Specific impulse

Power ranges specific to various engine types: explosion engine (1), turboreactor (2), statorreactor (3) and rocket engine (4).

The specific impulse is the period in seconds during which 1 kg of mass of propellant (the fuel and oxidizer together) will produce a thrust of 1 kg of force. Although specific impulse is a characteristic of the propellant system, its exact value varies according to some operating conditions and rocket engine design. For this reason different numbers are assigned to a certain propellant or combination thereof.

As an important characteristic of fuels, the number indicates the efficiency of the propellants. The higher the number, the higher the efficiency.

General considerations

The amount of propellant is usually measured in units of mass or weight. If mass is used, the specific impulse is one impulse per unit of mass, which dimensional analysis shows units of velocity, so specific impulses are often measured in meters per second and often referred to as exhaust velocity. effective. However, if the weight of the propellant is used, an impulse divided by a force (weight) turns out to be a unit of time, and therefore specific impulses are measured in seconds. These two formulations are widely used and differ from each other by a factor of g0, the dimensioned constant of gravitational acceleration at the Earth's surface.

Note that 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 needed to produce a given thrust for a given time. In this sense, a propellant is more efficient the greater its specific impulse. This is not to be confused with power efficiency, which can decrease as specific impulse increases, since propulsion systems that give high specific impulse require high energy to do so.

It is important that thrust and specific impulse are not confused. Specific impulse is a measure of the impulse produced per unit of propellant spent, while thrust is a measure of the momentary or maximum force delivered 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 a specific impulse, only propellant transported with the vehicle prior to use is counted. For a chemical rocket, the propellant mass would therefore include both fuel and oxidant; 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 inability of the engine to maintain a high specific impulse at a fast burn rate are the reasons propellant is not used as quickly as possible.

If an engine weighs more to gain a higher specific impulse, it may not be as efficient at gaining altitude, distance, or speed as a lighter engine that has a lower specific impulse.

If it weren't for air resistance and propellant reduction during flight, specific impulse would be a direct measure of the engine's efficiency in converting the weight or mass of the propellant into forward momentum.

Examples

An example of a specific impulse measured in time is 453 seconds, which is equivalent to an effective escape velocity of 4,439 m/s (when multiplied by Earth's gravity), for the space shuttle main engines when operated in a vacuum. A jet engine, or misnamed jet engine, typically has a much larger specific impulse than a rocket engine; For example a turbofan jet engine can have a specific impulse of 6,000 seconds or more at sea level, while a rocket would be around 200-400 seconds.

A combustion or jet engine is therefore much more propellant efficient than a rocket engine, because although the actual exhaust velocity is much lower, the air provides an oxidizer, and the air is used as the mass of reaction. Since the physical exhaust velocity is lower, the kinetic energy carried by the exhaust is less, and therefore the jet engine uses much less energy to generate thrust (at subsonic speeds). actual exhaust is less for these engines, the effective exhaust velocity is very high. This is because the effective escape velocity calculation essentially assumes that the propellant provides all the thrust, and is therefore not physically significant; However, it is useful for comparison with other engine types.

The highest specific impulse for a chemical propellant tested in a rocket engine was 542 seconds (5,320 m/s) with a tripropylant of lithium, fluorine, and hydrogen. However, this combination is not practical; 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. Tests in the 1960s produced specific impulses of about 850 seconds (8,340 m/s), about twice as long as space shuttle engines.

A variety of other non-rocket propulsion methods, such as ion thrusters, give much higher specific impulse but 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. The Variable Impulse Magnetoplasma Rocket Engine (VASIMR) currently under development it will theoretically produce 20,000-300,000 m/s and a maximum thrust of 5.7 newtons.