The story doesn't need speed, but because recent thinking about journeys to/from Mars has been about doing it in three or four months, I've been working my plot around that.
The timings of when there are launch windows is important for the plot, which would be easier if you could ignore launch windows. Even if you're not constrained by launch windows, it's much quicker to fly between the two planets when they're near, rather than far away. That means that people will tend to fly then.
Space isn't an important element of the economy in the story. Indeed, the difficulty of making money out of Mars is one of the elements of the plot. The real reason why countries have Mars bases is as a bolt hole for the political leaders and the super-rich in the event that a planetary disaster befalls Earth.
Spinning two cruising IPS-like ships on a tether was something I had planned originally, but you can't do that if you're using electric propulsion.
When I'd convinced myself that solar-electric would probably be the cheapest technology, I changed the plan to a rotating torus for the passengers, but solar-electric created plot difficulties in terms of avoiding interception.
I think the difficulty I'm having is that what I think would be most economic and the demands of the plot are pointing in different directions. Am I right or wrong to think that solar-electric will be the propulsion option with the lowest life-cycle costs?
I did include cyclers in my story originally, but after reading about them more I went off the idea. They seem too inflexible and too easy to miss.
One hundred years from now?First, start by thinking how people 100 years ago would envision transportation and technology today.That should put you in the imaginative frame of mind. Forget next 3-4 decades tech, like ITS family vehicles. Forget messy NTRs with their inherent problems of long time re-use.https://www.nasa.gov/directorates/spacetech/niac/2017_Phase_I_Phase_II/Fusion_Enabled_Pluto_Orbiter_and_LanderAt 100 years you should be firmly in the fusion camp*. Who is going to prove you wrong?
So what should I do? What kind of propulsion technology should I go for in my story? Chemical? Solar-electric? Chemical/solar-electric hybrid? Nuclear thermal? Nuclear-electric? Fusion? I'm sort of leaning towards a chemical/solar-electric hybrid, but I'm not at all sure.