Spacecraft Attitude Control and Intro to Space Power

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Spacecraft Attitude Controland Intro to Space
Power

• Space systems lecture 15

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Agenda

• Angular Momentum
• Spinning satellites
• Active Attitude Control
• Space Power
• Homework 4

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Angular momentum

• A satellite is a rigid body with an angular
  momentum H ?I (mivixri)M VCMx r
• Satellite has both internal ( about CM -Center of
  Mass) and external motion components of angular
  momentum
• Internal angular momentum and its changes
  determine attitude control
• Internal angular momentum is changed by torques

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Moment of Inertia
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Spinning satellites
H
Some satellite simply spin like gyroscopes to
maintain attitude and control precession to
control it
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Attitude Control Tools

• Reaction control jets
• Magnetic torquers
• Reaction wheels

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Key Equations
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Momentum Wheels
Many satellites use momentum wheels to control
attitude
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Momentum Wheels
N
S
Magnetic field
Many satellites use on board magnetic torque
rods to maintain attitude and control it
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Key Equations
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Momentum Dump
Momentum wheels
Desired range
Rotation frequency
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Momentum Dump
Satellites dump angular momentum by torquing the
momentum wheels and firing control thrusters to
produce an opposing torque
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Celestial Sphere
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Clementine
Clementine used full attitude determination and
control since it the spacecraft was made to
gather imaging data and all cameras were bolted
to the spacecraft frame
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Star tracker algorithm
Algorithm drew 100s of triangles among stars and
compared lengths of sides to table values to find
constellations
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Clementine Image of Earth Over Moon
Clementine achieved high camera pointing accuracy
due to good attitude control Attitude control is
vital to mission success in space as well as life
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Overshoot and correction
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Space Power Systems
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Agenda

• Space Power Systems Overview
• Batteries
• Solar Power
• Radioisotope Thermal Generators RTGs
• Fuel cells
• Auxiliary Power Units
• Space Nuclear power

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Batteries

• Nickel Cadmium
• Very Rechargeable
• Mature technology
• Nickel Hydrogen
• New
• Very Rechargeable
• Good Discharge Recovery

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Batteries

• Lead Acid
• Very Rechargeable
• Mature technology
• Heavy
• Lithium
• High energy
• Light
• Zinc Silver
• Very High Current

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Battery Comparison
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Nickel Hydrogen Battery
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Primary Power Sources
Inner planets Solar Panels
Outer planets RTGs
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Solar System
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Solar Batteries

• Oldest Source of Continual Power in Space
• 100W/ meter2
• Silicon
• Low Efficiency10
• Radiation Sensitive
• Low cost
• Gallium Arsenide GaAs
• Efficient20
• Radiation Insensitive
• Costly

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Solar Batteries
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Orbits Radiation Belts
Solar batteries are degraded by radiation in the
van Allen Belts and so must be either hardened or
kept out of van Allen penetrating orbits
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Radiation Degradation of Solar Battery Material
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Orbits Eclipse Duration
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Homework 4

• 1. (30 points)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.
• 2.(30points) How much battery mass at 40Whr/kg ,
  assuming 50 power margin, for a satellite in
  tight orbit around Phobos , the inner Moon of
  Mars, using 100W continuously ( assume spherical
  Phobos at 1.9gm/cc density)
• 3. (30points) How much solar panel area do we
  need for this probe to operate in low mars orbit
  with a 50 power margin.