Propulsion system

A Propulsion system (10) comprising: a propulsion gas turbine engine (11); and a compressor (59) that is configured to receive and compress air for distribution to an air collection and enrichment system, said compressor being part of a non-propulsive gas turbine engine (50), the air collection and enrichment system configured to generate liquid oxygen using the compressed air, said non-propulsive gas turbine engine configured to receive a flow of oxygen-depleted air (84) from the air collection and enrichment system.