"Rocket"s first firebox was of copper sheet and of a somewhat triangular shape from the side. The throatplate was of firebrick, possibly the backhead too. When rebuilt around 1831, this was replaced by a wrought iron backhead and throatplate, with a drum wrapper (now missing), presumed to be of copper, between them. This gave a larger internal volume and encouraged better combustion within the firebox, rather than inside the tubes. These early fireboxes formed a separate water space from the boiler drum and were connected by prominent external copper pipes.