What propulsion method did Hero develop for rockets?

Hero of Alexandria, an ancient Greek engineer and inventor, is credited with developing an early form of steam propulsion known as the aeolipile. This device operated using steam generated from boiling water, which would escape through nozzles, creating thrust and causing the device to spin. The fundamental principles behind this method demonstrate the use of steam as a propulsion system long before modern engines were invented.

Hero's innovations laid the groundwork for later developments in steam technology, influencing both engineering and the concept of propulsion in various applications, including rockets. This historical significance of steam propulsion is what makes it the correct answer to the question regarding the propulsion method developed by Hero.

The other options do not align with Hero's contributions; combustion engines and chemical propulsion systems came much later in development, while electric propulsion systems arose in the modern era.