Fuel-Rich Propellants

The selechon of fuel components to be mixed with oxidizer components is also an important issue in the development of pyrolants for various applications. Metal particles are used as fuel components to develop small-scale pyrolant charges as deployed in igniters, flares, and fireworks. Non-metal particles such as boron and carbon are used to formulate energetic pyrolants. Polymeric materials are commonly used as fuel components to develop relatively large-scale pyrolant charges, such as gas generators and fuel-rich propellants. 

Fuel-rich propellants - Sustainers of Kvadrat (Russian) and Akash (Indian)... 

Fuel-rich propellants (FRPs) with high metal content find use in ram-rockets which operate with the combustion of fuel-rich hot gases generated in the primary chamber or combustor and ram air inducted from atmosphere to a secondary chamber or combustor for full combustion. The rocket system where energy for propulsion is derived in such a manner is termed an integrated rocket-ramjet (IRR). The major benefit of a ram-rocket is substantial reduction in the weight of rocket or missile as the oxidizer need not be carried along with the propellant fuel. Fuel-rich propellant formulations for ram-rockets consist of metal fuels, binder... 

AP is the most commonly used oxidizer for fuel rich propellants. AN is used where a high rate of gas generation is the prime requirement. Sometimes, sodium nitrate also finds application due to its high density, oxygen availability, heat of combustion and affinity of its exhaust species with ram-air. HMX-based fuel-rich formulations which give better performance, are also available. 

Similar to composite propellants, flexibilized epoxy or novolac epoxy resins reinforced with fillers or fibers are used for inhibition of fuel-rich propellants. 

Liu, T.-K., Shyu, L-M. and Hsia, Y.-S. (1996) Effect of fluorinated graphit e on combustion of boron and boron-based fuel-rich propellants. J. Propul. Power,... 

Nitramine composite propellants composed of HMX or RDX particles and polymeric materials offer the advantages of low flame temperature and low molecular mass combustion products, as well as reduced infrared emissions. The reduced infrared emissions result from the elimination of COj and H2O from the combustion products. To formulate these composite propellants, crystalline nitramine monopropellants such as HMX or RDX are mixed with a polymeric binder. Since both HMX and RDX are stoichiometrically balanced, the polymeric binder acts as a coolant, producing low-temperature, fuel-rich combustion products. This is in contrast to AP composite propellants, in which the binder surrounding the AP particles acts as a fuel to produce high-temperature combustion products. 

MPa. A reddish flame is seen above the bluish flame. On the other hand, no bluish and reddish flames are seen when the p(0.80) propellant burns at a pressure of 0.1 MPa, as shown in Fig. 7.4 (c). The flame is entirely yellowish, which can be ascribed to the fuel-rich nature of the diffusion flame generated by the gaseous decomposihon products of the binder and the AP parhcles.l Indeed, the flame... 

Fig. 7.14 shows the effect of the AP particle size on the burning rate of mono modal AP composite propellants.1 1 The burning rate increases as the AP particle size, do, decreases. However, the effect of the particle size on the burning rate diminishes with increasing pressure. Though the propellants shown in Fig. 7.14 are all fuel-rich, with ap(0-65), the effect of AP particle size on burning rate is clearly evident. 

When some portion of the AP particles contained within an AP composite propellant is replaced with nitramine particles, an AP-nitramine composite propellan-tis formulated. However, the specific impulse is reduced because there is an insufficient supply of oxidizer to the fuel components, i. e., the composition becomes fuel-rich. The adiabatic flame temperature is also reduced as the mass fraction of nitramine is increased. Fig. 7.49 shows the results of theoretical calculations of and Tf for AP-RDX composite propellants as a function of Irdx- Th propellants are composed of jjxpb(0-13) and the chamber pressure is 7.0 MPa with an optimum expansion to 0.1 MPa. Both I p and T)-decrease with increasing Irdx- The molecular mass of the combustion products also decreases with increasing Irdx due to the production of Hj by the decomposition of RDX. It is evident that no excess oxidizer fragments are available to oxidize this H2. 

When large spherical AP particles dg = 3 mm) are added, large flamelets are formed in the dark zone.Pl Close inspection of the AP particles at the burning surface reveals that a transparent bluish flame of low luminosity is formed above each AP particle. These are ammonia/perchloric acid flames, the products of which are oxidizer-rich, as are also observed for AP composite propellants at low pressures, as shown in Fig. 7.5. The bluish flame is generated a short distance from the AP particle and has a temperature of up to 1300 K. Surrounding the bluish flame, a yellowish luminous flame stream is formed. This yellowish flame is produced by in-terdiffusion of the gaseous decomposition products of the AP and the double-base matrix. Since the decomposition gas of the base matrix is fuel-rich and the temperature in the dark zone is about 1500 K, the interdiffusion of the products of the AP and the matrix shifts the relative amounts towards the stoichiometric ratio, resulting in increased reaction rate and flame temperature. The flame structure of an AP-CMDB propellant is illustrated in Fig. 8.1. 

Since NC is a fuel-rich nitrate ester, a nitropolymer propellant with a high NC content generates black smoke as a combustion product. In addition, the combustion of nitropolymer propellants becomes incomplete at low pressures below about 3 MPa and black smoke composed of solid carbon particles is formed. This incomplete combustion is caused by the slow rates of the reactions of NO with aldehydes and CO in the combustion wave. Thus, the nitropolymer propellants are no longer smokeless propellants under low-pressure burning conditions. 

Fig. 12.11 shows the structure of a rocket plume generated downstream of a rocket nozzle. The plume consists of a primary flame and a secondary flame.Fil The primary flame is generated by the exhaust combustion gas from the rocket motor without any effect of the ambient atmosphere. The primary flame is composed of oblique shock waves and expansion waves as a result of interaction with the ambient pressure. The structure is dependent on the expansion ratio of the nozzle, as described in Appendix C. Therefore, no diffusional mixing with ambient air occurs in the primary flame. The secondary flame is generated by mixing of the exhaust gas from the nozzle with the ambient air. The dimensions of the secondary flame are dependent not only on the combustion gas expelled from the exhaust nozzle, but also on the expansion ratio of the nozzle. A nitropolymer propellant composed of nc(0-466), ng(0-369), dep(0104), ec(0 029), and pbst(0.032) is used as a reference propellant to determine the effect of plume suppression. The burning rate characteristics of the propellants are shown in Fig. 6-31. Since the nitropolymer propellant is fuel-rich, the exhaust gas forms a combustible gaseous mixture with the ambient air. This gaseous mixture is ignited and afterburning occurs somewhat downstream of the nozzle exit. The major combustion products in the combustion chamber are CO, Hj, CO2, N2, and HjO. The fuel components are CO and H2, the mole fractions of which at the nozzle throat are co(0.47) and iH2(0.24).

The principal infrared emissions from gaseous combustion products of propellants are caused by the high-temperature COj and H2O molecules. When nitropolymer propellants or AP composite propellants burn, large amounts of high-temperature CO2 and HjO molecules are formed. If these propellants burn incompletely due to their fuel-rich composihons, large amounts of hydrocarbon fragments and solid... 

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. 

The projectile acquires a specified supersonic flight speed after burn-out of the booster propellant. The booster nozzle is then ejected to the outside and the port cover is opened. The compressed air resulting from the aforemenhoned shock wave is then introduced through the air-intake. The booster chamber becomes a ramburner and the gas-generating pyrolant is ignited to produce fuel-rich combushon products. 

Nitropolymers composed of -O-NO2 functions and hydrocarbon structures are pyrolants that produce fuel-rich products accompanied by exothermic reaction. Typical nitropolymers are mixtures of nitrocellulose, nitroglycerin, trimethylolethane trinitrate, or triethylene glycol dinitrate, similar to the double-base propellants used in rockets and guns. Mixtures of these nitropolymers are formulated as fuel-rich pyrolants used in ducted rockets. This class of pyrolants is termed NP pyrolants. 

Tq, of gas-generating pyrolants such as fuel-rich AP-HTPB and fuel-rich nitropoly-mer pyrolants are lower than those of rocket propellants such as AP-HTPB and nitropolymer propellants. The gas-phase temperature is low and hence the heat flux feedback through the wires is low for the gas-generating pyrolants as compared with propellants. However, r /ro appears to be approximately the same for both pyrolants and propellants. The obtained burning-rate augmentations are of the order of 2-5. 

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