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However the amount of power generated by such systems is typically fairly low. Whereas the full "active" reactor system in a nuclear thermal rocket can be expected to generate over a gigawatt, a radioisotope generator might get 5 kW. This means that the design, while highly efficient, can produce thrust levels of perhaps 1.3 to 1.5 N, making them useful only for thrusters. In order to increase the power for medium-duration missions, engines would typically use fuels with a short [[half-life]] such as [[polonium|Po 210]], as opposed to the typical RTG which would use a long half-life fuel such as [[plutonium]] in order to produce more constant power over longer periods of time.
Another drawback to the use of radioisotopes in rockets is an inability to change the operating power. The radioisotope constantly generates heat that must be safely dissipated when it is not heating a propellant. Reactors, on the other hand, can be throttled or shut down as desired.
[[TRW]] maintained a fairly active development program known as '''Poodle''' from 1961 to 1965, and today the systems are still often known as '''Poodle thrusters'''. The name was a play on the larger systems being developed under '''Project Rover''', which led to NERVA. In April 1965 they ran their testbed engine for 65 hours at about 1500°C, producing a specific impulse of 650 to 700 seconds (6.5 to 7 kN·s/kg).
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