View source for Reaction engine

[[Image:LV-T30_Liquid_Fuel_Engine.jpg|right|thumb|[[LV-T30 Liquid Fuel Engine]]]]

A '''reaction engine''' is an [[engine]] that uses nothing but its onboard propellant to generate thrust. Unlike traditional air breathing [[jet engine]]s rocket engines are able to function where no oxygen is present. If there is also a combustion it is called a '''rocket engine'''.

== Physics ==
All reaction engine work with the same principle: They propel mass out of nozzles and accelerate because of Newton's third law of motion. The difference to jet engines is that it doesn't need a surrounding atmosphere. Also it doesn't need any surface which allows them to be used everywhere.

There are four different types available in the game which all use this basic principle. Apart from RCS thrusters their controls are the same. Except for the ion engine all engines are rocket engines as they burn up fuel instead of simply accelerating it. The RCS from the [[EVA]] suit is most likely a reaction engine which expels nitrogen gas without combusting it.

== Solid fuel rocket engines ==
:→ Main article: [[Solid rocket booster]]
The most ancient type is a [[solid fuel]] rocket which is as simple as igniting a solid, self-oxidizing compound or mixture within the casing of the rocket and allowing the escaping gases to be released through a nozzle at the rear.

=== Advantages ===

* Have a very high thrust to weight ratio.
* Engine and fuel tank are combined in one part, lowering part count, simplifying design and improving stability, which is especially important in the early stages of a rocket launch from the surface of a planet where large ammounts of more or less heavy parts and high-powered rockets cause stress both to the vessel itself and the player's computer.

=== Disadvantages ===

* Monolithic design, must be built in one piece.
* Cannot be throttled or switched off after ignition.
* Once spent, they are unable to use fuel from another source of the craft.

{{FlipBox
|title=Available solid fuel rocket engines
|content={{Stats Table Solid Fuel Boosters}}
}}

== Liquid fuel rocket engines ==
Liquid fuel engines are powered by a liquid propellant which is comprised of combustible [[liquid fuel]] and [[oxidizer]]. In the real world typical liquid fuels are often liquid hydrogen or kerosene, and typical oxidizers are liquid oxygen or nitrous-oxide. All engines use [[w:Bell nozzle|Bell nozzles]] except the only [[w:Aerospike engine|aerospike engine]].

The [[LV-N Atomic Rocket Engine]] is treated by the game as a liquid fuel rocket actually would be in it's own category and not a rocket engine.

=== Advantages ===

* Fuel and oxidizer can be moved between liquid fuel tanks, which means that spacecraft with liquid fuel engines and fuel tanks are easier and more resource-efficient to refuel than vessels with solid fuel engines, which required the entire engine bay to be replaced, while liquid fuel and oxidizer can quickly be moved between two docked vessels.
* Modular design: fuel and other components can be stored elsewhere in the craft.
* Throttle can be adjusted depending on thrust requirements.
* Can be switched off and re-ignited at will.
* Many nozzles can be gimballed for thrust vectoring.
* Efficiency largely dependent on design, with large variations. For early stages, high-output engines with low fuel-efficiency can be used to reach a low Kerbin orbit, while extremely efficient but low-power engines should be used for travelling between planets and moons to save fuel and extend the range of the craft.

=== Disadvantages ===

* Lower power to weight ratio compared to solid rocket engines.
* Complex design with fuel tanks and other additional components.  
* More points of possible failure such as if the parts tear apart from stress.

{{FlipBox
|title=Available liquid fuel rocket engines
|content={{Stats Table Liquid Fuel Engines}}
}}
{{FlipBox
|title=Available liquid fuel tanks
|content={{Stats Table Liquid Fuel Tanks}}
}}

== Ion engines ==
A ion engine uses ionized atoms, [[xenon gas]], and accelerate them in an electro-magnetic field and technically works different than the other rocket engines. This type of engine is very efficient but has a very low thrust making it only suitable for probes already in space.

=== Advantages ===
* A very high specific impulse.
* The [[xenon gas]] can be moved between tanks, which means that spacecraft with ion engines and tanks are easier and more resource-efficient to refuel than vessels with solid fuel engines, which required the entire engine bay to be replaced, while xenon gas can quickly be moved between two docked vessels.
* The fuel doesn't need to be moved as xenon gas is used evenly.
* Modular design: fuel and other components can be stored elsewhere in the craft.
* Throttle can be adjusted depending on thrust requirements.
* Can be switched off and re-ignited at will.
=== Disadvantages ===
* Very low thrust so it is impossible to lift off from the [[launch pad]] with ion engines only.
* No thrust vectoring is available.

{{FlipBox
|title=Available ion engines
|content={{Stats Table Xenon Engines}}
}}

== RCS thrusters ==
The [[RCS]] thrusters are allowing an craft to change the attitude, because their thrust is low allowing tiny changes in velocity. Usually it isn't used as a main engine because the specific impulse is lower than any other liquid fuel engine.

=== Advantages ===
* The [[monopropellant]] can be moved between tanks, which means that spacecraft with RCS thrusters and tanks are easier and more resource-efficient to refuel than vessels with solid fuel engines, which required the entire engine bay to be replaced, while monopropellant can quickly be moved between two docked vessels.
* The fuel doesn't need to be moved as monopropellant is used evenly.
* Modular design: fuel and other components can be stored elsewhere in the craft.
* Can be switched off and re-ignited at will.

=== Disadvantages ===
* Very low thrust so it is impossible to lift off from the launch pad with RCS thrusters only.
* No thrust vectoring is available.
* The user can only switch engines on or off, while the SAS can run them on different thrust levels in between.
* The specific impulse is, especially in atmosphere, low and comparable with solid rocket engines.

{{FlipBox
|title=Available RCS engines
|content={{Stats Table RCS Thrusters}}
}}

== See also ==
* {{Wikipedia|Reaction engine}}
* {{Wikipedia|Rocket engine}}

{{Parts}}
[[Category:Engine]]