Autoclavable foam core resists bucking and shortens production cycle on launch vehicle interstage

The February 26 launch of the Multi-functional Transport Satellite-1 Replacement (MTSAT-1R) communications satellite in Japan was successfully accomplished by the H-IIA No. 7 launch vehicle, developed by the Japan Aerospace Exploration Agency (JAXA). The H-IIA is Japan's heavy rocket, first launched in 2001. Many of the vehicle's components, including the engines and composite interstage sections, are manufactured by Mitsubishi Heavy Industries (MHI, Toyko, Japan).

In step with other space launch enterprises, MHI has reduced per-launch costs on its vehicle by adopting new materials and manufacturing methods that simplify composite components and reduce overall weight. An example is the vehicle's 4m/13-ft diameter cylindrical composite interstage section. Formerly fabricated from aluminum alloy, it now is manufactured as a composite sandwich laminate, incorporating high-performance, closed-cell polymethacrylimide (PMI) Rohacell foam core. Rohacell is manufactured by Degussa Rohm GmbH & Co. KG (Darmstadt, Germany).

The interstage sandwich structure is manufactured on a large layup mandrel. After placement of the carbon fiber/epoxy prepreg plies for the inner skin, precisely thermoformed segments of Rohacell WF foam are positioned over the uncured prepreg layers, temporarily held in place by the adhesive effect of the tacky prepreg and an elastic band fixture. The foam then functions as a layup mandrel for the outer skin plies. Because the foam is able to withstand the part's 180°C/358°F autoclave cure cycle, the entire part can be vacuum bagged and cured in one shot, saving several steps. (Previously, the inner skin was autoclaved, then an adhesive layer and foam core were placed and bonded. Afterward an additional adhesive layer was placed, followed by the outer skin and final cure). The foam resists compressive creep and helps ensure dimensional stability and laminate consolidation. Its high compressive strength improves the part's buckling resistance.

MHI reports that this and other manufacturing improvements implemented system-wide for the H-IIA have reduced costs by 30 percent, compared to the first generation of the launch vehicle's design.