How Ion Thrusters can Get us to Mars cheaper
The NASA Glenn Research Center has been a leader in ion propulsion technology development since the late 1950s, with its first test in space, the Space Electric Rocket Test 1, flying on July 20, 1964. From 1998 to 2001, the NASA Solar Technology Application Readiness (NSTAR) ion propulsion system enabled the Deep Space 1 mission, the first spacecraft propelled primarily by ion propulsion, to travel over 163 million miles and make flybys of the asteroid Braille and the comet Borelli.
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. These thrusters have high specific impulses (the 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. 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.
An ion thruster ionizes the 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. This is why the outer ring of the ion thrusters are made up of magnets.
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 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. The 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.
Although I have mentioned it, maybe some might be thinking why a xenon atom? To answer this question let me give an overview of xenon. Xenon is an inert gas or a noble gas. The specialty of these is that they do not undergo chemical reactions under a given set of conditions. Because of this, you don't need to worry about noble gasses reacting with sensors, electronics, etc. Noble gases have there outer shell full meaning that they are stable. And due to this, they don't react with any element. There are 6 Noble gases which are :
He(Helium)
Ne(Neon)
Ar(Argon)
Kr(Krypton)
Xe(Xenon)
Rn(Radon)
Now Xenon is not a metal at room temperature and it is also a gas at room temperature which makes it easy to handle. However, there are other noble gases like Krypton, Radon, Argon, etc. The first step of an ion thruster is actually knocking the electron out. The ionization energy is one of the most important. The bigger the atom, the less ionization energy is required. Xenon is the heaviest non-radioactive elemental inert gas. The added mass allows for denser packing at less pressure. The mass is one of the limiting factors, so having a more dense gas helps tremendously. But xenon has a 131.293 u atomic mass. If you still did not understand why mass is so important, let me explain.
Essentially, a heavier mass allows for more momentum to come from the overall system. The mass will take longer to accelerate, allowing more momentum to be exerted on the particle. There are other elements that are heavier then xenon, but they are radioactive. The reason we don't use radioactive metals is that radioactivity could cause all kinds of issues, as could something that would be reactive. Elemental is easier because it’s easier to manipulate, and as you have to make the gas ionic if it’s not elemental it will have a much higher potential to react with something. Thus, it is more efficient to use a heavy elemental non-radioactive inert gas like xenon.
Scientists are experimenting with other metals, so far, xenon is the only
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 ion propulsion system’s efficient use of fuel and electrical power enables modern spacecraft to travel farther, faster, and cheaper than any other propulsion technology currently available. Chemical rockets have demonstrated fuel efficiencies up to 35 percent, but ion thrusters have demonstrated fuel efficiencies over 90 percent. Currently, ion thrusters are used to keep communication satellites in the proper position relative to Earth and for the main propulsion on deep space probes. Several thrusters can be used on a spacecraft, but they are often used just one at a time. Spacecraft powered by these thrusters can reach speeds up to 90,000 meters per second (over 200,000 mph). In comparison, the Space Shuttles can reach speeds around 18,000 mph.
The trade-off for the high top speeds of ion thrusters is low thrust (or low acceleration). Current ion thrusters can provide only 0.5 newtons (or 0.1 pounds) of thrust, which is equivalent to the force you would feel by holding 10 U.S. quarters in your hand. They cannot be used to put spacecraft in space because large amounts of the thrust are needed to escape Earth’s gravity and atmosphere. But they can be operated at maximum efficiency in a vacuum.
To compensate for low thrust, an ion thruster must be operated for a long time for the spacecraft to reach its top speed. Acceleration continues throughout the flight, however, so tiny, constant amounts of thrust over a long time add up to much shorter travel times and much less fuel used if the destination is far away. Deep Space 1 used less than 159 pounds of fuel in over 16,000 hours of thrusting. Since much less fuel must be carried into space, smaller, lower-cost launch vehicles can be used, which decreases the price. Due to there efficiency and price, we can get to mars for cheaper.
Ion thrusters (based on a NASA design) are now being used to keep over 100 geosynchronous Earth orbit communication satellites in their desired locations, and three NSTAR ion thrusters that utilize Glenn-developed technology are enabling the Dawn spacecraft (launched in 2007) to travel deep into our solar system. Dawn is the first spacecraft to orbit two objects in the asteroid belt between Mars and Jupiter: the protoplanets Vesta and Ceres.
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