We are all painfully familiar with the cost of filling the tank – even with a hybrid, visiting the gas pump isn't fun. Similarly, NASA is familiar with the high cost of launching vehicles from Earth, as it takes about $10,000 per pound to launch things into space.
But, what about propulsion once the craft is up there? Satellites, spacecraft, and deep-space probes require thrusters to keep them in place or push them wherever they need to go – not an easy task when space, payload, and cost are incredibly finite.
NASA’s answer to these issues has been the development of ion propulsion engine technology, and projects like NASA's Evolutionary Xenon Thruster (NEXT) Project at the Glenn Research Center. The NEXT Thruster was recently sanctioned to be shut down after running for more than 48,000 hours, or somewhere around five and half years, as a test of the system.
The test was performed in a high-vacuum chamber where the engine used about 870 kg of xenon propellant.
The NEXT Thruster uses Xenon propellant, which is used for most electric propulsion systems. Xenon is an advantageous propellant because it’s inert, so it is non-reactive and non-contaminating to ground test facilities or spacecraft surfaces, and it has a high Atomic mass unit (AMU), which results in a high thrust-to-power ratio.
Also, Xenon propellant is highly condensable (about twice the density of water at high pressure), making it ideal when space is limited. Using Xenon propellant, this thruster technology could readily change a spacecraft velocity over the duration of the thruster lifetime by greater than 10 km/second.
The amount of impulse produced would have consumed more than 10,000 kg of conventional rocket propellant in similar applications. Ion propulsion is nothing new to NASA, but with NEXT, they are hoping to take science and observation to further and more challenging destinations than ever before.
The Key to Ion Propulsion
NEXT is an ion propulsion system that includes a 7-kw-class thruster intended to operate on solar electric power. Mike Patterson, principal investigator for the NEXT project, explains the electric propulsion process as accelerating charged particles (ions) with electric fields.
“With electric propulsion, your energy source comes from electricity," explains Patterson. Electric propulsion doesn’t produce very much thrust because the engine is not expelling a lot of propellant, but the exhaust velocity is very high – up to 90,000 mph with NEXT.
“Because we produce relatively low thrust, we have to operate for longer periods of time to get the same total impulse,” Patterson adds. Though the thruster requires a longer period to get where it’s going, the efficiency is greatly improved with this method of propulsion – hence the long-duration test. “Since we’re no longer energy limited by the chemical propellant, we’re only energy-limited by the amount of available solar power that can fit on the spacecraft.”
Ion propulsion engines, like the NEXT Thruster, have already been employed on numerous commercial spacecraft, as well as two previous NASA missions – Dawn and Deep Space 1. “The NEXT Thruster is the evolutionary higher-power version of the technology flown on those missions,” says Patterson.
NASA science missions go through a competitive process for technology. “They might have 30 proposals for one mission, from those 30 you have a down-select to maybe 3 for what’s called a 'Phase A study,' and then they’ll down-select one to go to 'Phase B.' From that, you’ll end up flying one mission four or five years later,” explains Patterson. NASA science missions are risk-adverse, so the researchers choose the lowest risk, lowest cost system to perform their objective.
NEXT, though it has been on the docket a few times, is still fairly new technology, making the risk higher than other, more tested technologies. “The easy stuff, the low-hanging fruit has been picked out of the solar system, so we’re getting to the point if you want to conduct a high-value science, you have to start relying on advanced propulsion, like NEXT,” says Patterson. “[NEXT] is the natural progression beyond the ion propulsion system flying on the Dawn Mission. NEXT has been identified as critical for missions in NASA’s Decadal Survey, which has identified planetary science missions that need to be performed over the next decade.”
Designing for Maximum Thrust
“Ion thrusters are conceptually simple, but difficult to manufacture,” explains Patterson. “I could make half or three quarters of the [NEXT] engine in my garage, but the rest is very complex and requires sophisticated manufacturing processes.”
The NEXT Thruster follows traditional, spherically domed, circular ion optics. Ion optics are two sheets of metal with tens of thousands of apertures (tiny holes) that are coaligned between two electrodes. Patterson explains, “The electrodes are sub-millimeter thickness, with sub-millimeter gap, with sub-millimeter apertures that have to be perfectly aligned to extract the ion from the plasma, to accelerate them to high voltage.” As ion thruster designs get larger in diameter for more power, it becomes increasingly difficult to keep the gap between the electrodes constant – a necessity for function.
“From a manufacturing standpoint, in terms of being able to control the material so you don’t have non-uniform stretching, it becomes very complex and very difficult,” says Patterson. Because of this, the team already has their sights set on the next ion thruster design, an annular ion engine, which should provide an answer to this problem.
Patterson explains that they didn’t use sophisticated prototyping techniques, like 3D printing, for the NEXT thruster. “Quite literally, I do sketches or use some commercial product and hand them to my technicians, who work with sheet metal. Typically, we can prototype a new idea or concept in three to six months, test and evaluate over the next six months, and then we’ll know whether it is going to work or not within a year.”
Once the prototype is justified with a high-fidelity unit, the team incorporates thermal and structural analysis using commercially available thermal software. “Then we’d move to a more sophisticated model that might take between nine months and a year to fabricate. Then assemble and test, which would take maybe two years in total.” The team then hands the design over to U.S. industry.
In NEXT’s case, Aerojet Rocketdyne took the engineering model and did additional design work to improve its manufacturability, and it’s thermal and structural design in order to meet the requirements for a launch and in-space operation. “We make sure that the commercial entity understands what the critical dimensions, parameters, and materials are so they can replicate them,” explains Patterson. “They take the design and mature it so that it can accommodate things like structures that will survive on a vibration table, to simulate launch.”
Patterson’s team is developing the entire thruster system to provide an ideal efficiency. “If you just have a good thruster, that’s not enough,” he says. “There are other subsystems that are necessary for the care and feeding of the thruster to build an integrated system for a successful spacecraft.”
The team is working on fine-tuning a power processing unit for the NEXT thruster, as well. “It pumps about 7.2 kw into the box and we get about 6.9 kw out, that powers the thruster. That is a dynamic throttling capability, so it can vary the output power to the thruster by an order of magnitude, and it can operate at input voltages of about 80 to 160 V.” The voltage varies so much because these thrusters are typically meant for outbound missions on solar arrays.
Expect the Unexpected
Because NASA has been building engines for decades, the NEXT team had a massive empirical database to reference while developing the thruster, allowing them to build upon prior design experiences. “Unexpected things come up all of the time. You learn because it’s a progression and development of technology,” Patterson says. “This thruster literally is a design evolved from the technology flown. At the beginning, we have a design that incorporates all of the lessons learned through the development of those prior thrusters, but we’re trying to push the technology harder.”
The team is trying to cut the design cycle as much as possible in the prototype and test phase, and then mature the concept. “Since we’re trying to push everything simultaneously, we’re trying to break stuff,” he says. “It only takes time and money. In five minutes, I can put forward work on a sheet of paper that could take a decade or more to implement. The lifecycle in the aerospace and advanced propulsion market is incredibly long from concept to development to implementation.”
There really is no question that advanced electric propulsion, like NEXT, is ready for the limelight, it is just a matter of when the opportune mission will fly the thruster.