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Specific impulse

## Summary

Specific impulse (usually abbreviated Isp) 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 the effective 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 delta-v,[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 by-pass, 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 Isp = 1/(go·SFC) for SFC in kg/(N·s) and Isp = 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 of mass, which dimensional analysis shows to have units of speed, specifically the effective exhaust velocity. As the SI system is mass-based, this type of analysis is usually done in meters per second. If a force-based 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 (g0) 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, LH2/LO2 bipropellant produces higher Isp but lower thrust than RP-1/LO2 due to the exhaust gases having a lower density and higher velocity (H2O vs CO2 and H2O). 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 thrust-to-weight 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 air-breathing 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

Various equivalent rocket motor performance measurements, in SI and English engineering 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 gas-generator cycle engines. For airbreathing jet engines, the effective exhaust velocity is not physically meaningful, although it can be used for comparison purposes.[6]

Metres per second are numerically equivalent to newton-seconds per kg (N·s/kg), and SI measurements of specific impulse can be written in terms of either units interchangeably. This unit highlights the definition of specific impulse as impulse per unit mass of propellant.

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 air-breathing jet engines.[7]

### Specific impulse in seconds

Specific impulse, measured in seconds, effectively means 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 delta-V 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 energy-dense propellant can burn for a longer duration than some less energy-dense 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 weight-on-Earth of propellant) in seconds can be defined by the following equation:[8]

${\displaystyle F_{\text{thrust}}=g_{0}\cdot I_{\text{sp}}\cdot {\dot {m}},}$

where:

• ${\displaystyle F_{\text{thrust}}}$  is the thrust obtained from the engine (newtons or pounds force),
• ${\displaystyle g_{0}}$  is the standard gravity, which is nominally the gravity at Earth's surface (m/s2 or ft/s2),
• ${\displaystyle I_{\text{sp}}}$  is the specific impulse measured (seconds),
• ${\displaystyle {\dot {m}}}$  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 g0 becomes unnecessary, because the slug is dimensionally equivalent to pounds divided by g0:

${\displaystyle F_{\text{thrust}}=I_{\text{sp}}\cdot {\dot {m}}\cdot \left(1\mathrm {\frac {ft}{s^{2}}} \right).}$

Isp 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, ${\textstyle \left(1\mathrm {\frac {ft}{s^{2}}} \right)}$ , is necessary for dimensional consistency (${\textstyle \mathrm {lbf} \propto \mathrm {s} \cdot \mathrm {\frac {lbm}{s}} \cdot \mathrm {\frac {ft}{s^{2}}} }$ )

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).

The specific impulse of various jet engines (SSME is the Space Shuttle Main Engine)

#### Rocketry

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 weight-on-Earth of the propellant:[9]

${\displaystyle I_{\text{sp}}={\frac {v_{\text{e}}}{g_{0}}},}$

where

• ${\displaystyle I_{\text{sp}}}$  is the specific impulse measured in seconds,
• ${\displaystyle v_{\text{e}}}$  is the average exhaust speed along the axis of the engine (in m/s or ft/s),
• ${\displaystyle g_{0}}$  is the standard gravity (in m/s2 or ft/s2).

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 g0 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, ve. "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 one-dimensional 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:

${\displaystyle v_{\text{e}}=g_{0}\cdot I_{\text{sp}},}$

where
• ${\displaystyle I_{\text{sp}}}$  is the specific impulse in seconds,
• ${\displaystyle v_{\text{e}}}$  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/s2),
• ${\displaystyle g_{0}}$  is the standard gravity, 9.80665 m/s2 (in United States customary units 32.174 ft/s2).

This equation is also valid for air-breathing 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 ${\displaystyle I_{\text{sp}}}$  might logically be used for specific impulse in units of (N·s3)/(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]

${\displaystyle F_{\text{thrust}}=v_{\text{e}}\cdot {\dot {m}},}$

where ${\displaystyle {\dot {m}}}$  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 gas-generator 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 air-breathing jet engines, particularly turbofans, the actual exhaust velocity and the effective exhaust velocity are different by orders of magnitude. This happens for several reasons. First, a good deal of additional momentum is obtained by using air as reaction mass, such that combustion products in the exhaust have more mass than the burned fuel. Next, inert gases in the atmosphere absorb heat from combustion, and through the resulting expansion provide additional thrust. Lastly, for turbofans and other designs there is even more thrust created by pushing against intake air which never sees combustion directly. These all combine to allow 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] Again, this is because the mass of the air is not counted in the specific impulse calculation, thus attributing all of the thrust momentum to the mass of the fuel component of the exhaust, and omitting the reaction mass, inert gas, and effect of driven fans on overall engine efficiency from consideration.

Essentially, the momentum of engine exhaust includes a lot more than just fuel, but specific impulse calculation ignores everything but the fuel. Even though the effective exhaust velocity for an air-breathing engine seems nonsensical in the context of actual exhaust velocity, this is still useful for comparing absolute fuel efficiency of different engines.

### Density specific impulse

A related measure, the density specific impulse, sometimes also referred to as Density Impulse and usually abbreviated as Isd is the product of the average specific gravity of a given propellant mixture and the specific impulse.[12] While less important than the specific impulse, it is an important measure in launch vehicle design, as a low specific impulse implies that bigger tanks will be required to store the propellant, which in turn will have a detrimental effect on the launch vehicle's mass ratio.[13]

## Examples

Rocket engines in vacuum
Model Type First
run
Application TSFC SI
(s)
EEV
(m/s)
lb/lbf·h g/kN·s
Avio P80 solid fuel 2006 Vega stage 1 13 360 280 2700
Avio Zefiro 23 solid fuel 2006 Vega stage 2 12.52 354.7 287.5 2819
Avio Zefiro 9A solid fuel 2008 Vega stage 3 12.20 345.4 295.2 2895
RD-843 liquid fuel Vega upper stage 11.41 323.2 315.5 3094
Kuznetsov NK-33 liquid fuel 1970s N-1F, Soyuz-2-1v stage 1 10.9 308 331[14] 3250
NPO Energomash RD-171M liquid fuel Zenit-2M, -3SL, -3SLB, -3F stage 1 10.7 303 337 3300
LE-7A liquid fuel H-IIA, H-IIB stage 1 8.22 233 438 4300
Snecma HM-7B cryogenic Ariane 2, 3, 4, 5 ECA upper stage 8.097 229.4 444.6 4360
LE-5B-2 cryogenic H-IIA, H-IIB upper stage 8.05 228 447 4380
Aerojet Rocketdyne RS-25 cryogenic 1981 Space Shuttle, SLS stage 1 7.95 225 453[15] 4440
Aerojet Rocketdyne RL-10B-2 cryogenic Delta III, Delta IV, SLS upper stage 7.734 219.1 465.5 4565
NERVA NRX A6 nuclear 1967 869
Jet engines with Reheat, static, sea level
Model Type First
run
Application TSFC SI
(s)
EEV
(m/s)
lb/lbf·h g/kN·s
Turbo-Union RB.199 turbofan Tornado 2.5[16] 70.8 1440 14120
GE F101-GE-102 turbofan 1970s B-1B 2.46 70 1460 14400
Tumansky R-25-300 turbojet MIG-21bis 2.206[16] 62.5 1632 16000
GE J85-GE-21 turbojet F-5E/F 2.13[16] 60.3 1690 16570
GE F110-GE-132 turbofan F-16E/F 2.09[16] 59.2 1722 16890
Honeywell/ITEC F125 turbofan F-CK-1 2.06[16] 58.4 1748 17140
Snecma M53-P2 turbofan Mirage 2000C/D/N 2.05[16] 58.1 1756 17220
Snecma Atar 09C turbojet Mirage III 2.03[16] 57.5 1770 17400
Snecma Atar 09K-50 turbojet Mirage IV, 50, F1 1.991[16] 56.4 1808 17730
GE J79-GE-15 turbojet F-4E/EJ/F/G, RF-4E 1.965 55.7 1832 17970
Saturn AL-31F turbofan Su-27/P/K 1.96[17] 55.5 1837 18010
GE F110-GE-129 turbofan F-16C/D, F-15EX 1.9[16] 53.8 1895 18580
Soloviev D-30F6 turbofan MiG-31, S-37/Su-47 1.863[16] 52.8 1932 18950
Lyulka AL-21F-3 turbojet Su-17, Su-22 1.86[16] 52.7 1935 18980
Klimov RD-33 turbofan 1974 MiG-29 1.85 52.4 1946 19080
Saturn AL-41F-1S turbofan Su-35S/T-10BM 1.819 51.5 1979 19410
Volvo RM12 turbofan 1978 Gripen A/B/C/D 1.78[16] 50.4 2022 19830
GE F404-GE-402 turbofan F/A-18C/D 1.74[16] 49 2070 20300
Kuznetsov NK-32 turbofan 1980 Tu-144LL, Tu-160 1.7 48 2100 21000
Snecma M88-2 turbofan 1989 Rafale 1.663 47.11 2165 21230
Eurojet EJ200 turbofan 1991 Eurofighter 1.66–1.73 47–49[18] 2080–2170 20400–21300
Dry jet engines, static, sea level
Model Type First
run
Application TSFC SI
(s)
EEV
(m/s)
lb/lbf·h g/kN·s
GE J85-GE-21 turbojet F-5E/F 1.24[16] 35.1 2900 28500
Snecma Atar 09C turbojet Mirage III 1.01[16] 28.6 3560 35000
Snecma Atar 09K-50 turbojet Mirage IV, 50, F1 0.981[16] 27.8 3670 36000
Snecma Atar 08K-50 turbojet Super Étendard 0.971[16] 27.5 3710 36400
Tumansky R-25-300 turbojet MIG-21bis 0.961[16] 27.2 3750 36700
Lyulka AL-21F-3 turbojet Su-17, Su-22 0.86 24.4 4190 41100
GE J79-GE-15 turbojet F-4E/EJ/F/G, RF-4E 0.85 24.1 4240 41500
Snecma M53-P2 turbofan Mirage 2000C/D/N 0.85[16] 24.1 4240 41500
Volvo RM12 turbofan 1978 Gripen A/B/C/D 0.824[16] 23.3 4370 42800
RR Turbomeca Adour turbofan 1999 Jaguar retrofit 0.81 23 4400 44000
Honeywell/ITEC F124 turbofan 1979 L-159, X-45 0.81[16] 22.9 4440 43600
Honeywell/ITEC F125 turbofan F-CK-1 0.8[16] 22.7 4500 44100
PW J52-P-408 turbojet A-4M/N, TA-4KU, EA-6B 0.79 22.4 4560 44700
Saturn AL-41F-1S turbofan Su-35S/T-10BM 0.79 22.4 4560 44700
Snecma M88-2 turbofan 1989 Rafale 0.782 22.14 4600 45100
Klimov RD-33 turbofan 1974 MiG-29 0.77 21.8 4680 45800
RR Pegasus 11-61 turbofan AV-8B+ 0.76 21.5 4740 46500
Eurojet EJ200 turbofan 1991 Eurofighter 0.74–0.81 21–23[18] 4400–4900 44000–48000
GE F414-GE-400 turbofan 1993 F/A-18E/F 0.724[19] 20.5 4970 48800
Kuznetsov NK-32 turbofan 1980 Tu-144LL, Tu-160 0.72-0.73 20–21 4900–5000 48000–49000
Soloviev D-30F6 turbofan MiG-31, S-37/Su-47 0.716[16] 20.3 5030 49300
Snecma Larzac turbofan 1972 Alpha Jet 0.716 20.3 5030 49300
IHI F3 turbofan 1981 Kawasaki T-4 0.7 19.8 5140 50400
Saturn AL-31F turbofan Su-27 /P/K 0.666-0.78[17][19] 18.9–22.1 4620–5410 45300–53000
RR Spey RB.168 turbofan AMX 0.66[16] 18.7 5450 53500
GE F110-GE-129 turbofan F-16C/D, F-15 0.64[19] 18 5600 55000
GE F110-GE-132 turbofan F-16E/F 0.64[19] 18 5600 55000
Turbo-Union RB.199 turbofan Tornado ECR 0.637[16] 18.0 5650 55400
PW F119-PW-100 turbofan 1992 F-22 0.61[19] 17.3 5900 57900
Turbo-Union RB.199 turbofan Tornado 0.598[16] 16.9 6020 59000
GE F101-GE-102 turbofan 1970s B-1B 0.562 15.9 6410 62800
PW TF33-P-3 turbofan B-52H, NB-52H 0.52[16] 14.7 6920 67900
RR AE 3007H turbofan RQ-4, MQ-4C 0.39[16] 11.0 9200 91000
GE F118-GE-100 turbofan 1980s B-2 0.375[16] 10.6 9600 94000
GE F118-GE-101 turbofan 1980s U-2S 0.375[16] 10.6 9600 94000
CFM CF6-50C2 turbofan A300, DC-10-30 0.371[16] 10.5 9700 95000
GE TF34-GE-100 turbofan A-10 0.37[16] 10.5 9700 95000
CFM CFM56-2B1 turbofan C-135, RC-135 0.36[20] 10 10000 98000
Progress D-18T turbofan 1980 An-124, An-225 0.345 9.8 10400 102000
PW F117-PW-100 turbofan C-17 0.34[21] 9.6 10600 104000
PW PW2040 turbofan Boeing 757 0.33[21] 9.3 10900 107000
CFM CFM56-3C1 turbofan 737 Classic 0.33 9.3 11000 110000
GE CF6-80C2 turbofan 744, 767, MD-11, A300/310, C-5M 0.307-0.344 8.7–9.7 10500–11700 103000–115000
EA GP7270 turbofan A380-861 0.299[19] 8.5 12000 118000
GE GE90-85B turbofan 777-200/200ER/300 0.298[19] 8.44 12080 118500
GE GE90-94B turbofan 777-200/200ER/300 0.2974[19] 8.42 12100 118700
RR Trent 970-84 turbofan 2003 A380-841 0.295[19] 8.36 12200 119700
GE GEnx-1B70 turbofan 787-8 0.2845[19] 8.06 12650 124100
RR Trent 1000C turbofan 2006 787-9 0.273[19] 7.7 13200 129000
Jet engines, cruise
Model Type First
run
Application TSFC SI
(s)
EEV
(m/s)
lb/lbf·h g/kN·s
Ramjet Mach 1 4.5 130 800 7800
J-58 turbojet 1958 SR-71 at Mach 3.2 (Reheat) 1.9[16] 53.8 1895 18580
RR/Snecma Olympus turbojet 1966 Concorde at Mach 2 1.195[22] 33.8 3010 29500
PW JT8D-9 turbofan 737 Original 0.8[23] 22.7 4500 44100
Honeywell ALF502R-5 GTF BAe 146 0.72[21] 20.4 5000 49000
Soloviev D-30KP-2 turbofan Il-76, Il-78 0.715 20.3 5030 49400
Soloviev D-30KU-154 turbofan Tu-154M 0.705 20.0 5110 50100
RR Tay RB.183 turbofan 1984 Fokker 70, Fokker 100 0.69 19.5 5220 51200
GE CF34-3 turbofan 1982 Challenger, CRJ100/200 0.69 19.5 5220 51200
GE CF34-8E turbofan E170/175 0.68 19.3 5290 51900
Honeywell TFE731-60 GTF Falcon 900 0.679[24] 19.2 5300 52000
CFM CFM56-2C1 turbofan DC-8 Super 70 0.671[21] 19.0 5370 52600
GE CF34-8C turbofan CRJ700/900/1000 0.67-0.68 19 5300–5400 52000–53000
CFM CFM56-3C1 turbofan 737 Classic 0.667 18.9 5400 52900
CFM CFM56-2A2 turbofan 1974 E-3, E-6 0.66[20] 18.7 5450 53500
RR BR725 turbofan 2008 G650/ER 0.657 18.6 5480 53700
CFM CFM56-2B1 turbofan C-135, RC-135 0.65[20] 18.4 5540 54300
GE CF34-10A turbofan ARJ21 0.65 18.4 5540 54300
CFE CFE738-1-1B turbofan 1990 Falcon 2000 0.645[21] 18.3 5580 54700
RR BR710 turbofan 1995 G. V/G550, Global Express 0.64 18 5600 55000
GE CF34-10E turbofan E190/195 0.64 18 5600 55000
CFM CF6-50C2 turbofan A300B2/B4/C4/F4, DC-10-30 0.63[21] 17.8 5710 56000
PowerJet SaM146 turbofan Superjet LR 0.629 17.8 5720 56100
CFM CFM56-7B24 turbofan 737 NG 0.627[21] 17.8 5740 56300
RR BR715 turbofan 1997 717 0.62 17.6 5810 56900
GE CF6-80C2-B1F turbofan 747-400 0.605[22] 17.1 5950 58400
CFM CFM56-5A1 turbofan A320 0.596 16.9 6040 59200
Aviadvigatel PS-90A1 turbofan Il-96-400 0.595 16.9 6050 59300
PW PW2040 turbofan 757-200 0.582[21] 16.5 6190 60700
PW PW4098 turbofan 777-300 0.581[21] 16.5 6200 60800
GE CF6-80C2-B2 turbofan 767 0.576[21] 16.3 6250 61300
IAE V2525-D5 turbofan MD-90 0.574[25] 16.3 6270 61500
IAE V2533-A5 turbofan A321-231 0.574[25] 16.3 6270 61500
RR Trent 700 turbofan 1992 A330 0.562 15.9 6410 62800
RR Trent 800 turbofan 1993 777-200/200ER/300 0.560 15.9 6430 63000
Progress D-18T turbofan 1980 An-124, An-225 0.546 15.5 6590 64700
CFM CFM56-5B4 turbofan A320-214 0.545 15.4 6610 64800
CFM CFM56-5C2 turbofan A340-211 0.545 15.4 6610 64800
RR Trent 500 turbofan 1999 A340-500/600 0.542 15.4 6640 65100
CFM LEAP-1B turbofan 2014 737 MAX 0.53-0.56 15–16 6400–6800 63000–67000
Aviadvigatel PD-14 turbofan 2014 MC-21-310 0.526 14.9 6840 67100
RR Trent 900 turbofan 2003 A380 0.522 14.8 6900 67600
GE GE90-85B turbofan 777-200/200ER 0.52[21][26] 14.7 6920 67900
GE GEnx-1B76 turbofan 2006 787-10 0.512[23] 14.5 7030 69000
PW PW1400G GTF MC-21 0.51[27] 14 7100 69000
CFM LEAP-1C turbofan 2013 C919 0.51 14 7100 69000
CFM LEAP-1A turbofan 2013 A320neo family 0.51[27] 14 7100 69000
RR Trent 7000 turbofan 2015 A330neo 0.506 14.3 7110 69800
RR Trent 1000 turbofan 2006 787 0.506 14.3 7110 69800
RR Trent XWB-97 turbofan 2014 A350-1000 0.478 13.5 7530 73900
PW 1127G GTF 2012 A320neo 0.463[23] 13.1 7780 76300
Specific impulse of various propulsion technologies
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 oxygen-liquid hydrogen
4,400 450 9.7
NSTAR[28] electrostatic xenon ion thruster 20,000-30,000 1,950-3,100
VASIMR predictions[29][30][31] 30,000–120,000 3,000–12,000 1,400
DS4G electrostatic ion thruster[32] 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 RS-25 engines when operating in a vacuum.[33] An air-breathing 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.[34]

An air-breathing 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.[35] While the actual exhaust velocity is lower for air-breathing 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 air-breathing engines; nevertheless, it is useful for comparison with other types of engines.[36]

The highest specific impulse for a chemical propellant ever test-fired 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.[37][38][39]

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.[40] 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.[41]

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 SMART-1 satellite has a specific impulse of 1,640 s (16.1 km/s) but a maximum thrust of only 68 mN (0.015 lbf).[42] 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).[43]

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