a cellular reservoir flexible pressure vessel is formed as a series of closely packed tubes fitted into a pair of opposing end caps . the end caps have individual receptacles sized and shaped to receive the tube ends that are secured with adhesive or radio frequency welding . at least one end cap has a passageway for connection of the vessel . the vessel may be formed in a variety of useful shapes and the tubes may have various internal and external cross - sections . the end caps may be filled with sintactic foam with canals leading to the passageway . microtubes through the syntactic foam may connect the tubes to the passageway . the vessel is further strengthened by overwrapping with high - strength braiding material , hoop winding or by overlayment with high - strength fabric . the vessel is further strengthened by coating with plastic resin . apparatus and methods for forming the cellular reservoir flexible vessels are described .