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.
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
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.
Specific Impulse: 100-400 sec
Thrust: 103-107 N
||Solid rockets are the simplest and earliest types of rocket propulsion dating back to the first rockets used by the Chinese. Solid rockets are filled with a solid mixture of a propellant and an oxidizer. Little else is actually required for these rockets. The designs are very simple and therefore very reliable. The main drawback of solid rockets is that once ignited, they burn until all of the fuel is gone. Because of this, they aren't used often in space where propulsion systems are usually required to be turned on and off many times. However, they are good for getting things into space. In fact, the space shuttles use solid rocket boosters (SRBs) during takeoff.
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
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.
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.