Cislunar cycler (1991)

The Space Exploration Initiative (SEI), launched on the 20th anniversary of the Apollo 11 lunar landing (July 20, 1989), differed from most other presidential space initiatives in that President George H. W. Bush and members of his Administration apparently truly believed in it. Bush, the 41st President of the United States, had been in office for only six months at the time he made his announcement on the steps of the National Air and Space Museum, so he did not perceive SEI mainly as a component in his re-election campaign. This was unlike President Richard M. Nixon, who delayed announcement of the Space Shuttle Program until January of the 1972 election year, President Ronald W. Reagan, who announced the Space Station in January of the 1984 election year, and President George H. W. Bush, who announced the Vision for Space Exploration in January of the 2004 election year.



Though among SEI's many failings was a flawed execution, Bush 41 did his best to support his initiative after he had announced it. Again, this differed from Nixon, Reagan, and Bush 43. Because of its rumored cost of a trillion dollars, SEI became a political liability for Bush; nevertheless, as late as the summer of 1991, he gave speeches about it and negotiated with Congress for funds to keep it alive in some form.



Bush was not reelected in 1992, and SEI ended soon after President William J. Clinton took office in January 1993. Bush's initiative was not, however, without result. It left behind it a substantial legacy in the form of a vast body of literature, a motherlode of ideas for ways to explore the moon, Mars, and, to a lesser extent, other destinations. Not since the 1960s space race had American aerospace engineers had the opportunity to be so creative.



In February 1991, for example, Chauncey Uphoff and Robert Crouch, engineers at Ball Space Systems Division in Boulder, Colorado, described a cycler system for regularly scheduled voyages between the Earth and its peculiarly oversized natural satellite. Cycling spacecraft take advantage of planetary gravity-assist flybys to continuously repeat voyages between at least two worlds with minimal use of rocket propellants. Almost all cycler designs proposed to date have been intended to link Earth and Mars.



Their cislunar cycler system would include at least two spacecraft, the design of which Uphoff and Crouch did not specify. It is logical to assume, however, that they would be derived from space stations meant to operate beyond the safety of low-Earth orbit (LEO). The images above show two such station designs. The middle image shows a 2002 NASA concept of an Earth-moon L1 "Gateway" station with many inflatable components, while the bottom image illustrates a North American Rockwell lunar-orbit station concept from the early 1970s.



The cyclers would carry the most massive equipment and structures needed for economical and safe cislunar transport - for example, a recycling closed-loop life support system and ample radiation shielding for protecting the passengers from solar flares and cosmic rays - plus a propellant farm for refueling small "taxi" spacecraft that would carry passengers and cargo to and from the cycler. The cycler would be launched from LEO only once, yet could complete many Earth-moon journeys. In effect, it would be a permanent space station following a complex path through the Earth-moon system. Compared with, say, an Apollo-type Earth-moon transportation system, the cycler system's propellant requirements would be negligible.



The twin cyclers would follow three kinds of paths through cislunar space. Used in combination, they would enable a lunar swingby every 14 days, with each cycler encountering the moon three times in two months.



The first of the three cislunar paths, an elliptical Earth-moon "transfer orbit," would need either nine days or 14 days to complete. The second, a novel circular high-inclination "BackFlip" moon-to-moon transfer, would need 14 days. The third, a circular "holding orbit" that would match the moon's orbit but be inclined slightly relative to its orbital plane, would need 28 days to complete.



Low-thrust electric (ion) thrusters would perform almost all routine cycler course adjustments once the pattern of Earth-moon transfers became established. These would have the advantage of requiring minimal propellant, thereby reducing the logistics burden of operating the cycler system. In short, electric propulsion would reduce the number of costly launches from Earth's surface that would be needed to keep the cislunar cycler system running.



The twin cyclers would also carry high-thrust rocket motors for emergency maneuvers. These would burn chemical propellants drawn from the taxi propellant farm.



Uphoff and Crouch described their cislunar cycler system in action by following the movements of one cycler spacecraft. At the beginning of its career, a chemical-propellant booster would launch the cycler from its LEO assembly orbit toward the moon on a trip that would last from 4.5 to seven days. As it flew past the moon, its passengers would enter taxis and undock to land at the moon base or capture into lunar orbit.



The cycler, with no one on board, would then perform a gravity-assist lunar swingby that would take it into a BackFlip inclined 46° relative to the plane of the moon's orbit about the Earth. The spacecraft would remain within 318,000 kilometers of the moon (78% of the Earth-moon distance of 397,000 kilometers) during the BackFlip. Because of this, it would undergo gravitational perturbations to its orbit that it would need to correct using its electric thrusters.



Fourteen days after beginning the BackFlip, the cislunar cycler would for the second time encounter the moon, this time on the opposite side of the moon's orbit from the place where the first swingby took place. By applying electric-propulsion thrust selectively during the BackFlip, the cycler would reach the moon for the second time already positioned to enter either a 14-day or nine-day Earth-moon transfer orbit or a 28-day holding orbit.



If the cycler aimed for Earth, then passengers bound for the homeworld would leave the moon in taxis and dock with it during the second lunar swingby. Uphoff and Crouch suggested that the cycler carry chemical-propellant tugs for rescuing taxis that suffered propulsion or docking failures. The voyage to Earth would require 4.5 days (for a nine-day transfer orbit) or seven days (for a 14-day orbit). As the cycler neared Earth, the taxis would undock and aerobrake in the atmosphere to shed speed and capture into LEO, where they would rendezvous with the space station.



The unmanned cycler, meanwhile, would swing by Earth and complete the second half of its nine-day or 14-day orbit, in theory reaching apogee (its farthest point from Earth) at lunar distance (in practice, some nine-day orbits would not reach apogee at lunar distance, Uphoff and Crouch noted). The moon would not be there when the cycler attained apogee: instead, it would be located either one-third (for a nine-day orbit) or halfway (for a 14-day orbit) around its 28-day orbit from the point where the cycler reached apogee.



The cycler would then return to Earth in 4.5 days or seven days. If in a 14-day orbit, passengers bound for the moon would fly taxis to the cycler during this second Earth swingby. The cycler would then re-encounter the moon seven days later and drop them off. If in the nine-day orbit, the cycler would swing past the Earth unmanned and again reach apogee without the moon nearby 4.5 days later. It would then fall back to Earth. During its third Earth encounter, it would collect passengers, which it would deliver to the moon 4.5 days later.



If no one had need of a ride to Earth when the first BackFlip brought the cycler back to the moon, the unmanned cycler would enter a 28-day holding orbit. The holding orbit was, Uphoff and Crouch explained, an unfortunate necessity, for orbital mechanics dictated that BackFlips not occur in succession. This was because the lunar swingby needed to begin a new BackFlip immediately after completing one would occur below the lunar surface. In other words, the attempt would crash the cislunar cycler into the moon.



At the end of its 28-day holding orbit, the cycler would re-encounter the moon and, if passengers needed to travel from the moon to the Earth, would commence another nine or 14-day Earth-moon transfer orbit. Alternately, the unmanned cycler would begin another BackFlip, swinging by the moon again 14 days later.



Once every six weeks, the twin cyclers would fly past the moon at the same time. They could then enter a nine-day or a 14-day Earth-moon transfer orbit together, allowing them to dock for 28 days before they encountered the moon again. During this period, astronauts would board the cyclers to perform repairs and maintenance and restock supplies.



To reestablish the cislunar cycler system's pattern of one lunar encounter every 14 days, the cyclers would undock when next they encountered the moon. One would then enter a 14-day BackFlip, while the other would enter a 28-day holding orbit.

"Lunar Cycler Orbits with Alternating Semi-Monthly Transfer Windows," C. Uphoff and M. A. Crouch, AAS 91-105, Spaceflight Mechanics 1991, pp. 163-176; paper presented at the AAS/AIAA Spaceflight Mechanics Meeting held in Houston, Texas, February 11-13, 1991.