Aluminim Rocket Fuel

[RogerInHawaii]

There is an article on the msnbc website (http://www.msnbc.msn.com/id/33604286/ns/technology_and_science-future_of_energy/) describing a new rocket fuel using aluminum as the primary component. Existing rockets, including the shuttle's solid rocket boosters use aluminum, but this is new in that it uses nanoparticles of aluminum. However, the article indicates that it uses aluminum and ICE (water ice). It states that "Theoretically you can get very high temperatures using aluminum and water". But there is no explanation of how combining aluminum and ice can produce a burnable compound.

Can someone provide an explanation of how this works?

 

[yevaud]

Aluminum reacts with water to form various oxides of aluminum plus hydrogen gas, with the release of a great deal of heat. The primary reactions are:

2Al + 6H2O ---> 2Al(OH)3 + 3H2
2Al + 4H2O ---> 2AlO(OH) + 3H2
2Al + 3H2O ---> Al2O3 + 3H2

 

[origin]

Aluminum reacts with water to form various oxides of aluminum plus hydrogen gas, with the release of a great deal of heat. The primary reactions are:

2Al + 6H2O ---> 2Al(OH)3 + 3H2
2Al + 4H2O ---> 2AlO(OH) + 3H2
2Al + 3H2O ---> Al2O3 + 3H2

It is kind of interesting how much heat is released by the oxidation reactor of aluminum - until you realize that most common heat producing reations are acutally just oxidation reactions, such as burning logs in a fire or gasoline for your car. Most people do not think of a fire as a chemical reaction.

I have been asked, "if aluminum can be used for rocket fuel why doesn't aluminum foil just burst into flames when it is put in the oven?" The reason is due to particle size. As a particle gets small the ratio of surface area to volume increases. This is because the surface area is the square of the particle radius and the volume is the cube of the particle radius. So if you were to oxidize a 1 cm block of aluminum only the outside 6 sq cm would oxidize which would evolve heat but very little heat relative to the volume of the block. Now if you had a tiny particle of aluminum and oxidized it there would be a huge amount of heat generated on the surface relative to the miniscule volume of the particle. So much so that the particle will glow red or white hot.

The 1 cm block that had 6 square cm oxidize generated just a tiny amount of heat, if that 1 square cm of aluminum is ground into fine particles and ignited it would be a rather spectacular fireworks display, because the surface area has increased a 1000 fold (not an actual number - it can be calculated but I am suppose to be working!).

I know that most people on this site already know this but - so what I think it is kinda cool how the relatively simply math can be applied to real world stuff - it is sorta like science.

edited to change the word alumina to the proper aluminum. Alumina is the oxidized form of aluminum.

 

[drwayne]

Note that iron-oxide is also in the mix, which can be fun to explain:

http://www.nasa.gov/returntoflight/syst ... m_SRB.html

 

[RogerInHawaii]

Thank you for the information on the reaction of aluminum with water. The original article indicated that they mix the nanoparticle aluminum with ice, and that actually starting the reaction is difficult, taking a very high temperature spark (or some such thing) to get it started. I would think that even if it were mixed with ice, you'd still get at least some reaction and that it might well ignite even without a starting spark. Is it really the cold temperature of the ice that keeps it "stable" until ignited? The article also mentioned the possibility of a gel form of the propellant. Would they need to add some other chemicals to inhibit self-ignition? How do they hold off the chemical reaction of the aluminum with the water until they actually want it to ignite

 

[origin]

Thank you for the information on the reaction of aluminum with water. The original article indicated that they mix the nanoparticle aluminum with ice, and that actually starting the reaction is difficult, taking a very high temperature spark (or some such thing) to get it started. I would think that even if it were mixed with ice, you'd still get at least some reaction and that it might well ignite even without a starting spark. Is it really the cold temperature of the ice that keeps it "stable" until ignited? The article also mentioned the possibility of a gel form of the propellant. Would they need to add some other chemicals to inhibit self-ignition? How do they hold off the chemical reaction of the aluminum with the water until they actually want it to ignite

Well, if you are in a room filled with 20% hydrogen it will not spontaneously ignite even though oxygen and hydrogen are very reactive - however lighting a match would be a very bad idea. There needs to be some energy put into the reaction to allow the Hydrogen and Oxygen to combine. The match will give off enough energy to cause Hydrogen and Oxygen to burn and form water, this reaction releases more energy or heat (exothermic reaction), which in turn causes more O-H combinations resulting in a runaway reaction (explosion). It is the same type of thing with the aluminum / ice mixture the ice is not reactive but once you start the reaction with a spark the ice melts and you have a reaction that releases heat and so on.

 

[origin]

Particle size is the driving force for all of the solid fuels. The aluminum and perchorate are mixed with thiokol silastic - as in the rocket manufacturer Morton-Thiochol.

Even grain dust can be explosive which is unfortuantely accidently demostrate almost every year. Sprinkling iron filings or an organic dust over a flame will demonstrate the reactive nature of small particles.

 

[kelvinzero]

It is still weird that burning hydrogen and oxygen is one of the most efficient reactions for propelling a rocket, yet aluminium and ice (the frozen result of burning hydrogen and oxygen) also makes a reasonable rocket fuel. In that last reaction it looks like water is your oxidizer. I can sort of understand it with aluminium atoms being heavier than hydrogen and I assume their combustion producing more energy on a molecule-by-molecule basis... still weird.