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Why hasn't anybody used superheated water as rocket fuel


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Distilled Water

I've seen youtube videos about how distilled water that has been super heated can explode and I wondered "wouldn't it be great if the used that to kill someone Use as kind of a pulse jet by releasing it into a combustio chamber, adding a contaminant (sugar for example) and watching it whisk you to the land of Oz

 

Well, anyway I'd like to here your thoughts on this.

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41 minutes ago, kunok said:

As principal source of propulsion? You will need a lot of energy to be usefull, but I can see using superheated steam in the RCS in the future for simplicity.

Yes you need a lot of energy, super heated steam splitting into hydrogen and oxygen and generate explosions in the pipes. 
This is also an problem for this design, you have an risk of explosion in tank / boiler or pipes.
Think this belong under dangerous rocket fuels. as it would be very unstable. 

On the other hand an cubesat engine used water it split into hydrogen and oxygen and burned in engine. Yes this has huge power demand so trust will be low, upside is that system is simple, small and cheap, water is storeable and non toxic. ISP is also better than other non ion engines. 

 

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The idea of flashing superheated water to steam by adding a nucleation site seems superfluous, it's more controllable and offers higher performance to just heat water under pressure then release that pressure. Or just plain boil water.

The first rocket motor in history used steam, Heron's Aeolipile almost 2000 years ago!

https://en.wikipedia.org/wiki/Steam_rocket

As for why it isn't more widely used, there are a few drawbacks:

How do you heat it? If you use fuel and oxidizer, well you're better off just burning and expelling that and not bothering with the steam. If you use an external heat source, you can then choose propellants other than water that might perform better. That said water is easier to store than say liquid hydrogen.

How hot can you get it? Probably nowhere near as hot as a chemical rocket engine, and a cooler exhaust moves slower. Very hot water is actually quite corrosive too.

What's it for? This I think is the big one. Chemical rockets have high thrust and decent efficiency, monopropellant thrusters and even cold gas thrusters have simplicity, ion engines have high efficiency and low thrust. Where's the niche for the steam rocket, which is less efficient than chemical rockets and probably more complex than monopropellant thrusters?

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5 hours ago, RainDreamer said:

Horrible ISP is one of the reasons. I am thinking it miiiight work with a NERVA engine in place of Liquid hydrogen for lower efficiency. Also I assume frozen bits of water expanding in pipes can be a bad thing.

Water is great- if you want to use it as a long-term storage mechanism for H2 O2 fuel.

Otherwise, forget it, when it comes to propulsion.

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3 minutes ago, pincushionman said:

Water injection has been used in the past to increase jet engine thrust. A good example is getting early BUFFs off the ground. But there are serious drawbacks in weight and efficiency for doing it.

Also in piston engines, like the one used in the P-47.

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In piston engine the water is a coolant for the intake air. After the supercharge/turbocharger compress the in coming air it becomes really hot, and if you then try to mix petrol with that it can cause problem with pre-detonation. The water gets injected after forced induction and before the engine so the intake air cools down, so for a given engine and fuel octane rating you can compress the air more without pre-detonation.

It's conceptually similar to Skylon's engine precooler or a normal engine intercooler, except it works on an open cycle so the cooling effect is very strong at the expensive of using up the coolant quickly.

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4 hours ago, pincushionman said:

Water injection has been used in the past to increase jet engine thrust. A good example is getting early BUFFs off the ground. But there are serious drawbacks in weight and efficiency for doing it.

I thought this was actually to cool the incoming air, the water itself is besides the point.  I don't know.

In the case of turbocharged piston engines, you inject water mist into the cylinders and adjust the turbocharger "boost" pressure.  Essentially, the air from the turbocharger is at higher pressure and so each cylinder-full has more actual air.  This lets you inject more fuel.  The water mist flashes to steam as the cylinder of fuel-air compresses, cooling it.

 The purpose of this is to let you have more fuel and air and not get predetonation from the "overfilled" cylinder heating up the air inside too fast during the compression stroke.

So you get more power: weight, but the energy lost from the water injection (and the weight of the water tank) reduces efficiency.  

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8 hours ago, SomeGuy123 said:

I thought this was actually to cool the incoming air, the water itself is besides the point.  I don't know…

From what I understand, the temperature reduction (throughout the engine, not just the incoming air) allows increased RPM and works for both piston and jets. But on turbojets and low-bypass-ratio turbofans, you also increase the mass flow rate, which is a key variable in the thrust equation.

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21 hours ago, RainDreamer said:

Horrible ISP is one of the reasons. I am thinking it miiiight work with a NERVA engine in place of Liquid hydrogen for lower efficiency. Also I assume frozen bits of water expanding in pipes can be a bad thing.

The [maximum] ISP of a NERVA using water would roughly be the same as the ISP of LH/LOX (same output), but I can't vouch for getting the same temperature (LH/LOX temperatures are limited by materials science, so that's your max ISP).  Presumably H2O is easier to find than H2, it might be worth it (well, easier to find outside of massive gravity wells).

Of course, if you didn't care how long it took your solar panels [or RTGs] to crack the water to hydrogen before starting your burn (it isn't efficient at all, and commercial H2 production cracks methane or other hydrocarbons), you would never bother with water.

Edited by wumpus
any idea why the above is struckthrough? I can't undo it.
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How about using it as a secondary propulsion system in a very fast jet as it wouldn't need intake air so say using two J58 turbo jets and linked to each other via a linear aerospike that release the water.

15 hours ago, Bill Phil said:

Wait, isn't this what happens to H2O2, the monoprop? It decomposes after running through a catalyst, I think.

 

But anyways, how would you heat the water? Hmm....

http://www.projectrho.com/public_html/rocket/enginelist.php#id--Beamed_Power--Solar_Moth

couple a super conductive material to cool off the J58s and run it through the water tank to warm up the water.

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  • 2 weeks later...
On 1/21/2016 at 9:30 AM, chadgaskerman said:

Distilled Water

I've seen YouTube videos about how distilled water that has been super heated can explode and I wondered "wouldn't it be great if they used that as a kind of a pulse jet by releasing it into a combustion chamber, adding a contaminant (sugar for example) and watching it whisk you to the land of Oz

 

Well, anyway I'd like to here your thoughts on this.

Using super-heated water as a propellant has been considered in the past.  The two difficulties are: 1. The energy to boil water is enormous!  It is over 700 Watt Hours per liter. 2. Water containers usually break when water freezes because of the water expansion.  Because of this, the water must be kept above freezing at all times.

Here is a Wikipedia article on steam powered rockets:

https://en.wikipedia.org/wiki/Steam_rocket

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[Why not use superheated water] The answer can be summed in the concept of ISP.  Understand ISP and you are on your way to understanding KSP and the rocket equation.

Assumption: these work similarly to those toy rockets (do they still have them?) that operate on the "supersoaker" principle (although they predate supersoakers by decades) that you fill with water and then pump in a bunch of air (and build up some pressure) and release?  

Probably the closest thing to a "real rocket" (i.e. one that while presently on paper but actively being developed with real money behind it) is the firefly alpha.  Of course, it doesn't use water at all: the similarity is that it uses pressurized fuel (and oxidizer) tanks to force the fuel (and oxidizer) into the rocket nozzle.  This requires a *huge* amount of pressure already (thus why other rocket manufacturers spend so much on the turbopumps), but is only used to force the fuel & oxidizer into position to burn.  The Ve (and thus ISP) all depend on the fuel burning and exiting the bell.

I'm curious how far you could "launch" a firefly alpha without ignition (which would strongly resemble the OP's suggestion in practice).  My guess is it might get out of sight (assuming you somehow removed a stage and probably a lot of fuel, obviously the thing isn't going to have the right TWR without ignition), but the delta-v would really be miserable for a rocket hoping for orbit.

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The specific impulse will be very low because the specific heat of water is much much lower that the heat of combustion of, say, kerosene. Hot liquid water can hold something like 5J/g/K of energy (so cooling it from say 300 C to 0C as it expands in the nozzel could give you, say, 1500J per gram of water . The heat of combustion of kerosene is about 50000 J per gram! Very rough numbers but good enough to make a rough estimate: heat capacity changes with temperature... and ok not considering the mass of oxygen to burn the kero, but lets say chop it in half, kerosense/oxygen still gives nearly 20 times the energy per gram of fuel, and you dont need to store it pressurised at an extreme high temperature! (liquid oxygen is cold, but not unmanagably cold, super heated waterr would probably be more difficult)

So in short: we want rocket fuels to hold as much energy as possible per unit mass, and an engineering tradeoff with ease of handling etc. thermal energy stored in water is nowhere near as good as run-of-the-mill chemical propellants!

Edited by jf0
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14 minutes ago, sevenperforce said:

Earth offers plenty of options which are much better than steam for rocket propulsion. However, using a tank of liquid water with a nuclear reactor or a solar-thermal rocket is a nice simple design if you can collect the water in space.

Or you know, just spend a bit more electricity to turn it into its components, and get more ISP that way.

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17 minutes ago, fredinno said:

Or you know, just spend a bit more electricity to turn it into its components, and get more ISP that way.

That's stupid. Converting energy directly to heat is way, way more efficient than electrolyzing water and burning the mix to generate the heat. If you are trying to use water as propellant, an NTR is going to be several times more efficient than what you are proposing.

Rockets aren't magic. They don't get ISP from some mystical reaction between fuel and oxidizer. Fuel and oxidizer burn, generating heat. Nozzle allows heated exhaust gas to expand and converts internal heat energy of the gas into thrust. If you can generate the same heat without bothering with a chemical reaction, you've just saved yourself some trouble.

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