Propulsion on a Manned Martian Expedition
written by Kevin Reimund on January 15, 2003 | contact me
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Fusion: To understand the problems and advantages of fusion propulsion, you first must understand what fusion is. In a relatively simple example, when a hydrogen-2 nuclei and a hydrogen-3 nuclei are placed under intense heat and pressure, the two atoms combine and form helium nucleus. Since a helium nucleus is smaller than 5 hydrogen nuclei, a neutron plus energy is released. This process is very powerful and produces enormous amounts of force. The reaction that takes place in the center of Sol, our sun, and heats up the planets and is visible all the way on other stars. Fusion can also occur between helium 3 and deuterium as well as other elements. There are 3 basic types of fusion proposed. They are continuous fusion, pulsed fusion and controlled fusion.
Continuous fusion uses hydrogen/plasma fuel to produce nuclear fusion. Extremely powerful electromagnets compress hydrogen to the point where it fuses. A magnetic "cap" prevents backflow and a directed magnetic "funnel" forces the fusion reaction toward the exhaust nozzle. At the very end, electromagnets act as a funnel and add extra thrust as they try to recompress the fusion flow. The fusion flow exits the nozzle at very high speeds and propels the spacecraft at high speeds.
Pulsed fusion involves a very large solar sail and a supply of fusion bombs. The fusion bombs are launched at the solar sail and detonated. The sail takes off at high speeds dragging cargo behind it. This is considerably cheaper and simpler, but has a more limited fuel supply.
Controlled fusion involves a very strong reinforced chamber in which a fusion bomb is detonated. Since fusion explosions are large and this chamber is not 5 miles wide, the chamber instantly become highly pressurized with a nuclear fusion explosion. As this explosion rushes out, it propels the spacecraft forward at high speeds.
There are many problems with fusion. For one, the fusion exhaust stream could read anywhere from 500,000 all the way up to 15,000,000 degrees fahrenheit. Nothing that we have can survive these temperatures. This has been solved by using electromagnets and the most efficient fusion reactor returns 65% of the energy used to create the fusion reaction. This heat is not a problem in space because the vacuums are very cold and we are exhausting the stream, not trying to control it. Fusion is also hard to start, since it requires immense amounts of pressure and heat. There is one distinct advantage with fusion "torch" ships. Since hydrogen is expelled in solar wind, a magnetic "funnel" of sorts can be used to collect hydrogen and use it as fuel. There is a similar design commonly known as the Bussard Ramscoop.
Antimatter: As far as we know, things are made out of molecules. Molecules are made out of atoms and, theoretically, atoms are made out of quarks. According to theory, each atoms has 3 quarks, all positive. Antimatter (which exists, even if quarks do not) is supposed to have 2 positive quarks and 1 negative quark. Apparently, this negative quark makes it impossible to have matter and antimatter touching each other. When these two matters come into contact with each other, they annihilate each other and the result is an explosion of pure energy. Antimatter is magnetic, so it can be controlled without coming into contact with matter, so it can be used as fuel. Magnetic antimatter and magnetic sand could be used to power a spacecraft. These two items would be slowly funneled into the center of a highly reinforced chamber. When they come into contact with each other, they will explode violently and exhaust through the opening into space. Since the energy of an object is the mass of the object times the speed of light squared, 1 gram of antimatter and 1 gram of matter can release 22,320,000 grams of thrust per minute of continuous anti/matter annihilation (120 grams of fuel), or 373,000 grams of thrust per second. This amount of thrust is unheard of in any other form of propulsion system that we at least have a general idea as to how it would work. Please note that a single gram does not weight that much. It takes around 3,000 grams to equal a pound.
A trip to Mars using an antimatter annihilation chamber could take as little as 24 hours, maybe less. Like all good things, there is 1 drawback. Antimatter costs around 10,000,000 per ounce. That means that it would cost about 10,000,000 for 187.5 grams of antimatter, or 3.125 minutes of thrust. The only way around this problem is to either construct large particle accelerators specifically designed to harvest antimatter, or to find a large deposit of antimatter within our solar system. This idea is on the back burner for now, but it has not been forgotten.
Photon Reaction: Until recently, scientists believed that photons (light) had no mass, but recently, it was discovered that they did have mass. Let's do this logically. Light has mass. Light is reflected by reflective surfaces. For every action, there is an equal and opposite reaction. That means that if you put a mirror in space facing the sun, it would slowly accelerate out of orbit as the photons pushed slowly. This is the idea behind solar sails. There are four proposed solar sails, the solar propeller, the solar sail, the laser stream and the laser sail.
A solar propeller involves a series of mirrors arranged in a pinwheel like formation. Light hits these and is reflected at an odd angle. This gives the spacecraft a spin and propels the craft forward.
A solar sail works on the same principal except with literally, a large sail made out of something similar to tin foil. Light reflects off of it and propels it forward.
A laser stream involves generating light, compacting it into a laser and shining it out the end of the spacecraft. This provides very little thrust, similar to a Hall Effect Thruster, except using photons instead of xenon.
A laser sail involves the same exact principles, except with a laser pushing the spacecraft instead of the light from the sun. Since lasers are compacted photon streams, they propel the spacecraft much faster than conventional solar sails could ever hope to achieve. An Earth mounted laser could push spacecraft as far as Mars without much loss of power.
Electrothermal: A lot of gasses expand when they are heated. This is the principal behind electrothermal propulsion. Gas is heated and expelled out the rear of the spacecraft. Nothing more, nothing less.
Electrodynamic: A little bit trickier. Xenon is positively ionized. Since it has a positive charge, it will move toward the negatively charged electromagnets which then propel the ionized gas out into space. A fifty gigawatt test module provides about as much thrust as a the weight of a sheet of paper, however, since it can fire continuously and does not need gravity assisted launches, it is faster.
Hypergolic Chemical Combustion: Nitrogen tetroxide and monomethal hydrazine come in contact with each other and explode using no oxygen. Simple as that.
Hydrogen/Oxygen Combustion: Hydrogen can be funneled into a fuel chamber using a ramscoop and oxygen can be obtained elsewhere or carried as fuel. These two get together in a chamber and are ignited. Exhausts rush out.
Methane/Oxygen Combustion: Same as hydrogen combustion except does not need oxygen, only carbon dioxide. CO2 is recycled and mixed with ramscoop found hydrogen. These form oxygen and methane which can be burned together.
Caution- You are now entering the theoretical zone. None of these drives have any actual testing behind them and we don't even know how some of them could even possible work, though they're worth mentioning.
Antigravity: Antigravity is the opposite of gravity. It repels and if we could generate it artificially, we could use it to propel a spacecraft.
Inertialess Drive: An idea presented by Arthur C. Clarke that one day, we may reverse inertia so that instead of an object remaining at motion, it continues to accelerate when force is applied.
Subspace/Hyperspace: As leading theoretical physicists Michio Kaku explains, Hyperspace is another level of space and time which is level with our space. Since space is (theoretically) curved, imagine we must go all the way around the curve while hyperspace links the two points directly.
Wormhole: Another hyperspace theory that holes in space can link 2 places instantly.
Warp Drive: Warping space to make travel time shorter. Equivalent of driving down a 10 mile road, bringing the end of the road to you, driving onto it and having it resume its original position. Effectively 0 travel time.
Some of these drives are theoretical while others have been used for years. Some of these drives have worked but are not currently used and others are in the process of getting "final touches." Which one will we use?