An improved isolation system is provided for attaching ceramic tiles of insulating material to the surface of a structure to be protected against extreme temperatures of the nature expected to be encountered by the space shuttle orbiter. This system isolates the fragile ceramic tiles from thermally and mechanically induced vehicle structural strains. The insulating tiles are affixed to a felt isolation pad formed of closely arranged and randomly oriented fibers by means of a flexible adhesive and in turn the felt pad is affixed to the metallic vehicle structure by an additional layer of flexible adhesive

Leger, L. J.

English

NASA Technical Reports Server

United States Pate [111 4,124,732 
Leger [451 Nov. 7, 1978 
[54] THERMAL INSULATION ATTACHING - -  
MEANS 
[75] Inventor: 
[73] Assignee: 
[21] Appl. No.: 
[22] Filed: 
Lubert J. Leger, Friendswood, Tex. 
The United States of America as 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
786,913 
Apr. 12, 1977 
Related U.S. Application Data 
[63] Continuation-in-part of Ser. No. 555,750, Mar. 5, 1975, 
abandoned. 
[51] Int. C1.2 ........................... BQ4C 1/00; B32B 7/00 
[52] U.S. CI .  ...................................... 428/77; 244/121; 
244/158; 244/160; 428/189; 428/212; 428/332; 
428/280; 428/285; 428/286; 428/447; 428/450; 
428/920 
Field of Search ................. 428/280, 235, 77, 447, 
428/450,234,252,298,332,286,300,920, 189; 
52/750; 244/117 R, 119, 121, 126, 158, 160 
[58]  
~561 References Cited 
U.S. PATENT DOCUMENTS 
3,082,611 3/1963 Alvis et al. ............................. 106/86 
3,920,339 11/1975 Fletcher ............................... 428/447 
FOREIGN PATENT DOCUMENTS 
2,238,631 2/1974 Fed. Rep. of Germany ........... 428/300 
Primary Examiner-George F. Lesmes 
Assistant Examiner-P. Thibodeau 
Attorney, Agent, or Firm-Russell E. Schlorff; John R. 
Manning; Marvin F. Matthews 
1571 ABSTRACT 
An improved isolation system is provided for attaching 
ceramic tiles of insulating material to the surface of a 
structure sought to be protected against extreme tem- 
peratures of the nature expected to be encountered by 
the space shuttle orbiter. This system isolates the fragile 
ceramic tiles from thermally and mechanically induced 
vehicle structural strains. The insulating tiles are affixed 
to a felt isolation pad formed of closely arranged and 
randomly oriented fibers by means of a flexible adhesive 
and in turn the felt pad is affixed to the metallic vehicle 
structure by an additional layer of flexible adhesive. 
4 Claims, 3 Drawing Figures 
2. 
https://ntrs.nasa.gov/search.jsp?R=19790004050 2020-03-20T18:04:20+00:00Z
U S .  Patent Nov. 7, 1978 4,124,732 I 
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4,124,732 
1 
THERMAL INSULATION ATTACHING MEANS 
ORIGIN OF THE INVENTION 
The invention described herein was made by an em- 5 
ployee of the United States Government and may be 
manufactured and used by or for the Government of the 
United States of America for governmental purposes 
without the payment of any royalties thereon or there- 
for. 10 
This is a continuation-in-part of Ser. No. 555,750, 
filed Mar. 5, 1975, now abandoned. 
BACKGROND OF THE INVENTION 
particularly, to improved means for attaching insulating 
tiles having one coefficient of thermal expansion to a 
structure having a markedly different coefficient of 
thermal expansion. 
to temperatures in the range of -250" F. to 2300" F., 
utilize tiles formed of substantially chemically pure 
silica fibers which are rigidized with a high purity silica 
binder. The tiles, known as L1-900, are manufactured 
bv Lockheed Missiles & &ace Co., Inc., have a density 25 
This invention relates to thermal insulation and, more 15 
Portions of the space shuttle orbiter which are subject 20 
0; 9 Ibs/cu. ft., relatively iow strength, extremely high 
temperature resistance, and extremely low coefficient 
of thermal expansion as compared to metals. Because of 
the relatively low strength, the tiles cannot be used for 
load-bearing and must be secured to the protected 
structure by an attachment system which will minimize 
transfer of strains from the metal structure to the tile. 
Ordinarily this requirement of minimizing load 
(strain) transfer into a low strength insulator is met by 
the use of a bond having a strain isolation capability, Le., 
a flexible bond. However, during operation in space, the 
space shuttle orbiter will be subject to -250" F., which 
is far below the glass transition temperature of known 
flexible bonds. At such temperature, the bonds lose 
their flexibility and behave as rigid connectors. As such, 
they permit enough strain to be passed through from the 
metal structure to the insulators so that the latter be- 
come mechanically loaded and as a result crack and 
spall. This result obtains whether the transferred strain 
is mechanical or thermal in nature, and whether it takes 
the form of a shock or is gradual in nature, as when the 
structure being protected contracts (or expands) as its 
temperature falls (or rises in the latter case). 
To overcome some of these problems, an attachment 
technique has been proposed in a co-pending U.S. pa- 
tent application Ser. No. 419,748 filed on Nov. 28, 1973, 
and entitled "Strain Arrestor Plate," (now U.S. Pat. No. 
3,920,339) wherein a rigid strain arrestor plate and sili- 
cone foam pad is interposed between the insulating tiles 
and the surface of the structure to be protected. This 
attachment system isolates thermal and mechanical 
strains from the tile down to --100" F. at which point 
the material modulus significantly increases as glass 
transition temperature (- 170" F.) is approached. Fur- 
ther, it is subject to certain disadvantages such as diffi- 
culty of assembly, additional weight, and expense. 
It has also been proposed to employ only the silicone 
foam pads as an attachment system for the orbiter. This 
technique is also limited to usage at temperatures above 
- 100" F. because of glass transition problems as previ- 
ously discussed. Consequently, use of this foam pad 
technique would require that the space shuttle orbiter 
be thermally conditioned by maintaining it in a rotating 
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attitude so that the temperature of the foam pad does 
not reach a temperature below approximately - 100" F. 
These disadvantages of the prior art are overcome 
with the present invention, and improved methods and 
apparatus are herewith provided for attaching rigid, 
fragile insulating tiles to a structure to be protected 
against the extreme low temperature (- 250" F.) en- 
countered by the space shuttle orbiter. 
SUMMARY OF THE INVENTION 
As hereinbefore explained, portions of the space shut- 
tle orbiter are protected from extreme temperatures 
resulting from reentry into the atmosphere by ceramic 
tiles which are laid in a closely abutting manner on 
portions of the surface of the structure to provide a 
protective shield or shell thereabout. Although the 
temperatures on the external surface of these ceramic 
tiles may reach 2300" F. during reentry, the temperature 
of these tiles and associated attachment system will be 
as low as -250" F. during orbital operations. Since the 
metallic structure has a thermal expansion coefficient 
which is markedly different from that of the ceramic 
material, the tiles must be isolated by an intermediate 
layer of material which yields to or absorbs the stresses 
which arise from expansion or contraction of the metal- 
lic structure, and/or mechanical deflections which will 
otherwise fracture the tiles. In the preferred embodi- 
ment of the present invention, this intermediate layer is 
composed of a strain isolation pad formed of fibers of a 
suitable material which are closely arranged and ran- 
domly oriented to provide a felt-like configuration. The 
pad is attached to the tile and to the structure by thin 
layers of a flexible adhesive. These fibers may be com- 
posed of organic or inorganic based materials which are 
sufficiently stable in the temperature range of interest 
(-250" F. to 500" F.) and capable of being felted with 
suitable mechanical properties. 
These and other features and advantages of the pres- 
ent invention will become apparent from the following 
detailed description, wherein reference is made to the 
figures in the accompanying drawings. 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a simplified pictorial representation of the 
space shuttle orbiter showing the various portions of the 
external surface to which insulating tiles of silica will be 
attached. The lower half of the FIG. illustrates the 
bottom side of the vehicle, and the upper half illustrates 
the top side of the vehicle. 
FIG. 2 is a simplified pictorial representation of an 
exemplary portion of the vehicle external surface de- 
picted in FIG. l and illustrating how the insulating tiles 
are attached to such portion. 
FIG. 3 is an exploded representation of the attaching 
means depicted in FIG. 2. 
, 
DETAILED DESCRIPTION 
Referring now'to FIG. 1, there may be seen a simpli- 
fied pictorial representation of a space vehicle 2 such as 
the space shuttle orbiter. The lower portion of the FIG. 
illustrates the bottom side of the vehicle, and the upper 
portion illustrates the top side of the vehicle. The vehi- 
cle is provided with wings 3 having leading edges 4 
which are susceptible to extremely high temperatures 
upon reentry into the atmosphere of the earth and 
which are protected by a high density refractory mate- 
rial. The bottom side § of the vehicle (other than leading 
edges 4 of wings 3) and certain portions 6 of the top side 
4.124,732 
3 
of the vehicle upon reentry reach temperatures ap- 
proaching 2300" F. These are as are protected by rigid- 
ized silica insulation. The remaining portions 7 of the 
top side of the vehicle, wings and fuselage, may reach 
temperatures approaching 1200" F. and are protected 
by other silica insulation and organic material with 
sufficient thermal stability. 
Referring now to FIG. 2, there may be seen a picto- 
rial representation of a portion of vehicle protected by 
high purity silica tiles and showing how the metallic 
outer surface 10 (aluminum alloy) of the vehicle may be 
covered with insulating tiles 11 laid thereon in a closely 
abutting manner. The tiles 11 are approximately 6 x 6 
inches, with varying thickness depending upon specific 
vehicle locations and heat load. Accordingly, it will be 
apparent from FIG. 2 that the tiles I1 form a thermal 
shield over the surface IO. Furthermore, it will also be 
apparent from consideration of the structures depicted 
in FIG. 2 that tiles 13. will be subject to damaging struc- 
tural stresses arising because of the difference in coeffi- 
cient of thermal expansion between the metal of the 
vehicle 2 and the material from which the tiles 11 are 
fabricated, as well as deflections of the structure due to 
mechanical (both aerodynamic and internal pressure) 
loads unless such loads are isolated. Accordingly, in 
attaching each of the tiles I1 to the metallic surface 10, 
an intermediate layer of material, such as a felt pad 12, 
is interposed to act as a strain isolator. Although the 
temperature during reentry may be 2300" F., the effec- 
tive temperature for the strain isolation pad will be 500" 
F. due to the insulating characteristics of the tiles. 
Referring now to FIG. 3, there may be seen a simpli- 
fied functional representation of the manner in which 
each tile 11 and pad 12 is affixed to the structure 10. 
More particularly, a thin layer (less than 10 mils) 14 of 
a suitable flexible adhesive such as a methylphenyl sili- 
cone (RTV 560 manufactured by the General Electric 
Company) attaches the insulating tile 1% to the strain 
isolation pad 12. A thin layer (less than 10 mils) 13 of the 
same adhesive attaches the isolation pad to the surface 
10. 
As more particularly shown in FIG. 3, the isolation 
pad 12 is preferably formed of fibers of a material hav- 
ing thermal vacuum stability and stability in air within 
the intermediate temperature range of -250" F. to 500" 
F. The fibers are closely arranged and randomly ori- 
ented whereby each filament fiber supports the tile 11 
independently of the other fibers in the isolation pad 12. 
Although fibers of various orgar,ic and inorganic based 
materials may be utilized in fcrming felt pad 12, it has 
been determined that aramid fibers sold by DuPont 
under their trademark NOMEX are particularly well 
suited for the application as the strain isolation pad for 
the space shuttle orbiter. As reported in the January 
1977 issue of Textile Research Jonrnal, pp 62-66, 
NOMEX is poly (1,3-phenylene isophthalamide). An- 
other fiber which may be used is novolocid, a highly 
cross-linked phenolic polymer, the constituents of 
which are carbon, hydrogen and oxygen and marketed 
by the Carborundum Corpolrtion under its tradeinark 
KYNOL. Another material is a treated aramid mar- 
keted under the trademark DURETTE. For applica- 
tions where the temperature and deflection require- 
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ments are less stringent other materials suitable fo; fclt- 
ing and having fibers capable of absorbing strain by 65 
bending may be utilized. 
The preferred embodiment of the isolation pad 12 
may be NOMEX formed into a felt approximately 160 
4 
mils thick. One such felt is made by Globe-Albany, Inc. 
and known as style number 2544 NRBZ. "NRBZ" 
stands for needled, calendered, heat set and water- 
proofed. Needled means that the fibers have been entan- 
gled during fabrication by a mechanical process. Calen- 
dered means that the material has passed between rol- 
lers heated to elevated temperatures under pressure. 
Heat set means that the material has been exposed to 
elevated temperatures in excess of 500" IF., but less than 
550" F., to achieve maximum fiber shrinkage prior to 
service use at that temperature. Waterproofed means an 
organic based coating has been added to the felt to 
provide water resistant characteristics. This material 
has been found to be especially suitable for space shuttle 
orbiter purposes. 
Isolation pad 12, formed of snch material, and ap- 
proximately 160 mils thick is capable of deflecting ap- 
proximately 30 mils under a tensile force of 5.3 psi, and 
has a tensile strength of substantially greater than or 
equal to 20 psi. Such material yeighs 1 0 f  1 oz/sq. yd. 
and is not a weight penalty on a weight sensitive system, 
particularly since it is waterproofed and will not absorb 
moisture. For areas of the vehicle for which the struc- 
ture is non-buckling, isolation pads of lesser thickness 
and lower deflection Characteristics may be used. For 
example, some pads are as thin as 90 mils with deflection 
less than 5 mils at 5.3 psi tensile loading. In all cases, the 
isolation pads accommodate shear and tensile structural 
strains by translating these strains to bending strains in 
the individual fibers of the felt pad. 
In constructing insulation for a structure such as the 
orbiter, strain isolation pads 12 are preferably first at- 
tached to silica insulating tiles PI by a thin layer of 
adhesive 14 and then the combination is affixed to slruc- 
ture 10 by means of another thin layer of flexible adhe- 
sive 13. The adhesive bond line on both sides of the 
strain isolation pad is preferably held to a thickness of 
7.552 mils, so that the adhesive will not penetrate deep 
enough into the isolation pad to recreate the problem of 
glass transition within the isolation pad. Although the 
layers of flexible adhesive bond will undergo glass tran- 
sition in outer space (- 170" F.) it has been determined 
that since the layers are very thin the transition will not 
be detrimental to the tile, whereas it was fou:id that a 
thick layer undergoing glass transition wovld damage 
the tile. 
Although the strain isolauon pad 12 of the present 
invention is preferably formed of NOMEX, as hereinbe- 
fore described, other materials heretofore mentioned 
will also be suitable. 
Many other variations and modifications may be 
made in the apparatus and techniques hereinbefore de- 
scribed, both by those having experience in this tecbnol- 
ogy, without departing from the concept of the present 
invention. Accordingly, it should be clearly understood 
that fhe apparatus and methods depicted in the accom- 
panying drawings and referred to in the foregoing de- 
scription are illustrative only and are not intended as 
limitations on the scope of the invention. 
What is claimed is: 
1. In an insulating system €or a space vehicle formed 
of a plurality of insulating tiles of substantially, chemi- 
cally pure rigidized silica tiles to p petal .sub- 
strate of the space vehicle, the ng a coqfficient 
of thermal expansion markedly lower than the coeffici- 
ent of the metal substrate, the improvement comprising: 
a strain isolation pad between each tile 2nd the metal 
substrqte, said pad attached to each tile and the metal 
I . ,  
I 4,124,732 
5 6 
substrate by a separate, layer of flexible adhesive bond, 2. The insulating system described in claim 1, wherein 
having a thickness less than 10 mils, each pad being said fibers in said strain isolation pad comprise aramid. 
formed of a plurality of high temperature resistant, 3. The insulating system of claim 2, wherein the strain 
flexible, plastic fibers randomly oriented and closely isolation pad is approximately 160 mils thick and de- 
arranged with respect to each other to form a needled 5 flects approximately 30 mils under a tensile force of 5.3 
felt configuration isolating said tile from thermal and psi with a tensile strength of substantially 20 psi. 
mechanical stresses generated by deformation of said 4. The insulating system of claim 3 wherein the thin 
metal substrate of the space vehicle, said fibers being layers of flexible adhesive bond are each approximately 
capable of absorbing within a temperature range of 7.5 mils. * * * * *  -250" F. to 500" F., said stresses by bending. 10 
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Thermal insulation attaching means

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