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Electric rockets get a boost.

Xenon-ion thrusters are expected to replace conventional chemical rockets in many nonlaunch propulsion tasks, such as controlling satellite orbits and sending space probes on long exploratory missions.

The space age dawned some four decades ago with the arrival of powerful chemical rockets that could propel vehicles fast enough to escape the grasp of earth's gravity. Today, chemical rocket engines still provide the only means to boost payloads into orbit and beyond. The less glamorous but equally important job of moving vessels around in space, however, may soon be assumed by a fundamentally different rocket engine technology that has been long in development--electric propulsion.

Rather than generating thrust from chemical reactions between fuels and oxidizers, electric rocket engine technology (specifically, xenon-ion propulsion) uses solar-powered electrode grids to create electrostatic fields that accelerate charged particles (xenon ions) to high exhaust velocities. Although solar-electric propulsion produces much lower thrust levels than traditional chemical rocket systems, it is more efficient and therefore requires substantially less propellant volume. This characteristic makes electric rocket engines desirable for many tasks other than launching, such as maintaining the position of satellites in orbit and propelling scientific probes out to study the solar system.

One of the first commercial spacecraft to use electrostation propulsion is scheduled to be launched in May 1997. The Galaxy 8-I communications satellite will sustain its precision geosynchronous orbit for more than a decade using xenon-ion electric thrusters. The new comsat, built by Hughes Space and Communications Co. in Los Angeles to provide direct-to-home broadcast capability throughout Latin America, swill be fitted with four small 500-watt ion engines.

The basic ion system consists of a xenon propellant source, an electrical power processor, and a cylindrically shaped thruster assembly 8.5 inches in diameter and 9 inches long. Each electric motor, which looks like a coffee can with a dish shaped screen on one end, generates a 13-centimeter-diameter ion beam with a thrust of 18 millinewtons (mere thousandths of a pound).

The miniscule thrust level is, however, sufficient for the important job of north-south stationkeeping (NSSK) in geosynchronous orbit, said John Beattie, manager of the plasma source research department at Hughes Research Laboratories in Malibu, Calif. NSSK is the process of maintaining a satellite's orbit in Earth's equatorial plane so it is always in a fixed location relative to a ground observer. "Orbital perturbations caused by the sun's and moon's gravitational fields tend to incline the satellite's orbit relative to the equator," Beattie said. "Twice per day at the equatorial crossing points, the electric thrusters will burn for two and a half hours, first in the north direction, then in the south." NSSK corresponds to a velocity change of 40 to 50 meters per second per year.

A satellite using traditional chemical thrusters for NSSK typically starts out with a quarter of its total mass allocated to hydrazine propellant. The use of ion thrusters saves hundreds of kilograms in launch weight.

In 1998, a more powerful Hughes xenon-ion rocket system will send a robotic survey vessel on a challenging rendezvous or flyby mission to an asteroid and a comet. It will be the first time a deep space probe will use electric rockets for primary propulsion. Depending on the launch date, the flight duration would be 12 to 18 months.

The mission will be the first of the New Millenium program, which was established by the National Aeronautics and Space Administration as a way to test available commercial technologies in the rigors of space. Successful technologies will be used in the development of more cost-effective scientific spacecraft. Ion drive "is the first thing that planetary scientists want," said Rex Ridenoure, a program architect for the New Millenium. "It gives them fast and flexible access to space." In the neighborhood of the Earth, an ion-drive-powered spacecraft could potentially achieve speeds of several hundred kilometers per second, fast enough to reach Mars, Venus, or Mercury in only a few weeks. Ion motors bring flexibility because they can enable a spacecraft to change trajectories.

"This idea has been around for decades, and the dramatic benefits of ion propulsion for a wide variety of deep space missions are well known," said Kane Casani, New Millenium program manager. "But NASA science mission managers have never felt that the technology was mature enough to be used for the first time on their missions. With important contributions from other technology development programs, New Millenium will take on this challenge and bring full-scale solar-electric propulsion out of the lab and into space once and for all."

Astronautical engineers say that these two missions are just the beginning of what they expect to be a tremendous increase in the use of electric rockets during the next century.

Electric vs. Chemical Thrust

Because of dissimilarities in the nature of the rocket thrust they produce, electric motors and their chemical counterparts accomplish their propulsive tasks quite differently. A chemical rocket burns strongly for a short time and then coasts for most of the remainder of the journey. Electric rockets, on the other hand, generate low thrust levels for extended periods--months, perhaps years, on end. A spacecraft with electric motors can attain the same (or greater) final velocity as a chemical rocket-powered vehicle because it can accelerate much longer. The result can be significant time savings for planetary missions that come from avoiding gravity-assist maneuvers that boost velocity.

Although the total propulsive impulse created by each engine type can be equivalent, chemical rockets drive large amounts of propellant mass out the exhaust nozzle at moderately high velocities, while electric systems impel small amounts of mass out at very high speeds. Electric rockets need much less propellant at launch than chemical systems and can therefore offer spacecraft designers significant savings in propellant mass over their chemical cousins.

"Chemical rockets are mostly fuel," said John Brophy, supervisor of the Advanced Propulsion Technology Group at the Jet Propulsion Laboratory (JPL) in Pasadena, Calif. "Since the energy in a chemical rocket is locked up in the propellant and oxidizer, there's only so much to be had. Electric propulsion decouples the energy source (in this case, photovoltaic arrays) from the mass you push out the back."

Engineers compare propulsion system performance with a useful parameter called specific impulse, a term defined as the ratio of rocket thrust to the mass flow rate of propellant. "Specific impulse is a figure of merit (measured in seconds) that describes propellant utilization efficiency," said Frank Curran, chief of the on-board propulsion branch at the NASA Lewis Research Center in Cleveland. "It's something like the gas mileage of a car."

High specific impulse means a lower propellant-mass flow rate to produce a given thrust and less propellant consumption to produce a given total impulse. High-specific-impulse solar-electric propulsion (SEP) systems need much less propellant than chemical systems to accomplish the same task.

For example, NASA's Solar Electric Propulsion Technology Application Readiness or NSTAR engine--the New Millenium power plant--has a specific impulse that varies from 1700 to 3300 see, depending on available power. The best chemical (space-storable) engine has a specific impulse of about 325 sec.

For the deep-space "planetary" missions planned for the near future, a specific impulse of 2500 to 3000 sec is needed, Brophy said. "Ion drive is pretty much the only engine technology that is efficient enough to do the job. We've already done the easy planetary missions [to large bodies with strong gravitational pull]; those requiring a change in velocity ([delta]V) less than 1 km/see," he said. "Today, we're attempting more difficult missions like rendezvousing with fast-moving, small-body (hence low-gravity) targets such as comets or asteroids, or going a long way quickly with few or no gravity assists." This kind of journey requires a [delta]V of from 5 to 10 km/see, Brophy said.

Faster, Cheaper, Better

Beyond the technical issue of propulsive efficiency, propellant utilization is a matter concerning size and money, NASA's Curran said. The initial mass of a spacecraft's on-board propulsion system is a major design driver that affects both mission cost and performance. Propulsion system mass-fraction can range from 20 to 40 percent for satellites in low- and mid-earth orbit (LEO/MEO) to more than 50 or 60 percent for geosynchronous (GEO) communications satellites and planetary exploration spacecraft. (LEO missions include big communications satellite constellations like the proposed Iridium system.)

Pressed by budgetary constraints and a general goal of "faster, cheaper, better," the trend in spacecraft design has moved toward more compact, though more capable, systems that can be launched with smaller boosters, Curran said. "Improving the mass efficiency of the propulsion system gets you fractionally more than improving any other subsystem. Lightweight electric propulsion systems give you more leverage in the design."

According to Beattie, the Hughes xenon-ion propulsion system (XIPS) provides nearly 10 times greater exhaust velocity than current bipropellant (monomethyl hydrazine /nitrogen tetroxide) chemical thrusters, which, he said, essentially allows Hughes' designers to trade 10 pounds of chemical fuel for 1 lb of the xenon propellant. "This translates into a savings of 300 to 400 kilograms on a three-axis stabilized bus for a GEO satellite," he said. "It costs $30,000 to get a kilogram of payload to geosynchronous orbit, so the switch to XIPS means dramatic launch-cost savings of from $9 to $12 million."

Because of market competition, Curran said, virtually all makers of GEO communications satellites offer or are considering offering electric propulsion. :'An executive from one of these companies said that they're afraid to use electric motors, but they're more afraid not to."

Ion propulsion will not only enable reductions in I launch vehicle-class size, it is also highly suitable for advanced earth-orbital missions with high [delta]V requirements such as repositioning large military reconnaissance I or communications satellites. Repositioning missions, I Curran explained, involve raising a satellite's orbit, letting Earth "catch up" in terms of relative drift, and then dropping down in the right place.

Chemical rocket systems, it should be noted, will not become obsolete. They will be used where relatively high-thrust maneuvers are required or where limited power availability precludes the use of electric propulsion, such as at distances greater than 2.5 to 3 astronomical units (AUs or sun-to-Earth distances) from the sun. As the energy for photovoltaic arrays decreases with increasing distance from the sun, an electric engine would have to throttle down correspondingly.

Electric Rocket History

The idea of electric rockets was conceived early this century by two famed astronautics pioneers, Russian space theoretician Konstantin Tsiolkovsky and American rocket engineer Robert Goddard. It has intrigued prospective spacecraft designers as well as science fiction writers ever since.

Laboratory work on ion-drive systems started in 1957 and soon bore fruit, but, Curran recalls, "for a long time afterwards it remained `the technology of the future,' and seemed like it was always going to stay that way." The slow introduction was due in large part to a lack of compelling economic or performance incentives in the space industry of that era. At the time, the space community developed the mission and designed the spacecraft; then a suitable launch vehicle was found. The cost of the booster was a secondary issue.

Nevertheless, research on the novel propulsive scheme continued in the U.S. at JPL, at NASA Lewis, and at TRW and Olin Aerospace. Meanwhile, engineers in the Soviet Union, Europe, Japan, and China also pursued electric motor research. From 1962 through 1985, some 77 flight tests of different kinds of electric propulsion units were conducted.

Though there was an explosion in the capability of electric propulsion early on, with systems demonstrating good performance at powers ranging from 7 watts to 130 kilowatts (kW), there were technical difficulties, Brophy said. "For a long time, the power for ion motors wasn't really there, because the amount of power available in space never met the expectations of the 1960s. Back then, it was projected that planetary spacecraft would have tens of kilowatts of power (from large solar arrays or nuclear reactors). Well, none of that materialized." The reality was a few hundred watts, he said. "Nowadays, power is still expensive, but having kilowatts in space is no longer unusual." Current commercial communications satellites have 5 to 7 kW, and the next-generation Hughes HS-702 satellite bus will have up to 15 kW of power.

Another problem hinders the use of electric engines for primary propulsion, Brophy added. "No one has yet demonstrated the full total impulse capability plus some safety margin that would make it easier for someone to use it on a real mission. It's not for lack of trying, though." In the 1970s, he said, several projects using electric propulsion were started, including a Halley's Comet Rendezvous probe and a substage mercury-ion orbital booster, both of which went uncompleted. "Risk-averse project managers would not take chances with unproven technology, and it took too long in the development process to implement a complete demonstration," Brophy said.

The NSTAR program was established as a result, according to Brophy. "We recognized the need to obtain the data required to baseline ion propulsion, first by assessing the service life of the engine and modeling to understand its wear-out modes, and second by flight testing." The NSTAR program is to validate ion-drive technology to confirm its potential for primary propulsion for prospective NASA, Department of Defense, and commercial customers.

Originally, an Air Force technology demonstration platform called ELITE was to serve as the host spacecraft for NSTAR, but the Air Force dropped the project. Then the New Millenium program, whose goal was to fly new technology developed elsewhere, came along and adopted NSTAR as its power plant. "It's a good match," Brophy said.

The first New Millenium mission will have a life-cycle cost of from $80 million to $100 million, including design and construction of the spacecraft bus, technical integration, the launcher, mission operations, data analysis, and reserves. The primary industrial partner on the program is Spectrum Astro Inc. in Gilbert, Ariz.

Right now, intense system design work is going on to develop the 340-kg spacecraft. The 11.8-inch-diameter SEP unit is estimated to weigh 135 kg, and the xenon propellant will weigh 40 kg.

To keep costs low, the program had to find a solar array that could be obtained essentially for free to power the probe. Fortunately, the Air Force Ballistic Missile Defense Organization (BMDO) agreed to provide a 2.6-kW solar array developed at the Air Force Phillips Laboratory. Called SCARLET (for solar concentrator array with reflective linear element technology), the radiation-shielded photovoltaic system uses a 10:1 linear concentrator that focuses sunlight on solar cells. The type of solar cell to be used is still under consideration. Silicon PVs could be used, or more advanced dual-band-gap gallium arsenide cells, which convert sunlight at two wavelengths with up to 26 percent efficiency, may be the choice. Jim Sovey, senior aerospace specialist at NASA Lewis, said that the BMDO solar array puts out an unregulated bus voltage that range from 80 to 160 V. The NSTAR power processing unit--a group of power converters--will tailor the electrical current for use by the ion engine.

The New Millenium spacecraft will also feature several other advanced technologies including a miniaturized deep-space antenna, lithium-ion batteries, low-mass space structures, a miniaturized imaging spectrometer that will make chemical maps of targets (one-tenth the mass of the spectrometer on the Voyager probe), as well as an advanced flight computer and autonomous navigation system capable of making independent decisions.

The NSTAR program was initiated in 1993, with JPL managing the effort and NASA Lewis developing the 30-centimeter-diameter ion engine. Hughes' Electron Dynamics Division has a X38-million contract for the ion thruster, power processor, and interface control unit, while Moog Inc.'s Space Products Division will supply the xenon feed system.

The result will be a 63-percent-efficient ion system that can be throttled from 0.5 to 2.5 kW. NSTAR is designed to run for 8000 hours with a 50 percent safety margin. Brophy reported that the prototype NSTAR motor was ground tested for 2000 hours at NASA Lewis to demonstrate engine life a year ago. After modifications, the unit was run at JPL for 1000 hours. At press time, a newly redesigned system is being tested for 8000 hours.

Electric Thruster Operation

There are three basic classes of electric thrusters: electrothermal, electromagnetic, and electrostatic. The electrothermal types are simple systems in which a fluid propellant is heated by an electrical discharge or gas-discharge arc (as in an arcjet), or by electrical heating through a wall (as in a resistoject), and expands through a nozzle. More than 140 hydrazine resistojets have been used for satellite NSSK since 1980. A 2-kW arcjet with a specific impulse of 600 sec. has been used for NSSK on a commercial comsat, said NASA's Sovey.

Electromagnetic units generate thrust by accelerating a conducting fluid (often of liquefied Teflon) with magnetic forces and pressures. They include Teflon pulsed-plasma thrusters and magnetoplasmadynamic systems. Both varieties have flown in space.

In electrostatic thrusters, of which NSTAR is a prime example, thrust derives from the acceleration of charged particles through a potential difference set up by perforated electrodes. There are three components to the NSTAR ion drive system: the ionizer or ion source, the ion extractor or accelerator, and the neutralizer.

In operation, xenon gas flows at 3 milligrams per second into the motor's ionization chamber through an annular plenum. The xenon atoms are ionized by bombarding them with energetic electrons emitted from a thermionic hollow cathode. The cathode filament emits electrons that are accelerated toward the chamber wall by a high positive-wall potential. To increase the likelihood of an electron-atom collision, a magnetic field is applied by powerful permanent magnets arranged in concentric rings around the chamber. The field causes the electrons to spiral toward the walls, increasing travel time. The collisions produce a plasma with an equal number of positive ions and electrons.

The positive ions in the plasma flow toward the extraction electrodes, where they are accelerated by the negative potential of grids at one end of the thrust chamber. The grids create thousands of exhaust beamlets as the ions pass through the screens.

Neutralization of the outgoing ion beam is needed because the continuous ejection of positive ions would gradually charge the vehicle negatively, making high-speed cation ejection increasingly difficult, explained Roger Myers, senior propulsion specialist at NASA Lewis. "Unless you squirt electrons away from the spacecraft, it'll become so negative it'll pull the positive ions back." The ion-beam field extracts electrons from a nearby neutralizer cathode, which is a hot filament that discharges electrons. Not many neutralization electrons are required as the beam amperage is only 1.8 amps at 2.5 kW (maximum) power.

There are two ways to throttle down an ion engine: one is to reduce the high grid voltage, the other is to reduce the propellant mass flow. NSTAR does the latter.

Ion drives have three principal loss mechanisms: the loss of energy during ion creation and ion-beam neutralization, and the loss of some un-ionized propellant (up to 10 percent) via leaks through holes in the electrode grids.

Xenon was not the first propellant used in electric motors, Hughes' Beattie said. Mercury and cesium were used because they were easy to ionize and store, and they have high atomic masses, which increases propulsive force, he said. But in the mid-1980s, there was a concern about using these toxic substances. "Mercury and cesium contaminate everything with which they come in contact, making safe ground testing difficult," Beattie said. In addition, they required a more complex power supply because they needed to be stored as room-temperature liquids, then vaporized for use. The required heaters constituted a power drain, Beattie said.

Though rare, the noble gas xenon has very nearly ideal properties for ion motors. Xenon has a high atomic mass and low ionization potential and is chemically inert. It can be stored for years at moderate 1200 pound-per-square-inch pressures (at a density 65 percent greater than liquid water) in basketball-size tanks. Its adoption removed the perceived safety concerns and simplified the propellant system.

Though ion drives bring many benefits, it is very difficult to make them last, Brophy said. The dominant life-limiters are the cathodes and accelerator grids. In the case of the grids, the main wear-out mode occurs when charge-exchange ions--those formed outside in the engine plume by ion-atom collisions--get drawn back in by the electric fields. "It's like sandblasting on an atomic scale," slowly wearing down the millimeter-thin molybdenum grids, Brophy said. The NSTAR SEP is slowed down to slow this erosion and help ensure its mission lifetime. Brophy said that research on longer-life grids with low sputter yield coatings and grids made of carbon-carbon composites is being pursued.

NASA is in the process of starting an effort to develop a new high-efficiency, low-mass propulsion ion-drive system capable of 15,000-hour operation for missions to be mounted five to 10 years from now. At least two electrostatic thruster concepts, an advanced "ridded ion system and a Hall-thruster-based system, will be evaluated. The advanced "ridded ion-thruster program will target a system with twice the NSTAR engine's life capability and less than half the power processing system mass.

Russian Hall thruster (RHT) technology, another kind of electrostatic motor, has generated significant interest in the last few years. With specific impulses of 1600 sec at 50 percent efficiency and demonstrated lifetimes of 6000 hours, RHT is promising for primary propulsion purposes. It remains to be fully developed, however. RHTs run similarly to ion motors, except that the electrode grids are replaced by a magnetic field that points a plasma-generated electric field out the back. The advantage of this ion acceleration scheme is that it allows higher current density than perforated electrodes.

Fakel Enterprises of Russia has built 0.66-kW and 1.35-kW RHT units that have operated on Russian satellites. The latter system is being readied for commercialization on Western GEO comsats. In the United States, the BMDO is developing a 1.5-kW RHT unit for various military missions.

With all this interest and activity, it is clear that electric rocket engines are no longer the sole province of astronautical visionaries and sci-fi authors. The long-promised place of electric motors in the firmament seems to be coming at last.
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Title Annotation:development of xenon-ion engines
Author:Ashley, Steven
Publication:Mechanical Engineering-CIME
Date:Dec 1, 1995
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