# What is the minimum transfer time for Earth/Mars cycler and still remain balllistic?

The classic Aldrin cycler doesn’t require onboard fuel when accounting for both earth and mars gravity assists. Which is fantastic. But it has a transfer time of roughly 5 months out to Mars. We could shave this time down, I’ve seen designs for 75 day transfers, but this increases the hyperbolic excess velocity at both planets, and consequently decreases the maximum deflection angle obtainable. At some point the transfer windows requires correction from onboard fuel, which isn’t great considering it would most likely have to use amyl, hydrazine mixture. So how much time can we shave off before we have to start carrying toxic explosives with us?

• Yes I know we could use other fuel types, that’s not what I’m asking. Commented Aug 22, 2022 at 11:55

To shorten the transfer time of a cycler, the perihelion and aphelion of the cycler orbit start drifting further and further away from the energetically favourable Hohmann ellipse.

Theoretically, the only hard limit to doing that is having a perihelion skimming the surface of the Sun and an aphelion far out in the outer solar system. This makes for a cycler travelling between Earth and Mars at approximately solar system escape velocity, meaning a transfer time of 24 days.

This is however impractical for several reasons.
Firstly, the long orbital period of the cycler gives very long turnaround times. Already in the 75 day scheme, a unit is only (re)usable once every decade.
Secondly, the hyperbolic excess velocity at both Mars and Earth becomes very high, requiring beefy propulsion to match velocities with the cycler.

The solution space is relatively "smooth", with solutions for every synodic period of cycler. Thus, you can pick a transfer time pretty much anywhere down to the 24 day limit and have a valid cycler orbit for it.

• This doesn't answer my question. What subset of the solutions are ballistic. On one end of spectrum you have the 24 day limit, which isn't ballistic since you can't deflect the orbit at all basically. On the other end is the the Aldrin cycler, which itself is pushing it, since without Mars' help, it'd be impossible getting the required deflection angle without slamming into Earth. How fast can we go whilst still being ballistic? Commented Aug 28, 2022 at 17:03
• The 24 day limit is ballistic. Significant deflection angles are only required for short periodic cyclers, in order for the orbits to line up quickly. Short periodic cyclers (like the Aldrin cycler) inherently have slow transfers. For a (very) long periodic cycler, deflection is a non-issue. Commented Aug 28, 2022 at 18:10
• Oh your right! The cycler would always needs to be deflected somewhat since the synodic periods aren't rational numbers. But some multiple of the synodic period would bring it close enough for low deflection requirement despite the higher hyperbolic excess velocities. Commented Aug 28, 2022 at 19:46