An aeolipile, also known as a Hero engine, is a rocket style jet engine which spins when heated. In the first century AD, Hero of Alexandria described the device, and many sources give him the credit for its invention. The name – derived from the Greek words “aeolos” and “pila” – translates to “the ball of Aeolus”; Aeolus being the Greek god of the wind.
The aeolipile consists of a vessel, usually a “simple” solid of revolution, such as a sphere or a cylinder, arranged to rotate on its axis, having oppositely bent or curved nozzles projecting from it (tipjets). When the vessel is pressurized with steam, steam is expelled through the nozzles, which generates thrust due to the rocket principleAeolipile as a consequence of the 2nd and 3rd of Newton’s laws of motion. When the nozzles, pointing in different directions, produce forces along different lines of action perpendicular to the axis of the bearings, the thrusts combine to result in a rotational moment (mechanical couple), or torque, causing the vessel to spin about its axis. Aerodynamic drag and frictional forces in the bearings build up quickly with increasing rotational speed (rpm) and consume the accelerating torque, eventually canceling it and achieving a steady state speed.
Typically, and as Hero described the device, the water is heated in a simple boiler which forms part of a stand for the rotating vessel. Where this is the case the boiler is connected to the rotating chamber by a pair of pipes that also serve as the pivots for the chamber. Alternatively the rotating chamber may itself serve as the boiler, and this arrangement greatly simplifies the pivot/bearing arrangements, as they then do not need to pass steam. This can be seen in the illustration of a classroom model to the right.
Read more about Aeolipile on Wikipedia.