High pressure heat exchangers used in closed cycle rocket engines and air-breathing propulsion pre-coolers are required to work at very high heat transfer rates. They work with high fluid flow rates and are fabricated from tubes or channels which have small hydraulic diameters. This increases the compactness of the unit and therefore reduces its mass. Novel designs of the manifold are required so that the pressure drop remains within acceptable limit. The paper reports on the progress of research work to investigate the manufacture of such heat exchangers and characterise their performance. The investigations centre on a heat exchanger constructed from tube of 0.4 mm diameter with potential heat transfer coefficients of up to 5000 W/m2/K. The heat exchanger is subjected to pre-cooler operating conditions of 1000K simulated air external flow and supercritical cryogenic internal flow. It seeks to validate extrapolations of aerodynamic and heat transfer design data under extreme temperatures and high mass flow rates. Due to the small size of the heat exchanger and the thin walls of the tubes, novel manufacturing methods are required. Work is being done to investigate compatibility of various high temperature brazing materials with thin walled tubes and special manufacturing automation processes to allow cost effective constant-quality fabrication of production units. It is concluded that heat exchangers capable of power transfer rates of up to 1 megaWatt per kilogram mass are capable of being manufactured and used operationally. This is a technology where production to satisfy future aerospace demands for single-stage-to-orbit and hypersonic propulsion can be envisaged.