An engine comprising: a rocket combustion chamber for the combustion of fuel and oxidant; a compressor for supplying pressurized oxidant to said combustion chamber; a first heat exchanger (29, 30) having an inlet and an outlet arranged for cooling oxidant to be supplied to said compressor using a heat transfer medium before compression by said compressor; a heat transfer medium loop (25a-25g) for said heat transfer medium; a fuel delivery arrangement for delivering fuel; a second heat exchanger arranged for cooling of said heat transfer medium by fuel delivered by said fuel delivery arrangement; a first circulator for circulating said heat transfer medium around said heat transfer medium loop (25a-25g) and delivering said heat transfer medium to said inlet of said first heat exchanger (29, 30); a second circulator arranged downstream of said outlet of said first heat exchanger (29, 30); said engine being configured to operate in a first operating mode, in which said second circulator is configured to deliver heat transfer medium from the outlet of said first heat exchanger (29, 30) to the inlet of said first heat exchanger (29, 30); and a third heat exchanger, the third heat exchanger <44^ being arranged for cooling said heat transfer medium from the outlet of the first heat exchanger (29, 30) using said fuel; and wherein the engine is configured to operate in a second operating mode, in which the second circulator is configured to deliver heat transfer medium from the outlet of the first heat exchanger (29, 30) to an intermediate point between the inlet and outlet of the first heat exchanger (29, 30) after cooling by said third heat exchanger; and wherein the engine further comprises a bypass which can be employed for bypassing fuel past said third heat exchanger in said first operating mode. a rocket combustion chamber for the combustion of fuel and oxidant; a compressor for supplying pressurized oxidant to said combustion chamber; a first heat exchanger (29, 30) having an inlet and an outlet arranged for cooling oxidant to be supplied to said compressor using a heat transfer medium before compression by said compressor; a heat transfer medium loop (25a-25g) for said heat transfer medium; a fuel delivery arrangement for delivering fuel; a second heat exchanger arranged for cooling of said heat transfer medium by fuel delivered by said fuel delivery arrangement; a first circulator for circulating said heat transfer medium around said heat transfer medium loop (25a-25g) and delivering said heat transfer medium to said inlet of said first heat exchanger (29, 30); a second circulator arranged downstream of said outlet of said first heat exchanger (29, 30); said engine being configured to operate in a first operating mode, in which said second circulator is configured to deliver heat transfer medium from the outlet of said first heat exchanger (29, 30) to the inlet of said first heat exchanger (29, 30); and a third heat exchanger, the third heat exchanger <44^ being arranged for cooling said heat transfer medium from the outlet of the first heat exchanger (29, 30) using said fuel; and wherein the engine is configured to operate in a second operating mode, in which the second circulator is configured to deliver heat transfer medium from the outlet of the first heat exchanger (29, 30) to an intermediate point between the inlet and outlet of the first heat exchanger (29, 30) after cooling by said third heat exchanger; and wherein the engine further comprises a bypass which can be employed for bypassing fuel past said third heat exchanger in said first operating mode.