A reaction engine is an engine that works via “equal and opposite reaction” as in Newton's third law of motion. Specifically, they generate thrust by expelling reaction mass in the opposite direction as their acceleration. In Kerbal Space Program, the reaction mass propelled outward is always a form of fuel, though not always one modeled on chemical reactions.
Different sources classify kinds of reaction engine a bit differently, but all contradistinguish those which must collect outside material from those using only onboard material as reaction mass.
Staying relevant to KSP, we separate out air-breathing jet engines which can only operate inside an oxygen-rich atmosphere from those able to operate in space. All use the same resources and are controlled the same way.
All KSP's other reaction engines can operate using only resources stored or generated onboard and are collectively called rocket engines. There are many types of rocket engines. They differ by the kind of fuel they use and the controls used to operate them in-game.
Listed below are general descriptions of jet engines and several broad types of rocket engines along with their advantages and disadvantages.
- → Main article: Jet engine
|Excellent fuel efficiency||Efficiency changes with altitude|
|Excellent thrust-to-weight ratio||Thrust output changes with speed|
|Some engines provide thrust vectoring||Does not respond rapidly to throttle changes due to turbo lag|
|Can only be used with an oxygenated atmosphere|
|Engines available only in 1.25m and 2.5m|
Jet engines use the same fuel as rocket engines, but unlike them jets draw oxygen from the atmosphere using air intakes rather than carrying the weight of oxidizer onboard. This is represented in-game by a much lower rate of fuel consumption. The classical rocket equation doesn't hold valid for them. In flight they face the trade-off between the lower atmosphere where intake air is easily available at lower speeds but air resistance is greater, and the upper atmosphere where there's less air resistance but higher speeds are required to collect sufficient intake air.
Solid fuel rocket engines
- → Main article: Solid rocket booster
|High thrust-to-weight ratio, especially in atmosphere||Cannot be throttled or switched off after ignition|
|Engine and fuel tank are a single unit||Cannot refuel or transfer fuel stored elsewhere on the craft|
|Cheap||Low efficiency compared to other types of engines|
|No thrust vectoring|
The most basic type of rocket engine is the solid fuel rocket. Solid fuel is simply a self-oxidizing compound or mixture within a casing with a nozzle at the end to direct the exhaust gases produced. It offers no real control beyond choosing when to ignite it, though tweakables enable altering the thrust limit and total fuel.
Liquid fuel rocket engines
|Work both in vacuum and in atmosphere||Less efficient in atmosphere than jet engines, and less efficient in space than ion engines|
|Can be throttled to allow fine tuning of thrust||Lower thrust-to-weight ratio compared to solid rocket engines|
|Fuel and oxidizer can be moved between tanks or vessels|
|Available in a wide range of shapes, sizes, and power levels|
|Frequently gimballed to allow flight control|
|Immediate throttle response|
Most liquid fuel engines utilize a mixture of liquid fuel and liquid oxidizer in a 9:11 ratio — generally called “rocket fuel”. The exceptions are the O-10 MonoPropellant Engine which uses monopropellant and the LV-N "Nerv" Atomic Rocket Motor which only consumes liquid fuel (heating it in the core of a nuclear reactor rather than combusting it with oxidizer). The IX-6315 "Dawn" Electric Propulsion System uses xenon gas and electric charge to power it. This engine has a very low thrust, but is very efficient, with an Isp of over 1000 seconds. In the real world, typical liquid fuels are liquid hydrogen and a highly-refined kerosene blend called RP-1, and typical oxidizers are liquid oxygen and nitrous oxide.
|Monopropellant is automatically routed||No throttle; either switched on or off|
|Offers rotation and translation maneuverability||Low thrust and low thrust-to-weight ratio|
|Offers additional thrust for SAS when reaction wheels cannot provide enough torque||Low fuel efficiency|
RCS thrusters make up the Reaction Control System designed primarily for translation maneuvers, especially useful during docking. They cannot be throttled and are controlled with a separate set of keys from the pitch-yaw-roll keys. They use only monopropellant fuel, except for the Vernor Engine which uses rocket fuel. Monopropellant flows automatically from tanks mounted anywhere on a vessel to thrusters mounted anywhere on a vessel, so there is no need to set up fuel lines or mount thrusters directly on tanks if it is inconvenient or unsightly.
|Extremely high efficiency||Extremely low thrust; inefficient for orbital transfers|
|Cool blue glow||Uncool excessive electric consumption|
|No engine gimbal|
|Xenon containers have lower mass ratio than rocket fuel tanks|
An ion engine uses electric charge to ionize atoms of xenon gas and accelerate them in an electrostatic or electromagnetic field to propel them as exhaust. Remarkably little xenon gas is needed, and probes equipped with just a few tanks are considered able to operate forever, or until something inevitably goes wrong. However, ion engines are very demanding on electrical generation and storage and very slow to accelerate.
In real-life, they are often not considered true “rocket” engines. But given that they require no outside material or mass to operate, for ease and simplicity they are included with rocket engines on this wiki.
Hall effect thruster on Wikipedia