>>13627706use a "space ship" that can thrust in specific and changeable directions.
speed and efficiency are mutually exclusive.
speed 2 chemical rockets into LEO one with the xfer stage and one for the habitat.
Burn at a mars xfer window and use atmospheric breaking at the Mars side, might take a few passes. can't come home.
efficiancy swap the xfer stage for nuke or xenon thrusters. the xfer window would be a different time and it would take a lot longer.
atmo breaking is still the most efficient at the far end.
i guess you could send a second xfer stage to mars orbit to come back home. but coming back isn't fast or efficient.