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INTRODUCTION
Spacecraft propulsion is used to change the velocity of spacecraft and
artificial satellites. There are many different methods. Each method has drawbacks
and advantages, and spacecraft propulsion is an active area of research. Most
spacecraft today are propelled by heating the reaction mass and allowing it to flow
out the back of the vehicle.For propulsion the required product is the velocity of
the exhaust products of the reaction. All current spacecraft use chemical rocket for
launch.
When in space, the purpose of a propulsion system is to change the velocity
of a spacecraft. When launching a spacecraft from the Earth, a propulsion method
must overcome the Earth's gravitational pull in addition to providing acceleration.
Interplanetary vehicles mostly use chemical rockets and this takes at least six
months to reach Mars. And this fusion propulsion system makes this possible for
humans to reach Mars within three months.
Fusion propulsion mainly uses fusion reactions to produce thrust to propel
rockets. It is a type of nuclear propulsion system derives its thrust from the
products of nuclear fusion.
Fusion reactions release an enormous amount of energy, which is why
researchers are devising ways to harness that energy into a propulsion system. A
fusion-powered spacecraft could move up NASA's schedule for a manned Mars
mission. This type of spacecraft could cut travel time to Mars by more than 50
percent, thus reducing the harmful exposure to radiation and weightlessness. The
building of a fusion-powered spacecraft
would be the equivalent of developing a car on Earth that can travel twice as fast as
any car, with a fuel efficiency of 7,000 miles per gallon. In rocket science, fuel
efficiency of a rocket engine is measured by its specific
impulse. Specific impulse refers to the units of thrust per the units of propellant
consumed over time.
A fusion drive could have a specific impulse about 300 times greater than
conventional chemical rocket engines. A typical chemical rocket engine has a
specific impulse of about 450 seconds, which means that the engine can produce 1
pound (.4539kg) of thrust from 1 pound of fuel for 450 seconds. A fusion rocket
could have an estimated specific impulse of 130,000 seconds. Additionally, fusionpowered rockets would use hydrogen as a propellant, which means it would be
able to replenish itself as it travels through space. Hydrogen is present in the
atmosphere of many planets, so all the spacecraft would have to do is dip down
into the atmosphere and suck in some hydrogen to refuel itself.
Fusion-powered rockets could provide longer thrust than chemical rockets,
which burn their fuel quickly. It's believed that fusion propulsion will allow rapid
travel to anywhere in our solar system, and could allow round trips from Earth to
Jupiter in just two years.
FUSION
Fusion comes from the word fuse. Fusion occurs when two nuclei combine
to from a new nucleus. In the diagram shown below, we are looking at a depiction
of two hydrogen nuclei fusing to form a Helium nucleus. We see fusion everyday.
Fusion causes the energy release, and therefore the light we see from the sun and
the stars. Nuclear fusion which is much cleaner which has no radioactive byproducts as that of fission and has a higher energy density, about 108 (10^8 or 100
million) times greater than current chemical systems. The energy released in most
nuclear reactions is much larger than that for chemical reactions, because the
binding energy that holds a nucleus together is far greater than the energy that
holds electrons to a nucleus. For example, the ionization energy gained by adding
an electron to hydrogen is 13.6 electron volts -- less than one-millionth of the 17
MeV released in the D-T (deuterium-tritium) reaction shown in below.
D
+
T
→
4He (3.5 MeV) + n (14.1 MeV)
Fusion occurs at a sufficient rate only at very high energies (temperatures) on earth, temperatures greater than 100 million Kelvin are required. At these
extreme temperatures, the Deuterium - Tritium (D-T) gas mixture becomes a
plasma (a hot, electrically charged gas). In a plasma, the atoms become separated electrons have been stripped from the atomic nuclei (called the "ions"). For the
positively charged ions to fuse, their temperature (or energy) must be sufficient to
overcome their natural charge repulsion
Since no fossil fuels are used, there will be no release of chemical
combustion products because they will not be produced. Similarly, there will be
no fission products formed to present a handling and disposal problem.
Radioactivity will be produced by neutrons interacting with the reactor structure,
but careful materials selection is expected to minimize the handling and ultimate
disposal of activated materials
Fusion fuel
Mainly uses deuterium and tritium as the fusion fuel
Deuterium
Heavy isotope of hydrogen in which the nucleus contains one proton and
one neutron (compared with ordinary hydrogen’s single proton). The abundance of
deuterium in interstellar space is about 1.4 × 10-5 that of hydrogen. Because
deuterium is difficult to manufacture and is quickly destroyed in stellar nuclear
reactions, one view is that most of the deuterium found in the universe today was
formed in the Big Bang. It is an important fuel for nuclear fusion
Electron
Positron
Neutron
Tritium
Tritium, as a form of Hydrogen, is found naturally in air and water. Most
hydrogen is made up of one proton, and an orbital electron, but tritium has two
extra neutrons in the nucleus.
Electron
Positron
Neutron
Plasma
Plasma is made up of atoms. Atoms are composed of one or more negatively
charged electrons that orbit the nucleus. The nucleus has positively charged
protons and neutral particles called neutrons. Atoms are electrically neutral, but
can become either positively charged, or negatively charged when exposed to
radiation. When this happens they become ions and negatively charged free
electrons. Another term for plasma is "ionized gas". Plasma is distinct from
common gaseous state because it consists of electrically charged particles.
Movement of charged particles in a plasma
a) In the absence of a confining magnetic field, hot plasmas tend to
spread and fill the space available;
b) If a linear magnetic field is applied, the particles move in helical
each encircling a line of force and thus remain radially
Plasma without magnetic
confinement
confined
Plasma with magnetic confinement
paths,
CONDITIONS FOR FUSION
Nuclei are always positive charged so the electrical repulsion prevents the
two nuclei from getting close to one another. A substantial energy barrier must be
overcome for fusion to occur. Nuclei repel one another because of the electrostatic
force between their positively charged protons. If two nuclei can be brought close
enough together, however, the electrostatic force is overwhelmed by the more
powerful strong nuclear force which only operates over short distances.
When a nucleon (proton or neutron) is added to a nucleus, the strong force
attracts it to other nucleons, but primarily to its immediate neighbors due to the
short range of the force. The nucleons in the interior of a nucleus have more
neighboring nucleons than those on the surface. Since smaller nuclei have a larger
surface-to-volume ratio, the binding energy per nucleon due to the strong force
generally increases with the size of the nucleus but approaches a limiting value
corresponding to that of a fully surrounded nucleon.
The electrostatic force, on the other hand, is an inverse-square force, so a
proton added to a nucleus will feel an electrostatic repulsion from all the other
protons in the nucleus. The electrostatic energy per nucleon due to the electrostatic
force thus increases without limit as nuclei get larger.
Generally the reactions take place at such high temperatures that the atoms
have been ionized, their electrons stripped off by the heat; Thus, fusion is typically
described in terms of "nuclei" instead of "atoms". To achieve fusion, you need to
create special conditions to overcome this tendency. Here are the conditions that
make fusion possible:
1. High temperature
The high temperature gives the hydrogen atoms enough energy to overcome
the electrical repulsion between the protons. Fusion requires temperatures about
100 million degrees Kelvin (approximately six times hotter than the sun's core). At
these temperatures, hydrogen is a plasma, not a gas. Plasma is a high-energy state
of matter in which all the electrons are stripped from atoms and move freely about.
The sun achieves these temperatures by its large mass and the force of gravity
compressing this mass in the core. We must use energy from microwaves, lasers
and ion particles to achieve these temperatures.
2. High pressure
Pressure squeezes the hydrogen atoms together. They must be within 1x1015 meters of each other to fuse. The sun uses its mass and the force of gravity to
squeeze hydrogen atoms together in its core. We must squeeze hydrogen atoms
together by using intense magnetic fields, powerful lasers or ion beams
MANETIC CONFINEMENT FUSION PROPULSION
Magnetic Confinement Fusion (MCF)
MCF sometimes referred to as continuous fusion, effectively tries to recreate
the Sun's method of achieving fusion, by super heating the fuel to hundreds of
millions of degrees by using a plasma. The theory is that as the fuel is heated the
atoms become much more excited and as they rush a round at high speeds there
is an increased chance that the nuclei will get close enough to fuse.
The major problem here is that plasmas are very difficult to create and even
more difficult to control, mainly because they would simply melt through any
structural confinement. The Sun overcomes this
simply by its immense
gravitational field strength, however this is not possible for us to mimic so the
fusion plasma is contained in extremely powerful magnetic fields.
This is
possible because the plasma is composed mainly of ions and electrons, which of
course have electromagnetic charges. Despite this it is still technically very
difficult to achieve for any useful length of time (i.e. for reactions to occur).
In MCF the hot plasma is confined by magnetic fields forming a magnetic
trap for the charged particles. In theory, a stationary burn is possible for as long as
the magnetic confinement is maintained.
Diagram illustrating the principle of magnetic confinement in a torus (in this
case a tokamak). The plasma is ring-shaped and is kept well away from the vessel
wall.
MCF Propulsion
MCF as a propulsion system is possibly not the optimum approach, although
it is quite possible that MCF will be the optimum for power generation, but we will
have to wait and see for a definitive answer. Before the development of a MCF
propulsion system fundamental scientific understanding of basic components must
be achieved through further research, for example a plasma diverter will be
necessary.
There are two primary problems with this system as a propulsion system
even if we can develop one. Firstly the weight of the reactors would be
prohibitively large due to the huge magnets that are required for containment. This
problem is further enhanced by the fact that MCF operates a very low density, that
MCF operates a very low density, which means that much larger reactors would be
required than for ICF.
Reactions take inside a magnetic bottle and release the plasma via a
magnetic nozzle, so that no solid matter need come in contact with the plasma. A
magnetic bottle contains the fusion reaction. Very difficult to do. Researchers in
this field say that containing fusion plasma in a magnetic bottle is like trying to
support a large slab of gelatin with a web of rubber bands. Making a magnetic
bottle which has a magnetic rocket exhaust nozzle is roughly 100 times more
difficult.
INERTIAL CONFINEMENT FUSION PROPULSION
Inertial Confinement Fusion (ICF)
ICF, sometimes referred to as pulsed fusion, is a different idea based on the
same principle, in this case a tiny plasma, a thousand trillion times more dense than
that used in MCF, is created by using blasts from lasers to rapidly superheat fuel
pellets. This plasma then rapidly expands and, due to an equal and opposite
momentum reaction, compresses the fuel pellets (which increases the fusion
reaction rate). This combination of laser and the compression produces enough
heat to induce fusion to occur.
This system requires no heat containment, as the reaction is so rapid. This
means there is no need for the magnetic fields; the pellet's own inertia should
confine the heat long enough for a fusion reaction. There also exists the possibility
that particle beams could be used instead of lasers, as they are more efficient.
A pellet of fusion fuel is bombarded on all sides by strong pulses from laser
or particle accelerators. The inertia of the fuel holds it together long enough for
most of it to undergo fusion.
In inertial confinement fusion (ICF), nuclear fusion reactions are initiated by
heating and compressing a target – a pellet that most often contains deuterium and
tritium – by the use of intense laser or ion beams. The beams explosively detonate
the outer layers of the target, accelerating the remaining target layers inward and
sending a shock wave into the center. If the shock wave is powerful enough and if
high enough density at the center is achieved some of the fuel will be heated
enough to cause fusion reactions, releasing energy. In a target which has been
heated and compressed to the point of thermonuclear ignition, energy can then heat
surrounding fuel to cause it to fuse as well, creating a chain reaction that burns the
fuel load, potentially releasing tremendous amounts of energy. Theoretically, if the
reaction completes with perfect efficiency , a small amount of fuel about the size of
a pinhead releases the energy equivalent to a barrel of oil.
ICF Propulsion
This propulsion system would operate by detonating pellets in a chamber at
the rear of the vehicle using lasers. Detonation will have to occur at a rate of
anywhere between 30 and 250 per second. And this ICF produce tremendous
amount of energy which is exhausted. ICF operates at a much higher density than
MCF but it should be noted that the required banks of lasers are likely to be heavy,
power intensive devices, though probably less so than the magnets in MFC.
MUON-CATALYSED FUSION PROPULSION
Muon-Catalysed Fusion
By contrast to these two approaches muon-catalysed fusion is much
different. A negative muon is an elementary particle similar to an electron, but
about 207 times as massive. Due to this much larger size the muon orbits much
closer to the nucleus than the electron. So, in terms of fusion, the idea is to
introduce these muons (replacing the electrons) and allow their negative charge to
effectively shield the positively charged nuclei from each other. This will eliminate
the electrical repulsion force and allow the nuclei to get close enough for the strong
nuclear force to fuse them. Finally the fusion energy ejects the muon and it goes
off to attach itself to another nucleus.
The big advantage here is that no superheating or confinement is needed,
indeed the reaction can occur at virtually any temperature. This new theory brings
new problems however, firstly it is extremely difficult to actually introduce these
muons and allow them to move into orbit around the nucleus. Muons also have a
very short lifetime, about 1 millionth of a second, so to be used to catalyse many
reactions the fusion itself will have to be extremely fast. There is also the problem
that no one is sure how much of the muon will be lost in each reaction it catalyses.
Finally it is a very expensive process producing muons, both in terms of energy
and money, for this system to break-even the muon must catalyse many
reactions for the system to produce as much energy as was put in producing and
using the muons.
Normally, two atoms of hydrogen will combine to make a molecule of
hydrogen gas and are held together by their electrons. However, the nuclei of
the atoms are not nearly close enough to fuse and their mutual repulsion keeps
them apart. The muon is part of the same family of particles as the but is 207
times heavier and is typically created in a nuclear accelerator. A negative muon has
similar properties as an electron and can also orbit an atom. The muon only lasts
about 2.2 microseconds before decaying and becoming an electron. When a
negative muon is fired at hydrogen (regular or heavy), the muon will knock the
electron out of a hydrogen atom and take its place, but since the muon is heavier it
orbits much closer to the nucleus than the electron and cancels some of the positive
charge of the nucleus. This muonic hydrogen atom can combine with another
hydrogen atom, forming a molecule where the two atoms are bound closely enough
by the muon to counteract the repulsion of the nuclei and fuse them. There are
many ways fusion can occur, producing atoms of deuterium, tritium, 3-helium, and
normal helium. One of the ways the fusion can occur is with deuterium and tritium
atoms where the muon binds a deuterium-tritium molecule tightly enough to fuse
them into a helium atom and an extra neutron is shown below. The muon then
leaves the new helium atom and continues on to fuse more hydrogen. Since the
muon is not consumed in the fusion reaction, it acts as a catalyst. Muons have
catalyzed up to 150 fusion reactions in some experiments before they decay. The
main problem with this approach is that the muon tends to stick around the new
helium nucleus, wasting part of its life before leaving to catalyze more reactions. In
order to be a viable energy source, the energy released in the fusion reactions must
be much more than the energy used in the accelerator to create the negative muons
and fire them at the hydrogen. The main stumbling block has been reducing the
stickiness
of
the
muons
further to enable many more reactions to occur. Finding less energy intensive ways
of producing muons is also being researched. Scientists reached energy break-even
in the 1980s and are continuing to try to harness this tantalizing method of fusion
Deuterium-Tritium Molecule
Muon catalyzed D-T Fusion
Muon-Catalysed Fusion Propulsion
This concept may not be the optimum method for a propulsion system, the
short lifetime of the muon would mean that they would have to be manufactured
on the spacecraft, and this would offset the weight saving of not needing magnets
or lasers. The controlled plasma is exhausted through the exhaust nozzle. Added to
this with current technology the energy required to produce muons is probably too
great for us to generate onboard a spacecraft, which means to become a realistic
possibility much easier and cheaper methods of production would need to be
found. It should be pointed out that if Zero Point Energy proves to be available the
energy problem could be avoided, but the mass problems would remain. Of course
the efficiency of this type of fusion is unknown at the present time, if a muon
proves unable to catalyse many reactions it is likely the system would be too
inefficient for consideration.
CONCLUSION
This concept increases the mission flexibility, enabling new science
missions and greater flexibility in reaching and exploring distant. It reduces the
time for the journey in space mission by 50% compared to the chemical rocket.
The specific impulse offered from fusion propulsion systems could be more than
1,000,000secs; this together with moderately high thrust levels allows this
propulsion system to open up the entire solar system to human exploration.
REFERENCES
1. www.spacesite.com.
2.www.wikipedia.com.
3. http://science.howstuffworks.com/fusion-propulsion.htm.
4. www.fusionpropulsion.com
5. Fuels and Combustion, author Samir Sarkar
ABSTARACT
A new concept for the interplanetary and interstellar mission engine. All
current spacecraft use chemical rocket for launch and this fusion propulsion system
uses fusion rockets for launch. Fusion propulsion has the potential to produce high
speed transportation any where in the universe. In this propulsion use fusion
reactions to produce thrust to propel rockets. In order to occur fusion we have to
create conditions like high temperature about 100 million degree celcius and high
pressure. At these conditions plasma is formed and the fusion reaction takes place
producing high amount of energy which is exhausted through the nozzle. It is very
difficult to confine the plasma and uses magnetic confinement, inertial
confinement methods for controlling the plasma. And the types of fusion
propulsion are magnetic confinement propulsion, inertial confinement propulsion
and the emerging type moun-catalyzed propulsion.
CONTENTS
1. INTRODUCTION
1
2. FUSION
3
3. CONDITIONS FOR THE FUSION
7
4. MANETIC CONFINEMENT FUSION PROPULSION
9
INERTIAL CONFINEMENT FUSION PROPULSION
12
6. MUON-CATALYSED FUSION PROPULSION
15
7. CONCLUSION
18
8. REFERENCES
19
5.