In his 1961 novel Stranger in a Strange Land, Robert Heinlein is clear about the real-world limitations of space travel to Mars with minimum energy paths:
At that time, only eight Terran years after the founding of the first human colony on Luna, any interplanetary trip made by humans necessarily had to be made in weary free-fail orbits, doubly tangent semi-ellipses--from Terra to Mars, two hundred fifty-eight days, the same for the return journey, plus four hundred fifty-five days waiting at Mars while the two planets crawled slowly back into relative positions which would permit shaping the doubly-tangent orbit-a total of almost three Earth years.
After making the trip the old fashioned way, a new technology becomes available:
The Federation Ship Champion, manned by an all-male crew of eighteen experienced spacemen and carrying more than that number of male pioneers, made the crossing under Lyle Drive in only nineteen days. The Champion landed just south of Lacus Soli, as Captain van Tromp intended to search for the Envoy.
This week, University of Washington researchers and scientists announced their progress in building components of a fusion-powered rocket that could drop the long months required for such a trip to mere weeks.
"Using existing rocket fuels, it's nearly impossible for humans to explore much beyond Earth," said lead researcher John Slough, a UW research associate professor of aeronautics and astronautics. "We are hoping to give us a much more powerful source of energy in space that could eventually lead to making interplanetary travel commonplace."
Slough and his team have published papers calculating the potential for 30- and 90-day expeditions to Mars using a rocket powered by fusion, which would make the trip more practical and less costly.
Slough and his colleagues at MSNW think so. They have demonstrated successful lab tests of all portions of the process. Now, the key will be combining each isolated test into a final experiment that produces fusion using this technology, Slough said.
(The Fusion Driven Rocket: Animation)
The Fusion Driven Rocket
Thin hoops of metal are driven at the proper angle and speed for convergence onto target plasmoid at thruster throat. A target Deuterium FRC plasmoid is created and injected into thruster chamber.
Target FRC is confined by axial magnetic field from shell driver coils as it translates through chamber eventually stagnating at the thruster throat.
Converging shell segments form fusion blanket compressing target FRC plasmoid to fusion conditions. The shell absorbs neutrons emitted during fusion.
Vaporized and ionized by fusion neutrons and alphas, the plasma blanket expands against the divergent magnetic field resulting directed flow of the metal plasma out of the magnetic nozzle.