Yeu-Cherng Lu

Regression Rate Behavior of Hybrid Rocket Solid Fuels

An experimental investigation of the regression-rate characteristics of hydroxyl-terminated polybutadiene (HTPB) solid fuel burning with oxygen was conducted using a windowed, slab-geometry hybrid rocket motor. A real-time, x-ray radiography system was used to obtain instantaneous solid-fuel regression rate data at many axial locations. Fuel temperature measurements were made using an array of 25- πm e ne-wire embedded thermocouples. The regression rates displayed a strong dependence on axial location near the motor head-end. At lower mass e ux levels, thermal radiation was found to signie cantly ine uence the regression rates. The regression rates werealso affected by theadditionofactivated aluminum powder.A 20%by weightaddition of activated aluminum to HTPB increased the fuel mass e ux by 70% over that of pure HTPB. Correlations were developed to relate the regression rate to operating conditions and port geometry for both pure HTPB and for HTPB loaded with certain fractions of activated aluminum. Thermocouple measurements indicated that the fuel surface temperatures for pureHTPBwerebetween930 and1190 K.TheHTPBactivationenergywasestimatedat11.5 kcal/mole,suggesting that the overall regression process is governed by physical desorption of high-molecular weight fragments from the fuel surface.

Thermal decomposition study of hydroxyl-terminated polybutadiene (HTPB) solid fuel

Thermal analysis has been conducted on R-45M resin, Isonate 143L crosslinking agent, and cured HTPB polymer to determine the thermal decomposition processes, energy changes, and kinetic parameters. The results show that thermal decomposition processes of the cured HTPB polymer are predominantly characterized by the R-45M resin, and confirm that the urethane bond cleavage is the first step in the decomposition. After the urethane bonds are cleaved and the diisocyanate volatilizes (above approximately 430°C), HTPB behaves as though it had never been crosslinked. Increasing the heating rate of HTPB polymer increases the extent of the first-stage decomposition and decreases its exothermicity, implying augmentation of the depolymerization process. The activation energies and pre-exponential factors of thermal decomposition for test samples were determined. Results also indicate that adding the curing agent to the polybutadiene resin has a greater influence on the energetics than on the kinetics of R-45M decomposition.

Fuel decomposition and boundary-layer combustion processes of hybrid rocket motors

Using a high-pressure, two-dimensional hybrid motor, an experimental investigation was conducted on fundamental processes involved in hybrid rocket combustion. HTPB (Hydroxyl-terminated Polybutadiene) fuel cross-linked with diisocyanate was burned with GOX under various operating conditions. Large-amplitude pressure oscillations were encountered in earlier test runs. After identifying the source of instability and decoupling the GOX feed-line system and combustion chamber, the pressure oscillations were drastically reduced from +/-20% of the localized mean pressure to an acceptable range of +/-1.5% Embedded fine-wire thermocouples indicated that the surface temperature of the burning fuel was around 1000 K depending upon axial locations and operating conditions. Also, except near the leading-edge region, the subsurface thermal wave profiles in the upstream locations are thicker than those in the downstream locations since the solid-fuel regression rate, in general, increases with distance along the fuel slab. The recovered solid fuel slabs in the laminar portion of the boundary layer exhibited smooth surfaces, indicating the existence of a liquid melt layer on the burning fuel surface in the upstream region. After the transition section, which displayed distinct transverse striations, the surface roughness pattern became quite random and very pronounced in the downstream turbulent boundary-layer region. Both real-time X-ray radiography and ultrasonic pulse-echo techniques were used to determine the instantaneous web thickness burned and instantaneous solid-fuel regression rates over certain portions of the fuel slabs. Globally averaged and axially dependent but time-averaged regression rates were also obtained and presented.

UV / Visible absorption spectroscopy of dark zones in solid-propellant flames

UV / Visible absorption spectroscopy, coupled with multiparameter least-squares analysis, was used to determine profiles of gas-phase temperature and NO concentration in the dark-zone region of two RDX-based solid propellants (XM39 and M43) at pressure up to 3.55 MPa. At low pressures (e.g. P < 1.6 MPa for M43 and P < 2.17 MPa for XM39), a uniform dark zone was observed between the burning surface and the luminous flame, with intermittent flamelet attachment to the burning surface. As pressure increased, flamelet attachment became very pronounced, and the uniform dark zone was rarely seen. Temperature and NO concentration profiles in the dark zone of both propellants were very uniform when a uniform dark zone existed. Under this situation, M43 and XM39 were found to have similar dark-zone temperatures (around 1050 K for M43 and 1180 K for XM39); however, M43 has about 5% more NO mole fraction than XM39 in the dark-zone region (20% vs 15%). On the other hand, when luminous flamelets were attached to the burning surface, line-of-sigh-averaged NO concentrations (where a fixed path length is used in data anlysis) exhibited a rapid reduction with increased distance from the burning surface. This reduction results from the shorter intercepted lengthy by the UV light beam in the dark zone as the vertical distance from the burning surface increases; the deduced NO concentration is thereby decreased if the whole propellant diameter is used as the path length in data analysis. According to video records, M43 has a smaller dark zone than XM39 at the same pressure, and its dark zone decreases more rapidly with an increase of pressure than XM39 in the pressure range studied. M43 was found to be able to maintain a stable “flame-burning” mode combustion at a lower pressure than XM39.

Ignition and combustion characteristics of RDX-based pseudopropellants

Burning rates, ignition delay times, temperature, and OH concentration profiles in the flame zone were determined for pure RDX and pseudopropellants containing RDX with CAB (cellulose acetate butyrate) binder having 8, 11, and 14% by weight. Deduced final flame temperatures ( T f ) at 0.45 MPa from UV/visible absorption spectroscopy measurements indicated a monotonic decrease of T f with an increase of CAB percentage from 3062 K for pure RDX to 2742 K for RDX/CAB (86/14%) as departure from stoichiometry became larger. The deduced final flame temperatures are in good agreement with equilibrium calculations. The measured burning rates of the propellants at 0.45 MPa decreased with increasing CAB content. No multistage flame structure was observed for either pure RDX or RDX/CAB pseudopropellants processed by shock-precipitation procedure. Measurements indicate that the ignition delay times increase with the increase of CAB content in pseudopropellants; this increase is partly due to the endothermic surface reactions of CAB and partly caused by the increase of specific heats of pseudopropellants by adding more CAB. The simple power-law curve-fitted to the ignition data indicated that the pure RDX sample surface is almost inert during the ignition period. During the tests, it was observed that the onset of light emission occurred in the gas phase above the sample surface; therefore, the gas-phase chemistry plays an important role in the ignition processes of RDX/CAB pseudopropellants.

Absorption spectroscopy of solid propellant flames

Absorption spectroscopy, incorporated with multiparameter least-squares analysis, was used to determine profiles of @-phase temperature and NO and OH concentrations of RDX, XM39 and M43 solid propellants at pressures up to 3.55 MPa. The deduced RDX f d flame temperatures at different pressures are very close to results of equilibrium calculations; however, the deduced OH concentratiom are smaller than equilibrium solutions. The major source of measurement error in OH concentrations is in the determination of light path length Temperature and NO concentration profiles of M43 and XM39 showed plateaus when a dark zone existed between the burning surface and luminous flame regions. However, when IuminoUS flamelets were attached to the burning surface, line-of-sight averaged NO concentrations (where a uniform path length is used in data analysis) exhibited a rapid reduction for in~eased istance from burning surface This reduction is caused by shorter intercepted distance by the W light beam in the dark zone as the vertical distance from the burning surface increases, and the deduced NO concentration is thereby decreased if the whole propellant diameter is used as the path length in data analysis. Even though M43 and XM39 have simila~ dark-zone temperatures (around 1,050 K forM43 and 1,150 K for XM39), M43 has about 5% more of NO concentration than XM39 in the darkzone region (21% vs. 16%). d

MEASUREMENT OF INTRINSIC BURNING RATE OF NITROMETHANE

An optically accessible, high-pressure liquid propellant strand burner (LPSB) was designed and established to measure the intrinsic burning rates of liquid propellants over broad ranges of pressures and initial temperatures. Compared to other methods, the current setup minimizes undesirable effects introduced by gelling agents and LP-containing tubes that may modify LP burning behavior. A temperature conditioning system allowed temperature sensitivity to be examined as well. To determine the intrinsic burning rate, chamber pressure was varied to achieve a steady-state position of the LP burning surface at a set actuator feeding rate. Operation of the LPSB was demonstrated with nitromethane monopropellant. Burning rates at three different initial temperatures were determined in an air environment. Over a pressure range of 2.5 to 15 MPa (250 to 2200 psig), the intrinsic burning rate of nitromethane was determined as a power law of chamber pressure: H, [mm/s] = 0.316 P1-02 [MPa] It was found that the burning rate of nitromethane was insensitive to the variation in initial temperatures tested (15 to 43 °C). The tube confinement encountered in conventional LP burningrate measurements was found to have non-negligible effects on the LP burning rate.