<div class="htmlview paragraph">The European Space Agency Far Infra-Red and Sub-millimeter Space Telescope (FIRST) is one of the four designated “Cornerstone” missions in the long term science plan “Horizon 2000”. FIRST is intended to open up the last major part of the electromagnetic spectrum, the sub-millimeter and far infra-red range, which is still mainly inaccessible to observational astronomers.</div> <div class="htmlview paragraph">In 1992 an industrial consortium led by Dornier was tasked with developing a spacecraft concept for a payload comprising one Far Infra-Red (FIR) direct detection instrument and one Multi-Frequency Heterodyne (MFH) instrument.</div> <div class="htmlview paragraph">In order to minimise the launch mass, a mechanically cooled concept was proposed in which the payload is cooled to 4K using the range of mechanical coolers originally developed by the Rutherford Appleton Laboratory (RAL) and industrialised by jointly funded ESA/BAe contracts. Cooling below 4K (down to 0.15K) required for the FIR instrument detectors is provided as part of the instrument by a Benoit Helium Dilution Refrigerator.</div> <div class="htmlview paragraph">This paper describes the concept development of the FIRST Cryogenic Cooling Sub-system and addresses the problems associated with the integration of such a complex cooling system and the proposed solutions.</div>