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A “planetary system of electric propulsion thrusters: four main types of electric propulsion systems currently tested and used on small satellites and cubesats. (credit: Appl. Phys. Rev. 2018, the authors)

Small thrusters for small satellites: trends and challenges

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For virtually all fields of technology, small is beautiful. From electronics to sensors, as users, we have come to associate small with fast, affordable, and efficient. For industry, small also means profitable, as portable and wearable devices, multifunctional smartphones, and crystal-size computers made possible by miniaturization create enormous new markets.

Even though miniaturization of space technology may sound straightforward, moving from big to small in space brings a multitude of unique challenges to the space industry.

A similar shift is taking place in space technology. Indeed, in space, space is at a premium. The truth of this statement holds on many levels. For one, miniaturization of satellites can make their development and launch less expensive. Then, in orbits where debris outnumbers operational satellites, having smaller assets can give us more room to maneuver, and free up the space for the use of very small satellites that can deliver new data transfer capabilities.

Miniaturization means lower cost and thus much higher affordability and access to space for those who cannot spend millions on buying an entire launch, like small research labs, private companies, and universities. Even for those who can afford launching conventional spacecraft, small satellites provide a potentially lucrative opportunity as they could form networked distributed systems. A dynamically changing, adaptive constellation of a large number of small satellites can perform mission-oriented, coordinated formation flights and feature technical capabilities not readily available in small networks of conventional satellites with the total mass comparable to that of the constellation.

The additional capabilities can mean faster data transfer and wider coverage of survey and information collection—and there is no secret that big data analytics has become central to the global economy. These capabilities will open new horizons for space exploration, as explained in a recent paper in Nature. Constellations are also not just for Earth orbit applications. Small, organized networks of tiny satellites orbiting Moon and Mars could provide a realistic alternative to large, heavy, and expensive spacecraft that require large launch vehicles. Deep space cubesats are fast becoming a reality.

Even though miniaturization of space technology may sound straightforward, moving from big to small in space brings a multitude of unique challenges to the space industry. Space technology miniaturization cannot be achieved by simply reducing the size of every component of the satellite. Certain components and subsystems are notoriously difficult to miniaturize, exposing significant limitations in the overall design of the device, as well as manufacturing and assembling processes. Not surprisingly, advances in certain aspects of space technology, such as electronics and sensors, leapfrog others, such as propulsion.

Miniaturization of propulsion technology presents a significant obstacle for the wider use of organized mini-satellites. Indeed, even though we can launch them a hundred at a time (104 satellites were deployed by the Polar Satellite Launch Vehicle C37 mission in February 2017), most of these satellites are passive spacecraft capable of orientation but not active maneuvering. Organized, coordinated flight requires highly efficient, reliable thruster systems capable of coherent maneuvering of small satellites within the formation. For a typical cubesat of form factors of 1–10U, this means a propulsion system that can deliver tens of watts up to one hundred watts. Yet, at these power levels, the efficiency of small thrusters is extremely low, sometimes not exceeding 10–15 percent. Such an engine would use considerable amounts of both propellant and electric power, potentially at the expense of payload and orbital service life.

How to proceed with specific space propulsion systems?

Cold gas and hydrazine systems, when scaled down to tens of watts, feature extremely low specific impulses. From a physical standpoint, this is hardly surprising because of high hydraulic losses in the accelerating channels and nozzles due to very high surface-to-volume ratios in tiny systems. Early attempts to enhance them by increasing specific impulse via electric heating resulted in hybrid thrusters that were rather cumbersome but still not particularly efficient. Furthermore, these thrusters used both electrical energy as well as chemical fuels, with the latter often being corrosive. Not surprisingly, such systems did not find much use in space technology, although they were used in several missions.

Three types of electric space thrust systems: a gridded ion thruster, an electrodeless Rotamak-type thruster with a rotating magnetic field, and a Hall-type thruster. (credit: I. Levchenko & K. Bazaka)

Current trend focuses on all-electric space thrust platforms, with many companies taking a keen interest in the technology, such as Airbus with its Eutelsat 172B launched in June 2017. This is because electric thrust platforms are very flexible, with an impressive range of possible device configurations that have been produced and tested in labs and in space. Among others, the following types of electric propulsion platforms are attracting considerable attention:

  • Hall type thrusters that use a closed electron drift and feature high thrust density and specific impulse;
  • Ion thrusters that accelerate ions by high electric potential applied to a metallic grid and feature low thrust density but very high specific impulse;
  • Pulsing and direct-current thrusters that use an electric dis- charge between two electrodes to accelerate plasma as a whole medium;
  • Electrodeless systems with rotating and complex-shape magnetic fields, such as Rotamaks, helicon thrusters and similar systems;
  • Electrospray thrusters of various configurations; and
  • Printable cathode arc thrusters.

Each of the above devices faces its own obstacles on the path to miniaturization.

The Hall-type thrusters are now considered the most promising platform for medium-sized satellites. Because of that, they were a natural choice to miniaturize them down to a cubesat form-factor. Development and testing so-called cylindrical Hall thrusters featuring simplified geometry was the first major step in that direction. Even though this new geometry slightly reduces the efficiency of the device, it also opens up opportunities to build a tiny accelerating channel of very small diameter. The pioneering work at the Princeton Plasma Propulsion lab has allowed these thrusters to gain wide recognition in the cubesat community, with efforts currently underway to adapt them to cubesats.

Hybrid Hall-type thrusters designed and tested at the Space Propulsion Centre, Singapore (SPCS), combine novel acceleration channel materials with exceptional properties, flexible permanent magnet-based magnetic circuits, and innovative miniaturized cathodes. It showed a record-breaking efficiency of 75 percent in the ultra-low power and thrust ranges (about ten watts), and the results have been reported in the journal Plasma Sources Science and Technology). This performance have warranted their installation on the 6U cubesats currently under development at the SPCS and scheduled for launch in 2019.

One more promising way to enhance the efficiency of small Hall thrusters and thus to ensure their miniaturization is using so-called wall-less configuration. Here, a sophisticatedly shaped magnetic field at the exit of the acceleration channel prevents the plasma from coming into contact with surrounding material and thus prevents the plasma from escaping to the walls. The pioneering work is led by Dr. Stephane Mazouffre (France, CNRS) and Prof. Yangdi Ding (Harbin, China) have been published in J. Appl. Phys. and Plasma Sources Science and Technology.

Ion thrusters constitute the second major type of electric propulsion system that can support active maneuvers, orbit keeping, and interplanetary maneuvers. With the ion flux being accelerated by the DC potential applied to grids, these thrusters feature very high specific impulse. But, in contrast to Hall thrusters, ion thrusters demonstrate low thrust density due to the electric space charge in the discharge chamber. Thus, their ability to be miniaturized is limited. However, the low value of the thrust density could be compensated by higher accelerating voltage applied to the grids. This is also not a trivial task for the miniaturized systems, where electric breakdowns are possible due to small gaps between the powered electrodes and thin insulators. In addition, robust and safe generation of high potentials in highly miniaturized systems on cubesats requires advanced semiconductor technology.

Thus, miniaturization of ion thrusters relies mainly on the progress in material science rather than advancement in plasma physics and technology. Nevertheless, some efforts are being made to design miniaturized ion thrusters; see, for example, the article in Journal of Spacecraft and Rockets. One company, Busek, has recently reported a miniature radio frequency ion thruster with a size of one centimeter and power of about ten watts that is designed for cubesats.

Miniaturization of propulsion technology presents a significant obstacle for the wider use of organized mini-satellites.

Pulsing and direct-current thrusters accelerate plasma as a whole medium, thus they do not suffer from low thrust density and are therefore quite suitable for applications in microthrust configurations. Indeed, they have multiple applications in small satellites and cubesats of various sizes, including 1U cubesats and even smaller spacecraft. However, these types of electric thrusters feature low power efficiency as compared with the Hall and ion types, mainly due to high energy losses on metal evaporation and ionization in non-equilibrium plasma and non-stationary discharge in the transient process. This makes them less suitable for applications requiring significant delta-V changes, such as orbit-raising and interplanetary maneuvers. They are currently considered mainly for precise attitude control and positioning. On the other hand, these operations require precision, thus small pulsing thrusters are irreplaceable for small satellites that are designed for highly organized formation flights. Moreover, further miniaturization is required to fill increasing demands for accuracy of attitude control in constellations. This is why a new generation of flat (material form-factor) printed arc thrusters has appeared.

Printable cathode arc thrusters are pulsed-type devices that utilize pulsed or DC discharge between the two electrodes, and the erosion of an electrode (usually a cathode) supplies the mass to be ionized, accelerated, and expelled to create a reactive thrust. In contrast to conventional pulsed thrusters, printed thrusters have a flat geometry of two or more concentric electrodes, printed by metallic ink on a dielectric wafer, usually a thin flexible polymer film. These thrusters are extremely small (typically several millimeters in diameter and fractions of a millimeter in thickness), very cheap, and simple. More importantly, they are capable of producing extremely small thrust impulses for the extra-precise attitude control of smallest, but still active satellites (for one, picosatellites weighing less than one kilogram.)

Further progress in printable thrusters would center on optimization of thruster geometry to enhance efficiency of the material ionization and acceleration in still under-explored flat discharges. Another area of focus is on discovering novel materials capable of efficiently supplying an easily ionizable propellant to the acceleration zone. These materials need to be able to withstand harsh open-space conditions, such as temperature cycles and ionized radiation, without disruption of its thin structure and exfoliation. The group led by Prof. M. Kim from the University of Southampton, UK, had recently produced printed cathode arc thruster with a size of about five centimeters.

Electrodeless systems with rotating and complex-shape magnetic fields are a very promising type of electric propulsion systems that does not involve current exchange between plasma and conductive electrodes, thus eliminating power and material losses associated with material heating, erosion, and metal-vacuum transitions. Being potentially highly efficient, these systems are only now at the initial stages of miniaturization, where exceptionally complex physics of the involved processes is the principal obstacle. However, several teams have achieved significant progress working on helicon thrusters, such as the group led by Prof. S. Shinohara at the Tokyo University of Agriculture and Technology (see the article in Physics of Plasmas). There are also Rotamak-type systems designed by SPCS, Singapore (see the Journal of Visualized Experiments) and radiofrequency electro-thermal thruster (the “Pocket Rocket” designed by the Space Plasma Power and Propulsion Group at the Australian National University, see the article in Plasma Sources Science and Technology journal.) Right now, it is difficult to say if these systems will scale down to a cubesat scale, or whether they will only occupy the niche of medium- and high-power thrusters.

Electrospray thrusters occupy a special niche between the flat printed systems that produce ultra-low thrust pulses and micro-pulsed thrusters. The thrust is produced by applying an electrostatic voltage to electrospray emitters that accelerate the liquid propellant exiting from a small diameter capillary. This is a relatively mature technology that is intrinsically miniature, and further scaling down could involve complex nanostructures to further miniaturize the needles. Porous tungsten emitters are used on the present-day electrospray systems, but silicon technology is still the promising approach for the micro-fabricated emitter tips.

This type is highly promising for the cubesat applications due to possible multiplexing of electrospray microthrusters. Further progress in the electrospray thrusters is anticipated by involving complex metamaterials and optimization of their geometry. Thrusters of this type were used in the Laser Interferometer Space Antenna (LISA Pathfinder) project.

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