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Name: Seth M.
Status: student
Age: 17
Location: N/A
Country: N/A
Date: 12/20/2004


Question:
How does rocket fuel burn without air?


Replies:
Hi Seth

The burning of fuel, especially hydrocarbon based fuel, with oxygen is an oxidation process where the carbon is combined with oxygen and much energy is released. On Earth, this oxidation occurs using free oxygen in our atmosphere to support combustion. For a spacecraft however, things become more complicated as there is no free oxygen in space. The solution is to take oxygen with you.

There are two ways to do this, first you can pack oxygen in the form of LOX or liquid oxygen. You then feed LOX with your fuel in the rocket engine and you have combustion. This can be difficult as LOX must remain under pressure, and can get very cold.

The second and sneakier way is to carry chemically bound oxygen. This is a stable compound which has oxygen as part of its chemical formula. We call these chemicals "oxidants." Examples are Potassium Permanganate KMnO4, Hydrogen Peroxide H2O2, and Nitric Acid HNO3. When kept by themselves, they are fairly easy to handle. When mixed with other compounds, violent reactions can occur.

Hope this helps

Bob Hartwell


Air is needed largely for the oxygen present. The oxygen aids combustion by acting as an electron acceptor (an oxidizing agent). Rocket fuel can be made such that other oxidizing agents are present (sometimes liquid oxygen).

Greg (Roberto Gregorius)


Rockets need to mix into the fuel, a second kind of fuel called an oxidizer, which takes the place of the oxygen in air.

Usually this oxidizer has a high percentage of oxygen in its molecule: Liquid O2, H2O2, N2O, N2O4, HNO3, HClO4, etc. A couple of other elements are electron-hungry enough to replace oxygen; the most energetic oxidizer of all is Fluorine.

Explosives usually work this way, too, except the fuel and oxidizer are mixed together in one material when manufactured, often side-by-side right in the same molecule. A very news-topical example is Ammonium Nitrate, NH4NO3. 3 of the 4 H's may be considered fuel, and the 3 O's are the oxidizer. The N's act as half-hearted carriers that hold their H or O for a while, but can let go easily when provoked.

Solid rocket propellants are like this, too, but not quite as closely mixed. Typically the oxidizer is salt-white crystals of NH4CLO4 (Ammonium PerChlorate), which fortunately does not easily explode, and the rubbery glue between the crystals is the extra fuel needed for complete burning.

Since we do not like our rockets to explode at odd moments, we usually do not put our liquid fuels and oxidizers in the same tank. So a liquid-fuel rocket almost always has two kinds of fuel on board, in two separate tanks. These are mixed by spraying them through separate nozzles into the combustion chamber during combustion.

Possibly the safest chemical rockets are the hobbyist's Hybrid rockets. "Hybrid", in this case, means neither "solid-fuel" nor "liquid-fuel" but half and half. The fuel is the very thick rubber walls of the long, tubular combustion chamber, and the oxidizer is N2O gas injected down the tube from a pressure-tank in front.

Someday we will not need to use a chemical reaction to make the energy to make hot gas that squirts out the back. For energy we might use nuclear power or stored antimatter or stable plasma toroids or sunlight or laser-beams from the ground or who knows what... Then our rockets will have energy source and a reaction fluid, but no fuel and oxidizer.

The energy source will take the place of fire.

Jim Swenson


A rocket carries its "air" with it, in a fuel tank. The space shuttle has a big fuel tank underneath it, that big orange thing. Half of it is filled with liquid oxygen (the "air"), half with liquid hydrogen, the fuel. Solid rockets have an oxidizing agent mixed in the solid propellant, which after all is what the air does.

Steve Ross


A rocket motor has to supply both fuel and oxidizer to burn. This increases the weight required to achieve a specific thrust relative to a jet which uses the oxygen in the air to burn the fuel.

Greg Bradburn



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