This episode is brought to you by Brilliant. Interstellar space is beyond enormous, in order to travel between the stars we will need the power of the stars. So today we will be taking a look at spaceship propulsion focusing on fusion-based drives, and we’ll be looking at both the various possible fusion drive systems and what they would really mean in practical terms. In that regard we will discuss the possible
speeds such ships might obtain and what sort of costs we might see for passengers or cargo, both for interplanetary travel and for interstellar travel, though for the latter especially we will have to be rather speculative. This is not our first time discussing fusion as a power source, so we will skim over some of the basic function and economic impacts that we explored in our Fusion Power and Impact of Nuclear Fusion episodes, but we have several different fusion drive scenarios and each would look very different. The key ones we will consider for today will be Fusion Torch Drives vs Fusion-powered Ion drives, the Bussard Ramjet, Deuterium-based fusion, Helium-3 Fusion, plain Hydrogen fusion, and Pulsed Fusion – which is essentially shooting hydrogen bombs out behind a ship. That last one, Pulsed Fusion, is the only one we currently have the technology for and folks are not overly anxious to construct a ship to Mars or Alpha Centauri that would leave Earth’s Orbit with a cargo bay full of nuclear bombs, and pushed from orbit by them – we’ll discuss how this might be done and possibly safely later today but for the moment it reminds us that while fusion often gets called the technology of future, and always will be, it is something we have actually had for an entire human lifetime, since the mid-20th century, just as bombs. We also need to start with two other reminders. First, while shooting bombs out the back of a ship sounds dangerous, it is essentially what we already do with jets and rockets, just scaled up, and any interstellar drive needs to be operating at those hydrogen bomb levels of energy anyway.
This is why we often say on the show that there is no such thing as an unarmed spaceship, since the energies needed to push them between worlds is easily convertible into a weapon and a terrifyingly powerful one at that. Second, just as any ship with such a spaceship drive is carrying a world-busting arsenal with it, those same engines should be available to their civilization for non-space uses. A fusion-powered economy is one of titanic capabilities. Not only can a fusion-powered ship reach Mars in days not
months, but the society with a fusion economy can mass-manufacture hundreds of ships hundreds of times bigger than what we tend to plan for that trip now. And when those ships land, they’ll each have a powerplant capable of brute force producing all the metal, glass, and air any growing domed colony could plausibly need. They could even brute force extract minerals by centrifuge or heating and 3D print stuff where that would be recklessly uneconomical to us. Fusion is the power of the Sun, something so potent it was routinely worshiped as a god by our ancestors, once harnessed to our control, it lets us perform tasks of unimaginable scope, not just run the ships using it. But let’s move on to those ships by quickly covering how fusion works. In the first place, we need to understand the kind of power levels involved. As Einstein showed with E=mc², mass and thus all matter is also energy,
and because of the multiplier and squared term, it ends up being quite a lot. Chemical fuels are incredibly powerful, but their energy has to do with changing around the relatively weak bonds between molecules and atoms. It's not something we tend to think of when burning gasoline, since it all burns away, but it is actually losing mass as it transforms into various byproducts. The amount of mass it loses in this transmutation is equal to the energy it emitted, in accordance with E=mc², and yet we’re not even losing a millionth of the starting mass through such a transaction. If you were to add up all the mass of air and fuel burned,
then add up all the gas and ash that fuel leaves, what you’re left with will be nearly the same as where you started, minus just a tiny fraction. Nuclear forces do around a million times better, as this involves a massive rearrangement of the vast sea of real and virtual quarks making up the core of an atom, not the tiny electrons orbiting at the periphery. However it takes vastly stronger forces to ignite these changes, not temperatures of hundreds of degrees but temperatures of millions of degrees and pressures far beyond what occurs on Earth or even in its crushing and scorching interior. These rearrangements of the nucleus of an atom, fission or fusion, produce far more energy but require far greater efforts to achieve.
Fission, the cutting of big atoms into small ones, differs from fusion – the combining of small atoms to make bigger ones – in two key ways. First, there are vastly more of these small atoms lying around, making it a cheaper and more abundant fuel in the same way seawater is more abundant than gold, and second, this energy is usually a decent fraction of a proton or neutron’s mass, so the energy released by fission of one atom of uranium – composed of hundreds of protons and neutrons, is on par with what we get from fusing a hydrogen or helium atom of just one or two protons and neutrons. So while the energy released per atom is similar for fusion and fission, the atoms involved in hydrogen or helium fusion are far more numerous per kilogram of fuel and there are just far more kilograms of that fuel lying around. There are other advantages too, but it is why we often see nuclear spaceships proposed, because while fission might not be as good as fusion, it's still vastly better than chemical fuels, see our episode the Nuclear Option for discussion of those. Even though Uranium and Thorium are rarer than Hydrogen and Helium, there’s still more than enough of the stuff available in our solar system to power millions of spaceships up to speeds permitting interplanetary and even interstellar colonization. We’ll be talking about the specific
ways a given drive aims to achieve fusion but this is why it is desired as a power source. On the show we often discuss alternatives like fission or solar power, or even antimatter and black holes for driving spaceships, but we do this while recognizing that we don’t have fusion yet and that even if it’s something we somehow never achieve, we can still journey to the stars through other means. Fundamentally though, it's always been fusion that offers that promise as the leading candidate, with the other runners-up getting discussed here as a ‘just in case’ or as hypothetical upgrades. Fusion, the power of the stars, offers us the chance to journey to those stars and does so by throwing out so much energy a spaceship in orbit of a world would look like a star when its engine came on. In fact that’s a bit of an understatement, since bigger ships might light the sky up like a second sun, but this is part of why certain types of fusion drives are known as a Torch Drive or Torchship, a term we get from Science Fiction Great Robert Heinlein, although the ships described as such in Heinlein’s works actually were using straight matter-to-energy conversion, not the roughly 1% conversion we see with hydrogen fusion, and most fusion types don’t do even that well. Torch drives are the muscle cars of rocketry, combining the high fuel efficiency of something like an ion drive with the high output-per-second of a rocket engine..
Folks tend to forget we don’t really need a higher thrust than what chemical fuels provide, we just need something that will keep producing it for days rather than seconds or minutes. The specific impulse of a fuel, normally given in seconds, is how long a given fuel could hold a ship up against the pull of Earth’s gravity. A decent rocket fuel can provide a few hundred seconds of that. We usually burn it even faster though, with enough force to pin people to their seats, so producing force faster than this isn’t usually a priority. Rather the hoped for upside of a torch drive is that it can produce this thrust not for hundreds of seconds but for millions of seconds. This is the other half of the game, protracted acceleration. Here on Earth almost every type of travel we do involves a brief period of speeding up and slowing down, and most of your time and energy is spent cruising along and maintaining that effort. Since space is fairly empty, very
little effort goes into maintaining speed, but it's also insanely huge so you have to spend a lot of time crossing it unless you can go ridiculously fast. Because of how fast you’d need to go, it takes a lot of time and a lot of energy to speed up or slow down to and from those insane speeds. Mars for instance is a long way off, varying from 55 million kilometers or 34 million miles at its closest to 400 million kilometers or 250 million miles at its furthest. This means even light takes 4 to 24 minutes to cross that distance, varying as Earth and Mars both orbit the Sun.
Our ships travel slow enough that we can’t send them in a fairly straight line either, but the hope is that a fusion ship could travel fast enough that it was a fairly straight course. Such being the case, how long would such a journey take? The thing is that humans don’t handle high accelerations well. It's essentially the same as gravity, so a ship just doing 1-gee of acceleration, akin to normal earth gravity where the rocket nozzle is essentially in your basement of your ship, not the back like an airplane, and is probably as fast as you would want to accelerate. Emergency or Military vessels might do more, but probably at the cost of fuel efficiency and maximum possible speed for longer trips. Incidentally, this means that such ships, these torchships able to maintain high thrust for long times, are closer to a skyscraper than an airplane. Your destination is upward toward
the roof, and the engine is down in the basement, not the back. All your gravity is being provided by thrust so if it turns down, gravity goes down, if it turns off, you’re in zero-g, and if it suddenly turns on again, or turns on sideways to dodge something, you can get smashed around and potentially killed. In fiction we often see ships with effectively infinite fuel that will burn the entire time, and they reverse at their halfway point, what gets called a Turnover, on the assumption the ship might just turn itself over to point the engine toward the destination and slow down. This is never the ideal way to use fuel efficiently,
in a lot of cases you would end up burning an order of magnitude more fuel than if you took twice as long to get there, and should only happen in cases where the maximum acceleration the ship can achieve, or its crew or cargo can survive, is a bigger constraint than the fuel supply. Where we see torch drives we rarely get descriptions of how they work, and given the origin of the name they need not necessarily even run on fusion, antimatter would fit better, but it’s the assumption you can find a way to make fusion occur and presumably spit its byproducts out at full speed behind the ship as its propellant. The exhaust velocity of the typical rocket flame is in the thousands of meters per second range, which is also the general zone of what orbital velocities of planets are, both to orbit them and in their own orbit around the Sun and why chemical rocket fuels can just barely get the job done, though often requiring virtually all of ship be composed of its fuel.
Ion Drives function by using energy to run magnets to spit particles out the back an order of magnitude faster, but a torch drive is usually assumed to be firing particles out the back at thousands of kilometers per second instead, at a small but significant percentage of light speed. In all other respects a torch drive is treated like any other rocket, burning hot flame out the back shoving the ship along till the fuel is spent. Although again in such cases it isn’t really the back of the ship, it is the floor of the ship. It would not be realistic to expect to get the maximum hypothetical exhaust velocities out of a fusion fuel but let’s list some major one’s theoretical maximums. The exhaust velocity for Deuterium-Deuterium Fusion - generally viewed as the easiest type of fusion to achieve – is 4.3% of light speed, which is 13,000 kilometers or 8000 miles per second. For comparison, this is about 3000 times faster than burning hydrogen as a chemical fuel, meaning that an otherwise identical rocket fusing deuterium would be able to move 3000x times faster than one burning hydrogen. Helium-3 is about half again as good at 6.8% of light speed,
Deuterium-Tritium comes in around double at 8.7%, and regular hydrogen proton-proton fusion comes in at a whopping 11.7%. Hypothetically you can do even better with multiple fusion chains running hydrogen potentially all the way up to iron. See the always excellent Project Rho website if you want a run down on how that’s all calculated and some of the other fusion options. And again
those are absolute maxes, Deuterium-Tritium for instance releases most of its energy as neutrons which are very hard to control and redirect so getting even half of that 8.7% exhaust velocity would be optimistic without some bit of Clarketech for redirecting neutrons, something we also cited as useful for creating neutron beams in our recent episode on Death Rays. It's an important reminder that just because you can slam a bunch of atoms together doesn’t mean you can make them and their byproducts all dance to your tune and exit out the back like a rocket flame. Now if we are treating it as just another rocket fuel, you’re probably aware that liquid hydrogen is one of the more popular rocket fuels in spite of being hard to handle exactly because it’s got such a high exhaust velocity. However, that exhaust velocity is still only about half Earth’s low orbit speed and even slower than the delta-v needed for going to higher orbits, the Moon, or other planets.
This is why we need most of a rocket to be its fuel. If the exhaust velocity is less than the delta-v needed this is not the case, and generally you can only get a ship up to around quadruple the exhaust velocity before your payload is so small it’s only a couple percent of the initial ship mass including the fuel. And only double if you need to use that rocket to slow down too. There’s no actual limit as to how fast any ship can go off a given fuel, but even by 8 times the exhaust velocity, or 4 times if you want to slow down, your payload mass is down to 0.03%, 3000 kilograms of fuel for every kilogram of payload, and it only gets worse from there.
Folks like to ask a lot how fast a fusion-powered ship could go and I think it irritates a lot of them when I or others ballpark it at 10% of light speed, they want something more precise, but that’s generally why we do. I’m assuming they aren’t getting that ideal proton-proton fusion at full efficiency and that they are planning to slow down and want to carry a lot of cargo. Hypothetically even 50% of light speed isn’t off the table though, especially for something like a missile. And while it might take ridiculous ratios of fuel to payload, it is worth noting that fusion fuels are beyond abundant, so you might not care if you use a million tons of fuel per ton of payload for a colony ship. 20% though is probably on the high
end of plausible for colony vessels and a nicely round number which is why I used it as the upgrade case for Unity in our old episodes following that hypothetical interstellar colony ship. I don’t want to say that a Torchship is unrealistic, not at all, but they are a pretty vague concept so it’s hard to call them realistic or unrealistic. The three leading ways to make one would be antimatter-catalyzed fusion, a micro-black-hole, or a Bussard Ramjet. We’ve discussed black hole powered ships in their own episode so we’ll bypass that today beyond saying this is probably the closest thing to what Heinlein original Torchship description would be with matter to energy conversion. However, there are a few ways to run ships on black holes. We’ll come back to the Bussard Ramjet, but antimatter catalyzed fusion is a good lead into pulsed fusion propulsion because it’s essentially the same thing.
This is where a little antimatter is used to set off fission or fusion fuel as a small nuclear bomb, most of the energy derives from that hydrogen or other fusion fuel though. See our Antimatter episode for a discussion of the specifics but the concept is lots of pellets containing antimatter and fusion fuel being spat out the back of the ship. This lets you use smaller explosions than setting of traditional Hydrogen Bombs and this ties in with what I was saying earlier about Turnover and why it isn’t desirable unless your maximum acceleration the crew and cargo can survive is a bigger bottleneck than your fuel supply. A nuclear explosion obviously releases a lot of energy from a little bit of mass, and a hydrogen bomb or fusion bomb does this even more energetically per unit of mass than a uranium or plutonium bomb does. Pulsed nuclear propulsion is the idea that you can set a bomb off and ride the wave, and the bigger the bomb is, the bigger the wave, but the harder it is to safely absorb. Hence we tend to consider using a large number of smaller fission bombs but they don’t carry as much energy per unit mass as a fusion bomb, and thus have a lower maximum speed.
Now that safe absorption issue isn’t actually about not being killed by the blast or radiation in a conventional sense. The general notion is to put a big thick metal plate behind the ship that’s wide enough to absorb a big fraction of the blast, which shoves the ship forward and of course the radiation will strip off some of the plate with each blast but that merely acts as additional propellant as hot vaporized layers of the pusher-plate fly out from the back of the ship. The bigger issue is that this giant plate absorbing all this energy is now ramming forward, so the idea is to have it on a great big spring that slows it and more slowly shoves the rest of the ship forward, and you detonate the next one after the pusher plate fully returns, ideally converting all that blast into good smooth push for those inside the main ship.
Now the issue there is that this requires huge ships, and even larger ones for those using fusion bombs instead of smaller fission nukes. But to emphasize there is nothing stopping you from building one today anymore than there’s anything stopping us from using this same sort of trick to make a fusion reactor by detonating nukes in very big tanks of water and collecting their steam then detonating another as it cools. If you don’t mind making your ship truly massive then you can run them smoothly on h-bombs, indeed we have contemplated running ridiculously large ships off this sort of pulsed propulsion scaled up to use cosmic blast like novas, supernovas, or even quasars. One caveat though, a very big and massive pusher plate could as easily be recast as an equally massive but vastly larger and thinner mirror or solar sail for absorbing sunlight or laser beams, or in reverse, you might ride laser propulsion out of a system to get up to speed then contract the sail to use as a pusher plate with pulsed propulsion to slow down at your destination. Hybrid ship drives have many advantages. Antimatter catalyzed nukes let us avoid using hundreds of kilograms of uranium or plutonium and conventional explosives for a fission bomb or even more for a fusion bomb, and thus a much smaller pusher plate more akin to a standard rocket nozzle.
We believe as little as a microgram of antimatter might do the trick to catalyze a very small nuke. Obviously antimatter is a way better rocket fuel if you can make and store it cheaply and safely, but it is quite probable that producing antimatter would always be an energy loser in terms of energy put in to make it, in which case using it as a fusion catalyst might be very appealing. I do rate this as one of the more probable drive systems though because if you can make a working fusion engine, it does not mean you necessarily have a torch drive, but it does mean you have a strong and steady power source which can divert some of its energy to making antimatter which could then be used to run a fusion drive by this antimatter catalyzed micro-fusion bomb approach. It's also a way to store energy for use when you want it which is tricky with a reactor you would expect to run constantly at a set rate. That sort of rate is fine for most spaceship missions, you have a destination in mind and you accelerate to a calculated amount then cruise till it’s time to slow down.
It doesn’t really contemplate much sudden need for higher acceleration and in different directions, and micro-nukes permit smaller ships but also smaller pusher plates which might emerge from various angles of a ship to be used for lateral motion like dodging space debris. Indeed there has been a range of approaches contemplated in the last few decades for propelling a ship via continuous explosions of small fusion pellets. It tends to give you the high specific impulse of classic fusion drives while accelerating closer to the relatively high rate of pulse drives, and it’s sometimes called micropulse. Daedalus can be considered an early example of this concept.
One other note, those big pusher plates can be turned around long before you reach your destination to serve as a forward shield against space radiation and dust particles. Two things I want to emphasize real quick. First, just because your fuel is a million times denser than chemical fuels doesn’t mean you can automatically tap that for anything like the implied speed boost. Our current fuels aren’t the only things made out of chemicals, the ships are too, and outside of the hypothetical torch drive the usual assumption is that fusion reaction, which is so hard to contain even compared to a normal fission reactor, is not something you can just vent out into space. You are extracting power, possibly as electricity,
and may just be using it to run an ion drive or big flashlight to push your ship along. Helium-3 is attractive in this regard because its considered our best option for aneutronic fusion, which is fusion which releases less neutrons which are absolute murder on any containment system you try to keep them in, which includes magnetic containment since neutrons are electrically neutral. Aneutronic reactions might spit out a proton rather than a neutron, which can be magnetically contained or magnetically bounced out the back of the ship and charged particles are also much easier to generate electricity with. So it is viewed as a nicer option for spaceships in that regard. We often tend to assume fusion ships, or fission ships for that matter, are more likely to be slowly but steadily putting out power as heat and electricity we can run more conventional engines off of, with that reaction carefully sustained and nurtured in a reactor.
One quick note. There is a tendency to assume a stable fusion reactor is gushing power out at rates far faster than a regular nuclear reactor let alone a gas engine, but this is not exactly true. The Sun is hot and runs on fusion and is insanely bright, so it's easy to forget that the Sun only releases about one watt of power for every 5 tons of mass it has, compared to your typical engine which usually produces several thousand watts of power per ton. It just that such an engine runs on fuel that it would
probably use around a ton a day, whereas the Sun burns its fuel over billions of years. Indeed only the brightest and shortest lived of stars even begin to have the kinds of internal pressures that permit fusion at such a rate that it would be handy for running a spaceship, and even then only in the kind of power to mass ratios of conventional engines or generators. When we talk about torch drives we’re talking about trying to simulate conditions in a supernova, not a regular star. We tend to think of non-torch fusion drives as fairly big and slow to accelerate
but able to achieve pretty high velocities. Now I mentioned that we might instead use electricity to shoot particles out the back or even a flashlight, and indeed photons of light are your optimal spaceship propellant in terms of mass. They move at light speed so have the maximum hypothetical exhaust velocity beyond something weirdly and probably non-existent like tachyons. Nuclear light bulbs or flashlights or laser-emitters or photon drives or photon rockets, many names or variations, but they are plausible. They would be pretty bright, especially a laser
drive approaching or departing a planet, though you could spit out two or more lasers at slightly different angles to avoid hitting your destination with your exhaust beam. I want to emphasize by the way how insanely bright and energetic a normal spaceship is if you are trying to run it up at 1-gee and it is some multi-megaton hulk dwarfing even an aircraft carrier or supertanker. Just giving it the delta-v needed to break orbit, maybe 10 kilometers per second, is implying a power release for around a thousand seconds of several trillion watts.
Thousands of times more than our biggest powerplants and on par with what our whole civilization is using at any given moment. And to get to 10% of light speed it is going to need to keep that up for over a month. We’ll discuss travel times for ships between planets in a bit. We wouldn’t expect all that energy to be in the visual spectrum, it would depend a lot on how it was being made what portion of it we could see with our naked eye, but putting that into visual light terms, you would be able to see that during the daytime quite easily while it was nearby and such a ship should be naked-eye visible at night even a million kilometers away or further. Needless to say that makes for a pretty powerful weapon with some modification, which is why we say there is no such thing as an unarmed spaceship on this show.
Though its not exactly stealthy either, if aiming to sneak up and attack, which is why another thing we say on this show a lot is that there is no stealth in space. One other option that might be a bit more stealthy, though still not stealthy, might be neutrino drives. A thing that gets produced a lot in nuclear processes are neutrinos and they go right through any containment we’ve got but they carry a lot of momentum. So one fusion-drive option, if we ever found a material, or metamaterial, that absorbed or reflected neutrinos, might be to wrap a ship’s reactor core in that material except for a hole out the back for neutrinos to fly out. Done right this also might seriously amplify your fusion reactor too. Now folks often wonder how long it takes to travel between planets with a given spaceship engine and of course. The answer really varies on where those planets are relative to each other but also on how fast you can speed up and how fast your maximum speed is. I mentioned near
the start of the episode that Mars at its closest is a mere 4 light minutes away, but even if your ship can achieve 1% of light speed you aren’t getting to Mars is 400 minutes, because even if your ship can pull 1-g acceleration indefinitely you need a few days to reach 1% of light speed. For those watching the episode, I’ve got the formulas up on the screen for calculating this for classic Newtonian speeds. You can get to the moon with a 1-g burn and turnover in 3-4 hours, Mars in 2 days at its closest, and similar for Mercury and Venus, and Mars at its furthest or any of the Asteroid Belt in under 4 days. Jupiter 6-7 days, Saturn 8-9, Uranus about 12, Neptune 15-16. For interstellar distances you can just divide the
distance in light years by the fraction of light speed that the maximum is, because virtually all of that is spent cruising not speeding up, and at 1-g every week of acceleration or deceleration will add or subtract about 2% of light speed. One caveat though, when trying to shave time off a voyage, all things being equal, you want to accelerate quickly. If you have the option of accelerating at 2-g for five hours or 1-g for 10 hours and burning the same fuel, 2-g is better because you will hit your max speed after five hours and only need to start slowing five hours out, not ten. And remember we do need fuel to slow, unless we’re a missile of course. In practice higher acceleration rates tend to be less fuel efficient but we shouldn’t assume that is automatically true of all drive types. This also assumes you have to carry all your fuel,
something that rarely seems to trouble ships in science fiction but is a big issue for ships in science fact. But you can also achieve higher speeds by not having to carry all your fuel with you, and this is where the Bussard Ramjet comes in, both to grab fuel for speeding up along the way and to avoid needing fuel to slow down. If you are already familiar with this device then that explanation likely included why it doesn’t work as planned, and that’s only sort-of true since it still might be great for slowing down, but it’s a claim that is focused on the specific suggested design for the Bussard Ramjet not the core concept, which is sucking matter in from the front and shooting it out the back while it fuses when passing through. This ignores alternatives like have a blackhole in its throat or keeping to slower speeds or using that interstellar gas simply as propellant heated by on board fuel. The core concept of the Bussard ramjet is that most of space is full of ionized gas particles, most of which are hydrogen, a ready source of fusion fuel, and if you can grab these magnetically and suck them down into your ship you can use them as fuel. The ramjet part comes from parallels to how an air-breathing ramjet works, sucking air in and superheating it then shooting it out the back, but here the power source for heating that gas up is the gas itself by fusion, and that fusion is ignited by sucking in interstellar gas at relativistic speeds and jamming them tightly down the throat of the ship to ram into other gas particles at high speeds, temperatures, and pressures. This seemed like a possible way to give ships an
infinite power supply, simply grabbing it out of space as they flew by like a ship sailing on a sea of diesel fuel. Indeed it was contemplated for a while as a way to keep accelerating indefinitely, something we see in the scifi classic novel Tau-Zero, but the math turned out not to work. Indeed some argue the method would actually require more energy than reaction released and result in the ship slowing. Two points on this though. First, slowing down is
awesome if you get to do it for free, and this is essentially a physical or magnetic sail being used to slow a ship as it approaches its destination by scattering its momentum into all the interstellar gas and dust around it. Your scoop can be magnetic but costs you power to run it, or it can be physical but get eroded as you travel by gas and dust collisions, both have advantages. Second, there’s debate about if it can work at very high speeds as a ramjet, but works just fine for an occasion where absorbing the passing gas costs you less energy than you can produce from it if you’re using more of a collector rather than a ramjet approach. As we were saying early, various fusion fuels have exhaust velocities in a torch drive of 4-12% of light speed so if your ship is designed to go slower than that, it ought to be able to suck hydrogen in and use it as fuel. This is an important distinction since different types of drives are converting this into thrust in different ways. As an example it works just fine with a black hole drive at fairly high speeds since black holes are converting at a much higher matter to energy ratio than fusion. So I don’t like to rule out some version of the ramjet,
given how tempting the free fuel along the way is, but even if none of those work, the option for slowing down for free by magnetically dragging on interstellar gas is also a vast savings in fuel. So there we have it, the major types of fusion propulsion discussed and roughly how they function. I said earlier that we often discuss alternatives to fusion not because it's not an awesome option but because it's such a good option folks fear there are no alternatives if we never get it working. Those options do exist and you can see those episodes for details, but fusion still offers the temptation of practical interplanetary and interstellar travel if we can but make it work. If we can harness the power of the stars,
we will be able to journey out and claim the stars. Today’s topic were rocket science and nuclear fusion, fascinating topics but legendary for being tricky to understand. Learning math and science can be pretty intimidating for many folks and if you’re trying to improve your own skills at those or have a loved one who is, I want to recommend Brilliant. If you've heard me talk about Brilliant before, then you know that it's a website and app built off the principle of active problem solving — because it takes more to learn something than just watching it — to really learn something, you have to do it. Over the last year, Brilliant has built a whole new platform for their courses that takes interactivity to the next level.
Pre-Algebra, Mathematical Fundamentals, and Algorithm Fundamentals were the first courses launched on this platform and they just released a new Scientific Thinking course with this Center of Mass lesson, one of those concepts that’s vital to making sure a spaceship goes in the right direction instead of tumbling, and one of those topics where a good hands on example is the difference between getting a quick and clear understanding of a topic or spending hours trying to grasp a topic when a little interactivity makes it plain in moments. You learn best while doing and solving in real-time, not by long lectures or memorising formulas and facts, and Brilliant understands that and has something for everybody — whether you want to start at the basics of math, science, and computer science, or go straight to advanced material. If you'd like to join me and a community of 8 million learners and educators today, click the link in the episode description down below or visit: brilliant.org/IsaacArthur. So this wraps us up for today but we’ll be back next week for a look at the Edge of the Universe, on Thursday, August 26th, then we’ll have our Monthly Livestream Q&A on Sunday, August 29th at 4 pm Eastern Time. That will finish us for August but we’ll leapright into September with a look at the Future of Thorium on September 2nd and Human-Machine Teaming on September 9th If you want alerts when those and other episodes come out, make sure to subscribe to the channel, and if you’d like to help support future episodes, you can donate to us on Patreon, or our website, IsaacArthur.net, which are linked in the episode description below,
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