Specific impulse
Specific impulse (usually abbreviated Isp) is a measure of how efficiently a
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.
Propulsion systems
For engines like cold gas thrusters whose reaction mass is only the fuel they carry, specific impulse is exactly proportional to the effective exhaust gas velocity.
In atmosphere
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, such as by fuel combustion or by external propeller.
For air-breathing engines, only the fuel mass 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 limiting factors to the propellant consumption rate. 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 mass into forward momentum.
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.
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
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
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
The use of
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 impulse in seconds
This section needs additional citations for verification. (August 2019) |
Specific impulse, measured in seconds, effectively means how many seconds a given propellant, when paired with a given engine, can accelerate its own initial mass at 1 g. The longer 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]
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/s2 or ft/s2),
- is the specific impulse measured (seconds),
- is the mass flow rate of the expended propellant (kg/s or slugs/s)
The
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, , 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).
Rocketry
In rocketry, the only reaction mass is the propellant, so the specific impulse is calculated using an alternative method, giving results with units of seconds. Specific impulse is defined as the thrust integrated over time per unit weight-on-Earth 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/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
This section needs additional citations for verification. (August 2019) |
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:
- 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/s2),
- 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 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]
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
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]
Specific fuel consumption
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).
Examples
Rocket engines in vacuum
| |||||||
---|---|---|---|---|---|---|---|
Model | Type | First run |
Application | TSFC
|
Isp (by weight)
|
Isp (by weight) | |
lb/lbf·h | g/kN·s | s | m/s | ||||
Merlin 1D | liquid fuel | 2013 | Falcon 9 | 12 | 330 | 310 | 3000 |
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 | cryogenic | 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 |
Model | Type | First run |
Application | TSFC
|
Isp (by weight)
|
Isp (by weight) | |
---|---|---|---|---|---|---|---|
lb/lbf·h | g/kN·s | s | m/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
|
Isp (by weight)
|
Isp (by weight) | |
lb/lbf·h | g/kN·s | s | m/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-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 |
General Electric 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
|
Isp (by weight)
|
Isp (by weight) | |
lb/lbf·h | g/kN·s | s | m/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 | 19–19 | 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 | |
General Electric 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[26] | 15.9 | 6410 | 62800 |
RR Trent 800 | turbofan | 1993 | 777-200/200ER/300 | 0.560[26] | 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[26] | 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[26] | 14.8 | 6900 | 67600 |
GE GE90-85B | turbofan | 777-200/200ER | 0.52[21][27] | 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[28] | 14.4 | 7100 | 69000 | |
CFM LEAP-1C | turbofan | 2013 | C919 | 0.51 | 14.4 | 7100 | 69000 |
CFM LEAP-1A | turbofan | 2013 | A320neo family | 0.51[28] | 14.4 | 7100 | 69000 |
RR Trent 7000 | turbofan | 2015 | A330neo | 0.506[a] | 14.3 | 7110 | 69800 |
RR Trent 1000 | turbofan | 2006 | 787 | 0.506[b] | 14.3 | 7110 | 69800 |
RR Trent XWB-97 | turbofan | 2014 | A350-1000 | 0.478[c] | 13.5 | 7530 | 73900 |
PW 1127G | GTF | 2012 | A320neo | 0.463[23] | 13.1 | 7780 | 76300 |
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[29] electrostatic xenon ion thruster | 20,000–30,000 | 1,950–3,100 | |
NEXT electrostatic xenon ion thruster
|
40,000 | 1,320–4,170 | |
VASIMR predictions[30][31][32] | 30,000–120,000 | 3,000–12,000 | 1,400 |
DS4G electrostatic ion thruster[33] | 210,000 | 21,400 | 22,500 |
Ideal photonic rocket[d] | 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
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.[36] 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.[37]
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.[38][39][40]
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.[41] 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.[42]
A variety of other rocket propulsion methods, such as
See also
- Jet engine
- Impulse
- Tsiolkovsky rocket equation
- System-specific 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
- Delta-v (physics)
- Rocket propellant
- Liquid rocket propellants
Notes
References
- ^ "What is specific impulse?". Qualitative Reasoning Group. Retrieved 22 December 2009.
- ^ Hutchinson, Lee (14 April 2013). "New F-1B rocket engine upgrades Apollo-era design with 1.8M lbs of thrust". Ars Technica. Retrieved 15 April 2013.
The measure of a rocket's fuel effectiveness is called its specific impulse (abbreviated as 'ISP'—or more properly Isp).... 'Mass specific impulse ... describes the thrust-producing effectiveness of a chemical reaction and it is most easily thought of as the amount of thrust force produced by each pound (mass) of fuel and oxidizer propellant burned in a unit of time. It is kind of like a measure of miles per gallon (mpg) for rockets.'
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- ^ Rocket Propulsion Elements, 7th Edition by George P. Sutton, Oscar Biblarz
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- ^ 10% better than Trent 700
- ^ 10% better than Trent 700
- ^ 15 per cent fuel consumption advantage over the original Trent engine
- ^ A hypothetical device doing perfect conversion of mass to photons emitted perfectly aligned so as to be antiparallel to the desired thrust vector. This represents the theoretical upper limit for propulsion relying strictly on onboard fuel and the rocket principle.