Hydrazine powered APU's
 

Hello,
I have been trying to look at Hydrazine powered APU's but I am having trouble understanding how they work. I know that the F-16's EPU and the three APU's on the orbiter are powered by hydrazine, but how? Something to do with catylitic action (whatever that is?). Anyone know?
Cheers
Rich

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

I'm not a chemist but as I understand it, Hydrazine is a mono fuel. When sprayed over a catalytic bed, it reacts and decomposes into a hot gas.
Very reliable as it doesn't require an ignition source. Very energetic and so power units are small and can be up and running very quickly.
Quite nasty to work with.

Sorry shuttle bus, your reply is much better and you beat me to it.

Brilliant thanks just what I was looking for.

Dimethyl hydrazine + nitrogen tetroxide have been used for years in Russian missiles. Very nasty stuff - not just toxic but also carcinogenic and teratogenic...

Used it years ago as an O2 scavenger in high pressure boiler systems - in, I would guess, a dilute form of N2H4.
No one told us it was dangerous - just as no-one mentioned the bone dry asbestos lagging around the steam pipes or that some of our oils and other chemicals caused skin irritation etc. etc. :eek:

Hydrazine is not nearly as bad as its methylated derivatives. On a related note, however, it is strange to observe how the perceived hazard of a given substance varies from country to country. In the U.S., methanol (lethal dose for ingestion = 20-40 grams) is available in some states as car fuel. In Russia, you need a special governmental authorization to buy anything containing methanol (which is probably the right thing, given the drinking habits of some people). In Russia, asbestos is a relatively common construction material. In the U.S., people would don a protective suit before touching it, blithely forgetful of the fact that there is no single mineral called "asbestos", but rather two very distinct ones: chrysotile asbestos and amphibole asbestos, the latter being some 100,000 times more dangerous than the former...

. . and worryingly some vermiculite insulation may contain amphibole asbestos fibres.