In the discussion of Specific Impulse, a review of units in Newton's Second Law is useful.
Four Sets of Units come to mind (All four of these sets are "coherent" with respect to f = m a): The preferred set is the SI Set- One newton is the force that accelerates one kilogram of mass one meter per second squared. One dyne is the force that accelerates one gram of mass one centimeter per second squared. (One dyne is equal to 1.000 E-05 newton) One poundal is the force that accelerates one pound of mass one foot per second squared. (One poundal is equal to 1.382 E-01 newton) One pound-force is the force that accelerates one slug of mass one foot per second squared. (One pound force (lbf) is equal to 4.448 E+00 newtons) ..... An incoherent set is: One pound-force is the force that accelerates one pound mass 9.80665 meters per second squared. i.e. (4.448 222 N) is the force that accelerates (0.4535924 kg) 9.80665 meters per second squared, where 4.448 222 = .4535924 times 9.80665 in good approximation. A ratio of units outside the SI relevant to the GRC definition of Isp is; pound-force per pound mass (thrust to mass ratio): One lbf/lbm is equal to 9.80665 E+00 newtons per kilogram. See NIST SP 811 for more details. Eugene Mechtly
