vladislav Volkov had undocked on 29 June 1971, after nearly 24 days in space (at the time, a new world record for human space endurance). The three cosmonauts had suffocated during reentry when a malfunctioning valve caused their capsule to lose pressure, so the Soviet Union halted all piloted missions while the Soyuz spacecraft underwent a significant redesign.

The third Earth-orbital CSM mission on Culbertson's list combined the first two missions. 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 CSM-119 dock first with a Salyut for a brief time, then undock and rendezvous with the dormant Skylab A Orbital Workshop. After docking with and reviving Skylab A, CSM-119's crew would live and work on board for an unspecified period.

Image credit: NASA
NASA planned that, during the three CSM 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). The Salyut-Skylab A mission, which would include no rescue CSM, was planned to begin 18 months after Skylab A reached orbit, or about nine months after the third Skylab A mission returned to Earth.

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 CSM modified to carry five astronauts stood by. NASA had funded partial assembly of Skylab B so that it would have a backup in the pipeline in case Skylab A failed. Reflecting uncertainty about the availability of Saturn rockets and CSMs, Culbertson gave no date for the Skylab B launch.

Of the five missions Culbertson declared to be on the table in August 1971, none flew. In January 1972, Nixon called on Congress to fund Space Shuttle development, and Congress agreed. Shuttle costs and continued NASA budget cuts pushed even the least complex and cheapest of Culbertson's five missions off the table.

For a short time, his second mission looked to be within reach. Formal joint U.S./U.S.S.R. planning for an Apollo docking with a Salyut was under way when Culbertson wrote his letter. In early April 1972, however, 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.

NASA was disappointed to lose an opportunity for an early post-Skylab space station visit; the Nixon White House, on the other hand, saw the mission as a poster child for its policy of detente with the Soviet Union, so any sort of piloted docking mission would do. At the superpower summit in Moscow on 24 May 1972, Nixon and Soviet Premier Alexei Kosygin signed the agreement creating the Apollo-Soyuz Test Project (ASTP).

Skylab A, redesignated Skylab I (but more commonly called simply Skylab), reached orbit on 14 May 1973 on a two-stage Saturn V. It suffered damage during ascent, but NASA and its contractors pulled it back from the brink.

Skylab in a photograph taken by the second crew to live on board. Signs of damage the Orbital Workshop suffered during ascent to low-Earth orbit are obvious: one solar array wing is missing (left) and a hastily improvised solar shield stands in for the reflective meteoroid shield that would have protected Skylab's crew volume from the Sun. Image credit: NASA
The three CSM missions to Skylab spanned 25 May-22 June 1973, 28 July-25 September 1973, and 16 November 1973-8 February 1974, respectively. Leaks in attitude control thrusters on the second CSM to dock with Skylab caused NASA to ready CSM-119 for flight, going so far as to roll it and its Saturn IB rocket out to the launch pad; the leaks stopped by themselves, however, so the rescue CSM remained earthbound.

In August 1973, with Skylab functioning well in Earth-orbit, NASA began to mothball its backup. Several plans were floated for putting Skylab B to use in Earth orbit. In December 1976, however, NASA donated its second space station to the newly opened Smithsonian National Air and Space Museum on the National Mall in Washington, DC.

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, officially designated "Apollo" (but sometimes informally called Apollo 18), docked with Soyuz 19 on 17 July 1975. CSM-111 did not include a SIM Bay. The last CSM to reach space undocked on 19 July and, after a period during which its crew performed experiments in the CM, splashed down in the Pacific Ocean near Hawaii on 24 July 1975, six years to the day after Apollo 11, the first moon landing mission, returned to Earth.

Artist concept of the Apollo-Soyuz docking in Earth orbit, 17 July 1975. Image credit: NASA

A Summary of NASA Manned Spacecraft Center Advanced Earth Orbital Missions Space Station Activity from 1962 to 1969, Maxime Faget & Edward Olling, NASA Manned Spacecraft Center, February 1969

Letter, Philip E. Culbertson to Rene A. Berglund, 27 August 1971

Skylab News Reference, NASA Office of Public Affairs, March 1973, pp. IV-6 - IV-8

Living and Working in Space: A History of Skylab, NASA SP-4298, W. David Compton and Charles Benson, NASA, 1983

Thirty Years Together: A Chronology of U.S.-Soviet Space Cooperation, NASA CR 185707, David S. F. Portree, February 1993, pp. 9-26 (http://ntrs.nasa.gov/search.jsp?R=19930010786 - accessed 10 May 2017)

Mir Hardware Heritage, NASA RP 1357, David S. F. Portree, March 1995, pp. 33-35, 65-72 (http://history.nasa.gov/SP-4225/documentation/mhh/mhh.htm - accessed 10 May 2017)

"Skylab B: Unflown Missions, Lost Opportunities," Thomas Frieling, Quest, Volume 5, Number 4, 1996

More Information

Space Station Resupply: The 1963 Plan to Turn the Apollo Spacecraft Into a Space Freighter

"Assuming That Everything Goes Perfectly Well In the Apollo Program. . ." (1967)

"A True Gateway": Robert Gilruth's June 1968 Space Station Presentation

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

A Bridge from Skylab to Station/Shuttle: Interim Space Station Program (1971)

Skylab-Salyut Space Laboratory (1972)

What If a Crew Became Stranded On Board the Skylab Space Station? (1972)

Reviving & Reusing Skylab in the Shuttle Era: NASA Marshall's November 1977 Pitch to NASA Headquarters