* Several other types of advanced rocket engines are now in laboratory development, but have never been flown on a space mission. They include the "magnetoplasmodynamic" drive; the "pulsed inductive" drive; and the "variable specific impulse magnetoplasmodynamic rocket (VASIMR)" drive.
In a magnetoplasmodynamic drive, sometimes also called a "Lorenz force accelerator", the propellant is accelerated by magnetic, rather than electric, fields. The engine consists of a thrust chamber with walls that act as an anode and a central rod that acts as a cathode. The propellant, which can be argon, lithium, or hydrogen, in increasing order of efficiency, is ionized, causing a very strong current to flow radially between the anode wall and the central cathode. While the current flow is provided by equal numbers of electrons and positively-charged ions, the ions are much heavier than the electrons and so move much more slowly.
A moving current sets up a magnetic field acting at right angles whose magnitude is proportional to rate of current flow, and so the fast-moving electron current sets up a magnetic field directed in concentric rings around the cathode. A magnetic field in turn accelerates charged particles at right angles to itself, and so the positive ions are driven out the exhaust nozzle.
* Conceptually, a pulsed inductive drive looks like a flat hoop coil with a fat spike in the center, with the coil connected to a bank of big capacitors. A puff of propellant, usually argon though many other propellants are possible, is injected inside the hoop, and then the capacitors are discharged into the coil. This sets up an intense magnetic field that ionizes the propellant, and the electric fields that are set up push the ions out the hoop. Specific impulse should be ten to twenty times that of LOX-RP propulsion.
Pulsed inductive drives do not have electrodes, which tend to be worn down by ion and electron bombardment, and engine thrust can be scaled up by increasing the pulse rate, which is on the order of several hundred times a second. TRW has been working on the concept using company funds, but no pulsed inductive drive has been flown in space yet.
* The VASIMR drive is one of the most exotic of the advanced propulsion concepts. It is the brainchild of Franklin Chang-Diaz, a Costa Rican-born American plasma physicist who was also a NASA space shuttle astronaut, performing seven space flights -- a record for number of flights at the time of his retirement from the agency. It uses hydrogen or argon as a propellant, first ionizing it with radio-frequency (RF) energy, and then injecting it into a thrust chamber where oscillating magnetic fields and RF energy heat it to millions of degrees Celsius. A magnetic choke controls the flow of the hot plasma to the exhaust nozzle.
If the magnetic choke is constricted, the flow of plasma is small, but the temperature remains high. This gives low thrust but extremely high efficiency, possibly a hundred times that of LOX-RP propulsion, useful for interplanetary cruise. If the magnetic choke is opened up, the flow of plasma is high, but the temperature is low. This gives high thrust and lower efficiency, about ten times that of LOX-RP propulsion, useful for initial boost out of planetary orbit.
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