Conceptual design of CFRP sandwich structures interfacing cryogenic tankages in rockets

Interstage and intertank structures of rocket's cryogenic stages face a critical design challenge due to the large thermal contractions in metallic cryogenic tanks after propellant filling. Existing design concepts such as metallic skirts, truss designs, corrugated shells, and special brackets are not mass-efficient. Their direct attachment to cryogenic tanks, combined with the inherently high thermal conductivity of metals, also causes significant heat leakage and necessitates additional thermal insulation. Overall, these configurations result in high mass penalties and reduced efficiency. To overcome these limitations, the present study proposes a Carbon Fiber Reinforced Plastic (CFRP) sandwich composite structure featuring specially designed slits near the tank interfaces. These slits enable controlled deformation and effective stress relief. Furthermore, to minimize the transfer of cryogenic temperatures into adjoining structural regions, thermally insulating Glass Fiber Reinforced Plastic (GFRP) end attachments are integrated into the sandwich configuration. Analytical design studies were carried out using MATLAB-based codes to establish the basic sandwich shell design, including the number of face skin layers, layup orientation, and core thickness. Parametric investigations using simplified shell element models were performed to systematically evaluate the effects of slit number, orientation, and dimensions, leading to optimized configurations. Additionally, detailed three-dimensional sector models incorporating cryogenic tanks and interface bolts were developed using solid elements. These refined models validated the adequacy of the proposed concept. Results demonstrate that incorporating slits and GFRP end attachments enables direct interfacing of interstage and intertank structures with cryogenic tanks, which is not feasible with conventional approaches. Proof-of-concept hardware studies further confirm a 35% improvement in mass efficiency compared to existing CFRP truss-based and metallic bracket-based designs. The combined use of structural slits and thermally insulating attachments thus offers a novel and efficient design strategy for next-generation lightweight cryogenic stage structures.