Hall Thruster Propellants: Finding the Best Fit

In the context of chemical propulsion, the selection of propellants is determined by a range of considerations, including their thermal and chemical properties, as well as their storage density. In contrast, electric propulsion, especially Hall thruster technology, considers a different set of criteria, such as corrosivity, atomic or molecular mass, ionisation and dissociation potentials, boiling and melting points, and so forth. To illustrate, the highest thrust-to-power ratio is attained with a high mass-to-charge ratio of ionic propellant; thus, xenon is frequently selected for ion propulsion. In addition to the necessity for pressurised storage tanks, xenon is regarded as an optimal propellant for electric propulsion, given its mass-to-charge ratio, chemical inertness and its natural gaseous state. In selecting an appropriate propellant for electric propulsion systems, a broad range of constraints must be considered, including storage limits, propellant management, thruster performance, and economic considerations. It identifies the most suited propellant with respect to these specific constraints. Currently, there are different classes of propellants: either solid-state, liquid-phase, gaseous, or supercritical-phase, and each has its own advantages and challenges.

Factors affecting Propellant Choice in Hall Thrusters

The selection of a suitable propellant for Hall thrusters involves evaluating various factors, including storage, performance, and cost. Understanding these parameters allows for the optimization of thruster efficiency and overall mission success.

1. Thrust per kW is a critical factor, it determines the operational time, especially in missions with limited fuel supplies. High thrust is preferred for shorter mission times, such as in satellite rescue operations. Xenon is particularly effective in such scenarios due to its ability to provide high thrust-to-power ratios.
2. Specific impulse (Isp) is another key parameter that reflects the efficiency of propellant use. A higher Isp means more efficient propellant consumption, making it favorable for long-duration missions where time is less critical. However, in time-sensitive missions, a balance between thrust and Isp must be found.
3. Power consumption is a limiting factor for electric propulsion systems. The available onboard electrical power constrains both thrust and specific impulse, and an optimized trade-off between energy usage (high thrust-to-power) and mass usage (high I_sp, or high power-to-thrust) is essential, particularly for missions with limited power availability.

But there are other factors…

The atomic mass of ionic propellant is very important for the propulsion system. The heavier the ion, the more thrust it will produce, and vice-versa. The lighter ions have more specific impulse compared to the massive ions, and therefore, will necessitate less propellant mass for the same impulse. Therefore, propellant atomic mass directly affects a propulsion system’s efficiency and thrust capabilities, but ionization potential – the energy cost of producing an ion – and ionization cross-section are also critical to the discharge efficiency. There are also availability issues related to the propellants, both on Earth and in space, which could have a bearing on their practicality for certain missions. For instance, the use of extraterrestrial sources of propellant would reduce dependence on terrestrial resources, while requiring extraction and refining processes, but would still allow a new range of exploration missions and interplanetary trips.

Propellant Storage and Handling

The storage and management of propellants in space introduce significant technical challenges, particularly due to the varying physical states of potential propellants.

1. Physical phase of the propellant, whether solid, liquid, or gaseous, greatly affects storage complexity. Solid propellants, such as Bi or Li, are challenging to handle due to the need for phase change systems. Similarly, cryogenic liquids like hydrogen require elaborate thermal management systems to maintain their phase, increasing mission complexity and power consumption.
2. Energy demands for propellant storage also present a challenge. Certain propellants, such as iodine and water, require temperature regulation to remain in a usable state, increasing the overall energy requirements of the system. Propellants stored at high pressure, such as argon, require careful management due to the added risks of high-pressure containment.

Mission Considerations and Propellant Availability

Mission time duration hardware constraints determine which propellant to use for its propulsion. The weight of the propulsion system (including the propellant, the storage tank and all the other subsystems like thrusters, power supplies, fluid control etc.) contributes the most to mission failure or success. This will require larger amounts of propellant and, therefore, more complicated solutions of its storage within the system design. On Earth, the availability of propellant is primarily defined by the price category, bearing limiting factors with the rates of production if the amount to be produced is quantitatively constrained. For noble gases, the best options seem to be Argon (Ar), Xenon (Xe), and Krypton (Kr).

Other propellants, either tested already with different types of thrusters or that may still be utilized within EP, include H2, I2, air, N2, O2, N2H4, Li, Cs, Cd, Hg, H2O, Mg, Zn, EMI-BF4, and Bi.

Cost of Propellants for Space Missions

Costs are incurred from production and delivery of the propellants. The noble gases, however, are by-products of distilling the air and are more expensive than argon, which is cheap and plentiful. However, the whole transportation costs are huge; in particular, costs of lifting propellant from Earth into space can get dis- proportionately high. Mission-specific costs depend on where the source of propellant is and how much ∆v it takes to bring the propellant to the spacecraft. For refueling missions with propellants, space-based ’service stations’ or hubs may be located at some place, such as low Earth orbit (LEO) or Lagrangian points, to lessen the expense associated with continuous Earth launches.

Propellant selection for Hall thruster propulsion systems remains a complex process, involving a balance between performance, storage complexity, and cost. Xenon, krypton, and argon are the most commonly used propellants, but other options, such as air-derived CO2 and water, show promise, particularly in the context of in situ resource utilization. Iodine is also a strong contender as a condensed-storage alternative. As missions extend deeper into space, propellant availability and storage technologies will continue to evolve, offering new possibilities for more efficient and cost-effective propulsion systems.

References

1. Consortium for Hall Effect in Orbit Propulsion System CHEOPS VHP
2. Dan M. Goebel and IraKatz ”Fundamentals of Electric Propulsion: Ion and Hall Thrusters”