You might be one step closer to owning your own TIE fighter a,s the Dawn spacecraft has proven the concept in its recent rendezvous with Ceres.
As most Star Wars aficionados know, TIE stands for “Triple Ion Engine” and although it sounds sexy, real-world ion thrusters don’t exhibit either the limber turn of speed exhibited in the movies or the distinctive growling roar.
Star Wars Ion Engine Sound:
Reality isn't quite as nice, is it?
Dawn does have three ion engines, but only used them one at a time on the long haul out to the asteroid belt. Although the mission was primarily designed to explore the asteroid belt, and particularly the protoplanets Ceres and Vesta, it is also a full-scale test-bed for ion thruster technology. And so far, it’s working… although “working” as designed only involves going from zero to sixty in about four days. Still, it is a notable first breakthrough.
Propulsion in a vacuum typically requires tossing something off the back of your spacecraft as quickly as possible and stealing the momentum, which is imparted by the inexorable logic of Newton’s Third Law: for every action, there is an equal and opposite reaction. Thus, the thrust in the direction you want to go is a fairly straightforward product of: The mass of whatever you threw backward times the velocity with which you threw it.
Of course, the ions that the thruster is throwing out the back are tiny and don’t have a lot of mass, but they’re going pretty fast. In fact, the charged ion stream comes out at a brisk 88,000 miles per hour. All the same, the total thrust is quite low, requiring very long firing periods to achieve the necessary velocities. For that reason, ion engines are all but useless for the bits of spaceflight that most people think of first — getting off the ground.
But the long haul to the final destination is another matter. The real advantage to ion thrusters lies in their efficiency. Using Xenon as a fuel source, the specific impulse (a measure of fuel efficiency) of the thrusters can range from 3,000 to 10,000 seconds. Compared to conventional rockets, which may only get 300 to 500 seconds, that’s a huge improvement. Ultimately, the amount of energy generated per kilogram of propellant allows the fuel payload to be dramatically lower.
Artist's impression of Dawn's Ceres mission: Image Credit: NASA
Dawn carries around 1,000 pounds of xenon and had a higher delta-v capacity than any previous ion-equipped craft. Despite running the thrusters for more than 200 days during the boost phase to get to Ceres, it used only around 160 pounds of that propellant. That all adds up to a long way on one tank of gas.
Ion thrusters have been around for a while. The original idea was put forward by Konstantin Tsiolkovsky, he of the famed
Tsiolkovsky rocket equation, in 1911. The first working model was produced at NASA in the late Fifties and a demonstration spacecraft, SERT 1, was launched in 1964 to prove the concept in space.
The devices have been used on Earth-orbitting satellites for station-keeping since the early 1970s. And a spate of missions launched around the turn of the century proved the utility of ion thrusters in deep space missions, ranging from NASA’s Deep Space 1 to Japan’s Hayabusa to ESA’s Smart 1.
Although they are light on the gas bill, ion thrusters do gulp a lot of one other resource: electricity.
There are a variety of methods used to accelerate the ions out the back of the thruster but
all of them require a lot of electricity in order to perform the trick. Solar panels can be used for missions which are sufficiently well-supplied with sunlight, but for excursions to the outer parts of the solar system, on board generating capacity is required.
The Dawn thrusters are 2.3 kilowatt NSTAR ion engines, but NASA has already moved the bar up a notch back here on Earth. The NEXT (NASA’s Evolutionary Xenon Thruster) Project has
just broken a record by operating at 7 kilowatts for over 43,000 hours — more than five years straight. With that performance, the next generation of deep space probes are all but guaranteed to take to the skies with ion thrusters propelling them.
NASA's NEXT ion thruster in action - (Image via NASA)
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