Energy conversion options that are usually considered are heat engines (Rankine cycle, Brayton cycle and Magnetohydrodynamic – MHD) and direct conversion engine (the so-called Traveling Wave Direct Energy Convertor – TWDEC).

There was a proposal for Traveling Wave Direct Energy Convertor that also surveyed advanced nuclear fission space power concepts.

Low specific mass (less than 3 kg/kW) in-space electric power and propulsion can drastically alter the paradigm for exploration of the Solar System, changing human Mars exploration from a 3-year epic event to an annual expedition. A specific mass of ~1 kg/kW can enable 1-year round-trips to Mars, regardless of alignment, with the same launch mass to low Earth orbit (350 mT) estimated by the Mars Design Reference Architecture 5.0 study for a 3-year conjunction mission. Key to achieving such a propulsion capability is the ability to convert, at high efficiency and with only minimal losses rejected as heat via radiators, the energy of charged particle reaction products originating from an advanced fission or aneutronic fusion source directly into electricity conditioned as required to power an electric thruster.