By John McHale
PASADENA, Calif. - The New Millennium Program`s Deep Space-1 (DS-1) craft will use a xenon ion rocket for the first time in a National Aeronautics and Space Administration (NASA) deep-space mission.
The DS-1 craft will use electrostatic propulsion, which derives its thrust from accelerating and expelling the positively charged xenon atoms, or ions.
Spacecraft using electrostatic propulsion systems can achieve high velocities from a small amount of propellant if the electric propulsion system thrusts continuously over a long period of time.
For future interplanetary and comet rendezvous missions, electric propulsion will result in decreased spacecraft mass and faster trips. Solar electric propulsion systems can be in the form of electrostatic, electrothermal, and electromagnetic.
NASA engineers have demonstrated ion engines already in their two Space Electric Rocket Test missions - SERT I and SERT II - and in the Applications Technology Satellite. The DS-1 flight, however, "will be the most extensively instrumented endurance test of an ion engine ever performed," says John F. Stocky, manager and principal investigator of the ion propulsion system project at the NASA Jet Propulsion Laboratory (JPL) in Pasadena, Calif.
The main advantage to the deep-space mission is the system`s high payload-to-propellant ratio, says Marc Rayman, chief mission engineer of the New Millennium Program, Deep Space Mission 1. The craft is slower than chemically power craft, but draws less power and is cheaper to power because it draws its energy from the sun.
Ion propulsion reduces by a factor of 10 the propellant necessary to provide a given impulse to a spacecraft as compared to a chemical-propulsion system. This benefit, however, comes at the cost of an increase in the dry mass of the propulsion system - an increase associated with such additional required design elements as solar arrays and power processors.
The thrusting action is similar to chemical propellant engines, Rayman says, which expel burning gases. Chemical- propellant engines, however, can produce millions of pounds of thrust - far more than ion propulsion.
In space, the 11.8-inch diameter ion-propulsion engine will use the heavy but inert xenon gas as fuel and be powered by more than 2,000 watts from large gallium indium arsenide solar arrays developed under supervision of the U.S. Defense Department`s Ballistic Missile Defense Organization in Arlington, Va., Rayman explains.
Most of the components for the system will come from the NASA Solar electric propulsion Technology Applications Readiness team (NSTAR). The NSTAR Validation Program is NASA`s initiative to validate low-power ion propulsion technology by obtaining the data needed by a project manager to specify solar-powered ion propulsion on a spacecraft.
The engine, built at the NASA Lewis Research Center in Cleveland, will be tested for 8,000 hours (330 days) in the space-like environment of JPL`s vacuum chamber. It weighs 17.6 pounds and is 15.7 inches in diameter and 15.7 inches long.
Ion engines work only in the vacuum of space, and the spacecraft moves only millimeters per second in its early stages of flight because their mass is so low. The low-thrust engine, however, will increase the spacecraft`s velocity over time to meet its celestial target at more than 22,000 miles per hour.
Powering the craft
The xenon propellant feeds from a tank, which can hold as much as 176 pounds of xenon, in the craft to an ion thruster (i.e. motor or engine), according to the New Millennium Solar Electric Propulsion Handbook . Once in the thruster, xenon atoms are ionized, and the ions accelerate through a voltage difference, which propels them to a high speed. The high exhaust velocity enables designers to use more payload and less propellant than chemical engines.
Deep Space-1 will consume only 99 pounds of xenon during its 2-year mission. A typical deep-space probe would have a prime mover and generator, but on DS-1 these components are replaced by solar arrays and a power processing unit.
Electrons emit from a neutralizer to keep the spacecraft electrically neutral - or to compensate for the thruster`s emission of xenon ions. The intent is for each positive xenon ion fired from the thruster, one electron is fired from the neutralizer.
A cathode filament emits electrons, which then accelerate toward the chamber walls. As propellant moves into the ionization chamber, these high-speed electrons bombard the xenon atoms and ionize the atoms by knocking off electrons.
The NSTAR thruster uses powerful permanent magnets arranged in concentric rings around its ionization chamber to increase the likelihood of an electron-atom collision.
While xenon is not the only fuel used in electrostatic propulsion, it is a good choice because at between 20 and 50 degrees Celsius it remains gaseous in space. A gaseous propellant is easy to carry because it doesn`t slosh. Xenon is also safe, unlike mercury and cesium, which are poison. Xenon also has a high ionization potential.
The NSTAR ion engine operates at several levels of thrust that change as the craft moves away from the sun. Solar panels also lose efficiency with age, thus forcing the craft operate at lower power.
The first of three deep-space missions to be flown by the year 2000, DS-1will feature a 1998 launch of a small spacecraft destined for a flyby of an asteroid and a comet. Spectrum Astro, Inc., of Gilbert, Ariz., will build the 220-pound minicraft under the direction of JPL team leader David Lehman.
The New Millennium deep-space probe from the NASA Jet Propulsion Laboratory, pictured above, will use a highly efficient form of solar electric power called electrostatic propulsion with its xenon ion engine, which accelerates and expels positively charged xenon atoms.
