Radiant Heat Transfer and Homework Discussion

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Radiant Heat Transfer and Homework Discussion

• Lecture 20
• Space Systems

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News From Mars

• Portion of Mars old Ocean found frozen at South
  Pole
• Enough to cover
• Mars with 10 meter layer

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Old Mars Ocean
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Possible Guitars on Mars
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Agenda

• The Blackbody Radiation Curve
• Thermal Radiation in Space
• The Cosmic Background
• Thermal Radiation in Space
• Homework Problems
• Attitude control problem
• Mars eclipse problem

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Why do the stars shine?
Because they are hot! Including the Sun!
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Blackbody Radiation Curve
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Blackbody Radiation Curve
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Black Body Radiation Curve
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Blackbody radiation

• If a body absorbs and radiates EM radiation
  perfectly, it is called a blackbody
• It then radiates power by the Stefan Boltzman Law
  W/m2 ??T4
• ?emissivity, 1 for a black body , ? gt1 for real
  surfaces
• ?5.76x 10 8 Wm 2 K-4
• ( Stefan-Boltzmann constant)

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Cosmic Background Radiation
Temperature of space is TCosmos2.72K (cold) (
This is why it is dark at night)
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Space Thermal Radiation problems

• Spacecraft receive and radiate heat by blackbody
  radiation
• They receive radiation because the sun is hotter
  than they are ( 10,000K)
• They can dump heat to space because it is colder
  than they are (2.7K)
• These two phenomenon are tools for engineers

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Homework 4 Solutions
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Attitude control problem

• Use the equation for angular momentum
  conservation to calculate the of rotations a
  reaction wheel of 1cm thickness and 10 cm
  diameter and mass 10 grams must turn in order to
  rotate a 1ton 1meter diameter 1meter long
  satellite through 30degrees

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Attitude control problem
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Attitude control problem
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Phobos Explorer Power System

• How much battery mass at 40Whr/kg , assuming 50
  power margin, for a satellite in tight orbit
  around Phobos using 100W continuously ( assume
  spherical Phobos at 5gm/cc density)
• Solution Find maximum shadow period due to Mars
  and Phobos combined assume 90 min orbit ( ?5g/cc
  )around Phobos

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Circular orbital period for tight orbit
Phobos actual density is 2.2g/cc
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Phobos power problem
R2.76 RMars
Phobos
Period 7.7hr
Mars shadow
Mars
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Phobos power problem
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Phobos power problem