Thrust-to-weight ratio A ratio of a propulsive device's thrust compared to its weight, and in the same terms. Turbojet ' A jet engine in which the energy of the jet operates the air compressor. Van Allen radiation belts Two broad zones of intense, natural radiation encircling the earth at varying levels in the upper atmosphere. Working fluid A fluid, either liquid or gaseous, which transfers heat in a thermal-energy system. Also known as a heat-transfer-fluid. Zenith The point, on any given observer's celestial sphere, which lies directly over his head.
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