South Korean Marines loading a 155mm shell inside a K9 Thunder self-propelled howitzer The K9 Thunder self-propelled howitzer was developed as a replacement for the M109A2K, also known as the K55, self-propelled howitzers. In the late 1980s Republic of Korea Army drew up its requirements for a new indigenous 155-mm tracked howitzer to meet the requirements of 21st century. Key operational requirements included a higher rate of fire, longer range, better accuracy and faster into and out of action