he massive cylindrical structure teased by Musk earlier this month is most likely a mandrel, a tool that can be spun on its horizontal axis to weave predetermined structures. In the case of the Mars rocket mandrel, it will likely be used to carefully wind dozens or hundreds of layers of carbon fiber (known as prepreg), interspersed with layers of laminate and various epoxies and resins. It’s also possible, however, that the massive tool is instead a multipurpose mold and autoclave, where the composite layers would be lain on the inside of the cylinder, allowed to set, and eventually sealed inside and heat/pressure treated.
Images of the machinery are fairly ambiguous: they show a structure that could have connection points one might find on an autoclave, as well as what appears to be a thick and well-insulated internal wall. However, the external skin appears to be a relatively thin sheet of metal, which would point more towards a traditional composite mandrel, where certain sheets could be removed or modified as needed to create desired shapes in the composite while it’s being formed, less risky than machining a completed segment.