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Unformatted text preview: AA 311 Lecture 3: Altitude Concepts; The Atmosphere Reading:  Chapter 3. The difference between atmospheric flight vehicles and space vehicles is that atmospheric vehicles always fly within the atmosphere. They need the surrounding air to produce the required lift force that keeps them in the air. As we saw in previous lectures, the lift force is produced either by buoyancy (in case of lighter-than-air vehicles), or aerodynamic forces (in case of heavier-than-air vehicles). Aside 1 Heavier-than-air vehicles need the atmosphere for another purpose as well: for their propulsion. Today, most atmospheric flight vehicles use some sort of air-breathing engine (namely, either a piston- cylinder reciprocating engine, or jet engine) that requires the oxygen in the air for its operation. Space vehicles, on the other hand, need to carry the oxidizer with them, because in space there is no oxygen available for combustion. The oxygen can be carried in liquid form (liquid fuel rocket), or in solid form (solid fuel rocket). During the design, performance evaluation, and operation of an atmospheric flight vehicle, the atmospheric properties need to be understood, and explicitly taken into account. Altitude There are several notions of altitude that are in common use in engineering practice. The Earth is not a perfect sphere, it resembles more to an ellipsoid (or more precisely, it is often referred to as a geoid). For a variety of applications, an accurate model of the Earth’s shape, rotation, and gravity has to be known. In the U.S., the currently used model is World Geodetic System - 1984 (WGS-84) that defines the Earth as an ellipsoid of revolution (a so-called spheroid) with semi-major axis a = 6 , 378 , 137 . m , and semi-minor axis b = 6 , 356 , 752 . 314245 m . For many guidance purposes such a two-parameter description is adequate. When calculating a detailed model of the Earth’s gravitational field, a more accurate description is necessary to acknowledge the fact that the Earth has an irregular mass distribution. Such vertical undulations are modeled in the Earth Gravitational Model 1996 (EGM96), which is a spherical harmonic expansion model with 130,317 coefficients (see Figure 3 showing the Earth gravity anomalies). Figure 1: Earth gravity anomalies. From . The Mean Sea Level (MSL) is the average of sea state reference. Equipotential surface of the Earth’s gravity field that coincides with the undisturbed mean sea level, extended continuously under the continents, is called a geoid . Due to differences in salinity, current, local air pressure, etc., MSL does not always correspond with the geoid, but it is a resonable approximation, the maximum error being on the order of ± 2 m ....
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- Fall '09
- geopotential altitude