A reaction engine is an engine that generates thrust by expelling reaction mass in accordance with Newton's third law. Their classification differs by sources a bit, but all contradistinguish the air dependent jet engines from the others, independently from the usage of the oxygen of the atmosphere or not. The rest of them use on-board propellant for expelling reaction mass, making them independent from the atmosphere - these are the widely defined “rocket engines”. The propellant can be expelled by outer energy source like the ion engines (electricity) or nuclear-thermal rocket motors (nuclear fission) do, or the expelled propellant is the source of that energy. This energy can derive from physical state of the propellant gas (cold rockets), or the chemical reactions of the propellant cover the energy need of the acceleration of the projected gas (these are the classical chemical rocket engines). There are many types of the propellants ensuring energy for these chemical rocket engines, but in the Kerbal Space Program the variety of the propellants are restricted to solid fuel, liquid fuel - oxidizer mix in 9:11 ratio, and the monopropellant.
All reaction engine work with the same principle: mass is propelled out of the nozzle which accelerates the vehicle due to Newton's third law of motion. Every engine in the game uses this basic principle, differing in the environments in which they will function (jet engines require an oxygen-rich atmosphere), the types of fuel they consume (ion engines require the vehicle to mount xenon tanks), and the game controls which activate them (RCS thrusters respond to directional controls, all others respond to the throttle).
Solid fuel rocket engines
- → Main article: Solid rocket booster
The most basic type is the solid fuel rocket, which is simply a solid, self-oxidizing compound or mixture within a casing with a nozzle at the rear to allow the gases produced to escape.
- Very high thrust-to-weight ratio.
- Engine and fuel tank are combined in one part, lowering part count and simplifying design.
- Cannot be throttled or switched off after ignition.
- Cannot be refueled or use fuel stored elsewhere on the craft.
- Low efficiency compared to other types of engines.
Liquid fuel rocket engines
Liquid fuel engines are powered by liquid fuel ignited with liquid oxidizer, except of the O-10 MonoPropellant Engine. In the real world, typical liquid fuels are liquid hydrogen or kerosene, and typical oxidizers are liquid oxygen or nitrous-oxide. All engines use Bell nozzles except the aerospike engine.
The LV-N Atomic Rocket Motor was inspired by real-world nuclear thermal rockets, such as the NERVA. Nuclear thermal rockets use a different propulsion model; the propellant (typically liquid hydrogen) is heated in a nuclear reactor rather than being combined with an oxidizer. However, in order to reduce development effort and to simplify gameplay, Squad chose to permit the LV-N to use the same fuel combination as other in-game liquid-fuel engines.
- Fuel and oxidizer can be moved between tanks. This opens many possibilities for fuel management and logistics, including replenishing the fuel supply of a craft already in flight.
- Engines and fuel need not be mounted in the same location on the ship, expanding design possibilities.
- Variable throttle allows different levels of thrust at different times, or for the engines to be shut down entirely and restarted later in the flight.
- Some liquid engines have gimbals which allows them to help steer the craft.
- Available in a wide range of thrusts and efficiencies, all of which may share fuel with each other.
- Lower thrust-to-weight ratio compared to solid rocket engines.
- Separation of engine and fuel leads to increased part count for all except the KR-1x2.
RCS thrusters use their own fuel, monopropellant (except of the liquid fuel - oxidizer consuming Vernor Engine), and do not require a separate oxidizer. RCS is primarily used for steering a craft or making very small positional adjustments during docking; it is too weak and inefficient to function as a main maneuvering engine.
- Monopropellant is automatically distributed throughout a vehicle and does not require crossfeeding to be manually set up.
- Very low thrust. The available RCS thrusters are too weak to lift off from most celestial bodies.
- No thrust vectoring is available (however, the most commonly use thruster, the RV-105 RCS Thruster Block, can thrust in 4 directions).
- The user can only switch engines on or off, while the SAS can run them on different thrust levels in between.
- Low efficiency.
An ion engine uses electricity to ionize atoms of xenon gas and accelerate them in an electrostatic or electromagnetic field to propel them as exhaust. Remarkably little xenon gas fuel is needed, but ion engines are very demanding on electrical generation and storage.
- Extremely high efficiency, which is unaffected by atmosphere.
- Extremely low thrust. Ion engines cannot be used to lift off from most celestial bodies, and most maneuvers will take tens of minutes to complete.
- High electric consumption.
- No thrust vectoring currently available.
- → Main article: Jet engine
Unlike rocket engines, jets propels the intake air (mainly), and draws oxygen from the atmosphere rather than taking it from an on-board tank. This is represented in-game by a much lower rate of fuel consumption. As they do so the classical rocket equation doesn't valid for them, and also they depends on the properly dense atmosphere consisting the vital oxygen.
- Provides excellent fuel efficiency within an atmosphere
- All current jet engines provide thrust vectoring for greater maneuverability
- Excellent power to weight ratio
- Cannot be used outside of an atmosphere that contains oxygen. In current version, it means they only function on Kerbin and Laythe.
- Thrust output changes depending on speed
- Engine requires time to spool up to maximum thrust potential