Two for the Price of One: 1980s Piloted Missions with Stopovers at Mars and Venus (1969)
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 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 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 mission design would see a piloted spacecraft depart a circular low-Earth orbit and capture into an loosely bound high-apoapsis (that is, orbit high point) elliptical orbit around Mars or Venus. It would then transfer to a loosely bound high-apoapsis elliptical orbit around Mars (if the first stopover were at Venus) or Venus (if the first stopover were at Mars). From there, the spacecraft 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, meanwhile, swing past Earth into a disposal orbit around the Sun.
The mission plan was designed to reduce the amount of energy required to move between worlds, thus conserving propellant. The piloted dual-stopover spacecraft would travel between planets only when an opportunity for a minimum-energy transfer occurred; that is, only when the planets moved into positions relative to each other necessary for a minimum-energy transfer. Loosely bound orbits would reduce energy needed to capture into and escape from orbit. Direct reentry into Earth's atmosphere would ideally require only enough energy to deflect the capsule's course so that it would intercept Earth after it separated from the dual-stopover spacecraft.
The LeRC mathematicians calculated the total "propulsive effort" necessary to carry out the seven dual-stopover missions in the 1979-1986 cycle. They measured propulsive effort in terms of the total velocity change firing the dual-stopover spacecraft's rocket motor or motors would produce. Propulsive effort would expend precious propellants, so most of the time small velocity changes were to be preferred over large ones.
They 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 most cases, the minimum propulsive effort needed to perform Earth-Venus-Mars-Earth dual-stopovers would be less than that needed for Earth-Mars-Venus-Earth dual-stopovers. In their paper, Willis and Padrutt emphasized the 6.4-year cycle that would begin in late 1979.
The second opportunity of the 6.4-year cycle would occur in the first half of 1980. The dual-stopover spacecraft would depart Earth on a minimum-energy path to 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.
Because short missions limited the time available for hardware breakdowns and the crew medical problems, they were to be preferred to long ones. Willis and Padrutt acknowledged, however, that the opportunity's short stopover at Mars would provide little time for exploration. Total propulsive effort would amount to 8.738 kps.
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). Total effort would equal 8.7 kps.
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). Total propulsive effort would equal 9.252 kps.
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). Total propulsive effort would total 8.896 kps. 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). Total propulsive effort would amount to 9.339 kps.
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. Total propulsive effort would amount to 9.321 kps.
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.
Willis and Padrutt compared the total propulsive effort necessary to accomplish four of the dual-stopover missions in the 1979-1986 period with that needed to carry out four Mars stopover/Venus swingby missions. They sought to reduce dual-stopover mission duration, however, so permitted increased propulsive effort. This would enable shorter stays at planets and shorter transfers between planets. 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 effort 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 propulsive effort of only eight kps. The same missions could be carried out in 575 days with propulsive efforts 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 effort 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 propulsive effort of about 10 kps.
The sixth dual-stopover mission (Earth-Mars-Venus-Earth) could be accomplished in just 625 days with a total propulsive effort 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 propulsive effort: 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 effort required to carry out 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 propulsive effort 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 effort of at least 17 kps; that is, nearly double the minimum propulsive effort of a typical dual-stopover mission.
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
After EMPIRE: Using Apollo Technology to Explore Mars and Venus (1965)
NASA Marshall's 1966 NERVA-Electric Piloted Mars Mission
To Mars by Way of Eros (1966)
Triple-Flyby: Venus-Mars-Venus Piloted Missions in the Late 1970s/Early 1980s (1967)
Humans on Mars in 1995! (1980-1981)
Footsteps to Mars (1993)