Electric Propulsion (Ion Drives)

Propulsion using electrical energy to obtain thrust

Gravity Tug
Image from Steve Bowers
This type of gravity tug uses magnetoplasmadynamic thrusters powered by photovoltaic collectors and on-board reactors to gently move asteroids

The electric or ion or plasma rocket turn the fuel into a plasma (ionized particles), and then uses electricity or ions to eject the fuel.

Basically the Ion rocket uses electrodes to turn the fuel into negative or positive ions. Then the positive ions are accelerated and ejected using an accelerating electrode. Ions are particles carrying negative or positive electrical charges that are attracted to each other. It is this characteristic of ions that is used to propel a rocket.

Power plants using almost any source of fuel (hydrogen being most common) become possible. Nuclear power sources can be used with ion drives, or solar power (photovoltaic cells) or even beamed power; this makes the ion thruster a versatile form of propulsion. However the thrust from an electric propulsion system is typically low.

Although ion drives produce high velocity, and due to the high isp it can continue for long periods of time and hence accelerate the ship to great speeds, the low thrust from the ion jet beam isn't strong enough to give a rapid acceleration or to lift a vehicles off even a moderate sized planet.

Electrostatic thrusters

This class has a single member, the ion thruster. Its key principle is that a voltage difference between two conductors sets up an electrostatic potential difference that can accelerate ions to produce thrust. The ions are then usually neutralized by electrons emitted from a hot filament.

Electrothermal thrusters

This class of thrusters (resistojet, arcjet, RF-heated thruster) does not achieve particularly high exhaust velocities. The resistojet essentially uses a filament to heat a propellant gas (not plasma), while the arcjet passes propellant through a current arc. In both cases material characteristics limit performance to values similar to chemical rocket values. The RF-heated thruster uses radio-frequency waves to heat a plasma in a chamber and has been modified by some early Space Adapted Human clans to reach somewhat higher exhaust velocities.

Electromagnetodynamic thrusters

Under this heading are a number of categories and technologies.

In the Magnetoplasmadynamic (MPD) thruster, a current along a conducting bar creates an azimuthal magnetic field that interacts with the current of an arc that runs from the point of the bar to a conducting wall. Erosion at the point of contact between the current and the electrodes generally was often a critical issue for MPD thruster design, and this type was discarded in favour of other versions.

In Hall-effect thrusters, perpendicular electric and magnetic fields lead to an ExB type of charged-particle drift. For a suitably chosen magnetic field magnitude and chamber dimensions, the ion gyroradius is so large that ions hit the wall while electrons are contained. The resulting current, interacting with the magnetic field, leads to a JxB Lorentz force, which causes a plasma flow and produces thrust. The early Russian projects during the late information and early interplanetary ages of old Earth, favoured this type of thruster.

The Helicon thruster uses a traveling electromagnetic wave that interacts with a current sheet to maintain a high JxB force on a plasma moving along an axis. The traveling wave can be created in a variety of ways, and a helical coil is often used.

VASIMR thrusters combine radio-frequency heating and magnetic fields to produce a variable thrust, variable impulse system. This versatile space drive can be used for low thrust applications and applications requiring somewhat greater thrust, replacing more expensive chemical pockets. However even the VASIMR has too low a thrust-to-weight ratio to be used for launches.

History and Uses

This is a cheap, very reliable, and low tech form of propulsion (Ion drive unmanned probes were used from Old Earth during the Information Revolution (late 20th/early 21st Century Old Earth calendar), and is hence favoured by paranoid, luddite, and anti-Ai groups, especially those for whom rapid acceleration is not important. The Space Adapted Humans for example favoured Ion-propulsion ships for some time. Unfortunately the fuel elements are fairly rare and hence expensive and hard to find among the iceballs of the Oort cloud zones. An ion drive engine using iron and an adamant-duralloy grid was developed during the late Interplanetary age but proved difficult and unreliable.

Ion Drive
Image from Anders Sandberg
Ion drive ship of the Kandefinizione AI clan in the vicinity of S106 IRS4 in the Cygnus region. Like most Kandefinizione ships it is elaborately sculpted, in this case in biont gothic. This type of ships are used for deeptime exploration, especially in dense interstellar media. Length: one meter. Mass: 870 kg. Crew: one intellect. Passengers: up to 170,000 fully active Second Singularity sentients in nanostorage.

Note: in order to image both the ship and nebula the image was overexposed, and the ion engine output appears very bright. In reality the engines do not produce more than a few watts of visible output.

Ion drive ships are still used by some deep space travelling daughter clades of Space Adapted Humans, especially the Imirodi Clade in the vicinity of Groombridge 34Aa, Ross 248, and Kruger 60 A, and also the Zain Beta Clade of the 36 Ophiuchi4 / C - CD-32 13297 - CD-34 11626 region. But since the centers of galactic civilisation have moved to the great Empire, trade, and knowledge capitals like Abydos (Xi Ursae Majoris A-II), Merrion (Alpha Mensae XII), Ken Ferjik, Alexandria, and Eta Sagittarius, less and less has been known about the activities of the Space Adapted Human, Backgrounder, and Genen clades that are still active in this region.

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Development Notes
Text by M. Alan Kazlev and Steve Bowers
Initially published on 03 December 2008.