Mission Challenges Spur Next Generation Missile Radome Materials Innovations
Despite their seemingly simple shapes, radomes are complex components that have competing design requirements. The radome forms a part of the aerodynamic structure of the missile, and must be able to support the aerodynamic and structural loads placed upon it. A radome must protect the sensitive guidance and electronic components from the captive carry or flight environment (e.g., sand, rain, hail), while resisting high flight temperatures. A radome must also provide an environment that the seeker can operate in, which may include low levels of moisture or a prescribed operating pressure. And, of course, it must be transparent to the radio frequency (RF) wavelengths of interest to meet the performance objectives of the missile. Finally, a radome is no different than any other missile component in that it must be affordable and lightweight — particularly important for a radome since it is located so far from the missile's center of gravity.
A successful radome design must optimize the balance of competing requirements. For example, although higher frequencies or multiband performance would favor thinner radome walls, thicker walls are desired to enable the missile to endure the higher speeds and the larger aerodynamic forces planned for next-generation missiles.
The ideal material for a radome has the following properties:
- It has a low dielectric constant and loss tangent, both of which are constant over the temperature range of interest.
- It is strong enough to sustain structural, aerodynamic and aerothermal loads, as well as resist impacts from adverse environmental agents over all speeds and flight durations.
- It is lightweight and affordable.
- It is impermeable to water and able to support a pressure differential between the radome's interior and exterior.
- It has consistent material properties from radome to radome and is easily manufactured.
The two materials that currently meet many, but not all, of the ideal characteristics described above are Pyroceram® and slip cast fused silica (SCFS).
Pyroceram (a glass-ceramic material with cordierite as its main crystalline phase) and SCFS (a porous material comprising small grains of silica glass sintered together) are currently used at Raytheon for high-temperature missile radomes. Pyroceram is strong and impact resistant, has good thermal shock resistance, is water impermeable, and has a reasonably low dielectric constant and a low loss tangent. SCFS has a low dielectric constant and a low loss tangent over a very wide temperature range, as well as excellent thermal shock resistance and low thermal conductivity.
New mission profiles for the next generation hypersonic interceptor missile push the radome requirements into trade spaces where Pyroceram and SCFS will not meet the system performance. The main disadvantages of Pyroceram are that the dielectric constant and loss tangent increase with temperature, and the changes become great enough at temperatures above approximately 800°C that compensation to account for boresight error is no longer accurate. The thermal shock resistance of Pyroceram is also limited and would not support the expected thermal profiles for planned new rocket motors. Thermal shock stress may be mitigated somewhat by controlling the flight speeds or flight profile, especially during the initial launch portion of the flight, but this is not a desirable alternative.
The main disadvantages of SCFS are the porous nature of the material and its mechanical properties. The mechanical properties of SCFS limit the aerodynamic forces that can be imparted to the radome, as well as the radome's performance when exposed to environmental agents, particularly rain. SCFS is not fully dense and can transmit water vapor readily from the atmosphere into the interior of the radome. Attempts to create a "hermetic" SCFS radome have not been entirely successful.
The drawbacks of Pyroceram and SCFS have led to research and development activities at Raytheon, and at several radome suppliers, with the goal of producing a material that meets all of the requirements imposed by a next generation high speed, all weather missile. Figure 1 shows the expected thermal environment that the radome must withstand based on the speed and altitude of a missile. As the speed increases, and the altitude decreases at a given speed, the expected temperature that the radome will be exposed to increases. The high temperatures that next generation missiles must endure limit the choice of available materials to ceramics or ceramic matrix composites. There are several materials being developed that may offer the potential for an improved radome. None of these yet meet all the desired requirements of an ideal radome. Three of them are based on composite technologies, and one is a monolithic ceramic material.
Polysiloxane is a polymer resin containing silicon that converts to silicon dioxide when exposed to high temperatures in an oxygen-containing environment. It can be used with quartz fibers to create a composite, and it can be processed in a number of different ways, like traditional organic composites. It has good dielectric properties and reasonable mechanical properties, but unproven high temperature, rain impact and hermetic performance.
A second intriguing material is a ceramic matrix composite (CMC) with an aluminophosphate matrix and alumina fibers.The matrix phase is a very stable high-temperature amorphous material that bonds well to the fiber reinforcement. This material has reasonably good dielectric and mechanical properties, but with yet unproven rain impact and hermetic performance ceramic.
A new oxide-based CMC based on a radome material used in production of the Advanced Anti-Radiation Guided Missile uses alumina fibers in an alumina matrix. This CMC overcomes the traditional challenge of fiber matrix interface issues by using nearly identical materials for both. It has reasonably good dielectric properties and good mechanical properties, but its cost may be high and its producibility and hermeticity are currently unproven.
Raytheon is developing a monolithic ceramic material called reaction bonded silicon nitride (RBSN) for high-temperature missile radomes. It is produced by forming fine grains of silicon in the shape desired, then carefully converting it to silicon nitride through an extended heating cycle in a nitrogen atmosphere. This is accomplished without causing significant changes in shape or dimensions. The resulting product is a strong, fracture-resistant material with excellent rain impact and thermal shock resistance. The material is porous, and due to this porosity has an effective dielectric constant similar to Pyroceram, which is adjustable by controlling the material's overall density. Since it is porous, RBSN requires a coating to provide hermeticity. Raytheon is developing several different methods to form the silicon nitride, including isostatic pressing and injection molding of the silicon powder.
Since it is likely that no material, by itself, can satisfy all of the desired radome requirements, the next-generation radome will likely be a material system composed of several technologies. Raytheon's system approach to developing such a high speed missile radome is illustrated in Figure 2. The radome will be based on reaction-bonded silicon nitride, as it can withstand the high temperatures, adverse environmental agent impacts and structural requirements. Hermetic coatings will be applied to limit the water vapor permeation for the expected environmental conditions over the life of the missile.
The expected temperatures that a high-speed missile radome will experience are above 1,000°C with flight times of several minutes. Heating of the entire radome will occur, which in turn will heat the interior space within the radome. Therefore, an insulation or thermal protection system will be needed in the radome interior to limit heat flow from the radome to the seeker. The insulation must be RF transparent and not produce debris during use, which would impair the performance of the seeker assembly.
To accommodate missiles that operate at multiple frequencies, properly designed tuning features can be incorporated into the radome structure to control its electrical characteristics. This will allow any combination of RF frequencies to effectively transmit through the radome to successfully guide the missile to its target.
Drawing on more than 50 years of radome development, design, manufacturing and fielding experience, Raytheon is developing radome materials, hermetic coatings, RF-transparent insulations and tuning features to develop and produce the best and most affordable radome assemblies that will meet the needs of our customers for the next generation of high speed missiles.
Approved for Public Release 12-S-2070 (23May12)
W. Howard Poisl, Christopher Solecki,
Joseph M. Wahl
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