The Aldrin cycler requires fuel to keep it going around the sun (with periodic flybys of Earth and Mars).
In principle, each Earth flyby is used to rotate the orbit so that it goes to Mars, but the problem with the Aldrin cycler is that the required flyby distance is too close to Earth: it's smaller than Earth's radius! As a result, rocket fuel is required to give it the required extra kick.
But there are other Earth-Mars cycler trajectories! One of my projects in grad school was to investigate them, and there are some that really can keep going without spending fuel (except for minor adjustments). In particular, the so-called "Ballistic S1L1" cycler is very promising. Here's a paper about it:
McConaghy, T. Troy, Landau, Damon F., Yam, Chit Hong, and Longuski, James, M., “Notable Two-Synodic-Period Earth-Mars Cycler,” Journal of Spacecraft and Rockets, Vol. 43, No. 2, March–April 2006, pp. 456–465.
In principle, each Earth flyby is used to rotate the orbit so that it goes to Mars, but the problem with the Aldrin cycler is that the required flyby distance is too close to Earth: it's smaller than Earth's radius! As a result, rocket fuel is required to give it the required extra kick.
But there are other Earth-Mars cycler trajectories! One of my projects in grad school was to investigate them, and there are some that really can keep going without spending fuel (except for minor adjustments). In particular, the so-called "Ballistic S1L1" cycler is very promising. Here's a paper about it:
McConaghy, T. Troy, Landau, Damon F., Yam, Chit Hong, and Longuski, James, M., “Notable Two-Synodic-Period Earth-Mars Cycler,” Journal of Spacecraft and Rockets, Vol. 43, No. 2, March–April 2006, pp. 456–465.