INTRODUCTION • Aircraft and launch vehicles fly mainly in the earth’s atmosphere.
CHAPTER IV EARTH’S ATMOSPHERE
• A knowledge of the pressure, temperature and density of the aircraft’s immediate surrounding is very essential for proper flight. • This chapter deals with a model of the earth’s atmosphere.
EARTH’S ATMOSPHERE
EARTH’S ATMOSPHERE
• The earth’s atmosphere consists of the following layers, • Troposphere • Stratosphere • Mesosphere • Ionosphere • Exosphere
TROPOSPHERE • This is the region closest to the earth’s surface, characterized by turbulent air.
STRATOSPHERE • Extends from tropopause up to about 50km.
• Height varies from 9km at poles to 16km at equator.
• It has high velocity winds ( but no gusts)
• Temperature drops at a nearly constant rate of 6.5 degrees Celsius per km.
• Temperature remains constant up to 25km and then increases (simplified model).
• The highest point is the Tropopause.
• The highest point is the stratopause.
1
MESOSPHERE • This extends from the stratopause upto 80km. • Temperature decreases. Pressure and density are very low, but air retains the composition at sea level.
IONOSPHERE • It extends from the mesopause to about 500km. • It consists mainly of ions and free electrons. • Temperature increases in this region.
• The region near the stratopause contains the ozone layer.
EXOSPHERE • This is the outermost region of the earth’s atmosphere. • It contains very few air molecules and merges into interplanetary space.
• Phenomena like aurora borealis occur here.
INTERNATIONAL STANDARD ATMOSPHERE (ISA) • The ISA is a simplified model of the earth’s atmosphere using which preliminary prediction of the earth’s atmosphere can be made.
• The mean free path of the molecules is of the order of a few meters.
• This does not represent the earth’s atmosphere exactly but only an averaged earth’s atmosphere.
ISA - STANDARD CONDITIONS
ISA - STANDARD CONDITIONS
• The following are defined as the standard reference conditions at sea level • Standard Temperature = 288.15 K • Standard Pressure = 101325 Pa • Temperature lapse rate = 6.5K/km upto 11km, 0 from 11km to 25km (simple) • Note : from these, the standard density is got as 1.2256 kg/m3.
• Air is treated as a perfect dry gas. • The composition of air is for practical purposes 78% Nitrogen, 22% Oxygen and traces of other gases like Hydrogen, Carbon dioxide etc.
2
BASIC EQUATIONS • The two main equations using which all the variations can be derived are • The equation of state, p = ρRT, where p: pressure, ρ: density, T:Temperature, R: gas constant. • Variation of pressure, dp/dh=-ρg, where g is the acceleration due to gravity. (is g constant?)
BASIC EQUATIONS (Linear T) T = T0 − Lh p ⎛T ⎞ =⎜ ⎟ p0 ⎜⎝ T0 ⎟⎠
g
• Using these two equations along with the characteristics of each region (like constant temperature in the lower stratosphere), the variations of pressure, temperature and density in each region can be derived.
BASIC EQUATIONS (constant T) T = T0
g LR
ρ ⎛ T ⎞ LR =⎜ ⎟ ρ 0 ⎜⎝ T0 ⎟⎠
BASIC EQUATIONS
g
− ( h − h0 ) p = e RT p0
ρ p = ρ 0 p0
−1
T0, p0, ρ0 at bottom of layer, h measured from bottom of layer
Standard (1976 US) model
T0, p0, ρ0 at bottom of layer, h measured from bottom of layer
SIMPLIFIED MODEL
• http://www.pdas.com/coesa.htm (geo-potential altitude)
h1 (km) 0 11 20 32 47 51 71
h2 (km) 11 20 32 47 51 71 84.85
L (K/km) -6.5 0 +1.0 +2.8 0 -2.8 -2.0
• This is the final set of equations modeling the earth’s atmosphere, • For h
