The popular Space Press has heard about VASIMR and its amazing potential for interplanetary travel for over a decade. What’s neglected in most presentations is the hefty power requirement of high-performance VASIMR systems. No current power source is up to the task of propelling vehicles to Mars in 39 days or so.
A paper that the amazing “39 days” time-frame might be quoted from is this one…
…but it’s a pretty hefty vehicle needed to achieve that performance, some 600 tons of which 476 tons is propellant and 22 tons of payload. Powering a 200 MW VASIMR is no small exercise either requiring considerable advancement in nuclear power in-space. Franklin Chang-Diaz, chief scientist working on VASIMR, discussed the kind of power-source needed in this paper…
…which describes a vapour or plasma core nuclear reactor with an MHD power-extractor/converter which gets a specific power of about 2 kWe/kg of power system. It’d mass about 100 tons to give a 200 MWe supply. Thus the 39 day VASIMR mass-breakdown is…
476 tons propellant
100 tons power-supply
24 tons payload/structure
…quite a hefty machine, but that’s the price of top performance.
High-power is no small task to supply for a reasonable power-supply mass in-space. A terrestrial power-reactor can’t just be deployed in space as many of its systems are designed requiring both gravity and open-cycle sources of water and air available as heat-sinks. Thermal power conversion of heat to electricity is inherently inefficient in space because the only way to dump waste heat – other than as a hot gas jet – is via radiators, and to be light-weight radiators operate best at about 75% the temperature of the heat-source. Thus the efficiency is usually less than 25%. Thermoelectric conversion might one day improve this figure out of sight, but currently 20-30% is the best performance squeezed out of Stirling cycles and similar thermal power conversion systems.
But what of non-thermal power conversion? Magneto-HydroDynamic (MHD) power converters have been researched for decades, but on Earth these have issues with sufficient ionizing of the working fluid stream. In space, using highly-ionizing systems like vapour, gas or plasma core reactors and MHD comes into its own, allowing very high conversion efficiencies for low system masses.
A popularized discussion from the University of Florida… NEP with Vapor Core Reactor & MHD