A gas turbine engine comprising: a first spool comprising a first compressor rotationally driven by a first turbine via a first shaft ; a primary combustor ; a diffuser having walls that diverge towards the primary combustor , wherein the first compressor , the diffuser , and the primary combustor are arranged in series to provide a compressed airflow discharged from the first compressor to the primary combustor via the diffuser ; and a second spool comprising a second compressor rotationally driven by a second turbine via a second shaft ; a secondary combustor disposed downstream from the second compressor and upstream from the second turbine ; wherein: the second compressor is fluidly coupled to the diffuser to receive at least a portion of the compressed airflow from the diffuser ; the second compressor , the secondary combustor , and the second turbine are arranged in series to provide a compressed boost airflow discharged from the second compressor to the secondary combustor and a combusted boost airflow from the secondary combustor to the second turbine ; and the second turbine is fluidly coupled to the diffuser to discharge an expanded airflow to the diffuser , characterized in that : the gas turbine further comprises a boost clutch selectively coupling the second spool to the first spool , wherein the second spool drives the first spool when the boost clutch is engaged and the second spool rotates independently of the first spool when the boost clutch is disengaged. a first spool comprising a first compressor rotationally driven by a first turbine via a first shaft ; a primary combustor ; a diffuser having walls that diverge towards the primary combustor , wherein the first compressor , the diffuser , and the primary combustor are arranged in series to provide a compressed airflow discharged from the first compressor to the primary combustor via the diffuser ; and a second spool comprising a second compressor rotationally driven by a second turbine via a second shaft ; a secondary combustor disposed downstream from the second compressor and upstream from the second turbine ; wherein: the second compressor is fluidly coupled to the diffuser to receive at least a portion of the compressed airflow from the diffuser ; the second compressor , the secondary combustor , and the second turbine are arranged in series to provide a compressed boost airflow discharged from the second compressor to the secondary combustor and a combusted boost airflow from the secondary combustor to the second turbine ; and the second turbine is fluidly coupled to the diffuser to discharge an expanded airflow to the diffuser , the second compressor is fluidly coupled to the diffuser to receive at least a portion of the compressed airflow from the diffuser ; the second compressor , the secondary combustor , and the second turbine are arranged in series to provide a compressed boost airflow discharged from the second compressor to the secondary combustor and a combusted boost airflow from the secondary combustor to the second turbine ; and the second turbine is fluidly coupled to the diffuser to discharge an expanded airflow to the diffuser , characterized in that : the gas turbine further comprises a boost clutch selectively coupling the second spool to the first spool , wherein the second spool drives the first spool when the boost clutch is engaged and the second spool rotates independently of the first spool when the boost clutch is disengaged. <EOS>