Espresso Insight

Ion propulsion is one of the top technologies that will enable deep space exploration.

The X3 ion thruster is currently the most advanced of its kind and capable of producing greater power and thrust. The X3 will further advance human space travel technology and our ability to embark on missions into the far depths of outer space.

The X3 Nested Channel Hall thruster is being developed in collaboration between NASA, the University of Michigan PEPL, Aerojet Rocketdyne, and the Air Force Research Laboratory.

X3 Ion Thruster Updates as of 2021

Additional developers include NASA GRC, NASA JPL and the Air Force Office of Scientific Research.

Ion propulsion uses electrostatic fields to ionize and accelerate a propellant.

More on ion thrusters here.

Specs for the X3 ion thruster:

1. Type: Hall-effect ion thruster
2. Size: 80 cm diameter
3. Weight: 230 kg (500 pounds)
4. Specific Impulse: 1800–2650 seconds
5. Force/Thrust: 5.4 Newtons
6. Power: 100mw
7. Discharge Current: 247 A
8. Discharge Voltage: 500 V at peak efficiency
9. Propellant: Krypton or Xenon compatible
10. Lifetime: over 50,000 hours
11. Speed: 40 km/s = 89,000 mph

What’s so special about the X3 ion thruster?

There are two key technological factors that make the X3 Ion thruster better, faster, and more efficient:

1. Hall Effect Thruster Technology

First of all, there are multiple types of ion thruster designs.

The best is the Hall effect ion thruster. The X3 Ion Thruster is designed based on the Hall effect.

Hall thrusters have been identified as the best approach to building better ion drives because of their longer lasting characteristic, as opposed to other plasma based ion thrusters.

Hall-effect ion thruster – What is it?

The Hall Effect describes how an electromagnetic field occurs perpendicular to the flow of current.

By using electricity to create a current in a circular shape, depending on whether current flows clockwise or counter-clockwise, the vector of the magnetic field will point either up or down.

The electromagnetic field gives ionized, or charged, particles kinetic energy, resulting in a force and causing the particles to accelerate in the given direction.

Based on Newton’s third law, the force of particles leaving the engine ultimately causes the spacecraft to move forward.

Why Hall effect ion thrusters last longer than plasma ion thrusters:

• Hall thrusters feature an innovative magnetic field configuration which prevents interaction and disturbances between ionized propellant and the engine components.
• In the case of plasma based ion thrusters, the ionized particles tend to quickly erode engine components after a year.
• The magnetic configurations in Hall thrusters produce a shielding mechanism so this does not happen.

2. Nested Ion Propulsion Channels

In addition to using the Hall effect, the second innovative differentiator in the X3 design is nested channels. The X3 has multiple rings, or discharge channels.

The nesting approach places multiple propulsion channels in a concentric-circle arrangement around a center-mounted cathode. Electric current flows around three circular pathways of different sizes, each producing the electromagnetic field perpendicular to the flow of current. By featuring additional channels, the magnetic field is stronger and thus produces more force to move a spacecraft.

From the 2017 tests at the NASA Glenn Research Center, the X3 demonstrated the ability to produce 5.4 newtons of thrust, which is almost 40% more than the previous best ion thruster, which was capable of producing 3.3 newtons.

Nested Hall Thrusters (NHTs) have a larger throttling range than traditional single-channel thrusters. By only engaging a single channel, a minimum amount of force can be produced. Alternatively, by engaging all three, more powerful configurations are possible.

As of 2018, the project is at a Technology Readiness Level 5, (TRL 5) meaning “component and/or breadboard validation in relevant environment”. This is a significant step on NASA’s 9 TRL levels, with number 9 being that the system is flight proven through successful mission operations.

From the Latest Journal Articles:

One of the main things that the July 2020 ion thruster paper found was that the X3 is likely able to operate more efficiently than expected.

In technical terms, the paper discovered that cathode flow as a fraction of anode flow can be as low as 4% in the X3 without having significant impact.

According to the paper, “due to the reduced flow rates, the total efficiency is slightly increased (although all values are within the measurement uncertainty)”.

Also, “These results suggest that low-TCFF operation is feasible for high-power Hall thrusters and can offer increased system efficiency as well as improved cathode lifetime, and can do so with little impact on thruster operation.”

Since the X3 Nested Hall Effect Thruster is more efficient, this means that it can be more conservative with fuel propellant, essentially getting more “miles per gallon”, to put it in terms used with automobiles. Saving propellant means that missions can go longer, further, and faster.

The article did not underplay the importance of unanswered questions that have yet to be resolved.

Why is the X3 ion thruster a big deal?

The short answer: the X3 is more powerful while at the same time, more efficient.

Nested-channel Hall thrusters have been identified as a means to increase Hall thruster power levels above 100 kW.

Given the X3’s capacity to produce a greater amount of force, the engine itself is also larger.

This will enable deep space travel:

• According to NASA’s technology roadmap, “This higher-power category [of ion drives] will be pertinent to human space exploration missions beyond LEO, and for rapid-transit science missions to the outer solar system and deep space destinations.”
• According to the research paper by Scott James Hall, if an ion propulsion system could produce over 300 kW of power, it would enable possible space missions to near-Earth asteroids as well as Mars.

So far, however, it has been challenging to reach this level of power. But the X3 has pushed the limits on what’s possible – although not yet in the 300 kW range, the technology is slowly progressing to higher levels, which may one day be attainable.

X3 Ion Propulsion Reducing Launch Mass

When you look at a traditional chemical rocket, the majority of the mass that is used to send it into orbit is fuel.

The large amounts of chemical fuel required for space missions is less efficient than the amount that would be needed by utilizing electric propulsion.

“high-power electric propulsion was key to allowing affordable travel to asteroids and near-Earth destinations by reducing launch mass”

NASA / Michigan PEPL

In the quotation above, “reducing launch mass” is referring to the absence of more heavy rocket fuel propellant as part of the payload. Ion thrusters carry a comparatively tiny amount of inert gas as propellant that allows the launch mass payload to be reduced.

“Large-scale cargo transportation to support human missions to the Moon and Mars will require next-generation, high-power Solar Electric Propulsion (SEP) systems capable of operating between 200 and 400 kW.” – American Institute of Aeronautics and Astronautics, Inc., 2018.

Sources:

• Scott Hall of the Plasmadynamics and Electric Propulsion Laboratory at U-M
• published 2020 by U. Michigan; https://pepl.engin.umich.edu/pdf/AIAA_2020_Hall_Jorns_gallimore.pdf
• https://pepl.engin.umich.edu/project/x3-nested-channel-hall-thruster
• https://www.nasa.gov/centers/glenn/about/fs21grc.html
• Space propulsion paper: https://www.lpi.usra.edu/sbag/goals/capability_inputs/2015_Tech_2_in_space_propulsion.pdf
• Propulsion methods: https://www.airuniversity.af.edu/Portals/10/AUPress/Papers/WF_0067_HANS_JEFFERSON_WEHRLE_MOVEMENT_AND_%20MANEUVER_IN_DEEP_SPACE.pdf
• MIT phd paper: https://dspace.mit.edu/bitstream/handle/1721.1/42043/228865572-MIT.pdf?sequence=2
• Scott James Hall paper: https://core.ac.uk/download/pdf/189597252.pdf
• NASA Glenn info: https://technology.nasa.gov/patent/LEW-TOPS-34