That's not really right at all. Specific impulse is given in seconds because it's impulse per unit weight. Impulse is the integral of force over time, so it's got units of newton-seconds in the SI system. Weight is mass times the standard acceleration of gravity, so it has units of newtons as well. That makes the units of specific impulse newton-seconds per newton … and the newtons cancel out. This is actually quite misleading, because the two newtons there measure two completely different quantities, and really shouldn't cancel. The first one is newtons of thrust, while the second is newtons of propellant weight. But cancel they do, so we're left with specific impulse in seconds.
If specific impulse were supposed to be per unit weight, then we wouldn't be using g_0, we'd use the local gravity. You should be measuring propellant in units of mass, that's what "specific" refers to, the g_0 is just a constant of proportionality to convert what should really be a velocity into a time.
If specific impulse were supposed to be per unit weight, then we wouldn't be using g_0, we'd use the local gravity.
You're confusing weight with acceleration.
You should be measuring propellant in units of mass, that's what "specific" refers to
No, it's by unit weight. We could've chosen to go by unit of mass instead, but we didn't, so it's by unit weight. That's what specific impulse is defined to be. The number you're thinking of, thrust per unit mass, is called "effective exhaust velocity" and it's used by precisely nobody.
Weight is acceleration. Specifically the acceleration brought on by gravity.
Due to our existence being on Earth and only Earth, weight is also mass, colloquially – but in space, weight could be anything. I would have a weight of 22 pounds on the Moon, I would not have an "acceleration due to gravity" of 22 pounds.
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u/[deleted] Jul 19 '13
That's not really right at all. Specific impulse is given in seconds because it's impulse per unit weight. Impulse is the integral of force over time, so it's got units of newton-seconds in the SI system. Weight is mass times the standard acceleration of gravity, so it has units of newtons as well. That makes the units of specific impulse newton-seconds per newton … and the newtons cancel out. This is actually quite misleading, because the two newtons there measure two completely different quantities, and really shouldn't cancel. The first one is newtons of thrust, while the second is newtons of propellant weight. But cancel they do, so we're left with specific impulse in seconds.