39 Day Trips to Mars

Maybe, with VASIMIR. [Link]

As hot as the sun

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.

High power

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.

One important thing about rockets, you can have high thrust (required to get from the ground to orbit), or you can have high efficiency (required to go any real distance on a reasonable schedule). The important number that describes this is Specific Impulse (Isp).

General considerations

Essentially, the higher the specific impulse, the less propellant is needed to gain a given amount of momentum. In this regard a propulsion method is more propellant-efficient if the specific impulse is higher. This should not be confused with energy-efficiency, which can even decrease as specific impulse increases, since many propulsion systems that give high specific impulse require high energy to do so (discussed later).
In addition it is important that thrust and specific impulse not be confused with one another. The specific impulse is a measure of the impulse per unit of propellant that is expended, while thrust is a measure of the momentary or peak force supplied by a particular engine. In many cases, propulsion systems with very high specific impulses (some ion thrusters reach 10,000 seconds) produce low thrusts.^{[citation needed]}
When calculating specific impulse, only propellant that is carried with the vehicle before use is counted. For a chemical rocket the propellant mass therefore would include both fuel and oxidizer; for air-breathing engines only the mass of the fuel is counted, not the mass of air passing through the engine.

[edit] Examples

Specific impulse of various propulsion technologies

Engine	"Ve" effective exhaust velocity (m/s, kg·m/s/kg)	Specific impulse (s)	Energy per kg (MJ/kg)
Turbofan jet engine (actual V is ~300)	29,000	3,000	43
Solid rocket	2,500	250	3
Bipropellant liquid rocket	4,400	450	9.7
Ion thruster	29,000	3,000	430
VASIMR	290,000	30,000	43,000