Rockets specific impulse

Fig. II. C. 3 Effect of oxidizer-fuel ratio on calculated H2-Oz rocket specific impulse for various nozzle flow conditions...

Other relevant keywords are -> Solid Propellant Rockets, -> Specific Impulse, -> Thermodynamic Calculation of Decomposition Reactions, -> Thrust. 

Other keywords in this connection -< Rocket Motor, -> Solid Propellant Rocket -> Specific Impulse. 

In rocket technology, the ratio between the burning surface of the propellant and the smallest cross-section of the nozzle. It determines the resultant pressure in the combustion chamber of the rocket (other relevant keywords -> Burning Rate, - Gas Jet Velocity - Rocket, Solid Propellant Rocket, Specific Impulse, - Thrust). 

Elemental fluorine has been studied as a rocket propellant as it has an exceptionally high specific impulse value. 

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. 

Propellants and Explosives. Hydrazine fuels include anhydrous hydrazine (AH), monomethyUiydrazine (MMH), and unsymmetrical dimethyUiydrazine (UDMH) for military and space programs. These compounds are used mainly as bipropeUant fuels, ie, with oxidizers, in rockets such as the Titan, MX missile, and the Ariane (UDA4H7X30. Using oxygen or fluorine as the oxidizer, hydrazine is exceeded only by hydrogen in specific impulse, ie, kilograms of thmst developed for each kilogram of fuel consumed per second (196). 

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)... 

Recent trends in rocket propint development and most of the recent developments in SP are for rocket and not gun proplnts, were directed toward the attainment of higher energy systems (actually higher specific impulse). Unfortunately, the use of exotic components presents grave stability problems which are yet to be overcome. Some energetic plasticizers have been developed and in some applications specific impulse has been increased substantially by substituting Be for Al. Much of this subject remains classified... 

Without the influence of burning rate catalysts most of these hydrocarbon prop bits have similar burning rates and ballistic behavior. They may differ significantly in mechanical properties, particularly as a function of temp. Most hydrocarbon-based composites are used in larger rockets because of their ease of fabrication and high specific impulse. Polaris first and second stages, the Titan 3C booster rocket and Mlnuteman are all powered with composite proplnts... 

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. 

The ratio of rocket thrust to propellant mass flow, commonly called the specific impulse (/9p) of the propellant, represents a measure of the force developed per unit mass flow of propellant. From Eq. (2), it is apparent that high propellant-flame temperatures and low molecular-weight combustion products are required to produce high 7sp. 

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],... 

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. 

Though the oxidation potentials of potassium nitrate (KN KNO3) and sodium nitrate (SN NaN03) are high, both metal nitrates generate combustion products of high Mg, Thus, the specific impulse becomes low when KN or SN is used in a rocket propellant KN and SN are used as major ingredients of explosives and in pyrotechnics. KN is a weU-known material as a major component of black powder. 

Since rocket propellants are composed of oxidizers and fuels, the specific impulseis essenhally determined by the stoichiometry of these chemical ingredients. Ni-tramines such as RDX and HMX are high-energy materials and no oxidizers or fuels are required to gain further increased specific impulse. AP composite propellants composed of AP particles and a polymeric binder are formulated so as to make the mixture ratio as close as possible to their stoichiometric ratio. As shown in Fig. 4.14, the maximum is obtained at about p(0.89), with the remaining fraction being HTPB used as a fuel component. 

When HNF or ADN particles are mixed with a GAP copolymer containing aluminum particles, HNF-GAP and ADN-GAP composite propellants are formed, respectively. A higher theoretical specific impulse is obtained as compared to those of aluminized AP-HTPB composite propellants.However, the ballistic properties of ADN, HNIW, and HNF composite propellants, such as pressure exponent, temperature sensitivity, combustion instability, and mechanical properties, still need to be improved if they are to be used as rocket propellants. 

Since the combustion temperature of AN pyrolants is very low compared with other composite pyrolants, their specific impulse when used as rocket propellants is also low. However, they are used as gas generators for the control of various types of mechanics owing to the low temperature and low burning rate characteristics. 

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 ... 

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. 

Nozzleless rockets are very simplified and low-cost rockets because no nozzles are used. Their specific impulse is lower than that of conventional rockets even when the same mass of propellant is used. Normally, a convergent-divergent nozzle is used to expand the chamber pressure to the atmospheric pressure through an isentropic change, which is the most effective process for converting pressure into propulsive thrust The flow process without a nozzle increases entropy and there is stagnahon pressure loss. 

Though the pyrolants used in gas-hybrid rockets burn in a similar manner as rocket propellants, their chemical compositions are fuel-rich. The pyrolants burn incompletely and the combustion temperature is below about 1000 K. When an atomized oxidizer is mixed with the fuel-rich gas in the secondary combustor, the mixture reacts to generate high-temperature combustion products. The combushon performance designated by specific impulse, is dependent on the combinahon of pyrolant and oxidizer. 

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