The objective of this project was to evaluate methods to improve the CH-53E Sea Stallion transmission housing service life and life-cycle costs by focusing on production yield, corrosion and maintainability.
Traditional magnesium transmission housings require replacement or intensive maintenance as often as every three to five years, mainly due to corrosion damage. Although corrosion-resistant coatings are used, they are often damaged during operation and maintenance.
Other alloys commensurate with magnesium that have a low corrosion rate lack ballistic strength and inflict a substantial weight penalty of 30%. For example, if you directly replace the magnesium with these corrosion-resistant alloys in one dozen castings, a weight penalty of several hundred pounds would ensue.
Cost avoidance would result from reduced cost of replacement transmission housings, including the cost of the housings themselves and the labor and out-of-service time required to remove, test and replace the housings. Additional cost avoidance would result from reduced removal of transmissions for corrosion repair.
Implementation is not planned for the CH-53E based upon the time and cost to complete prototype development (2011), non-recurring engineering and qualification of the alternative housing design (2014), and initiate acquisition (2015) relative to the anticipated fleet retirement, which is to begin in 2020. Therefore, the project terminated with the completion of Phase I. Phase I provided an assessment of corrosion-resistant aluminum alloys relative to the baseline magnesium housings for performance, weight and cost. Phase I also provided an assessment of available manufacturing processes based upon lead time and cost. Two aluminum alloys, A356 and E357 (non-beryllium), and a bottom pour sand casting process using rapid mold manufacturing was selected as candidate replacements for the baseline magnesium housing based upon life-cycle structural requirements, weight and geometry requirements, and life-cycle cost-reduction goals. Preliminary material data and the successful full-size trial casting of the tail rotor center housing provides a basis for consideration on other aircraft having magnesium cast components that are subjected to corrosive environments.