The X3 Ion Engine Test Break Thrust Records

When it comes to the future of space exploration, a number of new technologies are being investigated. Foremost among these are new forms of propulsion that will be able to balance fuel-efficiency with power. Not only would engines that are capable of achieving a great deal of thrust using less fuel be cost-effective, they will be able to ferry astronauts to destinations like Mars and beyond in less time.

This is where engines like the X3 Hall-effect thruster comes into play. This thruster, which is being developed by NASA’s Glenn Research Center in conjunction with the US Air Force and the University of Michigan, is a scaled-up model of the kinds of thrusters used by the Dawn spacecraft. During a recent test, this thruster shattered the previous record for a Hall-effect thruster, achieving higher power and superior thrust.

Hall-effect thrusters have garnered favor with mission planners in recent years because of their extreme efficiency. They function by turning small amounts of propellant (usually inert gases like xenon) into charged plasma with electrical fields, which is then accelerated very quickly using a magnetic field. Compared to chemical rockets, they can achieve top speeds using a tiny fraction of their fuel.

Artist’s concept of Dawn mission using its blue ion engine to reach Ceres in the distance. Credit: NASA/JPL

However, a major challenge so far has been building a Hall-effect thruster that is capable of achieving high levels of thrust as well. While fuel efficient, conventional ion engines typically produce only a fraction of the thrust produced by rockets that rely on solid-chemical propellants. Hence why NASA has been developing the scaled-up model X3 thruster in conjunction with its partners.

The development of the thruster has been overseen by Alec Gallimore, a professor of aerospace engineering and the Robert J. Vlasic Dean of Engineering at the University of Michigan. As he indicated in a recent Michigan News press statement:

“Mars missions are just on the horizon, and we already know that Hall thrusters work well in space. 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.”

In recent tests, the X3 shattered the previous thrust record set by a Hall thruster, achieving 5.4 newtons of force compared with the old record of 3.3 newtons. The X3 also more than doubled the operating current (250 amperes vs. 112 amperes) and ran at a slightly higher power than the previous record-holder (102 kilowatts vs. 98 kilowatts). This was encouraging news, since it means that the engine can offer faster acceleration, which means shorter travel times.

Scott Hall makes some final adjustments on the thruster before the test begins. Credit: NASA

The test was carried about by Scott Hall and Hani Kamhawi at the NASA Glenn Research Center in Cleveland. Whereas Hall is a doctoral student in aerospace engineering at U-M, Kamhawi is NASA Glenn research scientist who has been heavily involved in the development of the X3. In addition, Kamhawi is also Hall’s NASA mentor, as part of the NASA Space Technology Research Fellowship (NSTRF).

This test was the culmination of more than five years of research which sought to improve upon current Hall-effect designs. To conduct the test, the team relied on NASA Glenn’s vacuum chamber, which is currently the only chamber in the US that can handle the X3 thruster. This is due to the sheer amount of exhaust the thruster produces, which can result in ionized xenon drifting back into the plasma plume, thus skewing the test results.

NASA Glenn’s setup is the only one with a vacuum pump powerful enough to create the conditions necessary to keep the exhaust clean. Hall and Kamhawi also had to build a custom thrust stand to support the X3’s 227 kg (500 pound) frame and withstand the force it generates, since existing stands were not up to the task. After securing a test window, the team spent four weeks prepping the stand, the thruster, and setting up all the necessary connections.

All the while, NASA researchers, engineers and technicians were on hand to provide support. After 20 hours of pumping to achieve a space-like vacuum inside the chamber, Hall and Kamhawi conducted a series of tests where the engine would be fired for 12-hours straight. Over the course of 25 days, the team brought the X3 up to its record-breaking power, current and thrust levels.

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

Looking ahead, the team plans to conduct more tests in Gallimore’s lab at U-M using an upgraded vacuum chamber. These upgrades will are schedules to be completed by January of 2018, and will enable the team to conduct future tests in-house. This upgrade was made possible thanks to a $1 million USD grant, contributed in part by the Air Force Office of Scientific Research, with additional support provided by the Jet Propulsion Laboratory and U-M.

The X3’s power supplies are also being developed by Aerojet Rocketdyne, the Sacramento-based rocket and missile propulsion manufacturer that is also the lead on the propulsion system grant from NASA. By Spring of 2018, the engine is expected to be integrated with these power systems; at which point, a series of 100-hour tests that will once again be conducted at the Glenn Research Center.

The X3 is one of three prototypes that NASA is investigating for future crewed missions to Mars, all of which are intended to reduce travel times and reduce the amount of fuel needed. Beyond making such missions more cost-effective, the reduced transit times are also intended to reduce the amount of radiation astronauts will be exposed to as they travel between Earth and Mars.

The project is funded through NASA’s Next Space Technologies for Exploration Partnership (Next-STEP), which supports not just propulsion systems but also habitat systems and in-space manufacturing.

Further Reading: Michigan News

New Way to Make Plasma Propulsion Lighter and More Efficient

Plasma propulsion is a subject of keen interest to astronomers and space agencies. As a highly-advanced technology that offers considerable fuel-efficiency over conventional chemical rockets, it is currently being used in everything from spacecraft and satellites to exploratory missions. And looking to the future, flowing plasma is also being investigated for more advanced propulsion concepts, as well as magnetic-confined fusion.

However, a common problem with plasma propulsion is the fact that it relies on what is known as a “neutralizer”. This instrument, which allows spacecraft to remain charge-neutral, is an additional drain on power. Luckily, a team of researchers from the University of York and École Polytechnique are investigating a plasma thruster design that would do away with a neutralizer altogether.

A study detailing their research findings – titled “Transient propagation dynamics of flowing plasmas accelerated by radio-frequency electric fields” – was released earlier this month in Physics of Plasmas – a journal published by the American Institute of Physics. Led by Dr. James Dendrick, a physicist from the York Plasma Institute at the University of York, they present a concept for a self-regulating plasma thruster.

A 6 kW Hall thruster in operation at NASA;s Jet Propulsion Laboratory. Credit: NASA/JPL

Basically, plasma propulsion systems rely on electric power to ionize propellant gas and transform it into plasma (i.e. negatively charged electrons and positively-charged ions). These ions and electrons are then accelerated by engine nozzles to generate thrust and propel a spacecraft. Examples include the Gridded-ion and Hall-effect thruster, both of which are established propulsion technologies.

The Gridden-ion thruster was first tested in the 1960s and 70s as part of the Space Electric Rocket Test (SERT) program. Since then, it has been used by NASA’s Dawn mission, which is currently exploring Ceres in the Main Asteroid Belt. And in the future, the ESA and JAXA plan to use Gridded-iron thrusters to propel their BepiColombo mission to Mercury.

Similarly, Hall-effect thrusters have been investigated since the 1960s by both NASA and the Soviet space programs. They were first used as part of the ESA’s Small Missions for Advanced Research in Technology-1 (SMART-1) mission. This mission, which launched in 2003 and crashed into the lunar surface three years later, was the first ESA mission to go to the Moon.

As noted, spacecraft that use these thrusters all require a neutralizer to ensure that they remain “charge-neutral”. This is necessary since conventional plasma thrusters generate more positively-charged particles than they do negatively-charged ones. As such, neutralizers inject electrons (which carry a negative charge) in order to maintain the balance between positive and negative ions.

An artist's illustration of NASA's Dawn spacecraft approaching Ceres. Image: NASA/JPL-Caltech.
An artist’s illustration of NASA’s Dawn spacecraft with its ion propulsion system approaching Ceres. Credit: NASA/JPL-Caltech.

As you might suspect, these electrons are generated by the spacecraft’s electrical power systems, which means that the neutralizer is an additional drain on power. The addition of this component also means that the propulsion system itself will have to be larger and heavier. To address this, the York/École Polytechnique team proposed a design for a plasma thruster that can remain charge neutral on its own.

Known as the Neptune engine, this concept was first demonstrated in 2014 by Dmytro Rafalskyi and Ane Aanesland, two researchers from the École Polytechnique’s Laboratory of Plasma Physics (LPP) and co-authors on the recent paper. As they demonstrated, the concept builds upon the technology used to create gridded-ion thrusters, but manages to generate exhaust that contains comparable amounts of positively and negatively charged ions.

As they explain in the course of their study:

“Its design is based on the principle of plasma acceleration, whereby the coincident extraction of ions and electrons is achieved by applying an oscillating electrical field to the gridded acceleration optics. In traditional gridded-ion thrusters, ions are accelerated using a designated voltage source to apply a direct-current (dc) electric field between the extraction grids. In this work, a dc self-bias voltage is formed when radio-frequency (rf) power is coupled to the extraction grids due to the difference in the area of the powered and grounded surfaces in contact with the plasma.”
The hall-effect thruster used by the SMART-1 mission, which relied on xenon as its reaction mass. Copyright: ESA

In short, the thruster creates exhaust that is effectively charge-neutral through the application of radio waves. This has the same effect of adding an electrical field to the thrust, and effectively removes the need for a neutralizer. As their study found, the Neptune thruster is also capable of generating thrust that is comparable to a conventional ion thruster.

To advance the technology even further, they teamed up with James Dedrick and Andrew Gibson from the York Plasma Institute to study how the thruster would work under different conditions. With Dedrick and Gibson on board, they began to study how the plasma beam might interact with space and whether this would affect its balanced charge.

What they found was that the engine’s exhaust beam played a large role in keeping the beam neutral, where the propagation of electrons after they are introduced at the extraction grids acts to compensate for space-charge in the plasma beam. As they state in their study:

“[P]hase-resolved optical emission spectroscopy has been applied in combination with electrical measurements (ion and electron energy distribution functions, ion and electron currents, and beam potential) to study the transient propagation of energetic electrons in a flowing plasma generated by an rf self-bias driven plasma thruster. The results suggest that the propagation of electrons during the interval of sheath collapse at the extraction grids acts to compensate space-charge in the plasma beam.”

Naturally, they also emphasize that further testing will be needed before a Neptune thruster can ever be used. But the results are encouraging, since they offer up the possibility of ion thrusters that are lighter and smaller, which would allow for spacecraft that are even more compact and energy-efficient. For space agencies looking to explore the Solar System (and beyond) on a budget, such technology is nothing if not desirable!

Further Reading: Physics of Plasmas, AIP