In Chapter 3 there is further discussion of the specific impulse. For constant propellant mass flow m, constant thrust F, and negligibly short start or stop transients:
The product mpgQ is the total effective propellant weight w and the weight flow rate is w. The concept of weight relates to the gravitational attraction at or near sea level, but in space or outer satellite orbits, "weight" signifies the mass multiplied by an arbitrary constant, namely g0. In the Système International (SI) or metric system of units Is can be expressed simply in "seconds," because of the use of the constant g0. In the USA today we still use the English Engineering (EE) system of units (foot, pound, second) in many of the chemical propulsion engineering, manufacturing, and test operations. In many past and current US publications, data and contracts, the specific impulse has units of thrust (lbf) divided by weight flow rate of propellants (lbf/sec), simplified as seconds. The numerical value of Is is the same in the EE and the SI system of units. However, the units of Is do not represent a measure of elapsed time, but a thrust force per unit "weight"-flow-rate. In this book the symbol Is is used for the specific impulse, as listed in Ref. 2-1. For solid propellant systems the symbol Isp is sometimes used, as listed in Ref. 2-2.
In a rocket nozzle the actual exhaust velocity is not uniform over the entire exit cross-section and does not represent the entire thrust magnitude. The velocity profile is difficult to measure accurately. For convenience a uniform axial velocity c is assumed which allows a one-dimensional description of the problem. This effective exhaust velocity c is the average equivalent velocity at which propellant is ejected from the vehicle. It is defined as
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