Ask A Scientist Chemistry Archive

    
name         Seth M.
status       student
age          17

Question -   How does rocket fuel burn without air?
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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
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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)
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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
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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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