Propulsion Review

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Propulsion Review

• The Theory of Rocket Propulsion and Orbit changes

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Agenda

• The Tsiolkovsky equation for a space rocket
• Rocket Architecture
• Orbit Changes Using Rocket Thrust
• Summary

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The Tsiolkovsky Equation
Begin with momentum conservation for An
Isolated Body in free space
Rocket motion
Exhaust motion
No aerodynamic drag and no gravity
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The Tsiolkovsky Equation Cont.
Mass of the rocket is determined by what lies
within its mechanical envelope and rocket nozzle
Mass leaves envelope at Vex leading to mass
decrease within envelope
Mechanical envelope
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The Tsiolkovsky Equation Cont.
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The Tsiolkovsky Equation Cont.
In order to maximize the ?V we must maximize Vex,
the exhaust velocity ( it must be very explosive
fuel) and the ratio of fully fueled mass M0to
final or empty mass Mf ( the rocket must be a
fuel tank, composed of the lightest substance
possible that can withstand the stress)
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The Tsiolkovsky Equation Cont.
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Specific Impulse
Exhaust velocity is most often given in terms of
Specific Impulse Isp
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The Tsiolkovsky Equation Cont.
For the case of small rocket burns on satelites
in space, where the mass of fuel burned is small
relative to the satellite mass
The equation becomes simple
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Rocket Fuel Types
pressurant
oxidizer
fuel
Turbo pump
Thrust chamber
Liquid Fuel (turbo pump fed)
Liquid Fuel (pressure fed)
Solid Fuel
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Solid Fuel Rockets

• Solid fuel rockets are oldest (China in 200 AD)
• Simple and reliable
• Inexpensive, easy to Launch
• Part of Space Shuttle Booster
• Isp of solid fuel is low
• Isp 250 seconds compared to liquid fuels Isp
  300-450 seconds

Solid Fuel
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Solid fuels

• Solid fuels consist of a solid oxidizer plus a
  fuel example
• Oldest solid fuel Black Powder
• Saltpeter (KNO3) Carbon Sulfur
• Oxidizer KNO3 ( releases oxygen)
• Fuel Carbon (burns with oxygen)
• Burn accelerator Sulfur ( combines with left
  over potassium, releasing more oxygen)

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Liquid Fuel (turbo pump)

• Most commonly used space launch vehicle
• Highest Isp and M0/Mf
• Basic design unchanged since German V-2
• LOX (liquid oxygen) is common oxidizer
• Kerosene , Ethanol, or Liquid Hydrogen are common
  fuels

oxidizer
fuel
turbo pump
Thrust chamber
Liquid Fuel (turbo pump fed)
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Liquid Fuels

• Liquid fuels have lower average molecular weight
  exhaust than solid fuels
• They can be burned in thrust chamber of fixed
  geometry to maximize performance
• The liquid can be pure cryogenic gases such as
  oxygen and hydrogen

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Electric Propulsion

• Electric propulsion heats or accelerates gases
  electrically to achieve higher Isp than possible
  with chemical combustion
• Electro thermal uses electric arcs to heat gas to
  welding arc temperatures
• Ion thrusters electrically accelerate ionized gas
  to high velocity

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MET Thruster

• MET is an electrodeless arc-thruster
• MET can be operated at 915MHz, 30-50kW using
  water as fuel

• 915MHz can be generated at 95 efficiency
• Electrodeless design and operation on 915MHz make
  very high system efficiency

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Isp versus Power
Nuclear power required
Lox hydrogen
High fuel mass
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Orbits and Orbit Transfer
Orbits can be changed by adding or subtracting
energy by rocket burns that change velocity
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Orbit Changes Through Rocket Propulsion
Rocket burn
Initial circular orbit e0 E-½V2escape
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Transfer Between Circular Orbits
Initial orbit is circular and rocket burn turns
this into an elliptical transfer orbit A second
burn inserts the satellite into its final
circular orbit
2nd rocket burn
Final orbit
rf
ri
transfer orbit
1st rocket burn
Assume all burns parallel to V
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Orbit Raising Calculation
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Orbit Raising Calculation Cont.
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Orbital transfer Earth to Mars

• Start in low circular orbit do rocket burn to
  give ?V (add energy)
• This puts one into transfer orbit that is an
  ellipse
• At highest point of ellipse (aphelion) one
  performs rocket burn to add ?V to circularize
  orbit at Mars(add more energy)
• To return one reverses the procedure (subtract
  energy)

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WORK HARD AND GOOD LUCK