VARION: Plasma Modelling and Testing of a Atmospheric Ion Thruster for Orbital and Space Propulsion

Introduction

Space has the power to transform humanity. As we aim towards long-duration missions, interplanetary exploration, Mars colonization and commercialization of space, the need for efficient propulsion systems is greater than ever. In contrast to chemical propulsion, electric propulsion, particularly ion thrusters, is a highly fuel-efficient alternative. Ion thrusters achieve specific impulses up to 20 times higher than chemical rockets, expelling ions at velocities exceeding 50,000 m/s. Chemical propulsion is optimal for reaching space from Earth's surface, which requires high thrust, while ion thrusters dominate space travel and orbiting due to their high lifetime and efficiency.

Problem:

However, the greatest challenge remains the limitation imposed by finite onboard propellant. For satellites in low orbit, the most populated region and crucial for orbiting missions on Earth and Mars, onboard fuel determines operational lifetime, launch mass, mission cost, and sustainability, as continuous atmospheric drag causes orbital decay.

Solution:

Rather than relying on onboard propellant, my project, VARION, is a versatile atmosphere-breathing ion thruster that ionizes atmospheric gases using an intake system and produces thrust to sustain orbit and perform various missions on Earth, Mars and potentially other planets.

No spacecraft has ever used this system before. There is ongoing research on utilizing atmospheric gases as propellant by Project Sitael at the European Space Agency, but they do not focus on gridded electrostatic ion thrusters. My concept of using CO2 on Mars is completely novel and has not been researched before.

Analytical Modelling of Plasma Discharge Performance

A 0-dimensional analytical model developed by John R. Brophy, a NASA scientist, was used to calculate the performance of a gridded electrostatic ion thruster with xenon (Xe), nitrogen (N2) and carbon dioxide (CO2) at different thruster configurations.

Model Implementation and Code Process

The model takes the power inputs, thruster geometry, atmospheric densities, and 2000 data points of cross-sectional data for ionization, excitation and dissociation of N2 and CO2. The cross-sectional data were acquired from the LXCat database and past papers, while the data on Earth and the Martian atmosphere were taken from the NRLMSIS 2.0 atmospheric model of Earth and the Mars climate database.

The model calculates plasma parameters, including the beam ion energy cost (discharge loss), Maxwellian electron temperature, Ion production rate, energy losses, etc. Ultimately, the model calculates the discharge loss vs mass utilization efficiency curve, which is the golden standard across plasma thrusters to assess and compare the performance of different propellants and thruster configurations. The solution from the model is used to calculate the thrust, power consumption and efficiency of the thruster on Mars and Earth.

The model uses iteration due to the complexity, performing around 20 million iterations. The model uses Python and consists of 6300 lines of code.

Adapting the Model for Molecular Gases

Brophy's analytical model expects one excitation and one ionization rate coefficient, as the model was designed after traditional propellants that have always been noble gases like Xenon and Argon. Therefore, a process was developed to combine the numerous dissociation, fragment excitation and ionization reactions of molecular gases.

Prototype Atmospheric Ion Thruster

A prototype was modelled on Fusion 360 and 3d printed. The thruster utilizes a glow discharge mechanism to generate plasma and was tested in a vacuum simulating the atmosphere at 38 km.

Results from Analytical Modelling:

VARION can function on Earth and Mars with atmospheric propellant, producing 362.92 % more thrust than required on Earth at 190km altitude and 900.46% more thrust on Mars than required to sustain orbit at 160 km altitude. However, due to the higher power consumption on Earth, VARION can be used in a hybrid mode that carries a reduced amount of xenon onboard and alternates it with atmospheric propellant. On Mars, VARION achieves a self-sustainable orbit with manageable power consumption due to the more optimal ionization properties, molecular mass and gas densities of CO2.

The code was validated using the experimental data of the T-6 electrostatic ion thruster developed by ESA (European Space Agency). The experimental discharge loss was compared with my calculated discharge loss, which showed that VARION achieved an accuracy of 98.77% accuracy.

Unlike Noble gases like Xenon and Argon, CO2 is a molecular gas with more complex reactions and energy losses. The decreasing discharge loss of CO2 at 140 and 130km at higher mass utilization rates can be a result of ionizable fragments produced by dissociation, which would offset the decreasing neutral density, therefore, reducing energy loss as the ionization probability will be greater.

At lower altitudes, the required thrust and power are higher due to increased atmospheric drag. Mars requires much lower power due to its thinner atmosphere, demonstrating that atmosphere-breathing ion thrusters are more viable on Mars than on Earth.

Prototype Results

How it works:

Working Process:

1. Creating a Vacuum Environment

A rough vacuum was created (2700 torr) with a Vacuum chamber.

2. Plasma Formation (Glow Discharge)

A high voltage is applied between:

Cathode (−)

Anode (+)

Electrons accelerate and collide with gas particles, causing:

Ionization → creates positive ions

Excitation → produces purple glow due to the nitrogen present

3. Ion Acceleration

Positive ions are accelerated by the electric field:

Ions gain high speed and exit the chamber

This produces thrust

The prototype glow discharge thruster successfully produces plasma in recreated environments that simulate it at altitudes of 38 km. The ion beam current of the plasma could not be measured due to the high gas densities that would cause collisions with neutral particles. Testing at realistic vacuum levels will require advanced materials and very expensive resources. This glow discharge mechanism shows future promise in aiding VARION to function at lower altitudes where gas densities are drastically higher because of its higher efficiency in ionizing large amounts of propellant.

Practical Consideration:

Atomic oxygen formed in low Earth orbit has sufficient energy and flux to oxidize and erode most hydrocarbon polymers.  Protective coatings of metal oxides and metals can be used as a means of reducing oxidative attack on spacecraft. There is atomic oxygen present in the Martian atmosphere at higher altitudes(>180km), which is not in VARION's range.

Statistics and Tools used:

The analytical model was coded in Python (using libraries like numpy, scipy, matplotlib), and outputs 2100 rows of data on Excel and some graphs. Data analysis was performed on Excel.

So What?

1. VARION can be used for Earth observation, communications, space stations, and scientific missions across Low Earth Orbit. Depending on the Spacecraft, onboard propellant can be substantially decreased.

2. Exceptional for Mars orbiting missions due to CO2 being a better propellant. VARION's use in low-orbit missions will be crucial to establishing a base on Mars.

3. VARION’s adaptability enables deep space exploration and interplanetary missions because it allows a spacecraft to "refuel" in a planet's atmosphere, allowing us to travel further than ever and faster than ever.

4. VARION improves orbital sustainability by extending mission lifetimes and therefore reducing space junk production

5. VARION can revolutionize the space industry by making space cheaper and scalable, opening the industry to commercialization.

Future Work:

Using a more advanced and accurate analytical model or particle-in-cell simulation to investigate CO2 plasma in ion thrusters and account for the various gases in the atmosphere while modelling.

Model ion thruster in other planetary atmospheres like Venus, Titan, Pluto

Model heavier molecular propellants as an alternative fuel for ion thrusters

Develop AI that optimizes interplanetary travel by analyzing a planet's atmosphere and checking its potential as fuel.

Experimental validation of CO2 and N2 plasma discharge performance in high vacuum

Research on other electric plasma propulsion systems, including magnetoplasmadynamic thrusters

I want to thank my family for their constant support and encouragement throughout my science fair journey and for buying the equipment for my experimental testing.

I want to extend my sincere gratitude to my science teacher, Ms. Dragana Savic, for allowing me to participate in the science fair representing my school and supporting me.

Thank you to Dr. Goebel Dan, a senior scientist at NASA JPL, for answering some questions about his analytical model.

References

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