Impulse?

[Hiatsu2k8]

What does the impulse stat on the engines mean as opposed to thrust?

[phoenix_ca]

In very basic terms, specific impulse is a measure of fuel efficiency, in terms of how much delta-v a propulsion system can provide relative to how much propellant it uses. A higher number means the engine will use less fuel per volume of propellant.

Specific Impulse - Wikipedia

[Person012345]

Think of it like miles per gallon on a car engine. Thrust is more like horsepower, it tells you how powerful the engine is. Specific impulse tells you nothing about how powerful the engine is, but how far it'll go on the same amount of fuel.

Edited September 25, 2012 by Person012345

[Frederf]

Impulse is just the total momentum change. 10 newtons over 10 seconds, 100 newtons over 1 second, 1 newton over 100 seconds. All are the same total 100 Newton-seconds impulse.

Specific impulse means "amount of impulse you get, per unit." In this case it is per liter fuel. An engine that delivers more thrust x time or mass x speed on the same amount of fuel is sad to have "a higher specific impulse." It's a complex sounding term but broken down it is simple.

Another "specific" example is "specific heat capacity." 20 lbs of iron will have twice the heat capacity as 10 lbs, but their specific (per pound) heat capacity will be the same.

[maltesh]

It's not per liter of fuel. When specific Impulse winds up being measured in seconds (as it does in the game stats) its' "Per Newton of fuel weight measured under a standard gravity". Newton * second / Newton cancels out to seconds, and can be thought of as "The amount of time for which 1 Newton's weight of fuel can produce 1 Newton of thrust with this engine."

As for why you'd use weight under standard gravity here instead of mass, in the real world, rocket manufacturers have to deal both with clients that use the imperial system, and clients who use the metric system, and if you calculate "The amount of time for which 1 pound of fuel can produce 1 pound of thrust" you wind up with the same value in seconds.

If you go by unit mass of fuel instead, you wind up with Newton * second / kilogram, and since a Newton is a kilogram * meter / second ^2, you wind up with units of meters/second, which is the Effective Exhaust Velocity.

Edited September 25, 2012 by maltesh

[phoenix_ca]

You mean, clients who use the metric system, and clients who don't know how to measure things properly.

(No seriously though, metric/imperial conversion screw-ups destroyed a probe once.)

[Frederf]

What I said is true, just not handy. Since a L of fuel has the same mass any other (in KSP) they are exchangeable with each other and also with fuel weight. Specific impulse expressed in any of those three ways is the same measure, just not necessarily the same units of measure. A physicist such as myself can barely be bothered with such trivia as units. Indeed the convention in KSP is to express it in Newton-seconds-impule/Newtons-fuel (aka seconds).

What I wanted to show is the origin of the phrase "specific impulse" so it had more real meaning than "a crazy science term that I just have to memorize."

[maltesh]

A physicist such as myself can barely be bothered with such trivia as units.

I don't think I've ever met a physicist who'd agree with that statement.

[UmbralRaptor]

Since a L of fuel has the same mass any other (in KSP)

FL-T200: 200 L/tonne

Mk 1 fuselage: 167 L/tonne

And there were more differences in 0.14 and 0.15...

[bradley101]

The best moter shode idealy have the Thrust and Specific Impluse the same see Spacecraft Propulusion on Wikipedia for extra info.

[draeath]

Higher = better

Note that the impulse scales based on atmospheric pressure. On Kerbin, you hit 0 pressure at 75km altitude, but it's close enough to 0 for a good portion of 30km and up.

Note though - just because the impulse is high doesn't mean it will make you go fast. The thrust tells you that - the more thrust, the more push (and the lighter the load, the faster the resulting acceleration). That said, for two engines with identical thrust and mass, the one with the higher Specific Impulse for the atmospheric pressure you will be using it in is best.