A side shot of the X3 firing at 50 kilowatts. NASA

Ion thrusters are among the most exciting propulsion systems for future deep space exploration, and the technology's viability has already been demonstrated on dozens of spacecraft. The thrusters ionize a fuel source, generally xenon, and then accelerate the charged particles to tens of thousands of miles per hour using electric and magnetic fields. The beam of ions blasting out the back of the thruster is an efficient way to move satellites in the near-vacuum of space.

If scientists could scale ion thrusters to accelerate larger spacecraft, they could propel missions to Mars and beyond. That's what the X3 project is all about.

In a vacuum chamber at the NASA Glenn Research Center in Ohio, University of Michigan doctoral student Scott Hall and NASA Glenn research scientist Hani Kamhawi recently fired up the X3, a 500-pound Hall thruster (a type of xenon-fueled ion thruster) developed by the University of Michigan, NASA, and the Air Force. The X3 proceeded to break records for operating current, power, and thrust generated by ion propulsion. UM calls the X3 a prototype "Mars engine," one of three experimental propulsion systems funded under NASA's Next Space Technologies for Exploration Partnership.

"Mars missions are just on the horizon, and we already know that Hall thrusters work well in space," said Alec Gallimore, University of Michigan dean of engineering who led the development of the X3, in a press release. "They can be optimized either for carrying equipment with minimal energy and propellant over the course of a year or so, or for speed—carrying the crew to Mars much more quickly."

Scott Hall makes some final adjustments on the thruster before the test begins in NASA Glenn's vacuum chamber. NASA

In a series of tests carried out at NASA Glenn from July to August of this year, the X3 trounced the record for thrust produced by an ion thruster, pumping out 5.4 newtons of force, a more than 60 percent increase over the previous record of 3.3 newtons. The X3 also broke records for operating current, running at 250 amperes vs. 112 amperes, and power, producing 102 kilowatts vs. 98 kilowatts. The test fires in the vacuum chamber at Glenn are the result of more than five years building, refining, and tuning the X3.

Ion thrusters already have a proven track record in space, most notably on the Deep Space 1 craft that flew by the asteroid Braille and the comet Borrelly in 1999 and 2001, respectively, becoming the first spacecraft to rely primarily on ion propulsion. The Dawn spacecraft used ion propulsion to become the first spacecraft to orbit two celestial bodies: the large asteroid Vesta in 2011 and the dwarf planet Ceres in 2015, where the spacecraft is still at work today. In addition, more than 100 communications satellites use small ion thrusters to correct their orbital positions.

A Hall thruster relies on the Hall effect to accelerate ions and produce thrust. The effect is a potential difference across an electric current that is created when a magnetic field is placed perpendicular to the current, as first discovered by American physicist Edwin Hall in 1879. After the xenon gas is ionized by bombarding the atoms with electrons, knocking other electrons loose and creating positively charged particles, those ions are accelerated through the Hall thruster to incredible speeds. The beam of ions shooting out the thruster wouldn't do much in the atmosphere, but in space, the near-vacuum conditions allow ion thrusters to produce significant acceleration with tiny amounts of fuel compared to chemical rocket thrusters.

The vacuum chamber at Glenn is currently the only one in the country with a vacuum pump system that can handle the X3, which produces too much exhaust for other chambers. Scott Hall and NASA Glenn engineers built a rig just to hold the 500-pound X3 and withstand the forces it generates.

"The big moment is when you close the door and pump down the chamber," Hall says. Because the Glenn chamber takes 20 hours of pumping to create a space-like vacuum, every snag along the way turned into a major ordeal. Whenever the team needed to get in and tweak or repair the X3, they had to spend days gradually filling the chamber back up with air, make the fix, and then pump air out of the chamber yet again.

The time-intensive test runs had Hall and Kamhawi working 12-hour days during the 25 days available for X3 testing in the Glenn vacuum chamber. The vacuum chamber in Gallimore's own lab at the University of Michigan is currently being upgraded so that it can support X3 testing, and the UM chamber is expected to be ready by January 2018.

The team isn't done at NASA Glenn though. In spring 2018, they plan to return for a 100-hour test fire with the X3 that will use a power system currently being developed by Aerojet Rocketdyne. With any luck, next year's tests will produce even more encouraging results, and then NASA can start thinking about how they are going to mount one of these things on a spacecraft to blast it off to Mars.

Source: University of Michigan

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