Hi everyone. Happy new year!
I'm starting on my first .dll addon and have a question: Are jet or rocket engines rated at actual thrust as applied to a vessel or is at an ideal "bench number"? For example, specs read: "two turbojet engines... rated at 310 kN dry thrust each..." Does this equal 620 kN total thrust of the vessel, or is there some power loss?
You usually also apply the altitude reference for the thrust to it, since all rocket engines are suffering losses from ambient air pressure. Often you have two numbers given, the sea level thrust (maximum ambient pressure) and the vacuum thrust. With the thrust, that drop because of the ambient air pressure, you also loose specific impulse, the quality that measures how effective the rocket engine works. specific impulse is measured by impulse (Thrust over time) generated per propellant mass used. A lower thrust at the same fuel consumption results in lower specific impulse.
Specific impulse varies with the engines, and is not constant for a fuel combination, that quality of the propellant is described with the characteristic velocity of this fuel, and only depends on the fuels you use, their temperatures in the tanks and the mixture ratio.
The thrust does not vary much by the installation into the rocket, since rockets have no inlet for air, like jet engines. You can have a boat tail effect by having multiple engines form some sort of an aerospike, but the effect is often exaggerated in science-fiction, it can give you a few percent more thrust, but that is often eaten by the worse aerodynamics and less effective engines placement.
The thrust that you get by installing rocket engines on your rocket differently does of course change depending on how you place them and how you point them - but that is not different to aircraft then. usually you place the engines close together to have a smaller cross section, but far enough apart to permit control of the rocket by tilting the engines (thrust vector control).
---------- Post added at 08:41 PM ---------- Previous post was at 08:35 PM ----------
Impulse is the amount of fuel that the engine consumes to produce a given amount of force and is usually measured in "impulse seconds" or ISP. An engine with an ISP of 300 will produce 300 newtons for one second per kilogram of fuel consumed.
Wrong. ISP means "specific impulse" or [math]I_{sp}[/math] in mathematical notation. It is measured in seconds sometimes (inaccurately, but it works at least), but generally, it is impulse (Force * Time) per propellant mass.
if you have 300 seconds as poor US engineer, you would have 300 kg(f) thrust in your example - about 3000N. In SI units, 300 [math]\frac{N \cdot s}{kg}[/math] or short 300 m/s give you the 300 N thrust for one second per kg of propellant burned - or an engine burning one kg fuel every second makes 300 N thrust.
In Britain, you use furlongs per fortnight for describing the specific impulse, which generates a idle thrust already because of the zeros pouring out of the engine documentation.
