Teflon has been the historical standard fuel for ablative PPTs due to a high Isp, high impulse bit, and little surface charring compared with other propellants. Minor improvements to the efficiency and mass bit were found with Teflon variants and other plastic fuel cells, however, no published results with non-plastic propellants have been found. Flight PPTs typically ablate 1-2μg/J of fuel per discharge, though some laboratory experiments have measured higher; such as the 4.8μg/J measurement from the LES-6 PPT. In addition to ablative PPTs, substantial research has been invested into gas-fed versions. Higher thrust efficiencies and mass bits have been measured with gas-fed PPTs in the laboratory; however, the added complexity, mass, and size have prevented a gas-fed PPT from being flown. The use of a heavier fuel will increase a thrusters specific thrust at the expense of lower exhaust velocities. At constant energy (E=0.5mv^2), a doubling of the propellant mass will result in a reduction to the velocity by a factor of sqrt(2), resulting in a factor of 2 thrust (T=mv/t) increase. The optimum exhaust velocity is found by maximizing the energy efficiency and minimizing the energy required to produce the rocket exhaust. This occurs when the exhaust velocity matches the spacecraft’s speed, that is, when the exhaust is made to be at rest when viewed from a stationary reference frame. For this reason a variable speed PPT, (possible through variations in the propellant) would allow increased energy efficiency. If a mission’s power and time constraints are not critical, a lighter propellant is ideal as it will result in a higher ΔV for a given fuel mass. For missions which have time and power constraints, a heavier fuel with a lower mass efficiency is required.
The Micro Pulsed Inductive Thruster (μPIT) combines elements of pulsed plasma thrusters and inductive discharges to form a new alternative to the current generation of micropropulsion technologies. Beginning with an ablative arc as in standard PPTs, thruster operation continues as an RF antenna couples additional energy into the arc plasma and associated neutral gas. μPIT employs coaxial electrodes and solid Teflon fuel. The total system mass, including the power supply, is less than 500 grams. The RF antenna is proposed as a mechanism for increasing PPT efficiency by improving overall mass utilization.