Ion thruster; Electric engine technology, design and challenges
An ion thruster, ion drive, or ion engine is a form of electric propulsion used for spacecraft propulsion. It creates thrust by accelerating ions using electricity. An ion thruster ionizes a neutral gas by extracting some electrons out of atoms, creating a cloud of positive ions. Ion thrusters are categorized as either electrostatic or electromagnetic.
Electrostatic thruster ions are accelerated by the Coulomb force along the electric field direction. Temporarily stored electrons are reinjected by a neutralizer in the cloud of ions after it has passed through the electrostatic grid, so the gas becomes neutral again and can freely disperse in space without any further electrical interaction with the thruster.
By contrast, electromagnetic thruster ions are accelerated by the Lorentz force to accelerate all species (free electrons as well as positive and negative ions) in the same direction whatever their electric charge, and are specifically referred to as plasma propulsion engines, where the electric field is not in the direction of the acceleration.
Ion thrusters in operation typically consume 1–7 kW of power, have exhaust velocities around 20–50 km/s (Isp 2000–5000 s), and possess thrusts of 25–250 mN and a propulsive efficiency 65–80% though experimental versions have achieved 100 kW (130 hp), 5 N (1.1 lbf).
The Deep Space 1 spacecraft, powered by an ion thruster, changed velocity by 4.3 km/s (2.7 mi/s) while consuming less than 74 kg (163 lb) of xenon. The Dawn spacecraft broke the record, with a velocity change of 11.5 km/s (7.1 mi/s), though it was only half as efficient, requiring 425 kg (937 lb) of xenon.
Applications include control of the orientation and position of orbiting satellites (some satellites have dozens of low-power ion thrusters), use as a main propulsion engine for low-mass robotic space vehicles (such as Deep Space 1 and Dawn), and serving as propulsion thrusters for crewed spacecraft and space stations (e.g. Tiangong).
Ion thrust engines are generally practical only in the vacuum of space as the engine’s minuscule thrust cannot overcome any significant air resistance without radical design changes, as may be found in the ‘Atmosphere Breathing Electric Propulsion’ concept. MIT has created designs that are able to fly for short distances and at low speeds at ground level, using ultra-light materials and low drag aerofoils. An ion engine cannot usually generate sufficient thrust to achieve initial liftoff from any celestial body with significant surface gravity. For these reasons, spacecraft must rely on other methods such as conventional chemical rockets or non-rocket launch tecyhnologies to reach their initial orbit.
Rocket ion thruster electric technology
The thrusters work by using an electrical charge to accelerate ions from xenon fuel to a speed 7-10 times that of chemical engines. The electrical power level and xenon fuel feed can be adjusted to throttle each engine up or down in thrust. The engines are thrifty with fuel, using only about 3.25 milligrams of xenon per second (about 10 ounces over 24 hours) at maximum thrust. The Dawn spacecraft carried 425 kilograms (937 pounds) of xenon propellant at launch. Xenon was chosen because it is chemically inert, easily stored in a compact form, and the atoms are relatively heavy so they provide a relatively large thrust compared to other candidate propellants. At launch, the gaseous xenon stored in the fuel tank was 1.5 times the density of water. At maximum thrust, each engine produces a total of 91 millinewtons—about the amount of force involved in holding a single piece of notebook paper in your hand.
You would not want to use ion propulsion to get on a freeway — at maximum throttle, it would take Dawn’s system four days to accelerate from 0 to 60 MPH. As slight as that might seem, over the course of the mission the total change in velocity from ion propulsion will be comparable to the push provided by the Delta II rocket that carried it into space — all nine solid-fuel boosters, plus the Delta’s first, second and third stages. This is because the ion propulsion system will operate for thousands of days, instead of the minutes during which the Delta performs.
Solar Power
The electrical power system provides power for all onboard systems, including the ion propulsion system when thrusting. Each of the two solar arrays is 27 feet (8.3 meters) long by 7.4 feet (2.3 meters) wide. On the front side, 18 square meters (21.5 square yards) of each array is covered with 5,740 individual photovoltaic cells. The cells can convert about 28 percent of the solar energy that hits them into electricity. On Earth, the two wings combined could generate over 10,000 watts. The arrays are mounted on opposite sides of the spacecraft, with a gimbaled connection that allows them to be turned at any angle to face the sun.
Designs of ion thruster
Gridded ion thruster
The gridded ion thruster is a common design for ion thrusters, a highly efficient low-thrust spacecraft propulsion method running on electrical power by using high-voltage grid electrodes to accelerate ions with electrostatic forces.
Gridded ion thrusters (GITs) belong to the category of electrostatic ion engines and are composed of three main components: hollow cathodes, grids, and a discharge chamber in which plasma is generated. In an ion thruster, a propellant is injected from the propellant tank into an anode potential discharge chamber.
Propellant atoms are injected into the discharge chamber and are ionized, forming a plasma.
There are several ways of producing the electrostatic ions for the discharge chamber:
- electron bombardment (Kaufman type) by a potential difference between a hollow cathode and anode (NSTAR, NEXT, T5, T6 thrusters)
- radio frequency(RF) oscillation of an electric field induced by an alternating electromagnet, which results in a self-sustaining discharge and omits any cathode (RIT 10, RIT 22, µN-RIT thrusters)
- microwave heating (µ10, µ20)
Related to the electrostatic ion production method is the need for a cathode and power supply requirements. Electron bombardment thrusters require at the least, power supplies to the cathode, anode and chamber. RF and microwave types require an additional power supply to the rf generator, but no anode or cathode power supplies.
The positively charged ions diffuse towards the chamber’s extraction system (2 or 3 multi-aperture grids). After ions enter the plasma sheath at a grid hole, they are accelerated by the potential difference between the first and second grids (called the screen and accelerator grids, respectively). The ions are guided through the extraction holes by the powerful electric field. The final ion energy is determined by the potential of the plasma, which generally is slightly greater than the screen grids’ voltage.
The negative voltage of the accelerator grid prevents electrons of the beam plasma outside the thruster from streaming back to the discharge plasma. This can fail due to insufficient negative potential in the grid, which is a common ending for ion thrusters’ operational life. The expelled ions propel the spacecraft in the opposite direction, according to Newton’s 3rd law. Lower-energy electrons are emitted from a separate cathode, called the neutralizer, into the ion beam to ensure that equal amounts of positive and negative charge are ejected. Neutralizing is needed to prevent the spacecraft from gaining a net negative charge, which would attract ions back toward the spacecraft and cancel the thrust.
Challenges of ion thruster
One of the main drawbacks of ion propulsion is that it requires a high-power supply, such as a nuclear power source, to generate the high-energy ions needed for propulsion. Additionally, ion propulsion systems are relatively heavy and bulky, which can limit their use on smaller spacecraft or satellites. Another drawback is that ion propulsion requires a relatively long time to accelerate to high speeds, which can make it less suitable for some types of missions that require rapid acceleration or high speeds. Finally, the thrust generated by ion propulsion is low, which can make it less suitable for some types of missions that require high thrust or rapid maneuvering.
