|Fig. 1-4 Heros engine - probably the earliest|
form of jet reaction.
7. This same principle of reaction occurs in all forms of movement and has been usefully applied in many ways. The earliest known example of jet reaction is that of Hero’s engine (fig. 1-4) produced as a toy in 120 B.C. This toy showed how the momentum of steam issuing from a number of jets could impart an equal and opposite reaction to the jets themselves, thus causing the engine to revolve.
|Fig. 1-5 A garden sprinkler rotated by the|
reaction of the water jets.
firefightingequipment are an example of ’jet reaction’,for often, due to the reaction of the water jet, the hosecannot be held or controlled by one fireman. Perhapsthe simplest illustration of this principle is afforded bythe carnival balloon which, when the air or gas isreleased, rushes rapidly away in the directionopposite to the jet.
9. Jet reaction is definitely an internal phenomenon and does not, as is frequently assumed, result from the pressure of the jet on the atmosphere. In fact, the jet propulsion engine, whether rocket, athodyd, or turbo-jet, is a piece of apparatus designed to accelerate a stream of air or gas and to expel it at high velocity. There are, of course, a number of ways of doing this, as described in Part 2, but in all instances the resultant reaction or thrust exerted on the engine is proportional to the mass or weight of air expelled by the engine and to the velocity change imparted to it. In other words, the same thrust can be provided either by giving a large mass of air a little extra velocity or a small mass of air a large extra velocity. In practice the former is preferred, since by lowering the jet velocity relative to the atmosphere a higher propulsive efficiency is obtained.