[SHB] Two for the Price of One: 1980s Piloted Missions with Stopovers at Mars an

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 Two for the Price of One: 1980s Piloted Missions with Stopovers at Mars and Venus (1969)
29 April 2017 David S. F. Portree


The authors of the dual-stopover study did not design a spacecraft. The 6.4-year cycle of mission opportunities they identified repeats endlessly, however, so the NASA image above, which shows a present-day design for a piloted Mars spacecraft, can be pressed into service to illustrate this post. With relatively minor changes, this spacecraft might orbit both Mars and Venus during a single mission.

The piloted flyby missions NASA studied in the 1960s often included close encounters with both Mars and Venus. The October 1966 NASA Planetary Joint Action Group report Planetary Exploration Utilizing a Manned Flight System, for example, emphasized a piloted Mars flyby mission departing Earth orbit during the September 1975 free-return opportunity, but also noted an opportunity to launch a Earth-Venus-Mars-Venus-Earth flyby in February 1977 and an Earth-Venus-Mars-Earth flyby in December 1978.

Piloted flybys in the 1970s were intended to clear a path to piloted "stopover" missions in the 1980s. Stopovers - a category which included Mars and Venus orbiters and Mars landings - almost always emphasized a single objective. That is, each mission would travel to a single world, then return to Earth. The closest stopovers came to visiting more than a one planet was when a Mars stopover mission performed a Venus "swingby" to bend its course, slow its approach to Earth to enable a safe direct Earth-atmosphere reentry, or accelerate toward Mars.

During a Venus swingby, a Mars stopover spacecraft might explore the cloudy planet much as 1970s piloted Venus flybys were meant to do. That is, it might drop off probes insulated and armored against Venusian temperatures and pressures and scan the hidden Venusian surface with radar.

That a piloted spacecraft might stop at both Mars and Venus during a single mission was unthinkable. It was widely accepted that such a mission would demand enormous quantities of propellants, all of which would need to be launched into Earth orbit atop costly heavy-lift rockets.

In a brief September 1969 NASA Technical Memorandum, E. Willis and J. Padrutt, mathematicians at NASA's Lewis Research Center (LeRC) in Cleveland, Ohio, sought to overturn the prevailing view of what would be possible during 1980s stopover missions. Lead author Willis was no stranger to NASA piloted Mars mission planning; he had designed interplanetary trajectories at LeRC at least since early 1963.

Willis and Padrutt's calculations showed that a piloted spacecraft might capture into an elliptical orbit around Mars or Venus, then transfer to an elliptical orbit around Mars (if the first stopover was at Venus) or Venus (if the first stopover was at Mars). From there it would transfer back to Earth, where the crew would reenter the atmosphere directly in a small capsule. Its usefulness ended, the dual-stopover spacecraft would swing past Earth into a disposal orbit around the Sun.

The NASA LeRC mathematicians explained that they had discovered a repeating 6.4-year cycle of seven potentially useful dual-stopover mission opportunities. The seven opportunities varied only slightly from one 6.4-year cycle to the next. The first, fourth, and sixth opportunities would begin with an Earth-Mars transfer, while the second, third, fifth, and seventh would begin with an Earth-Venus transfer. In their paper, Willis and Padrutt emphasized the 6.4-year cycle that would begin in late 1979.


A hand-drawn illustration from Willis and Padrutt's NASA Technical Memorandum outlines the dual-stopover mission beginning in late 1979. 1 = Earth departure on a minimum-energy path to Mars. 2 = Arrival at Mars at the beginning of a 78-day stopover during which Mars's position shifts as it orbits the Sun. 3 = Mars departure on a minimum-energy path to Venus. 4 = Arrival at Venus at the beginning of a 177-day stopover during which Venus's position shifts as it orbits the Sun. 5 = Venus departure on a minimum energy path to Earth. 6 = Arrival at Earth. Image credit: NASA

A spacecraft launched during the late 1979 dual-stopover mission opportunity would spend 78 days at Mars and 177 days at Venus. The Earth-Mars, Mars-Venus, and Venus-Earth legs of its voyage would together require 638 days. Adding the time spent at Mars and Venus to the time spent between worlds would yield a mission duration of 894 days - that is, slightly less than two and a half years.

The second opportunity of the 6.4-year cycle would occur in the first half of 1980. The dual-stopover spacecraft would depart Earth for Venus. It would spend 180 days at Venus, 10 days at Mars, and 669 days between worlds, for a total mission duration of 860 days (two and a third years). This made it the shortest dual-stopover mission of the seven-mission cycle, and to avoid technical and biomedical problems short missions were preferable to long ones. Willis and Padrutt acknowledged, however, that its short stopover at Mars would provide little time for exploration.

The third opportunity would occur in late 1981. The dual-stopover spacecraft would leave Earth for Venus, where it would spend about 265 days. It would stop over for 133 days at Mars, and spend 629 days between worlds, yielding a total mission duration of 1027 days (nearly three years).

The fourth opportunity would occur at the end of 1981. The dual-stopover spacecraft would leave Earth for Mars. It would spend about 274 days at Mars, 340 days at Venus, and 680 days between planets, for a total duration of about 1294 days (a little more than three and a half years).

The fifth opportunity would occur in the first half of 1983. The dual-stopover spacecraft would leave Earth for elliptical Venus orbit, where it would spend just 10 days. It would spend 601 days at Mars and 619 days between worlds, yielding a mission duration of 1230 days (a little less than three and a half years). The short stopover at Venus might make the opportunity undesirable; on the other hand, the mission's Mars stopover would be the lengthiest in the 6.4-year cycle, enabling a long period of exploration.

The sixth opportunity would see the dual-stopover spacecraft depart Earth for elliptical Mars orbit in early 1984. The spacecraft would spend 200 days at Mars, 250 days at Venus, and 639 days between planets, for a total mission duration of 1089 days (a little less than three years).

The seventh and last opportunity of the 6.4-year cycle would occur in mid-1985. The dual-stopover spacecraft would spend 767 days in elliptical Venus orbit before voyaging to Mars for a 78-day stopover. It would spend 599 days between worlds - the shortest travel time of the seven opportunities. The long Venus stopover would, however, result in a mission duration of 1444 days (about four years), making it the lengthiest of the seven dual-stopover missions.

The 6.4-year-cycle Willis and Padrutt studied in detail would end just before the first dual-stopover opportunity of the next 6.4-year cycle. That opportunity, very similar to the late 1979 Earth-Mars-Venus-Earth opportunity, would occur in the first half of 1986.

The NASA LeRC mathematicians calculated the "propulsive effort" necessary to carry out the seven dual-stopover missions in the 1979-1986 cycle. They measured propulsive effort in terms of the spacecraft velocity change firing the dual-stopover spacecraft's rocket motors would produce. Firing the motors would expend precious propellants, so most of the time small velocity changes were to be preferred over large ones.

In order of Earth-departure date, the dual-stopover missions would need minimum total propulsive velocity changes of 9.382 kilometers per second (kps), 8.738 kps, 8.7 kps, 9.252 kps, 8.896 kps, 9.339 kps, and 9.321 kps. In most cases, the minimum velocity change needed to perform Earth-Venus-Mars-Earth dual-stopovers would be less than that needed for Earth-Mars-Venus-Earth dual-stopovers.

Willis and Padrutt compared the total propulsive velocity change necessary to accomplish four of the dual-stopover missions with that needed to carry out four Mars stopover/Venus swingby missions. They sought to reduce dual-stopover mission duration, so did not limit themselves to minimum propulsive velocity changes. The Mars stopover/Venus swingby missions - all of which would include a 30-day Mars stopover - were assumed to leave Earth on approximately the same dates as the dual-stopover missions.

They found that the first dual-stopover mission, the December 1979 Earth-Mars-Venus-Earth mission, would need a total propulsive velocity change of about 13 kps to reduce its duration to 700 days. A Mars stopover/Venus swingby mission launched at about the same time could be performed in 700 days with a total velocity change of only eight kps. The same missions could be carried out in 575 days with velocity changes of 20 kps and a little less than 11 kps, respectively. These numbers indicated that the first opportunity in the 6.4-year dual-stopover cycle was not a favorable one for dual-stopover missions of reduced duration.

Dual-stopover missions launched in the other three opportunities compared more favorably with Mars stopover/Venus swingby missions. The fourth mission of the 1980s dual-stopover cycle - another Earth-Mars-Venus-Earth mission - could be shortened to 700 days if a total propulsive velocity change of about 12 kps were permitted, while a 700-day Mars stopover/Venus swingby mission departing Earth at about the same time would need a velocity change of about 10 kps.

The sixth dual-stopover mission (Earth-Mars-Venus-Earth) could be accomplished in just 625 days with a total velocity change of a little more than 10 kps. Willis and Padrutt calculated that a 625-day Mars stopover/Venus swingby mission launched at the same time would actually need a greater total velocity change: a little less than 12 kps.

The seventh dual-stopover mission in the cycle - an Earth-Venus-Mars-Earth mission - could be shortened to 675 days with a total velocity change of about 10 kps. A 675-day Venus swingby/Mars stopover mission launched at the same time would need a velocity change of eight kps.

Willis and Padrutt conceded that the minimum propulsive velocity change of a dual-stopover mission would almost always exceed that of a single mission that traveled from Earth to either Venus or Mars and back to Earth. They noted, however, that the minimum velocity change of a separately launched Earth-Venus-Earth stopover mission and a separately launched Earth-Mars-Earth stopover mission combined would always exceed that of a single dual-stopover mission. The two separate missions would together need a minimum propulsive velocity change of at least 17 kps; that is, nearly double the minimum velocity change of a typical dual-stopover mission.

Source

Round Trip Trajectories With Stopovers At Both Mars and Venus, NASA TM X-52680, E. Willis and J. Padrutt, NASA Lewis Research Center, September 1969

Planetary Exploration Utilizing a Manned Flight System, NASA Office of Manned Space Flight, 3 October 1966

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