Launching From The Moon With Silicon Based Rocket Fuel And Other Ideas

"... Anyway, uh yeah, you're probably not going to use chemical propulsion to accelerate stuff inside barrels like guns on the moon simply because again, you don't have any ox you don't have any fuels to go with your oxidizers...."

I think he got tongue-tied on the way to saying gunpowder won't burn on the moon because there's no oxygen. "Smokeless" propellants made for use in firearms are based on nitroglycerine, which comes with its own oxygen.

"Smokeless" gunpowder burns so fast it would consume all the oxygen available locally and peter out, so it has to bring its own. So it will work as well on the moon as it does on earth.

Compressed gas propulsion has been used to fire (tiny) projectiles (into a near vacuum) at 40,000+ fps, but those were "total loss" systems, which (on the moon) makes replenishment a problem. The same propellants could be used in a captive gas system but I'd expect the wear rate would be pretty extreme.

NASA's Ames Research Center has a "light-gas" gun that can fire projectiles at 8.5 km/s (19,000 mph) but I have no idea if it's total loss.

In the 1960s, DoD ran an experiment called "Project HARP" with a giant 16" naval gun with a 119-foot long barrel that launched a 400 lb projectile at more than 7000 fps. It reached an altitude of about 111 miles, technically in "outer space." It would work the same from the moon but the problem with combustible solid fuel is somebody's got to swab out the barrel after.

But, as already noted, systems like this pose a problem with excessive acceleration. Even mechanical objects would have to be specially engineered to withstand those kinds of Gs.

The Apollo V had peak Gs of about 3.9, and Falcon 9's is 4.1. Acceleration isn't uniform and neither was at peak Gs for an extended period but it takes Falcon 9 about 12 minutes to reach orbital velocity.

To accelerate to lunar escape velocity (2.38 km/s) with a constant 4 G acceleration would take about 70 seconds, so obviously you'd need a SERIOUSLY long electromagnetic catapult (like 50 miles long). If a human crew is going to live through it, it'll require a very long acceleration phase.

This is why I've always argued that if we're going to use the moon for manned deep space exploration, we need a launch platform in lunar orbit. Transferring the crew from earth straight to the orbiting launch platform saves from having to slow for landing on the surface. They could resupply the orbiting launch pad from a depot on the lunar surface using launch means with accelerations no human could survive.

Lunar orbit velocity is about 43% of escape velocity from the surface. Escape velocity from a platform in high orbit will be somewhat less. None of the Apollo missions reached escape velocity on the way up because they went into orbit first, then blasted out of orbit, not unlike a Hohmann Transfer. Combined with lunar orbital velocity I'm guessing it should talk about half the energy required for a launch from the lunar surface to send it from orbit into deep space from a launch pad in orbit.

Transcript pt 1

Hello, it’s Scott Manley here and today I have for you an answer to a deep space question. A deeper answer to a deep space question. These are questions that are asked by supporters over at my Patreon, people that have signed up to support my, uh, random blitherings on the internet and occasionally asked questions. And I decided that I should probably do some research to actually answer these and give them their money’s worth.

So this question comes from Russell Nielsen who said, “Assuming that there will be mining and smelting operations on the moon, how would you launch the resulting products into lunar or even Earth orbit? Spin launch, chemical rockets, electromagnetic angled ramp, or other?” And I’m going to say we’re going to manage to get all of those categories covered. But let’s start by talking about what the moon is actually made of. What could you get from the moon and what could you use?

Now, the moon may not have an atmosphere. You might be surprised to know, however, that the majority of the moon is made of oxygen. It’s just oxygen that is bound up into various mineral oxides. Silicon dioxide is one of the most common. Aluminium oxide is pretty common, too. Whereas calcium oxide, iron oxide, magnesium oxide, titanium oxide, sodium oxide. Yeah, there’s also a little bit of potassium and stuff like this. But notably, one thing which is pretty rare on the moon is something called dihydrogen monoxide or what most people call water.

Now, while it does exist around the moon, it’s largely been baked out of the surface by those long hot days because there’s not a lot of shade on the moon. Uh, but it is found in some places such as permanently shadowed polar craters. A lunar prospect mission actually found the signature of hydrogen near the poles with its neutron spectrometer. And the Elcross mission slammed like a Centaur upper stage into a permanently shadowed crater on the moon and then a second spacecraft on the way in flew through the debris plume and it found about 5% water in that ejecta plume.

And then there’s also things like helium-3, which in science fiction words is of course needed for nuclear fusion, but it does actually get embedded in the lunar crust from, uh, like solar wind. And yeah, okay, it could be used hypothetically for some forms of nuclear fusion which would, uh, be neutral or a neutronic fusion, but I can’t say that in the short term it’s a mineral or material that’s going to necessarily make a difference. And also, I’m not even sure that in the long term, it’s going to make a difference. The moon isn’t really rich in precious metals or anything like that.

But whatever, for space flight, the resources on the moon have the massive advantage of being way outside Earth’s gravity well. In theory, the energy costs of delivering materials from the moon to cis-lunar space are about 5% of the costs of delivering the same materials from Earth. The kind of things that I can see launched from the moon will be structural materials in the form of aluminium, titanium, but we’ve also seen things like Blue Origin developing solar cells that could be manufactured from lunar resources.

So, how do we launch from the moon when we want to use lunar materials to be the cornerstone of our space industry? Well, the orbital, you know, the escape velocity from the moon is about 2.5 km/s and the orbital velocity is about 1.7 km/s. These are both massively lower than the Earth, which is about 7.8 km/s, I think, to get into low Earth orbit, 11 something or other to get out to the moon. The moon is going to be definitely a lot easier on that.

But the hard part about the moon using traditional rockets is, uh, the moon is very poor in volatiles. That is anything which tends to vaporize when it’s heated up. It’s stuff that will tend to evaporate away and the stuff that we use to make rocket fuel, uh, tends to be volatiles. Now to be clear, we have lots of the oxidizer side of the equation, right? When we are making these aluminium, silicon, whatever, we are taking the oxygen out of that and we probably will have lots of oxygen left. So you can have as much oxygen as you like, but the stuff that you would burn with oxygen that is vanishingly rare.

Right now, um, we do know yes that there is some water at the poles. We don’t know just how much is there, whether it’s enough to sustain industry for a long term, a short term, or just one launch. This is the kind of mission finding about this stuff would be the kind of mission that like a Viper would have been doing if it wasn’t cancelled. Uh, but yeah, it is on a small scale it’s absolutely viable that we could extract lunar water from wherever and then, uh, electrolytically split that into hydrogen and oxygen and then you would have, you know, rocket fuel, right? Hydrogen and oxygen, uh, and that has an existing heritage. You could take an engine from the Earth and create hydrogen and oxygen on the moon and run your rocket engine on it. Problem solved, right?

But if hydrogen is rare, what you might want to do is say combine it with carbon to make a hydrocarbon. Now, that won’t be as good specific impulse, but you’ll have a lot more fuel for your very limited supply of hydrogen. Now, classic hydrocarbons are things like methane, right? A carbon with four hydrogens around the outside. Yeah, you could definitely do better in terms of, uh, you know, your limited supply of hydrogen except that carbon is even rarer than hydrogen. And I think this is because carbon forms carbon dioxide and carbon monoxide and both of those are even more volatile than water. So those carbon-bearing materials, those disappear faster than water. We also don’t form things like carbonates on the moon because it doesn’t have like a water cycle or anything.

But if you wanted an alternate form of rocket fuel to, uh, make the most of that very limited amount of hydrogen you could have, well, if you look at the periodic table, right, carbon is useful for hydrocarbons because it has four chemical bonds right around it, roughly speaking. If you look below on the periodic table, there’s another element which has four bonds and that is silicon. You can actually make a lot of silicon analogs to carbon by just swapping out the carbon. So there is something called silene where you have a silicon atom and then four hydrogens around the outside. And this will absolutely burn like a rocket fuel. And silicon is everywhere on the moon. So you could make, you could use this to augment your limited supply of hydrogen, make a silene-based rocket. Um, and that’s great.

You know, one of the nice things about silene is that it’s actually hypergolic with oxygen at, you know, reasonable temperatures. It will spontaneously combust. So you could solve a lot of your combustion and ignition problems. Now, the downside is that when you burn silicon with oxygen, you get silicon oxide, silicon dioxide. You’re producing sand in your rocket exhaust. Now, it’s vaporized initially, but as it goes downstream, if it cools down, you can get sand, you know, quartz deposits forming on critical stuff, or you could just be sandblasting stuff downstream. But still, silane-based rockets are totally a viable option if you’re wanting to launch stuff off the moon using local materials.

Another type of rocket could use aluminium and oxygen. Now, it turns out that by the way that most solid rocket motors that we launch rockets with, most of the energy the burn comes from perchlorates combusting with aluminium because aluminium actually has a lot of energy in it when it burns. We don’t think of aluminium as something that’s burnt, but it totally will do that. So, uh, you can actually make a rocket motor with that. There’s like ideas called a flashbulb rocket where it’s like just like very fine wires and you blast through the oxygen. It burns very, very quick and you get a quick thrust to put yourself into orbit.

There have been designs which use slurries of, uh, liquid oxygen and very fine-grained aluminium powder and you can burn that like a solid rocket motor or you can even, you know, you can actually blow dust by having, uh, the oxygen flow through it as well. So this is totally a legit item which has been investigated making rockets from aluminium and, uh, you know, all liquid oxygen.

Now there’s some other option, another option I’ve heard is, uh, a liquid metal alloy rocket. So there’s potassium on the moon. It’s not that common, but there’s a decent amount of it. There’s sodium. If you mix potassium and sodium together in the right ratios, you actually get an alloy which is liquid at reasonable temperatures. So you could use that as a fuel. Now you could also, if you want to get a little more energy, you could probably mix in some aluminium, find your exact perfect ratios, and then burn that with liquid oxygen. Then you would have two propellants that you can in fact burn.

Again, this is something you can look at. The potassium, by the way, comes, uh, in the maria, you know, these large lava flat areas which were lava flows. The materials in it are called creep. It’s like K for potassium and rare earth elements and phosphate. Turns out that potassium is fairly radioactive, relatively speaking, and it forms a lot of heat. So, potassium-rich regions of the moon would remain liquid longer and you would get volcanic activity.

Anyway, yeah, you could absolutely build a rocket engine using liquid metal alloys and then you could actually control it by turning a valve on and off and maintaining the temperature. So, that’s another type of rocket engine you could build. And of course, if you’ve got oxygen, you could just have a cylinder, high-pressure cylinder, heat, you know, and let it out. Cold gas. You could heat it up if you want. There’s a limit to how much you want to heat up oxygen because if it gets too hot, it’s going to attack things because it’s an oxidizer and you’d rather it oxidize a fuel rather than your engine.

Transcript pt 2

Mass Driver Technology

So, now let’s get away from the rocket engines and start talking about a mass driver. Now, that’s really a general name for a whole bunch of different technologies that will take a payload and accelerate it up to at least orbital velocity of 1.7 km/s or, you know, um 2.5. Now, to get an idea of how fast this is, this is basically a space gun. And 1.7 km/s orbital velocity on the moon is about 5,500 feet per second. And that’s a strange number, but it’s actually the number that a lot of, you know, gun people talk. And I looked up and the fastest bullet that I can find that could be fired from a somewhat conventional gun is a round called the 22 splitting louden bulma. I think that’s how you’re supposed to say it, right? And even that was basically what they did was they took a 378 magnum case and charge and put a tiny 22 bullet, you know, on it, mated that to a suitable breach and barrel, and that was able to fire at 4,500 ft per second. So even the fastest bullets that were built for firearms are slower than the orbital velocity from the moon. Anyway, uh yeah, you’re probably not going to use chemical propulsion to accelerate stuff inside barrels like guns on the moon simply because, again, you don’t have any ox, you don’t have any fuels to go with your oxidizers. No, instead we’re probably going to be doing it with electromagnetic accelerators.

So the classic mass driver was envisaged by Gerald O’Neal in his High Frontier, and it was a coil gun. They actually built an example of this in the 1970s with MIT. So a coil gun essentially has a series of coils that get activated in sequence, and the changing magnetic field interacts with a projectile. It can either have like permanent magnets and/or it can generate eddy currents and then push it or pull it depending upon the design. The one that they built in the 1970s accelerated at 30g and shot the stuff out at, you know, 40 meters per second. So, if you just scale this up and make it longer, bigger and longer, you could eventually reach orbital velocity or even escape velocity.

Now, how big, how powerful? That’s a question you’re asking. Well, turns out there’s some very simple back of the envelope math that we can use to figure this out, right? So first of all, in terms of the amount of power required, it’s simply kinetic energy. 2.5 kilometers/s is about 3.1 megajoules per kilogram, and that is about 75% of the energy per unit mass that you get in TNT. Um, but yeah, that actually, if you think about it, then that means that the amount of fuel needed to accelerate 1 kilogram to this speed is generally going to be less than the mass of that object. Uh, by the way, yeah, 3.1 megajoules per kilogram, that’s equivalent to about six Twinkies. Yes, 135 kilocalories. Uh, of course, there are inefficiencies in the system. You might need more than six to feed them. And by the way, Twinkies aren’t actually something that is found on the moon naturally. So I don’t know why I went there. I just, when I speak about energies, look.

Okay, let’s talk about the size. The size of this, you can figure out the size of the gun by the amount of acceleration you want to give it. So you know the velocity it needs to get to. If you imagine it’s accelerating in a linear rate, then the length of the barrel becomes, or the length of the acceleration section is 1/2 of the ultimate velocity squared divided by the acceleration. So for about 2.5 km/s, that works out to 320 km divided by the acceleration in g. So if you had a human being accelerated at 1g on the moon, that would be 320 km, right? If you maybe shrunk it down to 3.2 g, which is quite doable, you could sit in a seat. You could accelerate to lunar escape velocity in 100 km. So that’s roughly the size of the thing you’re doing. But maybe you’re not launching a human. Maybe you’re just throwing chunks of rock up there. Well, let’s accelerate it at 32 gs. Then it only needs to be 10 km long. If you accelerate at 320 gs, that’s only one kilometer. So you can see how you can very quickly make these things compact. The size of these structures are not that far off from things that we have on the Earth. If you look at CERN, if you look at the Large Hadron Collider, it’s quite big compared to this. Granted, you know, it wasn’t having to be built on a moon that was a long way away and had very limited infrastructure, but again, it starts to seem fairly doable.

Now, powering this kind of thing would require a storage system which could charge up slowly using, say, solar panels or nuclear power, and then it would be able to rapidly release the necessary power in the time it took to accelerate the object. Now, batteries are great in terms of their energy density, the amount of power you can put in for a given size or a given mass, but the thing with mass drivers is that they need to release the power very, very quickly. And batteries don’t tend to release as quickly. So, you have to go with capacitors. Those are an option. They’re a lot bigger than batteries, but they can release the power very, very quickly. Uh, now there’s another option which is superconducting coil storage where you essentially have a big superconducting coil in, you know, cryogenics to keep it below its superconducting temperature, and if you put a magnetic field onto this, the electricity is flowing around, it’s creating a magnetic field, and since it’s superconducting, it doesn’t go away. You’ve stored the electrical power in the form of a magnetic field, and then if you change the contacts to flow a different way, the power will then come out. The magnetic field collapses and you get that energy out very, very quickly. So superconducting coil storage is better than capacitors in many ways, and that could absolutely be a viable way to store power for a mass driver on the moon. Um, again, we’re not going to go into too much detail as to the exact mechanism by which these things are launched, whether, you know, it’s a coil gun, a rail gun, or a linear motor or something like that.

Anyway, another thing about this, by the way, is that if you’re accelerating an object with magnetic fields, it needs to be able to interact with these electromagnetic fields. It wants to be a highly conductive substance, but maybe what you’re throwing to space isn’t going to be particularly conductive. So, what you’ll do is you’ll wrap it in like a payload container and accelerate that. Or you could have the payload container basically be a bucket, which you dump stuff into. The bucket gets accelerated down the line, and when it reaches a certain point, it starts decelerating and the material inside it shoots off into space. So you can then slow the bucket down, recover some of that energy, by the way, to your energy storage, and then return it to the start of the launch track getting ready to fire again. So that’s another thing to consider that you might have like a series of buckets that are just running and shooting stuff out into space.

Um, anyway, Spin Launch that was mentioned, and as ridiculous as Spin Launch is on the planet Earth, it starts to make a lot more sense when you’re dealing with the lower velocities of the moon and the fact that the moon is a vacuum and you don’t need a giant vacuum chamber. The nice thing about spin launch in this context is you don’t really need a massive energy storage system in the same way because you’re putting the solar power into the spin rotation system and it is basically storing all that power as kinetic energy. And now most of the energy is actually going into the arm because it’s quite heavy. Uh, and you’re spinning that up. You’ll probably actually have two masses on here which get released at the appropriate moment and they shoot off. Now, the accelerations for a spin launch system are lower than they need to be on Earth. Um, you could probably make it a bit lighter, a bit smaller, and since there’s no, you know, there’s well, since there’s nothing around, you can really have your pick for where you shoot stuff.

Uh, now after you’ve released your masses, you then do want to have a storage system to recover the energy back in. But yes, spin launch as a concept, it generates higher G’s than a linear accelerator, but it’s more compact. It’s something that you can fly down, you possibly put on a spacecraft and land on the moon and get going probably a little more quickly. Um, now, one other thing to realize is that these mechanisms by which you throw stuff off into space, you are going to need to catch them somehow. Uh, you also need to think about what way you’re throwing the objects. Now, if you throw it like straight towards the earth, that may not be best. Generally, you’re better to aim the stuff towards either the leading or the trailing Lagrange point, the L4 or the L5 point, partly because they’ll actually have to climb uphill and they’ll arrive with a little lower energy. And at that point, they’re sort of somewhat stable. So, it’s a good place to actually set up your industry up there. But to get there, you might need small corrections to your orbit. And that’s where these small thrusters, perhaps pressurized oxygen thrusters could work. Uh, another one, by the way, you could use is just have an arcjet. An arcjet which passes a lightning bolt through the oxygen and then shoots it out the back. Now, I don’t think that would last very long because it’ll tend to just destroy the rocket engine. But these are only needed for a very short amount of time to trim the velocity and put it in the right place so it can get caught by something else.

(the remaining 700 or so words? Wait a couple of hours.)

Transcript pt 3

Lunar Space Elevator Concept

Anyway, if the thought of building a 300 km long linear launch system doesn’t scare you, then there’s a more efficient possibility for the moon. You could actually build a lunar space elevator using pretty much existing materials that are available today. So, a space elevator, you probably know, is a cable that runs between the surface of a planet or body and some point out in space. And typically, there’s a counterweight out there. And if you imagine the earth, the earth is rotating, this gets the counterweight held out and it holds it tight. And you can run stuff up and down this cable.

Now, the moon is tidally locked to the earth. So, it doesn’t quite work in the same way. What you have to do is have your cable come up from the lunar near side, pass through the Lagrange point, the L1 point, the point of gravitational balance between the earth and the moon, and as you go over the other side at some point, you maybe go like another 35 km, thousand km, and you have your counterweight. So you’re talking a cable which is maybe 80 to 100,000 km long depending upon the size connects to the lunar near side and you can run spacecraft cars climbers up it. This actually, it’s actually slightly longer than an earth-style space elevator, but because the mass of the moon or the gravitational forces are so weak, the gradients are so small, it’s actually easier to build.

In many ways, the structural strength of the materials needed to build a lunar space elevator are a lot weaker, a lot, you know, less demanding than that is capable or is required for an earth-based space elevator. Like we could do this with existing polymers that you can buy off the shelf here. Things like Kevlar and Xylon absolutely work. You don’t need to get your dislocation-free perfect carbon nanotubes which you would need for something on earth. So yeah, you could build a space elevator.

Now, I looked at one particular example that said that if you have the near side surface of the moon and the earth, you would go out what, 85,000 km from the moon for every 1 kg of cable you would need 1 ton of counterweight out here. Now, a 1 mm square millimeter cross-section cable will carry about 5,800 newtons of tension, right, of load. If it’s 85,000 km long, that’s 133 tons. So, that would then, if you have a, you know, 133 tons times 133,000 tons, that’s a small asteroid. And it’s not even that small an asteroid. It’s like a Chelyabinsk, maybe a bit bigger than a Chelyabinsk-sized asteroid to put that amount of load out there and balance it.

The trick then is that you have to be able to capture the asteroid and put it in an appropriate orbit so you can lasso it. Also incidentally, we’ve talked about like the space elevator connecting from the point on the near side. You could absolutely have other connections that go to like the lunar south pole and stuff. And then yeah, you have a robot, a climber which can climb up this and drop the payloads, drop the materials into the appropriate orbit. This is something which is within our technological understanding right now.

Now, it’s not trivial. There’s all sorts of problems to solve. For example, the best polymer that we have in terms of strength to weight ratio is Xylon and that has issues with ultraviolet radiation. There was like a whole scandal where bulletproof vests that were using this were breaking, were, you know, failing well before their lifetime because moisture and ultraviolet was causing them to break down. In deep space, you won’t have the moisture, but you will have a lot of ultraviolet. So, it’s not entirely a solved problem, but we believe that it is well within humanity’s technological grasp to build a lunar space elevator.

And once you have a lunar space elevator, you can bring materials up, you know, to whatever place you need. And then depending upon where you release it on your elevator, it can go into specific orbits. And if you build one on the other side, you can actually go way, way out and release an object at the right time and it will basically get a free transfer to Mars if you make it big enough and long enough. So, this is an exceptionally neat thing if we could ever build it.

So yeah, Russell, I hope you found that interesting. I hope that all of you found this interesting, if a little bit rambly, and yeah, if you have questions like this that you think I can answer, again, go over support me on Patreon and I pick questions out of this list. It takes a while to sometimes find the good ones, but you know, you could be getting your name checked here, and maybe you could be the one that gets me to talk about a sandstorm powered rocket. I’m Scott Manley. Fly safe.

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