Pulsed plasma thrusters are a method of spacecraft propulsion also known as Plasma Jet Engines in general. They use an arc of electric current adjacent to a solid propellant, to produce a quick and repeatable burst of impulse. PPTs are excellent 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%. At present they are deployed in space vehicles and probes as space does not offer any frictional force when compared to that on earth. The extremely quick and repetitive thrust accelerates the space probe continuously. Thus it eventually reaches and goes beyond the speeds of conventional propulsion systems. The electrical energy required to operate the arc mechanism is abundantly available by harnessing the solar energy via self adjusting solar panels on the probe.
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 the electromagnetic acceleration of propellant via the Lorentz force to velocities of the order of tens of km/s - which is much higher than the thermal velocity of chemical engines. Chemical propulsion engines, with their limited rate of chemical reaction exhaust velocity (which is in the range of 2-4.5 km/s), become exponentially ineffective (see Tsiolkovsky equation) to achieve high interplanetary speeds (in the 20-70 km/s range, within the Solar System).
Pulsed plasma thrusters were the first electric thrusters to be deployed in space, used for attitude control on the Soviet probes Zond 2, from parking at Earth orbit to Mars on November 30, 1964, and Zond 3 in 1965. The active gases used in the Soviet plasma propulsion engines were argon and helium. 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 Earth Observing-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.
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