The BT-4 is a pressure-fed liquid rocket engine designed and manufactured by IHI Aerospace of Japan. It was originally developed for the LUNAR-A project, but it has been used as a liquid apogee engine in some geostationary communications satellite based on the Lockheed Martin A2100 and GEOStar-2 satellite buses. It has also been used on the HTV and Cygnus automated cargo spacecraft.