Until the beginning of the Jet Age, piston engines were common on propliners like the Douglas DC-3. Nearly all modern airliners are now powered by turbine engines, either turbofans or turboprops. Gas turbine engines operate efficiently at much higher altitudes, are more reliable than piston engines, and produce less vibration and noise. Prior to the Jet Age, it was common for the same or very similar engines to be used in civilian airliners as in military aircraft. In recent years, divergence has occurred so that it is now unusual for the same engine to be used on a military type as a civilian type. Usually military aircraft which share engine technology with airliners are transports or tanker types. The Jet Age is a period of history defined by the social change brought about by the advent of large aircraft powered by turbine engines. These aircraft are able to fly much higher, faster, and farther than older piston-powered propliners, making transcontinental and inter-continental travel considerably faster and easier: for example, aircraft leaving North America and crossing the Atlantic Ocean (and later, the Pacific Ocean) could now fly to their destinations non-stop, making much of the world accessible within a single day's travel for the first time. Since large jetliners could also carry more passengers, airfares also declined (relative to inflation), so people from a greater range of social classes could afford to travel outside of their own countries. In many ways, these changes in mobility are similar to those brought about by railroads during the 19th century. The introduction of the Concorde supersonic transport (SST) airliner to regular service in 1976 was expected to bring similar social changes, but the aircraft never found commercial success, and after a fatal crash in Gonesse, near Paris (the only loss of an SST in civ

lian service), flights were discontinued in 2003. The only other SST to enter service was the Tu-144. This only saw limited passenger services within Russia but proved expensive to operate and maintain and was retired from civilian service in 1978 (although it was later used for research flights). McDonnell Douglas, Lockheed and Boeing were three US manufacturers that had planned to develop SSTs since the 1960s, but these projects were abandoned for various developmental/cost or other reasons. A reciprocating engine, also often known as a piston engine, is a heat engine that uses one or more reciprocating pistons to convert pressure into a rotating motion. This article describes the common features of all types. The main types are: the internal combustion engine, used extensively in motor vehicles; the steam engine, the mainstay of the Industrial Revolution; and the niche application Stirling engine. There may be one or more pistons. Each piston is inside a cylinder, into which a gas is introduced, either already under pressure (e.g. steam engine), or heated inside the cylinder either by ignition of a fuel air mixture (internal combustion engine) or by contact with a hot heat exchanger in the cylinder (Stirling engine). The hot gases expand, pushing the piston to the bottom of the cylinder. The piston is returned to the cylinder top (Top Dead Centre) by a flywheel, the power from other pistons connected to the same shaft or (in a double acting cylinder) by the same process acting on the other side of the piston. In most types the expanded or "exhausted" gases are removed from the cylinder by this stroke. The exception is the Stirling engine, which repeatedly heats and cools the same sealed quantity of gas. In some designs the piston may be powered in both directions in the cylinder, in which case it is said to be double-acting.