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C. Low Observable Axisymmetric Nozzle Before the development of the F-35 low observable axisymmetric nozzle (LOAN), signature demands typically drove nozzles to fixed, structurally integrated affairs (e.g., F-117). They had to have very high aspect ratio designs (e.g., F-117) or highly capable but heavy two-dimensional systems (e.g., F-22), as illustrated in Image 1. Departing from what was then the state of the art, industry and CRAD efforts developed multiple nozzle configurations to create a LOAN for the F-35. The F135 engine with a LOAN balanced the requirements of LO and efficient aeromechanical performance. This resulted in a lightweight configuration with reduced radar cross-section. Image 1 A spectrum of fighter aircraft nozzle designs. 1. Background The F-16’s and F-15’s exhaust systems are examples of what was used in classic 4th Generation tactical supersonic aircraft. These systems are axisymmetric for low weight and structural efficiency, with variable geometry to maintain stable and efficient engine operation. Specifically, afterburning requires a large increase in the minimum nozzle flow area (throat area) to retain engine stability. This is particularly necessary when fuel is being dumped into the exhaust and the resultant flow density decreases. These convergent/divergent nozzles are composed of overlapping flaps and seals. The exit area and throat area mechanically linked and scheduled with a power setting for efficient flow expansion. Externally, overlapping flaps provide a fairing between the air vehicle aft body and nozzle exit for reduced drag. However, the nozzles were not generally considered capable of meeting LO requirements. The subsonic F-117 stealth fighter and B-2 bomber exemplify how exhaust system designs can be dominated by LO features. This effect results in nontraditional exhaust systems that are driven by air vehicle shaping. The F-117 incorporated an airframe-mounted, structurally integrated, fixed exhaust system with planform-aligned edges. The exhaust system transitions from axisymmetric to a high-aspect two-dimensional design and forgoes the variability needed to accommodate afterburning. Although these highly integrated designs can reduce drag, they are less aerodynamically and structurally efficient than axisymmetric designs. IRAD and CRAD investments were critical to positioning the industry to develop world-class solutions targeted at major program innovations. Well before the JSF X-planes were contracted, multiple vertical lift and exhaust system configurations were developed and matured. These were then available on call to meet the evolving needs of both new and existing aircraft designs. Some of the nozzle technologies explored were conformal fully fixed aperture nozzles, fixed aperture nozzles, and LOANs. Complexly shaped, fixed-geometry nozzles were matured for reduced drag and signature. These designs allowed flexibility to implement full line-of-sight obscuration. With few moving parts, they allowed for a great diversity of LO and thermally tolerant materials. Innovations in convergent section mechanical manipulation and shaping were explored as well. These were intended to reduce leakage, weight, and structural integration penalties while enabling the nozzle exit to remain motionless. Throat skewing and inducing shocks in the divergent section were assessed for thrust vectoring potential. Variable cycle engines were evaluated to reduce the need for large-scale nozzle throat area control, even during augmentation. During this period the notion that axisymmetric nozzles were not amenable to LO (or even thrust vectoring) was being challenged. A new generation of LO nozzles emerged that was characterized by shaping features to minimize radar reflections. Namely, they used a serrated trailing edge, serrated interface with the airframe, and interleaved external seals to complement the external flaps. They retained tight control of gaps and seals and had specialized high- temperature coatings on internal and external surfaces. In the years leading up to the X-35, various LOAN configurations were developed, ground tested, and flight tested on F-16s. Testing was done in concert with both Pratt & Whitney F100 and General Electric F110 engines. These versions incorporated a nod to an ejector feature that Approved for public release 5/8/18, JSF18-365 19PDF Image | F-35 Air Vehicle Technology Overview
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