YF-20
The YF-20 is a Chinese liquid-fuel rocket engine burning NO and UDMH in a gas generator cycle. It is a basic engine which when mounted in a four engine module forms the YF-21. The high altitude variation is known as the YF-22 is normally paired with the YF-23 vernier to form the YF-24 propulsion module for second stages. New versions when used individually for booster applications are called YF-25. Versions. The basic engine has been used since the Feng Bao 1 rocket and has been the main propulsion of the Long March 2, Long March 3 and Long  March 4 families. Modules. While the basic engine was used multiple times, it was only used as a single engine for booster application. It is usually bundled into modules of multiple engines. The relevant modules for first stage application are: The relevant modules for second stage application are: