| Acquisition Program: | |
Objective: | Produce an optically polished, tangent ogive, infrared-transmitting missile dome made of polycrystalline alumina.
| Description: | The fabrication of infrared-transparent polycrystalline alumina with a grain size of 0.5 um was reported in 2003[1-3]. This material has the same chemical composition as sapphire and similar physical properties. Sapphire is the most durable, commercially available infrared missile dome material. Polycrystalline alumina should be ~1/2 as expensive as sapphire and can be cast into difficult shapes, such as an aerodynamic ogive. Polycrystalline alumina is potentially superior to the competing material, aluminum oxynitride (ALON). Polycrystalline alumina has a longer wavelength infrared transmission window that does ALON and approximately 3 times greater thermal shock resistance that ALON.
In the past, infrared missile domes have always been made with a hemispheric shape because this shape introduces minimal optical distortion. The DARPA Conformal Optics program that was completed in 2000 demonstrated methods to correct the distortions introduced by aerodynamic shapes such as an ogive. The ogive offers improved aerodynamic performance compared to a hemisphere. An ogive allows some combination of increased range, speed, and payload because of reduced drag. The ogive also offers improved rain and sand erosion resistance and a greater unvignetted field of view.
GOALS: By the end of Phase II, the contract shall (1) Demonstrate the ability to fabricate polycrystalline alumina with proper grain size for casting optical quality alumina domes. (2) Demonstrate near net-shape casting of infrared-transparent polycrystalline alumina in the form of tangent ogive whose approximate finished dimensions will be a base diameter of 130mm, a thickness of 2mm, and a height of 195mm. Exact dimensions may be modified during the course of the program. (3) Demonstrate methods to grind and polish the outside and inside of the dome to an optical quality finish. An opening will be left at the nose for future attachment of a durable nose tip. Specifications for the optical figure, surface quality, surface metrics, and nose-tip dimensions will be determined in consultation with the Government during the performance period of the contract.
| | PHASE I: Demonstrate the ability to fabricate polycrystalline alumina with proper grain size for casting optical quality alumina domes. Demonstrate a process for near-net-shape casting of a polycrystalline alumina ogive dome. The infrared transmittance of polished, flat witness samples shall be within 2% of that of sapphire in the 3-5 µm wavelength region. Cast a subscale tangent ogive with a base diameter of 50 mm, a height of 50 mm, and a thickness of at least 2 mm. Demonstrate a method for optical polishing and figuring of the outside of the ogive. By the end of Phase I, develop a plan for how the inside of an ogive can be optically polished.
| | PHASE II: Demonstrate the optical polishing of the inside of the subscale ogive fabricated in Phase I. Cast 3 full scale, near-net-shape, tangent ogive domes with a base diameter of 130 mm, a thickness of 2 mm, and a height of 195 mm. Develop methods for optical polishing of the outside and inside surfaces of the full scale dome and deliver 2 fully polished, full-scale domes by the end of the contract. Provide measures of optical figure and surface quality.
| | PHASE III: Demonstrate commercial production capability for casting and polishing of full scale alumina ogive domes.
| | PRIVATE SECTOR COMMERCIAL POTENTIAL: Optical quality polycrystalline alumina has commercial potential as the lamp element for high intensity automobile headlights.
| References: |
1. A. Krell, P. Blank, H. Ma, T. Hutzler, M.P.B. van Bruggen, and R. Apetz “Transparent Sintered Corundum with High Hardness and Strength. “J. Am Ceram. Soc. 2003, 86, 12-18.
2. A. Krell, P. Blank, H. Ma, T. Hutzler, and M. Nebelung, “Processing of High-Density Submicrometer A12O3 for New Applications,” J. Am. Ceram. Soc. 2003, 86, 546-553.
3. A. Krell, G. Baur, and C. Däphne, “Transparent Sintered Sub-µm A12O3 with IR Transmissivity Equal to Sapphire, “ Proc. SPIE 2003, Volume 5078.
| | Keywords: | dome, missile dome, infrared dome polycrystalline alumina, polishing, dome polishing, ceramics, ceramic fabrication |