Reaction engine

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.

Available jet engines

Part	Radial size	Cost ()	Mass (t)	Max. Temp. (K)	Tolerance (m/s)	Tolerance (g)	Thrust (kN)	TWR	Fuel (/s)	Intake (/s)	I_sp (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^{[Note 1]}	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

↑ The R.A.P.I.E.R. Engine is a combination of liquid fuel and jet engine. Only the jet engine properties are shown.

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)	Liquid Fuel ()
	Engine Nacelle^{[Note 1]}	Small	600 (480)	0.925	0.15	2 000	10	50	150
	Engine Pre-cooler^{[Note 1]}	Small	1 650 (1 618)	0.375	0.15	2 000	10	50	40
	Mk1 Diverterless Supersonic Intake^{[Note 1]}	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^{[Note 2]}	X	2 800 (2 320)	3.78	0.78	1 200	15	50	600
	Big-S Wing Strake^{[Note 2]}	X	1 000 (920)	0.6	0.1	2 400	15	50	100
	Big-S Delta Wing^{[Note 2]}	X	3 000 (2 760)	2.0	0.5	2 400	15	50	300

↑ ^1.0 ^1.1 ^1.2 The Engine Nacelle, Engine Pre-cooler, and Mk1 Diverterless Supersonic Intake are a combination of air intake and liquid fuel tank. Only the tank properties are shown. They are located in the "Aerodynamic" category in the game.
↑ ^2.0 ^2.1 ^2.2 The FAT-455 Aeroplane Main Wing, Big-S Wing Strake, and Big-S Delta Wing are a combination of lifting surface and liquid fuel tank. Only the tank properties are shown. They are located in the "Aerodynamic" category in the game.

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.

Available solid fuel rocket engines

Solid Fuel Density is 7.5 kg/unit				Mass (t)					Fuel ()	Thrust (kN)		TWR				I_sp (s)		Burn (s)
Solid Fuel Density is 7.5 kg/unit				Mass (t)						Thrust (kN)		Full		Empty		I_sp (s)
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

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.

Available liquid fuel rocket engines

								Thrust (kN)		T/W ratio		Max. Fuel Consumption (/s)	I_sp (s)		TVC
Image	Part	Radial size	Cost ()	Mass (t)	Max. Temp. (K)	Tolerance (m/s)	Tolerance (g)	atm	vac	atm	vac	Max. Fuel Consumption (/s)	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^{[Note 1]}	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^{[Note 2]}	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^{[Note 3]}	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^{[Note 4]}	Small	6 000	2.00	2 000	20	50	162.30	180.0	8.27	9.18	12.036	275	305	3.0

↑ Consumes monopropellant. (the density of monopropellant is less: 4kg/unit)
↑ Consumes liquid fuel only.
↑ The LFB KR-1x2 is a liquid fuel booster -- a combination of a "normal" engine and a fuel tank.
↑ The R.A.P.I.E.R. Engine is a combination of liquid fuel and jet engine. Only the liquid fuel engine properties are shown.

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)	Liquid Fuel ()	Oxidizer ()
	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

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 I_sp profile as the RC-105 RCS Thruster Block.

Available RCS engines

Part	Radial size	Cost ()	Mass (t)	Max. Temp. (K)	Tolerance (m/s)	Tolerance (g)	Thrust (kN)	Fuel (/s)	I_sp (s) (atm)	I_sp (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^{[Note 1]}	Radial mounted	150	0.08	2 000	15	50	12.0	0.94	140	260

↑ The Vernor Engine uses a liquid fuel/oxidizer mixture.

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)	Monopropellant ()
	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

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

Available ion engines

								Thrust (kN)		TWR		I_sp (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)	Xenon ()
	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

Reaction engine

Contents

Types

Jet engines

Solid fuel rocket engines

Liquid fuel rocket engines

RCS thrusters

Ion engines

See also

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