Electric Propulsion
Electric (Ion) Propulsion
Electric propulsion works by using electrical energy to accelerate a propellant to much higher velocities than is possible using chemical reactions. The most common propellant used in ion engines is xenon.
Early experimental ion engines used mercury and caesium, but they proved hard to work with. At room temperature, mercury is liquid and caesium is solid; they both must be heated first to turn them into a gas. There is a risk that as the mercury or caesium exhaust cools, many of the atoms would condense on the exterior surface of the spacecraft, thus contaminating instruments and solar cells. Research is now concentrated on xenon, which is as a cleaner and simpler fuel for use in ion engines.
While current ion propulsion systems use solar panels to create the required electricity, there are plans for nuclear-powered ion systems. These would be of great use in exploring the outer solar system, where solar power is no longer efficient.
Thrusters
As stated above, ion propulsion involves ionising a gas to propel a craft. Instead of a spacecraft being propelled with standard chemicals, the gas xenon is given an electrical charge, or ionised. It is then electrically accelerated to a speed of about 30 km/second. When xenon ions are emitted at such high speed as exhaust from a spacecraft, they push the spacecraft in the opposite direction.
NASA's DS-1 probe is testing a new type of ion thruster, shown in the schematic above. This thruster is quite different from the thrusters found on satellites, in that is it being used as the spacecraft's primary propulsion system, rather than just a station-keeping device. DS-1 is also the first spacecraft to use continuous thrust. Rather than requiring a very high acceleration for a short period of time (as is normal in chemical propulsion), DS-1's ion drive was operated in a continuous mode for weeks and months at a time. Although the trust is very low, by accelerating steadily for long periods of time the spacecraft was able to reach very high velocities. The following description of DS-1's ion thrusters is from the official DS-1 Web site:
Its ion propulsion system (IPS) uses a hollow cathode to produce electrons, used to ionise xenon. The Xe+ is electrostatically accelerated through a potential in escess of 1 kV and emitted from the 30cm thruster through a molybdenum grid. A separate electron beam is emitted to produce a neutral plasma beam. The power-processing unit (PPU) of the IPS can accept as much as 2.5 kW, corresponding to a peak thruster operating power of 2.3 kW and a thrust of 92 mN. Throttling is achieved by balancing thruster and Xe feed system parameters at lower power levels, and at the lowest thruster power, 500 W, the thrust is 20 mN. The specific impulse decreases from 3100 s at high power to 1900 s at the minimum throttle level.
Although very efficient, an IPS does require a significant power source. Solar-powered IPSs are fine for small spacecraft such as satellites and probes, but if humans want to use this technology to explore the Solar System, power systems with greater output capacities will be required.
EIP Drives
Another advanced propulsion concept that has been around for about as long as NTR engines are "electrostatic ion propulsion (EIP)" drives, sometimes known as "electron bombardment ion engines". The first EIP drive was built in 1958 by NASA. The first flight test was in 1964, with the NASA "Space Electric Rocket Test (SERT)" suborbital flight, and was followed by further test flights, but the technology only reached full operational status in the 1990s. The EIP drive is one of several different types of electric rocket engines, and in fact is not the first type to be used in space. However, it is conceptually one of the simplest and one of the best-known.
The basic idea is to accelerate ions of a relatively heavy propellant, such as xenon, to high velocities, through grids charged to high voltages. The thrust is very low, but the efficiency is very high, with a specific impulse over ten times that of LOX-RP propulsion. Early experimental EIP drives used mercury or cesium as a propellant. They were troublesome since handling such toxic substances was tricky, and they had to be vaporized for use, involving energy loss through heat of vaporization. Although xenon is rare, it has almost ideal properties for EIP drives: it is normally a gas, has a high atomic mass and low ionization potential, is chemically inert, and is easy to handle. Hughes has been the leader in developing the technology, which they call the "Xenon Ion Propulsion System (XIPS)". Hughes had experimented with EIP drives since the 1960s, and judged xenon to be the propellant of choice in 1984.
A formal program to develop operational EIP drives was begun in 1992. The company attracted particular attention with the EIP drive developed for the NASA "Deep Space 1 (DS1)" probe, which was launched on 24 October 1998. The spacecraft was one of the first launched under the NASA "New Millennium" program. Although DS1 was to visit asteroids and comets, the goal of New Millennium is to develop and demonstrate advanced spacecraft technologies, and the probe incorporated twelve such advanced technologies, most significantly the EIP drive. The EIP drive was powered by the spacecraft's photovoltaic arrays, and so was referred to as a "solar electric propulsion (SEP)" drive.
DS1 was the first spacecraft to use an electric rocket engine of any type for its main propulsion system. DS1 weighed 490 kilograms, was 2.5 meters long, and was 1.7 meters wide. When its two solar panels were deployed, they spanned 11.8 meters. DS1's EIP drive was named the "NASA SEP Technology Application Readiness (NSTAR)". NSTAR weighed 8 kilograms, had a diameter of 30 centimeters, and used a supply of 81.5 kilograms of xenon propellant. Xenon atoms were fed from a propellant tank into an ionizing chamber, where an electron emitter ionized them, producing positive xenon ions. These xenon ions were then accelerated by two molybdenum grids with a potential of 1,300 volts between them, driving the ions out the exhaust at over 30 kilometers per second.
Electrons were injected into the external exhaust flow to neutralize the positive ions; this prevented the spacecraft from building up a negative charge, which would have attracted exhaust ions back to the grid and possibly disrupted the operation of spacecraft instruments. An EIP drive can be throttled either by adjusting the grid voltage or adjusting the propellant flow. NSTAR adjusted the propellant flow. EIP engine efficiency is reduced by the energy needed to ionize and then neutralize the propellant, and leakage of unionized propellant. NSTAR's conversion efficiency of electric power to thrust was 63%, and the engine could be throttled over a range of 2 to 9.4 grams of thrust. Design life of the drive was 8,000 hours. Development of NSTAR was begun in 1993 by NASA's Jet Propulsion Laboratory (JPL), which runs the New Millennium program. Hughes built most of NSTAR, though Moog INC built the xenon fuel system.
The DS1 mission went smoothly, though initial attempts to fire up the NSTAR EIP drive encountered difficulties. The first test was to last 17 hours, but the drive shut down after 4.5 minutes. The two ion-accelerating grids were only about 0.6 millimeter apart, and this narrow spacing could result in shorts and arc-overs, possibly due to molybdenum flaking off the grid or other contaminants. When a short or arc-over was detected, the power system "recycled" -- that is, it shut down for a second and then restarted, which usually cleared the fault. However, the EIP drive exceeded the limit on recycles and mission engineers were puzzled. They were not particularly alarmed, because such behavior was familiar from ground tests of the engine, and they knew that recycles were common during initial operation until the engine burned away contaminants. NSTAR eventually operated for much longer than its specified design life. DS1's advanced solar arrays used reflectors and lenses to concentrate sunlight on photovoltaic cells. The panels could generate 2.6 kilowatts for the NSTAR engine and the spacecraft's other systems at the distance of the Earth from the Sun, though power fell off as the probe moved farther into space.
Hughes has developed two versions of smaller EIP drives, which have been used operationally on communications satellites ("comsats") as station-keeping thrusters. Comsats are launched into "geostationary" orbit, at an altitude of about 36,000 kilometers. At this altitude, the satellite takes 24 hours to orbit the Earth. This matches the Earth's rate of rotation, and so the satellite hangs in the same position above the Earth. This is useful for a comsat, since ground-based antennas can be focused on the satellite and then left in place. The orbit has to be directly over the equator, or the satellite will move north and south over the course of the day. Since the Sun is not in the plane of the Earth's equator, its gravitational pull tends to nudge a geostationary satellite's orbit off its equatorial position. The Sun's influence is very small, and so even a weak thruster is powerful enough to keep the satellite on station.
The first operational EIP drives were flown on the "PanAmSat PAS-5" comsat in 1997, which was a Hughes (now Boeing) HS-601HP satellite fitted with four EIP drive thrusters for station-keeping. Each thruster was 13 centimeters in diameter and generated 1.3 grams of thrust. Design lifetime for the thruster system was 12 to 15 years. Only two thrusters were actually used, pointing north and south to keep the satellite in its orbit plane. The other two were backups. Each thruster operated for about five hours a day. Hughes also developed a more powerful EIP drive thruster for use with the larger HS-702 comsat. The bigger thruster has a diameter of 25 centimeters and produces 16.8 grams of thrust. As with the HS-601HP, four thrusters are used, but two are redundant. The greater thrust of the larger thruster means that a thruster only needs to be fired for about a half hour a day to keep the satellite on station. A number of comsats with EIP drive thrusters are now in service. The EIP drive thrusters cut fuel requirements from those of traditional hydrazine chemical thrusters by as much as 90%. This results in weight savings of hundreds of kilograms, and millions of dollars saved in payload launch costs.
Other nations are now developing EIP drives. AEA Technology of Culham, UK, is working on the "UK-10" and "UK-25" EIP drive thrusters, with diameters of 10 and 25 centimeters, for use on future British and European spacecraft. The Japanese developed a 12-centimeter EIP thruster that was flown on the "Engineering Test Satellite (ETS) VI" in 1995. ETS VI failed to reach its planned orbit, but tests of the thruster system proved satisfactory. Development of next-generation EIP drives is in progress. Operational lifetime is a major problem. EIP drives suffer from ionic and atomic erosion of the grids, which eventually wears the grids out. One engineer compares it to "sandblasting on the atomic scale." Advanced materials with longer lifetimes are being evaluated. EIP drives are now being developed with much greater lifetimes and lower costs, and in sizes ranging from smaller to larger than current EIP drives.
NASA is currently developing EIP drives that are an order of magnitude more power than the DS1 NSTAR and have two to three times the fuel economy. Scaling up the electrical grid for an EIP drives runs into some physical obstacles, and so a large spacecraft would use multiple EIP drives to obtain more thrust. For such big spacecraft, the solar panels required to provide power for the EIP drives would be prohibitively large, and so such a spacecraft would likely be powered by a nuclear reactor. Despite the political obstacles to development of space nuclear power systems, NASA is now engaged in a research program named "Prometheus" to define a nuclear-powered EIP spacecraft. The main focus of the Prometheus effort is the proposed "Jupiter Icy Moons Orbiter (JIMO)", which would be by far the biggest space probe ever flown, with a launch mass of 20 tonnes and a deployed length of 30 meters.
The JIMO design features a nuclear reactor with an output power of a few hundred kilowatts at its tip, linked to a spacecraft bus by a long boom flanked by heat radiators and with twin EIP engines at the end. JIMO's design is still being considered, with researchers evaluating different types of reactors and power-conversion technologies. It will not be launched before 2015.
Hall Effect Ion Drives
The European Space Agency (ESA) has flown a demonstrator spacecraft, the first "Small Mission For Advanced Research In Technology (SMART-1)", to test their own SEP drive. SMART-1 is a Moon orbiter, with a small payload of experimental instruments to perform lunar measurements to obtain some science value from the mission, and was launched as a secondary payload on an Ariane 5 booster in September 2003. The SMART-1 SEP system uses a "Hall effect drive", similar to an EIP but using magnetic fields and not electrostatic fields to accelerate heavy ions.
There are a number of different schemes for Hall effect drives, but one simple configuration consists of a cylinder ringed by one pole of a magnet, and with the other pole configured as a rod running the center of the cylinder. This configuration sets up a magnetic field running radially from the center rod to the ring. A propellant that can support an electric arc discharge, usually xenon, is injected into the inlet of the engine. A positive electrical anode is placed in the cylinder before the magnetic ring, and a negative cathode emitter is placed after the magnetic ring, possibly at the output. The potential between the anode and cathode ionizes the xenon. Since electrical charges move at a right angle to a magnetic field, the radial magnetic field causes the electrons and ions to circle around the center pole. An interaction between the electric and magnetic fields known as the "Hall effect" accelerates the xenon ions out the exhaust.
Hall effect drives have the advantage that they do not require grids, and so in principle can be more easily scaled up than EIP drives. They provide more thrust than EIP drives, but they are not quite as efficient. The SMART-1 SEP system is based on a French SNECMA PPS-1350 Hall effect thruster, built in collaboration with Russian propulsion organizations. The engine has 7 grams of thrust and is slowly nudging SMART-1's Earth orbit outward in a spiral to finally be captured by the Moon's gravity, with the probe going into orbit in March 2005. An improved Hall effect engine will be used on the ESA Bepi-Colombo Mercury probe, now in planning. The PPS-1350 is also being integrated into satellites for station-keeping and orbital adjustment.
Both the US and the USSR experimented with Hall effect drives in the 1960s. The US gave up on them in favor of EIP drives, but the Soviets continued their efforts, and have flown at least a hundred of them on various spacecraft. In the last few years, American researchers have evaluated Russian Hall effect drives and found them impressive, and the US is now developing their own Hall effect drives. In the summer of 2002, the NASA Glenn Research Center in Ohio announced that they had developed and ground-tested a new Hall-effect thruster, the "NASA-457M", claimed to be ten times more powerful than any other Hall-effect thruster built to date. The NASA-457M can provide over 300 grams of thrust.
ETR Ion Drives
In addition to the EIP and Hall-effect drives, simpler types of electric rocket engines were in use decades before EIP drives were employed operationally. "Electrothermal rocket (ETR)" drives operate on much the same principle as an NTR or STR, except that the propellant is heated electrically, instead of by a nuclear reactor or a solar mirror. There are two types of ETR drives, "arcjets" and "resistojets". In an arcjet, the propellant is heated by an electrical arc discharge, with the heated propellant blasting out the thruster nozzle. Specific impulse is on the order of two to four times LOX-RP propulsion. Propellants include hydrogen, ammonia, and hydrazine, but hydrazine is preferred because it is often used with other spacecraft systems.
Current arcjets use a DC arc discharge, but work has also been done on arcjets using an AC arc discharge, or RF or microwave heating. DC arcjets are used on the Lockheed Martin 7000-series comsat for station-keeping. A resistojet is one of the simplest conceivable rocket thrusters, though its specific impulse is not impressive, about 10% that of LOX-RP. All it does is use electrical resistance elements to heat a propellant for thrust. A resistojet can use the same types of propellant as an arcjet. Resistojet thrusters are used on some comsats for station-keeping. NASA JPL has been working on experimental matchbook-sized "microthrusters" for very small spacecraft that implement schemes similar to resistojets on the microscale, using micromachining fabrication techniques.
EMR Drives
There is also an "electromagnetic rocket (EMR)" drive that is similar to an ETR. It is fueled by a solid cylinder of teflon plastic, backed by a feed spring mechanism, with the output end of the block vaporized by high-intensity current pulse that also sets up an electromagnetic field to accelerate the vaporized teflon out the nozzle. The thrust can be very precisely controlled by varying the magnitude of the current pulse or its repetition rate, and the specific impulse is excellent, from about 2.5 to 5.5 times that of LOX-RP propulsion.
This type of drive is also known as a "pulsed plasma thruster (PPT)". The EMR drive has been around a long time, having been first used on the Soviet Zond 2 space probe in 1964. The PPT went out of fashion as satellites got bigger, but now that "smallsats" are making a comeback, interest in PPTs is reviving. NASA's "Earth Observation 1 (EO-1)" satellite, the first New Millennium spacecraft designed for the Earth observation mission, featured a 4.95 kilogram PPT, with enough Teflon fuel for 30 days of operations.
These types of engines have been used as thrusters, and they offer improved fuel efficiencies in comparison to traditional hydrazine thrusters. Scaling them up to any size seems to be out of the question.
VASIMR Ion Drives
Several other types of advanced rocket engines are now in laboratory development, but have never been flown on a space mission. They include the "magnetoplasmodynamic" drive; the "pulsed inductive" drive; and the "variable specific impulse magnetoplasmodynamic rocket (VASIMR)" drive. In a magnetoplasmodynamic drive, sometimes also called a "Lorenz force accelerator", the propellant is accelerated by magnetic, rather than electric, fields.
The engine consists of a thrust chamber with walls that act as an anode and a central rod that acts as a cathode. The propellant, which can be argon, lithium, or hydrogen, in increasing order of efficiency, is ionized, causing a very strong current to flow radially between the anode wall and the central cathode. While the current flow is provided by equal numbers of electrons and positively-charged ions, the ions are much heavier than the electrons and so move much more slowly.
A moving current sets up a magnetic field acting at right angles whose magnitude is proportional to rate of current flow, and so the fast-moving electron current sets up a magnetic field directed in concentric rings around the cathode. A magnetic field in turn accelerates charged particles at right angles to itself, and so the positive ions are driven out the exhaust nozzle. Specific impulse is on the order of ten times that of LOX-RP propulsion.
Conceptually, a pulsed inductive drive looks like a flat coil with a fat spike in the center, with the coil connected to a bank of big capacitors. A puff of propellant, usually argon though many other propellants are possible, is injected inside the hoop, and then the capacitors are discharged into the coil. This sets up an intense magnetic field that ionizes the propellant, and the electric fields that are set up push the ions out the hoop. Specific impulse should be ten to twenty times that of LOX-RP propulsion.
Pulsed inductive drives do not have electrodes, which tend to be worn down by ion and electron bombardment, and engine thrust can be scaled up by increasing the pulse rate, which is on the order of several hundred times a second. TRW has been working on the concept using private funds, but no pulsed inductive drive has been flown in space yet.
The VASIMR drive is one of the most exotic of the advanced propulsion concepts. It generally uses hydrogen as a propellant, first ionizing it with radio-frequency (RF) energy, and then injecting it into a thrust chamber where oscillating magnetic fields and RF energy heat it to millions of degrees Celsius.
A magnetic choke controls the flow of the hot plasma to the exhaust nozzle. If the magnetic choke is constricted, the flow of plasma is small, but the temperature remains high. This gives low thrust but extremely high efficiency, possibly a hundred times that of LOX-RP propulsion, useful for interplanetary cruise. If the magnetic choke is opened up, the flow of plasma is high, but the temperature is low. This gives high thrust and lower efficiency, about ten times that of LOX-RP propulsion, useful for initial boost out of planetary orbit. Such dual-mode operation is extremely attractive for a manned Mars mission, since attempting to boost out of low Earth orbit with a low-thrust engine would leave the Mars vehicle in the Earth's radiation belts for a long period of time, putting the crew at risk.
The VASIMR engine would allow the Mars vehicle to cross the radiation belts in a short time in the high-thrust mode, and then cruise to Mars in the low-thrust mode. Work has been conducted on VASIMR at the Massachusetts Institute of Technology, NASA JPL, and elsewhere. NASA is considering a test flight experiment.
Project Prometheus
This is an attempt by NASA / JPL to develop a hybrid drive using ion propulsion powered by a nuclear power source. See the Nuclear page for further details.
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