Rockets

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Rockets
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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.

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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

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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 rockets speed, if anything?
• Once in space, does a spaceship have a weight?
• What makes a spaceship orbit the earth?

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Question 1

• What pushes a rocket forward?

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Momentum Conservation

• A rockets 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
• Rockets total momentum remains zero

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Rocket Propulsion

• Neglecting gravity, then
• rockets total momentum is always zero
• momentumfuel momentumship 0
• The momenta of ship and fuel are opposite
• The ships momentum is equal but opposite to
• the velocity of the fuel
• times the mass of that fuel

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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.

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Question 2

• How does the rocket use its gas to obtain thrust?

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Rocket Engines

• Combustion produces hot, high-pressure gas
• The gas speeds up in a de Laval nozzle
• Gas reaches sonic speedin the nozzles throat
• Beyond the throat, supersonicgas expands to
  speed up further

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Question 3

• What keeps a rocket pointing forward?

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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
• Spaceships orientation doesnt affect its travel

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Question 4

• What limits a rockets speed, if anything?

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Ships 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

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Question 5

• Once in space, does a spaceship have a weight?

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Gravity (Part 1)

• The earths acceleration due to gravity is only
  constant for small changes in height
• When the distance between two objects changes
  substantially, the relationship is

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Gravity (Part 2)

• The ships weight is only constant for small
  changes in height
• When the ships height changes significantly

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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

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Question 6

• What makes a spaceship orbit the earth?

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Orbits (Part 1)

• An object that begins to fall from rest falls
  directly toward the earth
• Acceleration and velocityare in the same
  direction

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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

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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

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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