Editing Rocket engine (section)

===Energy efficiency===
{{Further|Rocket#Energy efficiency}}
[[File:Rocket propulsion efficiency.svg|thumb|Rocket vehicle mechanical efficiency as a function of vehicle instantaneous speed divided by effective exhaust speed. These percentages need to be multiplied by internal engine efficiency to get overall efficiency.]]
Rocket engine nozzles are surprisingly efficient [[heat engines]] for generating a high speed jet, as a consequence of the high combustion temperature and high [[compression ratio]]. Rocket nozzles give an excellent approximation to [[adiabatic expansion]] which is a reversible process, and hence they give efficiencies which are very close to that of the [[Carnot cycle]]. Given the temperatures reached, over 60% efficiency can be achieved with chemical rockets.

For a ''vehicle'' employing a rocket engine the energetic efficiency is very good if the vehicle speed approaches or somewhat exceeds the exhaust velocity (relative to launch); but at low speeds the [[Propulsive efficiency|energy efficiency]] goes to 0% at zero speed (as with all [[jet propulsion]]). <!--- it's very counterintuitive, a way to look at it is that energy= force x distance, but at zero speed you have no movement and you lose lots of energy in the jet --->See [[Rocket#Energy efficiency|Rocket energy efficiency]] for more details.
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