Lisa Grossman of New Scientist takes a closer look at Ad Astra’s VASIMR (Variable Specific Impulse Magnetoplasma Rocket), which uses a radio frequency generator to heat plasma:
VASIMR works something like a steam engine, with the first stage performing a duty analogous to boiling water to create steam. The radio frequency generator heats a gas of argon atoms until electrons “boil” off, creating plasma. This stage was tested for the first time on 2 July at Ad Astra’s headquarters in Webster, Texas.The plasma could produce thrust on its own if it were shot out of the rocket, but not very efficiently. To optimise efficiency, the rocket’s second stage then heats the ions to about a million degrees, a temperature comparable to that at the centre of the sun.
It does this by taking advantage of the fact that in a strong magnetic field – like those produced by superconducting magnets in the engine, ions spin at a fixed frequency. The radio frequency generator is then tuned to that same frequency, injecting extra energy into the ions.
Strong magnetic fields then channel the plasma out the back of the engine, propelling the rocket in the opposite direction.
Thanks to the radio frequency generator, VASIMR can reach power levels a hundred times as high as other engines, which simply accelerate their plasma by sending it through a series of metal grids with different voltages. In that setup, ions colliding with the grid tend to erode it, limiting the power and lifetime of the rocket. VASIMR’s radio frequency generator gets around that problem by never coming into contact with the ions.
“It’s the most powerful superconducting plasma source ever, as far as we know,” says Jared Squire, director of research at Ad Astra.
Scientists at Ad Astra began tests of the engine’s second stage – which heats the plasma – last week. So far, team members have run the two-stage engine at a power of 50 kilowatts. But they hope to ramp up to 200 kW of power in ongoing tests, enough to provide about a pound of thrust. That may not sound like much, but in space it can propel up to two tonnes of cargo, reaching Jupiter in about 19 months from a starting position relatively close to the sun, says Squire.
Of course, a starting position relatively close to the sun is half-way to anywhere, in Heinlein’s phrase, because the real challenge is getting out of earth’s gravity well, which requires more thrust than ion engines can generate:
At its current power level, VASIMR could be run entirely on solar energy. Squire says it would make a good Earth-orbit tugboat, pulling satellites to different orbits. It could also shuttle cargo to a lunar base, and because it could travel relatively quickly, it could be deployed to dangerous asteroids to gravitationally nudge them off course years before they would reach Earth.To travel to Mars in 39 days, however, the engine would need 1000 times more power than solar energy could provide. For that, VASIMR would need an onboard nuclear reactor. Early versions of the reactor technology were used from the 1960s to the 1980s by the Soviet Union, but have not been used in space since and would take time to develop. “That would be quite a ways down the line,” Squire says.