Powering VASIMR

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…

Andrew Petro’s Presentation from 2002 at NASA

…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…

Fast, Power-Rich Space
Transportation, Key to
Human Space Exploration
and Survival

…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

Some additional papers…
Vapor-Gas Core Nuclear Power Systems
with Superconducting Magnets

Development of Liquid-Vapor Core Reactors with MHD
Generator for Space Power and Propulsion Applications

4 thoughts on “Powering VASIMR”

  1. Consider, however, that the Saturn V came in at a hefty 3300 tons with a 50 ton payload. Granted, the Saturn is a full launch vehicle as compared to a VASIMR-driven ship that could never take off under its own power, but I think many people would a ship like that to be built in orbit anyway…

    1. Quite so Chris. But everything put into orbit has to be launched somehow. If a slower trip to Mars is acceptable, then the Mars ship can be smaller than the Shuttle Orbiter, but the trip takes ~115 days. It’s all in the Petro paper. Interestingly the power-to-mass ratio of thin-flim photovoltaics suited for space applications is dropping rapidly. If the desired power was ~12 MW like the ‘slow’ VASIMR to Mars then a solar power supply needs to only mass ~5.4 tons @ 5kWe/kg. Such ultra-thin PVs make Solar Power Satellites practical to launch via SpaceX’s Falcon-9 Heavy and one of Elon Musk’s stated goals is to go to Mars. I can’t think of a more perfect demonstration of the ‘power’ of thin-film PVs than to power a SpaceX VASIMR rocket to Mars.

  2. “power conversion system”? Gee, that sounds like you want technical details and I’m no engineer…

    The overall design of the light gathering part of the ship would be the same as a Cassegrain telescope, the secondary mirror would only need to be held in place with light cables under tension because of the light pressure from the main mirrors would push the two apart, the receiver would then be in the centre and behind the main mirror with the turbines behind that with the radiators attached.

    The further the ship was from the sun, the more mirrors would be deployed outside the primary dish to increase the light gathered, these outer system mirrors would obviously be of much lighter material.

    The ship would always retain the same orientation with regards to the sun, with several VASIMR (or similar performance) engines mounted around the centre of mass and gimballed.

    In principle the choice between PV vs thermal for the receiver would depend on the power/wt of the two options, I don’t know a lot about turbine systems, but I know for the turbines themselves there’s the potential of very high power/wt ratios, (think rocket engine turbo pumps).

    Perhaps a ship with a main mirror area of 4 square km turning the light gathered into a GW of power could be built for 200 tonnes dry mass, with tankage for 800 tonnes LH2 and lighter mirrors for the outer system, it could visit all the planets out to Saturn in just a few years, something a fission reactor powered ship couldn’t do.

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