Reusable Solid Rocket Motor Research Articles

Updated frequency-dependent directivity indices for large solid rocket motors

For many years, empirical models for rocket noise radiation relied on the directivity indices published in NASA SP-8072 (K. Eldred, 1971). Because these indices had known issues, NASA led a campaign to update the indices using a Space Shuttle reusable solid rocket motor (RSRM). The RSRM measurements, involving a polar arc at a radius of about 80 nozzle exit diameters, resulted in updated directivity indices (Haynes and Kenny, AIAA 2009–3160). However, because the arc origin was placed at the nozzle exit plane, James et al. [Proc. Mtgs. Acoust., 18, 040008 (2012)] corrected the low-frequency indices using an estimated dominant source position for each frequency and assumed spherical spreading. This paper revisits that correction effort by using near-field vector intensity measurements from a similar, but smaller diameter and lower thrust, GEM-60 motor to determine the frequency-dependent origin to adjust the apparent angles and distances of the measurements. Additionally, an effort is made to account for RSRM plume impingement downstream that likely resulted in lower high-frequency levels than would have been measured if the plume had been entirely free. This analysis results in updated, frequency-dependent directivity indices for a large solid rocket motor. Their applicability to other rockets will be discussed.

Solid rocket motor internal ballistics using an enhanced surface-vorticity panel technique

This work explores the use of surface-mesh flow solvers to model solid rocket internal ballistics with arbitrary grain geometry. Specifically, a surface-vorticity approach, originally intended for external flow applications, is adapted for internal flow analysis using boundary conditions that are suitable for solid rocket motors. In this study, an enhanced panel code is shown to be capable of resolving internal rocket flowfields with a striking level of fidelity and with such a degree of computational efficiency to make it valuable in the conceptual and preliminary design of rocket motors. In this process, the vortex paneling approach embodied within FlightStream® is refined using boundary conditions appropriate for solid rocket rotational flows. The simulation results are then compared to existing analytical solutions for cylindrical and planar chamber configurations exhibiting small taper angles and uniform headwall injection. For a more realistic validation case, the space shuttle's reusable solid rocket motor (RSRM) is examined. Guided by the analytical models, simple rotational and compressibility corrections are incorporated into the solver, and the results are subsequently compared to two other computational models and experimental measurements gathered from qualification motors. For the basic configurations, our results are shown to agree well with theoretical predictions. For the RSRM case, the corrected solution agrees well with the validation data in the first half of the motor; however, it becomes less robust in the aft region unless a recirculation zone boundary patch is applied; the latter approximates the added vorticity introduced by intersegmental gaps and the submerged nozzle effects.

Effect of Wollastonite on the ablation resistance of EPDM based elastomeric heat shielding materials for solid rocket motors

EPDM/aramid fibers ablatives represent the state of the art Elastomeric Heat Shielding Materials (EHSMs) for Solid Rocket Motors (SRMs). Due to their mechanical properties and excellent thermal stability, aramid fibers or pulp constitute the common reinforcement of EPDM based formulations. Aramid fiber was introduced by NASA and space companies such as Orbital-ATK with the aim to replace asbestos. Space Shuttle’s reusable SRMs and many ICBMs employed Nitrile butadiene rubber (NBR) loaded with asbestos (ASNBR) as a primary internal case insulation; however, due to cancerogenicity of asbestos, NASA sought a replacement for many years, but this goal has been very challenging to achieve since the comprehensive performance of ASNBR was and is still difficult to match. Wollastonite can be considered the only non-hazardous mineral/inorganic filler able to partially replace asbestos. Wollastonite is a calcium inosilicate mineral (CaSiO3) and the shape of the filler is acicular. Our study wanted to evaluate Wollastonite as a possible replacement of aramid pulp in the production of EPDM based EHSMs for solid rocket motors. The heat capacity, thermal and dimensional stability, mechanical properties, and ablation resistance of the produced materials were studied and here reported.

Far-field acoustical measurements during a Space Launch System solid rocket motor static firing

Acoustical measurements were made in the very far field during a recent test firing of the five-segment QM-1 Space Launch System solid rocket motor at Orbital ATK. Data were taken using 6.35 mm and 12.7 mm type-1 microphones at three far-field locations to the sideline and aft of the nozzle at a range of 650–800 nozzle diameters. The experiment setup, including the appreciable terrain changes, is first discussed. Spectral and autocorrelation analyses highlight the variation of the noise with respect to observation angle. In addition, high-frequency spectral characteristics and waveform statistics are evidence of the significant nonlinear propagation over the propagation range. Effects of microphone size, terrain effects, and data stationarity during the firing are discussed. This dataset is compared to measurements of other solid rocket motors at closer and farther ranges, including the GEM-60 and the four-segment Shuttle Reusable Solid Rocket Motor.

Comparison of the acoustical emissions of multiple full-scale rocket motors

Development of the next-generation space flight vehicles has prompted a renewed focus on rocket sound source characterization and near-field propagation modeling. Improved measurements of the sound near the rocket plume are critical for direct determination of the acoustical environment both in the near and far-fields. They are also crucial inputs to empirical models and to validate computational aeroacoustics models. Preliminary results from multiple measurements of static horizontal firings of Alliant Techsystems motors including the GEM-60, Orion 50S XLG, and the Reusable Solid Rocket Motor (RSRM) performed in Promontory, UT, are analyzed and compared. The usefulness of scaling by physical parameters such as nozzle diameter, velocity, and overall sound power is demonstrated. The sound power spectra, directional characteristics, distribution along the exhaust flow, and pressure statistical metrics are examined over the multiple motors. These data sets play an important role in formulating more realistic sound source models, improving acoustic load estimations, and aiding in the development of the next generation space flight vehicles via improved measurements of sound near the rocket plume.

Chemical Erosion of Carbon-Phenolic Rocket Nozzles with Finite-Rate Surface Chemistry

Ablative materials are commonly used to protect the nozzle metallic housing and to provide the internal contour to expand the exhaust gases in solid rocket motors. Because of the extremely harsh environment in which these materials operate, they are eroded during motor firing with a resulting nominal performance reduction. The objective of the present work is to study the thermochemical erosion behavior of carbon-phenolic material in solid rocket motor nozzles. The adopted approach relies on a validated full Navier–Stokes flow solver coupled with a thermochemical ablation model, which takes into account finite-rate heterogeneous chemical reactions at the nozzle surface, rate of diffusion of the species through the boundary layer, pyrolysis gas and char-oxidation product species injection in the boundary layer, heat conduction inside the nozzle material, and variable multispecies thermophysical properties. The results obtained with the proposed approach are compared with two sets of experimental data: subscale motor tests carried out for the space shuttle reusable solid rocket motor and the static firing tests of the second and third stage solid rocket motors of the European Vega launcher, which use carbon-carbon for the throat insert and carbon-phenolic for the region downstream of the throat.

Analysis of noise from reusable solid rocket motor firings

As part of investigations into the design of next-generation launch vehicles, near and far-field data were collected during horizontal static firings of reusable solid rocket motors. In addition to spectral analysis at individual microphone locations, the spatial variation of overall and one-third octave band pressure levels at sideline and polar arc arrays is considered. Analysis of the probability density functions reveals positively skewed pressure waveforms, but extreme skewness in the first-order estimate of the time derivative because of the presence of significant acoustic shocks.

Effects of ground impedance on large rocket motor noise measurements

Free field acoustic measurements have been collected on large solid rocket motors at the ATK test facility in Promontory, Utah. Ground effects have been measured to understand the impact on the overall data collection effort. Ground effects were measured at two test stands with vastly different terrain that hold the reusable solid rocket motor (RSRM) and RSRMV (five-segment RSRM) static test motors. Techniques for measuring and understanding ground effects are investigated, and examples presented from two different methods.

A review of large solid rocket motor free field acoustics

Approximately twice a year, ATK Aerospace Systems has static fired an assembled reusable solid rocket motor (RSRM) in a horizontal configuration at its test facility in Utah. The firings took place on elevated terrain with the nozzle exit plume mostly undeflected and the landscape allowing placement of microphones within direct line of sight to the exhaust plume. NASA and ATK underwent a significant effort over several years to collect acoustic data in the free field and on the motor itself during RSRM static test firings. These data were used to characterize the acoustic field and to update the prediction methodologies in the monograph NASA SP-8072 “Acoustic Loads Generated by the Propulsion System.” This work represents a review of the free field acoustics generated during large solid rocket motor firings and may be the only repeatable free field acoustic experiment on motors like these.

An acoustical comparison of sub‐scale and full‐scale.

Recently, members of the Marshall Space Flight Center (MSFC) Fluid Dynamics Branch and Wyle Labs measured far‐field acoustic data during a series of three reusable solid rocket motor (RSRM) horizontal static tests conducted in Promontory, UT. The test motors included the Technical Evaluation Motor 13 (TEM‐13), Flight Verification Motor 2 (FVM‐2), and the Flight Simulation Motor 15 (FSM‐15). Similar far‐field data were collected during horizontal static tests of sub‐scale solid rocket motors at MSFC. Far‐field acoustical measurements were taken at multiple angles within a circular array centered about the nozzle exit plane, each positioned at a radial distance of 80 nozzle‐exit‐diameters from the nozzle. This type of measurement configuration is useful for calculating rocket noise characteristics such as those outlined in the NASA SP‐8072 “Acoustic Loads Generated by the Propulsion System.” Acoustical scaling comparisons are made between the test motors, with particular interest in the overall sound power,...

Near‐field energy‐based measurements of small solid rocket motors.

Results of detailed near‐field energy‐based measurements of small solid rocket motors are described. The 5‐in. center perforated motors are tested to ensure thrust equivalence of Shuttle reusable solid rocket motor propellant samples. An array of four three‐dimensional acoustic probes, located as close as two nozzle diameters to the plume shear layer, was located at nine positions for a total of 36 measurement points. The data collected were used to create maps of near‐field energy‐based quantities in the vicinity of the plume. In particular, the time‐averaged vector intensity reveals the direction of the net acoustic energy flow away from the rocket plume, permitting direct characterization of the source region from these motors as a function of frequency. Further, these measurements establish the utility of energy‐based acoustic quantities in aeroacoustic source measurement and modeling. [Work supported by an STTR from NASA Stennis Space Center. The cooperation of ATK Space Systems Test Services is grat...

Rocket motor microphone investigation.

At ATK's facility in Utah, large full-scale solid rocket motors are tested. The largest is a five-segment version of the reusable solid rocket motor, which is for use on the Ares I launch vehicle. As a continuous improvement project, ATK and BYU investigated the use of microphones on these static tests, the vibration and temperature to which the instruments are subjected, and in particular the use of vent tubes and the effects these vents have at low frequencies.

Flame-Spreading Behavior in a Fin-Slot Solid Propellant Rocket Motor Grain (Part II)

To accurately predict the overall ignition transient for the reusable solid rocket motors of the space shuttle booster with head-end fin slots, it is necessary to have the knowledge of the flame-spreading rates in the fin-slot region. This paper is the second of a two part study and deals with the development of a flame-spreading correlation in the fin-slot region. A subscale (1:10) pie-shaped fin-slot motor was designed to perform diagnostic measurements for studying the flame-spreading behavior on the exposed propellant surface. Dynamic similarity was considered in the igniter design so the impinging jet had a similar exit angle onto the propellant surface in the fin-slot section. Flame-spreading measurements were gathered using a high-speed digital camera and nonintrusive optical measurement methods through an array of 36 near-infrared fast-response photodetectors installed perpendicular to representative regions of the propellant surface. Results showed that the flame-spreading phenomena was highly nonuniform, starting in the downstream portion of the fin-slot region before traveling back toward the igniter. A correlation was developed for the dimensionless flame-spreading time interval showing that it was inversely proportional to the pressurization rate to a power of 0.62, which depends strongly upon the flow parameters of the igniter induced flow and local propellant grain geometry.

Flowfield Structure in a Fin-Slot Solid Rocket Motor (Part I)

To accurately predict the overall ignition transient for the reusable solid rocket motor of the space shuttle booster with head-end fin slots, it is necessary to acquire detailed flowfield structure and energy transfer rates on the exposed inert fin-slot propellant surfaces. This paper is the first of a two-part study and deals with the internal flowfield structure and heat-transfer characteristics in the fin-slot region. A subscale (1:10) pie-shaped fin-slot motor was designed to perform diagnostic measurements. An array of 36 flush-mounted heat-flux gauges was installed to detect the local temperature-rise rates at representative regions perpendicular to the propellant surface. Flowfield visualizations were conducted by applying either a chalk-powder/kerosene mixture or many small threads taped to various locations on the inner surface of the sacrificial window of the fin-slot region for high-speed video camera recording. Computational fluid dynamics simulations were performed for modeling the internal flowfield of the test rig. Results were used to develop a heat-transfer correlation governed by the internal flowfield structure within the fin-slot region. The theoretically calculated and experimentally observed internal flowfield patterns were similar in nature. The heat-transfer rates determined from the developed correlation matched the measured data trend within the experimental error. The flowfield structure and heat-transfer rate distribution are mainly governed by the major recirculating flow induced by the igniter jet.

Convective Heat Transfer in the Reusable Solid Rocket Motor of the Space Transportation System

This simulation involved a two-dimensional axisymmetric model of a full-motor initial grain of the reusable solid rocket motor of the Space Transportation System. It was conducted with the computational fluid dynamics (CFD) commercial code FLUENT®. This analysis was performed to maintain continuity with most related previous analyses; serve as a non-vectored baseline for any three-dimensional vectored nozzles; provide a relatively simple application and test for various CFD solution schemes, grid sensitivity studies, turbulence modeling, and heat transfer; and calculate nozzle convective heat transfer coefficients. The theoretical prediction of turbulent convective heat transfer in supersonic nozzles is scarce and challenging. The accuracy of the present results and the selection of the numerical schemes and turbulence models were based on matching the rocket ballistic predictions of mass flow rate, head end pressure, measured chamber pressure drop and vacuum thrust, and specific impulse. The matching for these ballistic predictions was found to be good. This study was limited to convective heat transfer, and the results compared favorably with some of the methods cited. Good agreement with the backed-out data of the ratio of the convective heat transfer coefficient to the specific heat at a constant pressure was made at the nozzle throat. Qualitative agreement was achieved upstream and downstream of the nozzle throat due to effects that are absent in this study. These backed-out data were devised to match nozzle erosion that resulted from the combination of heat transfer (convective, radiative, and conductive), chemical (transpiration), and mechanical (shear and particle impingement forces) effects. To the author's knowledge, these effects have not been investigated/reported simultaneously.

Evaluation of a Multi-Axial, Temperature- and Time- Dependent Failure Mode

To obtain a better understanding of the response of the structural adhesives used in the space shuttle's reusable solid rocket motor (RSRM) nozzle, an extensive effort has been conducted to characterize in detail the failure properties of these adhesives. This effort involved the development of a failure model that includes the effects of multi-axial loading, temperature, and time. An understanding of the effects of these parameters on adhesive failure is crucial to the prediction of safety of the RSRM nozzle. The use of this newly developed multi-axial, temperature, and time-dependent failure model for modeling failure for the adhesives TIGA 321, EA913NA, and EA946 is documented. The development of the mathematical failure model using constant load rate data for both normal and shear loading is presented. Verification of the accuracy of the failure model is shown through comparisons between predictions and measured creep and multi-axial failure data. The verification indicates that the failure model performs well for a wide range of conditions (loading, temperature, and time) for the three adhesives. The failure criterion is shown to be accurate through the glass transition for the EA946 adhesive. Though this failure model has been developed and evaluated with adhesives, the concepts should be applicable for other isotropic materials.

Ablation, mechanical and thermal conductivity properties of vapor grown carbon fiber/phenolic matrix composites

The ablation, mechanical and thermal properties of vapor grown carbon fiber (VGCF) (Pyrograf III™ Applied Sciences, Inc.)/phenolic resin (SC-1008, Borden Chemical, Inc.) composites were evaluated to determine the potential of using this material in solid rocket motor nozzles. Composite specimens with varying VGCF loadings (30–50% wt.) including one sample with ex-rayon carbon fiber plies were prepared and exposed to a plasma torch for 20 s with a heat flux of 16.5 MW/m2 at approximately 1650°C. Low erosion rates and little char formation were observed, confirming that these materials were promising for rocket motor nozzle materials. When fiber loadings increased, mechanical properties and ablative properties improved. The VGCF composites had low thermal conductivities (approximately 0.56 W/m-K) indicating they were good insulating materials. If a 65% fiber loading in VGCF composite could be achieved, then ablative properties are projected to be comparable to or better than the composite material currently used on the Space Shuttle Reusable Solid Rocket Motor (RSRM).

Aging study of neoprene FB uncured rubber in support of an obsolescence issue for EPDM rubber insulation used in the Reusable Solid Rocket Motor of the Space Shuttle

Thiokol/Wasatch (TC/W) in Utah is the manufacturer of the Reusable Solid Rocket Motor (RSRM) for the Space Shuttle, and filled EPDM and NBR rubber insulators are used on RSRM for erosion resistance. Since DuPont/Dow is closing production lines for Neoprene FB and two Nordel EPDM polymers used in three EPDM insulators for RSRM, TC/W will stockpile 6–7 years of Neoprene FB and Nordel to support RSRM production until reformulated EPDMs can be developed and recertified. NASA/MSFC used several thermal analysis techniques to study the effect of aging on a lot of uncured Neoprene FB kept in cold storage at 40°F (4.4°C) for more than 11 years (‘old’). A lot of uncured Neoprene FB kept in cold storage for 8 months (‘new’) was used for comparison in the study. In processing of uncured calendered sheets by the Shuttle vendor that are later cured near 150°C as RSRM insulation by TC/W, Neoprene FB is mixed into the EPDM formulation at 105°C. Uncured Neoprene FB is a solid at room temperature with a texture similar to that of cheese, but was shown by DSC to have a peak melting endotherm at 50°C, becoming a low viscosity liquid until it begins to cross-link at higher temperatures. From a DMA study on the uncured ‘old’ and ‘new’ lots of Neoprene FB at cure temperatures of 95, 105 and 115°C, a linear Arrhenius relationship of the logarithm of the time to gelation versus reciprocal absolute temperature yielded activation energies for the two lots of rubber. When this linear relationship was extrapolated down to the cold storage temperature of 4.4°C, gel times of the ‘old’ and ‘new’ rubber lots were predicted to be 12.7 and 72.6 years, respectively. This data shows that the ‘old’ lot of Neoprene FB still has a considerably useful shelf life in cold storage.

Use of the TMA film tension technique for differentiating polymeric materials in the Space Shuttle program

During the recent processing of a forward segment of the Reusable Solid Rocket Motor (RSRM) for the Space Shuttle, the topcoat white paint turned light brown after autoclave cure of the case insulation. Thermomechanical analysis (TMA) in the film tension mode produced modulusvs. temperature data that helped to explain why a certain insulation vacuum bagging material contributed to the brown paint color. TMA film tension data was also obtained on lab-created white and brown topcoat paint samples (with and without primer). The brown/white ratios of moduli were about 2/1 and 4/1 for topcoat/primer and topcoat only samples, respectively, from 25 to 55‡C.

Reusable Solid Rocket Motor Case: Optimum Probabilistic Fracture Control

A methodology for the reliability analysis of a reusable solid rocket motor case is discussed in this paper. The analysis is based on probabilistic fracture mechanics and probability distribution for initial flaw sizes. The developed reliability analysis can be used to select the structural design variables of the solid rocket motor case on the basis of minimum expected cost and specified reliability bounds during the projected design life of the case. Effects on failure prevention plans such as nondestructive inspection and the material erosion between missions can also be considered in the developed procedure for selection of design variables. The reliability-based procedure that has been discussed in this paper can easily be modified to consider other similar structures of reusable space vehicle systems with different fracture control plans.