Element made of a reinforced low density heat protective material and method to obtain said element

An element made of a reinforced low density heat protective material including an elastomer and/or a silicon resin matrix loaded with organic and/or inorganic components, wherein the reinforcement is formed of glass or ceramic thread sections or the like or organic materials fitted in the mass of the matrix along directions approximately orthogonal to at least one of the faces of the element and being flush, at least at one of their extremities, with at least one of the faces. The method for obtaining this element, includes, after molding to the general dimensions and shapes of the element with the aid of a loaded silicon matrix, placing the thread sections by stitching.

FIELD OF THE INVENTION 
The present invention concerns a low density ablative material for heat 
protecting portions of space vehicles when they return into the earth's 
atmosphere. 
BACKGROUND OF THE INVENTION 
When returning into the atmosphere, a vehicle, such as a probe, capsule 
inhabited vehicle, etc., needs to confront the intense heat flows and 
consequently the most exposed portions need to be heat-protected. 
To combat the heat flows, thermic insulating materials, known as ablative 
materials, are used to coat the structures to be protected and whose 
gradual destruction under the action of the heat flow impelled by a 
re-entry into the atmosphere protects the coated structure from the heat 
by means of various mechanisms summed up as follows: 
storage of energy resulting in a rise of the internal temperature of the 
ablative material; 
endothermic reaction, namely : depolymerization, fusion, sublimation, 
vaporization; 
energy loss via radiation; 
flow of gaseous substances opposing the heat flow. 
This protection via the destruction of the thermic insulant is one of the 
most effective means available to combat the intense heat flows produced 
by an atmospheric return. 
This type of material, known for a large number of years, is formed of an 
elastomer and/or a silicon resin. There is a RTV (Room Temperature 
Vulcanization) type elastomer loaded with organic components (carbonated 
compounds, cork) or inorganic (SiC, silica, aluminium). 
This material is used as such and placed in the form of panels or mounted 
elements, especially glued elements, onto the surface to be protected. 
So as to prevent a possible flowing of the ablative material under the heat 
flow, the constitutive matrix of the material, composed, for example, of a 
RTV elastomer, silica ecospheres and phenolic microballoons and/or other 
loads, is inserted in a honeycomb type structure. 
Thus, it is possible to embody light, flat and mechanically resistant 
coating panels offering heat protection and good refractory properties. 
Furthermore, by using honeycomb structures being flexible in various 
directions; it is possible to embody bent structures. 
This technique consists of preparing the honeycomb structure, for example 
by indenting the walls of the cells so that the structure can be bent, 
followed by lining the cells of a siliconed matrix with a suitable 
formulation, of compacting the matrix and then shaping the entire unit in 
a press. 
However, the flexibility of this honeycomb structure has limits concerning 
the degree of bending of the panels able to be made according to this 
technique which moreover poses the problem of filling which needs to be 
thorough without having any vacuum in the cells of the honeycomb 
structure. 
Finally, the reinforcement constituted by this structure needs to be 
homogeneous concerning the entire weight of the final panel which does not 
make it possible to differentially reinforce the panel according to its 
various portions. For example, as it concerns a leading edge panel, it is 
not possible to significantly reinforce the most exposed portions of the 
panel, the reinforcement technique, as described earlier, proceeding by 
all or nothing. 
SUMMARY OF THE INVENTION 
The present invention offers a new technique for reinforcing low density 
ablative materials making it possible to obtain complex shapes and 
accentuated curves and the deliberate local modulation of the 
reinforcement on the shaped element. 
To this effect, the invention concerns an element made of a reinforced low 
density ablative heat protective material including an elastomer and/or 
silicon resin matrix loaded with organic and/or inorganic components, 
wherein the reinforcement is formed of glass or ceramic thread sections or 
the like or of organic materials disposed in the mass of said matrix along 
directions approximately orthogonal to at least one of the faces of the 
element and being flush, at least at one of their extremities, with at 
least one of said faces, said fibers in one particular embodiment being 
impregnated with a polymerized phenolic resin. 
The thread sections are preferably distributed regularly in zigzag fashion. 
If appropriate, the element may comprise zones, such as those most stressed 
by the heat flow, and having a number of thread sections per surface unit 
exceeding the number of the other less stressed zones. 
The invention also concerns a method for obtaining these elements, wherein, 
after molding to the general dimensions and shapes of said element with 
the aid of a loaded silicon matrix, said thread sections are placed by 
stitching. 
So as to preserve the face of the element directly struck by the needle of 
the stitching machine, said face is preferably coated with a fabric or 
silica or glass felt or even organic materials. 
According to one variant for implementing the method, threads are used 
pre-impregnated with a suitable phenolic resin, and after stitching, this 
resin is polymerized. 
According to another variant, non-impregnated threads are used, and after 
stitching, the element is impregnated with a suitable phenolic resin and 
said resin is polymerized. 
After polymerization of the phenolic resin, the faces of the element, 
possibly provided with said fabric or felt on one of its faces, are 
leveled so as to eliminate the projecting thread portions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows at 1 a parallelpiped block formed of a reinforced low density 
ablative material in accordance with the invention and including a silicon 
elastomer matrix 2 and loads of various natures, especially silica 
ecospheres and phenolic resin microballoons whose role are to reduce the 
density of the material and, by virtue of their heat properties, reduce 
heat conductivity and thus block the radiative heat in the material, and 
finally contribute by the reactions established inside the material during 
ablation in enriching the carbonated residual layer on the surface of the 
ablative element, that is of providing this layer with good structural 
strength and good refractory properties. 
A large number of formulations have been put forward for these matrix, 
possibly including additives for increasing the mechanical and refractory 
properties of the ablative material. The present invention does not target 
a particular type of formulation, but mainly concerns the reinforcement of 
these matrix, regardless to their composition. 
As can be seen on FIG. 1, the element 1 includes rigid picots 3 distributed 
regularly in the mass of the matrix. The picots 3 are rectilinear sections 
of ceramic, glass or organic threads disposed orthogonal to the upper flat 
face of the element 1. They extend into the entire thickness of the 
element and are thus flush with the two opposing faces. 
The grid for distributing the picots 3 is to the square pitch P of several 
millimeters, such as ten. 
According to a preferred placing embodiment of the present invention of 
this reinforcement, the picots 3 are inserted in the matrix 2 by stitching 
with the aid of a suitable thread. 
For example, it is possible to use ceramic/glass fibers known under the 
trade-mark 312 or 440 NEXTEL (sold by the 3M Company), KEVLAR thread type 
organic fibers (sold by the DUPONT DE NEMOURS Company). 
The stiffening of the thread sections of picots 3 is after stitching 
ensured via the polymerization of a suitable impregnation resin. 
As a suitable phenolic resin, it is possible to use resins from the family 
of resols or those from the family of novolaks, but also epoxy resins 
stitching is carried out with a suitable machine. 
FIG. 2 shows a diagram of a mode for stitching an element 1 of the type of 
FIG. 1. 
The needle (not shown on FIG. 2) appears on one of the faces of the element 
1 (the upper face on FIG. 2) perpendicularly and traverses the element on 
both sides by moving the thread 4 which thus on each to-and-fro movement 
of the needle through the element 1 makes a to-and-fro movement 3' which 
shall subsequently constitute a picot 3. 
Disposed on the upper face of the element 1 is a fabric or piece of felt 
formed of glass fibers so as to protect the matrix whose portions or 
particles could be pulled up by the needle coming out of the element. This 
fabric or felt moreover avoids incrusting in the material of the element 2 
of the thread 4 portions 6 for linking between two consecutive stitchings 
following the traction exerted on the thread by the needle driven into the 
element. 
The needle opens on the lower face of the element 1 and forms a loop on 
each stitching. 
Once the stitching operations have ended, the element 1 is impregnated with 
the appropriate resin, is polymerized and finally is machined. To this 
effect, the lower face of the element 1 of FIG. 2 is made flush at 9 so as 
to remove the loops 7 and also at 10 so as to remove the thread sections 6 
on the surface of the fabric or felt 5. This fabric or felt 5 is 
represented under the same numerical reference on FIG. 1 at the lower face 
of the element. 
The method of the present invention has the advantage that the 
reinforcement of the matrix is carried out after molding of the matrix to 
the shapes and dimensions of the element to be obtained and that this 
reinforcement can be adapted, that is accentuated in the most stressed 
zones of the element. 
By way of example, FIG. 3 illustrates a panel 11 intended to constitute a 
leading edge element and embodied via the molding of a matrix formed of a 
RTV 141 type silicon elastomer loaded with silica ecospheres and phenolic 
microballoons. 
The method for producing this panel formed of this material is well known 
and is accordingly not described in detail. 
FIG. 4 is a section of the panel of FIG. 3 illustrating a mode for 
implanting reinforcement elements constituted by picots similar to those 
of FIG. 1. 
After the element 11 has been removed from the mold, said element is 
stitched normally on the surface with the aid of a suitable thread of the 
type mentioned above and a stitching machine whose needle shall be 
available to penetrate into the matrix of the element on either of its 
opposing faces, depending on the type of reinforcement to be inserted. 
So as to embody the picots 12 or 13 being flush with the two opposing faces 
of the element 11, the needle shall attack the face affording the easiest 
access, the element to this effect being placed in a holding and 
positioning cradle. 
If desired, the method of the invention makes it possible to densify a 
particular more exposed zone of the element, such as the portion 11a, by 
implanting picots 12 with a pitch smaller than that of the picots 13 of a 
portion 11b confronting the heat flows under the lowest incidences. 
In the most curved portions 11c of the element, it is possible to insert 
picots 14 by means of stitching traversing the element on both sides and 
insert picots 15 in the gap between two adjacent picots, these latter 
picots being flush with the convex face of the element but not opening 
onto the concave face so as to keep an approximately constant gap between 
the threads in all directions. The penetration depth of these picots 15 
can of course be adjusted. 
The picots 12 to 15 can advantageously be distributed regularly in each 
zone and in zigzag fashion, the picots having a density per surface unit 
which varies from one zone to another. 
The face attacked by the needle can firstly be coated with a fabric or felt 
5. 
The stiffening of the picots 12 to 15 can be obtained in two ways. 
The thread used for stitching is pre-impregnated with a suitable resin, 
such as a phenolic resin. After the threads have been placed, the resin of 
the threads is polymerized and then the element is machined to the 
dimensions of the final element, as shown on FIG. 2 (making level of the 
faces of the element). 
According to a second method, the thread used is not pre-impregnated and 
after stitching, the element undergoes an impregnation operation with the 
aid of a suitable resin and this operation is followed by a polymerization 
of the resin and this element is finally machined as indicated above. 
Owing to the fact that the reinforcement is embodied after molding of the 
element, it is possible to give the latter a complex shape with large 
localized curves. It shall always be possible to gain access to one of the 
faces of the element so as to insert there threads perpendicularly or 
almost perpendicularly, possibly over the entire thickness of the element, 
and with any distribution pattern. 
Finally, the invention is not merely limited to the embodiment examples 
described above, but on the contrary covers all possible variants as 
regard the nature of the loaded silicon matrix, the nature of the threads 
stitched or introduced in another way into the mass of the matrix, the 
thread distribution pattern, the nature of the thread impregnation resin, 
the ways and means for placing and polymerizing this resin, as well as the 
shapes, dimensions and intended location of the elements able to be made 
of this reinforced ablative material.