A poodle thruster is a low-power NTR, or Nuclear Thermal Rocket, which relies on sub-critical masses (or smaller critical masses which don't risk overrun) of more exotic radioisotopes to provide heat rather than on critical masses of more conventional fuels such as uranium. This allows it to operate for far longer without degrading its core neutronics. NASA's advanced propulsion concepts group defines a poodle thruster specifically as one powered by a Radioisotope Heater1 (not a radioisotope thermoelectric generator - RTG - which is used to produce electricity). In any case, a propellent (hydrogen, usually, but anything would work, even water) is passed over the reacting core, heated by convection (and hence given energy) and then ejected from the rear of the rocket. This produces thrust.

Poodle thrusters are desirable for low-power, long-term applications such as long-term maneuvering of smaller craft. This is due to their fairly high Specific Impulse (Isp) ratings, which reflect the fact that the energy expended through their use derives from a fixed resource - the reactor core - rather than the expenditure of a consumable. While a propellent is required to accelerate, it is possible to engineer these engines to transfer large amounts of thermal energy to small quantities of propellent in order to achieve thrust - and since Isp considers the amount of propellent expended, they come off quite well. Although they become less efficient as the velocity of the ejecta goes up, the 'loss' is that of energy due to nuclear decay, which can be considered a 'fixed' resource.

The phrase poodle thruster derives from the original project which designed the first such engine - it was codenamed 'PROJECT POODLE' in reference to a concurrent project to build a much larger NTR named 'PROJECT ROVER'. According to a report2 from the Department of Energy, the original 'POODLE' project tested Polonium-210 "radioisotope heaters."

They should not be confused with other low-power nuclear-electric propulsion systems such as a RTG-powered, or other nuclear generator-powered ion drive.

1 http://sec353.jpl.nasa.gov/apc/Introduction/06.html
2 Report MLM-1261, U.S. DOE, 1965 (declassified 1971).