The primary function of the space propulsion system is to provide thrust, which helps in the functioning of the launch vehicle or satellite. In propulsion systems, the fluid (either solid, liquid, or electric) reacts to initiate acceleration and provide force in the system.
The use of electric propulsion (EP) for space applications is currently undergoing rapid expansion. There are hundreds of operational spacecraft employing EP technologies with industry projections showing that nearly half of all commercial launches in the next decade will have a form of electric propulsion.
An ion thruster is a form of electric propulsion used for spacecraft propulsion. It creates thrust by accelerating ions with electricity. As the ionised particles escape from the aircraft, they generate a force moving in the other direction. Power supplies for ion thrusters are usually electric solar panels, but at sufficiently large distances from the sun, nuclear power is used.
Ion thrusters are being designed for a wide variety of missions—from keeping communications satellites in the proper position (station-keeping) to propelling spacecraft throughout our solar system. “Ion propulsion is even considered to be mission-enabling for some cases where sufficient chemical propellant cannot be carried on the spacecraft to accomplish the desired mission,” says NASA. The technology could be used to power a return trip to Mars without refuelling, and use recycled space junk for the fuel.
Ion thrusters are used in the European Space Agency’s (ESA) mission to Mercury. BepiColombo is Europe’s first mission to Mercury. Launched on 20 October 2018, it is on a seven year journey to the smallest and least explored terrestrial planet in our Solar System. When it arrives at Mercury in late 2025, it will endure temperatures in excess of 350 °C and gather data during its one-year nominal mission, with a possible one-year extension.
Ion Propulsion Vs Chemical propulsion
As NASA explain: “An ion thruster ionizes propellant by adding or removing electrons to produce ions. Most thrusters ionize propellant by electron bombardment: a high-energy electron (negative charge) collides with a propellant atom (neutral charge), releasing electrons from the propellant atom and resulting in a positively charged ion. ” The gas produced consists of positive ions and negative electrons in proportions that result in no over-all electric charge. This is called a plasma. Plasma has some of the properties of a gas, but it is affected by electric and magnetic fields. Common examples are lightning and the substance inside fluorescent light bulbs. Ion thrusters have an input power need of 1–7 kW, exhaust velocity 20–50 km/s, thrust 25–250 millinewtons and efficiency 65–80%.
These thrusters have high specific impulses—ratio of thrust to the rate of propellant consumption, so they require significantly less propellant for a given mission than would be needed with chemical propulsion,” says NASA. These can be more than 10 times as fuel efficient as other rocket engines. Another attraction of using this kind of thruster is that it does not need the kind of high temperatures required by forms of chemical propulsion.
This kind of electric propulsion system is also lighter in weight, meaning that future space trips could be more feasible. A xenon based EPS can be five to six times more efficient than chemical-based propulsion on spacecraft and has many uses, according to Dr Annadurai, whose centre assembles all Indian spacecraft. A 3,500-kg EPS-based satellite, for example, can do the work of a conventional spacecraft weighing 5,000 kg, but cost far less.
The advantages include : Highest specific impulse offers substantial mass saving (>3000s); High performance at low complexity; Reduced power processing unit mass; Narrow beam divergence; Robust design concept with a large domain of operational stability; Large throttle range and adaptable to available electric power; Excellent thrust stability and fast thrust response and Highest growth potential with increasing electric power in near and medium-term future
However Ion thrust engines create small thrust levels (the thrust of Deep Space 1 is approximately equal to the weight of one sheet of paper ) compared to conventional chemical rockets. They are practical only in the vacuum of space and cannot take vehicles through the atmosphere because ion engines do not work in the presence of ions outside the engine. Besides, the engine’s minuscule thrust would not matter when air resistance comes into play.
Michael Patterson, senior technologist for NASA’s In-Space Propulsion Technologies Program compared ion and chemical propulsion with “Tortoise and the Hare”. “The hare is a chemical propulsion system and a mission where you might fire the main engine for 30 minutes or an hour and then for most of the mission you coast.” “With electric propulsion, it’s like the tortoise, in that you go very slow in the initial spacecraft velocity but you continuously thrust over a very long duration — many thousands of hours — and then the spacecraft ends up picking up a very large delta to velocity.”
Ion Thruster Technology
As NASA explain: “An ion thruster ionizes propellant by adding or removing electrons to produce ions. Most thrusters ionize propellant by electron bombardment: a high-energy electron (negative charge) collides with a propellant atom (neutral charge), releasing electrons from the propellant atom and resulting in a positively charged ion. ” The gas produced consists of positive ions and negative electrons in proportions that result in no over-all electric charge. This is called a plasma. Plasma has some of the properties of a gas, but it is affected by electric and magnetic fields. Common examples are lightning and the substance inside fluorescent light bulbs.
Ion thrusters are categorized by how they accelerate the ions, using either electrostatic or electromagnetic force. Electrostatic thrusters use the Coulomb force and accelerate the ions in the direction of the electric field. Electromagnetic thrusters use the Lorentz force. The most common propellant used in ion propulsion is xenon, which is easily ionized and has a high atomic mass, thus generating a desirable level of thrust when ions are accelerated. It also is inert and has a high storage density; therefore, it is well suited for storing on spacecraft. In most ion thrusters, electrons are generated with the discharge hollow cathode by a process called thermionic emission.
Electrons produced by the discharge cathode are attracted to the discharge chamber walls, which are charged to a high positive potential by the voltage applied by the thruster’s discharge power supply. Neutral propellant is injected into the discharge chamber, where the electrons bombard the propellant to produce positively charged ions and release more electrons. High-strength magnets prevent electrons from freely reaching the discharge channel walls. This lengthens the time that electrons reside in the discharge chamber and increases the probability of an ionizing event.
The positively charged ions migrate toward grids that contain thousands of very precisely aligned holes (apertures) at the aft end of the ion thruster. The first grid is the positively charged electrode (screen grid). A very high positive voltage is applied to the screen grid, but it is configured to force the discharge plasma to reside at a high voltage. As ions pass between the grids, they are accelerated toward a negatively charged electrode (the accelerator grid) to very high speeds (up to 90,000 mph).
“The positively charged ions are accelerated out of the thruster as an ion beam, which produces thrust. The neutralizer, another hollow cathode, expels an equal amount of electrons to make the total charge of the exhaust beam neutral. Without a neutralizer, the spacecraft would build up a negative charge and eventually ions would be drawn back to the spacecraft, reducing thrust and causing spacecraft erosion.”
The primary parts of an ion propulsion system are the ion thruster, power processing unit (PPU), propellant management system (PMS), and digital control and interface unit (DCIU). The PPU converts the electrical power from a power source—usually solar cells or a nuclear heat source—into the voltages needed for the hollow cathodes to operate, to bias the grids,and to provide the currents needed to produce the ion beam. The PMS may be divided into a high-pressure assembly (HPA) that reduces the xenon pressure from the higher storage pressures in the tank to a level that is then metered with accuracy for the ion thruster components by a low-pressure assembly (LPA). The DCIU controls and monitors system performance,and performs communication functions with the spacecraft computer.
The future of electric propulsion is mainly pushing in two directions: increasing the specific impulse and longevity of high-power technologies and improving the efficiency and reliability of low-power technologies. In the former, thrusters with longer lifespans and greater “fuel economy” will enable new deep space science missions, and thus are of primary interest to civilian institutions. In the latter, dropping launch vehicle costs has ignited interest in small-scale satellites and constellations for both commercial and scientific near-Earth applications. Naturally, a low-power electric propulsion solution is sought for this new wave of satellites.
Moderate power level Gridded ion thrusters
Gridded ion thrusters at 1–20 kW were developed heavily and flown extensively by the U.S. during the Twentieth Century; for example, the Xenon Ion Propulsion System family of GITs developed by (first Hughes and then) Boeing were flown extensively in the 1990s. Similarly, they have been flown in deep space missions like Dawn and (at slightly lower power levels) Hayabusa and, currently, the BepiColombo mission to Mercury. Further, they have undergone extensive wear testing in ground facilities, which has led to improvements in the understanding of the erosion of these thrusters.
The challenges facing GITs at moderate power levels are now largely related to incremental improvements in the testing and implementation of these devices. For instance, facility effects are known to influence GITs as they do Hall thrusters. For example, numerical modeling accompanying life tests of the NASA Evolutionary Xenon Thruster (NEXT) indicated that accelerator grid groove erosion could be reduced by 30% due to redeposition of sputtered beam dump material; this effect, if unaccounted for, could spuriously increase estimates of thruster longevity and therefore pose a significant risk for flight operation. Assessing the lifetime of moderately-powered GITs is another lingering obstacle. Although simulations can effectively reproduce erosion patterns in these devices, the causes of cathode erosion and failure are manifold and still under study, and the presence of long-duration failure mechanisms are difficult to explore without strenuous ground testing.
High powered gridded ion thrusters
The low thrust produced by modern electric propulsion devices has created a large divide between the applications for which they are suited and those where chemical propulsion is more fitting. For example, crewed missions prioritize short travel times, and thus, the large thrust produced by chemical systems is needed even at the expense of payload mass. However, with more power, EP systems may produce enough thrust to break into this niche, while retaining the propellant efficiency characteristic of this class of propulsion. As a result, there is continuing research into scaling EP devices to higher power and exploring new concepts that may excel at 20 kW.
Unlike many forms of electric propulsion, high-power gridded ion thrusters were explored several decades ago, including a 130 kW mercury thruster, and thus, they have long since climbed their s-curve.
Bloomberg Businessweek reports on Apollo Fusion, a new company designing a propulsion system for rocket engines that would use mercury as a fuel. Mercury has promise in this field, sure. But launching any rocket using this system would entail the risk of spreading a toxic substance through the atmosphere. The idea of using mercury as a spacecraft fuel is not exactly new. NASA experimented with mercury in the ’60s, during the SERT missions. The two spacecraft in this series, SERT-I and SERT-II, were designed to test the concept of ion propulsion. Mercury is much heavier than either xenon or krypton, so spacecraft carrying them would be able to generate more thrust. Of course, mercury is also a dangerous neurotoxin, so NASA stopped using it after SERT. Today’s ion engines commonly use krypton or xenon. Apollo Fusion is planning to bring mercury back, at least according to a collection of industry insiders talking to Bloomberg. If they’re successful, they could provide low-cost, high-power ion engines for satellites and spacecraft. But if they’re not, they could risk showering the atmosphere with toxic mercury.
There are several reasons for this relatively slow development of higher power GITs. GIT beam current is ultimately space-charge-limited, and thus, high-power thrusters must be physically larger and are therefore more challenging to test and fly. For example, the 130 kW
mercury GIT tested at Glenn Research Center in 1967 was a cumbersome 1.5 m in diameter, much larger than present-day flight EP systems. Second, the successful ground testing of such large GITs in the past was in part accomplished by using easily pumped propellants like mercury and cesium; however, such toxic and reactive fuels are mostly avoided in modern EP systems for both concerns of health and spacecraft interaction. As with other high-power EP technologies, then, the high gas throughput of these GITs easily overwhelms most ground vacuum facilities.
Sub-Kilowatt Gridded Ion Thrusters
There have been steady development of low-power devices like the Radio-frequency Ion Thruster (RIT) series originally developed by the University of Giessen and JAXA’s Kiku ion thrusters led to the early inflection of the s-curve. Since then, there has been steady development of low-power GITs, like the 350 W microwave discharge gridded ion engines on JAXA’s Hayabusa mission, which in total accumulated over 25,000 h of operation in flight. More recent very low-power GITs to be developed are the Busek BIT series, Astrium’s µNRIT, and UCLA’s Miniature Xenon Ion (MiXI) thruster.
Despite the recent development of these new systems, the majority of very low-power GITs remain less mature than higher power versions due to several major obstacles. Foremost is that ion thrusters require extensive power electronics to support the numerous electrodes involved in these designs. Naturally, then, significant development of more compact and efficient power processing systems is required to make sub-kW GITs practical. Second, the current that can be extracted from a gridded ion thruster is fundamentally space-charge-limited, such that for a given grid design, the beam current scales with the grid area of the device. Unlike in other technologies such as Hall thrusters where the current density can theoretically be increased for smaller thrusters to compensate for a lesser volume of plasma, this space-charge limitation constrains sub-kW GITs to low beam currents.
Australian student smashes NASA’s fuel efficiency record
University of Sydney doctoral candidate in Physics, Paddy Neumann, has developed a “new kind of ion space drive” that outperforms NASA’s in fuel efficiency and that can use a variety of metals, even those found in space junk, according to student newspaper Honi Soit.
NASA’s current record holder for fuel efficiency is its High Power Electric Propulsion, or HiPEP, system, which allows 9,600 (+/- 200) seconds of specific impulse. However, the new drive developed by Paddy Neumann, has achieved up to 14,690 (+/- 2,000), according to student newspaper Honi Soit.
“NASA’s HiPEP runs on Xenon gas, while the Neumann Drive can be powered on a number of different metals, the most efficient tested so far being magnesium,” the paper explains. “As it runs on metals commonly found in space junk, it could potentially be fuelled by recycling exhausted satellites, repurposing them into fresh fuel.”
China Ion thrusters
China’s upcoming Tiangong space station’s first module will be equipped with an ion propulsion system which will greatly improve energy efficiency and could slash journey times to Mars, the South China Morning Post (SCMP) reports. The space station’s core Tianhe module, is propelled by four ion thrusters. According to the Chinese Academy of Sciences, the ISS’s thrusters require four tons of rocket fuel to keep it afloat for a year, whereas ion thrusters would require only 882 pounds (400kg) to do the same.
These charged particles can degrade engine components, reducing satellite longevity and possibly putting astronauts at risk. Moreover, the thrust is usually fairly low. However, the Chinese Academy of Sciences says they found a way to make it work. The Chinese scientists put the thrusters through rigorous testing to make sure the engines could resist the damage caused by the particles. By putting a magnetic field over the engine’s inner wall to repel damaging particles, they were able to protect the engine from erosion. They also developed a unique ceramic material designed to withstand severe heat or radiation for an extended period of time.m adoption has been hampered by the fact that the thrust produced isn’t very significant. Their ion thruster has reportedly run non-stop for more than 11 months without a hitch.
In 2016, it was reported that China has finished building the world’s most powerful ion thruster and will soon use it to improve the mobility and lifespan of its space assets, according to a state media report. Researchers at the 502 research institute, which operates under the China Aerospace Science and Technology Corp. in Beijing, have delivered a new-generation Hall-effect thruster unit to Chinese customers in the space industry, the report by the Science and Technology Daily stated.
The machine will outperform all of the ion thrusters used on satellites or spacecraft that are currently in use, it added. The daily is run by the Ministry of Science and Technology. The most powerful ones in operation today can accelerate to 30 kilometres per second at their maximum thrust. But Mao Wei, chief designer of China’s Hall thruster, told the daily that the latest version will beat the current performance record of this kind of thruster by as much as 30 per cent. Gao Jun, another researcher involved in the project, said other countries were busy developing similar ion thrusters but that none had completed ground testing yet. As such, China should become “the first [country] to test the new technology on a high-altitude satellite,” he was quoted as saying by the newspaper.
Field-emission electric propulsion (FEEP)
Field-emission electric propulsion (FEEP) is an advanced electrostatic space propulsion concept, a form of ion thruster, that uses liquid metal (usually either caesium, indium or mercury) as a propellant. A FEEP device consists of an emitter and an accelerator electrode. A potential difference of the order of 10 kV is applied between the two, which generates a strong electric field at the tip of the metal surface.
The interplay of electric force and surface tension generates surface instabilities which give rise to Taylor cones on the liquid surface. At sufficiently high values of the applied field, ions are extracted from the cone tip by field evaporation or similar mechanisms, which then are accelerated to high velocities (typically 100 km/s or more).
A separate electron source is required to keep the spacecraft electrically neutral. Due to its very low thrust (in the micronewton to millinewton range), FEEP thrusters are primarily used for microradian, micronewton attitude control on spacecraft, such as in the ESA/NASA LISA Pathfinder scientific spacecraft.
Austrian startup ramping to mass produce tricky electric propulsion thrusters
Enpulsion is commercializing a Field Emission Electric Propulsion, or FEEP, thruster starting with small satellites ranging from 3 to 100 kilograms, Sypniewski said. ESA and industry have studied FEEP systems for well over a decade, but with limited success getting the technology beyond the laboratory.
The lure of FEEP thrusters is their ability to enable extremely precise movements or station-keeping while in space. ESA intended to use FEEP thrusters from Austria’s Fotec for the Lisa Pathfinder science mission, but production complications contributed meaningfully to the mission’s delays and ESA ultimately replaced the thrusters with more mature cold gas thrusters.
Enpulsion spun out of Fotec, a research division of the University of Applied Sciences Wiener Neustadt in Austria, to commercialize a breakthrough involving the use of a “porous tungsten crown emitter,” which Sypniewski said “provides a stable and repeatable technology that can be produced on a mass-production scale.”
“We have an enormous interest from worldwide small satellite manufacturers in our product,” Alexander Reissner, Enpulsion’s founder and CEO, said in a statement. “The key to this success is the concept of clustering pre-qualified building blocks, which is made possible by our proprietary Indium-FEEP technology. It seems that our offer of providing a custom propulsion solution at a catalog price and with less than two months lead time is really hitting a nerve of the industry.”
Sypniewski said the company plans to produce 100 to 200 thrusters per year, and has 150 pre-orders from customers in Europe and the United States. Among those customers is Iceye, a Finnish synthetic aperture radar startup that is flying a cluster of Enpulsion FEEP thrusters next year.
3D printed ion-emitting satellite propulsion system reported in Jan 2021
Researchers from the Massachusetts Institute of Technology (MIT) have designed and tested a novel 3D printed ion-emitting satellite propulsion system. The tiny thruster, which is believed to be the first fully-printed device of its kind, fires charged ionic particles from emitter cones along its outer shell, to give it a few micronewtons of propulsion. Within the frictionless environment of space, this power could prove sufficient to make it a low-cost and efficient alternative to conventional CubeSat engines.
“If you want to be serious about developing high-performance hardware for space, you really need to look into optimizing the shapes, the materials, everything that composes these systems,” said the project’s lead researcher Luis Fernando Velásquez-García. “3D printing can help with all of these things.”
In their study, the MIT team built two microelectromechanical system (MEMS) designs: one with a binder jetted SS 316L-based emitter array, and another created using an acrylic polymer. The devices themselves each featured a fluidic connector, liquid reservoir and an outer casing, which included an embedded array of external cone-shaped emitters.
During production, the team found that even though both emitters had the same basic design, the polymeric system still required the use of supporting materials. This led to slight differences in the products’ final dimensions, and the metal device’s shorter and sharper tips ultimately allowed it to emit higher thrust levels than the plastic iteration.
Over several hours of testing, both systems proved capable of operating without any performance dips, producing only a thin layer of ‘crust’ that could be easily removed. What’s more, the two engines produced a maximum thrust per emitter of 191.3 nN and 139.9 nN, giving them a higher ‘specific impulse’ than many state-of-the-art devices.
Although the metallic MEMS proved to be more powerful than the polymeric version, the team concluded that the latter could provide greater access to the tech in future. Given the cost advantages of their plastic electrode, the scientists hope that eventually, it provides a basis for a slew of new college-led designs and orbital space missions.
Despite the ongoing commercialization of EP systems, a number of requirements, somewhat similar to those of the established chemical systems, have to be met by these propulsion systems, such as reliability, robustness, electromagnetic compatibility (EMC), radiation hardness, non-hazardous interaction with the satellite, and energy efficiency. In addition, the speed of development, improvement, and adaptation of propulsion systems to the conditions of the specific mission has to increase for reasons of competitiveness. This will only be achieved if suitable test facilities with standardized measurement procedures and validated computer-aided modeling of the engines are both available for developers.
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