Transcript The Travels of the Water Rocket

The Travels of the Water
Rocket
Stretch out your hand this is going
to be a long one
Three Phases of Rocket Flight
X
a) Thrust
b
c
b) Coast
X
c) Re-entry/ Recovery
a
Thrust Phase
• Most Important of all the phases
• This phase is what gets the rocket to go
into the air
• The rocket achieves its lowest speed and
its highest speed in this phase
Thrust phase
• There are three main forces in the thrust
phase:
– Thrust
– Air resistance
– gravity
Thrust Phase
• Thrust is the strongest force in the rocket
flight
• This force pushes the rocket into the air
• Thrust comes from the fuel in the rocket.
– Our fuel was the water and the 40 psi of air
pressure
How does it work?
Thrust Phase
• Newton’s Third Law of Motion
– Whenever something pushes on an object,
the object pushes back with equal but
opposite force.
• Action/reaction
• The rocket pushes the fuel out, therefore
what must the fuel do to the rocket?
• But that is not all.
Thrust Phase
• Thrust also has to do with the BIG MOmomentum
• The momentum of the fuel leaving the
rocket is equal to the momentum of the
rocket going up
• Momentum is equal to mass X ________
• This is why we added water to the air
pressure. To give it more mass
Thrust Phase
• Gravity is another force acting on the
rocket
• It is the only force that does not change
strength (_____________) or direction
• It stays the same.
• The effect of gravity on the rocket will
change, but the actual force will not
Thrust Phase
• In the thrust phase gravity pulls the rocket
down towards the ground
• Its’ effect on the rocket is that it causes the
rocket to slow down
Thrust Phase
• Air resistance is the last force to talk about
in the thrust phase
• Air resistance increases as the speed of
the rocket gets faster.
• At the beginning of the thrust phase, air
resistance is zero
• At the end of the thrust phase, air
resistance is at its strongest
Thrust Phase
• Newton’s laws are essential for rocket
flight
– 1st law – the rocket is sitting on the launch
pad. It has inertia. The more mass, the more
inertia. The more it wants to stay where it is.
This is why heavy rockets don’t do so well.
– Light rockets have very little inertia so they
get going really fast. This has problems also
Thrust Phase
• Newton’s 2nd law
– The force that gets the rocket moving, the
faster and farther the rocket should go.
– This is why you want the maximum force
output on the fuel.
• Third law- already covered
Thrust Phase
• The timing of the thrust phase
– It begins the instant you pull the string
– The rockets speed at the beginning is zero
– The rocket continues to gain speed as long as
fuel is leaving the rocket.
– When the fuel runs out, the thrust phase is
over
• Then your rocket enters the coast phase
Coast Phase
• The coast phase begins when the fuel is
completely gone from the rocket
• There is no force pushing the rocket up
anymore.
• The rocket still goes up because of
momentum and inertia
– But these aren’t forces
• There are only two forces in the coast
phase
Coast Phase
• Gravity- it’s the same. It has the same effect
and the same strength and direction as before.
• Air resistance- at the beginning of the coast
phase, air resistance is at its strongest because
the rocket goes really, really fast
• As the coast phase continues, air resistance
gets less and less because both air resistance
and gravity are slowing the rocket down
• Eventually air resistance will be almost zero
Coast Phase
• 1st law – rocket continues up because it has
inertia. The more inertia, the less effect air
resistance will have on the rocket
– The greater the effect of air resistance, the faster the
rocket will slow down. Which brings us to…
• 2nd law – two forces are working on the rocket.
Both are trying to slow the rocket down. The
stronger the forces the faster the acceleration of
the rocket. In this case, acceleration is BAD
Coast Phase
• Third law- think about this for a minute
– What is the rocket pushing on
– What is pushing on the rocket
Coast phase
• Timing
– Coast phase begins when the rocket runs out
of fuel but continues to go high into the air.
– This phase is where your construction skills
really show off
• The better the rocket, the longer the coast phase
– The coast phase ends when the rocket hits
its highest point in the flight which is called its
apogee.
Re-entry/recovery
• This is the last phase of the rocket’s flight
– It starts when the rocket is at the apogee and
ends when the rocket hits the ground
R/R phase
• Two forces but they are slightly different
here
• Air resistance- remember that air
resistance is a friction, so therefore which
direction is air resistance going to be
working as the rocket is coming down?
• Air resistance increases as the speed of
the rocket increases.
R/R Phase
• If the rocket speed is high enough the
rocket might hit terminal velocity.
• There is only one way that your rocket hit
terminal velocity. What is it?
– Think of how the air resistance could really be
increased a lot
R/R phase
• Gravity still is pulling the rocket down with
the same strength it had before (unless
something fell off your rocket)
• But as I said before the effect of gravity is
different.
• What is gravity causing the rockets speed
to do?
R/R phase
• Newton’s laws here are essentially the
same as the other two phases.
– Just the effect of some forces will be different
or in different directions
• So I am not going to go into a lot of detail
here.