As threats increase in number and sophistication, Raytheon's missiles must also evolve to meet increasingly demanding requirements, and must do so cost effectively. The missile airframe, a major factor in performance and production cost, has benefited from a new Raytheon development approach.
To identify weight and cost reduction opportunities, component requirements are considered collectively as a system before component specifications are generated and flowed down to subject matter designers. This less regimented approach enables creativity and innovation to be achieved via multidiscipline requirements analysis; technology studies; and research into state-of-the-art (SOTA) developments from university, industry, government laboratory, and foreign sources. Certain industries — automotive, sports, watercraft, aviation, and satellites — can also inspire new solutions.
The critical feature of this strategy is to identify new applications for existing design and manufacturing solutions.
Applying the Approach
Raytheon has shown that missile interceptor airframe performance can be improved and costs reduced by integrating commercially available, advanced materials and manufacturing technologies. To do this, Raytheon produced advanced composite airframes for several missile programs.
SOTA supersonic missile airframes (Figure 1) typically involve exotic refractory materials and processing, and complex manufacturing and assembly processes, both of which incur high risk and expense. Although composite materials can be used to achieve specific performance requirements on airframe programs, until recently the precise integration of the technology or process know-how was not well understood. A research database established at Raytheon, after many system trade evaluations, has led to advanced airframe concepts; and Raytheon-initiated feasibility studies have led to innovative, low-cost airframe designs and a philosophy for integrating them into advanced missile systems.
The approach is to simplify manufacturing and consolidate parts by using composite material fabrication techniques. For example, substantial missile production cost savings result from integrating fuselage structures with components traditionally incorporated onto a vehicle as a secondary process. Consolidating common features and integrating fabrication steps simplify the design and streamline production. Product reliability and repeatability are also enhanced.
This approach also improves material efficiency, providing multifunctional airframe capabilities. Experience has shown that part consolidation leads to features of one component augmenting features of another component. For example, using a radome/ thermal protection system (TPS) continuous wrap not only provides a seal, but improves structural integrity. Features of an integral composite design are driven to be multifaceted; hence, redundant details are eliminated, airframe performance is robust and fabrication becomes more efficient. Moreover, as numerous components are integrated into the composite structures, fabrication processes and quality inspection steps previously done in parallel are integrated into a minimal number of manufacturing processes
Here are two developments that benefited from this approach.
Integral Missile Radome-Seeker Airframe (IMRSA)
SOTA tactical missile forebodies have typically incorporated ceramic radomes, metallic fuselages, and ablative TPS overwraps with numerous cut-outs and joint area reinforcements for side-viewing antennas and radomes. Some design features can be problematic, however. Joint O-rings and silicone beads that seal the SOTA forebody from external environments can allow moisture to leak into the internal electronics, hastening degradation. Teflon-based side-looking radomes have thermal limitations and are heavy. Metal fuselages with external ablative TPS laminates are heavy, expensive and incompatible with radomes. Bonded side-looking radomes can fail during flight.
To eliminate these issues, the IMRSA fore-body integrates the radomes, fuselage and TPS with a single high-temperature resin, but with different fibers for radio frequency (RF) transmittability, structural integrity and thermal insulation (see Figure 2). The external glass or quartz laminate layers perform multiple functions — as the forward- and side-looking radomes and the fuselage TPS — without breaking the external surface continuity. The internal graphite-reinforced laminates provide the load-carrying structure and internal mounting surfaces for the antenna trays and electronic assemblies. The IMRSA minimizes the need for fasteners, bonded joints and antenna cut-outs, providing greater environmental isolation to seal sensitive, active RF components while also providing greater load-carrying performance. Automated manufacturing processes, including resin-transfer-molding (RTM), filament winding or tape placement techniques, provide greater quality and repeatability. Because a single high-temperature resin is used throughout the coupled laminate structure, the entire IMRSA can be cured as a single piece, significantly reducing cost and weight, simplifying manufacturing, and improving structural integrity and production reliability.
Active Damped, Piezoelectric Composite Structures (ADPCS)
ADPCS uses commercially available technologies, found in the sporting and remote sensor industries, to preserve accurate inertial missile guidance by decreasing missile seeker loads and stabilizing inertial measurement units (IMUs). Missile vibration loads from captive-carry aero-buffeting during aircraft carriage, and shock loads from stage separation and rocket motor ignition may harm guidance functionality and reduce probability-of-kill (Pk) performance. Raytheon therefore investigated ways to reduce these vibration loads.
SOTA missile applications involve complicated mechanical shock absorption systems that are hard to design and to dynamically characterize. In most cases, the surrounding structures must be redesigned to accommodate the shock absorption systems.
ADPCS (Figure 3) integrates lightweight, power-generating piezoelectric fibers into composite structures for reliable environmental attenuation. This approach is easily characterized and can be electronically modified for any dynamic "tuning"; a major mechanical redesign is not needed. The piezoelectric current, generated during vibration, flows into a self-powered integrated circuit, is reconverted and supplied to the fibers to dampen the structure per the desired, pre-programmed frequencies.
A Final Word
A system emphasis on using common composite material systems, industry standard processing and multi-supplier availability is key to the success of this strategy. Moreover, because missile airframe applications are a small market fraction of the composite manufacturing industry, materials and processes dominated by other market applications must be used to ensure lower costs and reduced risk.