High clarity and concentricity are necessary features of aluminum oxynitride multimode seeker domes being developed for the Joint Air-to-Ground Missile. Credit: Surmet.
The business end of a missile is not the exploding part. Rather, it’s the guidance system that gets the missile to its target, or at least that’s what the guys who make the front end of the missile will tell you.
A critical piece of the guidance system is the dome on the leading tip of the missile. Multimode seeker domes are passive components (they don’t “do” anything), but they must meet stringent property and tolerance requirements. Multimode seeker domes are so named because they are optically transparent in the laser, IR and RF wavelength ranges.
There is significant interest in these domes because the Army and Navy have been developing a new weapon, the Joint Air-to-Ground Missile. In particular, aluminum oxynitride or spinel are candidate materials for JAGM dome systems.
The JAGM is being developed to replace the Hellfire, Javelin and TOW missiles according to an Aviation Week Aerospace Daily and Defense Report story. The military is designing the new missiles for air launch from various fixed-wing or rotary-wing aircraft mounts—six platforms in total.
Lockheed Martin and Raytheon-Boeing have been competing for the JAGM engineering and manufacturing development contract. An InsideDefense.com story reports that the Pentagon was planning to spend $1.7 billion for development and $6.5 billion to procure 20,000 missiles for the Army and 15,000 for the Navy/Marine Corps.
But, back in September, just when DOD was getting ready to decide which company would get the nod, the Army and Navy recommended terminating the program, apparently in response to budget constraints forced by the August debt ceiling agreement. Details are vague, but it looks like the JAGM program may have been downsized to an extended R&D program. At the very least, the Army is still awarding SBIR contracts, including one recently to Surmet of Burlington, Mass. to develop cost-effective multimode seeker domes.
According to Surmet chief technology officer, Lee Goldman, fabrication of domes is very costly because of the combined demands for very high optical clarity, homogeneous index of refraction and tight tolerance specifications.
Domes are made from powders pressed into net shape and fired. After firing, the domes are 100% dense and optically transparent, but the surfaces are rough, like ground glass. Surfaces are ground and polished to a mirror smooth finish. Also, the finished domes must have a high degree of concentricity to prevent distortion of the transmitted wavefronts.
“It is common practice in the industry, because of the tight tolerances, to do an inspection polish,” according to Goldman to evaluate the optical quality. Between the hardness of aluminum oxynitride or spinel and the extremely tight dimensional tolerances, inspection polishing adds significant cost to the domes.
Not surprisingly, the military is interested in know whether any cost-savings is possible. Thus, the goal of the Army SBIR is to develop a nondestructive evaluation method to assess the optical quality of the domes prior to inspection polishing. By doing so, the company believes it can reduce the cost of domes by up to 20%. There is a lot of potential savings because if the JAGM goes into production, 30,000-40,000 domes would be needed. With the NDE method, a representative sampling of domes could be testing quickly and cost-effectively.
Surmet has been working with the Army since 2002 on a variety of transparent ceramics for military applications.