Antenna window assembly for ablative heat shields

An antenna window for ablative type heat shield where the forward edge of the window is below the level of the heat shield and the trailing edge extends above the surface of the heat shield creating abrupt steps in the overall surface, precluding severe local material loss due to differences in ablation rates of the window and surrounding heat shield material.

BACKGROUND OF THE INVENTION 
This invention relates generally to ablative type heat shielding and more 
particularly to a window for such shielding. 
In space and other vehicles utilizing ablative heat shielding, it is 
frequently necessary to transmit and receive information and data via 
radio frequency instruments aboard the vehicle. These antennas must be 
provided with a window which will allow them to function while still 
maintaining the integrity of the heat shielding. Without such window, 
electromagnetic radiation to and from the antenna would be greatly impeded 
or blocked by the high density ablation heat shield. 
Varying density in the heat shield by the incorporation of windows or other 
devices necessarily tends to vary the ablation rate of the shield and in 
particular in those areas where substitute materials are used. In regions 
of varying ablation rates, material loss along the interface is severe and 
hence steps must be taken to prevent failure of the heat shield. 
One approach, to solve the problem of severe local material loss due to 
differences in ablation rates, has been the incorporation of a thicker end 
grain heat shield collar around the periphery of the window to allow 
extensive trailing edge ablation. This method has advantages with regard 
to the trailing edge but fails to correct deficiencies in the forward edge 
interface. Additionally, the collar is difficult to assemble in the heat 
shield and increases its cost. 
SUMMARY OF THE INVENTION 
The invention provides a simple and inexpensive means to preclude severe 
local material loss due to differences in ablation rates of antenna window 
material and the surrounding heat shield material. 
The heat shield is constructed with an appropriately shaped aperture 
consistant with the requirement for the antenna or other device whose 
needs the window is intended to serve. The window is placed in the 
aperture and so machined to provide a tight fit. It extends through the 
heat shield and is secured to the heat shield support by an appropriate 
mechanical assembly. The window may be constructed of fused silica and 
assembled with the antenna located therein for ease of assembly with the 
heat shield and improved radio frequency reception. 
The surface of the window subject to the ablating forces follows the radius 
of the heat shield in a direction transverse to the forces. In a direction 
along an axis parallel to the force of ablation, the forward edge of the 
window is depressed below the surface of the heat shield, forming an 
abrupt, rear facing step at the juncture. The rear or trailing edge of the 
window is raised to provide another abrupt step between the window and the 
heat shield. 
It is therefore an object of the invention to provide a new and improved 
window for ablative heat shields. 
It is another object of the invention to provide a new and improved window 
for ablative heat shields that prevents excessive heating at the leading 
and trailing heat shield-window interfaces. 
It is a further object of the invention to provide a new and improved 
window for ablative heat shields that is more easily assembled than any 
hitherto known. 
It is still another object of the invention to provide a new and improved 
window for ablative heat shields that is transparent to radio frequency 
electromagnetic energy. 
It is still a further object of the invention to provide a new and improved 
window for ablative heat shields that prevents excessive material loss due 
to differing ablation rates. 
It is another object of the invention to provide a new and improved window 
for ablative heat shields that is lower in cost than any known similar 
device. 
These and other advantages, features and objects of the invention will 
become more apparent from the following description taken in connection 
with the illustrative embodiment in the accompanying drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1 and the window and its securing assembly are shown 
generally at 10. The window 12 fits into an aperture in the heat shield 14 
supported by its backer 16. The window is held in position by a pair of 
retainers 18 and 20 mounted one on each side of the window and having a 
metal strap 22 connected therebetween. The strap 22 takes the general 
shape of the window 12 and through the appropriate cushioning, material 24 
applies a force against the window as the threaded fasteners 26 and 28 
take up on their respective retainers 18 and 20. The said fasteners 
transmit the force applied through the doublers 30, 32 which are held by 
rivits (34, 36) or otherwise securely attached to the heat shield backing 
plate 16. 
The window 12 is provided with shoulders 38, 40 which abutt the appropriate 
doubler and hold the window secure as the threaded fastener applies 
pressure through the strap 22. Although the window is so constructed to 
provide a close fit with the heat shield, an appropriate silicon sealant 
42 is placed around and between the periphery of the window and heat 
shield, doubler and retainer. 
An antenna 44 may be fabricated within the window and supplied with a 
connector means 46. 
Air flow across the heat shield is indicated by the arrow 47. The forward 
edge of the window 48 is depressed below the level of the heat shield 14 
to provide an abrupt, rear facing step whereby air flowing across the heat 
shield will strike the window a slight distance from the actual interface. 
Similarly, the trailing edge 50 of the window 12 is raised above the level 
of the heat shield 14 providing an abrupt step causing the air to strike 
the heat shield a small distance from the window-heat shield interface. 
The diversion of the air or other gas will preclude severe local material 
loss due to differences in ablation rates of the two materials. 
It should be understood, of course, that the foregoing disclosure relates 
to only a preferred embodiment of the invention and that numerous 
modifications or alterations may be made therein without departing from 
the spirit and scope of the invention as set forth in the appended claims.