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Main > Propulsion > Current Technology
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| Propulsion - Subsections | ||||
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Although the different propulsion systems in use today vary in design and usage, nearly all of them rely on the energy released in chemical reactions. Here we will introduce the basics of chemical reactions and discuss different types of rockets. Analysis
CHEMICAL REACTIONS So, what is a chemical reaction? It's basically a rearrangement of atoms. A basic reaction looks something like this:
The molecules on the left are the reactants. They are what you start with in a reaction. The molecules on the right are the products of the reaction. The letters represent different atoms: H is hydrogen, O is oxygen. The subscript indicates how many are in a particular molecule: H2O contains two hydrogens and one oxygen. The coefficients of each molecule indicate the ratios of how many molecules are needed in a reaction and how many are produced. So if you had 1,000 molecules of H2, you would need 500 molecules of O2 to get 1,000 molecules of H2O. As previously stated, a chemical reaction is a rearrangement of atoms. No matter is created or destroyed, so the number of atoms of each element on each side should be equal. In the above example, on the left side, there are two molecules of H2 (the coefficients can also be seen as the minimum number of molecules required for the reaction to take place), each containing two atoms of hydrogen, for a total of four atoms of hydrogen. On the right side, there are two molecules of H2O, each containing two atoms of hydrogen, again for a total of four atoms of hydrogen. Similarly, there are two atoms of oxygen on both sides. No atoms were gained or lost during the reaction. So now, the question is, if chemical reactions are just a rearrangement of atoms, where does the energy come from? To answer this, we need to take a look at the structure of molecules. A molecule is made up of two or more atoms of the same or different element bound together. When there are more than two atoms, there is usually one central atom with the other atoms arranged around it; although there are many examples of other structures such as chains and rings. The bonds that hold molecules together arise from the sharing of electrons between the different atoms. Two atoms can share up to three electrons. Bonds are much more stable than free atoms. Energy is released when they form, and energy is required to break them. The atoms involved and the number of electrons shared in the bond determine how much energy is required or released when breaking or forming a bond. The differences in these amounts of energy are where energy comes from in a chemical reaction. For example, again using the above reaction, 436 Joules are required to break the bonds of a mole of hydrogen-hydrogen bonds, and 145 Joules to break a mole of oxygen-oxygen bonds. If we wanted to react two moles of H2 with one mole of O2, would need about 2(436) + 145 = 1017 Joules. Each molecule of water formed contains two hydrogen-oxygen bonds, each of which releases 366 Joules per mole, for a total of 732 Joules per mole of H2O. Since there are two moles of H2O being formed, 1464 Joules are released. To find how much energy is released by the overall reaction, subtract the energy required from the energy released: 1464 - 1017 = 447 Joules released. So how does this energy help? In chemical-powered propulsion systems, the energy is released mainly in the form of heat. Heat is basically rapid, random movement of molecules. This movement causes the substances to expand, especially when they are gases (which is usually the case for the products of these reactions). The reactions usually take place in some sort of chamber where the expanding gases can be directed out, resulting in a forward thrust.
SOLID ROCKETS Specific Impulse: 100-400 sec Thrust: 103-107 N
Quick Fact : The SRBs are the largest solid-propellant motors ever flown and the first designed for reuse. Each is 149.16 feet long and 12.17 feet in diameter 3
MONOPROPELLANT ROCKETS Specific Impulse: 100-300 sec Thrust: 0.1-100 N Monopropellant rockets are simple propulsion systems that rely on special chemicals which, when energized, decompose. This decomposition creates both the fuel and an oxidizer (which allows the fuel to burn), which then react with each other. Because they only use a single propellant, monopropellant rockets are quite simple and reliable. Unfortunately, they are not very efficient. They are mainly used to make small adjustments such as attitude control. Main propulsion systems usually use some other technology.
BIPROPELLANT ROCKETS Specific Impulse: 100-400 sec Thrust: 0.1-107 N Bipropellant rockets separate the fuel and oxidizer and mix them in the chamber where they burn. Bipropellant rockets are widely used and more efficient than monopropellant rockets. The reaction given in the lesson on chemistry gives an example of a fuel(H2)/oxidizer(O2) combination. It's actually a very good conbination in that it releases a large amount of energy. It's the combination used by the space shuttle's main engines. Unfortunately, large tanks kept at extremely low temperatures are required to carry them. In fact, the main purpose of the giant red external tank attached to the space shuttle on take-off is to carry enough fuel to get the space shuttle into space. The main drawback of bipropellant rockets is that they are more complex than solid or monopropellant rockets. The fuel and oxidizer have to be stored separately and fed together in exactly the right ratios to achieve maximum efficiency. Despite the extra complexity, bipropellant rockets are still one of the preferred systems for primary propulsion.
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