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 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" or "liters per kilometer" is used for cars. A propulsion method with a higher specific impulse is more propellant-efficient.
Propellant is normally measured either in units of mass, or in units of weight at sea level on Earth. 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.
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.
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 (discussed later).
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.
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.
(For a more complete list see: Spacecraft propulsion)
An air-breathing jet engine typically has a much larger specific impulse than a rocket: a turbofan jet engine may have a specific impulse of 6000 seconds or more at sea level whereas a rocket would be around 200-400 seconds. Note that an air-breathing engine is thus much more propellant efficient; this is because the actual exhaust speed is much lower, because air provides oxidiser, and because air is used as reaction mass. Since the actual, physical exhaust velocity is lower, for at least subsonic speeds the kinetic energy the exhaust carries away is lower and thus the jet engine uses far less energy to generate thrust. This is so even allowing for the fact that more air must be exhausted at lower speeds to get the same thrust as a smaller amount of air at higher speeds.
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.
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.
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 through 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.
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.
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.)
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.
The lbf·s/lb unit sees little use but is covered in some textbooks.
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. This is used extensively for describing air-breathing jet engines.
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:
This Isp 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 Isp is the duration the propellant would last.
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).The specific impulse of various hydrocarbon fuelled jet engines
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:
Isp is the specific impulse measured in seconds
ve is the average exhaust speed along the axis of the engine in (ft/s or m/s)
g0 is the acceleration at the Earth's surface (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').
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 particular) in terms of thrust per unit mass flow of propellant (instead of per unit weight flow). 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. The two definitions of specific impulse are proportional to one another, and related to each other by:
This equation is also valid for airbreathing jet engines, but is rarely used in practice.
(Note that different symbols are sometimes used; for example, c is also sometimes seen for exhaust velocity. While the symbol Isp 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:
is the propellant mass flow rate, which is the rate of decrease of the vehicle's mass
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.
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.
Actual exhaust speed versus effective exhaust speed
Note that effective exhaust velocity and actual exhaust velocity are very often significantly different, for example when a rocket is run within the atmosphere, atmospheric pressure on the outside of the engine causes a retarding force that reduces the specific impulse and the effective exhaust velocity goes down, whereas the actual exhaust velocity is largely unaffected. Also, sometimes rocket engines have a separate nozzle for the turbopump turbine gas, and then the effective exhaust velocity is an average of the two mass flows.
For airbreathing jet engines, particularly, turbofans, the actual exhaust velocity and the effective exhaust velocity are different by orders of magnitude. This is because a good deal of additional momentum is obtained by using air as reaction mass. This allows for 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.
For rockets and rocket-like engines such as ion-drives a higher Isp implies lower energy efficiency: the power needed to run the engine is simply:
where Ve is the actual jet velocity.
whereas from momentum considerations the thrust generated is:
Dividing the power by the thrust to obtain the specific power requirements we get:
Hence the power needed is proportional to the exhaust velocity, with higher velocities needing higher power for the same thrust, and thus are less energy efficient.
However, this is not true in general, for example, air-breathing engines such as turbojets increase the momentum generated from their propellant by using it to power the acceleration of inert air rearwards. It turns out that the amount of energy needed to generate a particular amount of thrust is inversely proportional to the amount of air propelled rearwards, thus increasing the mass of air (as with a turbofan) both improves energy efficiency as well as Isp.
Figures for real engines
Published - July 2009
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