Reaction engine

LV-T30 Liquid Fuel Engine before 0.18

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.

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 All^[outdated] use the same resources and are controlled the same way.

All^[outdated] KSP's other reaction engines can operate using only resources stored or generated onboard and are collectively called rocket engines.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.

Jet engines

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→ 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.

Advantages

• Excellent fuel efficiency
• Excellent power to weight ratio
• All current^[outdated] jet engines provide thrust vectoring for greater maneuverability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
• Requires time to “spool up” thrust, lagging behind throttle setting
• Engines available only in 1.25m/Size 1^[outdated]

Available jet engines

Image	Part	Radial size	Cost ()	Mass (t)	Max. Temp. (K)	Tolerance (m/s)	Tolerance (g)	Thrust (kN)	TWR	Fuel (/s)	Intake (/s)	Isp (s)	TVC (°)	Reverse
	J-20 "Juno" Basic Jet Engine	Tiny	450	0.25	2 000	7	50	20.0 Mach 0 20.6 Mach 1.3	8.16 Mach 0 8.40 Mach 1.3	0.064	1.402	6 400	—	No
	J-33 "Wheesley" Turbofan Engine	Small	1 400	1.5	2 000	7	50	120.0 Mach 0	8.16 Mach 0	0.233	29.601	10 500	—	Yes
	J-404 "Panther" Afterburning Turbofan	Small	2 000	1.2	2 000	7	50	85.00 Mach 0 107.89 Mach 1.75 / 130.00 Mach 0 219.48 Mach 2.5	7.22 Mach 0 9.17 Mach 1.75 / 11.05 Mach 0 18.65 Mach 2.5	0.193 / 0.663	7.705 / 7.954	9 000 / 4 000	10.0	No
	J-X4 "Whiplash" Turbo Ramjet Engine	Small	2 250	1.8	2 000	7	50	130.00 Mach 0 386.66 Mach 3.0	7.36 Mach 0 21.90 Mach 3.0	0.663	5.303	4 000	1.0	No
	J-90 "Goliath" Turbofan Engine	Radial mounted	2 600	4.5	2 000	7	50	360.0 Mach 0	8.16 Mach 0	0.583	132.273	12 600	—	Yes
	CR-7 R.A.P.I.E.R. Engine	Small	6 000	2.0	2 000	20	50	105.00 Mach 0 465.64 Mach 3.75	5.35 Mach 0 23.74 Mach 3.75	0.669	4.015	3 200	3.0	No

Available "jet fuel" tanks

Liquid Fuel Density is 5 kg/unit				Mass (t)					Liquid Fuel ()
Image	Part	Radial size	Cost ()	Full	Empty	Max. Temp. (K)	Tolerance (m/s)	Tolerance (g)
	Engine Nacelle	Small	600 (480)	0.925	0.15	2 000	10	50	150
	Engine Pre-cooler	Small	1 650 (1 618)	0.375	0.15	2 000	10	50	40
	Mk1 Diverterless Supersonic Intake	Small	720 (560)	1.18	0.17	2 000	10	50	200
	Mk0 Liquid Fuel Fuselage	Tiny	200 (160)	0.275	0.025	2 000	10	50	50
	Mk1 Liquid Fuel Fuselage	Small	550 (230)	2.25	0.25	2 000	10	50	400
	Mk2 Liquid Fuel Fuselage Short	Mk2	750 (430)	2.29	0.29	2 500	50	50	400
	Mk2 Liquid Fuel Fuselage	Mk2	1 450 (810)	4.57	0.57	2 500	50	50	800
	Mk3 Liquid Fuel Fuselage Short	Mk3	4 300 (2 300)	14.29	1.79	2 700	50	50	2 500
	Mk3 Liquid Fuel Fuselage	Mk3	8 600 (4 600)	28.57	3.57	2 700	50	50	5 000
	Mk3 Liquid Fuel Fuselage Long	Mk3	17 200 (9 200)	57.14	7.14	2 700	50	50	10 000
	NCS Adapter	Small, Tiny	320 (256)	0.5	0.1	2 400	10	50	80
	FAT-455 Aeroplane Main Wing	X	2 800 (2 320)	3.78	0.78	1 200	15	50	600
	Big-S Wing Strake	X	1 000 (920)	0.6	0.1	2 400	15	50	100
	Big-S Delta Wing	X	3 000 (2 760)	2.0	0.5	2 400	15	50	300

Solid fuel rocket engines

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→ 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

• 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

Disadvantages

• 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

Available solid fuel rocket engines

Solid Fuel Density is 7.5 kg/unit				Mass (t)					Fuel ()	Thrust (kN)		TWR				Isp (s)		Burn (s)
												Full		Empty
Image	Part	Radial size	Cost ()	Full	Empty	Max. Temp. (K)	Tolerance (m/s)	Tolerance (g)		atm	vac	atm	vac	atm	vac	atm	vac
	RT-5 "Flea" Solid Fuel Booster	Small, Radial mounted	200 (116)	1.50	0.45	2 000	7	50	140	162.91	192.0	11.07	13.05	36.92	43.51	140	165	8.8
	RT-10 "Hammer" Solid Fuel Booster	Small, Radial mounted	400 (175)	3.56	0.75	2 000	7	50	375	197.90	227.0	5.66	6.50	26.91	30.86	170	195	23.7
	BACC "Thumper" Solid Fuel Booster	Small, Radial mounted	850 (358)	7.65	1.50	2 200	7	50	820	250.00	300.0	3.33	4.00	17.00	20.39	175	210	42.2
	S1 SRB-KD25k "Kickback" Solid Fuel Booster	Small, Radial mounted	2 700 (1 140)	24.00	4.50	2 200	7	50	2 600	593.86	670.0	2.52	2.85	13.46	15.18	195	220	62.8
	Sepratron I	Radial mounted	75 (70.2)	0.0725	0.0125	2 000	7	50	8	13.79	18.0	19.40	25.32	112.51	146.84	118	154	5.0
	FM1 "Mite" Solid Fuel Booster	Tiny, Radial mounted	75 (51.0)	0.375	0.075	2 200	7	50	40	11.012	12.5	2.93	3.33	14.68	16.66	185	210	49.44
	F3S0 "Shrimp" Solid Fuel Booster	Tiny, Radial mounted	150 (96.0)	0.875	0.155	2 200	7	50	90	26.512	30.0	3.22	3.65	17.1	19.35	190	215	47.44
	S2-17 "Thoroughbred" Solid Fuel Booster	Large, Radial mounted	9 000 (4 200.0)	70.00	10.00	2 200	10	50	8 000	1 515.217	1 700.0	2.16	2.43	15.15	17.0	205	230	79.6
	S2-33 "Clydesdale" Solid Fuel Booster	Large, Radial mounted	18 500 (8 660.0)	144.00	21.00	2 200	10	50	16 400	2 948.936	3 300.0	2.05	2.29	14.04	15.71	210	235	85.9

Liquid fuel rocket engines

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Liquid fuel engines utilize a mixture of liquid fuel and liquid oxidizer in a 9:11 ratio — generally called “rocket fuel”. The lone^[outdated] 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 Bell nozzles except the Toroidal Aerospike Rocket.

The LV-N Atomic Rocket Motor was inspired by real-world nuclear thermal rockets, such as the 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. The LV-N reflects this by consuming only liquid fuel.

Advantages

• 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 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 KR-1x2.

Available liquid fuel rocket engines

								Thrust (kN)		T/W ratio		Max. Fuel Consumption (/s)	Isp (s)		TVC
Image	Part	Radial size	Cost ()	Mass (t)	Max. Temp. (K)	Tolerance (m/s)	Tolerance (g)	atm	vac	atm	vac		atm	vac	Gimbal (°)
	LV-1R "Spider" Liquid Fuel Engine	Radial mounted	120	0.02	2 000	7	50	1.79	2.0	9.14	10.20	0.141	260	290	10.0
	24-77 "Twitch" Liquid Fuel Engine	Radial mounted	230	0.08	2 000	7	50	15.17	16.0	19.34	20.39	1.125	275	290	8.0
	Mk-55 "Thud" Liquid Fuel Engine	Radial mounted	820	0.90	2 000	7	50	108.20	120.0	12.26	13.60	8.024	275	305	8.0
	O-10 "Puff" MonoPropellant Fuel Engine	Radial mounted	150	0.09	2 000	7	50	9.60	20.0	10.88	22.66	2.039	120	250	6.0
	LV-1 "Ant" Liquid Fuel Engine	Tiny, Radial mounted	110	0.02	2 000	7	50	0.51	2.0	2.59	10.20	0.129	80	315	—
	48-7S "Spark" Liquid Fuel Engine	Tiny	240	0.13	2 000	7	50	16.56	20.0	12.99	15.69	1.275	265	320	3.0
	LV-909 "Terrier" Liquid Fuel Engine	Small	390	0.50	2 000	7	50	14.78	60.0	3.01	12.24	3.547	85	345	4.0
	LV-T30 "Reliant" Liquid Fuel Engine	Small	1 100	1.25	2 000	7	50	205.16	240.0	16.74	19.58	15.789	265	310	—
	LV-T45 "Swivel" Liquid Fuel Engine	Small	1 200	1.50	2 000	7	50	167.97	215.0	11.42	14.62	13.703	250	320	3.0
	S3 KS-25 "Vector" Liquid Fuel Engine	Small, Radial mounted	18 000	4.00	2 000	22	50	936.51	1 000.0	23.87	25.49	64.745	295	315	10.5
	T-1 Toroidal Aerospike "Dart" Liquid Fuel Engine	Small, Radial mounted	3 850	1.00	2 000	20	50	153.53	180.0	15.66	18.35	10.797	290	340	—
	LV-N "Nerv" Atomic Rocket Motor	Small	10 000	3.00	2 500	12	50	13.88	60.0	0.47	2.04	1.530	185	800	—
	RE-L10 "Poodle" Liquid Fuel Engine	Large	1 300	1.75	2 000	7	50	64.29	250.0	3.75	14.57	14.568	90	350	4.5
	RE-I5 "Skipper" Liquid Fuel Engine	Large	5 300	3.00	2 000	8	50	568.75	650.0	19.33	22.09	41.426	280	320	2.0
	RE-M3 "Mainsail" Liquid Fuel Engine	Large	13 000	6.00	2 000	7	50	1 379.03	1 500.0	23.44	25.49	98.683	285	310	2.0
	LFB KR-1x2 "Twin-Boar" Liquid Fuel Engine	Large, Radial mounted	17 000 (14 062.4)	42.50 (10.50)	2 000	20	50	1 866.67	2 000.0	4.48 (18.13)	4.80 (19.42)	135.964	280	300	1.5
	Kerbodyne KR-2L+ "Rhino" Liquid Fuel Engine	Extra large	25 000	9.00	2 000	7	50	1 205.88	2 000.0	13.66	22.66	119.968	205	340	4.0
	S3 KS-25x4 "Mammoth" Liquid Fuel Engine	Extra large	39 000	15.00	2 000	20	50	3 746.03	4 000.0	25.47	27.19	258.978	295	315	2.0
	CR-7 R.A.P.I.E.R. Engine	Small	6 000	2.00	2 000	20	50	162.30	180.0	8.27	9.18	12.036	275	305	3.0

Available liquid fuel tanks

Liquid fuel and oxidizer density are both 5 kg/unit				Mass (t)					Liquid Fuel ()	Oxidizer ()
Image	Part	Radial size	Cost ()	Full	Empty	Max. Temp. (K)	Tolerance (m/s)	Tolerance (g)
	R-4 'Dumpling' External Tank	X	50 (39.90)	0.1238	0.0138	2 000	5	50	9.9	12.1
	R-11 'Baguette' External Tank	X	50 (25.21)	0.3038	0.0338	2 000	5	50	24.3	29.7
	R-12 'Doughnut' External Tank	Small	147 (119.46)	0.3375	0.0375	2 000	6	50	27	33
	Oscar-B Fuel Tank	Tiny	70 (51.64)	0.225	0.025	2 000	6	50	18	22
	FL-T100 Fuel Tank	Small	150 (104.1)	0.5625	0.0625	2 000	6	50	45	55
	FL-T200 Fuel Tank	Small	275 (183.2)	1.125	0.125	2 000	6	50	90	110
	FL-T400 Fuel Tank	Small	500 (316.4)	2.25	0.25	2 000	6	50	180	220
	FL-T800 Fuel Tank	Small	800 (432.8)	4.5	0.5	2 000	6	50	360	440
	Rockomax X200-8 Fuel Tank	Large	800 (432.8)	4.5	0.5	2 000	6	50	360	440
	Rockomax X200-16 Fuel Tank	Large	1 550 (815.6)	9	1	2 000	6	50	720	880
	Rockomax X200-32 Fuel Tank	Large	3 000 (1 531.2)	18	2	2 000	6	50	1 440	1 760
	Rockomax Jumbo-64 Fuel Tank	Large	5 750 (2 812.4)	36	4	2 000	6	50	2 880	3 520
	Kerbodyne S3-3600 Tank	Extra large	3 250 (1 597.6)	20.25	2.25	2 000	6	50	1 620	1 980
	Kerbodyne S3-7200 Tank	Extra large	6 500 (3 195.2)	40.5	4.5	2 000	6	50	3 240	3 960
	Kerbodyne S3-14400 Tank	Extra large	13 000 (6 390.4)	81	9	2 000	6	50	6 480	7 920
	Mk2 Rocket Fuel Fuselage Short	Mk2	750 (566.4)	2.29	0.29	2 500	50	50	180	220
	Mk2 Rocket Fuel Fuselage	Mk2	1 450 (1 082.8)	4.57	0.57	2 500	50	50	360	440
	Mk3 Rocket Fuel Fuselage Short	Mk3	2 500 (1 352.5)	14.29	1.79	2 700	50	50	1 125	1 375
	Mk3 Rocket Fuel Fuselage	Mk3	5 000 (2 705)	28.57	3.57	2 700	50	50	2 250	2 750
	Mk3 Rocket Fuel Fuselage Long	Mk3	10 000 (5 410)	57.14	7.14	2 700	50	50	4 500	5 500
	C7 Brand Adapter - 2.5m to 1.25m	Small, Large	800 (433.0)	4.57	0.57	2 300	20	50	360	440
	C7 Brand Adapter Slanted - 2.5m to 1.25m	Small, Large	800 (433.0)	4.57	0.57	2 300	20	50	360	440
	Mk2 to 1.25m Adapter	Small, Mk2	550 (366.4)	2.29	0.29	2 500	50	50	180	220
	Mk2 to 1.25m Adapter Long	Small, Mk2	1 050 (682.8)	4.57	0.57	2 500	50	50	360	440
	Mk2 Bicoupler	Small, Mk2 x 2	860 (676.4)	2.29	0.29	2 500	50	50	180	220
	2.5m to Mk2 Adapter	Large, Mk2	800 (432.8)	4.57	0.57	2 500	50	50	360	440
	Mk3 to Mk2 Adapter	Mk2, Mk3	2 200 (1 282.0)	11.43	1.43	2 600	50	50	900	1 100
	Mk3 to 2.5m Adapter	Large, Mk3	2 500 (1 353.0)	14.29	1.79	2 600	50	50	1 125	1 375
	Mk3 to 2.5m Adapter Slanted	Large, Mk3	2 500 (1 353.0)	14.29	1.79	2 600	50	50	1 125	1 375
	Mk3 to 3.75m Adapter	Extra large, Mk3	2 500 (1 353.0)	14.29	1.79	2 600	50	50	1 125	1 375
	Kerbodyne ADTP-2-3	Large, Extra large	1 623 (246.0)	16.88	1.88	2 000	6	50	1 350	1 650

RCS thrusters

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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.

Advantages

• 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

Disadvantages

• 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

Available RCS engines

Image	Part	Radial size	Cost ()	Mass (t)	Max. Temp. (K)	Tolerance (m/s)	Tolerance (g)	Thrust (kN)	Fuel (/s)	Isp (s) (atm)	Isp (s) (vac)
	RV-1X Variable Thruster Block	Radial mounted	30	0.005	1 500	12	50	0.1	0.01	100	240
	Place Anywhere 1 Linear RCS Port	Radial mounted	15	0.001	1 500	12	50	0.2	0.02	100	240
	RV-105 RCS Thruster Block	Radial mounted	45	0.04	1 500	15	50	1.0	0.11	100	240
	Place-Anywhere 7 Linear RCS Port	Radial mounted	25	0.02	2 600	15	50	2.0	0.21	100	240
	Vernor Engine	Radial mounted	150	0.08	2 000	15	50	12.0	0.94	140	260

Available monopropellant tanks

RCS Fuel Density is 4 kg/unit				Mass (t)					Monopropellant ()
Image	Part	Radial size	Cost ()	Full	Empty	Max. Temp. (K)	Tolerance (m/s)	Tolerance (g)
	FL-R20 RCS Fuel Tank	Tiny	200 (176)	0.10	0.02	2 000	12	50	20
	FL-R120 RCS Fuel Tank	Small	330 (186)	0.56	0.08	2 000	12	50	120
	FL-R750 RCS Fuel Tank	Large	1 800 (900)	3.4	0.4	2 000	12	50	750
	Mk2 Monopropellant Tank	Mk2	750 (270)	1.89	0.29	2 500	50	50	400
	Mk3 Monopropellant Tank	Mk3	5 040 (2 520)	9.8	1.4	2 700	50	50	2 100
	Stratus-V Roundified Monopropellant Tank	X	200 (176)	0.10	0.02	2 000	12	50	20
	Stratus-V Cylindrified Monopropellant Tank	X	250 (190)	0.23	0.03	2 000	12	50	50

Ion engines

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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.

Advantages

• Extremely high efficiency
• Cool blue glow

Disadvantages

• Extremely low thrust; inefficient for Hohmann transfers
• High electric consumption
• No thrust vectoring currently^[outdated] available
• Xenon containers have poor mass ratio (2.27, compared to 8-9 for rocket fuel)

Hall effect thruster on Wikipedia

Available ion engines

								Thrust (kN)		TWR		Isp (s)		Max. Consumption
Image	Part	Radial size	Cost ()	Mass (t)	Max. Temp. (K)	Tolerance (m/s)	Tolerance (g)	atm	vac	atm	vac	atm	vac	Electricity (⚡/s)	Xenon (/s)
	IX-6315 "Dawn" Electric Propulsion System	Tiny	8 000	0.25	2 000	7	50	0.048	2.0	0.019	0.816	100	4 200	8.741	0.486

Available xenon tanks

Xenon density is 0.1 kg/unit				Mass (t)					Xenon ()
Image	Part	Radial size	Cost ()	Full	Empty	Max. Temp. (K)	Tolerance (m/s)	Tolerance (g)
	PB-X50R Xenon Container	Radial mounted	2 220 (600)	0.054	0.014	2 000	12	50	405
	PB-X150 Xenon Container	Tiny	3 680 (800)	0.100	0.024	2 000	6	50	720
	PB-X750 Xenon Container	Small	24 300 (1 500)	0.76	0.19	2 000	6	50	5 700

See also

• Reaction engine on Wikipedia
• Jet engine on Wikipedia
• Rocket engine on Wikipedia