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 (usually
cesium chloride but other fuels can also be used) 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. 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, and 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.
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.
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.
Electrodynamic thrusters
Under this heading are a number of categories and various
technologies, including
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.
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 a carbonite-duralloy grid was developed during the late
Interplanetary age but proved difficult and unreliable. 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.
Common Ion Drive efficiencies
| Type |
Specific Impulse |
Thrust-to-Weight Ratio |
| ARCJET (Electrothermal) |
800-1200 |
|
| MPD (Electromagnetic) |
2000-5000 |
|
| ION (Electrostatic) |
3500-10000 |
10-4 to 10-3 |
