I can't find a good definition of specific impulse

[travis575757]

So what is specific impulse, is it the amount of time in seconds it takes to burn a certain amount of fuel? The Amount of thrust produced in one second? It seems like i am getting a bunch of weird results from different sites.

[K^2]

The amount of impulse a rocket receives per weight of the fuel.

The reason it works out to be in seconds is that impulse has units of force multiplied by time, and you divide it by weight, which is a force, leaving you with units of time.

P.S. Impulse is the change in momentum, which for a classical rocket is it's mass times velocity.

[Nuke]

i want to say the amount of time to burn a unit of fuel producing a unit of thrust. but its one of those measurements that is hard to get your head around. needless to say, higher is better.

[travis575757]

That kind of makes sense, so is it like a ratio or something like that?

[K^2]

i want to say the amount of time to burn a unit of fuel producing a unit of thrust.

That is actually a very good way of putting it. I've never thought of it that way. Just so long as it's a unit of fuel by weight.

It makes the most sense in Imperial units, actually. If you have an engine with 300s of I_SP, then at 1lb of thrust, 1lb of fuel will last you 300 seconds. (And, of course, this is where we get such silly units from, anyways.) But this works in metric as well. If you have 1N of thrust, then 1N of fuel by weight will last you 300s.

Oh, and it's always going to be Earth/Kerbin weight. (Both have g₀ = 9.8m/s²) Just the way these units are defined.

[architeuthis]

That kind of makes sense, so is it like a ratio or something like that?

Sure, it's a ratio of exhaust velocity to the acceleration of gravity. Specific impulse is popular in part because the seconds unit means the value is the same irregardless of the dimensional system (I.e. no mucking about with meters or feet).

[travis575757]

Well also why is the acceleration due to gravity used no matter what, even when at a different planet, this probably makes the least amount of sense.

[K^2]

Well also why is the acceleration due to gravity used no matter what, even when at a different planet, this probably makes the least amount of sense.

Purely historical reasons. Rocket science started out in Germany and spread primarily to USSR and United States after WWII. Soviets continued to use metric system. So if you take any Soviet textbook on rockets, you'll see specific impulse defined per unit mass and the units are going to be m/s.

Americans have always favored using Imperial units in engineering, and rocketry was no exception. So instead of taking impulse per kilogram, they took impulse per pound. Now, it is what they call "pound of mass", which is property of material, not of the actual weight. So 1lb of mass on Earth is the same as 1lb of mass on the Moon. But the problem is that this value is defined as "Mass of an object that has weight of 1lb on Earth". And that introduces that factor of g into all of the equations that deal with I_SP using U.S. convention.

Furthermore, the unit is still the pound. So even though it's called "pound of mass", it's still formally a weight. Just that instead of current weight of object in whatever gravity, it's always the weight that object would have if it was placed on Earth's surface.

Hopefully that makes some sense. It is a bit confusing. I still mentally convert all specific impulse to m/s, because it makes more sense to me that way. But I'm going to arm myself with Nuke's method now as well.

[Red Iron Crown]

Specific impulse is a measure of how efficiently a jet converts propellant mass into delta-V. The higher the Isp, the more delta-V is produced for a given amount of propellant mass (or, alternately, the less propellant mass is needed for a given amount of delta-V).

Units for Isp can either be meters/second (more intuitive yet strangely less popular) or seconds, which must be multiplied by g in the rocket equation. When given in m/s it is the same as the jet's effective exhaust velocity.

[AngelLestat]

I never understand very well why with interstellar rockets (fussion, antimatter) the exhaust velocity is equal to its ISP.

I guess becouse there is not point to include the earth gravity, but It makes more difficult to compare. It would not be the same for rockets already in orbit?

[Nuke]

i surprise myself sometimes. i thought for sure someone was going to tear holes in my notion.

[travis575757]

okay that makes perfect sense, thanks every since i have started finding delta v i have been confused with the whole Isp thing. To make sure i was doing it right i went in ksp and made a simple test, i had a Rockomax poodle with 220kn of trust or 220000 Newtons then i got a 9 metric ton fuel tank and found the fuel mass to be 8 metric tons or 78453.2 Newtons, i then devided the newtons by the thrust to get .3566, then using this multiplied it by the Isp to get about 96 for about 1 minute 36 seconds burn time at max throttle. I then did this in game and got about right below 1 minute 38 seconds. After checking the engine though i realized the Isp was at 272 and not 270 as it said on the rocket description so i guess it works out if im doing this right.

[K^2]

I never understand very well why with interstellar rockets (fussion, antimatter) the exhaust velocity is equal to its ISP.

I guess becouse there is not point to include the earth gravity, but It makes more difficult to compare. It would not be the same for rockets already in orbit?

Because inter-stellar rockets are thought up by scientists who use metric system, and I_SP in seconds just doesn't make sense to them.

i surprise myself sometimes. i thought for sure someone was going to tear holes in my notion.

I was going to, but then I pulled up a piece of paper, made a few scratches, and realized that it's right. Happens. That's why checking yourself with maths is always a good policy.

[Nuke]

math is second nature to me, work ethic on the other hand.

[Red Iron Crown]

Because inter-stellar rockets are thought up by scientists who use metric system, and I_SP in seconds just doesn't make sense to them.

Isp in seconds is measurement system agnostic, as both metric and Imperial systems use the second. It makes as much (or as little) sense to metric users as Imperial users.

And I think you mean interplanetary.