It is given either in meters per second or feet per second. Since c and Is differ only by an arbitrary constant, either one can be used as a measure of rocket performance. In the Russian literature c is generally used.
In solid propellant rockets it is difficult to measure the propellant flow rate accurately. Therefore, the specific impulse is often calculated from total impulse and the propellant weight (using the difference between initial and final motor weights and Eq. 2-5). In turn the total impulse is obtained from the integral of the measured thrust with time, using Eq. 2-1. In liquid propellant units it is possible to measure thrust and instantaneous propellant flow rate and thus to use Eq. 2-3 for calculation of specific impulse. Eq. 2-4 allows another definition for specific impulse, namely, the amount of impulse imparted to a vehicle per unit sea-level weight of propellant expended.
The term specific propellant consumption refers to the reciprocal of the specific impulse and is not commonly used in rocket propulsion. It is used in automotive and duct propulsion systems. Typical values are listed in Table 1-2.
The mass ratio Mt of a vehicle or a particular vehicle stage is defined to be the final mass my (after rocket operation has consumed all usable propellant) divided by m0 (before rocket operation). The various terms are depicted in Fig. 4-1.
This applies to a single or a multi-stage vehicle; for the latter, the overall mass ratio is the product of the individual vehicle stage mass ratios. The final mass mf is the mass of the vehicle after the rocket has ceased to operate when all the useful propellant mass mp has been consumed and ejected. The final vehicle mass mf includes all those components that are not useful propellant and may include guidance devices, navigation gear, payload (e.g., scientific instruments or a military warhead), flight control systems, communication devices, power supplies, tank structure, residual or unusable propellant, and all the propulsion hardware. In some vehicles it can also include wings, fins, a crew, life support systems, reentry shields, landing gears, etc. Typical values of Mi can range from 60% for some tactical missiles to less than 10% for some unmanned launch vehicle stages. This mass ratio is an important parameter in analyzing flight performance, as explained in Chapter 4. When IVR is applied to a single stage, then its upper stages become the "payload."
The propellant mass fraction f indicates the fraction of propellant mass mp in an initial mass m0. It can be applied to a vehicle, a stage of a vehicle or to a rocket propulsion system.
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