Reusable launch vehicles provide a cost-saving solution for access to space, compared to expandable staged rockets. Their advantages are spread even further with the integration of advanced nozzle concepts. The altitude adaptation capability of nozzles has been sought since the beginning of rocket science. The aerospike engine comes as one of the best solutions. In this work, the model of an annular aerospike engine is designed and developed with the aim of a comparison with an octaweb configuration of conventional bell nozzles. The test case for the following experimental campaign is identified from the subsonic landing burn of the Horizon 2020 RETALT project, defining the same characteristic parameters of retro-flow phenomenology described in detail in this work.