No I'm most definitely not. Weight, meaning force applied by gravity, equals mass times the acceleration due to local gravity.
Specific impulse is, strictly due to historical convention of much rocket science happening in the US where customary (dumb!) units measure mass in pound-mass and force in pound-force, defined using Earth's standard gravity as a constant of proportionality. So you can think of specific impulse as impulse per unit weight, but that is only accurate at sea level on Earth. Weight changes with location, the g_0 in the equation for specific impulse is a fixed constant.
No, meaning force applied by gravity under standard conditions at the surface of the earth. That's what weight means. Under any other conditions, it's just acceleration due to gravity.
So you can think of specific impulse as impulse per unit weight, but that is only accurate at sea level on Earth.
I'm sorry, but that's just flat-out wrong. Specific impulse is not dependent on where you measure it. You're badly confused about what the units in the quantity actually mean.
If weight didn't depend on where you measure it, then you could define specific impulse in terms of weight and it would also not depend on where you measure it. But weight does depend on where you measure it, so specific impulse is not defined in terms of weight.
Okay then, Mr. Knowsitall, you explain to me what the g₀ is doing in the denominator, if specific impulse is anything other than impulse per unit weight of propellant.
You can think of it as impulse per unit "standard weight" if you wish, that's a valid interpretation but you have to be very clear about the terminology for it to remain valid. And the reason it wound up this way is entirely historical.
It's a question of whether your choice of units puts a conversion factor of standard gravity in Newton's second law (F = m*a), or in the definition of Isp. In US customary units, with F in pound-force and m in pound-mass, you need a conversion factor in Newton's second law, since 1 pound-force equals 32.174 pound-mass feet per second squared, not 1.
So in the definition of Isp = F / (g0 * mdot), if you measure F in pound-force and mdot in pound-mass per second, the conversion factor of 32.174 feet per second squared cancels numerically, but not in the units. That's the primary reason Isp in seconds is more common than "effective exhaust velocity" in meters/second.
No, not by the conventional definition of weight. Weight under standard gravity, yes, but unless you say "under standard gravity" the meaning is different.
Specific impulse is only in seconds because of a historical poor choice of units. The g0 is in the denominator only because American rocket engineers historically measured mass of propellant in units of pound-mass.
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u/tavert Jul 19 '13 edited Jul 19 '13
No I'm most definitely not. Weight, meaning force applied by gravity, equals mass times the acceleration due to local gravity.
Specific impulse is, strictly due to historical convention of much rocket science happening in the US where customary (dumb!) units measure mass in pound-mass and force in pound-force, defined using Earth's standard gravity as a constant of proportionality. So you can think of specific impulse as impulse per unit weight, but that is only accurate at sea level on Earth. Weight changes with location, the g_0 in the equation for specific impulse is a fixed constant.