The Uphoff-Crouch Cycler Orbit was developed in 1991 by C. Uphoff and M. A. Crouch in their paper published as 'Spaceflight mechanics 1991; Proceedings of the 1st AAS/AIAA Annual Spaceflight Mechanics Meeting, Houston, TX, Feb. 11-13, 1991'
A Lunar cycler or Earth–Moon cycler is an orbit which periodically passes close by the Earth and the Moon, using gravity assists and occasional propellant-powered corrections to maintain its trajectories between the two. If the fuel required to reach a particular cycler orbit from both the Earth and the Moon is modest, and the travel time between the two along the cycler is reasonable, then having a spacecraft in the cycler can provide an efficient and regular method for space transportation.
Earth --> Moon --> (deep space out of Earth-moon plane) --> Moon --> Earth --> (deep space in the Earth-moon plane) --> Earth -->(repeats)
As the Moon doesn't stay still, they can't just go back and forth.
The concept was developed in 1991 by Uphoff and Crouch
This paper is a presentation of a new type of cislunar transfer orbit that has encounters with the moon twice per month every other month. The use of this technique is suggested for earth-to-moon Cycler spacecraft that contain the heavy and expensive life support equipment for human transfer from low earth orbit to the moon and for logistical supply of lunar bases. The basis for the technique is a 180 deg near-circular moon-to-moon transfer orbit that is inclined to the earth-moon plane by an angle that is compatible with a low-inclination, near-minimal energy earth-to-moon transfer orbit. Also included are preliminary discussions of Cycler spacecraft logistics for extensive manned operations on the moon. Numerical studies are included to verify the usefulness of the technique in a realistic cislunar dynamic environment and estimates of navigation propellant requirements are given.