NOVELTY - The vehicle has a source of a propellant and an engine carried by a body reacting on the propellant to produce thrust. A heat exchanger transfers heat from the reaction to a component of the propellant and generates electricity thermoelectrically. The exchanger has cooling channels (310) defined within conduits (312). The conduits are elongate in the transverse cross-section and oriented parallel to a surface (304). Thermoelectric device arrays (320) are mounted on the hot side of the conduits. An electrical insulator layer (322) electrically insulates the arrays from the conduits.    USE - Vehicle e.g. on-board piloted aircraft, warhead-carrying missile, disposable launch vehicle, and hybrid turbine and ramjet aircraft such as a dual mode scramjet (DMSJ) ramjet.    ADVANTAGE - The cooling channels are defined within conduits, which are elongate in the transverse cross-section and are oriented parallel to the surface so improving the heat transfer rate, and cooling the combustion chamber and nozzle besides maintaining the engine life. The thermoelectric device arrays mounted on the hot side of the conduits are used to monitor the operational and conditional parameters of the engine such as a local power increase which indicates local thinning, cracking, wear and damage condition and provides a warning to a pilot to ensure the maintenance of the heat exchanger for a reusable craft and to change operational conditions.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for the following:    (1) a heat exchanger comprising a heat transfer surface for receiving heat;    (2) a method for operating a vehicle; and    (3) a method for manufacturing a thermoelectric generator heat exchanger.    DESCRIPTION OF DRAWING(S) - The drawing shows a partial section of a heat exchanger of a propulsion system of an aircraft.    Hot panel (302)    Surfaces (304, 316)    Back structure (306)    Fasteners (308)    Cooling channels (310)    Conduits (312)    Open channels (314)    Thermoelectric device arrays (320)    Electrical insulator layer (322)    Thermal insulation layer (330)    Air flowpath (500)