The LE-5B engine is a derivative of the LE-5 engine, originally used on the H-II launcher in the 1990s. Some of the changes between the LE-5 and LE-5B are intended to lower the cost of serial production. The LE-5B engine has a unique design in which the hydrogen used for the cooling of the thrust chamber is used as the gas powering the turbopump.
First flown in 2001 on the first H-IIA launcher's flight, in continues in service today. It has been integrated as the second-stage engine for the H-IIA, H-IIB, and H-III launchers.
