High temperature heat shield system

A high temperature heat shield system for re-entry vehicles and high speed ircraft is assembled from separate insulating panels made of metal foil layers diffusion soldered together and with a ceramic fiber material filled into interspaces between the metal foil layers. In this arrangement structural metal foil layers carry mechanical loads while separate reflecting foil layers and the ceramic fiber material are non-load bearing.

FIELD OF THE INVENTION 
The invention relates to a high temperature heat shield system, especially 
for spacecraft re-entering the earth's atmosphere, and for high speed 
aircraft. The system is assembled of multiple layers of metal foil and 
fiber layers, such as ceramic fiber mats arranged between the metal foil 
layers. 
BACKGROUND INFORMATION 
U.S. Pat. No. 4,456,208 discloses a heat shield having a metal box 
construction with a filling of fiber material and possibly also including 
reflecting foil layers. The known heat shield does not provide for any 
easy reusability of the separate elements of the heat shield. Further, the 
material expenditure and the weight of the known heat shield system are 
quite high. The thermal insulation of the known heat shield is provided by 
the fiber material filling. The metal cover of the box must be relatively 
strong, stiff, and therefore, heavy, for mechanical reasons, but provides 
only insignificant additional thermal insulation. 
U.S. Pat. No. 4,344,591 discloses a similar heat shield system having 
several layers of metal panels which are constructed as a metal multiwall 
structure combined with one or two layers of high density ceramic fiber 
mats. For high temperatures these known panels are either relatively heavy 
or relatively thick so that they lead to a disadvantageous increase of the 
outer diameter of the aircraft or spacecraft or instead, require a 
corresponding decrease of the usable inner space of the aircraft or 
spacecraft. 
OBJECTS OF THE INVENTION 
In view of the foregoing it is the aim of the invention to achieve the 
following objects singly or in combination: 
to construct a high temperature heat shield system especially for high 
speed aircraft and spacecraft including re-entry vehicles, which system 
uses removable and reusable components; 
to construct such a heat shield system with load bearing metal foil layers 
having a high strength to thickness ratio achieved by corrugating or 
dimpling the foil layer in combination with non-load bearing ceramic 
layers which provide good thermal insulation; 
to combine the advantages of the load bearing layers providing mechanical 
strength with the advantages of non-load bearing layers providing heat 
insulating characteristics; 
to construct such a heat shield system in such a manner and using such 
materials that a lightweight and small thickness is achieved for the 
insulation panels; 
to allow the specific insulating characteristics of a panel of such a heat 
shield system to be adjusted by appropriately selecting the materials of 
the panel combination while maintaining the same construction; and 
to increase the insulating value of such insulating panels by reducing the 
thermal conduction through gas in the fiber material and to construct the 
panels so that heat which is stored in the heat shield system during 
re-entry into the earth's atmosphere is to a greater extent discharged or 
given off externally during flight at lower flight altitudes. 
SUMMARY OF THE INVENTION 
The above objects have been achieved in a high temperature heat shield 
system according to the invention, wherein separate layered panels are 
assembled to form the heat shield. Each panel includes a plurality of at 
least three different layered structures. A first or outer layered 
structure includes an outer metal foil, an inner or inwardly facing metal 
foil and a dimpled metal foil sandwiched between the outer and inner metal 
foil. A second layered structure is a stack of corrugated metal foil 
layers with the direction of corrugation rotated by 90.degree. between 
neighboring layers and with a ceramic fiber layer having a specific 
thermal insulating value arranged between neighboring corrugated foils. A 
third layered structure includes an integrally stiffened substrate or 
baseplate including two metal foil layers with a dimpled metal foil 
sandwiched between the metal foil layers in a construction similar to the 
first or outer layered structure. A fourth optional layered structure in a 
panel of the invention is a stack of ceramic fiber layers separated from 
one another by respective non-load bearing infrared reflecting, foil 
layers. If the fourth layered structure is used it is arranged between the 
second and third layered structures. The load bearing foil layers of the 
heat shield system may be connected to each other, for example, by means 
of diffusion soldering. 
The materials for the insulation panels are selected according to the 
expected operating temperatures, whereby the metal foil layers may be 
selected from, for example titanium alloys, nickel alloys, cobalt alloys, 
or other alloys including refractory metal alloys and the highly 
reflective radiating foils may be made of aluminum coated Kapton (R.T.M.), 
aluminum foils, gold foils, nickel foils, copper foils or gold coated 
ceramic foils. The ceramic fiber filling has fibers with a higher 
proportion of Al.sub.2 O.sub.3 on the hot side or outer side of the panel 
and fibers with a higher proportion of SiO.sub.2 on the cold side or inner 
side of the panel. The density of the fiber mats increases from the outer 
side to the inner side while the average fiber diameter decreases from the 
outer side to the inner side. The fiber material is selected due to the 
lowest specific fiber density for the envisaged temperature range. 
Thermal expansion joints are provided between neighboring panels of the 
heat shield system. Flexible ceramic mats fill and insulate the expansion 
gaps. Snap catch fasteners or catch hooks attach to the aircraft cell 
structure and engage the separate panels in a removable manner. A 
polyimide foam layer or e.g. a Homers felt, fills a space between the 
baseplate of the separate panels and the outer skin of the aircraft 
fuselage or spacecraft body wall.

DETAILED DESCRIPTION OF PREFERRED EXAMPLE EMBODIMENTS AND OF THE BEST MODE 
OF THE INVENTION 
FIGS. 1a, 1b, and 1c show four different "heat load zones" on the outer 
surface of a spacecraft 10 during re-entry into the earth's atmosphere or 
analogously for a high speed aircraft. The areas identified as zone 1, 
that is, the nose leading edges, and the control surfaces of the craft 10 
are subjected to the highest temperatures and are generally realised as 
hot structures. Strippled zones 2 are at a slightly lower temperature 
while zones 3 are at an even lower temperature and zones 4 are at the 
coolest temperature. Accordingly, these zones 2, 3, 4 require a 
respectively decreasing level of heat shielding. The invention provides a 
high temperature heat shield system made of separate insulating panels 11 
which are described in detail below with reference to FIGS. 2, 2a, 3, and 
4. Each panel is made up of at least three layered structures. 
The separate panels 11 are arranged to form a continuous shield attached to 
the cell or body structure of the craft. The basic construction of all the 
separate layered panels 11 remains the same but the number of layered 
structures in each panel may vary and so may the materials in the layered 
structures which are selected for providing the appropriate degree of heat 
shielding protection in the different heat load zones 2 to 4 and 
accordingly for the control surfaces of zone 1 if there control surfaces 
are not realised as hot structures. The required protection is provided in 
each of the zones without a significant weight addition and without 
dimensional penalties. 
As shown in FIG. 2 a panel 11 of a heat shield system of the invention has, 
for example, four main layered structures 12, 13, 14 and 15. The first 
layered structure 12 comprises an outer metal foil layer 12a with a wall 
thickness of approximately 100 .mu.m, a nubbed or dimpled metal foil layer 
12b with a wall thickness of approximately 70 .mu.m and an inner metal 
foil layer 12c with a wall thickness of approximately 50 .mu.m. The 
dimpled metal foil layer 12b is sandwiched between the metal foil layers 
12a and 12b and the respective foil layers may be connected to each other, 
for example by means of diffusion soldering at points or regions where the 
foils contact each other. The spacing between dimples of the dimpled foil 
layer 12b is approximately 20 mm and the total thickness of the first 
layered structure 12 is approximately 4 mm. 
The second layered structure 13 comrpises a series of stacked corrugated 
metal foil layers 13a which are successively rotated by 90.degree. so that 
the valleys and ridges of one foil extend perpendicularly to the valleys 
and ridges in the other neighboring foil. The foils are also diffusion 
soldered together where the perpendicularly crossing corrugations of 
neighboring layers 13a contact each other. The interspaces between the 
corrugated layers 13a are filled with ceramic fiber layers 13b having a 
specific thermal insulating value selected in accordance with the 
requirements of the several zones 1 to 4. Each corrugated metal foil layer 
13a has a wall thickness of approximately 50 .mu.m and a corrugation 
height of approximately 6 mm while the on-center corrugation spacing from 
peak to peak is approximately 30 mm. 
The third layered structure 15 is an integrally stiffened substrate or 
baseplate comprising a metal foil layer 15a approximately 30 .mu.m thick. 
A nubbed or dimpled metal foil intermediate layer 15b approximately 70 
.mu.m thick and a metal foil layer 15c approximately 50 .mu.m thick. In 
this baseplate layer 15 the dimpled spacings, the layer thickness and the 
interconnection of adjacent layers is essentially the same as that 
described above for the first layered structure 12. In this panel 
structure the first layered structure 12 forms an outer skin layer while 
the third layered structure 15 is a base layer facing the aircraft 
fuselage. 
Depending on the heat insulation requirements, a fourth layered structure 
14 comprises a series of stacked ceramic fiber layers 14a separated from 
one another by loosely arranged non-load bearing highly reflecting foil 
layers 14b. This fourth layered structure is surrounded or enclosed by a 
corrugation stiffened metal foil frame 14' only shown as a line in FIG. 2 
for transmitting the occurring mechanical loads from the second layered 
structure 13 to the third layered structure 15 so that no loads are 
applied to the ceramic fiber layers 14a or the reflecting foil layers 14b 
of the optional fourth layered structure. This load transferring by the 
frame 14' can also be achieved by an appropriate grid structure or 
multi-walled structure, depending on weight considerations. FIG. 2a shows 
perspectively and in broken away fashion, a frame 14' between the base 
plate layer 15 and the layered structure 13. 
The appropriate material for constructing the panel 11 must be selected 
depending on the specific application of each panel 11, namely the 
expected operating temperatures and depending upon the specific aircraft 
and the respective heat zone 2, 3, or 4. For example, the various metal 
foils of the layered structures 12, 13, and 15 may be made--as referenced 
elsewhere--of titanium alloys such as Ti.sub.6 Al.sub.4 V for operating 
temperatures up to 500.degree. C., or nickle alloys such as Inconel 218 
(RTM), for temperatures up to 900.degree. C., or cobalt alloys such as 
Flayner 788 (RTM) for temperatures up to 1050.degree. C., or nickle chrome 
alloys such as TDNiCr (Thorium detached Nickel-chrome alloy), for 
temperatures up to 1200.degree. C., or molybdenum alloys such as TZM 
(Titanium-Zirconium-Molybdenum alloy) for temperatures up to 1300.degree. 
C. The latter (TZM) has to be coated by an oxidation protective material. 
The radiation reflecting foil layers 14b are made of non-load bearing, high 
reflecting and extremely thin foils selected from, for example aluminum 
coated Kapton (R.T.M.) polyimide foils 6 .mu.m thick for temperatures up 
to 400.degree. C.; aluminum foils 5 .mu.m thick for temperatures up to 
550.degree. C., or gold foils, nickle foils, copper foils, or gold coated 
ceramic foils of minimum available thickness for temperatures up to 
900.degree. C. 
The fibers of the ceramic fiber fillings 13b and 14a have a higher 
proportion of Al.sub.2 O.sub.3 such as, for example the fiber material 
referred to as "Fiberfrax H" (R.T.M.) ceramic fibers made of Al.sub.2 
O.sub.3 and SiO.sub.2 on the hotter side or toward the outside of the 
insulating panel 11. On the other hand, on the colder or inner side of the 
insulating panel 11 the fiber material has a higher proportion of 
SiO.sub.2 (up to 99.9%) or a higher proportion of borosilicate glass such 
as, for example microlite fibers (by Johns Manville Company). The density 
of the fiber filling increases from the outer layer to the inner layer 
from a value of approximately 8 kg/m.sup.3 to 40 kg/m.sup.3. The average 
fiber diameter of the fiber material decreases from the outside toward the 
inside from approximately 4 .mu.m to approximately 0.4 .mu.m. This 
variation of material characteristics across the thickness of the 
insulating panel 11 measured perpendicularly to the surface of the layered 
structures achieves the desired results that for flight altitudes less 
than 50 km and a correspondingly reduced aero-thermic heating, the heat 
shield preferably externally discharges or gives off previously stored 
heat. 
The outermost metal foil 12a is coated on its outer surface with a coating 
or film having a high thermal optic emission coefficient which may be as 
high as .epsilon..gtoreq.0.9 depending on the coating material such as 
Al.sub.2 O.sub.3 /SiO.sub.2 layers of a few microus thickeners, whereby, a 
large portion of the incident heat is immediately radiated off. The inner 
surface of the metal foil 12a as well as all of the other interior foil 
layers such as 12b, 12c, and 13a have a low emission coefficient with 
.epsilon.&lt;0.1-0.3. Such emission coefficient values can be achieved by an 
appropriate surface treatment making the foil surfaces oxidation 
resistant, e.g., by an Pt- or Au-vapor coating. 
Referring to FIGS. 3 and 4, the different thermal expansions of the 
material of the heat shield relative to the material of the air frame of 
the aircraft are compensated in so-called thermal expansion gaps 16 
dimensioned so that when the heat shield is at its maximum temperature and 
corresponding maximum expansion, the gaps 16 are nearly closed to the 
outside. The outermost metal foil 12a has an extension 12d which covers 
the expansion gap 16 by reaching over the adjacent panel 11. A flexible 
ceramic mat 17a such as a (Johns Manville) Dynaflex mat is glued to the 
lateral edges of the panels 11 so that the respective adjacent ceramic 
mats 17a fill the space of the expansion gap 16. The ceramic mats 17a are 
flexible and compressible so that, as the width of the expansion gap 16 
changes in response to the changing expansion of the panels 11, the mats 
17a expand or compress to fill the expansion gap 16 at all times. 
The layered structures 12 to 15 of each panel 11 are depressurised and 
vented through a relatively low number of perforations provided in the 
foils 17a. A compressed ceramic paper combination strip 18 approximately 1 
to 3 mm thick is arranged below each expansion gap 16. Each ceramic paper 
strip 18 is approximately 5 cm wide. The rest of the surface area between 
the panel baseplate 15 and the air frame structure 20 is, for example, 
filled with a Nomex (R.T.M.) felt of heat resistant nylon of uniform 
thickness or with a polyimide foam layer 19. 
As especially shown in the detail of FIG. 4, snap catch fasteners 21 such 
as spring loaded catch hooks are provided on the flying body such as the 
frame structure 20 of the craft for securing the separate insulation 
panels 11. Any type of conventional fastener may be used that is suitable 
for securing the flat panels in a releasable manner to the surface of the 
flying body, for example slide and lock rails, bayonet type fasteners, 
Velcro (R.T.M.) or the like are suitable for this purpose. The snap catch 
fasteners 21 are, for example, made of a titanium alloy, engage an edge 
15' of the lower layered structure 15 forming a baseplate of each panel 
11. A slight free play is allowed for easily operating the snap catch 
fastener 21 and for allowing a compensation for differing thermal 
expansions of the baseplate structure 15 and the air frame structure 20, 
but the free play remains small enough that an accidental disengaging of 
the snap catch fastener 21 is not possible. The panels 11 may easily be 
snapped into place by engaging one end of the panel 11 under the cover 
plate extension 12d of the adjacent panel and then snapping the baseplate 
structure 15' under the snap catch fastener 21. The panels may easily be 
removed to be exchanged or reused by pushing an appropriate tool through 
the expansion gaps 16 to push back and disengage the snap catch fasteners 
21. In this case, simply the flexible ceramic mats 17a may have to be 
replaced. 
Incidentally, any of the ceramic fiber material layers may comprise fibers 
coated with a highly reflective coating. Further, any of the ceramic fiber 
materials may comprise highly reflective particles interspersed among the 
ceramic fibers. 
References: 
Ti6A14V is a titanium alloy supplied e.g. by ZAPP (Federal Republic of 
Germany), Contimet (Federal Republic of Germany); 
IN718 is a nickel alloy supplied by INCO (Great Britain), 
ZAPP (Federal Republic of Germany); 
HS188 is a cobalt alloy supplied by ZAPP (Federal Republic of Germany), 
Cabot (U.S. of America); 
TD NiCr is a thorium dotated nickel-chrome alloy supplied e.g. by ZAPP 
(Federal Republic of Germany); 
TZM is a molybdenum alloy supplied e.g. by Plansee (Austria), Climax (U.S. 
of America); 
Fiberfrac H is an alumina fiber with 53% Al.sub.2 Ol.sub.3 content supplied 
by Carborundum (U.S. of America); 
Dynaflex is a silica fiber mat supplied by Johns Manville (U.S. of 
America); 
Microlite is a borosilicate glass fiber matt supplied by Johns Manville 
(U.S. of America). 
Although the invention has been described with reference to specific 
example embodiments, it will be appreciated, that it is intended to cover 
all modifications and equivalents within the scope of the appended claims.