US3292544A - Hyper-environmental radome and the like - Google Patents

Description

Dec. 20, 1966 o, CALDWELL ETAL. 3,292,544

HYPER-ENVIRONMENTAL RADOME AND THE LIKE Filed May 5, 1964 M w 0 E E L L W m w Z L @//H/////// ///.H/ V

firmeA/Em.

United States Patent 0 f 3,292,544 HYPER-ENVIRGNMENTAL RADOME AND THE LIKE Orval G. Caldwell, Los Angeles, and Leon J. Le Clercq,

Glendale, Calif., assiguors, by mesne assignments, to Douglas Aircraft Company, Inc, Santa Monica, Calif., a corporation of Deiaware Filed May 5, 1964, Ser- No. 364,949 16 Claims. (Cl. 10Z-92.5)

This invention pertains to rigid unitary nose cones, rad-omes and other formed objects through which high frequency energy must be transmitted, the objects of the present invention having highly desirable electrical characteristics, high strength and ability to withstand thermal shock without being excessively heavy. The invention also pertains to methods whereby the unitary objects herein disclosed can be effectively and economically manufactured, as well as data concerning compositions, ingredients, materials and techniques of manufacture.

Radomes have been previously made of resinous and plastic materials, but such radomes are unsuited for use at supersonic speeds or on missiles and space vehicles, primarily because the materials are incapable of fulfilling the mechanical, thermal and aerodynamic and erosion considerations while maintaining their electrical characteristics. Attempts have been made to utilize refractory compositions which can withstand rap-id and widely ranging temperature changes, but these materials are heavy (have a high specific gravity) and in order to provide a radome having suflicient strength to withstand maximum anticipated loads without the use of reinforcing ribs, disconformities, the use of binders, al kalies and other materials which affect other properties such as electrical characteristics, a radome would have to be made with walls that are thick and diamond ground after firing to precise $0.001 in dimensions. The radome is difficult and expensive to make and will now add an excessive dead load to the missile or vehicle. A heavy radome, when used on a rocket, missile or other space vehicle requires a great increase in the thrust generating, propelling facility of the missile or vehicle, it being estimated that an added ten pound load requires an added 100 pounds of fuel. Dense radomes of refractory oxides, even of one-half wave thickness, were either excessively heavy or incapable of withstanding hyper-environmental a plications, and at all events had to be made to conform to critical thickness tolerances which rendered their manufacture uneconomical and their use restricted to a precisely maintained frequency.

By following the teachings of this invention, it is now possible to produce, from highly refractory ceramic compositions, aerodynamically sound, streamlined, relatively light-weight radomes, nose cones and other objects which have electrical characteristics of exceptional utility. Their power transmission coefficients are high and losses attributable to reflection and absorption are reduced to a minimum: they exhibit broadband electrical performance. These and other characteristics such as strength, resistance to thermal shock, stability under adverse conditions of use and ease of manufacture industrially render the products of this invention particularly adapted for use in vehicles and equipment operating at high speeds, in rapidly varying media under changing load conditions and/ or under rapidly changing temperature conditions.

Generally stated the final product of this invention be it a radome, nose cone, scanner housing or other object, is composed essentially of refractory inorganic oxides or mixtures of oxides which are virtually inert and the final product is able to maintain its shape and strength at very high temperatures (1400-1600 0.). Although the chemical composition is homogeneous Patented Dec. 20, 1966 "ice throughout the body of the product, its apparent density is varied in a controlled and predetermined manner from one specific zone to another, thereby developing adjacent zones of different, predetermined physical and electrical properties in such product.

For example, the final product can have thin surface layers of high density, high dielectric coefficient and low loss tangent and an intermediate layer of predetermined lower dielectric coefficient and lower density, without the presence of boundary layers, adhesives or cements which might tend to impair, distort or otherwise detract from desired electrical and physical characteristics. The entire formed'object, even though it includes 'layers or wall portions differing from each other in electrical characteristics, is of substantially uniform inorganic chemical composition. By properly proportioning the thicknesses of the intimately united layers of the unitary product, the thicknesses bearing a predetermined and controlled rela tionship to the wave length or frequency of the signal or energy which is to be transmitted through the final object or product as hereinafter disclosed, undesired reflection and absorption is minimized and the range and efficiency of the equipment associated with the radomehoused unit is maximized. Such radome is of high strength and resistant to thermal shock; it is capable of being manufactured economically since total wall thickness is not critical as the radome exhibits broad band electrical performance.

An object of the present invention therefore is to provide formed objects adapted to effectively transmit high frequency energy, such objects being relatively light in weight, of high mechanical strength and the ability to withstand thermal shock as well as possessing highly efficient electrical characteristics.

A further object of the invention is to disclose and provide compositions and methods whereby o give or streamlined symmetrical radomes, windows, nose cones and the like may be made of highly refractory ceramic ingredients and compositions without burdensome weight.

Another object is to provide housings, electromagnetic windows and radomes which are of laminated character, but of uniform inorganic chemical composition and of homogeneous structure electrically, such housings and radomes being exceptionally well adapted for hyper-environmental applications.

A still further object is to disclose proportions, ratios, materials and characteristics which permit the production of housings and radomes having optimum electrical performance coupled with resistance to thermal shock.

Other objects of the invention as well as the attendant advantages will become apparent from the following detailed description of exemplary methods of manufacture, compositions and forms of the invention. In order to facilitate description, reference will be had to the appended drawings in which:

FIG. 1 is a side elevation partly broken away of a typical nose cone or radome housing embodying the present invention;

FIG. 2 is an enlarged section through the wall of a radome such as is illustrated in FIG. 1;

FIGS. 3 and 4 are somewhat diagrammatic views of a plaster mold in which the object of the present invention can be manufactured, at different stages of such manufacture.

FIGS. 5 and 6 are diagrammatic representations of a somewhat modified form of mold and die which may be used in carrying out the method of the invention.

A typical nose cone or radome made in accordance with this invention is illustrated in FIG. 1, and is provided with a wall 10 of generally uniform thickness having a smooth, generally convex outer surface 11 and a concave inner surface 14. As better illustrated in the enlarged partial section of FIG. 2, the wall includes a thin outer layer 12 and a thin inner layer 13. These innerand outer layers are of unitary width and intimately bonded to a central thicker core 15. This Wall (and therefore all layers and the entire object) is of virtually homogeneous chemical composition and more than 90% thereof (preferably 91% to 97%) is composed of an inorganic refractory such as alumina, mullite or calcined sillimanite. The remaining solids contents totalling less than 10% includes bonding minerals such as clays, crystal growth inhibitors and mineralizers.

The particles of refractory (such as alumina) used in making the wall are finely milled to a grain size of 60 microns and smaller. In the outer and inner layers 12 and 13 these particles of alumina are in closely adjacent relation and such layers are dense, weighing between about 200 and 235 pounds per cubic foot. In the central core the particles of alumina are arranged to form uniformly microporous structure which may weigh 45 to 120 pounds per cubic foot. At all events, the dense outer and inner surface layers have a dielectric constant of 8 to 12, whereas the central core has a dielectric constant of less than 8, preferably 2 to 5 or 2.5 to 4.

Although materials having a dielectric constant of 8 or more normally have a low loss tangent and introduce reflection losses, the construction here described utilizes the excellent strength, erosion resistance, and thermal shock resistance of dense alumina and eliminates reflection losses by making these outer and inner layers 12 and 13 so thin that they are virtually invisible to the energy Waves being transmitted. Pursuant to this invention, the thickness of each of layers 12 and 13 is only 0.06 to about 0.04 of the full wave length of the frequency which the housing or radome is to transmit or receive. The core 15, on the other hand, has a low dielectric contsant and a low refiec tion coeificient and the thickness of such core is not critical, but preferably is on the order of 0.3 to 0.7 of the full wave length of the energy to be transmitted through the wall.

The term full wave length, as used herein, is the actual wave length of the energy to be received or transmitted (in centimeters or inches) divided by the dielectric constant of the wall or densest component of such wall.

An electromagnetic window made as herein described has numerous advantages. It is to lighter in weight, has a very high strength to weight ratio, is resistant to erosion and thermal shock (the porous core acts as a heat sink), is homogeneous chemically and electrically, exhibits very low absorption of energy and therefore has a high electromagnetic transmission, and exhibits improved broad band performance; it is effective even when the frequency of the energy transmitted varies 15%. Moreover, the objects can be reproducibly manufactured in a simple and efiicient manner, without resort to delicate and accurate finishing. A consideration and appreciation of the objectives and definitions of this invention permits ready application of electromagnetic theory and geometrical optics to the design of radomes having optimum electrical performance, tremendous stability and adaptability for utilization in extreme environmental conditions. Attention is again called to the fact that the overall thickness of the wall (indicated at T in FIG. 2) is preferably between 0.3 and about 0.7 of the full wave length of the energy to be transmitted; the thickness of the central core is preferably between about 0.4 and 0.6 of the full wave length. Differently stated, the core thickness (T is between about 70% and 90% of T and is not critical.

A preferred method of making the objects of this invention involves casting the layer sequentially in porous, absorbent molds. A female mold made from gypsum (such as a mold used for casting cups and dinnerware bowls from clay-containing slips) may be used; such mold should be dried to a free moisture content of say 1%3% before use, and at such time will have the capacity to absorb 18%25% of water by weight. FIG. 3 illustrates an exemplary mold 20 having an inner surface 21 which should be smooth, free from dust and of a contour corresponding to the outer contour of the finished radome or other object, allowance being made for shrinkage which may take place when the cast object is fired.

A casting slip or suspension is prepared containing, as inorganic refractory oxides and solids, from to about 97% by weight of finely milled alumina (although mullitc or calcined sillimanite or other refractory oxides could be used) and from about 3% to 10% by weight of binding minerals such as clays and mineralizers and crystal growth inhibitors. The alumina should have a grain size not exceeding 60 microns; classified mixtures can be used. Binding clays preferably include those from the montmorillonite group, particularly of the trioetahedral type, such as hectorite from which substantially all normal CaCO contaimination has been removed, and ball type clays. Minor quantities of magnesium and calcium silicate minerals and mineralizers and crystalgrowth inhibitors such as BaSiF can be used. Typical solids compositions are s The finely ground olay type minerals are preferably first suspended in water and the alumina and mineralizers then added and intimately mixed to form a smooth slurry or suspension. Water content is adjusted to produce a slip-of desired viscosity and specific gravity (between about 2.0 and 2.2). The slip may be deareated or may contain minute quantities of modifying agents such as polyvinyl alcohol or gums to control rheological properties. The slip is then SlOWllY poured into the mold (which may be axially rotated during the casting) to form a skin or layer of desired thinness on the inner surface of the mold. This layer, corresponding to the outer surface layer 12 of FIGS. 1 and 2, is permitted to partially dry so that surface moisture is not visible and the inner core is then rapidly cast directly upon the layer 12.

The suspension used in casting the core is preferably identical in inorganic SOllldS content to that used for the outer layer 12. In addition, such suspension new contains a predetermined proportion of organic, preferably hollow particles of volatilizable, low ash material such as synthetic resin (for example, urea-formaldehyde 0r phenol-formaldehyde spheres), such particles having imperforate walls and being of virtually uniform selected size. Preferably these hollow balls are smaller than the refractory grain; hollow spheres of 20 and 25 micron size have been used successfully. These spheres may be prewetted before being added to the suspension of the inorganic refractory materials. The utilization of such hollow spheres in producing porous refractory bodies is fully disclosed in a copending application filed by Orval G. Caldwell, Ser. No. 182,921. By the use of these hollow plastic spheres of predetermined and substantially uniform size, it is possible to accurately control the porosity and therefore the density of the porous structure desired for the core. Ordinarily, from about 6% to 25% by weight of the solid components of the slip constitutes the quantity of such h'OlilOW spheres added to the slip. In some instances it is desirable to lightly spray the plastic spheres with a minute proportion of glycerin or ethylene glycol before adding them to the slip.

After the core has been cast to a desired thickness (which, as previously indicated, is not critical), the partially finished casting is again permitted to dry partially and the inner dense layer 13 is slowly cast, this dense layer being cast with a suspension identical to that utilized in castin the outer layer 12. T hereafter, the casting is allowed to dry until it releases from the mold, removed, dried and then fired to a temperature of between about 2600 F. and 2900 F., such temperatures being generally reached in about 18 hours. The radome or other object is now in finished form, although in some instances it is desirable to true up and grind the base of the cone to insure a tight fitting with its mount.

For purpose of information, it may be noted that the dense layer obtained by utilizing a suspension or slip having the inorganic composition given in the first column of the table hereina bove when cast at a slip specific gravity of 2.15 produced an extremely strong fired layer having a dielectric constant of 12.2. Dielectric constants of 9 and higher are readily obtained for the dense, thin layers; the spherical, plastic particles burn out without leaving residues during the firing and produce an electrically homogeneous, uniformly porous core whose density may be very accurately controlled. Such layers may have dielectric constants of 2.5 to 4.5, depending upon their apparent density. A typical electromagnetic window may have dense inner and outer layers measuring 0.025

inch in thickness and an intervening, intimately bonded,

porous core appnoximateily 0.250 inch in thickness, such electromagnetic window being particularly well adapted for use with 3 cm. wave lengths.

It may be noted that combined drying and firing shrinkages on the order from about 1.3% to about 10%12% may be expected, depending upon the density of the finished object and the firing temperatures employed.

Separate male and female molds may also be employed in making the objects of the present invention. FiG- URE 5 illustrates a female mold 29' having a suitably contoured surface 21 adapted to impart the desired configuration to the nose cone or other object. Whereas FIGURE 6 diagrammatically represents a male absorptive mold 34 having a suitably contoured surface arranged to pnovide the interior configuration of the nose cone. This male mold is shown provided with a head 35 which extends outwardly, this head being adapted to rest upon the upwardly facing shoulders of the mold 20' so as to position the male mold 34 in spaced relation with the internal surface 21 of the female mold. Locating pins may be employed (not shown) to insure proper positioning of the male mold wit-h respect to the female lTlOtld.

After the two mold portions 20 and 34 are placed in position, one extending into the other, the thin inner and outer dense surface layers of the refractory composition may be cast on the internal surface 21 of the female mold and the external surface of the male mold 34. This can be readily accomplished by providing one or more supply lines 36 through which this suspension may be rapidly fed into the cavity between the two mold surfaces. After a predetermined length of time the molds are again separated and the slip or suspension poured off, thereby leaving a layer of desired thickness on each of the two surfaces. The central microporous core can then be cast in a similar manner thereby uniting the surface layers. The male mold 34 may also include an air supply line 37 terminating insuitable perforated pipe sections within the male mold so that after the entire object has been cast compressed air may be supplied through line 37 in order to facilitate the separation of the mold from the cast object.

When very large radomes, cones or other objects are. being made the mold 20 may be used for forming the thin outer layer of the object and the male absorptive mold 34 may be dipped into a suspension or slip of the same composition so as to separately form on such male mold the thin dense layer of refractory. Thereafter, the now coated male mold 34 may be placed in position upon the female mold 20' (whose inner surface has already been coated with a dense layer) and then the core of microporous refractory composition may be cast in position to form a unitary object.

In some instances greater homogeneity is assured by casting the thin dense outer layer upon the surface of a female mold then placing a nonporous male mandrel or die within the female mold and casting the microporous low density core into the space between the mandrel and coated female mold. After this core is cast the mandrel or die is removed and the inner dense .layer is applied to the inwardly facing surface of the microporous layer. From these various alternative methods of casting it will be evi cut that many objects can be readily made by successive casting or refractory compositions difiering in content of hollow resinous particles, the number of layers and the respective thickness and densities being under accurate control. It is to be noted that the adjacent layers are parallel to a major surface of the object different in density and electrical characteristics and are intimately bonded to each other.

Radomes and other electromagnetic windows designed and manufactured in the manner herein described have capabilities adapting them for use to hyper-environmental conditions. They are capable of being used for communication, navigation, guidance, identification, tracking, telemetry, radar, comimand-destruct systems and in many other applications wherein the windows are subjected to thermal shock, erosion and other adverse conditions resulting from supersonic speeds and adverse environments. Although a particular laminated arrangement has been disclosed, the presence of an additional very thin and dense layer medially of the porous core layer, and multilayered objects of other arrangements can be made by the methods here described and are within the contemplation of this invention.

We claim:

1. A strong, unitary radome, nose cone and similar formed object adapted to effectively transmit high frequency energy and hawin g the ability to withstand thermal shock comprising: a rigid contoured object including a wall having a generally convex outer surface and a generally concave inner surface, said wall being composed essentially and virtually homogeneously of particles of refractory oxide bonded together by a small quantity of fritted minerals including clay of the montmorillonite group; the particles of refractory oxide being in compact, closely adjacent relation at said inner and outer surfaces to form thin dense layers of predetermined and virtually uniform thickness at said surfaces, said surface layers having a dielectric constant of 9 and higher and a low loss tangent; the particles of refractory oxide in the wall portion between said surface layers being arranged and bonded together to form a uniformly porous structure of low apparent density having a predetermined dielectric constant of less than 9; the overall thickness of said wall being between about 0.3 and 0.7 of the full Wave length of the energy to be transmit-ted through the wall of the object, said low density wall portion constituting between 70% and of said overall wall thickness.

2. A unitary object as stated in claim 1, wherein each of the dense layers has a thickness not exceeding about 0.06 of the full wave length of the energy to 'be transmitted.

3. A unitary radome as stated in claim 1, wherein the wall is composed of more than 90 percent alumina and less than 10 percent of minerals including hectorite clay.

4. A unitary radome as stated in claim 1, wherein the wall of the radome is composed of more than 93 percent of a refractory particle having a grain size smaller than 60 microns and less than 6 percent of bonding minerals and crystal growth inhibitors.

5. A strong, lightweight, unitary 'radome, nose cone, electromagnetic window and similar formed object adapted to effectively transmit high frequency energy and having the ability to withstand thermal shock comprising: a rigid contoured object including a wall having a generally convex outer surface and a generally concave inner surface, said wall being of virtually uniform overall thickness; said wall being com-posed essentially and virtually homogeneously of particles of refractory oxide bonded together by a small quantity of fired products of minerals including clay; the particles of refractory oxide being in compact, closely adjacent relation at said inner and outer surfaces to form thin dense layers having a dielectric constant of 9 and higher and a low loss tangent; the particles of refractory oxide in the wall portion between said surface layers being arranged and bonded together to form a uniformly porous structure of low apparent density having a predetermined dielectric constant of between about 2 and 5, the thickness of said low density centrally disposed wall portion being between about 0.4 and 0.6 of the full wave length of the energy to be transmitted through the wall of the object; the overall thickness of the wall being between about 0.3 and 0.7 of said full wave length. r

6. An electromagnetic window as stated in claim 5, wherein said wall is chemically homogeneous, is free from organic components and contains more than 90% alumina, and each of said thin dense layers has a thickness not exceeding about :06 of the full wave length of the energy to be transmitted.

7. A method of making unitary, refractory objects composed essentially of particles of refractory oxides bonded by small quantities of frit-ted minerals including clay, said object being of virtually uniform chemical composition but having continuous adjacent layers, differing in density and integrally bonded to each other, comprising: making a first aqueous suspension composed of finely divided refractory oxides in major proportion and a minor proportion of bonding minerals; making a second aqueous suspension composed of timely divided refractory oxides and bonding minerals in substantially the same proportions as the first suspension, but containing a predetermined added amount of hollow synthetic resin composition particles; sequentially casting said aqueous suspensions to form adjoinin contacting layers of desired thickness, and then firing the cast object to bond said layers together.

8. A strong, unitary radome, nose cone and similar formed object adapted to effectively transmit high frequency energy and having the ability to withstand thermal shock comprising: a rigid contoured object including a wall having a generally convex outer surface and a generally concave inner surface, said wall being composed essentially and virtually homogeneously of particles of refractory oxide bonded together by a small quantity of fired products of minerals including clay; the particles of refractory oxide being in compact, closely adjacent relation at said inner and outer surfaces to form thin dense layers of predetermined and virtually uniform thickness at said surfaces, the particles of refractory oxide in the Wall portion between said surface layers being arranged and bonded together to form a uniformly porous structure of lower density than said surface layers, said surface layers having a substantially higher dielectric constant than said wall portion, said wall portion constituting between 70% and 90% of said overall wall thickness.

9. A unitary object as defined in claim 8, wherein each of the dense surface layers has a dielectric constant of 8 to 12, and said wall portion between said layers has a dielectric constant of less than 8.

.10. A unitary object as defined in claim 9, wherein said wall portion has a dielectric constant of 2 to 5.

11. A unitary object as defined in claim 8, wherein the overall thickness of said wall is between about 0.3 and 8 0.7 of the full wave length of the energy to be transmitted through the wall of said object, and each of said thin dense surface layers has a thickness not exceeding about 0.06 of said full Wave length.

12. A method as defined in claim 7, wherein said first and second aqueous suspensions each contain from to about 97% by weight of said refractory oxides and from about 3% to 10% by weight of said bonding minerals, and wherein said second aqueous suspension also contains from about 16% to 25% of said hollow synthetic resin composition particles, by weight of the solid components of said second suspension.

13. A method as defined in claim 7, wherein said refractory oxides are selected from the group consisting of alumina, mullite and calcined sillirnanite, and said bonding minerals include a clay from the montmorillonite group.

14-. A method as defined in claim 7, wherein said refractory oxides are alumina having a grain size not in excess of 60 microns, said bonding minerals include hectorite, and said hollow synthetic resin composition particles are selected from the group consisting of urea formaldehyde and phenol-formaldehyde spheres.

15. A method of making unitary, refractory objects composed essentially of particles of refractory oxides bonded by small quantities of fritted minerals including clay, said object being of virtually uniform chemical composition but having continuous adjacent layers, differing in density and integrally bonded to each other, comprising: making a first aqueous suspension composed of finely divided refractory oxides in major proportion and a minor proportion of bonding minerals; casting said first aqueous suspension on a surface to form a first :thin layer, making a second aqueous suspension composed of finely divided refractory oxides and bonding minerals in substantially the same proportions as the first suspension, but containing a predetermined added amount of hollow synthetic resin composition particles; casting said second aqueous suspension upon said first layer to form a core of substantially greater thickness than said first layer, and casting said first aqueous suspension on said core to form a second thin layer, and then firing the cast object to bond said layers and core together.

16. A unitary, strong, refractory object composed essentially of particles of refractory oxide bonded by small quantities of minerals including clay, said object being of substantially uniform chemical composition throughout but composed of continuous, adjacent layers including thin dense inner and outer layers and an intermediate thicker core layer of porous structure having substantially lower density than said thin inner and outer layers, said inner and outer layers having a higher dielectric constant than said intermediate core layer, said layers being intimately bonded together.

References Cited by the Examiner UNITED STATES PATENTS 2,962,717 11/1960 Kofoid 343872 BENJAMIN A. BORCHELT, Primary Examiner.

FRED C. MATTERN, 112., Examiner.

R. F. STAHL, Assistant Examiner.

Claims (1)

1. A STRONG, UNITARY RADOME, NOSE CONE AND SIMILAR FORMED OBJECT ADAPTED TO EFFECTIVELY TRANSMIT HIGH FREQUENCY ENERGY AND HAVING THE ABILITY TO WITHSTAND THERMAL SHOCK COMPRISING: A RIGID CONTOURED OBJECT INCLUDING A WALL HAVING A GENERALLY CONVEX OUTER SURFACE AND A GENERALLY CONCAVE INNER SURFACE, SAID WALL BEING COMPOSED ESSENTIALLY AND VIRTUALLY HOMOGENEOUSLY OF PARTICLES OF REFRACTORY OXIDE BONDED TOGETHER BY A SMALL QUANTITY OF FRITTED MINERALS INCLUDING CLAY OF THE MONTMORILLONITE GROUP; THE PARTICLES OF REFRACTORY OXIDE BEING IN COMPACT, CLOSELY ADJACENT RELATION AT SAID INNER AND OUTER SURFACES TO FORM THIN DENSE LAYERS OF PREDETERMINED AND VIRTUALLY UNIFORM THICKNESS AT SAID SURFACES, SAID SURFACE LAYERS HAVING A DIELECTRIC CONSTANT OF 9 AND HIGHER AND A LOW LOSS TANGENT; THE PARTICLES OF REFRACTORY OXIDE IN THE WALL PORTION BETWEEN SAID SURFACE LAYERS BEING ARRANGED AND BONDED TOGETHER TO FORM A UNIFORMLY POROUS STRUCTURE OF LOW APPARENT DENSITY HAVING A PREDETERMINED DIELECTRIC CONSTANT OF LESS THAN 9; THE OVERALLY THICKNESS OF SAID WALL BEING BETWEEN ABOUT 0.3 AND 0.7 OF THE FULL WAVE LENGTH OF THE ENERGY TO BE TRANSMITTED THROUGH THE WALL OF THE OBJECT, SAID LOW DENSITY WALL PORTION CONSTITUTING BETWEEN 70% AND 90% OF SAID OVERALL WALL THICKNESS.

Images