NationMaster - Encyclopedia: Specific impulse

Specific impulse (usually abbreviated I_sp) is a way to describe the efficiency of rocket and jet engines. It represents the impulse (change in momentum) per unit mass of propellant. The higher the specific impulse, the less propellant is needed to gain a given amount of momentum. Isp is a useful value to compare engines, much like "miles per gallon" is used for cars. A propulsion method with a higher specific impulse is more propellant-efficient. A cold (un-ignited) rocket engine test at NASA A rocket engine is a reaction engine that can be used for spacecraft propulsion as well as terrestrial uses, such as missiles. ... A Pratt and Whitney turbofan engine for the F-15 Eagle is tested at Robins Air Force Base, Georgia, USA. The tunnel behind the engine muffles noise and allows exhaust to escape. ... In classical mechanics, the impulse of a constant force is the product of the force and the time during which it acts. ... A propellant is a material that is used to move an object by applying a motive force. ...

Depending on whether the amount of propellant is expressed in mass or by weight (conventionally sea level weight on the Earth) the dimensions of specific impulse is either that of impulse per unit mass, or time, respectively, differing by a factor of g, the gravitational acceleration at the surface of the Earth. Adjectives: Terrestrial, Terran, Telluric, Tellurian, Earthly Atmosphere Surface pressure: 101. ... Dimensional analysis is a conceptual tool often applied in physics, chemistry, and engineering to understand physical situations involving a mix of different kinds of physical quantities. ... g (also gee, g-force or g-load) is a non-SI unit of acceleration defined as exactly 9. ...

General considerations

Essentially, the higher the specific impulse, the less propellant is needed to gain a given amount of momentum. In this regard a propulsion method is more propellant-efficient if the specific impulse is higher. This should not be confused with energy-efficiency, which can even decrease as specific impulse increases, since many propulsion systems that give high specific impulse require high energy to do so.

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 most cases, propulsion systems with very high specific impulses (such as ion thrusters: 3000 seconds) produce low thrusts. Thrust is a reaction force described quantitatively by Newtons Second and Third Law. ... This article or section does not cite its references or sources. ...

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. An oxidizing agent is a substance that oxidizes another substance in electrochemistry or redox chemical reactions in general. ...

Examples

An example of a specific impulse measured in time is 453 seconds, or, equivalently, an effective exhaust velocity of 4500 m/s, for the Space Shuttle Main Engines when operating in vacuum. A Pratt and Whitney turbofan engine for the F-15 Eagle is tested at Robins Air Force Base, Georgia, USA. The tunnel behind the engine muffles noise and allows exhaust to escape. ... The Space Shuttle Columbia is initially launched with the help of solid-fuel boosters A solid rocket or a solid fuel rocket is a rocket with a motor that uses solid propellants (fuel/oxidizer). ... F-1 rocket engine (The kind used by the Saturn V.) A bipropellant rocket engine is a rocket engine that uses two fluid propellants stored in separate tanks that are injected into, and undergo a strong exothermic reaction, in a rockets combustion chamber. ... This article or section does not cite its references or sources. ... VASIMR test bed The Variable specific impulse magnetoplasma rocket (VASIMR) is a hypothetical form of spacecraft propulsion that uses radio waves and magnetic fields to accelerate a propellant. ... Look up second in Wiktionary, the free dictionary. ... Metre per second (U.S. spelling: meter per second) is an SI derived unit of both speed (scalar) and velocity (vector), defined by distance in metres divided by time in seconds. ... Space Shuttle Main Engine cluster The Space Shuttle Main Engines (SSMEs) are the three main engines on the Space Shuttle orbiter. ...

An air-breathing engine typically has a much larger specific impulse than a rocket: a jet engine may have a specific impulse of 2000−3000 seconds or more at sea level. Note also that it is more efficient; mainly due to the actual exhaust speed being lower, particularly for turbofan jets, and thus the jet engine uses far less energy to generate thrust. Schematic diagram of high-bypass turbofan engine CFM56-3 turbofan, lower half, side view. ...

In some ways, comparing specific impulse seems unfair in the case of jet engines and rockets. However in rocket or jet powered aircraft, specific impulse is approximately proportional to range, and suborbital rockets do indeed perform much worse than jets in that regard.

The highest specific impulse for a chemical propellant ever test-fired in a rocket engine was lithium, fluorine, and hydrogen (a tripropellant): 542 seconds (5320 m/s). However, this combination is impractical; see rocket fuel. A Tripropellant rocket is a form of spacecraft propulsion that uses two fuels and one oxidizer. ... Rocket fuel is a propellant that reacts with an oxidizing agent to produce thrust in a rocket. ...

Nuclear thermal rocket engines differ from conventional rocket engines in that thrust is created strictly through thermodynamic phenomena, with no chemical reaction. The nuclear rocket typically operates by passing hydrogen gas over a superheated nuclear core. Testing in the 1960s yielded specific impulses of about 850 seconds (8340 m/s), about twice that of the Space Shuttle engines. In a nuclear thermal rocket a working fluid, usually hydrogen, is heated in a high temperature nuclear reactor, and then expands through a rocket nozzle to create thrust. ...

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 1640 s (16 100 m/s) but a maximum thrust of only 68 millinewtons. The hypothetical Variable specific impulse magnetoplasma rocket (VASIMR) propulsion should yield a minimum of 10 000−300 000 m/s but will probably require a great deal of heavy machinery to confine even relatively diffuse plasmas, so they will be unusable for very-high-thrust applications such as launch from planetary surfaces. This article or section does not cite its references or sources. ... 2 kW Laboratory Hall Thruster in operation at the Princeton Plasma Physics Laboratory A Hall effect thruster is a type of ion thruster in which the propellant is accelerated by an electric field in a plasma discharge with a radial magnetic field. ... SMART-1 was a Swedish-designed European Space Agency satellite that orbited around the Moon. ... VASIMR test bed The Variable specific impulse magnetoplasma rocket (VASIMR) is a hypothetical form of spacecraft propulsion that uses radio waves and magnetic fields to accelerate a propellant. ...

Units

By far the most common units 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 is 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.

The effective exhaust velocity 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 not precisely the same as the actual exhaust speed due to, for example, fuel and oxidizer that is dumped overboard after powering turbopumps. For airbreathing engines it is not physically meaningful although can be used for comparison purposes nevertherless.

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.) Newtons laws of motion are the three scientific laws which Isaac Newton discovered concerning the behaviour of moving bodies. ...

The units of ft/s were used by NASA during Apollo, but seems to have fallen into disuse, and NASA are moving towards using SI units wherever possible.

Specific impulse in seconds

For all vehicles specific impulse (impulse per unit weight-on-Earth of propellant) in seconds can be defined by the following equation^[1]:

(When working with English units, it is conventional to divide both sides of the equation by g₀ so that the left hand side of the equation has units of lbs rather than expressing it in poundals.) The newton (symbol: N) is the SI unit of force. ... The poundal is a non-SI unit of force. ... Mass flow rate is the movement of mass per time. ... English unit is an American term that refers to a unit in one of a number of systems of units of measurement, some obsolete, and some still in use. ...

This I_sp in seconds value is somewhat physically meaningful—if an engine's thrust could be adjusted to equal the initial weight of its propellant (measured at one standard gravity), then I_sp is the duration the propellant would last. g (also gee, g-force or g-load) is a non-SI unit of acceleration defined as exactly 9. ...

The advantage that this formulation has is that it may be used for rockets, where all the reaction mass is carried onboard, as well as aeroplanes, 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).

Image File history File links Download high resolution version (800x648, 58 KB) Summary From wikibooks JetPropulsion/Specific Impulse Licensing Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1. ... Image File history File links Download high resolution version (800x648, 58 KB) Summary From wikibooks JetPropulsion/Specific Impulse Licensing Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1. ...

Rocketry - specific impulse in seconds

In rocketry, where the only reaction mass is the propellant, an equivalent way of calculating the specific impulse in seconds is also frequently used. In this sense, specific impulse is defined as the change in momentum per unit weight-on-Earth of the propellant: A spring scale measures the weight of an object In the physical sciences, weight is a measurement of the gravitational force acting on an object. ...

v e

is the average exhaust speed along the axis of the engine in (ft/s or m/s)

In rockets, due to atmospheric effects, the specific impulse varies with altitude, reaching a maximum in a vacuum. It is therefore most common to see the specific impulse quoted for the vehicle in a vacuum; the lower sea level values are usually indicated in some way (e.g. 'sl').

Rocketry - specific impulse as a speed (effective exhaust velocity)

Because of the geocentric factor of g₀ in the equation for specific impulse, many prefer to define the specific impulse of a rocket in terms of thrust per unit mass flow of propellant (instead of per unit weight flow). This is an equally valid (and 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, v_e. The two definitions of specific impulse are proportional to one another, and related to each other by:

v_e is the specific impulse measured in metres per second, which is the same as the effective exhaust velocity measured in metres per second (former usage in the U.S. was feet/second, though that is now obsolete) Metre per second (U.S. spelling: meter per second) is an SI derived unit of both speed (scalar) and velocity (vector), defined by distance in metres divided by time in seconds. ...

g₀ is the acceleration due to gravity at the Earth's surface, 9.81 m/s² (in English units 32.2 ft/s²).

(Note that different symbols are sometimes used; for example, c is also sometimes seen for exhaust velocity. While the symbol I_sp might logically be used for specific impulse in units of N•s/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: Thrust is a reaction force described quantitatively by Newtons Second and Third Law. ...

$frac {Delta m} {Delta t}$ is the mass flow rate, which is minus the time-rate of change of the vehicle's mass, since propellant is being expelled.

A rocket must carry all its fuel with it, so the mass of the unburned fuel must be accelerated along with the rocket itself. Minimizing the mass of fuel required to achieve a given push is crucial to building effective rockets. Using Newton's laws of motion it is not difficult to verify that for a fixed mass of fuel, the total change in velocity (in fact, momentum) it can accomplish can only be increased by increasing the effective exhaust velocity. Newtons First and Second laws, in Latin, from the original 1687 edition of the Principia Mathematica. ... The velocity of an object is its speed in a particular direction. ...

A spacecraft without propulsion follows an orbit determined by the 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. In general physics, delta-v is simply the change in velocity. ...

References

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