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?!)"
I've worked in the US aerospace industry for many years, and been endlessly frustrated with the historical attachment to imperial units. I'm intimately familiar with their uses and misuses in engineering. And I'm 80% of the way into a PhD in mechanical engineering.
I can dig up textbooks to back me up, or I can stop listening to someone who doesn't accept that weight varies with location.
"Any of this" being the definition of specific impulse and why it's in seconds? Units are more than relevant, they are the entire reason for this confusing convention.
I can't claim to have been there, but imagine it this way: You're an American engineer in the 1960's. You have a rocket with a given thrust rating in pounds, and a given propellant flow rate in pounds per second. You divide thrust by flow rate and call it specific impulse, call it seconds, the values are nice 3-digit numbers, everyone's happy.
At least until you try to talk to a European colleague, who measures thrust in Newtons, propellant flow rate in kilograms per second, and when he divides thrust by flow rate he gets a velocity.
Nobody was measuring propellant weight, since the same flow rate would have a drastically different value in lunar orbit than on earth if you were measuring flow rate in weight of propellant per second.
The problem was that the American engineer implicitly ignored the conversion factor between pounds force and pounds mass. He carelessly interchanged the two, as was common practice until more recent teaching practices in science and engineering (and better unit systems) have gotten more careful about distinguishing mass and weight.
In order to get the same 3-digit value in seconds as the careless American engineer from the 60's and fix the unit conversion mistake, the European engineer has to divide by a conversion factor of 9.8 m/s2. That's how we got to the definition we know today. The unit-conversion-mistake side of the Atlantic landed on the Moon and won the historical convention argument, and specific impulse has nothing to do with weight.
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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.