Fusion Propulsion

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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,  

along with all of our various social media forums  where you can get updates and chat with others   about the concepts in the episodes and many other  futuristic ideas. You can also follow us iTunes,   Soundcloud, or Spotify to get our  audio-only versions of the show.   Until next time, thanks for  watching, and have a great week!

2021-08-23

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