Original URL: http://www.theregister.co.uk/2010/11/25/vasimr_vx_200_fullbore_efficiency/

Plasma space-drive aces efficiency numbers: Set for ISS in 2014

Electro-rocket to open up Mars, the Belt, Jovian moons

By Lewis Page

Posted in Science, 25th November 2010 13:04 GMT

Top boffins working at a NASA spinoff company are thrilled to announce that their plasma drive technology – potentially capable of revolutionising space travel beyond the Earth's atmosphere – has checked out A-OK in ground tests.

The VX-200 blasting Argon at full bore in ground trials. Credit: Ad Astra Rocket Co

What interplanetary spaceship exhaust plumes really look like

According to the Ad Astra Rocket Company, building the Variable Specific Impulse Magnetoplasma Rocket (VASIMR), the firm's VX-200 prototype engine has just completed its latest round of trials with flying colours.

“Many of the flight applications at the heart of our business model – orbital debris removal, satellite servicing, cargo flights to the Moon and Mars, and ejecting fast probes to the outer solar system – have required that the propulsion system achieve 60 per cent system efficiency," explains Ad Astra's Dr Tim Glover.

"DC electrical power has to be converted to radiofrequency (RF) power, which is then absorbed by the plasma. The fraction of the RF power that is converted to thrust is called the “thruster efficiency”. Now we have demonstrated in the lab the 70 per cent thruster efficiency we need to achieve the 60 per cent end-to-end system efficiency, at the required exhaust speed," adds the doc.

The VASIMR works by flinging reaction mass out of its exhaust, in a similar fashion to conventional chemical rockets used today. The difference is that the VASIMR blasts the gas out hugely more violently, hurling out its argon reaction mass as plasma hot as the interior of the Sun and moving at no less than 50 kilometres per second.

This means that a VASIMR plasma drive gets a lot more poke from a given mass of propellant than a relatively feeble chemical rocket possibly can, and thus that a VASIMR spacecraft can be accelerated to hugely greater velocities while still carrying decent amounts of payload. As an example, a practical VASIMR ship would be able to carry astronauts to Mars in just 39 days: a chemically-powered vessel, compelled to coast most of the way, would take six months.

The downsides of VASIMR are twofold. Firstly it requires a plentiful supply of electricity to convert argon into plasma and expel it with such terrific violence. Secondly the drive can handle only a relatively limited amount of power and fuel flow for a given mass of engine. While hugely efficient in terms of the results it gets for a given amount of fuel, a VASIMR able to lift itself off the ground is not feasible - the present small-car sized test engine can produce a maximum thrust of just 5.7 Newtons, barely enough to support half a kilogramme against Earth's gravity.

Electric plasma tortoise beats chemical rocket hare

Artist's pic of a solar-powered VASIMR moon mission

Solar plasma-craft: Good for the Earth-Moon vicinity or sunwards

Where a VASIMR comes into its own is in space, in Earth orbit or beyond, where it can keep on exerting its relatively tiny push for days or weeks where a chemical rocket with more poke can make only brief burns due to massive fuel consumption. VASIMR, given time to work, can gradually boost a spacecraft up to velocities that chemically-propelled ships could never dream of - and still have reaction mass left with which to slow down again, all the while carrying plenty of cargo.

In the Earth-Moon neighbourhood, according to VASIMR inventor Franklin Chang Díaz, plasma-propelled craft could be usefully powered by solar panels. The International Space Station, and perhaps in time more ordinary satellites, could maintain or shift their orbits far more economically than they do at present. NASA plans to try out a flight-rated VASIMR unit for manoeuvring the ISS in 2014. This should mean far less space on supply capsules taken up by chemical propellant, permitting more experiments or other payload to be delivered to the station.

A 10MW nuclear VASIMR ship concept

A nuclear-powered plasma ship able to reach Callisto, ice moon of Jupiter

Chang Díaz, a plasma physicist and former NASA astronaut with seven Shuttle missions to his credit, believes that solar-powered VASIMR craft would be a massive shot in the arm for space operations in the vicinity of Earth and the Moon. Further out from the Sun, however - towards Mars, the main Asteroid Belt and the outer planets - some other source of power would be needed as solar panels struggle to produce much juice out there. (It will not have escaped the notice of solar-powered Mars rover fans that despite the machines' tremendous longevity they have still not travelled as far as the much shorter-lived Soviet Moon rovers of the 1970s did.)

The only other long-term option for electrical power generation in space is nuclear, which tends to cause a lot of technofear alarmism when deployed (except when this is secret, as it has often been in the case of classified spy satellites).

But if concerns over nukes in space could be eased, nuclear-powered VASIMR could potentially open up the solar system to humanity. Chang Díaz's proposed 39-days-to-Mars astronaut ship would use already existing dustbin-sized reactors of the sort already used to propel submarines: more ambitious plasma-drive ships would make manned journeys to the Belt or even the gas-giant moons of the outer system practical.

Now that the VX-200 prototype has been shown to work, Ad Astra will proceed with developing the VF-200 flight version for the space station. ®