Ion drives use ionized gas to propel a craft through space. The first ion engines were sponsored by NASA and built at the Glenn Research Center in 1960, and have been occasionally tested in limited orbital trials since then. However, it wasn’t until the launch of the comet-rendezvous mission DS1 in 1998 that one was used as the primary propulsion system for a spacecraft.
Tech Lvel: 10
On the DS1 engine, Xenon gas molecules were ionized (electrically charged) using a cathode (like what’s used in a TV), accelerated using electromagnetic means to about 30 km/sec, then basically spat out the back of the spacecraft.
Ion drives provide only extremely gentle thrust (About 1/50th of a pound of force at full throttle, or about the force a sheet of paper exerts on a table) compared to chemical rockets, but at the same time are vastly more fuel efficient. It would take the DS1 probe 20 months of constant thrust to use all 81.5 kilograms of its fuel. A chemical rocket would rip through an equivalent amount of fuel in a few minutes, at best. Yet, at the end of those 20 months, the DS1 probe would obtain a velocity of 4.5 km/sec, about ten times as fast as a chemical rocket of the same size using the same amount of fuel. Ion engines are therefore most appropriate for missions where fuel efficiency and/or final velocity is more important than high acceleration or quick travel times.
One interesting problem that arises with an ion drive is that exhaust is highly electrically charged, and could result in the spacecraft itself becoming electrically charged and causing all sorts of problems with delicate instruments. In order to avoid this, a free-electron beam "injects" electrons into the ion exhaust flow, neutralizing it electrically just after it leaves the exhaust port.
Unlike a chemical rocket, and ion engine’s power depends not on the type of fuel used but on the amount of power available to it. The greater energy available to the engine, the higher it can accelerate the gas ions, and greater specific impulse the engine can generate.
DS1 was meant primarily as a testbed for ion engine technology. Future missions will use arrays of multiple ion engines, such as Deep Space 4 (DS4) which is scheduled to be launched in 2004 with an array of four ion engines that will help it keep pace and eventually land on a comet.
Tech Level: 10
A type of ion plasma thruster has been in use for about 10 years now, employed mostly on Soviet/Russian satellites: the Hall thruster.
A Hall thruster accelerates the plasma into a jet by passing it through electric and magnetic fields generated by electrodes strung across the plasma flow chamber. Fuel used is typically Xenon, but other gasses, such as Krypton, Hydrogen, Nitrogen, or Argon, can be used. The electrical fields accelerate the ionized gas, while the magnetic fields constrict and direct the plasma flow. The plasma is then further accelerated electrostatically out of the thruster.
Compact and fuel efficient (Specific Impulses are typiclaly in the range of 1000 to 3000), Hall thrusters are considered to be a very promising technology and efforts are being made to scale them up for full-scale space exploration applications. Typical modern-day Hall thrusters operate at about 1.35 kilowatts; experiments are now being carried out with models capable of up to 72 kilowatts and nearly 3 Newtons of thrust. Though some problems do arise with larger Hall thrusters, including controlling differing plasma flow/plasma plume geometries and wear and tear on the plasma flow chamber, Hall thrusters capable of hundreds of kilowatts or more will probably be developed in the coming years.
Whereas baseline ion engines are meant for deep-space missions, Hall thrusters are more desirable for operations around planets, as their somewhat higher thrust capacity is more suited for maneuvering around gravity wells.
MINI ION DRIVE MODULES
Tech Level: 13
Some scientists envision small, modularized ion engines the size of soup cans driving a fleet of similarly-sized microprobes around the solar system to fully explore and map the outer planets and other bodies. Similarly, larger interplanetary vessels could have arrays of hundreds of such interchangeable ion engines, allowing for very fuel-efficient cargo transfers around the solar system. A similar scheme could also be used to herd asteroids for various purposes.
NASA’s FAQ on the DS1’s Ion Drive:
A more technical article on the Ion Drive:
More information on the Hall thruster:
An article focusing on the ESA’s upcoming ion-driven SMART-1 lunar probe:
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