Snake Paint-R: Robotic Coating for Confined Aircraft Ducts

Modern F-22 fighters use long, twisting Environmental Control System (ECS) and Air-Cycle Flow-Control (ACFC) ducts that carry conditioned air through the airframe. The openings are only 3–4 inches wide, yet the passages run 12–20 feet and turn through several tight 90-degree elbows. To stop corrosion and preserve radar-absorbent finish, every internal surface must receive both primer and top-coat. Maintainers now push a hand-held spray wand through the inlet, judging distance by feel, watching a borescope screen, and marking the hose with tape to guess where the tip is. Runs, sags, and thin spots are common, so jets often move to an off-station depot for stripping and rework. The task ties up aircraft, costs hundreds of labor hours per ship-set, and places technicians in confined-space, Class I Division 1 fuel-vapor environments where powered devices are normally forbidden.Snake Paint-R removes these limits by adapting aRoboticsCompany’s proven Clean-R GP1 cable-driven inspection arm into an intrinsically safe painter. All motors and electronics stay outside the duct; motion travels through steel tendons that slide inside a one-inch-diameter flexible spine, much like a surgical robot but built for industrial duty. At the tip, a pneumatic rotary-union head spins an airless nozzle and lays a continuous 360-degree film while a motorized rail feeds the snake forward or pulls it back. A desktop “digital puppet” controller lets one technician guide a teach-in pass; the system then repeats that path with millimeter accuracy, no robot code required. Because the body carries no ignition sources, the design meets Class I Div 1 rules and can work in fuel-vapor spaces. Phase I, a six-month effort, will prove technical and economic feasibility. The team will start by capturing detailed duct geometry, film-thickness targets, and safety criteria with the Air Force engineering authority, then flow those into system requirements. Engineers will size cables, spools, bearings, and explosion-proof drive motors; analyze tendon loads, backlash, and minimum bend radius; and design the pneumatic spray head with interchangeable tips for primer and top-coat. Parallel work will tune rotary speed, pressure, and flow windows for MIL-PRF-23377 primer and other top-coats, and develop a flush cycle that lets the same head switch materials without disassembly. The article will then run in a transparent mock-up of an F-22 duct to measure coverage, film-thickness uniformity, and cycle time. Success is defined as navigating the full path, holding ±3 mm standoff, and depositing a uniform 1.2–1.6 mil film that passes adhesion tests. Final deliverables will include test data, refined analytical models, a hazard analysis, and a TRL5 design ready for fabrication in Phase II, positioning the Air Force to replace slow, manual spray wands with a repeatable, intrinsically safe robotic process that returns jets to the flight schedule faster and at lower cost.