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. For our purposes, the reaction mass propelled outward is always a form of fuel, though not always one modelled on chemical reactions.
- 1 Types
- 2 See also
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 All[outdated] use the same resources and are controlled the same way.
All[outdated] KSP's other reaction engines can operate using only resources stored or generated onboard and are collectively called rocket engines.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
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.
- Excellent fuel efficiency
- Excellent power to weight ratio
- All current[outdated] jet engines provide thrust vectoring for greater maneuverability
- Cannot be used outside of an oxygenated atmosphere.
- Thrust output changes depending on speed
- Efficiency changes depending on altitude
- Requires time to “spool up” thrust, lagging behind throttle setting
- Engines available only in 1.25m/Size 1 and 2.5/Size 2
Solid fuel rocket engines
- → Main article: Solid rocket booster
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.
- Very high thrust-to-weight ratio, especially in-atmosphere (vs. high-efficiency vacuum-optimized liquid-fuel engines)
- Very cheap
- 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 transfer fuel stored elsewhere on the craft
- No thrust vectoring
- Low efficiency compared to other types of engines
Liquid fuel rocket engines
Most liquid fuel engines utilize a mixture of liquid fuel and liquid oxidizer in a 9:11 ratio — generally called “rocket fuel”. The exceptions[outdated] 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). 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.
- Work both in vacuum and in atmosphere
- 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
- Full thrust output on demand without “spool up” time like jets
- Some have gimbals giving thrust vectoring to help steer the craft
- Available in a wide range of thrusts and efficiencies, all using the same fuel supply
- Fuel and oxidizer can be moved between tanks or refueled from another vessel during missions
- Engines and fuel need not be mounted in the same location on the ship, expanding design possibilities
- Lower thrust-to-weight ratio compared to solid rocket engines
- Less efficient in atmosphere than jet engines
- Less efficient in space than ion engines
- Separation of engine and fuel leads to increased part count for all except the KR-1x2.
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 is automatically distributed throughout a craft, so neither crossfeeding or fuel lines have to be set up
- Only engines that respond to translation controls
- Provide additional thrust for SAS to stabilize a craft
- Very low total thrust and thrust-to-weight ratio — too weak to escape from most celestial bodies.
- Low efficiency
- No thrust vectoring (however, the most commonly used thruster, the RV-105 RCS Thruster Block, can thrust in 4 directions)
- The user can only switch engines on or off (unless precision control is enabled with the caps lock key), while the SAS can run them on different thrust levels in between
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.
- Extremely high efficiency
- Cool blue glow
- Extremely low thrust; inefficient for Hohmann transfers
- High electric consumption
- No thrust vectoring currently[outdated] available
- Xenon containers have poor mass ratio (2.5-2.27, compared to 8-9 for rocket fuel)
Hall effect thruster on Wikipedia