Hydrazine powered APU's

[shuttlebus]

jau,

Generally, for combustion to take place you need two things: -

FUEL and OXIDISER

Now in your normal jet engine, the components are JetA1 (Fuel) and oxygen (oxidiser).

This is not satisfactory for a rocket as there is a lack of oxygen at higher altitudes.

Therefore most rockets use cryogenic propellants. Cryogenic propellants are liquid oxygen (LOX), which serves as an oxidizer, and liquid hydrogen (LH2), which is a fuel. The word cryogenic is a derivative of the Greek kyros, meaning "ice cold." LOX remains in a liquid state at temperatures of minus 298 degrees Fahrenheit (minus 183 degrees Celsius).

LH2 remains liquid at temperatures of minus 423 degrees Fahrenheit (minus 253 degrees Celsius). In gaseous form, oxygen and hydrogen have such low densities that extremely large tanks would be required to store them aboard a rocket. But cooling and compressing them into liquids vastly increases their density, making it possible to store them in large quantities in smaller tanks.

The distressing tendency of cryogenics to return to gaseous form unless kept super cool makes them difficult to store over long periods of time and also fairly impractical for use in APUs etc.

So, the other alternative for the space shuttle was to use hydrazine as the fuel for the APU, which is a hypergolic fuel.

Hypergolic propellants are fuels and oxidizers which ignite on contact with each other and need no ignition source. This easy start and restart capability makes them attractive for both manned and unmanned spacecraft maneuvering systems.

Another plus is their storability — they do not have the extreme temperature requirements of cryogenics.

The fuel is monomethyl hydrazine (MMH) and the oxidizer is nitrogen tetroxide (N2O4).

Hydrazine is a clear, nitrogen/hydrogen compound with a "fishy" smell. It is similar to ammonia. Nitrogen tetroxide is a reddish fluid. It has a pungent, sweetish smell. Both fluids are highly toxic, and are handled under the most stringent safety conditions.

Hope this is clear enough!

Regards,

Shuttlebus