Editing Radioisotope rocket

{{Refimprove|date=September 2010}}
A '''radioisotope rocket''' or '''radioisotope thermal rocket''' is a type of [[Thermal rocket|thermal]] [[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 [[Nuclear thermal rocket|nuclear thermal rockets]] such as [[NERVA]], but are considerably simpler and often have no moving parts. Alternatively, radioisotopes may be used in a '''radioisotope electric rocket''',<ref>{{cite journal |last1=Schmidt |first1=George R. |last2=Manzella |first2=David H. |last3=Kamhawi |first3=Hani |last4=Kremic |first4=Tibor |last5=Oleson |first5=Steven R. |last6=Dankanich |first6=John W. |last7=Dudzinski |first7=Leonard A. |title=Radioisotope electric propulsion (REP): A near-term approach to nuclear propulsion |journal=Acta Astronautica |date=1 February 2010 |volume=66 |issue=3 |pages=501–507 |doi=10.1016/j.actaastro.2009.07.006 |bibcode=2010AcAau..66..501S |hdl=2060/20110016114 |hdl-access=free }}</ref> in which energy from nuclear decay is used to generate the electricity used to power an [[Spacecraft electric propulsion|electric propulsion system]].

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 {{cvt|1500|to|2000|C|F|-2}} are possible in this system, allowing for [[specific impulse]]s of about 700 to 800&nbsp;seconds (7 to 8&nbsp;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&nbsp;kW. This means that the design, while highly efficient, can produce thrust levels of perhaps {{cvt|1.3|to|1.5|N|lbf}}, 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-210]], as opposed to the typical RTG which would use a long half-life fuel such as [[plutonium-238]] in order to produce more constant power over longer periods of time.<ref>[http://pdf.aiaa.org/preview/CDReadyMSPACE06_1393/PV2006_7272.pdf AIAA meeting paper comparing fermium, polonium and plutonium as power sources]{{dead link|date=November 2017 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>

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.

==Technology development==

[[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&nbsp;hours at about {{cvt|1500|C|F|-2}}, producing a specific impulse of 650 to 700&nbsp;seconds (6.5 to 7&nbsp;kN·s/kg).

==Photon pressure==
{{see also|Nuclear photonic rocket}}
Even without an exhaust, the [[Radiation pressure|photon pressure]] of the energy emitted by a thermal source can produce thrust, although an extremely tiny amount. A famous example of spacecraft thrust due to photon pressure was the [[Pioneer anomaly]], in which photons from the onboard radioisotope source caused a tiny but measurable acceleration of the Pioneer spacecraft.

A similar phenomenon occurred on the [[New Horizons]] spacecraft; photons (thermal infrared) from the RTG, reflected from the spacecraft's antenna, produced a very small thrust which propelled the spacecraft slightly off course.<ref>[http://pluto.jhuapl.edu/News-Center/PI-Perspectives.php?page=piPerspective_05_21_2010 New Horizons official website article mentioning the thrust from the RTG]</ref>

== See also ==

* {{annotated link|Nuclear thermal rocket}}
* {{annotated link|Radioisotope heater unit}}
* {{annotated link|Spacecraft propulsion}}
* {{annotated link|Thermal rocket}}

== References ==
{{reflist}}

==External links==
*[http://pdf.aiaa.org/getfile.cfm?urlX=75%26%5D0%3BU%2BDN%26S7S%20%27MWT%5BBQ%3A%2B94%5B8%20%5F%5F%3C%20%0A&urla=%25%2B2L%25%22PH%20%0A&urlb=%21%2A%20%20%20%0A&urlc=%21%2A0%20%20%0A&urld=%28%2A%22D%26%23%20JGUA4%20%0A&urle=%27%282P%24%21P6JU0%20%20%0A AIAA meeting paper study comparing Poodle thrusters to a chemical (hydrogen/fluorine) option for upper stage propulsion]{{dead link|date=January 2018 |bot=InternetArchiveBot |fix-attempted=yes }}
*United States Patent 3315471; Direct cycle radioisotope rocket engine; 1967; Lee, Dailey Charles, Verdes, Estates Palos
*United States Patent 3306045; Radioisotope rocket; 1967; Buford Jr., William H. Thomas Jr., Arthur N

{{spacecraft propulsion}}
{{nuclear propulsion}}

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[[Category:Nuclear spacecraft propulsion]]