The Athena mission in general and the x-ray integral field unit (X-IFU) instrument in particular are designed to address a wealth of scientific questions related to the science theme of the hot and energetic universe.

X-IFU provides medium spatial resolution and high resolving power by means of a calorimetric detector.

As the X-IFU detector needs to be operated at 50mK, the instrument is contained in a dewar. The aperture cylinder consists of a set of structural and thermal elements that carry and position at correct distances from the detector the first three dewar thermal filters, provide adequate thermal interfaces and protect the filters from contamination.

In this paper, we present the Phase A-B1 contribution of Centre Spatial de Liège (CSL) to the X-IFU consortium. This summarizes to the elaboration of a baseline design for the aperture cylinder, and several demonstration models as a de-risking activity with the intent to increase the aperture cylinder maturity.