Difference between revisions of "Reaction engine"

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[[File:LV-T30 Liquid Fuel Engine.jpg|right|thumb|[[LV-T30 Liquid Fuel Engine]] before [[0.18]]]]
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[[File:LV-T30 Liquid Fuel Engine recent.png|right|thumb|200px|[[LV-T30 Liquid Fuel Engine]]]]
A '''reaction engine''' is an [[engine]] that works via “equal and opposite <u>reaction</u>” as in [[w:Newton%27s_laws_of_motion#Newton.27s_third_law|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.
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A '''reaction engine''' is an [[engine]] that works via “equal and opposite reaction” as in [[w:Newton%27s_laws_of_motion#Newton.27s_third_law|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.
  
 
== Types ==
 
== Types ==
 
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.  
 
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 [[intake air|air]]-breathing '''jet engines''' which can only operate inside an [[oxygen]]-rich [[atmosphere]]. All{{check version||0.90.0}} use the same [[resources]] and are controlled the same way.
+
Staying relevant to KSP, we separate out [[intake air|air]]-breathing '''jet engines''' which can only operate inside an [[oxygen]]-rich [[atmosphere]] from those able to operate in space. All{{check version||1.12.3}} use the same [[resources]] and are controlled the same way.
  
All{{check version||0.90.0}} 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.  
+
All{{check version||1.12.3}} 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.
 
Listed below are general descriptions of jet engines and several broad types of rocket engines along with their advantages and disadvantages.
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----
 
----
 
{{main article|Jet engine}}
 
{{main article|Jet engine}}
 +
{| class="wikitable floatright"
 +
|-
 +
! Advantages !! Disadvantages
 +
|-
 +
| 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 0.625m, 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 intake]]s 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.
 
Jet engines use the same fuel as rocket engines, but unlike them jets draw oxygen from the atmosphere using [[air intake]]s 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.
 
==== Advantages ====
 
* Excellent fuel efficiency within an oxygenated atmosphere
 
* Excellent power to weight ratio
 
* All current{{check version||0.90.0}} jet engines provide thrust vectoring for greater maneuverability
 
 
==== Disadvantages ====
 
* Cannot be used outside of an oxygenated atmosphere.
 
* Thrust output changes depending on speed
 
* Efficiency changes depending on altitude
 
* Engine requires time to “spool up” to maximum thrust potential
 
  
 
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}}
 
}}
 
{{FlipBox
 
{{FlipBox
|title=Available "jet fuel" tanks
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|title=Available jet fuel tanks
 
|content={{Stats Table Fuselage}}
 
|content={{Stats Table Fuselage}}
 
}}
 
}}
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----
 
----
 
{{Main article|Solid rocket booster}}
 
{{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.
 
  
==== Advantages ====
+
{| class="wikitable floatright"
* Very high [[thrust-to-weight ratio]]
+
|-
* Engine and fuel tank are combined in one part, lowering part count and simplifying design
+
! Advantages !! Disadvantages
 +
|-
 +
| 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
 +
|}
  
==== Disadvantages ====
+
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.
* 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
 
  
 
{{FlipBox
 
{{FlipBox
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=== Liquid fuel rocket engines ===
 
=== Liquid fuel rocket engines ===
----
 
Liquid fuel engines utilize a mixture of [[liquid fuel]] and liquid [[oxidizer]] in a 9:11 ratio — generally called “rocket fuel”. The lone{{check version||0.90.0}} exception is the [[O-10 MonoPropellant Engine]] which uses [[monopropellant]].In the real world, typical liquid fuels are liquid hydrogen or kerosene, and typical oxidizers are liquid oxygen or nitrous-oxide.
 
  
All liquid fuel rocket engines can be staged and respond to [[throttle]] controls. All engines use [[w:Bell nozzle|Bell nozzles]] except the [[Toroidal Aerospike Rocket]].
+
{| class="wikitable floatright"
 +
|-
 +
! Advantages !! Disadvantages
 +
|-
 +
| 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 || Separation of engine and fuel leads to increased part count |
 +
|-
 +
| Available in a wide range of shapes, sizes, and power levels || |
 +
|-
 +
| Frequently [[gimbals|gimballed]] to allow flight control || |
 +
|-
 +
| Immediate throttle response || |
 +
|}
  
The [[LV-N Atomic Rocket Motor]] was inspired by real-world nuclear thermal rockets, such as the [[w:NERVA|NERVA]]. In these, the propellant (typically liquid hydrogen) is heated by a nuclear reactor, rather than being combusted with an oxidizer, expanding into a high velocity jet of gaseous hydrogen. However, in order to reduce development effort and to simplify gameplay, [[Squad]] chose to have the LV-N to use the same rocket fuel mixture as other 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{{check version||1.0.4}} 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.  
  
==== Advantages ====
+
All liquid fuel rocket engines can be staged and respond to [[throttle]] controls. All engines use [[w:Bell nozzle|Bell nozzles]] except the [[Toroidal Aerospike Rocket]].
* 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 [[Gimbal|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 refuelled from another vessel during missions
 
* Engines and fuel need not be mounted in the same location on the ship, expanding design possibilities
 
 
 
==== Disadvantages ====
 
* 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 [[LFB KR-1x2|KR-1x2]].
 
  
 
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=== RCS thrusters ===
 
=== RCS thrusters ===
----
 
RCS thrusters make up the [[Reaction Control System]] designed primarily for translation maneuvers, especially useful during [[docking]]. They cannot be [[throttle]]d 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.
 
  
==== Advantages ====
+
{| class="wikitable floatright"
* [[Monopropellant]] is automatically distributed throughout a craft, so neither crossfeeding or [[fuel line]]s have to be set up
+
|-
* Only engines that respond to translation controls
+
! Advantages !! Disadvantages
* Provide additional thrust for [[SAS]] to stabilize a craft
+
|-
 +
| [[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 [[throttle]]d 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.
  
==== Disadvantages ====
+
In addition to separate thruster parts, the [[Mk1-3 Command Pod]] has its own built-in RCS thrusters, with the same power and I<sub>sp</sub> profile as the RC-105 RCS Thruster Block.
* Very low total thrust and thrust-to-weight ratio — too weak to escape from most [[celestial body|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, while the SAS can run them on different thrust levels in between
 
  
 
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=== Ion engines ===
 
=== Ion engines ===
----
+
 
 +
{| class="wikitable floatright"
 +
|-
 +
! Advantages !! Disadvantages
 +
|-
 +
| 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.
 
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.
 
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.
 
==== Advantages ====
 
* Extremely high efficiency
 
* Cool blue glow
 
==== Disadvantages ====
 
* Extremely low thrust; inefficient for Hohmann transfers
 
* High electric consumption
 
* No thrust vectoring currently{{check version||1.0.2}} available
 
* Propellant containers have poor mass ratio
 
  
 
{{Wikipedia|Hall effect thruster}}
 
{{Wikipedia|Hall effect thruster}}

Latest revision as of 10:46, 30 November 2022

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.

Types

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.

Jet engines


→ Main article: Jet engine
Advantages Disadvantages
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 0.625m, 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.

[Expand] Available jet engines
[Expand] Available jet fuel tanks

Solid fuel rocket engines


→ Main article: Solid rocket booster
Advantages Disadvantages
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.

[Expand] Available solid fuel rocket engines

Liquid fuel rocket engines

Advantages Disadvantages
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[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). 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.

All liquid fuel rocket engines can be staged and respond to throttle controls. All engines use Bell nozzles except the Toroidal Aerospike Rocket.

[Expand] Available liquid fuel rocket engines
[Expand] Available liquid fuel tanks

RCS thrusters

Advantages Disadvantages
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.

In addition to separate thruster parts, the Mk1-3 Command Pod has its own built-in RCS thrusters, with the same power and Isp profile as the RC-105 RCS Thruster Block.

[Expand] Available RCS engines
[Expand] Available monopropellant tanks

Ion engines

Advantages Disadvantages
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

[Expand] Available ion engines
[Expand] Available xenon tanks

See also