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Rockets. Introductory Question. If there were no launch pad beneath the space shuttle at lift-off, the upward thrust of its engines would be approximately unchanged. approximately half as much. approximately zero. Observations about Rockets. Plumes of flame emerge from rockets

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Presentation Transcript
introductory question
Introductory Question
  • If there were no launch pad beneath the space shuttle at lift-off, the upward thrust of its engines would be
  • approximately unchanged.
  • approximately half as much.
  • approximately zero.
observations about rockets
Observations about Rockets
  • Plumes of flame emerge from rockets
  • Rockets can accelerate straight up
  • Rockets can go very fast
  • The flame only touches the ground initially
  • Rockets can apparently operate in empty space
  • Rockets usually fly nose-first
6 questions about rockets
6 Questions about Rockets
  • What pushes a rocket forward?
  • How does the rocket use its gas to obtain thrust?
  • What keeps a rocket pointing forward?
  • What limits a rocket’s speed, if anything?
  • Once in space, does a spaceship have a weight?
  • What makes a spaceship orbit the earth?
question 1
Question 1
  • What pushes a rocket forward?
momentum conservation
Momentum Conservation
  • A rocket’s momentum is initially zero
  • That momentum is redistributed during thrust
    • Ship pushes on fuel; fuel pushes on ship
    • Fuel acquires backward momentum
    • Ship acquires forward momentum
  • Rocket’s total momentum remains zero
rocket propulsion
Rocket Propulsion
  • Neglecting gravity, then
    • rocket’s total momentum is always zero

momentumfuel + momentumship = 0

  • The momenta of ship and fuel are opposite
  • The ship’s momentum is equal but opposite to
    • the velocity of the fuel
    • times the mass of that fuel
introductory question revisited
Introductory Question (revisited)
  • If there were no launch pad beneath the space shuttle at lift-off, the upward thrust of its engines would be
  • approximately unchanged.
  • approximately half as much.
  • approximately zero.
question 2
Question 2
  • How does the rocket use its gas to obtain thrust?
rocket engines
Rocket Engines
  • Combustion produces hot, high-pressure gas
  • The gas speeds up in a de Laval nozzle
  • Gas reaches sonic speedin the nozzle’s throat
  • Beyond the throat, supersonicgas expands to speed up further
question 3
Question 3
  • What keeps a rocket pointing forward?
stability and orientation
Stability and Orientation
  • On the ground, a rocket needs static stability
  • In the air, a rocket needs aerodynamic stability
    • Center of aerodynamic forces behind center of mass
  • In space, a spaceship is a freely rotating object
    • Orientation governed by angular momentum
    • Small rockets are used to exert torques on spaceship
    • Spaceship’s orientation doesn’t affect its travel
question 4
Question 4
  • What limits a rocket’s speed, if anything?
ship s ultimate speed
Ship’s Ultimate Speed
  • Increases as
    • the ratio of fuel mass to ship mass increases
    • the fuel exhaust speed increases
  • If fuel were released with the rocket at rest,
  • Because rocket accelerates during thrust, ultimate speed is less than given above
question 5
Question 5
  • Once in space, does a spaceship have a weight?
gravity part 1
Gravity (Part 1)
  • The earth’s acceleration due to gravity is only constant for small changes in height
  • When the distance between two objects changes substantially, the relationship is:
gravity part 2
Gravity (Part 2)
  • The ship’s weight is only constant for small changes in height
  • When the ship’s height changes significantly:
gravity part 3
Gravity (Part 3)
  • Even far above earth, an object has weight
  • Astronauts and spaceships have weights
    • weights are somewhat less than normal
    • weights depend on altitude
  • Astronauts and spaceships are in free fall
    • Astronauts feel weightless because they are falling
question 6
Question 6
  • What makes a spaceship orbit the earth?
orbits part 1
Orbits (Part 1)
  • An object that begins to fall from rest falls directly toward the earth
  • Acceleration and velocityare in the same direction
orbits part 2
Orbits (Part 2)
  • An object that has a sideways velocity follows a trajectory called an orbit
  • Orbits can be closedor open, and areellipses, parabolas,and hyperbolas
current rocket technology
Current Rocket Technology
  • X-Prize Rockets
  • Single State to Orbit Rockets
  • Improbable Dreams
    • Rockets that rarely require refueling
    • Rockets that can land and leave large planets
    • Rockets that can turn on a dime in space
summary about rockets
Summary About Rockets
  • Rockets are pushed forward by their fuel
  • Total rocket impulse is basically the product of exhaust speed times exhaust mass
  • Rockets can be stabilized aerodynamical
  • Rockets can be stabilized by thrust alone
  • After engine burn-out, spaceships can orbit