posted August 19, 2014
Aerospace, in partnership with NASA, is on the cutting edge of the next big thing in ion engine propulsion – the aptly named NASA Evolutionary Xenon Thruster (NEXT).
What is the NEXT?
The NEXT, which was developed at the NASA Glenn Research Center, is the newest generation of electric ion thrusters and has a fuel efficiency that is 5-to-20 times greater than a chemical thruster. Fuel efficiency can be “chosen” to best match mission needs.
Ion thrusters have a lower level of thrust than their chemical counterparts, but each exhaust particle moves much more quickly as it exits, so less propellant mass can do the same job. While they are not powerful enough to launch a vehicle into space, their higher specific impulse (a measure of fuel efficiency) opens up new possibilities for future deep space exploration and longer trips to explore far off asteroids or planets, as well as new motion-intensive applications in near-Earth space.
“This is one of the ways to get very far in a feasible way,” said Mark Crofton, senior scientist, Propulsion Science Department. “You could say it’s got great gas mileage and can run forever.” Crofton is one of the handful of Aerospace scientists who has worked with the NEXT.
The uniqueness of the solar-powered NEXT engine, however, has mostly to do with its very long lifespan compared to any other thruster, chemical or electric, and improved performance capability relative to other ion engine designs. It can operate at high power levels – up to seven kilowatts and beyond, and process large amounts of propellant over its lifetime.
NEXT set a world endurance record when it operated for about 50,000 hours, equal to more than five years continuous running. During the test, a total of about 2,000 pounds of xenon propellant was run through the engine, setting another world record, this time for throughput. A typical chemical engine would have had to use more than 20,000 pounds of conventional rocket fuel to produce the same amount of total impulse. These performance levels exceed the anticipated requirements for any proposed space missions in the near future, according to a NASA press release.
The NEXT at Aerospace
Aerospace’s role with the 7-kilowatt ion thruster is running a series of tests on NEXT to characterize its capabilities, and conducting research development for the technology that comes after.
Aerospace has been working with NASA on its ion engine on and off for five years, two or three months each year. During that time, a litany of testing in Aerospace’s vacuum chamber has occurred, with “different members of the department handling various aspects of the testing,” Crofton said.
The most recent of those tests was to help NASA, potential future manufacturers, and purchasers understand the thruster’s life-limiting component. When the thruster expels ions, they pass through many small holes in an exit plate, or grid, at very high velocity. A small fraction of the ions will collide directly with the grid and change its surface by creating microscopic gouges.
“It changes the geography of the grid,” Crofton said. “If too many ions are impacting the surface directly, we have to tailor operating conditions to keep that from happening. There is a background of neutral propellant atoms. Ions that undergo collisions with neutral will have a reaction and form stationary ions. Instead of leaving, these stationary ions can seek out the negative potential of the grid and cause some damage.”
Testing things like this as accurately as possible, without being in the vacuum of space, is what Aerospace does best. Through a method using laser-induced fluorescence of molybdenum atoms, Crofton’s team assessed the lifetime potential of many possible throttling levels much more quickly than a conventional lifetest can do. The Aerospace lifetest ran more than five years and only looked at five throttle levels.
“We’re the only game in town as far as measuring the molybdenum,” he added. “Overall, Aerospace has done the most complete beam ion and plume characterization [measuring all relevant parameters] that’s been done for any thruster.”
The good news is lifetime figures completed on Earth’s surface will actually improve once in space. The vacuum of space has the power to pull away neutral, wayward propellant atoms faster and more completely than any vacuum chamber on Earth can mimic, so there will be less around to cause trouble.
What’s next after the NEXT?
The NEXT is in its final stages of testing and is considered a “well-developed article,” Crofton said. With that, it will not be changed before its first flight sometime in the near future. Proposed missions that use NEXT are being submitted this year for NASA’s Discovery program.
In the arena of ion engine technology, the Propulsion Science Department has now turned its attention toward technologies that could eventually replace the NEXT – annular and hybrid ion engines – using the knowledge gained while studying the NEXT and other ion thrusters.
“What we’ve done with NASA is typical of what we do – make electric thruster measurements,” Crofton said. “All sorts of companies come in with thruster systems. We have seen tons of civil and commercial work for thrusters. This is our bread and butter.”
There are two main classes of ion thrusters that can be used for sizeable spacecraft – the Hall current thruster and the ion engine. The possibility of a hybrid engine, one that pairs aspects of NEXT with portions of the Hall current thruster, is an exciting one for Crofton and his team because it means being able to combine the best aspects of both technologies into something that cannot be matched by anything on the market today.
The Hall thruster can generate higher thrust, while ion engines have higher exhaust velocities and better mileage. The ion engine under development for the hybrid has an annular rather than round shape, and is itself a big step forward.
“With these combined, we would have a much wider range over which you can operate the system,” Crofton said. “Each thruster has a sweet spot for exhaust velocity, where they work best. We want to improve the performance and increase the operating envelope at the same time. It is only a perceived advantage. Nobody’s even built one of these yet,” he added. “We are partnering with NASA for that work. NASA has been developing ion engines since the 1960s. NASA-Glenn is the world’s expert on ion engines and it is good for us to get to partner with them.
“Right now we’re building and testing annular ion engines as the first step,” Crofton added. “Raising the thrust to power metric over a wide specific impulse range is a big goal for us, because of the Earth orbit applications.”
There are big benefits to a hybrid, but combining the two types of engines would also mean the interactions would become more complicated and difficult to measure and interpret. Studying NEXT on its own has also opened up new lines of questioning for which the team is still trying to find answers. Crofton said this is no bad thing.
“There are also new physics there that you can come to grips with,” he said. “There are some measurements of the NEXT that we made, even four years ago, that we are still struggling to understand.
“When these measurements are made, you try to understand them,” he added. “If it is precise enough, you get in a regime where interpreting what you find can be a huge challenge. As you understand these new levels, that feeds into the next generation of research.”