B. N. Raghunandan

Flame Spread with Sudden Expansions of Ports of Solid Propellant Rockets

Detailed theoretical and experimental studies on flame spread over nonuniform ports of solid propellant rockets have been carried out. An idealized two-dimensional laboratory motor was used for the experimental study with the aid of cinematography. A detailed numerical simulation of the flame spread has also been carried out with the help of a two-dimensional Navier-Stokes solver. Experimental results showing the phenomenon of secondary ignition have been reported earlier and also reviewed here with the inclusion of additional results of a three-dimensional geometry closer to a dual-thrust motor. In this paper more tangible results including the numerical modeling of flame spread have been reported. It has been shown conclusively that under certain conditions of step location, step height, and port height, which govern the velocity of gases at the step by the partially ignited propellant surface or by the igniter gas Row, secondary ignition can occur far downstream of the step, This is very likely to be within the recirculating Row region. The secondary ignition gives rise to two additional flame fronts, one of which spreads backward st a relatively lower velocity, presumably as a result of low reverse velocities present in the separation zone. This phenomenon is Likely to play an important role in the starting transient of solid propellant rockets with nonuniform ports.

Fluid-Throat-Induced Shock Waves During the Ignition Transient of Solid Rockets

PREDICTION and control of pressure and pressure-rise rate during the ignition transient of solid-propellant rocket motors with a nonuniform port are of topical interest. In certain designs, an ignition pressure spike and a high rate of pressure rise may adversely affect the steadiness and stability of burning, thermoviscoelastic response of the grain and inhibitors, and the dynamic response of the hardware parts.1 An excessive pressurization rate can cause a failure even when the pressure is below the design limit.2,3 Although, a great deal of research has been done in the area of solid rocket motors (SRMs) for more than six decades, the accurate prediction of the ignition transient in ports of high-performance solid rocket, with sudden expansion and/or steep divergence/convergence or protrusions has not previously been accomplished.

Starting Transient Flow Phenomena in Inert Simulators of Solid Rocket Motors with Divergent Ports

The basic idea behind a solid rocket motor (SRM) is simple but its design is a complex technological problem requiring expertise in diverse subdisciplines to address all of the physics involved. The design optimization of high-performance rockets is more complex when the mission demands dual thrust. The motivation for the present study emanates from the desire to explain the phenomena or mechanism(s) responsible for the high ignition peak pressure (pressure peak), pressure-rise rate, instabilities, and pressure oscillations often observed during the static tests and the actual flights of certain class of high-performance SRMs with nonuniform ports [1–9]. In the SRM industry many dual-thrust motors (DTMs) are known to have experienced abnormal high ignition peak pressure often on the order of 5 times the steady state value [6]. Various measures were taken to eliminate the peak pressure, but none of the conventional remedies seemed to help. Nevertheless, through the empirical techniques increasing the port area of the motor has been proposed as one of the remedies for reducing the unusual ignition peak of the DTM. Although such a remedy could negate the unacceptable peak pressure, it has affected the high-performance nature of the motor. Hence the elimination of the unusual ignition peak and the pressure-rise rate without sacrificing the basic grain configuration or the volume loading became a meaningful objective for further studies.

Studies on Internal Flow Choking in Dual-Thrust Motors

Adetailed picture of the internal flow during the starting transient of high-performance solid rocket motors (SRMs) is of topical interest for several reasons in addition to the motor performance itself [1–12]. Despite the fact that many of the existing models could predict the internal flow features of certain classes of SRMs, none of these models could capture the unusual starting transient flow features such as pressure overshoot and pressure-rise rate often observed during the initial phase of operation of the dual-thrust motors (DTMs) [1]. Ikawa and Laspesa [8] reported that during the first launching of the space shuttle from the Eastern Test Range, the launch vehicle experienced the propagations of a strongly impulsive compression wave. This wave was induced by the SRM ignition and was emanating from the large SRM duct openings. The analysis further showed that the compression wave created by ignition of the main grain was the cause of the ignition overpressure on the launch pad [9]. Alestra et al. [10] reported that Ariene 5 launcher experienced overpressure load during the liftoff phase. The overpressure is composed of the ignition overpressure, which emanates from the launch pad, and the duct overpressure, which emanates from the launch ducts. Of late, Sanal Kumar et al. [1,2] reported that abnormal high-pressure overshoot in certain class of DTMs during the startup transient is due to the formation of shock waves because of the fluid-throat effect, which has received considerable attention in the scientific community. This manuscript is the continuation of the previous connected note for establishing the intrinsic flow physics pertinent to internal flow choking in inert simulators of dual-thrust motors [1]. Note that the illustration of ignition pressure spike is deliberately set aside in this note for explaining the intrinsic flow physics pertinent to internal flow choking without complications arising from the propellant combustion.

Boundary-Layer Effects on Internal Flow Choking in Dual-Thrust Solid Rocket Motors

Theoretical studies have been carried out to examine internal flow choking in the inert simulators of a dual-thrust motor. Using a two-dimensional k-omega turbulence model, detailed parametric studies have been carried out to examine aerodynamic choking and the existence of a fluid throat at the transition region during the startup transient of dual-thrust motors. This code solves standard k-omega turbulence equations with shear flow corrections using a coupled second-order-implicit unsteady formulation. In the numerical study, a fully implicit finite volume scheme of the compressible, Reynolds-averaged, Navier-Stokes equations is employed. It was observed that, at the subsonic inflow conditions, there is a possibility of the occurrence of internal flow choking in dual-thrust motors due to the formation of a fluid throat at the beginning of the transition region induced by area blockage caused by boundary-layer-displacement thickness. It has been observed that a 55% increase in the upstream port area of the dual-thrust motor contributes to a 25% reduction in blockage factor at the transition region, which could negate the internal How choking and supplement with an early choking of the dual-thrust motor nozzle. If the height of the upstream port relative to the motor length is too small, the developing boundary layers from either side of the port can interact, leading to a choked,flow. On the other hand, if the developing boundary layers are far enough apart, then choking does not occur. The blockage factor is greater in magnitude for the choked case than for the unchoked case. More tangible explanations are presented in this paper for the boundary-layer blockage and the internal flow choking in dual-thrust motors, which hitherto has been unexplored.

Ignition Transient of Dual-Thrust Solid Propellant Rocket Motors - A Review

This paper addresses the future design challenges associated with the development of high-performance dual-thrust solid propellant rockets, because of their large size, high volumetric loading density, high length-to-diameter ratio, and demanding thrust time trace shape requirements; after a close perusal of the earlier theoretical and experimental findings. We had observed and reported earlier that, at the subsonic inflow conditions, there is a possibility of the occurrence of internal flow choking in dual-thrust motors due to the formation of a fluid throat at the beginning of the transition region induced by area blockage caused by boundarylayer-displacement thickness. Through empirical techniques, increasing the upstream port area of the motor has often been proposed as one of the remedies for negating the internal flow choking and further eliminating the unusual ignition pressure spike often observed during the ignition transient of dual-thrust motors. Unfortunately, this reduces the propellant loading density and affects the high performance nature of the rocket motor due to the envelop restriction. The objective of this review is to verify the applicability of the existing concepts before embarking on the formulation of a new model and a code of solution for high performance solid propellant rockets with non-uniform port geometry.

Simulation of Flame Spread and Turbulent Separated Flows in Solid Rockets

Theoretical studies have been carried out to examine the flame spread and the turbulent separated flows in solid rocket motors with non-uniform port geometry. Detailed parametric studies have been carried out to examine the influence of port geometry on flow separation and reattachment using a standard k-omega turbulence model. In solid rockets, the flow separation and the reattachment will cause secondary ignition followed by multiple flame fronts. An error in pinpointing the location of secondary ignition can lead to significant errors in the thrust transient prediction of solid rockets. The present study is expected to aid the designer for conceiving the physical insight on secondary ignition into problems associated with the prediction and the reduction of the peak pressure and the pressurization rate during the starting transient period of solid rocket motors with non-uniform ports.

Diagnostic Investigation of Instabilities and Pressure Oscillations in High-Performance Solid Rockets

Using a standard k-ω turbulence model, in this paper detailed numerical computations have been carried out in inert simulators to examine the geometric dependence of transient flow features of solid rocket motors. We observed that the damping of the temperature fluctuation is faster in solid rocket with convergent port than with divergent port geometry. We inferred that an early damping of the transient flow fluctuations using the port geometry is a meaningful objective for the suppression and control of the instability and/or pressure/thrust oscillations during the staring transient of solid rockets.

A Phenomenological Introduction of Fluid-Throat in High-Velocity Transient Motors

Using a standard k-omega turbulence model, the influence of geometry-dependent driving forces on the formation of shock waves during the ignition transient of solid rocket motors have been examined. It was observed that, in addition to the igniter induced shock waves, at subsonic inflow conditions there is a possibility of the occurrence of shock waves in high-velocity transient motors with divergent port due to the formation of a fluid-throat at the beginning of the transition region induced by area blockage caused by boundary layer displacement thickness. As a result the upstream narrow port of the rocket motor will act like a second igniter to the downstream port, leading to the formation of possible shock waves inside the rocket motor during the ignition transient period. I. Introduction UANTITATIVE prediction/knowledge of the maximum pressure and pressurization rate during the ignition transient of solid rockets, allows and justifies the use of small margin of safety for the engine parts, thus result in high motor mass ratio, in addition to the control and guidance requirement of the vehicle. Ignition transient is defined in this paper as the time interval between the application of the ignition signal and the instant at which motor attains its equilibrium or designed operating conditions. In certain design, the ignition pressure spike (not the initial response function of the pressure transducer!) and the rate of pressure-rise rate may adversely affect the steadiness and stability of burning, thermoviscoelastic response of the grain and inhibitors and the dynamic response of the hardware parts. An excessive pressurization rate can cause a failure even when the pressure is below the design limit. The use of new grains with higher performance or more environmentally benign propellants will require systematic combined thermoviscoelastic and fluid dynamics analysis, and further redesign to account for higher structural loads and temperatures, and resulting changes in system instabilities. 1 Although, a great deal of research has been done in the area of solid rockets for more than six decades, the accurate prediction and the reduction of unusual pressure spike and pressure rise rate often observed during the ignition transient of high-performance solid rockets, have remained an intangible problem.

Ratiocinative Approach to Ignition Transient Modeling in Solid Rockets

Detailed theoretical studies have been carried out to examine the variation of ignition transient on identical motors with different ignition delay. Igniter ballistics and propellant properties are varied for getting different ignition delay. We observed that the altered variation of ignition delay will alter the flame spread and thereby the overall ignition transient history of identical solid rockets. Significant peak pressure variation is observed in the case of solid motors with non-uniform ports. We concluded that, after maintaining the constant igniter ballistics, the designer can reduce the peak pressure by altering the surface condition by pasting high conducting material over the surface of the propellant. Such a change in thermal conductivity can be easily achieved by slightly altering the metal loading in the propellant. This ratiocinative approach will help the designer in reducing the peak pressure by altering the propellant ignition properties and the igniter characteristics without sacrificing the basic grain configuration or the volume loading.