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					                  Star Wars Episode 3 – The Revenge of the Sith                          1
                         Yoda´s escape from Kashyyyk

Scientific subject and topic:
Physics / Mechanics

Title / year:
Star Wars Episode 3 – The Revenge of the Sith / 2005

Movie producer:
Lucasfilm Ltd.

George Lucas

Hayden Christiensen, Natalie Portman, Ewan McGregor Ian McDiarmid. Christopher
Lee, Samuel L. Jackson

Websites of movie:
Filmography links and data courtesy of The Internet Movie Database

Description of movie:
After three years of fighting the clone wars come to an end. To finally conquer the
separatists Obi Wan Kenobi is ordered to punish General Grevious and Count Dooku.
Anakin Skywalker could be a Jedi Master but the council of the Jedi declines his
desire. The chancellor Palpatine, who changed to the dark side, tries to deduce
Anakin also to the dark side of power. Anakin Skywalker now is perplexed between
the bad and the good side of power. He helps chancellor Palpatine killing a Member of
the council of the Jedis and now he is the subject of chancellor Palpatine and changed
to he identity of “Darth Vader“.

The Sith Lords begin a revenge campaign against the Jedi knights and start with
killing the young Jedi children by Darth Vader. Anakin is commanded to the hot planet
„Mustafar“, where he finally mutates to Darth Vader. Padme returns to Polis Massa.
She expects two children. After the birth of Luke and Lea Skywalker, Padme dies on
the planet Polis Massa. The two children get separated and brought to different
parents on different places.

Link to Trailer Site:
Filmography links and data courtesy of The Internet Movie Database

DVD: Star Wars Episode 3 – The Revenge of the Sith (U.K.)

DVD: Star Wars Episode 3 – The Revenge of the Sith (U.S.A.)
Title of scene:                                                                             2
Yoda’s escape from Kashyyyk


DVD cover scan from the movie Star Wars Episode 3 – The Revenge of the Sith
claimed as fair use

Time interval:
01:22:40 - 01:22:57

Fidler Martin []

Heinz Oberhummer []

Scientific Keywords:
Recoil, Rocket, Rocket Equation

Description of scene:
The clone army gets instructions from Darth Sidious to kill all the Jedi knights in the
Galaxy. Also Yoda is endangered. He just spends his time on the planet „Kashyyk“,
the home world of the Wookies. Suddenly an army of clone warriors appears, trying to
kill him.

A short time before Yoda has held a holographic conference with the other Jedi
knights. Now he is on the run from the clone army. He flees in the mountains, because
there is a hidden flight capsule ready for him. The Wookies escort him to this capsule.
Yoda says good bye to the Wookies and enters the capsule. After that he ignites the
engine of the capsule and lifts of with a blue-greenish jet. He is on his journey back to
Author:         Fidler Martin

Movie:          Star Wars Episode 3 – The Revenge of the Sith
Movie scene:    01:22:40 - 01:22:57
Director:       George Lucas
Film Studio:    20 Century Fox.

Basic description

To accelerate an object like a car or a rocket, it is necessary that a force is acting. On
Earths’ surface this force easily can be exerted for instance through the wheels on the

In the atmosphere air can be used as the medium causing the recoil. The air is
accelerated by a propeller or a turbine and creates this way recoil for the object.
Joda’s journey is very long and he also has to pass through the vacuum in space.
In outer space the principle of recoil is different: There is no ground or another object
where it is possible to repel. The outer space is finally mass less (exceptions are
objects like stars, fogs or planets). The most common techniques to move forward is
to use the recoil of mass that is carried along. One can imagine recoil by the example
of an air balloon, which is filled with air and loosened. The air is escaping out of the
balloon and pushes him in the opposite direction.This works as long as air is escaping
from the balloon.

In modern astronautics not air is used but material that is extremely flammable
Then the combustion generates a very high temperature. When the temperature
increases, also the pressure is enhanced and the hot material escapes with a very
high velocity. It is not impossible to reach such velocities with mechanical methods
like air balloons or propellers. A propeller would not work in the outer space.

The material that is burnt expands very quickly and streams out through a jet
generating recoil. The velocity a rocket or a space shuttle has to reach is very high
because of to reasons:
1) to escape from the gravity of a planet
2) to get over the large distances in outer space (already 400000 km to the moon)
Recoil drive on the basis of combustion is actually the only technology that is used in
modern astronautics. There are alternative power units that are already working.
However, combustion-based drive will be the primary technology for still a long time.
Author:         Fidler Martin                                                                4

Movie:          Star Wars Episode 3 – The Revenge of the Sith
Movie scene:    01:22:40 - 01:22:57
Director:       George Lucas
Film Studio:    20 Century Fox

Advanced description

The principle of recoil
In physics the principle of recoil is a specific application of the 3 axiom of Newton.
It can be stated as follows:
Mechanical forces always appear in pairs. If an object named A exert a force F on an
object named B the same force acts on B in the opposite direction. This force has the
same value but opposite direction.

The corresponding equation reads:

In rockets the combustion of fuel generates a force .This force acts on the rocket in
the opposite direction. This is named „recoil“. The recoil forces accelerate the rocket to
a certain velocity.

We can also introduce the idea of momentum The momentum p is defined as the
product  of    mass     m    and   velocity v     through   the    equation:

The value of the momentum of the ejected burned fuel is the same as for the rocket.

The rocket equation
Now what is the relation between
- start mass m of the rocket
- mass loss in a time interval dm(t)/dt
- velocity oft he burned fuel vg
- and velocity the rocket reaches after a time t, v(t)?

We start with the formula oft he shear force:

We set the left side equal zero and integrate the whole equation and obtain:

After the integration is carried out, we express the velocity of the rocket v(t) as a
function of m, m(t) and vg:
This is the famous rocket equation that was found by Konstantin Ziolkowski in the year
1903. The quantity m(0) is the mass oft he rocket at the begin, m(t) is the mass of the
rocket after a time t, vg is the velocity of the burned fuel, and v(t) is the velocity of the
rocket after a time t.

Typical numerical values in the case of a „Saturn V“ rocket are as follows:
The start mass is about 2900 tons. After 150 seconds the rocket has burnt 2000 tons
of fuel so the mass m(t=150) is about 900 tons. The emission velocity of the burned
chemical fuel is about 2km/s to 4 km/s.

The emission of fuel:
A rocket drive is based on the combustion of fuel. The fuel is burned in a combustion
chamber very quickly and accelerates the rocket by emitting a jet.

The emitted velocity of the fuel is given by the formula:

There vs is the emitted velocity, pi and pa are pressures in the combustion chamber
and outside, and I is the density of the ejecting gas. In outer space pa is equal to zero.

How is it possible to take off from a planet and travel trough outer space?
To get to a higher orbit it is necessary to leave the gravitation field of a planet. After
leaving the gravitation field normally there is no influence of gravitation on the rocket
by another object. To leave the gravitation field of a planet like Earth it is necessary to
reach a certain velocity called „escape velocity“.

To calculate the escape velocity the potential energy (left side) and the kinetic energy
(right side) are set equal:

and so the velocity v is given by:

With the numerical values for he mass oft he Earth M, the gravitational constant G and
the radius of the Earth r, the velocity is calculated to 11.2 km/s.

Is it possible to rocket a person in outer space in such a small object?
In the described scene Yoda is entering a capsule, where there is not much place for
fuel. At there moment there are devices, which can be strapped around the body and
holding up somebody in the air for a few minutes. These devices work with little
nozzles for holding the position, but are not able to carry someone outer space.

The following rough estimate makes the described scene of Yoda´s escape
questionable. We start with the rocket equation in the gravitation field of Earth:

Now the question is: How much fuel is necessary to hold up somebody for 5 minutes
(equal to 300 seconds ) in the air ?

The numerical values that are used, are as follows:
t = time in the air = 5 inutes = 300 seconds
vg = streamingt velocity = 4500 m/sec. (fastest possible value today)
g(t) = acceleration of free fall = 9.81 m/s (at the surface oft he earth)
v(t) = velocity of the capsule = 0 m/s because it staying still in the air
m(t) = 150 kg = mass of the capsule after 5 minutes of burning

With these values the mass of the object at the time t = 0 is calculated to 700kg. This
is the lower bound for the starting mass, because the object has no velocity in this
case                                                                           (v(t)=0).

A device being able to carry a human body in outer space would be much larger than
the capsule in Star Wars 3, if the drive is based on the recoil of burned fuel.

How is a usual rocket engine working?
Basically there are different kinds of recoil–based rocket drives. At the moment the
most commonly used drive is the “chemical drive“. The other kinds of rocket drive are
in experimental stage. If they will be used in future depends on the effectiveness and
the expensive development of such drive units.

Here is a shortly overview about different kinds and functions of rocket drive units:

Chemical combustion:
There are 3 categories of combustion: The liquid, the solid material and the hybrid
rocket. The solid rocket: The whole fuel is covered by a tank. It burn down from the
bottom to the top and cannot be stopped anymore after ignition.

The liquid material rocket consists of two liquid components that are separated
covered by two tanks. With pumps the liquids arte transported to the combustion
chamber through tubes, where they interact and generate the chemical combustion.
Hybrid rockets have liquid and also solid material aboard which interact together.

Electrical rocket drives:
The most well-known electrical drive is the ion-drive. The ion – drive works as follows:
First gas-particles (mostly xenon) get ionized meaning that they get electrically
charged. After that they are accelerated in an strong electric and magnetic field at high
velocity. After the last acceleration section they get neutralized, because after leaving
the drive unit they would come back to the rocket and there would not be a recoil
effect. This part is called „neutralizer“.

Nuclear rocket drive
In a nuclear rocket drive unit the recoil material is get heated by a nuclear reaction. In
this reaction an atomic nucleus get split in tow pieces by a neutron and so a lot of
energy is getting free. The heated material is passing a nozzle generating the recoil
effect. The nuclear drive is forbidden by law and it is much more dangerous than
atomic plants.
What is the future of rocket drive units?
There are many of visions to make rockets faster and to make their drives more
effective. The ion drive seems to be quite advanced, but in practice it doesn´t generate
enough power. It can only be used for positioning corrections in satellites but not for
accelerate thousand of tons.

The sun sail drive:
The sun sail drive uses the momentum of solar photons pressure on the sail. The time
after the object reaches a higher velocity can be very long but the reached velocity
can be very high.

The solar thermal drive:
The solar thermal drive works as follows: A parabolic reflector bunches the sun light
on a graphite block. Through this the graphite block is heated to 2400 Kelvin
generating a recoil effect. This is used for changing positions of satellite.
Author:        Martin Fidler                                         8

Movie:         Star Wars Episode 3 – The Revenge of the Sith
Movie scene:   01:22:40 - 01:22:57
Director:      George Lucas
Film Studio:   20 Century Fox

Scientific description

Wikipedia-websites about recoil, rocketry and the rocket equation:

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