Science Explosive water

[wehaveaproblem]

So it occured to me that rocket fuel is essentially water, albeit in it's seperate component parts and chillled right down. I think it's fascinating that 2 of the most explosive elements come together to be one of the most inert molecules. Water is a truely amazing molecule for that and other fascinating reasons.

But anyway, the vague point of this thread... I'm no scientist, so hopefully someone can set me straight. But, if you mix hydrogen and oxygen you get water. So a rocket engine essentially sets fire to the fuel before it has a chance to become water? before it becomes inert. If you fired a rocket engine without the ignition source, would you essentially end up with a huge puddle of water under it? I'm sure there is some complex chemistry I am not understanding, but the whole concept did make me chuckle.

 

[garyw]

Not all rocket engines are LOX/LOH. For example, Soyuz uses Kerosene in the mix and the shuttle has aluminium powder for the SRB's.

 

[cinder1992]

you NEED the heat to make water. let me see...

BTW, O2 is the stable form of oxygen found in the atmosphere. (the same applies for H2)
H2O= H2 + O2 - O1.

Heat + H2 + O2 = H2Ox2 or H2O2(hydrogen peroxide)?

I think my brain just let off flatulence.

 

[Artlav]

If you fired a rocket engine without the ignition source, would you essentially end up with a huge puddle of water under it? I'm sure there is some complex chemistry I am not understanding, but the whole concept did make me chuckle.

Hydrogen and oxygen burn to produce water. Without an ignitions source, they will just went out of the nozzle, with almost no water made. Sooner or later, the vented explosive gas mix cloud will find an ignition source around the pad and go off, MOAB style.

If both are put into a single tank, they will eventually recombine coldly, but it will take quite some time.

Think it like that - there is energy in separation of H and O, making them H2O releases that energy. If you release it fast, you get a combustion. If you release it slow, you get a warm pond some long time later (can't find the figures).

 

[Artlav]

Heat + H2 + O2 = H2Ox2 or H2O2(hydrogen peroxide)?
I think my brain just let off flatulence.

Second one.
Normally, it's H2+H2+O2=H2O+H2O+heat.

 

[tblaxland]

Without an ignitions source, they will just went out of the nozzle, with almost no water made.

And the graph below shows why (except in the case of H2+O2->H20 the reaction path would be right to left). So you start with some energy in the chemical mix (H2+O2) and you need to add some energy to get the reaction started. Once the reaction is started, you get a net increase in energy given off to the environment.
http://en.wikipedia.org/wiki/File:Activation_energy.svg

 

[Nerull]

So it occured to me that rocket fuel is essentially water, albeit in it's seperate component parts and chillled right down. I think it's fascinating that 2 of the most explosive elements come together to be one of the most inert molecules. Water is a truely amazing molecule for that and other fascinating reasons.

But anyway, the vague point of this thread... I'm no scientist, so hopefully someone can set me straight. But, if you mix hydrogen and oxygen you get water. So a rocket engine essentially sets fire to the fuel before it has a chance to become water? before it becomes inert. If you fired a rocket engine without the ignition source, would you essentially end up with a huge puddle of water under it? I'm sure there is some complex chemistry I am not understanding, but the whole concept did make me chuckle.

It's not really uncommon. Many chemical reactions are essentially reactive compounds attempting to become unreactive, essentially canceling each other out. Hydrogen and oxygen form covalent bonds, filling their electron orbitals, the source of the reactivity. All salts are formed from reactive ions forming ionic bonds, producing a molecule with a neutral charge.

And no, mixing hydrogen and oxygen does not produce water. Combusting hydrogen and oxygen produces water. If you 'cold fired' a LOX/LH2 rocket, you'd probably do some major damage to the surrounding area when the mix found an ignition source.

 

[RisingFury]

You can also think of it this way:

Two highly reactive molecules combine with huge energy output. That means it'll be hard to find another substance so reactive, that it would create even greater energy output.

In general, reactions tend to go to the state of lowest energy - releasing as much as possible. Look:

When burning wood, you get CO and CO2. CO2 is really stable and won't react further, but CO will. That's because adding one more oxygen atom onto the thing will release even more energy, creating an even more stable molecule because nothing else can make it even more stable.

Same goes with water... so it's not that surprising after all

 

[Urwumpe]

if you keep water pressurized and heat it past the critical temperature, you also have something quite explosive, if you depressurize it.

 

[cinder1992]

I did that once. ended up blowing my toupee off.

 

[ar81]

LOX/LH2 combustion produces a high octane reaction that has a high specific impulse. But the problem is that to go faster you need more fuel.

VASIMR has a variable ISP, so going faster could be achieved with the same amount of fuel. This is why VASIMR is the engine of the future right now. You need less fuel to do the same. The only problem is that you can only turn it on in space, and it is still experimental.

 

[Urwumpe]

Octane number does not apply to other chemicals than gasoline. It is simply not needed there. The octane number defines the combustion purity of a mixture of different hydrocarbon molecules.

 

[wehaveaproblem]

Ahhh, ok, cheers for the lesson guys! I've always tended to, probably stupidly, think that it's the seperation or splitting of things that produces energy... prolly just too influenced by the good ol' atom bomb.

So, I have another follow-up question. Where does this energy actually come from? If two things combine to become one, how does that actually generate energy? Is just the fact that the molecule is bigger so, carries more moleculer momentum so-to-speak?

It's amazing how much I realised I don't know when I actually pull apart what limited schooling science knowledge I posess.

 

[Urwumpe]

Well, think about H2 and O2 having energy stored in the connection between each atom.

To break this connection you need activation energy. Once you have broken it, you can recombine them. Giving each oxygen atom two hydrogen atoms instead of another oxygen atom needs less energy for the two connections, so you gain energy, which is stored as thermal energy inside the molecule.

 

[wehaveaproblem]

Cheers Urwumpe. This whole thing shas got me thinking, I'm off to go learn some chemistry basics again.

 

[RisingFury]

Just like in nuclear reaction, there is some mass deficit in chemical reactions. You'll remember E=mc^2. While the mass deficit is so small we can't even measure it....... the energy output, compared to nuclear reactions, is small.

 

[cinder1992]

(the following equation is in the format type-unit)

energy J= Mass KG x Speed-of-light Kps^2

this only applies for reactions that have 100% energy output, such as an
antimatter-matter reaction. common nukes have a reaction efficiency of ~2%.

 

[RisingFury]

No, it doesn't apply to only 100% efficiency. You just have to use the mass deficit and not starting mass in the equation to calculate the released energy.

 

[tblaxland]

energy J= Mass KG x Speed-of-light Kps^2

If you want the energy in Joules, you'll need to use m/s as your units for the speed of light. Kps^2 is not even a valid unit for a speed or velocity (perhaps you meant km/s?).

 

[Linguofreak]

So it occured to me that rocket fuel is essentially water, albeit in it's seperate component parts and chillled right down. I think it's fascinating that 2 of the most explosive elements come together to be one of the most inert molecules. Water is a truely amazing molecule for that and other fascinating reasons.

But anyway, the vague point of this thread... I'm no scientist, so hopefully someone can set me straight. But, if you mix hydrogen and oxygen you get water. So a rocket engine essentially sets fire to the fuel before it has a chance to become water? before it becomes inert. If you fired a rocket engine without the ignition source, would you essentially end up with a huge puddle of water under it? I'm sure there is some complex chemistry I am not understanding, but the whole concept did make me chuckle.

Another point you've missed here is that not all rockets burn H2 and O2. Some rockets don't even burn anything. A nuclear thermal rocket simply pumps a liquid through a nuclear reactor, and the heat causes it to become a gas and shoot out the back.