Specific impulse
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Specific impulse (usually abbreviated Isp) is a way to describe the efficiency of rocket and jet engines. It represents the impulse (change in momentum) per unit amount of propellant used.[1] The unit amount may be given either per unit mass (such as kilograms), or per unit Earth-weight (such as kiloponds, since g is used for the latter definition).[2] The higher the specific impulse, the less propellant is needed to gain a given amount of momentum.
The actual exhaust velocity is the average speed that the exhaust jet actually leaves the vehicle. The effective exhaust velocity is the speed that the propellant burned per second would have to leave the vehicle to give the same thrust. The two are about the same for a rocket working in a vacuum, but are radically different for an air breathing jet engine that obtains extra thrust by accelerating air. Specific impulse and effective exhaust velocity are proportional.
Specific impulse is a useful value to compare engines, much like miles per gallon or litres per 100 kilometres is used for cars. A propulsion method with a higher specific impulse is more propellant-efficient.[1] Another number that measures the same thing, usually used for air breathing jet engines, is specific fuel consumption. Specific fuel consumption is inversely proportional to specific impulse and effective exhaust velocity.
General considerations
Rocket motor propellant is normally 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 be a unit of speed, and so specific impulses are often measured in meters per second and are often termed effective exhaust velocity. However, if propellant weight is used instead, an impulse divided by a force (weight) turns out to be a unit of time, and so specific impulses are measured in seconds. These two formulations are both widely used and differ from each other by a factor of g, the dimensioned constant of gravitational acceleration at the surface of the Earth.
Essentially, the higher the specific impulse, the less propellant is needed to gain a given amount of momentum. In this regard a propellant is more efficient if the specific impulse is higher. This should not be confused with energy efficiency, which can even decrease as specific impulse increases, since propulsion systems that give high specific impulse require high energy to do so.[3]
In addition it is important that thrust and specific impulse not be confused with one another. The specific impulse is a measure of the impulse per unit of propellant that is expended, while thrust is a measure of the momentary or peak force supplied by a particular engine. In many cases, propulsion systems with very high specific impulses—some ion thrusters reach 10,000 seconds—produce low thrusts.[4]
When calculating specific impulse, only propellant that is carried with the vehicle before use is counted. For a chemical rocket the propellant mass therefore would include both fuel and oxidizer; for air-breathing engines only the mass of the fuel is counted, not the mass of air passing through the engine.
Examples
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[5] 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[6][7][8] | 30,000–120,000 | 3,000–12,000 | 1,400 |
DS4G electrostatic ion thruster[9] | 210,000 | 21,400 | 22,500 |
Ideal photonic rocket[a] | 299,792,458 | 30,570,000 | 89,875,517,874 |
- For a more complete list see: Spacecraft propulsion#Table of methods
An example of a specific impulse measured in time is 453 seconds, which is equivalent to an effective exhaust velocity of 4,440 m/s, for the Space Shuttle Main Engines when operating in a vacuum.[10] 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 around 200–400 seconds.[11]
An air-breathing engine is thus much more propellant efficient than a rocket engine, because the actual exhaust speed is much lower, the air provides an oxidizer, and air is used as reaction mass. Since the physical exhaust velocity is lower, the kinetic energy the exhaust carries away is lower and thus the jet engine uses far less energy to generate thrust (at subsonic speeds).[12] 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 essentially assumes that the propellant is providing all the thrust, and hence is not physically meaningful for air-breathing engines; nevertheless, it is useful for comparison with other types of engines.[13]
The highest specific impulse for a chemical propellant ever test-fired in a rocket engine was lithium, fluorine, and hydrogen (a tripropellant): 542 seconds (5,320 m/s). However, this combination is impractical; see rocket fuel.[14]
Nuclear thermal rocket engines differ from conventional rocket engines in that thrust is created strictly through thermodynamic phenomena, with no chemical reaction.[15] The nuclear rocket typically operates by passing hydrogen gas through a superheated nuclear core. Testing in the 1960s yielded specific impulses of about 850 seconds (8,340 m/s), about twice that of the Space Shuttle engines.
A variety of other non-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 (16,100 m/s) but a maximum thrust of only 68 milliNewtons.[16] The hypothetical Variable specific impulse magnetoplasma rocket (VASIMR) propulsion would theoretically yield a minimum of 10,000−300,000 m/s but would probably require a great deal of heavy machinery to confine even relatively diffuse plasmas, and so would be unusable for high-thrust applications such as launch from planetary surfaces.[17]
Model rocketry
Specifc impulse is also used to measure performance in model rocket motors. Following are some of Estes' claimed values for specific impulses for several of their rocket motors:[18] Estes Industries is a large, well knows American seller of model rocket components. The specific impulse for model rocket motors is much lower than for many other rocket motors because the manufacturer uses black powder propellant. The burn rate of model rocket motors is also tightly controlled.
Engine | Total Impulse (Ns) | Fuel Mass (N) | Specific Impulse (s) |
---|---|---|---|
Estes A10-3T | 2.5 | .0370 | 67.49 |
Estes A8-3 | 2.5 | .0306 | 81.76 |
Estes B4-2 | 5.0 | .0816 | 61.25 |
Estes B6-4 | 5.0 | .0612 | 81.76 |
Estes C6-3 | 10 | .1223 | 81.76 |
Estes C11-5 | 10 | .1078 | 92.76 |
Estes D12-3 | 20 | .2443 | 81.86 |
Estes E9-6 | 30 | .3508 | 85.51 |
Larger Rocket Engines
Here are some example numbers for larger rocket engines:
Rocket engines in vacuum | |||||||
---|---|---|---|---|---|---|---|
Model | Type | First run |
Application | TSFC | Isp (by weight) | Isp (by mass) | |
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[19] | 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[20] | 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 | Isp (by weight) | Isp (by mass) | |
lb/lbf·h | g/kN·s | s | m/s | ||||
Turbo-Union RB.199 | turbofan | Tornado | 2.5[21] | 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[21] | 62.5 | 1632 | 16000 | |
GE J85-GE-21 | turbojet | F-5E/F | 2.13[21] | 60.3 | 1690 | 16570 | |
GE F110-GE-132 | turbofan | F-16E/F | 2.09[21] | 59.2 | 1722 | 16890 | |
Honeywell/ITEC F125 | turbofan | F-CK-1 | 2.06[21] | 58.4 | 1748 | 17140 | |
Snecma M53-P2 | turbofan | Mirage 2000C/D/N | 2.05[21] | 58.1 | 1756 | 17220 | |
Snecma Atar 09C | turbojet | Mirage III | 2.03[21] | 57.5 | 1770 | 17400 | |
Snecma Atar 09K-50 | turbojet | Mirage IV, 50, F1 | 1.991[21] | 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[22] | 55.5 | 1837 | 18010 | |
GE F110-GE-129 | turbofan | F-16C/D, F-15EX | 1.9[21] | 53.8 | 1895 | 18580 | |
Soloviev D-30F6 | turbofan | MiG-31, S-37/Su-47 | 1.863[21] | 52.8 | 1932 | 18950 | |
Lyulka AL-21F-3 | turbojet | Su-17, Su-22 | 1.86[21] | 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[21] | 50.4 | 2022 | 19830 |
GE F404-GE-402 | turbofan | F/A-18C/D | 1.74[21] | 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[23] | 2080–2170 | 20400–21300 |
Dry jet engines, static, sea level | |||||||
---|---|---|---|---|---|---|---|
Model | Type | First run |
Application | TSFC | Isp (by weight) | Isp (by mass) | |
lb/lbf·h | g/kN·s | s | m/s | ||||
GE J85-GE-21 | turbojet | F-5E/F | 1.24[21] | 35.1 | 2900 | 28500 | |
Snecma Atar 09C | turbojet | Mirage III | 1.01[21] | 28.6 | 3560 | 35000 | |
Snecma Atar 09K-50 | turbojet | Mirage IV, 50, F1 | 0.981[21] | 27.8 | 3670 | 36000 | |
Snecma Atar 08K-50 | turbojet | Super Étendard | 0.971[21] | 27.5 | 3710 | 36400 | |
Tumansky R-25-300 | turbojet | MIG-21bis | 0.961[21] | 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[21] | 24.1 | 4240 | 41500 | |
Volvo RM12 | turbofan | 1978 | Gripen A/B/C/D | 0.824[21] | 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[21] | 22.9 | 4440 | 43600 |
Honeywell/ITEC F125 | turbofan | F-CK-1 | 0.8[21] | 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[23] | 4400–4900 | 44000–48000 |
GE F414-GE-400 | turbofan | 1993 | F/A-18E/F | 0.724[24] | 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[21] | 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[22][24] | 18.9–22.1 | 4620–5410 | 45300–53000 | |
RR Spey RB.168 | turbofan | AMX | 0.66[21] | 18.7 | 5450 | 53500 | |
GE F110-GE-129 | turbofan | F-16C/D, F-15 | 0.64[24] | 18 | 5600 | 55000 | |
GE F110-GE-132 | turbofan | F-16E/F | 0.64[24] | 18 | 5600 | 55000 | |
Turbo-Union RB.199 | turbofan | Tornado ECR | 0.637[21] | 18.0 | 5650 | 55400 | |
PW F119-PW-100 | turbofan | 1992 | F-22 | 0.61[24] | 17.3 | 5900 | 57900 |
Turbo-Union RB.199 | turbofan | Tornado | 0.598[21] | 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[21] | 14.7 | 6920 | 67900 | |
RR AE 3007H | turbofan | RQ-4, MQ-4C | 0.39[21] | 11.0 | 9200 | 91000 | |
GE F118-GE-100 | turbofan | 1980s | B-2 | 0.375[21] | 10.6 | 9600 | 94000 |
GE F118-GE-101 | turbofan | 1980s | U-2S | 0.375[21] | 10.6 | 9600 | 94000 |
General Electric CF6-50C2 | turbofan | A300, DC-10-30 | 0.371[21] | 10.5 | 9700 | 95000 | |
GE TF34-GE-100 | turbofan | A-10 | 0.37[21] | 10.5 | 9700 | 95000 | |
CFM CFM56-2B1 | turbofan | C-135, RC-135 | 0.36[25] | 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[26] | 9.6 | 10600 | 104000 | |
PW PW2040 | turbofan | Boeing 757 | 0.33[26] | 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[24] | 8.5 | 12000 | 118000 | |
GE GE90-85B | turbofan | 777-200/200ER/300 | 0.298[24] | 8.44 | 12080 | 118500 | |
GE GE90-94B | turbofan | 777-200/200ER/300 | 0.2974[24] | 8.42 | 12100 | 118700 | |
RR Trent 970-84 | turbofan | 2003 | A380-841 | 0.295[24] | 8.36 | 12200 | 119700 |
GE GEnx-1B70 | turbofan | 787-8 | 0.2845[24] | 8.06 | 12650 | 124100 | |
RR Trent 1000C | turbofan | 2006 | 787-9 | 0.273[24] | 7.7 | 13200 | 129000 |
Jet engines, cruise | |||||||
---|---|---|---|---|---|---|---|
Model | Type | First run |
Application | TSFC | Isp (by weight) | Isp (by mass) | |
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[21] | 53.8 | 1895 | 18580 |
RR/Snecma Olympus | turbojet | 1966 | Concorde at Mach 2 | 1.195[27] | 33.8 | 3010 | 29500 |
PW JT8D-9 | turbofan | 737 Original | 0.8[28] | 22.7 | 4500 | 44100 | |
Honeywell ALF502R-5 | GTF | BAe 146 | 0.72[26] | 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[29] | 19.2 | 5300 | 52000 | |
CFM CFM56-2C1 | turbofan | DC-8 Super 70 | 0.671[26] | 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[25] | 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[25] | 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[26] | 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[26] | 17.8 | 5710 | 56000 | |
PowerJet SaM146 | turbofan | Superjet LR | 0.629 | 17.8 | 5720 | 56100 | |
CFM CFM56-7B24 | turbofan | 737 NG | 0.627[26] | 17.8 | 5740 | 56300 | |
RR BR715 | turbofan | 1997 | 717 | 0.62 | 17.6 | 5810 | 56900 |
GE CF6-80C2-B1F | turbofan | 747-400 | 0.605[27] | 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[26] | 16.5 | 6190 | 60700 | |
PW PW4098 | turbofan | 777-300 | 0.581[26] | 16.5 | 6200 | 60800 | |
GE CF6-80C2-B2 | turbofan | 767 | 0.576[26] | 16.3 | 6250 | 61300 | |
IAE V2525-D5 | turbofan | MD-90 | 0.574[30] | 16.3 | 6270 | 61500 | |
IAE V2533-A5 | turbofan | A321-231 | 0.574[30] | 16.3 | 6270 | 61500 | |
RR Trent 700 | turbofan | 1992 | A330 | 0.562[31] | 15.9 | 6410 | 62800 |
RR Trent 800 | turbofan | 1993 | 777-200/200ER/300 | 0.560[31] | 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[31] | 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[31] | 14.8 | 6900 | 67600 |
GE GE90-85B | turbofan | 777-200/200ER | 0.52[26][32] | 14.7 | 6920 | 67900 | |
GE GEnx-1B76 | turbofan | 2006 | 787-10 | 0.512[28] | 14.5 | 7030 | 69000 |
PW PW1400G | GTF | MC-21 | 0.51[33] | 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[33] | 14.4 | 7100 | 69000 |
RR Trent 7000 | turbofan | 2015 | A330neo | 0.506[b] | 14.3 | 7110 | 69800 |
RR Trent 1000 | turbofan | 2006 | 787 | 0.506[c] | 14.3 | 7110 | 69800 |
RR Trent XWB-97 | turbofan | 2014 | A350-1000 | 0.478[d] | 13.5 | 7530 | 73900 |
PW 1127G | GTF | 2012 | A320neo | 0.463[28] | 13.1 | 7780 | 76300 |
Units
|
Specific Impulse (by weight) |
Specific Impulse (by mass) |
Effective exhaust velocity |
Specific fuel consumption |
---|---|---|---|---|
SI | =X seconds | =9.8066 X N•s/kg | =9.8066 X m/s | =(101,972/X) g/kN•s |
English units | =X seconds | =X lbf•s/lb | =32.16 X ft/s | =(3,600/X) lb/lbf•h |
By far the most common unit used for specific impulse today is the second, and this is used both in the SI world as well as where English units are used. Its chief advantages are that its units and numerical value are identical everywhere, and essentially everyone understands it. Nearly all manufacturers quote their engine performance in these units and it is also useful for specifying aircraft engine performance.[34]
The effective exhaust velocity in units of m/s is also in reasonably common usage. For rocket engines it is reasonably intuitive, although for many rocket engines the effective exhaust speed is considerably different from the actual exhaust speed due to, for example, fuel and oxidizer that is dumped overboard after powering turbopumps. For airbreathing engines the effective exhaust velocity is not physically meaningful, although it can be used for comparison purposes nevertheless.[35]
The N•s/kg is not uncommonly seen, and is numerically equal to the effective exhaust velocity in m/s (from Newton's second law and the definition of the Newton.)
Another equivalent unit is specific fuel consumption. This has units of g/kN.s or lbf/lb•h and is inversely proportional to specific impulse. Specific fuel consumption is used extensively for describing the performance of air-breathing jet engines.[36]
References
- ^ a b "What is specific impulse?". Qualitative Reasoning Group. Retrieved 22 December 2009.
- ^ Benson, Tom (11 July 2008). "Specific impulse". NASA. Retrieved 22 December 2009.
- ^ http://www.geoffreylandis.com/laser_ion_pres.htp
- ^ "Mission Overview". exploreMarsnow. Retrieved 23 December 2009.
- ^ In-flight performance of the NSTAR ion propulsion system on the Deep Space One mission. Aerospace Conference Proceedings. IEEExplore. 2000. doi:10.1109/AERO.2000.878373.
- ^ Glover, Tim W.; Chang Diaz, Franklin R.; Squire, Jared P.; Jacobsen, Verlin; Chavers, D. Gregory; Carter, Mark D. "Principal VASIMR Results and Present Objectives" (PDF).
- ^ Cassady, Leonard D.; Longmier, Benjamin W.; Olsen, Chris S.; Ballenger, Maxwell G.; McCaskill, Greg E.; Ilin, Andrew V.; Carter, Mark D.; Gloverk, Tim W.; Squire, Jared P.; Chang, Franklin R.; Bering, III, Edgar A. (28 July 2010). "VASIMR R Performance Results" (PDF). www.adastra.com.
- ^ "Vasimr VX 200 meets full power efficiency milestone". spacefellowship.com. Retrieved 2021-05-13.
- ^ "ESA and Australian team develop breakthrough in space propulsion". cordis.europa.eu. 18 January 2006.
- ^ http://www.astronautix.com/engines/ssme.htm
- ^ http://web.mit.edu/16.unified/www/SPRING/propulsion/notes/node85.html
- ^ http://www.dunnspace.com/isp.htm
- ^ http://www.britannica.com/EBchecked/topic/198045/effective-exhaust-velocity
- ^ ARBIT, H. A., CLAPP, S. D., DICKERSON, R. A., NAGAI, C. K., Combustion characteristics of the fluorine-lithium/hydrogen tripropellant combination. AMERICAN INST OF AERONAUTICS AND ASTRONAUTICS, PROPULSION JOINT SPECIALIST CONFERENCE, 4TH, CLEVELAND, OHIO, Jun 10-14, 1968.
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- ^ a b c d e f g h i j k l m n o p q r s t u v w x y z aa ab ac ad ae af ag Nathan Meier (21 Mar 2005). "Military Turbojet/Turbofan Specifications". Archived from the original on 11 February 2021.
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External links
- RPA - Design Tool for Liquid Rocket Engine Analysis
- List of Specific Impulses of various rocket fuels
See also
- Jet engine
- Impulse - the change in momentum
- Tsiolkovsky rocket equation
- System-specific impulse
- Specific energy
- 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)
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