US3520478A - Rocket nozzles - Google Patents

Description

July 14, 1970 A SHALER 3,520,478

ROCKET NOZZLES Filed June 6, 1966 If. G F O O 0 O I H... I \0 J G O I I 111. III.

INVENTOR AMOS J. 579141.51?

ATTORNEYS.a

United States Patent US. Cl. 239265.15 5 Claims ABSTRACT OF THE DISCLOSURE Rocket nozzles and nozzle inserts of the autotranspirational cooling type comprise a porous body of refractory material, for example, graphite, refrectory metal, or a refractory metal carbide. Hot propellant gases pass through a bore which provides a flame-contacting face. The outside of the body forms its back face. In accordance with the invention the body is provided with an array of closely spaced passages, or holes, radially disposed relative to the nozzle axis; they extend from the flame face to the back face. The porosity and passages are infiltrated in the region of said back face to a depth between onetenth and one-quarter of the thickness of the body at its throat with a substance that is solid, non volatile and of low coeflicient of thermal expansion at the operating temperature of the back face of the insert and which acts to reinforce the body against stresses due to the differential expansion during firing, and the passages and remaining porosity are then infiltrated with a metal which melts above about 1000 F., exerts high vapor pressure at the flame temperature and is non-reactive with the refractory body at temperatures, the latter reaches in service.

This invention relates to rockets, missiles, and other jet propulsion devices, and more particularly to the nozzles and nozzle inserts of such devices which are subjected to extremely high temperatures and other damaging conditrons.

The propellants used with devices of the type mentioned develop combustion products at temperatures of the order ofat least about 5000 F., and recently the trend has been toward the development of still higher temperatures, approaching 7000 and even 7500 F. Such temperatures impose severe requirements upon the materials used. First, it is requisite not only that the nozzle insert be refractory in the sense that in operation it will not melt or soften or erode to an extent such that would result in failure, but also that it will not crack or distort under the stresses created by the flame.

The refractory materials available for such purposes are generally poor conductors of heat, and large temperature gradients, which may amount to several thousand degrees Fahrenheit between the flame face and the back, or outer face, are created in a few seconds after firing. This tends to create thermal stress conditions which may, and commonly will, cause cracking, and even failure, of the nozzle part under the stress pattern created by the thermal conditions and by the flame pressure. Also, the material at the flame surface must resist erosion and corrosion by the combustion gases and the liquid and solid combustion products present in the flame. By resist is meant that the erosion and corrosion must be kept below a maximum acceptable value, which desirably is as low as a surface recession of a few thousandths of an inch per minute under flame temperatures of the order of 6000 to 7000 F., flame pressures of the order of 1000 p.s.i., and flame compositions which include substantial quantities of, for example, carbon dioxide, water vapor, hydrogen chloride, and oxides of aluminum and other metals.

Obviously, one way of minimizing the foregoing objectionable consequence of the flame temperature and pres- "ice sure is to provide some means of cooling the nozzle or its insert, and various ways of doing so have been proposed and tried. One of the most promising of these proposals is by autotranspirational cooling, the principle of which is well known. It involves a nozzle or insert comprising a coherent skeleton of refractory material having a multiplicity of interconnected pores of fine dimensions which have been infiltrated with a solid phase which melts under the flame heat to produce liquid of high vapor pressure; the vapor formed then moves through the pores into the flame. Heat is absorbed by this material while it is solid, liquid, and gaseous during the time it is being heated to the temperature of the flame, as well as when it is undergoing a transformation, such as in melting and vaporizing, and often, if the infiltrant is a compound, in dissociating it. In addition, cooling is provided by the reduction of the convective coeflicient of heat transfer from the flame through the boundary layer, by its constant dilution with the outpouring vapor of the infiltrant.

Such autotranspiring inserts may be provided by carboas, graphites, by such carbides as monotantalum carbide or mixed carbides of tantalum or zirconium and hafnium, or they may be made from refractory metals such as tungsten, and the pores infiltrated with, for example, silver or copper, or a combination of one of these with a lower-boiling material such as lead, polyethylene, or a fluorocarbon polymer, which under the influence of the heat of the flame passing through the nozzle will melt and be vaporized through the pores into the flame. The transformation of the infiltrant from its solid phase condtion to a hot vapor phase results in the absorption of heat from the porous material with consequent reduction in the rate of temperature increase of the insert whereby operation is permitted for a sufficient period of time in the presence of the flame, the temperature of which is such that without this cooling effect the insert would fail by softening, cracking, melting, erosion, or a combination of these factors. Thus autotranspirational cooling is capable of maintaining the integrity of the nozzle and its insert for the length of time required for the mission of the device.

Such autotranspirational articles may be made and infiltrated by well-known powder-metallurgy methods, the size and extent of the interconnected pores being controllable by the particular practice applied.

This cooling principle has been applied to tungsten inserts infiltrated with silver. Experience has shown that such inserts leave much to be desired because of objectionable factors and disadvantages. In the first place, the temperature of the insert may become so rapidly elevated at the flame surface, that the tungsten skeleton of that region will sinter to a substantially impenetrable skin so that the pores beneath it are no longer open to the flame surface. If, then, the evaporating phase is still substantially solid or liquid it will expand thermally faster than the tungsten skeleton with creation of pressure causing internal cracks under the sintered skin which is lifted by the pressure to form blisters. Blisters are objectionable because the smoothness of the flame surface is interrupted which prevents ideally smooth passage of the flame with reduction of nozzle efliciency.

Secondly, after a period of service the level of the infiltrant from which evaporation is taking place has receded into the insert far away from the flame surface, and because the resistance of the fine pores to the movement of the infiltrant vapor through them increases rapidly as their effective length increases, the quantity of vapor entering the flame per unit of time may fall off to an undesirably low value.

Thirdly, if the evaporating phase expands thermally faster than the tungsten skeleton, which is commonly the case, a portion of the infiltrant may be extruded from the flame surface as a result of the stress thus created and enter the flame as liquid or incompletely vaporized liquid with consequent impairment of the intended cooling effect.

It is among the objects of this invention to provide nozzles and nozzle inserts of refractory materials, as that term is used herein, for jet populsion devices, such as rockets and missiles, which provide the advantages of autotranspirational cooling while suppressing the objectionable factors that have been encountered with autotranspirational cooling.

Another object is to provide nozzles of the type contemplated by the foregoing object in the use of which erosion is minimized and internally induced stresses are resisted, and which also are strengthened against stresses created at the outer, or back face, that is, the face opposite the flame face.

A further object is to provide nozzles in accordance with the foregoing objects and which have the porosity adjacent the back face occupied by a strength-reinforcing infiltrant, particularly one that is non-volatile, and having the pores adjacent the flame face occupied by one, or more than one, vaporizable infiltrant.

Yet another object is to provide such nozzle inserts which provide improved movement of vaporizable infiltrant to the flame face and reduce the tendency of the nozzle to crack and blister in service.

Yet another object is to provide carbon or graphite or tungsten or refractory carbide nozzle inserts embodying the features and advantages of the foregoing objects.

A still further object is to provide nozzles in accordance with the last-named object having carbon or graphite as the refractory material, silicon carbide as the reinforcing infiltrant, and silver as the evaporating infiltrant.

Still another object is to provide nozzle inserts for use in rockets of the solid-propellant type.

Other objects will be recognized from the following specification.

The invention will be described with reference to the accompanying drawing which shows a vertical sectional view through a rocket nozzle insert in accordance with the invention. It comprises a porous body 1 of refractory material such as graphites, a refractory metal such as tungsten, or refractory metal carbides. Body 1 has a Venturi-type bore such as is shown in Pat. 3,145,529, the surface of which provides a flame-contacting face 2 for the hot propellant flame which passes through the bore in the direction of arrows 3. The outside surface of the body 1 constitutes a back, or outer, face. The microporosity of the insert in indicated by stippling 4. In accordance with the invention, the insert is provided with an array of closely spaced small diameter passages 5 laterally disposed relative to the axis of the nozzle and which extend from the flame face to the back face.

In accordance with this invention its objects are attained by porous nozzle inserts of the types and materials described above having an array, or multiplicity, of closely spaced, small diameter passages radially disposed relative to the axis of the nozzle and extending from the flame face to the back face. The said passages are infiltrated with an evaporable coolant or coolants.

My novel throat inserts are made, as heretofore, by practices known in the art of carbon and graphite manufacture, and in the art of powder metallurgy. The passages which characterize this invention are formed in them in any desired manner, and the microor macroporosity 4 and the passages 5 are infiltrated in known ways. Various practices may, of course, be used such as those applied to the production of graphite articles in general, or to the production of tungsten articles or carbide articles from powdered materials. Thus, a conventional hollow cylindrical billet of tungsten powder is isostatically pressed and presintered, drilled with the desired multiplicity of passages, and sintered to a density of 70- 80% of full density; or a conventional carbon rnix containing binder is extruded, molded, or isostatically molded into a hollow cylindrical billet, baked, for instance at 2000 F. with formation of an internal porosity of from 4 10 to 40% of the volume, graphitized, for instance at 4500" to 5000 F., then drilled with the desired multiplicity of passages.

Infiltration of the microporosity 4 and of the passages 5 may be accomplished after the aforementioned steps. Although a single evaporative infiltrant may be used for both the passages and the pores, it has been found that much of an evaporative coolant in the pores near the back face is not effective for that function. In accordance with the invention I prefer, accordingly, to infiltrate the fine pores in the region of the back face only with a reinforcing agent to minimize the danger of cracking as a result of thermal and flame-pressure stresses created during firing.

Preferably this is accomplished by first infiltrating the finest porosity to a depth of a tenth to a quarter of the thickness of the insert at its throat, measuring from the back face, with a material M of low expansivity, most suitably one which is non-volatile at the maximum temperature reached by the back face during the mission and remains solid during the use of the nozzle. This infiltrant should have a low coeflicient of thermal expansion; it should have a melting point and a boiling point higher than those of the autotranspiring infiltrant, and its surface tension and interfacial tension with respect to the refractory material should both be such that it will infiltrate the fine pores spontaneously, or under moderate pressure. Some glasses, and metals such as chromium, columbium, iridium, molybdenum, osmium, zirconium, and silicon and some of their alloys, fit these requirements. Especially suitable for this purpose if the refractory is graphite are lithium-silicon alloys and calciumsilicon alloys, especially lithium and calcium disilicides, as disclosed in application Ser. No. 519,945, filed Jan. 11, 1966, by John C. Kosco. In the molten and superheated state those materials will infiltrate spontaneously to the desired depth and are then converted to silicon carbide, with evolution of the calcium or lithium as gases, by subsequently heating to a higher temperature. For example, the infiltration may be done at 2000 F. and the subsequent conversion at 2500 F. Or there may be used one of the other metals or glasses mentioned, in conjunction with pressure or with wetting alloying agents that will cause them to infiltrate graphite spontaneously. If the refractory material is tungsten or a carbide, a similar method known in the art may be utilized, taking into account their different abilities to be infiltrated spontaneously. This infiltrant, being solid and non-volatile at the maximum backface service temperature, increases the resistance of the refractory material to the stresses due to differential thermal expansion which arise during firing.

The autotranspiring infiltrant, or, if there are more than one, at least one of them, should have a melting point above 1000 F., a boiling point of at least about 2100 F. and preferably between 3000 and 5000 F., a high vapor pressure at the operating flame temperature, say 5500 F. or more, a high latent heat of vaporization, and a low thermal expansion coeflicient. Likewise, this infiltrant should have a high enough surface tension and a low enough interfacial tension with the refractory material to permit it to be completely infiltrated, with only moderate application of pressure, if any, into the passages 5 and the porosity 4 of the refractory material remaining open after the first infiltration to provide a body of the autotranspiring infiltrant. Furthermore, the autotranspiring infiltrant should not react with the refractory material at any temperature below at least 4500 F. For missions of medium severity silver, copper, and barium fit these requirements reasonably well. For missions of high severity the passages are infiltrated with a metal meeting the characteristics stated, the voids resulting from its solidification then being infiltrated with a lower melting and high boiling metal examples of which are lead, magnesium and zinc to provide a zone containing both the high melting point and the lower melting point autotranspiring metals.

The evaporation of the evaporating infiltrant from the passages and bores during service can lower the temperature at the flame face of the nozzle by 400 to 800 F., with consequent lowering of the stresses at the back face, as is desirable for increasing the resistance of the refractory material at the flame face to erosion, softening, and blistering, and to prevent cracking at the back face. The filling of the pores near the back face with silicon carbide or other reinforcing material can raise the resistance to cracking at this face, where cracks are commonly initiated, by a factor of two or three.

The passages which characterize this invention provide, as an additional, but minor function, paths of highly conductive metal along which heat can flow rapidly to the back face, thus decreasing to some extent the thermal gradient which tends to cause cracking, but, more importantly, they provide paths of larger diameter than the fine pores whereby to improve release of the evaporating infiltrant and thus to improve cooling efficiency. This improved release of the evaporating infiltrant occurs in two ways: first, directly, by providing such larger paths for the infiltrant that is originally in the passages themselves, and, secondly, by providing a much shorter smalldiameter path followed by a longer and relatively larger diameter path for the vapor evaporating from the pores in the material between passages.

As exemplifying the benefits of the passages which characterize this invention, inserts with passages inch in diameter on inch centers increase the evaporative release of silver by about 33 percent in comparison with a similar nozzle insert without the passages, each having pores averaging two microns in diameter. As the passages are disposed closer together, the comparative increase in release rate goes up rapidly. For instance, with inch diameter passages on inch centers the release of silver in one minute approaches 90 percent of that present in inserts 1% inch thick at the throat, with a flame temperature near 6000" F., while it is only approximately 50% without the passages. Although the presence of the infiltrant-filled passages does weaken the refractory material to a minor extent, the elfect of the extra cooling in lowering the stresses at the back face more than offsets this minor weakening. Thus it is evident that not only does the use of passages in accordance with my invention decrease stress and improve reliability, but also such nozzles permit the use of propellants of higher flame temperature than nozzles without the passages.

In high severity tests of nozzles according to the invention the best results with graphite were had with Stackpole Carbon Co. Grade 2000 electrographite (porosity 13.5%) infiltrated with copper and then lead or with silver and then lead, while the best results with tungsten were had with metal of 20 percent porosity and infiltrated with silver.

Nozzle inserts of graphite possess various advantages. Thus, in comparison with tungsten and other refractory metals they are of lighter weight, which is advantageous for missile purposes. Too, graphite is not subject to the surface sintering with its undesirable consequences alluded to above. Despite its outstanding properties at high temperatures, especially strength, the idea of using graphite has often been rejected for use as nozzle inserts because of poor low temperature strength. Thus, firing stresses reach a maximum when the back face is still relatively cool, thus posing a problem of low temperature strength. However, this problem becomes of minor importance with the inserts of this invention which are strength-reinforced as described above. Experience has shown that graphite inserts of this invention, infiltrated with silver, or with copper, or with one of these metals and lead, are as satisfactory for some high-performance missions as is silver-infiltrated tungsten in regard to resistance to firing stresses, erosion and 6 corrosion, and are far superior in regard to weight and cost.

It will be understood that porous carbon, graphite, tungsten, or refractory-carbide bodies having an array of infiltrated passages as described above may be treated in similar manner for handling combustion products or hot gases or liquids under other conditions, either more or less severe than prevail with rocket nozzles, as for examples in re-entry bodies, hot-gas deflectors and plumbing in rockets, and nozzles and plumbing used in metallurgical and chemical processes.

According to the provisions of the patent statutes, I have explained the principles of my invention and have described what I now consider to represent its best embodiment. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

I claim:

1. A rocket nozzle or rocket nozzle insert having a flame-contacting face and a back face comprising a shaped body of refractory material of the group consisting of carbons, graphites, tungsten, and refractory carbides, said body being porous and provided with a multiplicity of closely spaced, radially disposed, smalldiameter passages extending between said faces, the porosity and passages being infiltrated in the region of said back face to a depth between one-tenth and one-quarter of the thickness of the article at its throat with a substance that is solid, non-volatile, and of low coeflicient of thermal expansion at the operating temperature of the back face of the nozzle insert and acting to reinforce the article against stresses due to dilferential expansion between the said faces, and the porosity and passages then remaining open being infiltrated with a metal melting above about 1000 F., exerting high vapor pressure at the flame temperature and being non-reactive with the refractory material at the temperatures the later reaches in service.

2. An article according to claim 1, the porosity being infiltrated in the region of said back face to a depth between one tenth and one quarter of the thickness of the article at its throat with a substance that is solid, nonvolatile, and of low coefficient of thermal expansion at the operating temperature of the back face of the nozzle insert and acting to reinforce the article against stresses due to differential expansion between the said faces.

3. An article according to claim 1, said substance being silicon carbide, and said refractory material being graphite.

4. An article according to claim 1, said body being graphite, said passages being infiltrated with silver, the porosity in the region of said back face being infiltrated to a depth between one tenth and one fourth of the thickness of the article with silicon carbide, and the remaining porosity of the body being infiltrated with silver.

5. An article according to claim 1, said body being graphite, said passages being infiltrated with copper, the porosity in the region of said back face being infiltrated to a depth between one tenth and one fourth of the thickness of the article with silicon carbide, and the remaining porosity of the body being infiltrated with copper.

References Cited UNITED STATES PATENTS 3,069,847 1'2/1962 Vest 6020O 3,137,995 6/1964 Othmer et al. 239-265.15 3,145,529 8/1964 Maloof 60-200 3,253,405 5/1966 Kropa 239265.15

ALLEN N. KNOWLES, Primary Examiner US. Cl. X.R. 60200

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