This innovation integrates existing highperformance metallic materials and manufacturing technologies (all of which are now certified and used to produce thinner stiffened panels for launch vehicle structures) in a novel manner to allow fabrication of more structurally efficient panels with stiffeners that are substantially deeper than existing plate stock materials.
First, a pattern of stiffener stubs is machined into currently available plate stock, along with the desired panel skin thicknesses. These partial-depth stiffener stubs should have the maximum height possible, up to the raw plate thickness. So far, the fabrication process is essentially identical to that currently used to make the Shuttle LH2 tank barrel panels or similar structures, with comparable scrap rates and machining times.
However, in this new process, the next step is to extrude or stamp stiffener caps from compatible materials. These stiffener caps would have whatever optimal shapes (e.g. blade, J, T) and dimensions that are predicted from structural design and sizing analyses. To complete panel fabrication, friction stir welding (FSW) is then used to permanently join the stiffener caps and stubs. The completed stiffened panel now has the desired full-depth stiffener cross-sections that are much deeper than the original plate stock used for fabrication of the panel skin and stiffener stubs. The completed stiffened panel also has a higher structural efficiency than would be possible using the original thinner plate stock thickness, without the waste and inefficiency of machining it from a much thicker plate.
Instead of relying only on axial stiffeners for optimal structural efficiency, an isogrid or orthogrid stiffener pattern may also be desirable. If so, these additional transverse stiffeners would be machined into the plate stock at the same time as the partial-depth axial stiffener stubs. To compensate for their reduced height, thicker or differently spaced transverse stiffeners could be incorporated, depending on the structural analysis results. These transverse stiffeners would have to terminate some distance below the intersecting axial stiffener stubs, thus allowing for clear passage of the FSW head during continuous joining of the axial stiffener caps and stubs. Any reductions in FSW head dimensions and clearances would permit corresponding increases in transverse stiffener height.