Triple-planet manned flybys (1967)

In the 1960s, NASA devoted nearly as much study effort to manned Mars and Venus flybys as it did to manned Mars landings. Manned flybys were viewed as the most ambitious planetary missions possible in the 1970s using upgraded Apollo moon program hardware and a natural stepping stone between Earth-orbiting space station missions and manned Mars landings and Venus orbiters.

The NASA-wide Planetary Joint Action Group (JAG) proposed in its October 1966 report [read] that the first manned planetary flyby mission leave Earth in September 1975. The four-man Apollo-derived flyby spacecraft would swing past Mars in late January-early February 1976, and return to Earth in July 1977. Except for modest course adjustments, no propulsion would be needed after the spacecraft left Earth orbit. As it flew past Mars, its crew would release a variety of automated probes, including at least one which would land, collect samples of Mars rocks, dirt, and air, and return them to a laboratory on board the piloted flyby spacecraft (top image above).

The Planetary JAG described candidate follow-ons to the 1975 mission for the remainder of the 1970s. Of great scientific interest was a "triple-flyby" that would see the manned spacecraft fly past Venus, then Mars, then Venus again. Unfortunately, the only known opportunity to begin a triple-flyby in the late 1970s was poorly timed. The spacecraft would need to leave Earth in February 1977, while the 1975 Mars flyby was still underway. No other opportunity to begin a triple-flyby mission was predicted before 1983. Planetary JAG planners assumed that by then manned Mars landings and Venus orbiters would have superseded manned flybys.

In September 1967, J. Bankovskis and A. Vanderveen, engineers with NASA planning contractor Bellcomm, identified a triple-flyby opportunity with an optimum Earth-departure date of May 26, 1981. A spacecraft launched on that date would fly past Venus on December 28, 1981, past Mars on October 5, 1982, and past Venus again on March 1, 1983. It would return to Earth on July 25, 1983. Mission duration for the May 26 departure would total 790 days. Departures on other dates within the 30-day launch window would yield durations of from 720 to 850 days.

Discovery of the 1981 triple-flyby opportunity led Vanderveen to look for other triple-planet opportunities previous researchers had missed. In October 1967, he announced that he had found that a previously identified November 1978 "dual-planet" (Venus-Mars) flyby opportunity could be modified slightly to create a triple-flyby (bottom image above).

Vanderveen determined that, if one assumed a launch from Earth orbit on November 28, 1978, then the triple-flyby spacecraft would pass Venus on May 11, 1979, Mars on November 25, 1979, and Venus again on January 29, 1980. Return to Earth would occur on January 31, 1981, yielding an 800-day mission. Earth departure on other dates within the mission's 35-day launch window could reduce duration to as little as 760 days.

Vanderveen explained that the two Venus flybys would have "very diverse. . .characteristics," so would need different scientific programs. In both, the flyby spacecraft would pass about 1200 miles from Venus. The May 1979 flyby would see it speed past the center of the dayside hemisphere, its ground track nearly paralleling the equator. This might favor visible-light mapping through breaks in the dense Venusian clouds, Vanderveen wrote.

The January 1980 flyby, on the other hand, would see the spacecraft slowly approach Venus's dayside southern hemisphere, its ground track passing near the south pole. It would reach closest approach 30° south of equator near the terminator (the line between day and night), then would recede from Venus's nightside hemisphere. Vanderveen recommended that the flyby crew turn infrared sensors and mapping radar toward the night side as they began their year-long return to Earth.

"The Existence of a 1981 Triple-Planet Ballistic Flyby - Case 103-2, A. Bankovskis and A. Vanderveen, Bellcomm, Inc., September 19, 1967.

"Verification of the Existence of the 1978 Triple-Planet Flyby Opportunity - Case 720," A. Vanderveen, Bellcomm, Inc., October 19, 1967.

Douglas conjunction-class Mars mission (1965)

In September 1964, the Douglas Aircraft Company began a nine-month study of a conjunction-class Mars mission on contract to NASA Headquarters. Wernher von Braun's 1950s Mars studies and Philip Bono's 1960 Mars plan [read] (bottom image above) described conjunction-class expeditions, but in the 1960s most Mars mission plans were opposition class. The names refer to the position of Mars relative to Earth about halfway through the expedition. In the former, Mars would pass behind the Sun as seen from Earth (that is, would reach conjunction); in the latter, Mars would be opposite the Sun in Earth's skies (that is, would reach opposition).

A conjunction-class expedition would include low-energy transfers to and from Mars, each lasting about six months, and a long stay at Mars - roughly 500 days. Expedition duration would total about 1000 days. An opposition-class expedition would have one low-energy and one high-energy transfer separated by a short stay at Mars - typically around 30 days. Duration would total about 600 days.

Because it would require more energy, the opposition-class Mars expedition would require more propellants. All else being equal, a purely propulsive opposition-class expedition might need more than 10 times more propellants than a purely propulsive conjunction-class expedition. Most or all of the extra propellants would need to be lifted from Earth's surface using costly heavy-lift rockets. Hence, the conjunction-class expedition plan is attractive; however, the long mission duration would demand great endurance and reliability from both machines and men, expose the crew to risk from micrometeoroids and radiation for a longer period, and require a complex Mars surface exploration program to enable productive use of the 500-day stay at Mars.

Douglas's June 1965 final report listed the major NASA-provided "ground rules" that guided its study. The conjunction-class expedition would rely on chemical propulsion systems expected to be available by the early 1970s, not nuclear systems. It would economize on propellants by using aerodynamic braking to the fullest extent possible at Mars and upon return to Earth. NASA had also asked Douglas to look at whether its Mars spacecraft might depart Earth on a launch vehicle of the Saturn family. The space agency had stressed, however, that this last request was not meant to dictate the design of its conjunction-class Mars spacecraft. In general, the purpose of the study was to explore the possibilities and problems of a conjunction-class Mars mission, not to develop a realistic spacecraft design.

Douglas's conjunction-class Mars spacecraft had a novel compact design. The 120.7-foot-long spacecraft would include three main parts. These were the Earth Departure Step (EDS), the Mars Orbit Module (MOM), and the Mars Excursion Module (MEM) lander.

The EDS, a cylinder 54.7 feet in diameter and about 32 feet long, would include a doughnut-shaped liquid hydrogen fuel tank surrounding a spherical liquid fluorine oxidizer tank. A support structure attached to the fluorine tank would hold four advanced-design plug-nozzle rocket engines. The latter would lack the bells seen on most rocket engines, significantly reducing their length.

The conjunction-class Mars spacecraft's second part, the MOM, would take the form of a truncated cone 39.7 feet long with a maximum diameter of 54.7 feet where it joined the front of the EDS. The MOM was the mothership that would carry the crew to and from Mars and house the orbital crew while the surface crew explored the planet's ochre landscapes. Heatshield material would cover the MOM hull to protect it during aerobraking in Mars's atmosphere. The MOM would comprise three main parts: a Mars Capture and Escape Propulsion (MCEP) stage closely resembling the EDS, though much smaller; the doughnut-shaped two-deck Life Support System (LSS) module for housing the 10-person crew; and a conical Earth Entry Module (EEM).

The 15,830-pound EEM would resemble the Apollo Command Module, but would be larger (14 feet in length to Apollo's 10 feet) in keeping with the greater number of crew it would transport. The astronauts would shelter in the EEM during propulsive and aerobraking maneuvers and during solar flares. A "water wall" 4.4 inches thick would surround the EEM to serve as radiation shielding. The EEM would nest in the hole of the LSS doughnut for additional radiation protection.

The third section of the Douglas conjunction-class Mars spacecraft was the MEM lander, which would form its nose. Together the MEM and and MOM formed a squat cone about 89 feet long. Heatshield material would cover the MEM; because it would enter Mars's atmosphere twice (first during aerobraking into Mars orbit and again during descent to the surface), its heatshield would be more robust than that of the MOM.

The MEM would be home to six astronauts during their 500-day stay on Mars's surface. Measuring 35.3 feet across its base and 49.8 feet tall, the 71-ton lander would comprise three main parts: the MEM Landing Module (MLM) with a plug-nozzle rocket engine, fluorine/hydrogen propellant tanks, extendible landing legs, and parachutes; the MEM Life Support Step (MLSS), a four-deck pressurized doughnut housing the crew and their exploration equipment; and the MEM Take-Off Module (MTOM), which would include fluorine/hydrogen propellant tanks, a plug-nozzle engine, and the MEM Command Center (MCC).

The MCC, a doughtnut-shaped crew compartment surrounding the MTOM's spherical fluorine tank, would be staffed by at least one crewmember at all times during the surface mission. The conical MTOM hydrogen tank would nest on top of the MCC, forming the MEM's nose. The surface team would ride in the MCC during landing on Mars and again during ascent to Mars orbit after they completed their surface mission.

Douglas warned that its spacecraft design was provisional; it explained that it could not be finalized until automated probes had returned additional data on the meteoroid environment near Mars and the composition and density of the martian atmosphere. The company noted that as "little as a 10% uncertainty in one term of the meteoroid flux equation would cause a 35% change in the thickness of meteoroid protection at Mars and almost a 10% change in vehicle gross weight." "Because of uncertainties [about] the martian atmosphere," it added, "heat flux methods and heat shield material optimization is poorly developed." (The first successful Mars flyby spacecraft, Mariner IV, left Earth on November 28, 1964, as Douglas conducted its study. The spacecraft flew past the planet on July 14, 1965, a month after Douglas completed its study, revealing a martian atmosphere one-tenth as dense as the company had supposed.)

Douglas's conjunction-class Mars expedition would begin with assembly in Earth orbit. The company's spacecraft would not reach orbit on top of a 33-foot-diameter two-stage Saturn V rocket, for this would form an ungainly "bulbous payload." Instead, the MOM and MEM would launch on a new-design 74-foot-diameter post-Saturn launcher capable of placing one million pounds into Earth orbit. A second, similar launcher would then boost the EDS into Earth orbit. The EDS would dock with the MOM/MEM, then the EDS engines would ignite, placing the conjunction-class Mars spacecraft on a low-energy course to Mars. The EDS would then separate.

About 200 days after Earth orbit departure, the MOM/MEM combination would perform a "skipping aerodynamic-braking maneuver" in the upper martian atmosphere in a nose-forward attitude so that it could slow down and enter an elliptical Mars orbit. The MCEP's four plug-nozzle engines would then ignite at apoapsis to raise the spacecraft's periapsis, placing it into a 500-mile-high circular orbit.

After initial orbital reconnaissance of potential landing sites, the surface crew would board the MEM and separate from the MOM. They would fire the MLM engine to slow the lander, then would turn its nose forward for entry into Mars's atmosphere. Following entry, parachutes would deploy, then the MLM engine would ignite again for final hover and landing. In the event of trouble during descent, the astronauts could blast free of the MLM/MLSS in the MTOM and return to Mars orbit.

The astronauts would step onto the martian surface from a cylindrical airlock that would lower like an elevator from the MLSS. They would then deploy a nuclear power system, twin three-man pressurized rovers, and other surface exploration gear. Douglas wrote that its surface science payload and research focus would be the same as those proposed in the 1963 Philco Aeronutronic MEM study [read] (top image above). The 8700-pound nuclear-powered rovers would be capable of two-week traverses ranging up to 300 miles from the MEM.

After about 500 days on Mars, the MTOM engine would ignite to return the crew to Mars orbit. Once there, the MTOM would rendezvous and dock with an airlock hatch on the MOM's side. The crew would transfer a total of 1580 pounds of Mars samples and other items to the MOM. After casting off the spent MTOM, they would fire the MOM's MCEP engines to leave Mars orbit for Earth.

As Earth loomed large ahead, the crew would separate from the MOM in the EEM capsule. The EEM would enter Earth's atmosphere, deploy parachutes, and bump down on dry land. The abandoned MOM, meanwhile, would swing past Earth and enter solar orbit.

Douglas ended its report by listing significant conclusions it reached concerning long-duration Mars stays. It found, for example, that space suits being built for Apollo lunar missions could not be used during long-duration Mars expeditions. This was because water evaporation in vacuum would cool the Apollo suits. The water would be vented out of the Portable Life Support System backpack and lost at the rate of eight pounds every 3.5 hours. Douglas estimated that, if evaporative cooling were used in Mars suits, then the water required over 500 days would cause a 15% increase in the overall mass of its Mars expedition.

The company noted also that radio communication between the MEM, MOM, and Earth would not be continuous during the long stay at Mars because of the constantly changing Earth-Mars-Sun-MEM-MOM geometry. For example, when the Earth passed behind the Sun as seen from Mars (that is, when it entered solar conjunction), no direct communication between Earth and Mars would be possible for from 10 to 40 days. Communication between the MEM on the surface and the MOM in orbit would be possible for a total of about two hours each day, with communications opportunities lasting about 15 minutes each. Douglas suggested that Mars-orbiting and Sun-orbiting relay satellites be established to permit more continuous communications.

Douglas proposed a 15-year development program for its conjunction-class Mars mission. The company would manufacture 16 spacecraft, of which six would constitute flight articles. Total program cost over 15 years would come to $17.5 billion.

Study of Conjunction Class Manned Mars Trips, Douglas Aircraft Company, SM-48661, Summary; SM-48662, Technical Details, Parts I & II, June 1965.

Elliptical versus circular (1967)

In October 1966, NASA's Planetary Joint Action Group (JAG) completed a report on a proposed class of piloted space mission for the 1970s: manned Mars/Venus flybys based on 1960s Apollo moon program technology [read]. The Planetary JAG's report was not intended as the final word on the subject; it was meant to put in place a basic scenario that could serve as a point of departure for more detailed studies.

One such detailed study, completed in June 1967, looked at increasing the efficiency of the Planetary JAG's Earth-orbital launch method. According to its author, Bellcomm engineer H. London, an upgraded two-stage Saturn V rocket would be capable of placing up to 170 tons into a circular 100-mile-high Earth orbit. The Planetary JAG's piloted flyby spacecraft would, however, have a mass of only 90 tons, and its three interplanetary booster stages would each have a mass of only 143 tons.

The Planetary JAG designated its interplanetary booster stage the MS-IVB, and London called his the S-IVC. The stages were, however, identical. Both were derived from the Apollo Saturn V S-IVB third stage (top image above). In its lunar program role, the S-IVB injected the Apollo spacecraft into a circular Earth orbit, then restarted to boost it out of Earth orbit toward the moon. The S-IVB also served as the second stage of NASA's Saturn IB rocket.

The Planetary JAG had its separately launched flyby spacecraft and booster stages coming together in a 263.5-mile-high circular orbit about the Earth because the circular orbit would facilitate rendezvous and docking. London argued, however, that this approach would waste the excess Saturn V capacity resulting from the flyby spacecraft and booster stage mass shortfalls. He proposed that the upgraded Saturn V rockets instead launch the flyby spacecraft and booster stages into an elliptical orbit so that their excess capacity could be applied to launching the flyby spacecraft out of Earth orbit.

London's scenario would start with launch of a upgraded Saturn V bearing the flyby spacecraft. After completing one 100-mile-high circular orbit, the rocket's S-II second stage (bottom image above) would restart, boosting the flyby spacecraft into an elliptical orbit with a 100-mile perigee and a 6000-mile apogee. S-II restart would need to be timed precisely so that the flyby spacecraft's elliptical orbit would be properly aligned when the time came for it to depart Earth orbit for its destination planet. Its propellants exhausted, the S-II would then detach.

A second upgraded Saturn V would launch an interplanetary booster stage into a 100-mile-high circular orbit. After completing one orbit, the S-II would restart. Because a booster stage fully loaded with liquid oxygen and liquid hydrogen propellants would have a mass 53 tons greater than the flyby spacecraft, the S-II would deplete its propellants by the time it attained a 1000-mile apogee. To match orbits with the flyby spacecraft, London proposed that the booster stage should separate from the spent S-II and fire its own engine at perigee to raise its apogee. It would then use auxiliary propulsion to rendezvous and dock with the waiting flyby spacecraft.

Additional booster stages would be launched and added to the flyby spacecraft as needed. When the launch window opened for interplanetary transfer, the flyby spacecraft would ignite its first booster stage at perigee, further raising its apogee. After it expended its propellants, the booster stage would detach. The second booster stage would then ignite, and so on, until all the booster stages were expended and the flyby spacecraft was on course for its destination.

London concluded that the performance advantage gained through the elliptical orbit assembly and launch technique could reduce the number of Saturn V launches required to carry out a piloted flyby mission while enabling a greater flyby spacecraft mass. He estimated, for example, that a 1975 Mars flyby mission assembled in and departing from elliptical Earth orbit would need just three Saturn V launches: one for the flyby spacecraft and two for booster stages. The flyby spacecraft could have a mass of from 87.5 to 105 tons. The same mission assembled in and departing from the Planetary JAG's circular Earth orbit would need four Saturn V launches: one for the flyby spacecraft, which could have a mass no greater than 65 tons, and three for booster stages.

"Comparisons of Circular and Elliptic Orbit Rendezvous for Manned Planetary Flyby Missions - Case 233," H. S. London, Bellcomm, Inc., June 8, 1967.