Evaluation of ‘Design for Assembly’ Performance of Undercut Finocyl Grain Casting Mandrels

Advanced space launch vehicles that employ solid rocket boosters are found to use undercut finocyl-shaped propellant grains. The undercut features, however, cannot be easily manufactured using conventional methods of casting propellant around rigid monolith mandrels or by machining the fin slots. To solve the problem, many non-rigid type mandrels were experimented with without wide acceptance due to functional, performance, or safety issues. Alternatively, in recent times, a few types of rigid undercut finocyl grain mandrels have been developed by different space agencies. While each claims superior performance, there has been no study measuring or quantifying their relative performance. The purpose of this study was to evaluate and compare the assembly performance of a new mandrel called ‘NEX’ using Design for Assembly (DFA) tools against state-of-the-art Vega mandrels employed in the making of some of the largest and highest-performing solid boosters worldwide. Mandrel parts and their assembly sequences were reconstructed using patent literature and published data. Boothroyd-Dewhurst DFA Product Simplification software was used for analyzing both mandrels and quantifying the difficulty of total assembly and disassembly. It is found that in comparison with Vega, NEX has 60 % fewer part counts, 80 % fewer fastener activities, and 41 % reduced assembly man-hours. The estimated DFA index for Vega is 3.57 and NEX is 5.78. Using the newer and safer mandrel, space launch vehicle booster manufacturing costs can be lowered.