"Rocket" uses a multi-tubular boiler design. Previous locomotive boilers consisted of a single pipe surrounded by water (though the "Lancashire Witch" did have twin flues). "Rocket" had 25 copper fire-tubes that carry the hot exhaust gas from the firebox, through the wet boiler to the blast pipe and chimney. This arrangement resulted in a greatly increased surface contact area of hot pipe with boiler water when compared to a single large flue. Additionally, radiant heating from the enlarged separate firebox helped deliver a further increase in steaming and hence boiler efficiency.