Pulsed Inductive Thruster (PIT)
According to a 1993 NASA document [1], prepared by TRW and having the identification NASA CR-191155: " The pulsed inductive thruster (PIT) is an electrode-less, magnetic rocket engine that can operate with any gaseous propellant. A puff of gas injected against the face of a flat (spiral) coil is ionized and ejected by the magnetic field of a fast rising current pulse from a capacitor bank discharge."
The 50 page report discusses the TRW PIT model mkVa. It is a unit with a 1 meter diameter coil and a 4,000 Joule capacitor bank, taking either ammonia or hydrazine as its working fluid. The report suggests that the design should scale well from the low kilowatt range to the low megawatt range. At 20KW, the design gains increasing advantages in mass savings with design lifetime as compared to ion engines. For example, with a design lifetime of 10,000 hours the two should have the same mass, while at 100,000 the PIT's mass should be 1/3.
The report states that there are two advantages of PIT as compared to other electric thruster designs then being researched. The first advantage is that the design is an electrode-less system, in which the hot plasma never comes in contact with the engine. This prevents engine corrosion and increases engine lifetime. (Since then, newer electrostatic ion engines have begun to incorporate this advantage as well by microwave or radio wave excitation of a plasma.) The second advantage is that the nature of the design makes it very forgiving about how the electric power is supplied since electric discharge into the plasma occurs through a bank of capacitors. Currently existing turbo-electric power supplies can supply the capacitors with electricity, as the capacitors don't care how they are charged. (This is an advantage that other forms of electric propulsion cannot duplicate.)
In comparing the mkVa to earlier models, the authors note that an important efficiency issue of the pulsed inductive thruster had been identified and corrected. It was discovered that the presence of inductance in the electric circuit external to the thruster had been the key element in prior engine inefficiency.
The report notes that due to an evolving understanding of future NASA mission requirements, the specific impulse (Isp) had been raised from the 1500 - 2000 second range (in the mark I) to the 4000 - 8000 second range in the current model. This is the Isp range that, according to the report, is ideal for the kind of interplanetary missions that NASA was beginning to specify for the future. (The increase in Isp translates as an increase in voltage and decrease in capacitance.)
The report also notes that, as a pulsed device, thrust and power output can be increased by increasing the pulse frequency. Thus the model tested could operate from tens of kilowatts to over a megawatt with output in Newton-seconds being directly proportional to input power.
Most of the body of the report details the physical and electric design of the tested model and the output of said model with the tested propellants at different power levels.
--Mweiss 28 November 2008 06:11 (EST)
