Pulsed plasma thrusters are a method of spacecraft propulsion which use an arc of electric current adjacent to a solid propellant, to produce a quick and repeatable burst of impulse. PPTs are great for attitude control, and for main propulsion on particularly small spacecraft with a surplus of electricity (those in the hundred-kilogram or less category). However they are also one of the least efficient electric propulsion systems, with a thrust efficiency of less than 10%. PPTs have much higher exhaust velocity than chemical propulsion engines. According to the Tsiolkovsky equation this results in proportionally higher final velocity of propelled craft. The principle of operation is acceleration of ions in a strong electric field to the velocities of the order of hundreds km/s - which is much higher than the thermal velocity of chemical engines. Chemical propulsion engines with their limited by rate of chemical reaction exhaust velocity (which is in the range of 2-3 km/s) become exponentially ineffective (see Tsiolkovsky equation) to achieve high interplanetary speeds (which in Solar system is in 20-70 km/s range). Pulsed plasma thrusters were the first electric propulsion system to be deployed in space, on the Soviet probes Zond 2, propelling the vehicle from parking at Earth orbit to Mars in Nov 30, 1964, and Zond 3 in 1965. The active gas used in the Soviet plasma propulsion engines was argon and helium. Used as an experimental system for spacecraft orientation control, Soviet engineers subsequently returned to the use of high-pressure nitrogen jets. Pulsed plasma thrusters were flown in November, 2000 as a flight experiment on the NASA Goddard Space Flight Center EO-1 spacecraft. The thrusters successfully demonstrated the ability to perform roll control on the spacecraft and also demonstrated that the electromagnetic interference from the pulsed plasma did not affect other spacecraft systems. These experiments used Teflon as the propellant.
