Liquid Aluminum Fueled Rocket

TwinBeam · 2007-04-06 21:44:34

There's been some discussion about the difficulty of using lunar aluminum as a fuel.

:idea: Why couldn't a lunar rocket use liquid aluminum, mined from the moon?

Load aluminum in powdered form into a thin steel fuel tank, possibly surrounded by a light-weight insulative blanket - but unlike on Earth, the native vacuum could be the main insulator. Use electric heaters to raise the aluminum to abut 665 degrees C - just above it's melting point - typically using power supplied from solar electric collectors. Probably have some pressure relief valves, in case contaminants create too much pressure in the tank.

(Alternatively, you could have equipment to store and handle liquid aluminum, but that would be more dangerous for human operators, and requires more complex infrastructure. Loading powdered aluminum into a tank might not take much more than a shovel and a storage bin to keep the aluminum pure.)

The liquid Aluminum would be pushed or drawn through steel flow control valves (gas pressure or simply gravity/acceleration), into a lower chamber, where it would be blown under high pressure into a fine mist (likely by a small hydrogen-oxygen rocket) into a reaction chamber. LOX would be pre-warmed to its boiling point, then sprayed under high pressure into the reaction chamber to ignite the aluminum, generating the rocket exhaust.

I'm sure it'd end up being more complex than that, but that'd be the basic scheme. It could serve as a cheap first stage for launching to lunar orbit or directly into Earth return.

Are there fundamental flaws in the basic scheme?

Major complications that'll need to be overcome to make it work?

The advantages seem pretty obvious, in terms of reduced Earth launch costs. Especially if you manage to make a re-usable lunar launcher/lander using liquid aluminum fuel.

cjchandler · 2007-04-07 09:04:03

I suppose that might work, you'd need pumps though, a low pressure engine would be pretty big and heavy. However if you really wanted to avoid pumps, how about freezing a mixture of oxygen and aluminum dust, in a solid fuel like slug, there wouldn't be a big problem with insulation if you lanched it at night. It wouls be simpiler with no pluming. Basalt fiber from the moon might make a pretty lighweight tank and it's good at high temperatures.

SpaceNut · 2007-04-07 11:53:43

[url=http://www.space-rockets.com/lsp.html]Lunar Soil Propellant
LSP[/url]

The problem was to find a way to inject the aluminum powder into the combustion chamber in a steady, controlled manner. He came up with the idea of mixing the aluminum powder into the liquid oxygen (LOX) to form a monopropellant. Under NASA contract, the safe mixing and storage of aluminum/LOX monopropellant or LSP was demonstrated.

cjchandler · 2007-04-07 14:33:36

Wow, I glad some one has tried that out, any idea what the isp is like?

SpaceNut · 2007-04-07 15:43:37

From the
Martian Jet Engine page:

The rocket engine had a thrust level of 40 lbs.
The thruster produced a thrust level of one pound and was fired in pulses simulating a thruster firing on a deep space mission.

[url=http://www.space-rockets.com/moon1.html]Using Lunar Soil For
Propellants[/url]

Phosphorus and sulfur could actually be the easiest as they both melt and could be fed as liquids. Phosphorus melts at around 111o F. It has the advantage of auto-igniting with oxygen. Sulfur melts at 239o F, but if heated above 482o F, it will also auto-ignite with oxygen. For either fuel, electrical heaters would preheat the propellant tank, feed lines and valves prior to using the engine. Once the engine was operating, heat from the combustion process could be used to continue heating the fuel so that it would stay molten.

TwinBeam · 2007-04-07 19:06:27

I suppose that might work, you'd need pumps though, a low pressure engine would be pretty big and heavy.

Perhaps - but I was thinking in terms of the small H2/O2 rocket going through a tube, with the AL fed in from the sides. The AL would be drawn into the high velocity flow by the Bernoulli effect.

cjchandler · 2007-04-08 18:00:26

That's an interesting pump idea, however, I can see a few problems, one, at supersonic speeds, the speed of a gas next to a solid dosen't have to equal any more, so you wouldn't get as much push as expected. Second, what is the viscosity of liquid aluminum? You would have quite a bit of drag from the other wall. So, I'm not sure it would work, possibly you could just increase the pressure and it would run backwards.

TwinBeam · 2007-04-09 01:33:53

That's an interesting pump idea, however, I can see a few problems, one, at supersonic speeds, the speed of a gas next to a solid dosen't have to equal any more, so you wouldn't get as much push as expected. Second, what is the viscosity of liquid aluminum? You would have quite a bit of drag from the other wall. So, I'm not sure it would work, possibly you could just increase the pressure and it would run backwards.

Sorry - I think you left something out of that - the speed of a gas...doesn't have to equal what? Equal the supersonic flow velocity?

That's probably true, but the velocity could still be high enough to create quite a bit of Bernoulli pressure, even if not as fast as the exhaust flow. And I suppose the AL injection tube could extend out a little bit into the flow, away from the walls - though that'd introduce other potential issues, like erosion. But I suspect it wouldn't be necessary.

From a googled source, it looks like liquid AL near 700 deg C would be around or under 1.5e-3 Pa-s, about 50% more viscous than water at room temp - but it doesn't seem so different that the Bernoulli effect wouldn't work.

But in any case, if pumps are needed, I presume something could be made to work at liquid aluminum temperatures. I'm just trying to see if there might be something a bit simpler.

cjchandler · 2007-04-10 13:43:00

Yes, I'm sorry. In normal fluid flow, at the walls, or the thicker liquid exterior the fuilds are moving the same speed, however in a supersonic flow there is a good deal of slip, I think. I'm not sure how much of a differanc this would make. The main problem I can see is the small rocket just increasing the pressure inside the pumping area and forcing the aluminum back into the tanks. I found that the isp is about 286, but batter is you can add hydrogen or some other light gas to the exhast.

RobS · 2007-04-10 17:07:58

I don't think there would be a Bernoulli effect; this is a rocket being launched on the moon, in a vacuum. The aluminum will be in a vacuum unless there is gas prssure in the tank pushing it out of the tank and mixing with it as it flows.

-- RobS

TwinBeam · 2007-04-11 14:42:30

I don't think there would be a Bernoulli effect; this is a rocket being launched on the moon, in a vacuum. The aluminum will be in a vacuum unless there is gas prssure in the tank pushing it out of the tank and mixing with it as it flows.

Hmm - I didn't think the Bernoulli effect depended on gas pressure "from behind"? If so, yes, I suppose it'd be necessary to add a source of gas pressure.

cjchandler · 2007-04-11 15:11:32

I don't think it does. What you are really trying to do is make something like a tesla turbine, see wikipedia, using the boundary layer to accelerate the fuel. It's really a moot point though, why not just use the aluminum dust suspended in LOX thats already been tested? A presure fed engine would work alright on the moon, assuming you had a strong enough tank.

RobS · 2007-04-11 21:31:02

The bernoulli effect is a differential pressure effect, is it not? If the aluminum powder is coming from a tank that has no gas pressure in it, we then have a stream of moving gas that would represent a lower pressure than still gas in the aluminum, but you can't get a pressure lower than zero, so if there is no gas pressure in the aluminum there is no meaningful differential pressure.

We've already talked about the potential "boom boom" danger of aluminum powder and LOX monopropellant. I doubt it's safe. But does aluminum burn in hydrogen? Someone check. It might not.

-- RobS

TwinBeam · 2007-04-12 00:50:12

why not just use the aluminum dust suspended in LOX thats already been tested? A presure fed engine would work alright on the moon, assuming you had a strong enough tank.

I think liquid AL as fuel would be simpler - no pre-mixing to get right. It should be a bit safer - no chance that a spark or blow-back will ignite the fuel at an inappropriate time or place - e.g. during fuel mixing. It might get a bit more thrust, since it starts out about 700 degrees hotter. There'd be no requirement of getting the AL dust powder grains to a specific fine-ness in order to assure a clean fast burn.

So I guess I'd ask the question the other way around - why bother mixing LOX and AL powder? It's not going to make a simpler rocket, I suspect - possibly more complex. And I also suspect that any pumps or valves would get a lot of wear from AL in LOX flowing through - hurting potential re-useability.

cjchandler · 2007-04-12 12:42:04

Well, I guess I'd still rather fly on the Al/Lox mix. A single tank would be simpler, and if you froze the Lox (50 K) you could have a solid fuel rocket with absolutly no moving parts. Pumps wise, it'll probably be presure fed any way you cut it, now that I think about it more a high thrust to weight ratio engine probably isn't going to be nessisary in 1/6 of a g. Something like 40 would probably be fine.

JoshNH4H · 2008-03-06 08:53:34

I've thought about this before, and this definitely seens like a good idea to me, especially because we're willing to use LH2 in rockets. I personally would go with the SOX/SAL mixture. (Solid OXygen/Solid ALuminum) Does anyone know what the Isp would be like? Come to think of it, are there possibilities for use as a SRB on earth? One of the biggest benefits is that Al2O3 is present in soil all over the solar system.

cjchandler · 2008-03-06 14:03:54

I don't see why not on eath except that the exhuast would probably be toxic and the isp is pretty low. Insulation is a problem if you to hold it around for a while, you can't just let a little boil off with a solid rocket. I would like to see a solid oxygen/ hydrocarbon rocket. If you had a large carrier plane with liquid oxygen tanks inside it, could you spray the oxygen out into the empty tank with maybe methane and the rapid expansion cause it to freeze right before you are ready to launch? We do this all the time in the lab with CO2 to make dry ice, but it's probably be harder with LOX. Maybe if you were spraying liquid hydgrogen in at the same time, and the lox could crystlize around the liquid hydrogen. That would give you excellent isp and the lack of moving parts might make it worth considering.

GCNRevenger · 2008-03-06 15:00:04

Using gas expansion to create solid cryogens isn't going to work, CO2 makes dry ice because there isn't that much energy difference between gas and solid. The difference for Oxygen, Hydrogen, etc is much larger, so gas expansion isn't going to cause this change over a practical pressure drop. High pressures and/or throwing a lot of other cryogen at it is the only easy way to solidify these sorts of things.

Solid Oxygen-Aluminum or Oxygen-Hydrocarbon rockets are too dangerous, it is simply too easy to ignite the mixture, leading to catastrophic failure (boom).

Liquid aluminum is a neat idea, but it will require too much energy to liquify and keep liquid. The heaters would be too heavy, need too much power, and insulation would be difficult simply from radiant cooling to put on a rocket. Furthermore, while steel won't melt at liquid Al temperature, softening would be a definite problem. The real killer though is the viscosity of the liquid aluminum, the stuff is going to be too thick and gooey to pump with the needed speed.

The solution is to stick with solid aluminum, but to grind it into a very fine powder, fine enough to flow like Talc or glass powder or even finer. The catch is figuring out how to push this into the combustion chamber.

Unfortunately the Isp will never be very good, since much of the Aluminum will remain unburned since Oxygen can only react with the metal on the surface. But, the stuff is literally lying around on the Moon by the megatonne.

JoshNH4H · 2008-03-06 15:37:29

could mercury be used somehow to make the aluminum completely oxidisable? on another note, there is always AlH3, which both melts at 150 C, and gets us right back where we started.

GCNRevenger · 2008-03-06 16:19:30

Mercury? Not really, I doubt it would soften the aluminum oxide coating fast enough and Mercury would have the worst Isp imaginable due to its very high mass. Getting a lot of it on the Moon would be a big deal too.

Aluminum hydride as a fuel kind of defeats the purpose too, that if you are going to use Hydrogen to make it, you might as well just burn the Hydrogen directly.

cjchandler · 2008-03-08 09:43:27

If pure gas expansion is not going to work, then why not use liquid hydrogen and liquid oxygen together? The hydrogen would ceirtinlly vaporize, but giving it's high heat capacity, it would definetly freeze the oxygen, no? If they were sprayed into a vessel at the same time, at a high pressure, would it be possible to have the hydrogen gas trapped in the solid oxygen, so that it could a chunk of solid fuel? I realize that oxygen is non-polar, but is there anything akin to a hydrate that could be formed, or maybe just many small bubbles in the ice?

Trebuchet · 2008-03-08 10:53:04

Why not build it hybrid? Make a strong pressure chamber, put a grate in the bottom to let hot gases out but not fuel, and fill the chamber with aluminum balls. Feed in LOX at a safe rate.

GCNRevenger · 2008-03-08 11:29:03

Two reasons:

1) Because, when you oxidize ("burn") solid aluminum, it forms a layer of tough, unreactive aluminum oxide ("alumina") that adheres strongly to the surface. This coating won't burn nor come off, and it prevents more aluminum metal under it from oxidizing.

So, if you have anything but ultra-fine aluminum powder, the vast majority of the aluminum won't burn since it becomes coated with the oxide. All thus unburnable aluminum then becomes dead weight, and keeps your rocket on the ground.

2) Whatever you burn in a rocket, you have to push it out the back to get any thrust, and if you can get aluminum to burn efficiently what you get is very fine aluminum oxide powder. Pushing this through a grating under pressure will be hard.

Trebuchet · 2008-03-08 12:28:20

What's generating the thrust, anyways? As you pointed out, the outputs are solid. High temperature oxygen exhaust?

Edit: aluminum wire 'yarn balls', GCN, loosely wound enough to provide lots of surface area.

JoshNH4H · 2008-03-08 12:46:35

the exaust wouldn't be solid at the temperatures it's being burned at. BTW, what's the exaust in a SS SRB

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#1 2007-04-06 21:44:34

Re: Liquid Aluminum Fueled Rocket

#2 2007-04-07 09:04:03

Re: Liquid Aluminum Fueled Rocket

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Re: Liquid Aluminum Fueled Rocket

#4 2007-04-07 14:33:36

Re: Liquid Aluminum Fueled Rocket

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#6 2007-04-07 19:06:27

Re: Liquid Aluminum Fueled Rocket

#7 2007-04-08 18:00:26

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#8 2007-04-09 01:33:53

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Re: Liquid Aluminum Fueled Rocket

#10 2007-04-10 17:07:58

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#11 2007-04-11 14:42:30

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#12 2007-04-11 15:11:32

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#13 2007-04-11 21:31:02

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#14 2007-04-12 00:50:12

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