Aeronautics and astronautics – Aircraft structure
Reexamination Certificate
1996-09-10
2001-06-05
Mojica, Virna Lissi (Department: 3614)
Aeronautics and astronautics
Aircraft structure
C244S131000, C244S133000, C244S121000, C244S158700, C343S872000
Reexamination Certificate
active
06241184
ABSTRACT:
This invention relates to a vehicle having a ceramic radome, and, more particularly, to the attachment of the ceramic radome to the vehicle.
Outwardly looking radar, infrared, and/or visible-light sensors built into vehicles such as aircraft or missiles are usually protected by a covering termed a radome. The radome serves as a window that transmits the radiation sensed by the sensor. It also acts as a structural element that protects the sensor and carries aerodynamic loadings. In many cases, the radome protects a forward-looking sensor, so that the radome must bear large aerostructural loadings.
Where the vehicle moves relatively slowly, as in the case of helicopters, subsonic aircraft, and ground vehicles, some radomes are made of nonmetallic organic materials which have good energy transmission and low signal distortion, and can support small-to-moderate structural loadings at low-to-intermediate temperatures. For those vehicles that fly much faster, such as hypersonic aircraft or missiles flying in the Mach 3-20 range, nonmetallic organic materials are inadequate for use in radomes because aerodynamic friction heats the radome above the maximum operating temperature of the inorganic material.
In such cases, the radome is made of a ceramic material that has good elevated temperature strength and good energy transmission characteristics. Existing ceramics have the shortcoming that they are relatively brittle and easily fractured. The likelihood of fracture is increased by small surface defects in the ceramic and externally imposed stresses and strains. The ceramic radome is hermetically attached to the body of the missile, which is typically made of a metal with high-temperature strength, such as a titanium alloy.
The ceramic has a relatively low coefficient of thermal expansion (“CTE”), and the metal missile body has a relatively high CTE. When the missile body and radome are heated, the resulting CTE-mismatch strain between the radome and the missile body can greatly increase the propensity of the radome to fracture in a brittle manner, leading to failure of the sensor and failure of the missile. Such heating can occur during the joining operation, when the missile is carried on board a launch aircraft, or during service.
There is a need for an approach to the utilization of ceramic radomes in vehicles, particularly high-speed missiles, wherein the tendency to brittle fracture and radome failure is reduced. The present invention fulfills this need, and further provides related advantages.
SUMMARY OF THE INVENTION
The present invention provides a vehicle, such as a missile, having a ceramic radome affixed to the vehicle body. The attachment structure is such that the thermal straining in the radome due to thermal expansion coefficient differences is reduced or avoided. The attachment structure itself does not tend to cause premature failure in the ceramic material, as has been the case for some prior attachment approaches. The attachment may be hermetic if desired, so that the delicate sensor is protected against external environmental influences, as well as aerodynamic and aerothermal loadings. The attachment is accomplished economically, because in the preferred embodiment the two braze joints are formed simultaneously in a single brazing cycle.
In accordance with the invention, a vehicle having a ceramic radome comprises a vehicle body having an opening therein, a ceramic radome sized to cover the opening of the vehicle body, and an attachment structure joining the radome to the vehicle body to cover the opening. The attachment structure includes a compliant metallic transition element disposed structurally between the radome and the body, and having a first end and an oppositely disposed second end. There is a first butt-joint attachment between the radome and the first end of the transition element, and a second butt-joint attachment between the vehicle body and the second end of the transition element. In this context, a butt joint where loads are transferred in tension is to be contrasted with a lap joint, where loads are transferred in shear.
The butt-joints are preferably made by brazing, most preferably using an active braze alloy. The use of the expensive active braze alloy is typically required for the ceramic-to-metal seal of the radome to the transition element. It is not required for the metal-to-metal seal of the transition element to the vehicle body. However, in this case it is preferred to use the active braze alloy in the metal-to-metal seal because the active braze alloy flows only sluggishly at the brazing temperature and therefore does not flow from its initially established position in the second butt joint.
The transition element is in the form of a ring for the preferred case of the circular nose opening. In cross section, the transition element is preferably an “I” beam having a web section that operates as a compliant arm region to absorb thermally induced strains, an upper crossbar region, and a lower crossbar region which is typically of different length than the upper crossbar region. Optionally, a centering lip extends upwardly from an inside end of the upper crossbar region toward the radome and adjacent to the inside surface of the radome. The lower margin surface of the radome is adjacent to an upper side of the upper crossbar region of the “I” beam. The centering lip serves to align the radome but does not enter into the attachment function. A brazed first butt joint, preferably made of an active brazing alloy, lies between the lower margin surface of the radome and the upper side of the upper crossbar region of the transition element, but it does not lie between the centering lip and the inside surface of the radome. A brazed second butt joint lies between the vehicle body and a lower side of the lower crossbar region of the transition element.
The compliant arm of the transition element flexes outwardly and inwardly to accommodate thermal coefficient mismatch strains, which result from heating and cooling of the vehicle body and radome during processing and service. The continuous transition element structure and brazed attachments provide a strong, hermetic, and compliant support for the radome.
Lap joints are often used for joining structural elements in other applications, because they spread structural loadings over large areas to reduce the incidence of joint failures. However, the lap joint has the undesirable effect of reducing the side-look angle of the sensor. For a sapphire radome having a crystallographic c-axis lying generally perpendicular to the lower margin surface of the radome, a lap joint made to the sides of the radome may also induce premature cracking and failure of the sapphire material.
In the present approach, the carefully made first butt joint between the lower margin surface of the ceramic radome and the upper side of the upper crossbar region of the transition element provides a strong, hermetic structural bond. The butt joint is preferably made by brazing, most preferably with an active braze material. The second butt joint between the lower side of the lower crossbar region and the portion of the opening of the vehicle body that faces (but is spaced apart from) the lower margin surface of the radome provides sufficient strength but does not adversely limit the length of the web section available to flex to absorb thermally induced strains.
The present approach provides an attachment of the ceramic radome to the vehicle body that is strong and hermetic, and minimizes the effects of thermal expansion coefficient mismatches. The attachment approach does not weaken the ceramic material. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.
REFERENCES:
patent: 2937597 (1960-05-01), Winn et al.
patent: 3177811 (1965-0
Hanian Oscar O
Liguori Edward
Sunne Wayne
Alkov Leonard A.
Collins David W.
Lenzen Glenn H.
Mojica Virna Lissi
Raytheon Company
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