A gyrobus is an electric bus that uses flywheel energy storage, not overhead wires like a trolleybus. The name comes from the Greek language term for flywheel, gyros. While there are no gyrobuses currently in use commercially, development in this area continues. Development The concept of a flywheel-powered bus was developed and brought to fruition during the 1940s by Oerlikon (of Switzerland), with the intention of creating an alternative to trolleybuses for quieter, lower-frequency routes, where full overhead-wire electrification could not be justified. Rather than carrying an internal combustion engine or batteries, or connecting to overhead powerlines, a gyrobus carries a large flywheel that is spun at up to 3,000 RPM by a "squirrel cage" motor. Power for charging the flywheel was sourced by means of three booms mounted on the vehicle's roof, which contacted charging points located as required or where appropriate (at passenger stops en route, or at terminals, for instance). To obtain tractive power, capacitors would excite the flywheel's charging motor so that it became a generator, in this way transforming the energy stored in the flywheel back into electricity. Vehicle braking was electric, and some of the energy was recycled back into the flywheel, thereby extending its range. Fully charged, a gyrobus could typically travel as far as on a level route at speeds of up to , depending on the total weight of passengers, as top speeds varied as passenger levels varied from stop to stop. The installation in Yverdon-les-Bains (Switzerland) sometimes saw vehicles needing to travel as far as on one charge, although it is not known how well they performed towards the upper end of that distance. Charging a flywheel took between 30 seconds and 3 minutes; in an effort to reduce the charge time, the supply voltage was increased from 380 volts to 500 volts. Given the relatively restricted range between charges, it is likely that several charging stops would have been required on longer routes, or in dense urban traffic. It is not clear whether vehicles that require such frequent delays would have been practical and/or suitable for modern-day service applications. The demonstrator was first displayed (and used) publicly in summer 1950 and, to promote the system, this vehicle continued to be used for short periods of public service in myriad locations at least until 1954. In 1979, General Electric was awarded a $5 million four-year contract by the United States