12-01-2011, 07:49 PM
ABSTRACT
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.
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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, fusion-powered 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 by-products 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 plasma (a hot, electrically charged gas). In 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
Commonly 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
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.
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 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 paths, each encircling a line of force and thus remain radially confined
Plasma without magnetic Plasma with magnetic confinement
confinement
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 1x10-15 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
MAGNETIC 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, 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-Catalyzed Fusion
By contrast to these two approaches muon-catalyzed 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 electron, 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-Catalyzed 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
Fusion has enormous potential as an energy source but the set up is too heavy to be used in propulsion at present. If we can reduce the weight of the reactor nuclear fusion can be a very efficient energy source for rocket propulsion. 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. Ignition on the National Ignition Facility: a path towards inertial fusion energy Edward I. Moses 2009 Nucl. Fusion 49 104022
2. spacesite.com.
3. wikipedia.com.
4. http://science.howstuffworksfusion-propulsion.htm.
5. fusionpropulsion.com
