In the Secondary Fluid Recuperator (SFR) concept, two heat exchangers are installed inside the aero engine core. The first-one (cold-SFR, blue colour) is installed right before the combustor and the second one (hot-SFR, red colour) is installed downstream the low pressure turbine (LPT), inside the hot-gas exhaust nozzle. The two SFR heat exchangers are linked together by a separate internal closed circuit which is occupied by a secondary working fluid (i.e. liquid metal), that flows independently from the core engine air and gas flow, and combines favourable heat transfer and pressure loss characteristics. The design of SFRs is based on circular tubes placed in a counter-flow staggered arrangement. Each SFR is optimized independently based on available space and operational conditions for achieving high effectiveness and low pressure losses. Heat is transferred with the SFRs from the exhaust hot-gas region after LPT, to preheat the cold air before the combustor, resulting to significant reduction of fuel consumption and pollutant emissions.