Rockets: Rockets
Question:: 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 operate well in empty space
Rockets usually fly nose-first
Momentum Conservation: Momentum Conservation A rocket’s momentum is initially zero
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 The momentum of the ship is opposite
the momentum of the ejected fuel
or, equivalently,
the velocity of that fuel and
the mass of that fuel
Question:: 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.
Rocket Engines: Rocket Engines Chemical reactions produce hot, high-pressure gas
Gas speeds up in nozzle
Gas reaches sonic speed in throat of de Laval nozzle
Beyond throat, supersonic gas expands to speed up further
Stability and Orientation: Stability and Orientation On ground, rocket needs static stability
In air, rocket needs aerodynamic stability
Center of dynamic pressure behind c.o.m.
In space, rocket is a freely rotating object
Orientation governed by angular momentum
Rocket’s orientation doesn’t affect its travel
Ship’s Ultimate Speed: Ship’s Ultimate Speed Increases as
ratio of fuel mass to ship mass increases
fuel exhaust speed increases
If fuel were released with rocket at rest, Because rocket accelerates during thrust, ultimate speed is less than given above
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 An object’s weight is only constant for small changes in height
When its height changes significantly, the relationship is:
Gravity, Part 3: Gravity, Part 3 Even far above earth, an object has weight
Astronauts and satellites have weights
weights are somewhat less than normal
weights depend on altitude
Astronauts and satellites are in free fall
Orbits, Part 1: Orbits, Part 1 An object that begins to fall from rest falls directly toward the earth
Acceleration and velocity are 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 closed or open, and are ellipses, 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 the product of exhaust speed times exhaust mass
Rockets can be stabilized aerodynamical
Rockets can be stabilized by thrust alone
After engine burn-out, rockets can orbit