Have you ever wondered what chemicals are used for the propellants to launch the shuttle? Or what the reactions were like? Or how much energy they really produce? This article discusses the chemistry involved in launching the shuttle. We shall look at the reactions of the solid rocket boosters (SRBs), the main engines, and the orbital maneuvering system (OMS) engines.
Combustion reactions are oxidation-reduction reactions; that is, electrons are transferred from one chemical element to another. Under many circumstances, combustion takes place in the presence of molecular oxygen (O2), which combines with a fuel to produce gases. Under ideal circumstances, a combustion reaction will produce only the reactants predicted; in reality, however, combustion reactions are not very clean. The combustion of gasoline in an automobile engine theoretically should produce only water (H2O) and carbon dioxide (CO2) but this is not what really happens. Lots of other, less desirable, products are created, like carbon monoxide (CO). If the gasoline combustion reaction were perfectly clean, we would not have any need for catalytic converters or any other anti-pollution measures on our cars. The reactions given below show the ideal products from the combustion.
The SRBs are solid rockets, meaning that the fuel that they use is in solid form. Solid-fuel rockets will, once ignited, continue to burn until all of the fuel is gone. Once the SRBs are ignited, the orbiter WILL launch. There is no way to stop the reaction once it is initiated. The two main chemicals in the SRBs are aluminum (Al) powder, which is the fuel, and ammonium perchlorate, NH4ClO4, as the oxidizing agent. A small amount of iron oxide is present as a catalyst for the reaction and the chemicals are bound together using a binding agent and an epoxy-like plastic.
The combustion reaction looks like this:
8 Al + 6 NH4ClO4 —> 4 Al2O3 + 3 Cl2 + 12 H2O + 3 N2
This reaction heats the interior of the SRBs to a temperature of 3475° K (5800° F). The heat generated during the combustion causes the rapid expansion of the chlorine gas, the nitrogen gas, and the water vapor. This expansion provides the lifting force for the SRBs. At ignition there are over one million pounds of propellant in each SRB; it takes only two minutes for all of it to burn. The white smoke that billows away from the pad during launch is aluminum oxide (Al2O3) generated from the SRBs.
The main engines use liquid fuel, which has the advantage of being more controllable—the reaction can be stopped at any point by simply shutting off the flow of liquid. The main engines use the fuel that is carried in the external tank (which is manufactured a few miles away from where Mark Trotter and I work). The external tank contains liquid hydrogen (H2) and liquid oxygen (O2).
This reaction is interesting because the combustion product is water:
2 H2 + O2 —> 2 H2O
This is an incredibly energetic reaction. The combustion generates temperatures of about 3600° K (6000° F) in the main engines. The water vapor expands rapidly at this temperature, generating lifting force. The external tank carries enough fuel to keep the main engines running for 8.5 minutes—enough time to get the orbiter to orbit. Mark Trotter and I were given a tour of the facility where the fuel for the main engines is made (which is also manufactured just a few miles from where we work).
The OMS engines provide thrust for orbit insertion, orbit change, transfer, rendezvous, and de-orbit burn. They are also liquid-fueled, which makes sense since they will be used repeatedly. They burn monomethylhydrazine and nitrogen tetroxide. This mixture is hypergolic—it will spontaneously ignite as soon as the chemicals come into contact with one another.
4 N2H3CH3 + 5 N2O4 —> 9 N2 + 12 H2O + 4 CO2
In the vacuum of space, this reaction generates a thrust of 27,000 newtons in each of the two OMS engines, sufficient to maneuver the vehicle. This is in comparison with the main engines, which generate a thrust of over 1.5 million newtons each.
References:
Angelo Jr., Joseph A., The Dictionary of Space Technology, New York: Van Nostrand Reinhold Company, 1982.
Brady, James E., and Gerard E. Humiston, General Chemistry: Principles and Structure, 4th Ed., New York: John Wiley and Sons, 1988.
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