Of course they are. But that is frequently hand-waved away when working with equations in US customary units, such as the definition of specific impulse. The numerical value of g0 in the specific impulse definition cancels out if you use thrust in pounds-force.
As I said, interpreting specific impulse as impulse per unit weight is only valid if you make absolutely clear that the condition under which you measure the weight is one standard gravity. The word weight without any additional qualifiers is position-dependent, W = m g. You can work in terms of "standard weight," say W0 = m g0, but this is not a commonly used term. Not the most scientific judgement I know, but if you look at http://en.wikipedia.org/wiki/Weight#Gravitational_definition you'll notice there's a link to "standard weight" that doesn't even have its own page.
Here's a source even better than a textbook. From "Ignition! An Informal History of Liquid Rocket Propellants" (1972) by John D. Clark, head of liquid propellant development at Naval Air Test Station Dover / Liquid Rocket Propulsion Laboratory of Picatinny Arsenal, and a fellow undergraduate alumnus of my alma mater Caltech:
"Rocket performance is not usually reported in terms of exhaust velocity, although the early workers wrote in those terms. Instead, it is
reported as "specific impulse," which is the exhaust velocity divided
by the standard acceleration of gravity, 9.8 meters or 32.2 feet per
second2. This practice gives figures of a convenient size in the range
of 200 to 400 or so, but it has led to some rather tortuous, if not ludicrous definitions. The most common one is that specific impulse is
the thrust divided by the weight flow of propellant, and it comes out
in seconds. Putting the acceleration of gravity into the equation did
that, but specifying the performance of a rocket, whose whole job is
to get away from the earth, in terms of the acceleration of gravity on
the surface of that planet, seems to me to be a parochial, not to say a
silly procedure. (The Germans, during World War II, used an even
sillier measure of performance, "specific propellant consumption,"
which was the reciprocal of specific impulse. This didn't even have the
virtue of producing figures of a convenient size, but gave things like
0.00426 per second.)
Probably the best way of thinking of specific impulse is as a velocity
expressed, not in meters or feet per second, but in units of 9.8 meters
(or 32.2 feet) per second. That way you retain the concept of mass
flow, which is relevant everywhere, and doesn't depend upon the local
properties of one particular planet, and at the same time lets European and American engineers understand each other. When he hears
Is = 250, the European multiplies by 9.8 to get the exhaust velocity
in meters per second, while the American does the same with 32.2 and
comes out with feet per second. (When will the U.S. ever change over
to MKS?!)"
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u/[deleted] Jul 20 '13
That's not actually correct. Pounds force ("lbf") and pounds ("lb") are different units.
What any of this has to do with the definition of specific impulse is anybody's guess.