Aeolipile
From Wikipedia, the free encyclopedia
An aeolipile (or aeolipyle, or eolipile), is a rocket style[1] jet engine[2] described in the first century BC by Vitruvius in his treatise De architectura.[3] Later, in the first century AD, Hero of Alexandria also described the instrument,[4] and many sources mistakenly give him the credit for its invention.
The aeolipile is considered to be the first recorded steam engine or reaction steam turbine.[5] The name – derived from the Greek words "aeolos" and "pila" – translates to "the ball of Aeolus"; Aeolus being the Greek god of the wind.
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[edit] Description and physics
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, with bent or curved nozzles projecting from it (tipjets), through which steam is expelled. These nozzles generate thrust due to the rocket principle[6] (which is a consequence of the 2nd and 3rd of Newton's laws of motion). When the nozzles produce forces that are perpendicular to the axis of the bearings, and arranged so that the thrusts act in different directions, the thrusts combine to result in a couple, and the resulting torque then causes the device to start to spin. Aerodynamic drag, and frictional forces in the bearings, build up quickly with increasing rpm, oppose the accelerating torque, and eventually cancel it out. The rpm at which the forces cancel out governs the steady state rpm.
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.
[edit] History
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Both Hero and Vitruvius draw on the much earlier work by Ctesibius (285-222BC). Since it is unknown whether or not Ctesibius himself was the inventor, awarding proper credit for creating the first aeolipile may never be achieved.
[edit] Vitruvius's description
Vitruvius (c. 80 BC – c. 15AD) mentions the aeolipile by name, while describing the layout of towns to minimize disturbance from the wind:
"Æolipylæ are hollow brazen vessels, which have an opening or mouth of small size, by means of which they can be filled with water. Prior to the water being heated over the fire, but little wind is emitted. As soon, however, as the water begins to boil, a violent wind issues forth." [3]
[edit] Hero's description
Hero (c. 10 - 70 AD) takes a more practical approach, in that he gives instructions how to make one:
| “ | No. 50. The Steam-Engine. PLACE a cauldron over a fire: a ball shall revolve on a pivot. A fire is lighted under a cauldron, A B, (fig. 50), containing water, and covered at the mouth by the lid C D; with this the bent tube E F G communicates, the extremity of the tube being fitted into a hollow ball, H K. Opposite to the extremity G place a pivot, L M, resting on the lid C D; and let the ball contain two bent pipes, communicating with it at the opposite extremities of a diameter, and bent in opposite directions, the bends being at right angles and across the lines F G, L M. As the cauldron gets hot it will be found that the steam, entering the ball through E F G, p asses out through the bent tubes towards the lid, and causes the ball to revolve, as in the case of the dancing figures.[4] |
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[edit] Practical usage
It is not known whether the aeolipile was put to practical use as an 'engine' in ancient times. Hero's drawing shows a stand-alone device, and was presumably intended as a temple 'wonder', like many of the other devices described in Pneumatica. [4]
Vitruvius, on the other hand, mentions use of the aeolipile for demonstrating the physical properties of the weather. He describes the aeolipile as
| “ | ...a scientific invention [to] discover a divine truth lurking in the laws of the heavens.[3] | ” |
After describing the device's construction (see above) he concludes:
| “ | ...Thus from this slight and very short experiment we may understand and judge of the mighty and wonderful laws of the heavens and the nature of winds.[3] | ” |
[edit] See also
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Wikimedia Commons has media related to: Aeolipile |
[edit] References
- ^ NASA Glenn Learning Technologies Project (LTP)
- ^ jet engine
- ^ a b c d "De Architectura": Chapter VI (paragraph 2)
from "Ten Books on Architecture" by Vitruvius (1st century BC), translated by Bill Thayer, published 17, June, 08 [1] accessed 2009-07-07 - ^ a b c "Pneumatica: Section 50 – The Steam Engine"
from The Pneumatics of Hero of Alexandria (1st century AD), translated from the original Greek by Bennet Woodcroft (Professor of Machinery in University College London) published 1851 by Taylor Walton and Maberly (London) – online edition accessed 2009-07-03 - ^ "turbine." Encyclopedia Britannica. 2007. Encyclopedia Britannica Online. 18 July 2007 <http://www.britannica.com/eb/article-45691>.
- ^ Aeolipile
