I
GEORGE EPSTEIN and HARRY A. KING Aerojet-General Corp., Azwsa, Calif.
lastics for Heat-resistant resins and fiber reinforcements are needed in reinforced nozzles. High refractory materials are more desirable than lower melting and organic fibers
space age demand an ever-increasing improvement in materials, processes, and designs for rocket motors and components thereof (7). I n the case of rocket nozzles, it is extremely desirable to achieve lighterweight units that will maintain their dimensions during firing. In solid-propellant rocket motors, the nozzle is generally uncooled-i.e., the propellant or heat transfer liquid is not available for channeling through the nozzle walls to absorb the heat, as in many liquid-propellant engines. Consequently, exposed surfaces of the rocket nozzle are subjected to severe thermal fluxes and mechanical erosion from the extremely hot, high-velocity, propellant combustion gases. Indeed, this environment produces possibly the most severe service conditions experienced to date. Various limitations are presently encountered in the use of current nozzle materials. Some are heat-sink materials suitable only for short durations; some are brittle and subject to spalling during operation; some are very costly, available only in limited quantities; and many are extremely difficult to fabricate into required configurations Previous experience (2, 3) had indicated considerable promise in the use of reinforced plastics under ultra-highA D V A N C E S IN THE
temperature, high-gas-velocity environments. Accordingly, a nozzle development program was initiated to establish materials, processes, and design criteria to provide lighter-weight nozzles that are dimensionally stable under the necessary service conditions. Further, these materials and concomitant processes should readily permit fabrication of reproducible, reliable units.
Test Methods I t was found that reinforced plastics can b e used successfully in nozzles for rocket motors and engines, provided that the design takes into consideration the dimensional changes that occur d u e to ablation. A permanent testing apparatus (gaseous hydrogen-oxygen rocket motor, SPAR) provides a useful tool for such investigations. For a given nozzle material a n d design, ablation rate is dependent upon time, gas pressure, a n d stagnation temperature. With regard to nozzle composition, the rate and extent of ablation depends upon various factors-primary resin, reinforcement, a n d orientation of the reinforcement. Heat-resistant resins appear desirable. Nonfibrous fillers impair nozzle durability. U n d e r the conditions of a typical rocket motor firing (about 5400' F. in this case), high-
The SPAR motor is controlled automatically o r manually, depending on data desired
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melting refractory fiber reinforcements a r e preferable to lower-melting a n d organic fibers. Orientation of reinforcement$ to provide an edge-grain effect is desirable. Thin refractory coatings have not proved to be beneficial in protecting plastic rocket nozzles.
INDUSTR~ALAND ENGINEERING CHEM~STRY
of
During a previous study (4)of rocket insulation materials, an oxyacetylene torch had been employed for screening-test purposes. Erosion and degradation were determined for a fixed torch condition; and the values were used for selecting materials to insulate interior portions of solid-propellant rocket motors. Some indications were also presented of potential nozzle materials. To simulate conditions encountered by a nozzle during motor operation, however, a thermal environment substantially more severe and more discriminating than the oxyacetylene torch test was needed. Various tests have been considered for this purpose (10). The test unit designed for this purpose, called the SPAR motor (structural plastics ablative rocket), utilizes gaseous oxygen and gaseous hydrogen as the oxidizer and fuel, respectively. The SPAR motor was designed to be fired with a stagnation temperature controllable from 2000' to 6000° F., and a chamber pressure controllable from 50 to 800 p.s.i.g. Prospective materials are fabricated into small test nozzles fitted directly into the aft end of the motor. Throat diameters of test nozzles can be varied from 0.25 to 0.75 inch. The firing duration of the SPAR motor is controlled automatically or manually, depending on the type of data desired. Typical test specimens, sectioned axially after firing, are shown on page 765. Ablation is defined as "The thermal degradation and continuous removal of surface material during the interaction of material with its high temperature environment" (10). For evaluation of nozzle materials, the actual rate of ablation must be determined, For this purpose, the decrease of chamber pressure with time is measured in the SPAR tests. AS the nozzle ablates, the throat diameter increases and chamber pressure correspondingly decreases.
P L A S T I C S A N D E L A S T O M E R S IN R O C K E T S
SPAR motor in operation. When total ablation is desired, the motor is fired under specified operating conditions, and the over-all change in performance (pressure drop) i s o f major interest If the performance of a specific rocket nozzle or design is under examination, the total ablation is desired. In such cases the SPAR motor is fired under specified operating conditions, and the over-all change in performance (pressure drop) is of major interest. A typical curve of chamber pressure us. time is shown below. Investigation Previous experience at Aerojet General had shown that high-temperature-resistant, glass-reinforced phenolic resins were reasonably satisfactory for many thermal insulation applications in shortduration rocket motors. However, there was little information available on the performance of these and similar materials when tested under the severe environments encountered by the nozzle during motor operation. Accordingly, a program was initiated to evaluate the resins, reinforcements, fillers, protectivecoatings, molding and postcuring conditions, and orientation and form of reinforcements. Initial SPAR motor investigations were designed to establish testing variables and procedures, and to provide information on the general characteristics of specific classes of materials. A portion of this phase of the work is reported here. More definitive test series are now being conducted, which are designed to examine more thoroughly the more important variables. The theoretical aspects are being examined in the belief that a sound understanding of ihe mechanisms involved will aid in development of superior plastic-base nozzle materials and concomitant designs.
The ablation rate of a given nozzle material and configuration has been found to be dependent on firing time, chamber pressure, and stagnation temperature. Regarding the relative performance ratings of nozzle materials tested, the effects of exhaust gas composition appear to be of a low order when plastic materials are used. Resins. Representative commercially available resins were fabricated into test nozzles, primarily using chopped Refrasil (a fiber consisting of 96 to 99% silica; H. I. Thompson Fiber Glass Co., Los Angeles, Calif.) fabric. These resins included heat-resistant phenolic, phenolic-silicone, and amine-cured epoxy. Curing cycles employed were essentially those recommended by the resin manufacturers. A commercially available melamine-glass fabric laminate was also tested and compared with
Typical test specimens after firing, sectioned axially A: Light layers, nylon-phenolic resin; dark layers, asbestos-phenolic resin. 6: Rockide alumina coating on Refrasil-phenolic resin. C: Stainless steel-screen phenolic
phenolic-glass. Results, as shown in Table I, demonstrate that the heatresistant phenolic and phenolic-silicone resins are essentially equivalent in performance, and substantially superior in performance to those melamine and epoxy resin systems evaluated. I n this initial study, no attempt was made to investigate thoroughly the various types of epoxy resins and curatives. Improvements may be anticipated with the more heat-resistant
A typical curve of chamber pressure vs. time for an ablating nozzle VOL. 52, NO. 9
SEPTEMBER 196Cf
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Table I.
thermal expansion. a. of silica glass may also be a significant factor, resulting in a lower thermal stress factor (product of Q and modulus of elasticity) ( 2 ) . I t is believed that the superior reinforcements form a protective surface layer which actS as a heat-absorptive thermal barrier. In addition. the reinforcement fibers maintain continuity and mechanical strength in the regions below this protective surface layer. Representative properties of E glass, 96% silica
glass, and silica glass arc presented in Table 11; and tend to support these hypotheses. The relatively poor performance of the phenolic-impregnated nylon fabric nozzle may be attributed to the particularly high temperature conditions encounteredLe., 5400' F. These results are in agreement with those of other investigators (5) who found that organic fibers such as nylon are optimum at substantially higher temperatures (about 1O,OOOo F.) than those encountered within a rocket motor. T o examine further this behavior, an experiment was conducted in which a test nozzle was fabricated of alternating '/s-inch thick layers of nylon and asbestos fabric reinforcements, impregnated with a heat-resistant phenolic resin. A significant improvement (approximately 40'%) was observed over the nylon fabric-reinforced phenolic nozzle. In addition, upon sectioning of this test specimen after firing, it was noted (top right, page 765) that substantially greater ablation had occurred in the nylon fabric layers than in those of asbestos fabric. I t is also interesting to note that the life of nozzles containing the various reinforcements appear to be inversely related to the thermal conductivity of the reinforcing agent. From these and subsequent tests (Table 111), it appears that the best results can be expected using a fabric reinforcement rather than random fibrous materials. Fillers. During a previous study (4) of rocket insulation materials at Aerojet-General Corp., it was observed that inert filler materials markedly affect performance of a reinforced plastic material when subjected to the very high temperatures of an oxyacetylene torch. .4ccordingly, a number of filler materials were investigated in representative formulations using the SPAR nozzle test. Results of these tests ('Table 111) demonstrate that fibrous fillers are preferable to nonfibrous filler materials.
Table I l l . Best Results Can Be Obtained Using a Fabric Reinforcement
Table IV. Thin Sprayed Refractory Coatings W e r e Detrimental to Nozzle Performance
Relative Performance of Resins and Reinforcements in Spar Nozzle Test (Resin content, about 45
+ 5%
by volume)
High silica glass gave superior performance
Resin Phenolic Phenolic-silicone EPOXY Melamine
Relative Performance Rating'
Resin Type High temperature-resistant High temperature-resistant Standard; cured with aromatic amine Standard Reinforcements
Refrasil Fib erfrax
E glass Glass-copper Stainless steel screen Asbestos Brass screen Nylon
96-99% SiOe; Type 182 fabric in '/*-in. squares Ceramic (alumina-silicate) fibers; tape in '/pin. squares Type 181 fabric in I/*-in. squares Special tape consisting of glass and copper fibers 100-Mesh screen, Type 304 stainless Mat, R-M 9517 100-Mesh screen SN4316,heat stabilized, 48 X 64 thread count; square weave fabric, 1/2-in. squares
5 3 2 2 1
1 1 1
a Based on firing duration under controlled motor operating conditions. Theoretical flame temperature, 5400' F.; initial chamber pressure, 525 p.s.i.a. ; nominal shut-down chamber pressure, 300 p.s.i.a.; 0.25-in. throat diameter. The best material is rated 5 ; others are rated proportionately lower, depending upon firing duration.
Table II.
Representative Properties o f Glass Compositions" 967, Silica Glass
E Glass Softening point, F. Linear coefficient of thermal expansion*, F. Modulus of elasticityc, p.s.i. Density, g./cc.
1680
2730
2.33 X 10-6 12.7 X 106 2.53
0.44 X 10-6 9.7 X 106 2.18
*
a Adapted from (6). For temperatures of 0'-25' vicinity of room temperature.
grades. Based on data obtained from these tests, heat-resistant resins appear most desirable for use in nozzles. However, there is not yet sufficient evidence to conclude if the observed differences in performance are due to the molecular structure of the resin or mechanical strength a t elevated temperatures. Reinforcements. While glass fabric is the principal reinforcement material used in thermal insulation of rocket motors, asbestos, Refrasil, and organic fibers have indicated promise in similar applications (5-7, 9 ) . In addition, fibers of metals and ceramics were considered worthy of investigation as reinforcements for plastic nozzles. As shown in Table I, of the reinforcements listed, Refrasil displayed the most outstanding performance in SPAR rests. This was further demonstrated by visual examination of the nozzles sectioned after firing. Specimens fabricated using Refrasil retained considerably more carbonaceous (charred) material than those using the standard E glass reinforcements. I t is believed that the superior performance of the highsilica glass is directly related to its higher softening temperature, melt viscosity, and possibly superior wetting or surface tension at high temperatures, compared to E glass. The lower coefficient of
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C. to 20O0-3OO0 C .
Silica Glass 3030 0.30 X 10-6 10 x 106 2.20 Determined in the
Filler/lieinforcementa No filler (Type 182 Refrasil fabric, i/n-in. squares), 55 vol. % Phenolic microballons (35 wt. % ) and chopped glass fibers (random), 10 wt. % MgO powder and Type 182 Refrasil fabric (l/s-in. squares) ZrO? powder, 90 wt. % MgO powder, 90 wt. yo
Relative Performance Ratingb 3
None CrzNiBl
&os
Alzos 1 1
