Can someone explain what specific impulse means?

[Ydoow]

I thought I knew this :-X

I wiki\'d it a bit, but kinda got lost.

What confuses me in-game is that there is still a unit of thrust, but then different units for Isp. So I don\'t know what I should be looking at when building a rocket, the two Isps, or the thrust.

I kinda get that Isp is a derivative of the thrust per kilogram, but I guess I don\'t know how to use this in my rocketry building logic

[vexx32]

If you don\'t quite get it, use thrust for your logic. It\'s essentially the same thing.

Isp (as I understand it) is basically, as you said, thrust per unit... so, essentially its a ratio of thrust to weight or mass. The higher it is, the more 'efficient' the engine will be, in a way -- it needs less fuel to lift its own weight, so is better for lifting the rocket as a whole.

[naryl]

Specific Impulse is how much throttle the engine can generate per mass unit of *fuel*. The higher it is the less fuel the engine burns to generate a unit of throttle.

[Ydoow]

Oh so truly, it\'s a measurement of efficiency and not 'How fast it goes'

Although technically, if you were to make all engines burn 1 unit of fuel per second, then I guess the one with the highest Isp would give off the must thrust.

I think I got it now, thank you

[NeverDrinkBleach]

Different fuel sources have a different specific impulse too. 8)

I guess maybe later on in development Harv and the troops will add more fuels to the game, that can produce more thrust per unit of fuel used.

[TheKerbinator]

If the above is correct, than Isp x burn rate = thrust

[PeriapsisPrograde]

An ISP of 1 means that if the engine produces 1 N of thrust for 1 second, it will burn 1 kg of fuel. An ISP of 500 means that if the engine produces 1 N of thrust for 500 seconds, it will burn 1 kg of fuel. Modern rockets have an ISP of anywhere from 250 to 400 ISP.

[dikkjo]

Although technically, if you were to make all engines burn 1 unit of fuel per second, then I guess the one with the highest Isp would give off the must thrust.

This is not entirely true, if you take for instance ion engines they typically have ISP\'s bigger than 1000, but have very low thrust compared to LFE\'s.

ISP is a consumption indicator which may allow you to estimate how much fuel you need for a given thrust, or deltaV providing you know the mass of your beast.

The propulsion efficiency of a jet engine is dependent on your vessel\'s speed and the exhaust speed at the nozzle, which i doubt is simulated in the game mechanics.

I guess maybe later on in development Harv and the troops will add more fuels to the game, that can produce more thrust per unit of fuel used.

I think something like this might happen too ;P

[Ziff]

I thought I knew this :-X

I wiki\'d it a bit, but kinda got lost.

What confuses me in-game is that there is still a unit of thrust, but then different units for Isp. So I don\'t know what I should be looking at when building a rocket, the two Isps, or the thrust.

I kinda get that Isp is a derivative of the thrust per kilogram, but I guess I don\'t know how to use this in my rocketry building logic

Specific impulse is basically the time for which an engine provides thrust equal to the weight of the propellant consumed. Basically, it\'s efficiency. You can\'t just ignore thrust over Isp because the unit of thrust helps you figure out how many engines are required to move a specific amount of mass. It doesn\'t help to put a fuel efficient engine with low thrust under a large mass if you plan on landing anywhere. (You\'ll need a lot more time to slow the mass, if you can, before you kerplode on the surface.)

I hope that helps.

-Ziff