Using Water As Rocket Fuel - Is This The Future of Space Exploration? [20:13]

Transcript

Hello, it’s Scott Manley here. Can you use water as a rocket fuel and not have it suck? Of course, I can feel the doubt from a bunch of you already, especially those of you who remember the steam-powered eco rocket. But there are literal rocket scientists out there who know what they’re talking about, and they will tell you that you can not only use water as a rocket fuel, but it might actually be the propellant of the future. So, rocket fuels are generally something that always needs special handling. Some are cryogenic, some are toxic. All of them are highly energetic when they get together with the oxidizers or the right kind of catalyst.

Well, except for xenon, which its special handling requirements involve throwing lots of money at it because it’s stupidly expensive. But what if we could just use water? It sounds almost absurdly simple. Water is the most common liquid on Earth. It’s non-toxic. It’s a dense liquid at room temperature and at one atmosphere, and you can handle it with your bare hands. Well, unless it’s scaldingly hot because somebody’s trying to use it in an eco rocket. I mean, look, you’d think if this was a genuinely good idea, it would have been done right now already. Well, there are problems, engineering issues that have to be solved. And there’s a company that’s ready to launch their first mission to demonstrate this. And they think that water is going to be a key technology in opening up the solar system to exploration. And no, I’m [clears throat] not talking about the Eco Rocket. I’m talking about General Galactic, who are sponsoring this video.

They came to me with the idea to talk about water as a rocket propellant. And after investigating them, I realized they were legit. And I was excited to dive in. I mean, literally, judging based on that GoPro footage founded by Halen Madison and Luke Nice. Halen worked at SpaceX, and he was really interested in building the technology required to refuel spacecraft on Mars or other off-world destinations, but those solutions wouldn’t be needed by Starship for a really long time. So that wasn’t his job. So he sort of left. He teamed up with Luke, who came from VA, and they decided to forge their own path into the future. Water is found all over the solar system. It’s widely seen as a key resource that could be used to refuel spacecraft and, of course, be used for many other processes, but I’m most interested in its potential as a propellant.

Missions today have to load up all their resources at launch and deplete them as time goes on. The idea of refueling while far away from home using locally available resources is still very much something from science fiction. But for a more near-term advantage, water is also incredibly interesting to small sat developers where many launch partners won’t allow chemically active propellants and payloads because of their potential to escape in highly energetic fashion and damage other satellites on the same deployer. Even chemically inert high-pressure gases are frequently forbidden because a pressure vessel failure releases a lot of energy. Water isn’t stored under pressure, and it won’t burn, so it’s possibly acceptable to some providers.

Water is commonly found in the exhaust of rocket engines from propellants using hydrogen, hydrocarbons, hydroine, hydrogen peroxide, basically almost every chemical fuel that’s used. And that’s not an accident. It’s a relatively light molecule, so it tends to go fast when kicked. And in turn, that makes the rocket go faster. But of course, water is a reaction product. Reactions like burning hydrogen and oxygen release a lot of energy. If you use water as a propellant, you somehow need to get that energy from somewhere else. And [clears throat] again, look, okay, the eco rocket pitch was that they would make a steam rocket storing the energy as high temperature, high pressure water in the tanks. In this state, it does contain energy, but it’s no longer the safe benign water that we use every day. It’s a raging, scalding beast that will strip the flesh from your bones in seconds if it escapes in your general direction. And even then, it is just thermal energy stored in vibrating molecules. It’s on the low end of energy availability. The water might be stored at 200 C, but burning hydrogen and oxygen produces a flame 10 times hotter, and that extra energy translates directly into specific impulse.

Certainly, you can make a case that hot water rockets could be used as first stage boosters because they can quick release the energy, but they simply don’t have the performance needed to go all the way to space without requiring a silly number of stages. So, let’s forget about water as a first stage propellant. I mean, I only did this so you could remind you all about the eco rocket and we could have some laughs, you know, comic relief before we got on to the real science.

Once we get to orbit, the equation changes. You don’t have to constantly fight against gravity. There’s no need for minimum levels of thrust. You can use solar panels to power electrothermal thrusters. That is, thrusters that use electricity to heat the propellant to a much higher temperature at the moment it’s used, replacing chemical energy with electrical energy. Resistor jets are the simplest practical implementation. Liquid water is fed into a vaporization chamber heated by electrical elements, and the resulting vapor is expanded through a nozzle. Now, resistor jets can maybe run at temperatures of like 500 Celsius, drawing maybe, you know, 10 to 20 watts of power, getting million levels of thrust on tiny satellites. You’d get about 90 seconds of specific impulse, which loses out to every sensible chemical propellant, but again, you might not be able to use those propellants.

Now, arcjets try to push the temperature even higher. They run an electric arc through the propellant, and they’re also well-tested technology with other propellants. But with water, the arcjets haven’t really been made to work well because the free oxygen attacks the electrodes and causes rapid erosion. There is laboratory work on water-based arcjets, but it’s immature, and the electrode lifetimes are definitely a question. And then we get to microwave electrothermal thrusters, and those avoid the electrodes entirely. Microwaves are focused into a resonant cavity. It creates a plasma that heats the water vapor.

Now, we’ve had ground tests of these that show that specific impulses of 700 to 900 second ranges are available. Uh, this technology has actually flown as well, and it hasn’t got quite that performance in space yet, uh, depending upon the power or mass flow settings available, but they’re still definitely better than the simple resisto jets. Uh, so it’s demonstrated repeated firings and actual orbit raising maneuvers with some test spacecraft. And because there’s no electrodes for the oxygen to destroy, they don’t have the lifetime problems of arc jets. So yes, water is already being used as a propellant in space flight.

But this is not what General Galactic is working on. And so I see a bunch of you out there think that you’ve already figured it out. We could use water in an electric thruster, say a Hall effect thruster. And those have actually been designed and demonstrated and tested water vapor Hall effect thrusters. It turns out their performance is kind of lousy right now, like the anode efficiency is something like 5%, whereas with xenon you get something like 50%. So it is a massive downgrade in capability both in performance thrust specific impulse, and it’s not what General Galactic is doing.

General Galactic’s Genesis system takes the next step. Instead of merely heating the water, they electrolyze it. Taking that electrical energy and putting it back in as available chemical energy. You apply electrical power, and you split H2O into hydrogen and oxygen gas, putting the energy into separating the molecules into the elemental components which are, of course, a conventional bipropellant rocket fuel.

On paper, it takes about 1.3 mega to split 1 kg of water. But realistically, there are inefficiencies that could require much more energy, and that gets you gases which you can store or use as a raw propellant in the engine. Now you need to store the propellants, and those are going to be as gases. They can take a lot of space unless you can compress them or liquefy them, and that does take extra hardware, adds complexity and energy to your processing. The more storage you have, however, the more flexibility you’re going to have depending upon the requirements of the mission.

Now speaking of hardware and complexity, zero gravity makes engineering a whole lot harder. On Earth, buoyancy helps separate the gas bubbles from the liquid in the electrolyte. In microgravity, the bubbles are going to stay where they are unless you force them to move. So, electrolyzer designers can use things like electrode surfaces that are hydrophilic and hydrophobic coatings, porous membranes, active flow management, all different tricks to keep the gases moving in the right places and prevent the cell from basically choking on its reaction products. General Galactic has been developing space-rated electrolysis cells specifically for this reason. The same cells are, of course, intended to become the building blocks of future propellant factories that could process, say, lunar or Martian ice.

So now let’s talk about how those gases are actually used. And you probably already guessed the first way. In pure chemical mode, you feed the hydrogen and the oxygen into a bipropellant thruster at high pressure and you burn them. You recover the chemical energy you just invested. Peak thrust can be relatively high because, unlike the electrothermal thrusters that are limited by the electrical power, the chemical mode is limited by how fast you can pump the propellants into the engine.

This makes it useful for rapid orbit raising, collision avoidance, or time-critical maneuvers. Because electrolysis can run continuously at low power from the solar arrays, you can accumulate propellant over hours or days and then fire a short high thrust pulse. The average thrust over long periods of time is limited by your electrical power budget, but the peak thrust is set by the thruster design. Small electrolytic hydrogen-oxygen gas engines of the type have already been flown on the punch mission.

So, the concept has some spaceflight heritage. Hydrolocks is pretty much the highest specific impulse chemical propellant in use today, and it’ll be better than the storable hydro hypergolics that dominate existing space propulsion. But given the small engine sizes, the performance won’t of these won’t match the large engines like the RL10. Also, because Hydrolocks isn’t hypergolic, your engines have to be more complex to add the ignition system.

So, all these video clips I’m showing are from General Galactic’s tests of their hydrogen oxygen engine. And they claim that they’re getting specific impulses of about 425 seconds, which is significantly more than the 270 odd that you get from traditional hydroine biopropellant thrusters. And you know the tyranny of the rocket equation really favors having higher specific impulse. It’s not as high as say as an RL10, but again this is a huge upgrade over other technologies that are used with storable propellants. This will enable a whole bunch of new missions, right, to either have much more delta V capability or to reduce the overall wet mass. Both of these are huge wins for spacecraft designers.

But there’s some other interesting facts like first of all the these are using gas gas thrusters and because they are getting their gas from electrolysis that means the gases that are coming out of the electrolyer are you know stoichiometric ratio. You’ve get eight kilograms of oxygen for every 1 kg of hydrogen. Most thrusters that use hydrogen and oxygen don’t actually use this ratio. They will use much more fuel, right? Because having the extra hydrogen in there helps with the cooling and it helps with specific impulse. So, they’ve had to solve all the engineering challenges to make their little thruster work with this 8:1 mixture ratio, which will mean higher temperatures in the combustion chamber. And also, because they’re cooling with gas rather than liquid, they are going to have a harder time cooling. That they think they’ve solved that problem.

The propellant is stored as gas at pressure. The electrolyer will naturally pressurize up to hundreds of atmospheres of pressure which allows them to store enough to make long duration burns. Some of the thrusters that we’ve seen say in the punch spacecraft have very small storage area volumes and they only allow thrust of a few seconds. This, you know, the design that’s going to be used with Genesis and Trinity is supposed to be a lot larger and hold a lot more. Also, because you’re only pressurizing the propellant that you’re about to use rather than your store of water, you save on tank mass overall at launch, right? You only need to pressurize the volume for the storage for the burn.

And I said that was the first way because the neat trick is that General Galactic planned to offer a dual mode propulsion capability with a second electric mode which uses the oxygen and the hydrogen as propellant in Hall effect thrusters. Now, as I said previously, using water vapor in a Hall effect thruster doesn’t work particularly well. And I think that’s possibly down to the complex set of species that you get when you try to ionize water. But a lot of these problems are apparently fixed when you just try to use oxygen. And this is the conclusion that came to, uh, people have come to. And so people have been developing thrusters that actually work on electrolyzed oxygen. Imperial College has the wet head and the aquaet devices. And they’ve shown that the concept works. You can get specific impulses from like 1,000 to 3,000 seconds range depending upon your power level and propellant combinations.

The performance is lower than a mature xenon Hall effect thruster. But again, we come back to water being a far more convenient and lower cost propellant. Materials for this, however, are hard. One of the reasons that noble gases are preferred in ion propulsion is their lack of chemical interactions. But oxygen plasma is chemically active. Tungsten anodes will form an insulating oxygen oxide layer and that will fail. Stainless steel will form a conductive oxide and that’ll continue to work. The channel wall ceramics and magnetic circuit components all need careful selection and shielding to reduce the erosion. Lifetime under pure oxygen operation is absolutely still a big long-term question. But the basic physics have been demonstrated and even made to work with electrolyzed gas.

So the dual mode architecture is the real payoff. One propellant tank full of water, one electrolyer, high thrust chemical burns when you need to move quickly, long sustained high specific impulse burns when you need an efficient total delta V. And I want to make it clear that spacecraft designers that choose to use water electrolysis propulsion don’t really need to compromise on specific impulse. You need a lot of power for sure, but, uh, this chart here shows the performance of various thrusters. Was put together by Halen on one of his blogs. But in the bottom axis you have thrust. On the vertical axis you have specific impulse. And you have the dark blue dots those are water vapor propulsion systems including things like resisto jets and plasma systems. Uh, uh, the light blue ones, the cyan, those are water electrolysis propulsion systems.

So in the bottom right you have things like the hydros thruster at one newton and then they have the higher propulsion higher thrust ones that are being developed by General Galactic and in the top left you have the scion ones, the wet head thruster from Imperial College and also on this you have an orange line showing where hydroine is that is the limit on the specific impulse. Obviously, you can scale the thruster up and get a lot of thrust and you’ll see that the water electrolysis propulsion systems, yes, they can sit above that in terms of specific impulse and they can definitely get better thrust than water vapor propulsion systems. Uh, the main thing is if you want a high thrust long duration water electrolysis system, you’re going to have to have a lot more propellant storage.

And in the vertical, if you look up at that, uh, you know, the wet head, it’s getting comparable specific impulses to xenon thrusters. However, I’m going to say xenon thrusters are definitely a lot better researched at this point and so they are going to get better, you know, thrusts for the same amount of power. So, you know, they will still have an edge if you’re willing to pay for the propellant. So, General Galactic believes that since they’re offering this dual mode capability and the higher performance chemical thrust that this will unlock a whole bunch of new missions, right? More capabilities for dog fighting in space is one thing that Space Force would love because when you are performing maneuvers, proximity operations around potential adversaries, you want as much delta V as you can get, but you also want to make sure you can move fast when you need to and move efficiently when you can.

The same hardware that lets a satellite perform dynamic space operations in low Earth orbit is also the core of a system that can eventually be refueled from lunar or Martian resources. General Galactic’s near-term plan is the Trinity mission, a roughly 500 kg spacecraft scheduled to fly in a transporter mission potentially later this year with the intent to demonstrate the full Genesis multiode system in orbit. That’ll demonstrate the technology and maybe it’ll be adopted by some missions longer term. They talk about operational mobility services. They’re hoping to build Seldon, a standalone spacecraft which can act as a space tug and move payloads around LEO, MEO, GEO, wherever the customer needs it to be. Then SIS lunar cargo and eventually propellant factories whose building blocks are the same electrolysis technology.

But the real long-term enabling technology is the electrolyer. Engineers have been designing architectures for exploring the solar system for decades. And electrolying water keeps turning up as a component that solves many problems. In video game speak, it’s a technology that unlocks huge parts of the tech tree. If you’re on Mars and you find water, split it to hydrogen and oxygen, and then you can feed it into a reaction vessel with carbon dioxide to make methane that could fuel Starship. The hydrogen and oxygen can be used in a fuel cell to generate electrical power again. So water electrolyer paired with a fuel cell could offer long-term power solutions, say for lunar night, creating the fuels during the day and then consuming them at night.

The energy density of hydrogen oxygen is higher than that of batteries. If the engineering closes, the business model becomes interesting. You’re not merely selling thrusters anymore. You’re selling the ability to move mass around the Earth moon system and beyond using a propellant that can be harvested locally. Launch water from the Earth in the early days. Later on, you can mine it on the moon or process Martian ice. The same infrastructure supports commercial satellite operations for those who want longer life and more maneuverability or defense customers who need responsive space. And eventually, it’ll support the logistics network that lets human missions stop carrying every kilogram of return propellant from Earth. It’ll be a railroad to Mars. This is the classic science fiction vision.

Spacecraft that travel from world to world, topping up with local resources instead of flying one-way missions with enormous mass ratios. Water is the most obvious common currency because it is abundant, easy to store, and can be turned into both high thrust chemical propellant and high specific impulse electric propellant with the same core technology. But none of this is easy. Microgravity electrolysis, oxygen compatible electric thrusters, systems mass fractions that actually beat simply carrying xenon or a conventional bipropellant and the operational reliability required for commercial service are all real engineering problems that still need solving. But the underlying idea is sound. The propellant is ubiquitous and the operational advantages are large enough that multiple groups are investing serious effort. So yes, we can use water as a rocket propellant. We just have to be clever about putting the energy back into it first and then clever again about how we extract that energy as thrust. General Galactic is one of the companies trying to turn that sequence into a practical scalable system. I’m Scott Manley. Fly safe.

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