Specific impulse
Specific impulse (usually abbreviated I_{sp}) is a measure of how efficiently a reaction mass engine (a rocket using propellant or a jet engine using fuel) creates thrust. For engines whose reaction mass is only the fuel they carry, specific impulse is exactly proportional to exhaust gas velocity.
A propulsion system with a higher specific impulse uses the mass of the propellant more efficiently. In the case of a rocket, this means less propellant needed for a given deltav,^{[1]}^{[2]} so that the vehicle attached to the engine can more efficiently gain altitude and velocity.
In an atmospheric context, specific impulse can include the contribution to impulse provided by the mass of external air that is accelerated by the engine in some way, such as by an internal turbofan or heating by fuel combustion participation then thrust expansion or by external propeller. Jet engines breathe external air for both combustion and bypass, and therefore have a much higher specific impulse than rocket engines. The specific impulse in terms of propellant mass spent has units of distance per time, which is a notional velocity called the effective exhaust velocity. This is higher than the actual exhaust velocity because the mass of the combustion air is not being accounted for. Actual and effective exhaust velocity are the same in rocket engines operating in a vacuum.
Specific impulse is inversely proportional to specific fuel consumption (SFC) by the relationship I_{sp} = 1/(g_{o}·SFC) for SFC in kg/(N·s) and I_{sp} = 3600/SFC for SFC in lb/(lbf·hr).
General considerations
The amount of propellant can be measured either in units of mass or weight. If mass is used, specific impulse is an impulse per unit mass, which dimensional analysis shows to have units of speed, specifically the effective exhaust velocity. As the SI system is massbased, this type of analysis is usually done in meters per second. If a forcebased unit system is used, impulse is divided by propellant weight (weight is a measure of force), resulting in units of time (seconds). These two formulations differ from each other by the standard gravitational acceleration (g_{0}) at the surface of the earth.
The rate of change of momentum of a rocket (including its propellant) per unit time is equal to the thrust. The higher the specific impulse, the less propellant is needed to produce a given thrust for a given time and the more efficient the propellant is. This should not be confused with the physics concept of energy efficiency, which can decrease as specific impulse increases, since propulsion systems that give high specific impulse require high energy to do so.^{[3]}
Thrust and specific impulse should not be confused. Thrust is the force supplied by the engine and depends on the amount of reaction mass flowing through the engine. Specific impulse measures the impulse produced per unit of propellant and is proportional to the exhaust velocity. Thrust and specific impulse are related by the design and propellants of the engine in question, but this relationship is tenuous. For example, LH_{2}/LOx bipropellant produces higher I_{sp} but lower thrust than RP1/LOx due to the exhaust gases having a lower density and higher velocity (H_{2}O vs CO_{2} and H_{2}O). In many cases, propulsion systems with very high specific impulse—some ion thrusters reach 10,000 seconds—produce low thrust.^{[4]}
When calculating specific impulse, only propellant carried with the vehicle before use is counted. For a chemical rocket, the propellant mass therefore would include both fuel and oxidizer. In rocketry, a heavier engine with a higher specific impulse may not be as effective in gaining altitude, distance, or velocity as a lighter engine with a lower specific impulse, especially if the latter engine possesses a higher thrusttoweight ratio. This is a significant reason for most rocket designs having multiple stages. The first stage is optimised for high thrust to boost the later stages with higher specific impulse into higher altitudes where they can perform more efficiently.
For airbreathing engines, only the mass of the fuel is counted, not the mass of air passing through the engine. Air resistance and the engine's inability to keep a high specific impulse at a fast burn rate are why all the propellant is not used as fast as possible.
If it were not for air resistance and the reduction of propellant during flight, specific impulse would be a direct measure of the engine's effectiveness in converting propellant weight or mass into forward momentum.
Units
Specific impulse  Effective exhaust velocity 
Specific fuel consumption 


By weight  By mass  
SI  = x s  = 9.80665·x N·s/kg  = 9.80665·x m/s  = 101,972/x g/(kN·s) 
English engineering units  = x s  = x lbf·s/lb  = 32.17405·x ft/s  = 3,600/x lb/(lbf·hr) 
The most common unit for specific impulse is the second, as values are identical regardless of whether the calculations are done in SI, imperial, or customary units. Nearly all manufacturers quote their engine performance in seconds, and the unit is also useful for specifying aircraft engine performance.^{[5]}
The use of metres per second to specify effective exhaust velocity is also reasonably common. The unit is intuitive when describing rocket engines, although the effective exhaust speed of the engines may be significantly different from the actual exhaust speed, especially in gasgenerator cycle engines. For airbreathing jet engines, the effective exhaust velocity is not physically meaningful, although it can be used for comparison purposes.^{[6]}
Meters per second are numerically equivalent to Newtonseconds per kg (N·s/kg), and SI measurements of specific impulse can be written in terms of either units interchangeably.^{[citation needed]} This unit highlights the definition of specific impulse as impulseperunitmassofpropellant.
Specific fuel consumption is inversely proportional to specific impulse and has units of g/(kN·s) or lb/(lbf·hr). Specific fuel consumption is used extensively for describing the performance of airbreathing jet engines.^{[7]}
Specific impulse in seconds
Specific impulse, measured in seconds, can be thought of as "for how many seconds can one pound of fuel produce one pound of thrust," or more precisely, "how many seconds this propellant, when paired with this engine, can accelerate its own initial mass at 1 g." The more seconds it can accelerate its own mass, the more deltaV it delivers to the whole system.
In other words, given a particular engine and a mass of a particular propellant, specific impulse measures for how long a time that engine can exert a continuous force (thrust) until fully burning that mass of propellant. A given mass of a more energydense propellant can burn for a longer duration than some less energydense propellant made to exert the same force while burning in an engine. Different engine designs burning the same propellant may not be equally efficient at directing their propellant's energy into effective thrust.
For all vehicles, specific impulse (impulse per unit weightonEarth of propellant) in seconds can be defined by the following equation:^{[8]}
where:
 is the thrust obtained from the engine (newtons or pounds force),
 is the standard gravity, which is nominally the gravity at Earth's surface (m/s^{2} or ft/s^{2}),
 is the specific impulse measured (seconds),
 is the mass flow rate of the expended propellant (kg/s or slugs/s)
The English unit pound mass is more commonly used than the slug, and when using pounds per second for mass flow rate, the conversion constant g_{0} becomes unnecessary, because the slug is dimensionally equivalent to pounds divided by g_{0}:
I_{sp} in seconds is the amount of time a rocket engine can generate thrust, given a quantity of propellant whose weight is equal to the engine's thrust. The last term on the right, , is necessary for dimensional consistency ()
The advantage of this formulation is that it may be used for rockets, where all the reaction mass is carried on board, as well as airplanes, where most of the reaction mass is taken from the atmosphere. In addition, it gives a result that is independent of units used (provided the unit of time used is the second).
In rocketry, the only reaction mass is the propellant, so an equivalent way of calculating the specific impulse in seconds is used. Specific impulse is defined as the thrust integrated over time per unit weightonEarth of the propellant:^{[9]}
where
 is the specific impulse measured in seconds,
 is the average exhaust speed along the axis of the engine (in m/s or ft/s),
 is the standard gravity (in m/s^{2} or ft/s^{2}).
In rockets, due to atmospheric effects, the specific impulse varies with altitude, reaching a maximum in a vacuum. This is because the exhaust velocity isn't simply a function of the chamber pressure, but is a function of the difference between the interior and exterior of the combustion chamber. Values are usually given for operation at sea level ("sl") or in a vacuum ("vac").
Specific impulse as effective exhaust velocity
Because of the geocentric factor of g_{0} in the equation for specific impulse, many prefer an alternative definition. The specific impulse of a rocket can be defined in terms of thrust per unit mass flow of propellant. This is an equally valid (and in some ways somewhat simpler) way of defining the effectiveness of a rocket propellant. For a rocket, the specific impulse defined in this way is simply the effective exhaust velocity relative to the rocket, v_{e}. "In actual rocket nozzles, the exhaust velocity is not really uniform over the entire exit cross section and such velocity profiles are difficult to measure accurately. A uniform axial velocity, v _{e}, is assumed for all calculations which employ onedimensional problem descriptions. This effective exhaust velocity represents an average or mass equivalent velocity at which propellant is being ejected from the rocket vehicle."^{[10]} The two definitions of specific impulse are proportional to one another, and related to each other by:
where
 is the specific impulse in seconds,
 is the specific impulse measured in m/s, which is the same as the effective exhaust velocity measured in m/s (or ft/s if g is in ft/s^{2}),
 is the standard gravity, 9.80665 m/s^{2} (in Imperial units 32.174 ft/s^{2}).
This equation is also valid for airbreathing jet engines, but is rarely used in practice.
(Note that different symbols are sometimes used; for example, c is also sometimes seen for exhaust velocity. While the symbol might logically be used for specific impulse in units of (N·s^3)/(m·kg); to avoid confusion, it is desirable to reserve this for specific impulse measured in seconds.)
It is related to the thrust, or forward force on the rocket by the equation:^{[11]}
where is the propellant mass flow rate, which is the rate of decrease of the vehicle's mass.
A rocket must carry all its propellant with it, so the mass of the unburned propellant must be accelerated along with the rocket itself. Minimizing the mass of propellant required to achieve a given change in velocity is crucial to building effective rockets. The Tsiolkovsky rocket equation shows that for a rocket with a given empty mass and a given amount of propellant, the total change in velocity it can accomplish is proportional to the effective exhaust velocity.
A spacecraft without propulsion follows an orbit determined by its trajectory and any gravitational field. Deviations from the corresponding velocity pattern (these are called Δv) are achieved by sending exhaust mass in the direction opposite to that of the desired velocity change.
Actual exhaust speed versus effective exhaust speed
When an engine is run within the atmosphere, the exhaust velocity is reduced by atmospheric pressure, in turn reducing specific impulse. This is a reduction in the effective exhaust velocity, versus the actual exhaust velocity achieved in vacuum conditions. In the case of gasgenerator cycle rocket engines, more than one exhaust gas stream is present as turbopump exhaust gas exits through a separate nozzle. Calculating the effective exhaust velocity requires averaging the two mass flows as well as accounting for any atmospheric pressure.^{[citation needed]}
For airbreathing jet engines, particularly turbofans, the actual exhaust velocity and the effective exhaust velocity are different by orders of magnitude. This is because a good deal of additional momentum is obtained by using air as reaction mass. This allows a better match between the airspeed and the exhaust speed, which saves energy/propellant and enormously increases the effective exhaust velocity while reducing the actual exhaust velocity.^{[citation needed]}
Examples
Engine type  First run  Scenario  Spec. fuel cons.  Specific impulse (s) 
Effective exhaust velocity (m/s) 
Mass  Thrustto weight ratio (sea level) 


(lb/lbf·h)  (g/kN·s)  
Avio P80 solid fuel rocket motor  2006  Vega first stage vacuum  13  360  280  2700  16,160 lb (7,330 kg) (Empty)  
Avio Zefiro 23 solid fuel rocket motor  2006  Vega second stage vacuum  12.52  354.7  287.5  2819  4,266 lb (1,935 kg) (Empty)  
Avio Zefiro 9A solid fuel rocket motor  2008  Vega third stage vacuum  12.20  345.4  295.2  2895  1,997 lb (906 kg) (Empty)  
RD843 liquid fuel rocket engine  Vega upper stage vacuum  11.41  323.2  315.5  3094  35.1 lb (15.93 kg) (Dry)  
Kouznetsov NK33 liquid fuel rocket engine  1970s  N1F, Soyuz21v first stage vacuum  10.9  308  331^{[12]}  3250  2,730 lb (1,240 kg) (Dry)  136.8 
NPO Energomash RD171M liquid fuel rocket engine  Zenit2M, Zenit3SL, Zenit3SLB, Zenit3F first stage vacuum  10.7  303  337  3300  21,500 lb (9,750 kg) (Dry)  79.57  
LE7A liquid fuel rocket engine  HIIA, HIIB first stage vacuum  8.22  233  438  4300  4,000 lb (1,800 kg) (Dry)  62.2  
Snecma HM7B cryogenic rocket engine  Ariane 2, Ariane 3, Ariane 4, Ariane 5 ECA upper stage vacuum  8.097  229.4  444.6  4360  364 lb (165 kg) (Dry)  43.25  
LE5B2 cryogenic rocket engine  HIIA, HIIB upper stage vacuum  8.05  228  447  4380  640 lb (290 kg) (Dry)  51.93  
Aerojet Rocketdyne RS25 cryogenic rocket engine  1981  Space Shuttle, SLS first stage vacuum  7.95  225  453^{[13]}  4440  7,004 lb (3,177 kg) (Dry)  53.79 
Aerojet Rocketdyne RL10B2 cryogenic rocket engine  Delta III, Delta IV, SLS upper stage vacuum  7.734  219.1  465.5  4565  664 lb (301 kg) (Dry)  37.27  
Ramjet  Mach 1  4.5  130  800  7800  
NERVA NRX A6 nuclear thermal rocket engine  1967  vacuum  869  40,001 lb (18,144 kg) (Dry)  1.39  
TurboUnion RB.19934R04 Mk.103 turbofan  Tornado IDS GR.1/GR.1A/GR.1B/GR.4 static sea level (Reheat)  2.5^{[14]}  70.8  1440  14120  2,107 lb (956 kg) (Dry)  7.59  
GE F101GE102 turbofan  1970s  B1B static sea level (Reheat)  2.46  70  1460  14400  4,400 lb (2,000 kg) (Dry)  7.04 
Tumansky R25300 turbojet  MIG21bis static sea level (Reheat)  2.206^{[14]}  62.5  1632  16000  2,679 lb (1,215 kg) (Dry)  5.6  
GE J85GE21 turbojet  F5E/F static sea level (Reheat)  2.13^{[14]}  60.3  1690  16570  640 lb (290 kg) (Dry)  7.81  
GE F110GE132 turbofan  F16E/F Block 60 or 129 upgrade static sea level (Reheat)  2.09^{[14]}  59.2  1722  16890  4,050 lb (1,840 kg) (Dry)  7.9  
Honeywell/ITEC F125GA100 turbofan  FCK1 static sea level (Reheat)  2.06^{[14]}  58.4  1748  17140  1,360 lb (620 kg) (Dry)  6.8  
Snecma M53P2 turbofan  Mirage 2000C/D/N/H/TH/5/9/retrofit static sea level (Reheat)  2.05^{[14]}  58.1  1756  17220  3,307 lb (1,500 kg) (Dry)  6.46  
Snecma Atar 09C turbojet  Mirage IIIE/EX/O(A)/O(F)/M, Mirage IV prototype static sea level (Reheat)  2.03^{[14]}  57.5  1770  17400  3,210 lb (1,456 kg) (Dry)  4.13  
Snecma Atar 09K50 turbojet  Mirage IV, Mirage 50, Mirage F1 static sea level (Reheat)  1.991^{[14]}  56.4  1808  17730  3,487 lb (1,582 kg) (Dry)  4.55  
GE J79GE15 turbojet  F4E/EJ/F/G, RF4E static sea level (Reheat)  1.965  55.7  1832  17970  3,850 lb (1,750 kg) (Dry)  4.6  
Saturn AL31F turbofan  Su27/P/K static sea level (Reheat)  1.96^{[15]}  55.5  1837  18010  3,350 lb (1,520 kg) (Dry)  8.22  
J58 turbojet  1958  SR71 at Mach 3.2 (Reheat)  1.9^{[14]}  53.8  1895  18580  6,000 lb (2,700 kg) (Dry)  
GE F110GE129 turbofan  F16C/D/V Block 50/70, F15K/S/SA/SG/EX static sea level (Reheat)  1.9^{[14]}  53.8  1895  18580  3,980 lb (1,810 kg) (Dry)  7.36  
Soloviev D30F6 turbofan  MiG31, S37/Su47 static sea level (Reheat)  1.863^{[14]}  52.8  1932  18950  5,326 lb (2,416 kg) (Dry)  7.856  
Lyulka AL21F3 turbojet  Su17M/UM/M2/M2D/UM3/M3/M4, Su22U/M3/M4 static sea level (Reheat)  1.86^{[14]}  52.7  1935  18980  3,790 lb (1,720 kg) (Dry)  5.61  
Klimov RD33 turbofan  1974  MiG29 static sea level (Reheat)  1.85  52.4  1946  19080  2,326 lb (1,055 kg) (Dry)  7.9 
Saturn AL41F1S turbofan  Su35S/T10BM static sea level (Reheat)  1.819  51.5  1979  19410  3,536 lb (1,604 kg) (Dry)  8.759.04  
Volvo RM12 turbofan  1978  Gripen A/B/C/D static sea level (Reheat)  1.78^{[14]}  50.4  2022  19830  2,315 lb (1,050 kg) (Dry)  7.82 
GE F404GE402 turbofan  F/A18C/D static sea level (Reheat)  1.74^{[14]}  49  2070  20300  2,282 lb (1,035 kg) (Dry)  7.756  
Kuznetsov NK32 turbofan  1980  Tu144LL, Tu160 static sea level (Reheat)  1.7  48  2100  21000  7,500 lb (3,400 kg) (Dry)  7.35 
Snecma M882 turbofan  1989  Rafale static sea level (Reheat)  1.663  47.11  2165  21230  1,978 lb (897 kg) (Dry)  8.52 
Eurojet EJ200 turbofan  1991  Eurofighter, Bloodhound LSR prototype static sea level (Reheat)  1.66–1.73  47–49^{[16]}  2080–2170  20400–21300  2,180.0 lb (988.83 kg) (Dry)  9.17 
GE J85GE21 turbojet  F5E/F static sea level (Dry)  1.24^{[14]}  35.1  2900  28500  640 lb (290 kg) (Dry)  5.625  
RR/Snecma Olympus 593 turbojet  1966  Concorde at Mach 2 cruise (Dry)  1.195^{[17]}  33.8  3010  29500  7,000 lb (3,175 kg) (Dry)  
Snecma Atar 09C turbojet  Mirage IIIE/EX/O(A)/O(F)/M, Mirage IV prototype static sea level (Dry)  1.01^{[14]}  28.6  3560  35000  3,210 lb (1,456 kg) (Dry)  2.94  
Snecma Atar 09K50 turbojet  Mirage IV, Mirage 50, Mirage F1 static sea level (Dry)  0.981^{[14]}  27.8  3670  36000  3,487 lb (1,582 kg) (Dry)  2.35  
Snecma Atar 08K50 turbojet  Super Étendard static sea level  0.971^{[14]}  27.5  3710  36400  2,568 lb (1,165 kg) (Dry)  
Tumansky R25300 turbojet  MIG21bis static sea level (Dry)  0.961^{[14]}  27.2  3750  36700  2,679 lb (1,215 kg) (Dry)  
Lyulka AL21F3 turbojet  Su17M/UM/M2/M2D/UM3/M3/M4, Su22U/M3/M4 static sea level (Dry)  0.86  24.4  4190  41100  3,790 lb (1,720 kg) (Dry)  3.89  
GE J79GE15 turbojet  F4E/EJ/F/G, RF4E static sea level (Dry)  0.85  24.1  4240  41500  3,850 lb (1,750 kg) (Dry)  2.95  
Snecma M53P2 turbofan  Mirage 2000C/D/N/H/TH/5/9/retrofit static sea level (Dry)  0.85^{[14]}  24.1  4240  41500  3,307 lb (1,500 kg) (Dry)  4.37  
Volvo RM12 turbofan  1978  Gripen A/B/C/D static sea level (Dry)  0.824^{[14]}  23.3  4370  42800  2,315 lb (1,050 kg) (Dry)  5.244 
RR Turbomeca Adour Mk 106 turbofan  1999  Jaguar retrofit static sea level (Dry)  0.81  23  4400  44000  1,784 lb (809 kg) (Dry)  4.725 
Honeywell/ITEC F124GA100 turbofan  1979  L159, X45 static sea level  0.81^{[14]}  22.9  4440  43600  1,050 lb (480 kg) (Dry)  5.3 
Honeywell/ITEC F125GA100 turbofan  FCK1 static sea level (Dry)  0.8^{[14]}  22.7  4500  44100  1,360 lb (620 kg) (Dry)  4.43  
PW JT8D9 turbofan  737 Original cruise  0.8^{[18]}  22.7  4500  44100  3,205–3,402 lb (1,454–1,543 kg) (Dry)  
PW J52P408 turbojet  A4M/N, TA4KU, EA6B static sea level  0.79  22.4  4560  44700  2,318 lb (1,051 kg) (Dry)  4.83  
Saturn AL41F1S turbofan  Su35S/T10BM static sea level (Dry)  0.79  22.4  4560  44700  3,536 lb (1,604 kg) (Dry)  5.49  
Snecma M882 turbofan  1989  Rafale static sea level (Dry)  0.782  22.14  4600  45100  1,978 lb (897 kg) (Dry)  5.68 
Klimov RD33 turbofan  1974  MiG29 static sea level (Dry)  0.77  21.8  4680  45800  2,326 lb (1,055 kg) (Dry)  4.82 
RR Pegasus 1161 turbofan  AV8B+ static sea level  0.76  21.5  4740  46500  3,960 lb (1,800 kg) (Dry)  6  
Eurojet EJ200 turbofan  1991  Eurofighter, Bloodhound LSR prototype static sea level (Dry)  0.74–0.81  21–23^{[16]}  4400–4900  44000–48000  2,180.0 lb (988.83 kg) (Dry)  6.11 
GE F414GE400 turbofan  1993  F/A18E/F static sea level (Dry)  0.724^{[19]}  20.5  4970  48800  2,445 lb (1,109 kg) (Dry)  5.11 
Kuznetsov NK32 turbofan  1980  Tu144LL, Tu160 static sea level (Dry)  0.720.73  20–21  4900–5000  48000–49000  7,500 lb (3,400 kg) (Dry)  4.06^{[14]} 
Honeywell ALF502R5 geared turbofan  BAe 146100/200/200ER/300 cruise  0.72^{[20]}  20.4  5000  49000  1,336 lb (606 kg) (Dry)  5.22  
Soloviev D30F6 turbofan  MiG31, S37/Su47 static sea level (Dry)  0.716^{[14]}  20.3  5030  49300  5,326 lb (2,416 kg) (Dry)  3.93  
Snecma Turbomeca Larzac 04C6 turbofan  1972  Alpha Jet static sea level  0.716  20.3  5030  49300  650 lb (295 kg) (Dry)  4.567 
Soloviev D30KP2 turbofan  Il76MD/MDK/SK/VPK, Il78/M cruise  0.715  20.3  5030  49400  5,820 lb (2,640 kg) (Dry)  5.21  
Soloviev D30KU154 turbofan  Tu154M cruise  0.705  20.0  5110  50100  5,082 lb (2,305 kg) (Dry)  4.56  
IshikawajimaHarima F3IHI30 turbofan  1981  Kawasaki T4 static sea level  0.7  19.8  5140  50400  750 lb (340 kg) (Dry)  4.9 
RR Tay RB.1833 Mk.62015 turbofan  1984  Fokker 70, Fokker 100 cruise  0.69  19.5  5220  51200  3,185 lb (1,445 kg) (Dry)  4.2 
GE CF343 turbofan  1982  CRJ100/200, CL600 series, CL850 cruise  0.69  19.5  5220  51200  1,670 lb (760 kg) (Dry)  5.52 
GE CF348E turbofan  E170/175 cruise  0.68  19.3  5290  51900  2,600 lb (1,200 kg) (Dry)  5.6  
Honeywell TFE73160 geared turbofan  Falcon 900EX/DX/LX, VC900 cruise  0.679^{[21]}  19.2  5300  52000  988 lb (448 kg) (Dry)  5.06  
CFM CFM562C1 turbofan  DC8 Super 70 cruise  0.671^{[20]}  19.0  5370  52600  4,635 lb (2,102 kg) (Dry)  4.746  
GE CF348C turbofan  CRJ700/900/1000 cruise  0.670.68  19  5300–5400  52000–53000  2,400–2,450 lb (1,090–1,110 kg) (Dry)  5.76.1  
CFM CFM563C1 turbofan  737 Classic cruise  0.667  18.9  5400  52900  4,308–4,334 lb (1,954–1,966 kg) (Dry)  5.46  
Saturn AL31F turbofan  Su27/P/K static sea level (Dry)  0.6660.78^{[15]}^{[19]}  18.9–22.1  4620–5410  45300–53000  3,350 lb (1,520 kg) (Dry)  4.93  
RR Spey RB.168 Mk.807 turbofan  AMX static sea level  0.66^{[14]}  18.7  5450  53500  2,417 lb (1,096 kg) (Dry)  4.56  
CFM CFM562A2 turbofan  1974  E3D, KE3A, E6A/B cruise  0.66^{[22]}  18.7  5450  53500  4,819 lb (2,186 kg) (Dry)  4.979 
RR BR725 turbofan  2008  G650/ER cruise  0.657  18.6  5480  53700  3,605 lb (1,635.2 kg) (Dry)  4.69 
CFM CFM562B1 turbofan  KC135R/T, C135FR, RC135RE cruise  0.65^{[22]}  18.4  5540  54300  4,672 lb (2,119 kg) (Dry)  4.7  
GE CF3410A turbofan  ARJ21 cruise  0.65  18.4  5540  54300  3,700 lb (1,700 kg) (Dry)  5.1  
CFE CFE73811B turbofan  1990  Falcon 2000 cruise  0.645^{[20]}  18.3  5580  54700  1,325 lb (601 kg) (Dry)  4.32 
RR BR710 turbofan  1995  C37, Gulfstream V, G550, E11, Project Dolphin, Saab Swordfish, Global Express/XRS, Global 5000/6000, Raytheon Sentinel, GlobalEye (original) cruise  0.64  18  5600  55000  4,009 lb (1,818.4 kg) (Dry)  3.84 
GE F110GE129 turbofan  F16C/D/V Block 50/70, F15K/S/SA/SG/EX static sea level (Dry)  0.64^{[19]}  18  5600  55000  3,980 lb (1,810 kg) (Dry)  4.27  
GE F110GE132 turbofan  F16E/F Block 60 or 129 upgrade static sea level (Dry)  0.64^{[19]}  18  5600  55000  4,050 lb (1,840 kg) (Dry)  
GE CF3410E turbofan  E190/195, Lineage 1000 cruise  0.64  18  5600  55000  3,700 lb (1,700 kg) (Dry)  5.2  
TurboUnion RB.19934R04 Mk.105 turbofan  Tornado ECR static sea level (Dry)  0.637^{[14]}  18.0  5650  55400  2,160 lb (980 kg) (Dry)  4.47  
CFM CF650C2 turbofan  A300B2203/B42C/B4103/103F/203/203F/C4203/F4203, DC1030/F/CF, KC10A cruise  0.63^{[20]}  17.8  5710  56000  8,731 lb (3,960 kg) (Dry)  6.01  
PowerJet SaM1461S18 turbofan  Superjet LR cruise  0.629  17.8  5720  56100  4,980 lb (2,260 kg) (Dry)  3.5  
CFM CFM567B24 turbofan  737700/800/900 cruise  0.627^{[20]}  17.8  5740  56300  5,216 lb (2,366 kg) (Dry)  4.6  
RR BR715 turbofan  1997  717 cruise  0.62  17.6  5810  56900  4,597 lb (2,085 kg) (Dry)  4.554.68 
PW F119PW100 turbofan  1992  F22 static sea level (Dry)  0.61^{[19]}  17.3  5900  57900  3,900 lb (1,800 kg) (Dry)  6.7 
GE CF680C2B1F turbofan  747400 cruise  0.605^{[17]}  17.1  5950  58400  9,499 lb (4,309 kg)  6.017  
TurboUnion RB.19934R04 Mk.103 turbofan  Tornado IDS GR.1/GR.1A/GR.1B/GR.4 static sea level (Dry)  0.598^{[14]}  16.9  6020  59000  2,107 lb (956 kg) (Dry)  4.32  
CFM CFM565A1 turbofan  A320111/211 cruise  0.596  16.9  6040  59200  5,139 lb (2,331 kg) (Dry)  5  
Aviadvigatel PS90A1 turbofan  Il96400/T cruise  0.595  16.9  6050  59300  6,500 lb (2,950 kg) (Dry)  5.9  
PW PW2040 turbofan  757200/200ET/200F, C32 cruise  0.582^{[20]}  16.5  6190  60700  7,185 lb (3,259 kg)  5.58  
PW PW4098 turbofan  777300 cruise  0.581^{[20]}  16.5  6200  60800  36,400 lb (16,500 kg) (Dry)  5.939  
GE CF680C2B2 turbofan  767200ER/300/300ER cruise  0.576^{[20]}  16.3  6250  61300  9,388 lb (4,258 kg)  5.495  
IAE V2525D5 turbofan  MD90 cruise  0.574^{[23]}  16.3  6270  61500  5,252 lb (2,382 kg)  4.76  
IAE V2533A5 turbofan  A321231 cruise  0.574^{[23]}  16.3  6270  61500  5,139 lb (2,331 kg)  6.42  
GE F101GE102 turbofan  1970s  B1B static sea level (Dry)  0.562  15.9  6410  62800  4,400 lb (2,000 kg) (Dry)  3.9 
RR Trent 700 turbofan  1992  A330, A330 MRTT, Beluga XL cruise  0.562  15.9  6410  62800  13,580 lb (6,160 kg) (Dry)  4.975.24 
RR Trent 800 turbofan  1993  777200/200ER/300 cruise  0.560  15.9  6430  63000  13,400 lb (6,078 kg) (Dry)  5.76.9 
Motor Sich Progress D18T turbofan  1980  An124, An225 cruise  0.546  15.5  6590  64700  9,000 lb (4,100 kg) (Dry)  5.72 
CFM CFM565B4 turbofan  A320214 cruise  0.545  15.4  6610  64800  5,412–5,513 lb (2,454.8–2,500.6 kg) (Dry)  5.14  
CFM CFM565C2 turbofan  A340211 cruise  0.545  15.4  6610  64800  5,830 lb (2,644.4 kg) (Dry)  5.47  
RR Trent 500 turbofan  1999  A340500/600 cruise  0.542  15.4  6640  65100  11,000 lb (4,990 kg) (Dry)  5.075.63 
CFM LEAP1B turbofan  2014  737 MAX cruise  0.530.56  15–16  6400–6800  63000–67000  6,130 lb (2,780 kg) (Dry)  
Aviadvigatel PD14 turbofan  2014  MC21310 cruise  0.526  14.9  6840  67100  6,330 lb (2,870 kg) (Dry)  4.88 
RR Trent 900 turbofan  2003  A380 cruise  0.522  14.8  6900  67600  13,770 lb (6,246 kg) (Dry)  5.466.11 
PW TF33P3 turbofan  B52H, NB52H static sea level  0.52^{[14]}  14.7  6920  67900  3,900 lb (1,800 kg) (Dry)  4.36  
GE GE9085B turbofan  777200/200ER cruise  0.52^{[20]}^{[24]}  14.7  6920  67900  17,400 lb (7,900 kg)  5.59  
GE GEnx1B76 turbofan  2006  78710 cruise  0.512^{[18]}  14.5  7030  69000  2,658 lb (1,206 kg) (Dry)  5.62 
PW PW1400G geared turbofan  MC21 cruise  0.51^{[25]}  14  7100  69000  6,300 lb (2,857.6 kg) (Dry)  5.01  
CFM LEAP1C turbofan  2013  C919 cruise  0.51  14  7100  69000  8,662–8,675 lb (3,929–3,935 kg) (Wet)  
CFM LEAP1A turbofan  2013  A320neo family cruise  0.51^{[25]}  14  7100  69000  6,592–6,951 lb (2,990–3,153 kg) (Wet)  
RR Trent 7000 turbofan  2015  A330neo cruise  0.506  14.3  7110  69800  14,209 lb (6,445 kg) (Dry)  5.13 
RR Trent 1000 turbofan  2006  787 cruise  0.506  14.3  7110  69800  13,087–13,492 lb (5,936–6,120 kg) (Dry)  
RR Trent XWB97 turbofan  2014  A3501000 cruise  0.478  13.5  7530  73900  16,640 lb (7,550 kg) (Dry)  5.82 
PW 1127G geared turbofan  2012  A320neo cruise  0.463^{[18]}  13.1  7780  76300  6,300 lb (2,857.6 kg) (Dry)  
RR AE 3007H turbofan  RQ4, MQ4C static sea level  0.39^{[14]}  11.0  9200  91000  1,581 lb (717 kg) (Dry)  5.24  
GE F118GE100 turbofan  1980s  B2A Block 30 static sea level  0.375^{[14]}  10.6  9600  94000  3,200 lb (1,500 kg) (Dry)  5.9 
GE F118GE101 turbofan  1980s  U2S static sea level  0.375^{[14]}  10.6  9600  94000  3,150 lb (1,430 kg) (Dry)  6.03 
CFM CF650C2 turbofan  A300B2203/B42C/B4103/103F/203/203F/C4203/F4203, DC1030/30F/30F(CF), KC10A static sea level  0.371^{[14]}  10.5  9700  95000  8,731 lb (3,960 kg) (Dry)  6.01  
GE TF34GE100 turbofan  A10A, OA10A, YA10B static sea level  0.37^{[14]}  10.5  9700  95000  1,440 lb (650 kg) (Dry)  6.295  
CFM CFM562B1 turbofan  KC135R/T, C135FR, RC135RE static sea level  0.36^{[22]}  10  10000  98000  4,672 lb (2,119 kg) (Dry)  4.7  
Motor Sich Progress D18T turbofan  1980  An124, An225 static sea level  0.345  9.8  10400  102000  9,000 lb (4,100 kg) (Dry)  5.72 
PW F117PW100 turbofan  C17 static sea level  0.34^{[20]}  9.6  10600  104000  7,100 lb (3,200 kg)  5.416.16  
PW PW2040 turbofan  757200/200ET/200F, C32 static sea level  0.33^{[20]}  9.3  10900  107000  7,185 lb (3,259 kg)  5.58  
CFM CFM563C1 turbofan  737 Classic static sea level  0.33  9.3  11000  110000  4,308–4,334 lb (1,954–1,966 kg) (Dry)  5.46  
GE CF680C2 turbofan  747400, 767, KC767, MD11, A300600R/600F, A310300, A310 MRTT, Beluga, C5M, Kawasaki C2 static sea level  0.3070.344  8.7–9.7  10500–11700  103000–115000  9,480–9,860 lb (4,300–4,470 kg)  
EA GP7270 turbofan  A380861 static sea level  0.299^{[19]}  8.5  12000  118000  14,797 lb (6,712 kg) (Dry)  5.197  
GE GE9085B turbofan  777200/200ER/300 static sea level  0.298^{[19]}  8.44  12080  118500  17,400 lb (7,900 kg)  5.59  
GE GE9094B turbofan  777200/200ER/300 static sea level  0.2974^{[19]}  8.42  12100  118700  16,644 lb (7,550 kg)  5.59  
RR Trent 97084 turbofan  2003  A380841 static sea level  0.295^{[19]}  8.36  12200  119700  13,825 lb (6,271 kg) (Dry)  5.436 
GE GEnx1B70 turbofan  7878 static sea level  0.2845^{[19]}  8.06  12650  124100  13,552 lb (6,147 kg) (Dry)  5.15  
RR Trent 1000C turbofan  2006  7879 static sea level  0.273^{[19]}  7.7  13200  129000  13,087–13,492 lb (5,936–6,120 kg) (Dry) 
Engine  Effective exhaust velocity (m/s) 
Specific impulse (s) 
Exhaust specific energy (MJ/kg) 

Turbofan jet engine (actual V is ~300 m/s) 
29,000  3,000  Approx. 0.05 
Space Shuttle Solid Rocket Booster 
2,500  250  3 
Liquid oxygenliquid hydrogen 
4,400  450  9.7 
NSTAR^{[26]} electrostatic xenon ion thruster  20,00030,000  1,9503,100  
VASIMR predictions^{[27]}^{[28]}^{[29]}  30,000–120,000  3,000–12,000  1,400 
DS4G electrostatic ion thruster^{[30]}  210,000  21,400  22,500 
Ideal photonic rocket^{[a]}  299,792,458  30,570,000  89,875,517,874 
An example of a specific impulse measured in time is 453 seconds, which is equivalent to an effective exhaust velocity of 4.440 km/s (14,570 ft/s), for the RS25 engines when operating in a vacuum.^{[31]} An airbreathing jet engine typically has a much larger specific impulse than a rocket; for example a turbofan jet engine may have a specific impulse of 6,000 seconds or more at sea level whereas a rocket would be between 200 and 400 seconds.^{[32]}
An airbreathing engine is thus much more propellant efficient than a rocket engine, because the air serves as reaction mass and oxidizer for combustion which does not have to be carried as propellant, and the actual exhaust speed is much lower, so the kinetic energy the exhaust carries away is lower and thus the jet engine uses far less energy to generate thrust.^{[33]} While the actual exhaust velocity is lower for airbreathing engines, the effective exhaust velocity is very high for jet engines. This is because the effective exhaust velocity calculation assumes that the carried propellant is providing all the reaction mass and all the thrust. Hence effective exhaust velocity is not physically meaningful for airbreathing engines; nevertheless, it is useful for comparison with other types of engines.^{[34]}
The highest specific impulse for a chemical propellant ever testfired in a rocket engine was 542 seconds (5.32 km/s) with a tripropellant of lithium, fluorine, and hydrogen. However, this combination is impractical. Lithium and fluorine are both extremely corrosive, lithium ignites on contact with air, fluorine ignites on contact with most fuels, and hydrogen, while not hypergolic, is an explosive hazard. Fluorine and the hydrogen fluoride (HF) in the exhaust are very toxic, which damages the environment, makes work around the launch pad difficult, and makes getting a launch license that much more difficult. The rocket exhaust is also ionized, which would interfere with radio communication with the rocket.^{[35]}^{[36]}^{[37]}
Nuclear thermal rocket engines differ from conventional rocket engines in that energy is supplied to the propellants by an external nuclear heat source instead of the heat of combustion.^{[38]} The nuclear rocket typically operates by passing liquid hydrogen gas through an operating nuclear reactor. Testing in the 1960s yielded specific impulses of about 850 seconds (8,340 m/s), about twice that of the Space Shuttle engines.^{[39]}
A variety of other rocket propulsion methods, such as ion thrusters, give much higher specific impulse but with much lower thrust; for example the Hall effect thruster on the SMART1 satellite has a specific impulse of 1,640 s (16.1 km/s) but a maximum thrust of only 68 mN (0.015 lbf).^{[40]} The variable specific impulse magnetoplasma rocket (VASIMR) engine currently in development will theoretically yield 20 to 300 km/s (66,000 to 984,000 ft/s), and a maximum thrust of 5.7 N (1.3 lbf).^{[41]}
See also
 Jet engine
 Impulse
 Tsiolkovsky rocket equation
 Systemspecific impulse
 Specific energy
 Standard gravity
 Thrust specific fuel consumption—fuel consumption per unit thrust
 Specific thrust—thrust per unit of air for a duct engine
 Heating value
 Energy density
 Deltav (physics)
 Rocket propellant
 Liquid rocket propellants
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