Towards measurements of CMB spectral distortions

High-precision spectroscopy of the Cosmic Microwave Background (CMB) is one of the three themes that have been selected by the ESA Voyage 2050 programme. Several space missions dedicated to the measurement of the CMB spectral distortions such as PIXIE, PRISTINE, and FOSSIL have been proposed since 2011. Additionally, balloon-borne projects, such as BISOU, have also been considered as a pathfinder for a future space mission. This paper presents an effort undertaken to define future missions and instruments dedicated to the measurement of the CMB spectral distortions. For this purpose, models allowing to optimize both instrument concept and mission parameters (mission duration, scanning strategy, ...) have been developed. It has been applied to two specific cases, FOSSIL, a space mission proposed to answer the ESA M7 call in 2022, and BISOU, a CNES Phase 0 study for a balloon-borne mission.