The electric arc water engine

[automcdonough]

This is a concept for a rocket engine which uses water as fuel. This engine assumes that you have a large quantity of electricity available.

It's actually pretty simple. Instead of doing electrolysis then combusting the HHO, a huge arc of electricity does both at the same time. Basically with enough input energy the water literally explodes.

Whatever water in the area doesn't explode will turn into superheated steam, which also expands in volume and serves to increase the air density of the output area.

Depending on the source of electricity this likely will not be the most weight efficient approach, but it should be very simple and clean to refuel. Also the output is only heat and water.

optionally, other fuels given the same approach may have more bang for the buck. like metal pellets or some other liquids which contain a lot of potential energy but are not normally combustible.

[Idobox]

So basically, you use electricity to heat water. There are easier ways to do it, and I think it's better to use as light a working fluid as you can, so either hydrogen or helium.

There is also a concept called an arcjet that uses electric arcs to heat air in a jet engine. I've been tinkering with the idea of building one powered by microwaves, that way it only needs to carry an antenna, but it's a very large project that goes beyond my budget, skill and free time.

[automcdonough]

Will heat by itself be enough to separate water into HHO? Like lasers or something

[Deadpangod3]

So basicly its a water-go-boom engine?

[Ralathon]

In essence you're just talking about a thermal rocket engine, like a NERVA. You heat some substance in a reactor chamber and spew it out. The hotter the better and the lighter the ejected particles the better. As such water is not a terrible choice, but H2 would be way better. Not to mention that if you have enough energy to split water you could just split the H2 into H radicals which are the best reaction mass without delving into subatomics.

[Tarrow]

Will heat by itself be enough to separate water into HHO? Like lasers or something

It can be enough, yes. Water starts to thermally decompose at about 2000c but you want it a thousand degrees hotter for there to be a good percentage of the water converted to baser components. As daft as it sounds this is why the fire extinguishers used around molten metals (or worse, burning metals) don't contain any water.

I wonder what the nozzle could be made out of? Would graphite work at those sort of temps if there's a bit of excess oxygen floating around?

[automcdonough]

In essence you're just talking about a thermal rocket engine, like a NERVA. You heat some substance in a reactor chamber and spew it out. The hotter the better and the lighter the ejected particles the better. As such water is not a terrible choice, but H2 would be way better. Not to mention that if you have enough energy to split water you could just split the H2 into H radicals which are the best reaction mass without delving into subatomics.

Well, not exactly.

The electricity primarily only serves to split the water into H2 and O, and be a source of ignition. As a result some heat will be added, yes, so to that extent you are right. But now we have hot HHO gas, it explodes and we have typical chemical rocket action from that reaction.

Using electrolysis to pre-separate the gas and have the H2+O reaction later on is different in that we would have isolate and store both H2 and O (storing them together would be highly unstable and probly eventually just turn back to water). It would work better if the power source is lower and steady, but now you have a water tank, electrolysis setup, gas tanks, etc. I want to explore the option that if the power is available, can we just do it all at once and make water explode?

[Idobox]

You end up with hot water in the exhaust. Splitting water into HHO and then burning it is just an especially complex way to go from cold water to hot water, and you can get the same result by using electricity to heat the water in a simpler way, for example magnetic induction (like a microwave oven).

Arcs generate very high temperatures, and are a decent way to heat water. Some energy will be lost electrolyzing water, but as you pointed out, the products will combust almost immediately, giving that energy back, so you don't get more energy than any other heating method, but you get very high temperatures (high ISP) for a rather low weight.

Water as a huge advantage over hydrogen : it's easier to store and handle, and it rarely explodes (unless you're playing with cesium or something). For a first stage, that might be enough to compensate the lower ISP compared to H2.

[magnemoe]

Heat split water into H2 and O, this would work, downside is that the ISP would not be impressive, upside is that water is easy to store and pretty easy to get.

[automcdonough]

You end up with hot water in the exhaust. Splitting water into HHO and then burning it is just an especially complex way to go from cold water to hot water, and you can get the same result by using electricity to heat the water in a simpler way, for example magnetic induction (like a microwave oven).

Arcs generate very high temperatures, and are a decent way to heat water. Some energy will be lost electrolyzing water, but as you pointed out, the products will combust almost immediately, giving that energy back, so you don't get more energy than any other heating method, but you get very high temperatures (high ISP) for a rather low weight.

In the big picture, yes. But we aren't sticking around to wait for it to cool back down into water. It's being ejected not as hot water but as an H2+O fueled fireball. Superheated steam just isn't going to be as much temperature..

[falofonos]

Unless you can power the arc with solar cells, you're going to be using some form of nuclear reactor. Possibly simpler just to use the reactor coolant to heat your water then hit it with the arc. Not sure if the additional boom would add much in the way of ISP though.

[Idobox]

The temperature at which you get more HHO than water is about 3000°C, not a lot of materials can survive that. Anyway, as the exhaust expands, it cools down, and most of the exhaust will be water. If it isn't, it will be because of bad design.

The concept of a thermal electric engine makes sense with non thermal electric sources, like solar arrays, or beamed power. Once in space, you would get much better ISP from ion drives for the same power, at the cost of lower thrust (which is not very important for probes)

[Bunsen]

The slow electrolysis/fast burn version is already in commercial development. http://www.tethers.com/HYDROS.html

It's made for cubesats where several factors make regular rockets impractical: the propellants need to be storable (no H₂), the materials need to be safe (hazmat handling can be brutally expensive, so no hydrazine), and there isn't enough power (or the service life is too short) to make an ion engine practical.

[Tommygun]

This reminds me of those seam power nuclear rockets people where theorizing about back in the 50s.

One of the ideas was that they could use almost any liquid or ice that they could mine, not just water based for reaction mass.

You don't have to process the fuel just basic fileting.

It's not the most efficient fuel, but if you can get the fuel off world, it has big advantages.

[K^2]

Basically with enough input energy the water literally explodes.

Whatever water in the area doesn't explode will turn into superheated steam, which also expands in volume and serves to increase the air density of the output area.

Depending on the source of electricity this likely will not be the most weight efficient approach, but it should be very simple and clean to refuel. Also the output is only heat and water.

H2O is not the best propellant. You are much better off with H2. Basically, the lighter the gas, the faster the molecules are at the same temperature. So you get better I_SP.

The other problem is that an electric arc is a horribly inefficient way to heat something up. You'll be getting more heat deposited in your wiring than propellant. You should be able to do much, much better with a microwave. Set up a cavity to heat up the propellant with RF radiation and use the heated gas for propulsion. You won't beat an ion drive in efficiency this way, but you might be able to get better TWR without huge losses.

This reminds me of those seam power nuclear rockets people where theorizing about back in the 50s.

Well, these theories lead to NERVA with the same modification. Use H2 gas instead of H2O for better I_SP.

Edited November 5, 2013 by K^2

[Nuke]

i dont thing this would give you any more isp over a conventional chemical engine. the only thing it really does is make the fuel take up less volume. but i have seen a thing about a microwave electrothermal engine that actually gets you better isp. a few kilowatts of microwave power is all you need, it just flash vaporizes the water (or any other propellant, it can run on lots of things) and spews it out the tailpipe with nerva-esque performance (~800s). its sort of a happy medium between plasma engines and chem engines that wont need a polywell or a couple football fields of solar panels to operate.

http://alfven.princeton.edu/projects/microwave_thruster.htm

Edited November 5, 2013 by Nuke

[Winter Man]

Here's a weight saving refinement - pre-split the water on the ground into its constituent elements, then store them aboard the rocket. Like a normal rocket. Seriously, we don't have any kind of electrical storage that can beat good ol' chemical energy for a rocket. Maybe in the distant future when we've cracked antimatter tech or vacuum energy or whatever, but not now.

[Crush]

Whew, when I read the title I thought it was about one of these pseudoscientific water engines which supposedly create infinite energy out of water unless someone is looking.

But gladly, it's just ineffective application of ordinary science.

You can't generate energy by separating water into oxygen and hydrogen and then combusting it, because the energy you need to invest to split water into oxygen and hydrogen is exactly the same energy you get when they react with each other. Minus any losses and inefficiencies, of course. Think of a hydrogen fuel cell not as a generator, think of it as a battery.

Edited November 5, 2013 by Crush

[automcdonough]

well yeah. this would be a situation where there's nuclear energy source or something.

Electrolysis works on electricity and is reasonably efficient at what it does. It doesn't have to heat the water into steam and cook it out, it just turns straight from liquid to gas. Using an arc isn't a great heat source. High temps but not a lot of heat (not really feeling your arc jet engine idea for that reason). Combusting that gas is where the vast majority of the heat comes from..

[Idobox]

Microwave antennas can be used to beam power from a base to a craft with great efficiency, but the required size becomes impractical when you get beyond Earth orbit (and probably before, I need to do the math).

At some point, lasers become better, and you just need a big sheet of mylar in a parabolic shape to focus it.

If you have to generate your own electricity, you will probably prefer a drive with higher ISP and lower thrust.

[Bunsen]

Using an arc isn't a great heat source. High temps but not a lot of heat (not really feeling your arc jet engine idea for that reason).

Arcjet thrusters have been used on satellites for quite a while. Like the electrolysis system, it's because not all rocket engines are used as boosters, and there's more to whole-system performance than just thrust. When you're looking at a spacecraft that generates and uses tons of electricity anyway, you can get a lot more mileage (well, lifetime of attitude control and station keeping) out of a tank of propellant and simple, lightweight plumbing by adding electrical energy rather than depending on stored chemical energy. Yes, there are other ways of electrically expelling propellants, and they can have better-looking theoretical performance numbers, but messy practicalities can create niches where simple approaches like arcjets are a better fit.

[lajoswinkler]

Wow, crackpottery is really popular around here, ain't it?

[JebidiahsBigSister]

Since it takes more energy to break down the water than you can get from the products (see Second Law of thermodynamics) the idea is impractical. You just end up losing energy. If you had a "large quantity of electricity available" it would actually be more efficient to just heat up the water with the original electricity.

[Dispatcher]

If you had a long enough extension cord, it might be doable.