Control apparatus for the formation of a tube (referred to as clad) and subsequent injection of material into the tube (referred to as core) to create a unified product (referred to as preform) while in a microgravity environment. The apparatus permits control of a plurality of key variables during the manufacturing process including heating, cooling, and holding temperature in various parts of the instrument, keeping a precise rotation schedule, maintaining a dry atmosphere, managing any chemical effluent, and ensuring all surfaces are unreactive.

A computer implemented method for measuring the viscosity of a fluid stream flowing from an opening with constant acceleration, wherein the method takes as input at least one image of an area of interest of a fluid stream, the section, s.sub.0, of the opening and the output volume flow rate, U.sub.0, of the fluid stream from said opening; wherein the method provides as output the viscosity, , of the fluid; wherein the method includes (a) modelling the geometrical profile of the fluid stream trough an digital processing operation of the input image, and (b) computing the viscosity of the fluid with a mathematical or physical model from the modelled geometrical profile, the section, s.sub.0, and the output volume flow rate, U.sub.0.