1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-03-08a71 Center Point of Contact: MSFCa71 Submitted by: Wil HarkinsSubject: Application of Ablative Composites to Nozzles for Reusable Solid Rocket Motors Practice: Practice: Fabrication of ablative composite materials for so
2、lid rocket motor nozzles requires a precision, integrated, multi-disciplinary, multivendor approach to design and manufacture. Creation of the material requires stringent process controls during manufacture of the rayon fiber, weaving the rayon fiber into cloth, carbonizing the rayon cloth, impregna
3、tion of carbon cloth with resin and filler, wrapping the carbon-phenolic onto a mandrel to the proper thickness, curing, nondestructive inspection and final machining to the designed configuration. Environmental conditions and cleanliness levels must be closely monitored when bonding the ablative ma
4、terial to the metal housing. The critical material properties for acceptance of carbon cloth-phenolic prepreg material are cloth content, dry resin solids content, volatile content, carbon filler content, and resin flow. Use of certified and highly skilled tape wrapping operators, bonding technician
5、s, machinists, and destructive and nondestructive testing personnel, is a must.Programs that Certify Usage: This practice has been used on Mariner Mars 64, Voyager, Galileo, Ulysses.Center to Contact for Information: MSFCImplementation Method: This Lessons Learned is based on Reliability Practice No
6、. PD-ED-1218; from NASA Technical Memorandum 4322A, NASA Reliability Preferred Practices for Design and Test.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Benefits:Adhering to proven design practices and process controls during manufacture of ablat
7、ive composite nozzle components will result in a high quality product with few rejects. Successful design and manufacturing of ablative composite materials for solid motor nozzles provides for proper transfer of the combustion gases from the burning propellant surface through the nozzle without dama
8、ge to the metal structure. Use of a properly controlled manufacturing process will result in the proper density, percent resin content, compressive strength, interlaminar shear strength, thermal conductivity, coefficient of thermal expansion, and tensile strength.Implementation:The increased interes
9、t in carbon-phenolic composite materials at NASA is due to the use of these materials as the ablative materials in the Solid Rocket Motor (SRM) nozzle of the Space Shuttle and potential follow-on solid rocket motor upgrades. The practices discussed here are the current industry standards, and, howev
10、er successful, much work is yet to be done. Most of the manufacturing practices have evolved by the trial and error method and could benefit greatly from further scientific investigation. Almost all of the nozzle materials and processes need continued research and development efforts as NASA strives
11、 for optimal performance from advanced materials. In the design of ablative nozzle components, considerable attention should be given to thermo-structural, thermochemical, process modeling and other computer predictive and analysis codes. Various computer based analytical and simulation programs are
12、 available and have been used successfully in characterizing ablative materials and their erosion, char, and thermal protection characteristics. Typical programs that have been used for this purpose are: Charring Material Ablator (CMA), Aerothermal Chemical Equilibrium (ACE), Momentum and Energy Int
13、egral Techniques (MEIT) (ACE and MEIT are used in conjunction with the CMA program), and PATRAN (geometry of nozzle, a general model for structural and thermal analysis). Computational Fluid Dynamics (CFD) is a discipline that is finding increasing usage in evaluating the exhaust flow in SRM nozzles
14、 and in determining potential heat transfer to nozzle components. These techniques are important tools for the designer who is applying ablative composites to solid rocket motor nozzles.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-refer to D descr
15、iptionD As shown on Figures 1 and 2, the aluminum structure of the SRM nozzle is protected from the heat of the expanding gasses by a series of carbon cloth phenolic rings backed up by glass or silica cloth phenolic rings. The glass and silica cloth phenolic backup rings are provided for structural,
16、 insulation, and galvanic corrosion protection. In lieu of one single ring, a series of rings is used for ease of manufacture and handling. Factors of safety for ablative carbon cloth phenolic vary between approximately 1.5 and 2.0 for erosion and are usually about 1.25 for char, depending upon the
17、location of the ablative carbon cloth phenolic in the nozzle. Generally, the factors of safety in the entrance section of the nozzle are higher than in the exit section.The optimum angle between the plies and the flame surface in SRM nozzles using ablative carbon cloth phenolic has been proven to be
18、 between 30 degrees and 60 degrees, depending upon the location, contour and heating conditions at various sections of the nozzle.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-refer to D descriptionD Manufacture of Carbon Cloth:The carbon cloth use
19、d to fabricate composite solid rocket motor nozzles is impregnated with the binder or matrix prior to wrap and cure. This preimpregnated material is commonly called “prepreg“ in the composite industry. The diversity of the manufacturing process requires six different vendors before final material is
20、 produced. These vendors: 1) produce rayon thread; 2) weave cloth; 3) carbonize cloth; 4) produce resin; 5) produce carbon fillers; and 6) impregnate carbon cloth with resin and filler (production of prepreg). Constant monitoring of all phases of the manufacturing process is required to ensure satis
21、factory quality. The rayon thread is manufactured, then woven into cloth 60 in. wide. The rayon cloth is carbonized by slowly heating to 1000 deg. to 1500 deg. C in an inert atmosphere. Critical factors to be controlled in this process are the rate of temperature increase, time, and maintenance of a
22、n inert atmosphere in the oven.After carbonization, the carbon-cloth is impregnated by drawing it through a heated container of phenolic resin and carbon filler to form prepreg. Critical factors that must be carefully controlled in the preimpregnation process are temperature of the resin/filler mixt
23、ure, tension and speed of the cloth through the resin/filler mixture, pressure on the roller to ensure penetration of resin into the cloth, oven temperature after impregnation to remove volatiles, and control staging of the resin. The prepreg should meet the uncured material acceptance test data pro
24、perties shown in Table 1 and the cured material acceptance test data properties shown in Table 2. However, it should be pointed out that since the structural minimum - maximum properties shown on Table 2 vary by factors greater than 2 indicate that additional material, process and cure controls impr
25、ovements are required to narrow the variation in material capabilities.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Table 1. Uncured Material Properties of Carbon Phenolic PropertyPercent by Weight Retest Properties Percent by Weight 1/Minimum Max
26、imum Minimum MaximumCloth content Dry resign solids Volatile content Carbon filler content Resin flow47.0 32.0 3.5 8.0 10.0 60.0 37.0 6.0 16.0 20.0 3.5 8.0 6.5 23.0 1/ Procuring activity verification of potential extension of material shelf life after six months from date of supplier manufactureThe
27、prepreg material has a six-month shelf life from date of manufacture. Potential shelf life may be extended if the material meets the retest properties of Table 1. The prepreg tape rolls should be cut on a 45 deg. bias in widths of 3 in. and 12 in. The bias cut allows the cloth to stretch and conform
28、 to the different mandrels, and the narrow widths help avoid wrinkling and folding of the material during the wrapping process. The rolled prepreg is bagged with desiccant and stored in an environmentally controlled cold storage until it is required for processing.Glass Cloth Phenolic:The glass clot
29、h phenolic shown in Figure 2 is used to provide structural integrity, a corrosion barrier, and insulation. It is fabricated in a manner similar to that of carbon cloth phenolic. The product should conform to the uncured material acceptance test data properties and the cured material acceptance test
30、data properties shown in reference 5.Table 2. Cured Material Physical and Mechanical Properties of Carbon Phenolic (At Room Temperature Unless Otherwise Specified) PropertyLimitsMinimum MaximumProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Density,
31、grams per cubic centimeter (g/cc) Resin content, percent Compressive strength, psi (edgewise)warp direction fill directionInterlaminar double shear strength, psi Thermal conductivity, Btu/ft-hr-degrees F at 250 deg. Facross ply with plyCoefficient of thermal expansion in/in-degrees F x 10 exp -6 at
32、400 deg. Facross ply with ply Flexural strength, psi warp direction fill directionTensile strength, psi (edgewise)warp direction fill direction1.4 30.0 25,000 20,000 3,500 0.10 0.10 5.0 2.0 25,000 20,000 15,000 10,000 1.52 38.5 65,000 55,000 8,000 1.1 1.1 20.0 9.0 55,000 55,000 40,000 35,000 Silica
33、Cloth Phenolic:The silica cloth phenolic shown in Figure 2 is also used to provide structural integrity, a corrosion barrier, and insulation. It is fabricated in a manner similar to that of carbon cloth phenolic. The product should conform to the uncured material acceptance test data properties and
34、the cured material acceptance test data properties shown in reference 6. The silica cloth phenolic is a higher purity material than the glass cloth phenolic, and is only used in the cowl ring.Nozzle Ablative Composites Fabrication:The fabrication process for the components of the nozzle includes two
35、 tape wrappings and two machining operations. Because of the sensitive variables occurring during tape wrapping of the prepreg tape, a highly skilled operator is required who understands what is required when one of the variables changes. The critical manufacturing variables are heat input, cooling
36、input, roller pressure, machine speed, and tape tension. There are many other random variables that play a less significant role. The material is wrapped onto a mandrel which has been machined to the desired contour of the component as specified in the manufacturing plan. Depending upon the componen
37、t location, the first wrap could be either bias-cut carbon cloth phenolic, glass cloth phenolic, or silica cloth phenolic tape wrapped on the mandrel at a specified angle (which depends upon the nozzle component) to the centerline of the nozzle. Enough tape should be wrapped to provide machining sto
38、ck and tag ends after cure.The tape wrapping process places the tape at the proper angle and debulks the tape material to minimize movement of the tape during cure. Debulking of the tape should be achieved by applying Provided by IHSNot for ResaleNo reproduction or networking permitted without licen
39、se from IHS-,-,-heat and pressure at the point of contact with the mandrel or the previous ply. Heat should be applied prior to wrapping to make the tape tacky. The pressure roller forces the fibers to nest and compact (debulk) within the resin/fabric matrix. CO2is used to cool the resin to stop fur
40、ther cure, and dimensionally and thermally stabilizes the billet for further processing. All ablative components are cured in a hydroclave except the aft exit cone, outer boot ring, and the cowl, which are cured in an autoclave.After wrapping, the billet and mandrel are vacuum bagged for waterproofi
41、ng, then installed in a hydroclave for final cure. Curing of the carbon-cloth liner requires a pressure of 1000 psi at 310 deg. F for a minimum of five hours. After the cure cycle, a test ring should be removed and tested to verify the properties of the cured carbon cloth phenolic. The carbon cloth
42、is then machined to configuration.In preparation for the second wrap, a coat of phenolic resin is applied to the machined surface of the first layer of phenolic. The second layer of phenolic tape is wrapped at the desired angle to the nozzle centerline using the same process as described for the car
43、bon-cloth phenolic. The component is vacuum bagged and cured in an autoclave at 250 psi at a temperature of 310 deg. F for approximately four hours. Carbon and glass rings should be removed and tested to verify properties of the cure. The billet is final machined, removed from the mandrel and x-raye
44、d to verify absence of voids, delaminations and low density indications. Following x-ray, the machined component is ready for bonding to the metal housings. Bonding of the machined ablative component to the metal housing is accomplished in a clean environment and requires strict process control.Insp
45、ection, Destructive, And Non-Destructive Evaluation:A very important development in nondestructive testing that promises to help maintain the meticulous cleanliness required to ensure bonding of composites to adjacent metallic structures is computer controlled scanning of the metal surface with devi
46、ces based on Optically Stimulated Electron Emission (OSEE) principle. Computer controlled contamination scanning (CONSCAN) is a system developed by MSFC using the OSEE technique for automated scanning of metal surfaces prior to critical bonding operations. The CONSCAN system provides the sensitivity
47、 and spacial resolution necessary when scanning large areas to ensure that surfaces are sufficiently free of contaminants that reduce bond strength. Scanning the surface with CONSCAN provides a map of surface contamination levels, which clearly identifies those areas that require further cleaning. R
48、escanning after the additional cleaning operation provides results of that recleaning. These data provide a permanent record of surface cleanliness levels. The data records may be used at a later date to help identify the specific process during which contamination occurred and to correlate surface
49、cleanliness levels with subsequent debond locations.The carbon cloth prepreg, glass cloth prepreg, and silica cloth prepreg should be visually inspected to ensure that there are no broken fibers, voids, or lack of resin or filler. Coupons of these materials should also be tested to verify uncured and cured properties. After final machining of both glass Provided by IHSNot for ResaleNo reproduction
