Specific impulse
For any chemical rocket engine, the momentum transfer efficiency depends heavily on the effectiveness of the nozzle; the nozzle is the primary means of converting reactant energy (e.g. thermal or pressure energy) into a flow of momentum all directed the same way.
Therefore, nozzle shape and effectiveness has a great impact on total momentum transfer from the reaction mass to the rocket.
In chemical and cold gas rockets, the shape of the nozzle has a high impact on the energy-to-momentum conversion, and is never perfect, and there are other sources of losses and inefficiencies (e.g. the details of the combustion in such engines).
Ion engines operate without a nozzle, although they have other sources of losses such that the momentum transferred is lower than the physical exhaust velocity.
Although the car industry almost never uses specific impulse on any practical level, the measure can be defined, and makes good contrast against other engine types.
Car engines breathe external air to combust their fuel, and (via the wheels) react against the ground.
As such, the only meaningful way to interpret "specific impulse" is as "thrust per fuelflow", although one must also specify if the force is measured at the crankshaft or at the wheels, since there are transmission losses.
In many cases, propulsion systems with very high specific impulse—some ion thrusters reach 25x-35x better Isp than chemical engines—produce correspondingly low thrust.
For a chemical rocket, unlike a plane or car, the propellant mass therefore would include both fuel and oxidizer.
The first stage can optimised for high thrust to effectively fight gravity drag and air drag, while the later stages operating strictly in orbit and in vacuum can be much easier optimised for higher specific impulse, especially for high delta-v orbits.
The problem with weight, as a measure of quantity, is that it depends on the acceleration applied to the propellant, which is arbitrary with no relation to the design of the engine.
But since technology has progressed to the point that we can measure Earth gravity's variation across the surface, and where such differences can cause differences in practical engineering projects (not to mention science projects on other solar bodies), modern science and engineering focus on mass as the measure of quantity, so as to remove the acceleration dependence.
As such, measuring specific impulse by propellant mass gives it the same meaning for a car at sea level, an airplane at cruising altitude, or a helicopter on Mars.
No matter the choice of mass or weight, the resulting quotient of "velocity" or "time" has no physical meaning.
However, the choice of reference acceleration conversion, (g0) is arbitrary, and as above, the interpretation in terms of time or speed has no physical meaning.
For airbreathing jet engines, the effective exhaust velocity is not physically meaningful, although it can be used for comparison purposes.
[6] 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.
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.
where In rockets, due to atmospheric effects, the specific impulse varies with altitude, reaching a maximum in a vacuum.
The specific impulse of a rocket can be defined in terms of thrust per unit mass flow of propellant.
A uniform axial velocity, ve, is assumed for all calculations which employ one-dimensional problem descriptions.
Minimizing the mass of propellant required to achieve a given change in velocity is crucial to building effective rockets.
Calculating the effective exhaust velocity requires averaging the two mass flows as well as accounting for any atmospheric pressure.
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.
[14] 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.
An example of a specific impulse measured in time is 453 seconds, which is equivalent to an effective exhaust velocity of 4.440 km/s (14,570 ft/s), for the RS-25 engines when operating in a vacuum.
This is because the effective exhaust velocity calculation assumes that the carried propellant is providing all the reaction mass and all the thrust.
[39] 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.
[45] The variable specific impulse magnetoplasma rocket (VASIMR) engine currently in development will theoretically yield 20 to 300 km/s (66,000 to 984,000 ft/s), and a maximum thrust of 5.7 N (1.3 lbf).
