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New Dawn for Electric Rockets

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New Dawn for Electric Rockets SPACE TECHNOLOGY New Dawn for KEY CONCEPTS Ef!cient electric plasma engines are ■ Conventional rockets propelling the next generation of space generate thrust by burning chemical fuel. Electric probes to the outer solar system By Edgar Y. Choueiri rockets propel space vehicles by applying electric or electromagnetic !elds to clouds of charged lone amid the cosmic blackness, NASA’s ing liquid or solid chemical fuels, as convention- particles, or plasmas, to Dawn space probe speeds beyond the al rockets do. accelerate them. A orbit of Mars toward the asteroid belt. Dawn’s mission designers at the NASA Jet Launched to search for insights into the birth of Propulsion Laboratory selected a plasma engine ■ Although electric rockets the solar system, the robotic spacecraft is on its as the probe’s rocket system because it is highly offer much lower thrust levels than their chemical way to study the asteroids Vesta and Ceres, two ef"cient, requiring only one tenth of the fuel cousins, they can even- of the largest remnants of the planetary embry- that a chemical rocket motor would have need- tually enable spacecraft os that collided and combined some 4.57 billion ed to reach the asteroid belt. If project planners to reach greater speeds years ago to form today’s planets. had chosen to install a traditional engine, the for the same amount But the goals of the mission are not all that vehicle would have been able to reach either of propellant. make this !ight notable. Dawn, which took off Vesta or Ceres, but not both. ■ Electric rockets’ high-speed in September 2007, is powered by a kind of space Indeed, electric rockets, as the engines are also capabilities and their propulsion technology that is starting to take known, are quickly becoming the best option for ef!cient use of propellant center stage for long-distance missions—a plas- sending probes to far-off targets. Recent success- make them valuable for ma rocket engine. Such engines, now being de- es made possible by electric propulsion include a SAIC deep-space missions. veloped in several advanced forms, generate visit by NASA’s Deep Space 1 vehicle to a comet, a thrust by electrically producing and manipulat- bonus journey that was made feasible by propel- —The Editors ing ionized gas propellants rather than by burn- lant that was left over after the spacecraft had ac- RAWLINGSPAT 58 SCIENTIFIC AMERIC AN © 2009 SCIENTIFIC AMERICAN, INC. February 2009 New Dawn for complished its primary goal. Plasma engines turned the concept into a practical technology in NASA’S DAWN SPACE PROBE, which have also provided propulsion for an attempted the mid-1950s. A few years later engineers at the is propelled by an electric rocket landing on an asteroid by the Japanese Hayabusa NASA Glenn Research Center (then known as called an ion thruster, nears the probe, as well as a trip to the moon by the Euro- Lewis) built the "rst operating electric rocket. asteroid Vesta in this artist’s pean Space Agency’s SMART-1 spacecraft. In That engine made a suborbital !ight in 1964 on- conception. Vesta is its initial light of the technology’s demonstrated advantag- board Space Electric Rocket Test 1, operating for survey target; the asteroid Ceres, its second destination, es, deep-space mission planners in the U.S., Eu- half an hour before the craft fell back to Earth. "oats in the far distance in the rope and Japan are opting to employ plasma In the meantime, researchers in the former image (bright spot at upper drives for future missions that will explore the Soviet Union worked independently on con- right). A conventional chemical outer planets, search for extrasolar, Earth-like cepts for electric rockets. Since the 1970s mis- rocket engine would be able to planets and use the void of space as a laboratory sion planners have selected the technology be- carry enough fuel to reach only in which to study fundamental physics. cause it can save propellant while performing one of these asteroids. such tasks as maintaining the attitude and or- A Long Time Coming bital position of telecommunications satellites Although plasma thrusters are only now mak- in geosynchronous orbit. ing their way into long-range spacecraft, the technology has been under development for that Rocket Realities purpose for some time and is already used for The bene"ts afforded by plasma engines become other tasks in space. most striking in light of the drawbacks of con- As early as the "rst decade of the 20th centu- ventional rockets. When people imagine a ship ry, rocket pioneers speculated about using elec- streaking through the dark void toward a distant tricity to power spacecraft. But the late Ernst planet, they usually envision it trailing a long, Stuhl inger—a member of Wernher von Braun’s "ery plume from its nozzles. Yet the truth is alto- legendary team of German rocket scientists that gether different: expeditions to the outer solar spearheaded the U.S. space program—"nally system have been mostly rocketless affairs, © 2009 SCIENTIFIC AMERICAN, INC. www.SciAm.com © 2009 SCIENTIFIC AMERICAN, INC. SCIENTIFIC AMERIC AN 59 [COMPARISON] motor would typically have no fuel left for brak- Chemical vs. ing. Such a probe would need the ability to "re its rocket so that it could slow enough to achieve Electric Rockets END orbit around its target and thus conduct extend- Thrust: low Chemical and electric propulsion ed scienti"c observations. Unable to brake, it systems are suited to different END Speed: very high Thrust: zero Tank: one-third full would be limited to just a !eeting encounter kinds of missions. Chemical Speed: high (enough for a with the object it aimed to study. Indeed, after rockets (left) produce large Tank: empty second mission) a trip of more than nine years, New Horizons, amounts of thrust quickly, so they can accelerate to high a NASA deep-space probe launched in 2006, will speeds rapidly, although they get only a brief encounter of not more than a sin- burn copious quantities of fuel gle Earth day with its ultimate object of study, to do so. These characteristics the recently demoted “dwarf planet” Pluto. make them appropriate for MIDDLE MIDDLE relatively short-range trips. Thrust: zero Thrust: low The Rocket Equation Electric rockets (right), which Speed: high Speed: high For those who wonder why engineers have been Tank: empty Tank: two-thirds full use a plasma (ionized gas) as unable to come up with ways to send enough propellant, generate much less chemical fuel into space to avoid such dif"cul- thrust, but their extremely frugal ties for long missions, let me clarify the immense consumption of propellant allows hurdles they face. The explanation derives from them to operate for much longer periods. And in the frictionless what is called the rocket equation, a formula environment of space, a small START START used by mission planners to calculate the mass Thrust: high Thrust: low of propellant required for a given mission. Rus- force applied over time can Speed: low Speed: low eventually achieve similarly Tank: full Tank: full sian scientist Konstantin E. Tsiolkovsky, one of high or greater speeds. These the fathers of rocketry and space!ight, "rst features make plasma rockets introduced this basic formula in 1903. well equipped for deep-space CHEMICAL ELECTRIC In plain English, the rocket equation states missions to multiple targets. ROCKET ROCKET the intuitive fact that the faster you throw pro- pellant out from a spacecraft, the less you need to execute a rocket-borne maneuver. Think of a because most of the fuel is typically expended in baseball pitcher (a rocket motor) with a bucket the "rst few minutes of operation, leaving the of baseballs (propellant) standing on a skate- spacecraft to coast the rest of the way to its goal. board (a spacecraft). The faster the pitcher !ings True, chemical rockets do launch all spacecraft the balls rearward (that is, the higher the ex- from Earth’s surface and can make midcourse haust speed), the faster the vehicle will be travel- [THE AUTHOR] corrections. But they are impractical for power- ing in the opposite direction when the last ball is ing deep-space explorations because they would thrown—or, equivalently, the fewer baseballs Edgar Y. Choueiri teaches astro nautics and applied physics require huge quantities of fuel—too much to be (less propellant) the pitcher would have to hurl at Prince ton University, where he lifted into orbit practically and affordably. Plac- to raise the skateboard’s speed by a desired also directs the Electric Propulsion ing a pound (0.45 kilogram) of anything into amount at any given time. Scientists call this and Plasma Dynamics Laboratory Earth orbit costs as much as $10,000. incremental increase of the skateboard’s velocity (http://alfven.princeton.edu) To achieve the necessary trajectories and high “delta-v.” and the university’s Program in Engineering Physics. Aside from speeds for lengthy, high-precision journeys with- In more speci"c terms, the equation relates ) plasma propulsion research, he out additional fuel, many deep-space probes of the mass of propellant required by a rocket to is working on mathematical the past have had to spend time—often years— carry out a particular mission in outer space to techniques that could enable detouring out of their way to planets or moons two key velocities: the velocity at which the rock- illustration accurate recording and reproduc- that provided gravitational kicks able to accel- et’s exhaust will be ejected from the vehicle and tion of music in three dimensions. erate them in the desired direction (slingshot the mission’s delta-v—how much the vehicle’s ve- );KEVIN HAND ( moves called gravity-assist maneuvers).
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