A gas turbine engine, comprising: at least first and second turbo-compressor spools, each of the at least first and second turbo-compressor spools comprising a centrifugal compressor in mechanical communication with a radial turbine; at least one intercooler positioned in a fluid path between the at least first and second turbo-compressor spools; a recuperator operable to transfer thermal energy from an output gas of a power turbine to a compressed gas produced by the at least first and second turbo-compressor spools and thereby form a further heated gas; and a combustor operable to combust a fuel in the presence of the further heated gas; wherein: a first inlet gas enters a first compressor in the first turbo-compressor spool along a first axis; a first compressed gas from the first compressor exits the first compressor, wherein the first compressor is rotatable about the first axis; the first compressed gas enters a second compressor in the second turbo-compressor spool along a second axis; and a second compressed gas from the second compressor exits the second compressor, wherein the second compressor is rotatable about the second axis; characterised in that the gas turbine engine further comprises: a free power spool associated with the radial turbine of one of the first and second turbo-compressor spools comprising a radial turbine, a variable area nozzle and a mechanical power output shaft; an inlet gas to the free power turbine enters the free power turbine along a third axis; an expanded gas from the free power turbine exits the free power turbine via the mechanical power output shaft; and wherein the free power turbine is rotatable about the third axis and a rotational direction of the mechanical power output shaft can be reversed. at least first and second turbo-compressor spools, each of the at least first and second turbo-compressor spools comprising a centrifugal compressor in mechanical communication with a radial turbine; at least one intercooler positioned in a fluid path between the at least first and second turbo-compressor spools; a recuperator operable to transfer thermal energy from an output gas of a power turbine to a compressed gas produced by the at least first and second turbo-compressor spools and thereby form a further heated gas; and a combustor operable to combust a fuel in the presence of the further heated gas; wherein: a first inlet gas enters a first compressor in the first turbo-compressor spool along a first axis; a first compressed gas from the first compressor exits the first compressor, wherein the first compressor is rotatable about the first axis; the first compressed gas enters a second compressor in the second turbo-compressor spool along a second axis; and a second compressed gas from the second compressor exits the second compressor, wherein the second compressor is rotatable about the second axis; a first inlet gas enters a first compressor in the first turbo-compressor spool along a first axis; a first compressed gas from the first compressor exits the first compressor, wherein the first compressor is rotatable about the first axis; the first compressed gas enters a second compressor in the second turbo-compressor spool along a second axis; and a second compressed gas from the second compressor exits the second compressor, wherein the second compressor is rotatable about the second axis; characterised in that the gas turbine engine further comprises: a free power spool associated with the radial turbine of one of the first and second turbo-compressor spools comprising a radial turbine, a variable area nozzle and a mechanical power output shaft; an inlet gas to the free power turbine enters the free power turbine along a third axis; an expanded gas from the free power turbine exits the free power turbine via the mechanical power output shaft; and wherein the free power turbine is rotatable about the third axis and a rotational direction of the mechanical power output shaft can be reversed. a free power spool associated with the radial turbine of one of the first and second turbo-compressor spools comprising a radial turbine, a variable area nozzle and a mechanical power output shaft; an inlet gas to the free power turbine enters the free power turbine along a third axis; an expanded gas from the free power turbine exits the free power turbine via the mechanical power output shaft; and wherein the free power turbine is rotatable about the third axis and a rotational direction of the mechanical power output shaft can be reversed.