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The '''radioisotope rocket''' is a type of [[rocket engine]] that uses the heat generated by the decay of [[radioactive]] elements to heat a [[working fluid]], which is then exhausted through a rocket nozzle to produce [[thrust]]. They are similar in nature to the [[nuclear thermal rocket]]s such as [[NERVA]], but are considerably simpler and often have no moving parts.
The basic idea is a development of existing [[radioisotope thermoelectric generator]], or RTG, systems, in which the heat generated by decaying nuclear fuel is used to generate power. In the rocket application the generator is removed, and the working fluid is instead used to produce thrust directly. Temperatures of about 1500 to 2000 °C are possible in this system, allowing for [[specific impulse]]s of about 700 to 800 seconds (7 to 8 kN·s/kg), about double that of the best chemical engines such as the [[LH2]]-[[LOX]] [[Space Shuttle Main Engine]].
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
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 Inc.|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).
The inadvertent construction of a radioisotope rocket is one of the suggested solutions to the [[Pioneer anomaly]]. The Pioneer space probes are powered by [[radioisotope thermal generator]]s located on the end of a long arm to keep their radiation away from the spacecraft electronics. In this position the back of the main radio dish is preferentially exposed, meaning that radiation (primarily infra-red) scattered by the dish would tend to be scattered to the rear of the spacecraft. This could lead to a ''tiny'' amount of net thrust being generated, although all calculations to date suggest it is not enough in itself to fully explain the effect.{{Citation needed|date=November 2008}}
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== References ==
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