Lightweight Heat Exchangers are considered for hypersonic airbreathing propulsion. The cycle being studied is a condensing cycle which liquefies inlet air. Achievement of low heat exchanger mass requires advances in heat transfer performance, which are predicted to be possible through enhanced geometry and high-Reynolds-number operation. The system under consideration cools and liquefies air using hydrogen, which is isolated from the air by a helium secondary heat transfer loop for safety reasons. This results in three distinct heat exchangers: An air cooler, and air condenser, and a hydrogen boiler/heater. Design studies for this system require methods for predicting the required masses of these units. The paper first discusses approaches for increasing the heat transfer coefficient to reduce heat exchanger mass. Effects of heat exchanger operating conditions, heat exchanger material, and heat exchanger geometry are considered. A methodology for heat exchanger sizing is presented, and heat exchanger mass is predicted.