Nitropolymer Propellants

Molecules in which fuel and oxidizer components are chemically bonded within the same structure are suitably predisposed for the formulation of energetic materials. Nitropolymers are composed of O-NO2 groups and a hydrocarbon structure. The bond breakage of O-NO2 produces gaseous NO2, which acts as an oxidizer fragment, and the remaining hydrocarbon structure acts as a fuel fragment. NC is a typical nitropolymer used as a major component of propellants. The propellants composed of NC are termed nitropolymer propellants . 

Since the energetics of nitropolymer propellants composed of NC-NG or NC-TMETN are limited due to the limited concentration of oxidizer fragments, some crystalline particles are mixed within these propellants in order to increase the thermodynamic energy or specific impulse. The resulting class of propellants is termed composite-modified double-base (CMDB) propellants . The physicochemical properhes of CMDB propellants are intermediate between those of composite and double-base propellants, and these systems are widely used because of their great potential to produce a high specific impulse and their flexibility of burning rate. 

The reaction rate is seen to increase linearly in an In [mj versus In p plot, and the overall order of the reaction in the gas phase is determined to be m = 1.78 based on the relationship m=n- d. This indicates that the reaction rate of TAGN in the gas phase is less pressure-sensitive than that of nitropolymer propellants for example, m = 2.5 for double-base propellants.PS]... 

The combustion of H2, CO, and hydrocarbons with NO is important in both the dark zone and the flame zone of nitropolymer propellants. It is well known that NO behaves in a complex way in combustion processes, in that at certain concentrations it may catalyze a reaction to promote a process, while at other concentrations it may inhibit the reaction. Sawyer and GlassmanP attempted to estabUsh a measurable reaction between H2 and NO in a flow reactor at 0.1 MPa. Over a wide range of mixture ratios, they found that the reaction did not occur readily below the temperature of NO dissociation, except in the presence of some radicals. Mixtures of CO and NO are also difficult to ignite, and only mixtures rich in NO could be ignited at 1720 K. 

In summary, gas-phase reactions between aldehydes and NOj occur readily and with strong exothermicity. The rate of reaction is largely dependent on the alde-hyde/N02 mixture ratio, and is increased with increasing NO2 concentration for aldehyde-rich mixtures. On the other hand, no appreciable gas-phase reactions involving NO are likely to occur below 1200 K. The overall chemical reaction involving NO appears to be third order, which impUes that it is sensitive to pressure. The reactions discussed above are important in understanding the gas-phase reaction mechanisms of nitropolymer propellants. 

Table 6.12 Chemical compositions of nitropolymer propellants catalyzed with KNO3 and K2SO4.

Fig. 7.46 shows the burning rates of the catalyzed HMX propellants and demonstrates a drastically increased burning rate, i. e., super-rate burning. However, LiF or C alone are seen to have little or no effect on burning rate. The super-rate burning occurs only when a combination of LiF and C is incorporated into the HMX propellant. The results indicate that LiF acts as a catalyst to produce super-rate burning of the H MX propellant only when used in tandem with a small amount of C. The C (carbon black) is considered to act as a catalyst promoter. A similar superrate burning effect is observed when the same catalysts are added to nitropolymer propellants. 

HMX composite propellants are composed of crystalline HMX particles and polymeric materials, and so their physical structures are heterogeneous. On the other hand, nitropolymer propellants are composed of mixtures of nitrate esters such as NC and NG, and their physical structures are homogeneous. Moreover, HMX pro-... 

It is well known that the super-rate burning of nitropolymer propellants diminishes with increasing pressure in the region 5-100 MPa and that the pressure exponent of burning rate decreases. - ] This burning rate mode is called plateau burning. As for these nitropolymer propellants catalyzed with LiF and C, HMX propellants catalyzed with LiF and C also show plateau burning. 

The combustion wave structure of HMX propellants catalyzed with LiF and C is similar to that of catalyzed nitropolymer propellants the luminous flame stands some distance above the burning surface at low pressures and approaches the burning surface with increasing pressure. The flame stand-off distance from the burning surface to the luminous flame front is increased at constant pressure when the propellant is catalyzed. The flame stand-off distance decreases with increasing pressure for both non-catalyzed and catalyzed propellants. 

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).

Potassium salts are known to act as suppressants of spontaneous igmtion of hydrocarbon flames arising from interdiffusion with ambient air. It has been reported that potassium salts act to retard the chemical reaction in the flames of nitropolymer propellants. Two types of potassium salts used as plume suppressants are potassium mtrate (KNO3) and potassium sulfate (K2SO4). The concentration of the salts is varied to determine their region of effectiveness as plume suppressants. 

Fig. 12.12 shows a typical set of flame photographs of a nitropolymer propellant treated with potassium nitrate. From top to bottom, the photographs represent KNO3 contents of 0.68%, 0.85%, 1.03%, and 1.14%. Each of these experiments was performed under the test conditions of 8.0 MPa chamber pressure and an expansion ratio of 1. Though there is little effect on the primary flame, the secondary flame is clearly reduced by the addition of the suppressant The secondary flame is completely suppressed by the addition of 1.14% KNO3. The nozzle used here is a convergent one, i. e., the nozzle exit is at the throat... 

Fig. 12.17 shows a typical set of afterburning flame photographs obtained when a nitropolymer propellant without a plume suppressant is burned in a combustion chamber and the combustion products are expelled through an exhaust nozzle into the ambient air. The physical shape of the luminous flame is altered significantly by variation of the expansion ratio of the nozzle. The temperature of the combustion products at the nozzle exit decreases and the flow velocity at the nozzle exit increases with increasing e at constant chamber pressure. 

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... 

This criterion is the so-called T combustion instability. The stability criterion expressed by < 1 is not sufficient to obtain stable combustion when the flame temperature is dependent on pressure.lO In general, m is approximately zero in the high-pressure region for most propellants. However, l/of nitropolymer propellants such as single-base and double-base propellants decreases with decreasing pressure below about 5 MPa. Since direct determination of m is difficult, the heat of explosion, is evaluated as a function of... 

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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