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Ask A Scientist

What is the chemical reaction that causes a rocket to take off?

Asked by: Thomas Davis
School: St. James Middle School
Grade: 5
Teacher: Mr. Martinkovic
Hobbies/Interests:  Play sports, reading, studying
Career Interest: Teacher, lawyer

Answer from Jim Clum

Professor Emeritus, Watson School of Engineering and Applied Science

Research area: Materials Engineering
Interests/hobbies: Travel; volunteer Wisconsin public broadcasting &
UW Arboretum; gardening

If you have ever ignited a sparkler on the 4th of July you have used the essential chemical reaction that fires the rocket engines of the NASA space shuttle.  That reaction is, in the simplest form written as:

2 Al + 3/2 O2 è Al2O3              

Which is the reaction for the oxidation of aluminum; a common reaction occuring whenever a clean piece of aluminum is exposed to oxygen in the air. That reaction, when employed to fire rocket engines, uses special forms of aluminum and a compound containing oxygen which make both the aluminum and oxygen more reactive than say a piece of aluminum foil sitting at room temperature.  For example, the aluminum in the rocket engine or the sparkler is in the form of small particles which have a large amount of surface area where the reaction can take place with the oxygen from the oxidizer compound.  The oxidizer compound (usually a form of the molecule perchlorate, ClO4-, for example, ammonium perchlorate (NH4ClO4), or potassium perchlorate (KClO4)) is also very reactive.  In your sparkler, the aluminum pieces and the perchlorate oxidizer compound are not as reactive as the reacting aluminum and oxidizer compound in the space shuttle engine, nor are they as confined. 

The confinement also is important since it makes the burning reaction behave more like the explosive reactions that occur when the gun powder in a bullet burns and creates the “thrust” which launches the projectile bullet out of the gun barrel.

If you burn a sparkler this 4th of July remember that you are seeing essentially the same reaction that occurs in the space shuttle rocket engine.  Your version burns less violently, but it is the same fundamental process.

Last Updated: 9/18/13