Specific impulse of a rocket

It is rather interesting that the maximum specific impulse of a rocket propellant system occurs when (T2iMW2)1/2 is maximized, even though the rocket combustion process is not one of detonation [17],... 

The specific impulse of a rocket motor, I, as defined in Eq. (1.75), is dependent on both propellant combushon efficiency and nozzle performance. Since is also defined by Eq. (1.79), rocket motor performance can also be evaluated in terms of the characterishc velocity, c, defined in Eq. (1.74) and the thrust coefficient, Cp, defined in Eq. (1.70). Since c is dependent on the physicochemical parameters in the combustion chamber, the combushon performance can be evaluated in terms of c. On the other hand, Cp is dependent mainly on the nozzle expansion process, and so the nozzle performance can be evaluated in terms of Cp. Experimental values of and Cpgxp are obtained from measurements of chamber pressure, p, and thrust, F ... 

A more detailed description to derive the thrust or the specific impulse of a rocket is given below In order to propel a rocket, a rocket engine ejects combustion gases with low molecular mass but high velocity z through the nozzle. The rocket has the mass M and initially moves with velocity u. If the rocket ejects combustion gases of the mass of Am within the time At and with the velocity 2 , a decrease in mass M - Am and an increase in the velocity to u + Au results. Due to the conservation of the impulse, the following can be derived ... 

Oxidizers. The characteristics of the oxidizer affect the baUistic and mechanical properties of a composite propellant as well as the processibihty. Oxidizers are selected to provide the best combination of available oxygen, high density, low heat of formation, and maximum gas volume in reaction with binders. Increases in oxidizer content increase the density, the adiabatic flame temperature, and the specific impulse of a propellant up to a maximum. The most commonly used inorganic oxidizer in both composite and nitroceUulose-based rocket propellant is ammonium perchlorate. The primary combustion products of an ammonium perchlorate propellant and a polymeric binder containing C, H, and O are CO2, H2, O2, and HCl. Ammonium nitrate has been used in slow burning propellants, and where a smokeless exhaust is requited. Nitramines such as RDX and HMX have also been used where maximum energy is essential. 

Though the specific impulse of a solid rocket is determined by the energetics of its propellant, the specific impulse of a ducted rocket is determined by the mixture ratio of the fuel flow rate from the gas generator and the airflow rate induced from the atmosphere. The ram pressure in the rambumer is raised by the shock wave formed at the air-intake. The combustion in the rambumer further increases the... 

Boron is one of the essential materials for obtaining high specific impulse of a ducted rocket However, the combustion efficiency of boron-containing gas-generating pyrolants is low due to incomplete combustion of the boron particles in the ramburner.[i3-i l Fig. 15.20 shows the combustion temperature ofa boron-containing pyrolant with and without boron combustion as a function of air-to-fuel ratio, 8. A typical boron-containing pyrolant is composed of mass fractions of boron particles b(0-30), ammonium perchlorate ap(0.40), and carboxy-terminated polybutadiene ctpb(0-30). If the boron particles burn completely in the ramburner, the maximum combustion temperature reaches 2310 K at 8 = 6.5 and v = Mach 2 p =... 

The specific impulse of a propellant or a pair of reacting liquids in rocket motors is the most important characteristic of the performance. It is the - Thrustxtme (i.e., the impulse) per unit weight of propellant ... 

The mass of heat sink material required to satisfactorily condense the rocket gas flow for a specified run time is a function of several rocket parameters and heat sink material properties. To a suitable approximation, the specific impulse of the rocket fuel determines the amount of energy per unit fuel mass that must be removed by the heat sink, and the fuel mass flow is determined by the ratio of rocket thrust to specific impulse. The heat capacity of the heat sink material along with the allowable material temperature rise during the rocket firing imply the required heat sink mass ... 

One measure of propellant performance is specific impulse,, which is defined as pounds of thrust delivered by a flow of one pound per second of gas through the rocket nozzle. While the specific impulse of operating rockets has increased about 25 since the Second World War, theory indicates a further possible increase of approximately 50 [31. Liquid-propellant rockets in use today, LOX-gasoline systems, are achieving a specific impulse of 264 Ib-sec/lb. Theoretical Jgp possible with liquid-fluorine—liquid-hydrogen systems is 373 Ib-sec/lb. Small increases in are important in terms of range and payload. As an example, it can be calculated that a increase in of a typical ballistic missile can result in a 9 increase in payload or a 6 increase in range [4]. 

Uses. A review of its use as an oxidizer in rocket proplnts is given in Ref 6. A 50 50 mixt of N trifluoride F, with ammonia as fuel, yields a specific impulse of 295sec and a d impulse (specific impulse X d) of 340 (Ref 6). Mixts with methyl fluoride, 1,1 -difluoropropane, and 1,3-dlfluoropropane have been patented as monoptoplnts(Ref 9). AN trifluoride/H flame has been proposed as a cutting and welding torch (Ref 8)... 

A relatively new class of fluorocarbon composite nronlrits with relatively high specific impulse and a high density impulse have been tested as booster rockets for 5-inch projectiles. 

Most composite propellants in wide use today use ammonium perchlorate as the oxidizer component. While such propellants benefit from high specific impulse and a lower vulnerability than double-base propellants based on nitroglycerine, they have a significant environmental impact, the exhaust fumes being rich in corrosive hydrogen chloride leading to acid rain after the launch of missiles/rockets. 

It should be noted that the gas flow process in the port is not isentropic because mass and heat addihons occur in the port. This implies that there is stagnation pressure loss and so the specific impulse is reduced for nozzleless rockets. When a convergent nozzle is attached to the rear end of port, the static pressure at the port exit, Pj, continues to decrease to the atmospheric pressure and the specific impulse of the nozzleless rocket motor is increased. The expansion process in a divergent nozzle is an isentropic process, as described in Section 1.2. 

Fig. 14.25 Specific impulse of an N204/pyrolant gas-hybrid rocket as a function of oxidizer/fuel ratio.

Representative LP Applications. Before proceeding with a description of the specific uses of LP, it is instructive to compare them with solid propellants. The choice between LP and a solid proplnt for a given application depends on several factors such as specific impulse, safety, storability, rocket engine design special requirements... 

Specific impulse, Isp, probably the most accepted measure of LP performance, was defined in Sect 1 and tabulated in Tables 1 2. The thrust, F, of a rocket is given by... 

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