Interplanetary Flight Times.

Too often when we talk about future interplanetary flight we assume the trajectories used will be Hohman transfer orbits, elliptical orbits that intersect of both the orbit of departure and the destination tangentially, in the case of Mars this involves flight times of around 250 days. But even with todays chemical rockets the mass ratios for flight times of 150 days may be optimal for cost, and with thermal systems using hydrogen as the propellant flight times of just 70 days to Mars may not only be reasonable, but sensible, with hydrogen propellant and exhaust velocities of 10km/s a delta v of 6.5km/s is achievable with a mass ratio of only 2. All it requires is a thermal energy source to heat the propellant, aerobraking at Mars, and a propellant source on Mars for the return.