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Rockets

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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Rockets

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  1. Rockets

  2. 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.

  3. 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

  4. 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?

  5. Question 1 • What pushes a rocket forward?

  6. 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

  7. 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

  8. 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.

  9. Question 2 • How does the rocket use its gas to obtain thrust?

  10. 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

  11. Question 3 • What keeps a rocket pointing forward?

  12. 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

  13. Question 4 • What limits a rocket’s speed, if anything?

  14. 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

  15. Question 5 • Once in space, does a spaceship have a weight?

  16. 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:

  17. Gravity (Part 2) • The ship’s weight is only constant for small changes in height • When the ship’s height changes significantly:

  18. 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

  19. Question 6 • What makes a spaceship orbit the earth?

  20. Orbits (Part 1) • An object that begins to fall from rest falls directly toward the earth • Acceleration and velocityare in the same direction

  21. 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

  22. 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

  23. 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

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