The Spaceship Propulsion Compendium
Today’s episode, the Spaceship PropulsionCompendium, is going to be a long one, sinceI am going to try to touch at least brieflyon every system including some of the moreunlikely ones. I will leave out any that are just not groundedin science at all, though if I skip one donot assume that means I am including it inthat category. Also some we have covered in more detail inother episodes so for those I will keep itbrief and mostly just refer you to those episodes. For those of you new to the channel, if myspeech impediment is giving you problems Iencourage you to turn on the closed captions,it takes most folks a little bit of time toadjust to it, though by the end of this videoyou probably will be as again this will bea long one. With that in mind you also might want to graba snack and a drink. I also want to add that this topic was selectedfrom those submitted by the channels patronsover at Patreon and is the first of at leastthree we will do this way. I only committed to doing three this way butit was a lot of fun working with the firstwinner, Drew McTygue, who selected the topicfor this episode, so I am considering makingthis a regular thing. The goal today is not to discuss basic rocketscience or review the history of spacecraft,and our interest in existing propulsion methodsalready in regular use is minimal, but rocketsmade spaceships possible for one key reasonthat separates them from two other methods,they provide fast thrust to the ship, butat the cost of devoting virtually all theship’s takeoff mass to fuel. This is because rockets do not have a veryhigh specific impulse. Specific Impulse, often called effective exhaustvelocity, is the total change to momentum,or speed, delivered to a ship per unit ofpropellant mass used. A rocket can deliver virtually all of thatin mere seconds or minutes, making it greatfor clawing your way up through an atmosphereand gravity well. Unfortunately it means that around 90% ofa ship’s take off mass is rocket fuel, justto get it a couple hundred miles up into space. Alternatively your car can take you a couplehundred miles off just part of a tank of gas,and a one ton vehicle usually only has 10-20gallons of gas, or about 60 to 120 poundsor 30 to 60 kilograms more or less. Cars take you hundreds of miles while usingless than 10% of their mass for fuel, rocketsoften do not get to use even 10% of theirmass for payload. If they had a higher specific impulse, orpower to mass ratio, we could carry up waymore. Double your effective exhaust velocity anda rocket that originally weighed, say 1000tons, 90% fuel and 10% for the rest, suddenlybecomes one that is only 70% fuel and 30%for the rest. If you quadrupled that exhaust velocity yourship that weighed a thousand tons at launchwould now not be 90% fuel but less than halffuel and more than half for the ship and itscargo. That is important for spaceships because youalways want your highest specific impulse,or effective exhaust velocity, but most alternativeswe have now that can go higher have to provideit at so slow a pace your ship could nevertake off, which does not mean you could notuse it once you got up into space to go further. But our ideal space ship drive would havea very high exhaust velocity and also be ableto deliver all that thrust quite quickly. We tend to use rocket science as synonymouswith very hard things, like brain surgery,“This isn’t rocket science or brain surgery”or as one of my sergeants used to say “Thisisn’t rocket surgery” but of course todaywe are doing rocket science, and I want toemphasize it isn’t as bad as all that. The basic equation is simple and the reallyhard part is calculating all the transferorbits and performance changes of specificfuel and rockets, we will not be looking atthose today and indeed there are lots of niceonline calculators you can use to do the gruntwork for you. So that was about the only math I plan tobring up and you do not need to have followedit to follow the basic principle. Some of the systems we will be looking attoday provide way higher exhaust velocitiesbut take longer to do it, others provide highervelocities and just as fast as a chemicalrocket. Now I have been asked a lot to cover the EMdrive or EmDrive many time recently so I willget to that at the end so I can give it alittle more time but I want warn people inadvance that I am not going to be pronouncingit a working or non-working system, I am justgoing to be filling in some explanation ofthe terms and concepts involved so folks knowwhat all this Q-cavity this and reactionlessdrive that stuff is talking about. This is a broad survey of mechanisms for spacepropulsion, some of which we have alreadycovered in more detail in other episodes,not a focused in look at all of them let alonea declaration of which is best or impossible. It is also worth mentioning that a spaceshipdoes not have to have a single drive, it mightemploy multiple types, in particular it mightuse different methods to take off or land. We have discussed launch assist options likeSpace Elevators, Orbital Rings, Sky Hooks,Mass Driver, Launch Loops, and Space Fountainsway back at the beginning of the megastructuresseries. We have not discussed landing and brakingmechanisms much so I will give a quick overviewof those in a minute. If you want to know more about those launchassist mechanisms you can go back and checkout those episodes but in summary form, rocketsuse a lot of fuel, and they use almost allof it getting out of our atmosphere. Getting a team of astronauts to Mars and backhome to us uses a lot of fuel and most ofthat gets used up in the first few minutesgetting all that into orbit. Fuel is also expensive, so anything that letsus get things into orbit with less fuel andenergy, or let’s use do the same for shipsleaving anywhere, is obviously quite handy. Of course once you get to your destinationyou have to slow down too. A problem not helped by both how fast youare going while making the journey and thatas you approach the target, if it is massive,you will start picking up speed from its gravity. Good timing can arrange to help reduce yourrelative speed but you will always arrivewith an awful lot of it. For Earth that is not too big a deal. Earth has a lot of gravity but it also hasa lot of atmosphere, so we can brake our speedoff the air, unsurprisingly called aerobraking. What is a big problem for leaving Earth, gettingthrough all that air, is quite handy for gettingback down to Earth. If you have a lot of speed, or the place hasa thin atmosphere, you can make multiple wideelliptical orbits, passing through the upperatmosphere repeatedly, until you slow enough. Even on places without much air this sortof trick can work, as can more conventionalconcept like parachutes or hybrids of themlike Ballute, a mix of a balloon and parachute. Of course you can also use balloons like airbags,as we did when Pathfinder landed on Mars. Trying to slow down after interstellar voyagesat relativistic speeds can also be done usingequivalents of aerobraking, in theory youcan loop around a solar system using the dustin space and solar wind to slow down. Space is not a vacuum, even in the intergalacticvoid, but solar systems are much less a vacuumthe interstellar space which is in turn lessthan intergalactic space. It’s not much but it would help you slowdown somewhat without using as much fuel. And of course you need to slow down beforereaching your destination or you will smashinto it. Though lithobraking, ramming into somethingat high speed to stop, is also a method oflanding if your ship is sturdy enough. Generally to save fuel and mass you can alwayshit with some speed and anything that savesfuel and mass is worth doing, assuming youdo not overdo it and turn yourself into apancake. Saving on board fuel takes us onto our nextsubject which is reactionless and quasi-reactionlessdrive. Now you will often hear folks say a reactionlessdrive is impossible but this is an oversimplificationand an incorrect one unless you are carefulwith your definition. As mentioned, rockets work by spitting matterout in one direction and shoving the shipin the other, conserving momentum in the process. Many of the other systems we will look atwork the same way, and many others are doingit by hiding the matter that is going in theopposite direction. For instance when I brake using the air duringre-entry, momentum is all being conservedbut we do not notice it since all that airgoes flying away and eventually settles inas heat, with the entire planet being shovedjust a little bit. If you jump up in the air the planet actuallydoes go the other direction, it is just thatthe planet out masses you by around a 100billion trillion, so even though you addedjust as much momentum to it when you jumpedup as it did to you, you move up a couplefeet and it in turn moves distances that wouldmake an atom look large. You also do not have to lose your reactionmass in all cases, same as you don’t whenjumping in the air. If I am standing on ice and bounce a basketballoff a wall and catch it, I will be shovedthe other way by tossing it then shoved againwhen it bounces back and I catch it. You can use tricks like this as a launch assistmechanism when you are near something to pushthings off of, and that would include laserbeams. Light has momentum and kinetic energy, indeedthat is essentially all it has. So bouncing stuff back and forth, be it basketballsor photons, is an example of a system essentiallywithout propellants, or at least a propellantyou can reuse, but you can only get so manybounces before it is not practical anymoreand it still costs energy. But we also have the notion of propellantsthat just are not internal. I mentioned earlier that one of the problemswith rockets is that you need more and morefuel to get to higher speeds because you haveto accelerate not just the ship but the fuelyou will be using to accelerate more withlater and the fuel you will use to slow down. That is core problem of any system with aninternal propellant. If you are pushing something with a lasergenerated elsewhere for instance, you do nothave to pay all that extra energy to pushon board fuel up to speed too. I will discuss this a bit more when we getto photons rockets. Hybrids of this, where you are carrying apropellant but the energy is coming from outside,like a ship that has solar panels it usesto heat up propellant to plasma temperatures,lets you get a lot more thrust from the samemass. This is the basic concept to a lot of thenew propulsion concepts we will look at andthe big problem is that external sources likethis tend to mean you can get to a higherspeed, which is great, but that it takes along time, which means it is no good for takeoffor landing. Ion drives, electrodynamic tethers, or electricpropulsion just does not give you the thrustyou need to climb out of a steep gravity welllike a planet particularly when you need toclaw your way through several kilometers ofair. This is the exact opposite of various nuclearpowered drives which can often give you allthe thrust you need you just do not want touse them in your own atmosphere. We spent a lot of time in the InterstellarColonization video talking about Project Orionand Daedelus, which basically operate by propellinga ship by blowing up nukes behind it. That’s great in deep space but not for takingoff from Earth. That is not the only nuclear option though. First you can just use it to power the electricalpropulsion drives we will get to momentarily,but you can also use what is called a NuclearThermal Rocket, backwards of a ThermonuclearBomb. These come in a lot of variations and aredevices we have actually built and tested,and essentially you have a fission reactorthat you are cooling with some substance,usually liquid hydrogen, then venting thehot hydrogen out the back. Thermal Rockets are themselves, nuclear ornot, are a fairly classic type of propulsion. You externally heat some gas rather than combustingit, or in the case of a cold gas thrusterare just letting something warm up and shiftfrom being a solid or liquid into a gas, nodifferent from putting dry ice in a bottle. In a nuclear thermal rocket you are just lettingfission do the heating. They are decently more efficient than chemicalrockets but usually seen as not enough tojustify the additional hazards and risks. That is debatable but Nuclear has bad PublicRelations issues and a Nuclear thermal rocketsare not the safest device in the world. Same issue for Nuclear Electric propulsion,and that is just where nuclear is the powersource. Electric propulsion is neither new as a conceptor a technology. The option was kicked around by Robert Goddardover a century ago. They tend to offer a nice slow thrust thatis good for stationkeeping and a lot of Russiansatellites use this, and it is the basis forconcepts like the ion drive. One alternative before we get to ion drivesthough is electrodynamic tethering. Way back in the Skyhooks episode I mentionedthis in comments, annotations, and FAQ asa way to re-generate momentum and altitudeon skyhooks but I skipped including it inthe episode because I was trying for shorterepisodes then and it still holds the recordfor shortest video on the channel. It is pretty novel concept though. Objects with powerful magnetic fields, likeour planet, can be pushed off of, and whilethis is not a good approach for take off vehiclesit works just fine for things in orbit wherethey just need a little thrust here and therefor station keeping. Lots of thrust is good but often we just wantsmall amounts that use little or no propellant. Which brings us to ion thrusters. These actually come in a lot of types butthe two most well-known are HET, the HallEffect Thruster, and VASIMR, the VariableSpecific Impulse Magnetoplasma Rocket, andwe will focus on those two. But in a nutshell you are taking power, whetherit is obtained from solar or nuclear powersources, and using that to speed up ions andshoot them out as your propellant. These are almost always very slow systemsin terms of their acceleration so you woulduse them on things already in orbit, not toget up in orbit. They get up to a higher speed than a chemicalrocket but take way longer to do it. Let us start with the Hall Effect Thruster. The Hall Effect was discovered by Edwin Hallof John Hopkins University way back in 1879before we even knew what an electron was,so it is old tech and designs for using itfor spacecraft propulsion go back to the earlydays of the Space race, but the designs wereso inefficient it mostly got shelved. Now in a nutshell a Hall Effect Thruster usesa magnetic field to accelerate plasma up tomuch higher exhaust velocities than chemicalrockets produces. It just does this as a thin trickle for daysor months instead of seconds or minutes likea rocket thruster. No good for launch but great for interplanetarywork. The preferred fuel is xenon, which is fairlyabundant on Mars, and which we usually geton Earth, or Mars, by distilling it out ofthe atmosphere. Xenon is a good fuel because it has a highatomic weight, about 130 times hydrogen’s,and a low ionizing potential, meaning it iseasy to strip an electron off so it will havea charge and respond to the electric field. There’s other things you can use but Xenonhas numerous advantages. The ionization is achieved by slamming electronsoff the Xenon so it knocks free an electron. VASIMR, the Variable Specific Impulse MagnetoplasmaRocket, uses radio waves to ionize its propellantand to heat it too. Proposed by Franklin Chang Díaz is 1977,this system does not use an anode which isgood because anodes tend to corrode quicklyduring use, a problem with many similar typesof ion thrusters. It heats the propellant by radio waves, usuallyXenon again though most testing is done withArgon because it is cheaper. In a nutshell VASIMR works off concepts developedfor nuclear fusion and it heats its plasmato around a million kelvin, and it can launchthose particles at speeds of up to 50 km/s,faster than other ion thrust systems. Unfortunately it also builds up a lot of heat. You can also vary the specific impulse itgives off which is handy. VASIMR is a very attractive system but isnot without its problems, and Robert Zubrinof the Mars Society has been strongly criticalof the system on several points, and one ispower consumption. With both systems, or any of the other variantsessentially running on electricity, the questionis always where you get your power supply. Which mostly comes down to how do you squeezethe most energy out of the least mass. This usually excludes chemical fuels becausewe could just set them on fire like a classicrocket, and also batteries since we have yetto develop one that can push out as much energyper unit mass as chemical fuels. This leaves us solar power or nuclear power. Ideally a nice compact fusion reactor wouldbe great, and we have talked about fusiona lot in previous episodes, but we do nothave that yet. So the question is, what is better, solaror nuclear?The answer to this is not as obvious as itmight sound like. In space the sun is always shining, thereis no night or clouds. Nuclear reactors produce huge amounts of powerfrom tiny amounts of fuel, around a milliontimes what chemical fuels release, but theytend to be massive when you include all theequipment and shielding. For a ground based reactor we do not careabout mass, so they are quite massive, butyou could possibly get reactors potentiallygenerating around a kilowatt per kilogramof reactor, maybe more. In practice, as Zubrin pointed out, the largestreactor ever put in space only produced about10 watts per kilogram. Solar power on the other hand, in our generalregion of the sun, can do a few hundred wattsper kilogram with the newest systems. Of course they are also more exposed to damagein space from micro-meteors since they arerather large and fragile, while a fissionreactor is mostly shielding already. Both of these technologies have tons of roomsfor improvement and it will be interestingto see who wins the race on power to massratio. If and when fusion gets made practical itwould probably replace both, but we couldeasily see both win, after all solar getsrather ineffective the deeper you go out inspace away from the sun, so you might seefission powered ion drives out there and solarnearer in. Ion Drives are very promising technology thoughespecially as we improve our power to massratio of both solar and nuclear systems. Now before we get to the EM Drive, I wantedto cover some other hypothetical high-techdrive systems. Quite a few of these we have talked aboutbefore in greater depth, for instance we dida whole video on the Alcubierre Warp Drivein the FTL, Faster Than Light, series. Similarly we did a video on using artificialblack holes to power starships. So I will just refer you to those if you areinterested. In the episode on black hole powered starshipsI mentioned that they give similar performanceto antimatter but are less prone to explode. Antimatter is often regarded as the ultimaterocket fuel, though of course it is actuallya bipropellant fuel, matter and antimatter. Antimatter, when combined with normal matter,turns both of them into raw energy, and youcould source your normal matter from the containmentsystem for the antimatter or by sucking inrandom space gas. As fuels go, when you have to carry your fuelwith you, nothing beats antimatter. The issues with antimatter are two-fold, productionand containment. It currently takes huge amounts of energyto produce antimatter, orders of magnitudemore than it releases, and storing it is atricky proposition usually assumed to involvekeeping it in a magnetic bottle. Obviously you cannot keep it in a normal bottleor it would explode when it touched the matterin the bottle. If you had a way to produce antimatter forsimilar amounts of energy to what it released,and if you could safely store it, then thisis the ultimate rocket fuel and allows speedsgenuinely close to the speed of light. Other than production and containment it isthe same as any other rocket fuel too, thoughit is arguably a photon rocket since it isreleasing its energy as photons. A photon rocket, sometimes jokingly calleda flashlight drive, is where you are justemitting photons to push you away. If you drifted away from your spaceship youcould use your flashlight to push you backtoward it, same as you could vent some airfrom your tanks to push you back too. The problem is it take a lot of energy todo this and a battery does not have much. Batteries do not store as many joules of energyin them as an equal weight of gasoline forinstance or other rocket fuels. The other problem is the produce virtuallyno thrust at all. A flashlight, say a 30 watt flashlight, andone that also radiates all that light in thesame direction, produces a thrust of its powerdivided by the speed of light. Conveniently that would be 30 Watts dividedby 300 Million meters per second, or one tenten millionth of newton. 10^-7 newtons or 100 nano newtons or . 1 micronewtons. Now if you and your spacesuit have a combinedmass of 100 kilograms, f = ma, force = masstimes acceleration, or acceleration equalsforce divided by mass. 10^-7 divided by 100equals 10^-9 m/s² or a nano-meter per secondsecond. Not very fast. If I had been driftingaway at a meter per second it would take mea billion seconds, or a few decades to bringmyself to a relative stop, all the while I’vebeen drifting further away. Obviously my batteryand oxygen ran out way before that. But there are two tricks of note for this. First, I don’t need any fuel on board,I can just have solar panels drinking in sunlight. Let say I had nice thin efficient solar panelof a couple square meters giving me 3000 wattsof power out of my bigger flashlight, andthe whole thing only weighed a kilogram. Onehundred times the power as our last examplepushing one hundredth of the mass, that givesme 10,000 times the acceleration. Now thatis not as bad. After a few decades you arenot going one meter per second but 10 kilometersper second. Some of you might be saying now, “Hey, whyeven bother with solar panels and a flashlight,why not just use a mirror? You could makethat even thinner and not even need the flashlight?”and that is true, except it’s even bettersince when light hits and reflects off somethingyou get double the effect. So a very thinmirror being hit by sunlight has a much higheracceleration than the flashlight since youcan use all that mass as just simple thinmirrors, getting more light, and getting doublethe push from it. You can also bounce thatlight at an angle to produce thrust in differentdirections. But that’s not the end of it, because wecould bounce more than once. Sort of likeour earlier example where we bounced a basketballoff the wall then caught it and threw it again,just with light instead. If I have a nicereflective surface, and a laser with a reflectivesurface around it to, I can shine that laseron the first mirror which bounces it backto the mirror around the laser which bouncesit back to the first mirror and so on. A spaceship and some space station bouncinga laser back and forth between them couldget that ship up to a pretty good speed ifyou can keep that beam contained, which ofcourse gets harder and harder the more timesit bounces and the further apart they getfrom scattering and diffusion, plus each bouncetakes longer. But such a thing as a launch assist systemhas some possibilities, and we explored laserand light propulsion in the Interstellar Colonizationand Nicoll-Dyson Beam episodes in more detail. I discussed there the notion of a laser highwaybetween solar systems where you have manystations along the way using fusion to powerthemselves and bouncing laser off ships asthey passed by, possibly back and forth afew times if you can aim and focus the reflectedlaser well enough. Now if we have the ability to alter eitherwhat the physical constants are, like gravity,or make them not work symmetrically, say gravitythat emitted like a cone not a sphere, otheroptions become available. Imagine for themoment that a large and massive object didnot emit gravity in all direction but justin one direction. Something like a gravityflashlight or laser. We have no idea how todo such things yet and maybe never will, butit may be possible in the future. Certain hypothetical materials, like negativematter, a type of exotic matter we have discussedbefore in the Faster Than Light Series, couldlet us achieve such effects or equivalentones. These sort of ideas are used for conceptslike the Diametric Drive or Pitch or BiasDrives. These are totally hypothetical craftrelying on science we do not have yet butI feel they deserve a mention. Lastly we have the idea of picking up fuelin route, gathering up either space gas ordark matter to use as fuel. This is the basicidea of the Bussard Ramjet, which we discussedin the Interstellar Colonization episode,and for how to refuel a black hole poweringa spaceship, which we discussed in Black HoleSpaceships. Using Dark Matter as a potentialfuel is something we looked at in the DarkMatter episode and has to be classified astotally hypothetical since we still do notknow the properties of Dark Matter or anyway to manipulate it, but we discussed itmore in that episode if you are curious. Okay, on to the EM Drive at last. I savedthis one till the end so we could spend abit more time on it since it is a big newsitem with an awful lot of confusing and oftencontradictory reporting. It hit the news againrecently when someone said that NASA wouldbe releasing a peer-reviewed paper on it. At the time of this episode that paper hasnot been confirmed to be released, let alonereleased, but the assumption it was beingreleased is what rekindled interest. Now the EM drive, or just EmDrive, is oftencalled a radio frequency, or RF, resonantcavity thruster. Its sibling device, the CannaeDrive or Q-Drive works off the same concept. This is a type of electromagnetic thrusterin which electromagnetic radiation, photons,are confined to a microwave cavity, and providesthrust to the cavity in a particular directionas the radiation reflects within the cavity. Now what is a Microwave cavity? Well basicallya box with mirrors inside that reflect thatfrequency of radiation, in this case microwaves. You probably have one in your kitchen, a microwaveoven is a simple microwave cavity. There are two special notes about this though. First, a terminology one. Whenever we discusscavities you will hear people refer to itsQ-Factor, and irritatingly most do not botherexplaining what this mundane thing is so manyfolks assume it is some weird physics thingy. A Q-Factor is just short for Quality Factor,and it basically measures how good the cavityis at keeping the waves bouncing around insideit rather than dissipating as heat. It isn’t quite as simple as just sayingthat a perfectly reflective mirror would haveinfinite Q, but for basic conceptual purposesit amounts to how good a reflector the cavityis. That’s important since as I mentionedback when we were discussing reflecting lasersrepeatedly, every time you can get it to bounceyou get more thrust imparted. So higher Qis good, it is also very hard to get froma practical standpoint when you are tryingto dump tons of power into a cavity. Somethingwith really high Q would make an excellentbattery for instance. You could just keepdumping more and more microwaves in and losevery little of it to heat dissipation. Now the other difference is that these kindof cavities are usually cylinders, for theEmDrive though it is a tapered cylinder, wideron one side than the other, and the basicnotion is that the wider end will get moreforce exerted on it, generating a net thrust. Now in principle this device produces thrust,but it does not seem to have any sort of propellant,which would make it violate conservation ofmomentum. Remember even our laser or solarsail drives use photons as their propellant. The EM drive is not supposed to be emittingany, just bouncing them around inside. Sothis drive got dismissed as total nonsenseby most until one got built by NASA and seemedto produce some thrust. Now it wasn’t much thrust, so little itmight even be noise, and actually less thanyou would get by shining a flashlight usingthe same power. Indeed there is a concernthat it is just asymmetrically emitting radiation,again like a flashlight, since it is hardlya nice symmetric sphere. At this point many would say “Okay, so evenif it does produce thrust, it is less thana flashlight or laser, so what good is it?”. Fair point, BUT, it would still be interestinguntil we knew how it was producing any thrustand the notion is that this device does notnecessarily scale its thrust linear to itspower. By which I mean, if we double the powerto it we would expect to get more than doublethe thrust. So you would next say, “Why have we notjust done that then, this experiment was onlyusing around a kilowatt of power, why notthrow in 10 kilowatts, or a 100, and see whathappens?”The simple answer is that high Q-factor cavitiesthan can also handle tons of power withoutmelting are very, very hard to make. Whichobviously also means very expensive. Now a few more notes. First, the EmDrive isnot the quite the same as the Cannae Drive,but the two get used synonymously a lot soyou should probably treat them that way whenreading about either one, because folks usethem interchangeably. Second, it is in noway a warp drive. That was just bad reporting. Eagleworks is NASA’s department for lookingat novel and sometimes fringe propulsion systems,headed up by Sonny White, so they are alwayslooking at various strange systems and SonnyWhite also examined the Alcubierre Warp Drivearound then and lots of bad reporting jumbledthem together. If you see an article talkingabout EmDrive and warp drive, you can prettymuch just skip it. Either the author is usinga clickbait title or they didn’t do theirresearch, so either way it probably is notworth your time. I get asked for my opinion on the EmDrivea lot and personally I do not expect it toactually work. Or if it does then the thrustwill turn out to be something quirky whereit is being provided by photons leaving thedrive but interfering with each other so wehave problems detecting them or bouncing offQuantum Vacuum Plasma, or a Q-Thruster, wherein this case the Q is for Quantum not Quality. Sonny White tossed that notion out there asan option but the idea of Virtual Plasma,as a type of Virtual particles which we havediscussed before, is not too popular withtheoretical physicist at the moment. Fundamentally we are at the wait and see stage,nobody has gone and proved the EmDrive works,that is just sensationalist journalism, butnobody has successfully killed it yet either. I would not suggest holding your breath itwill work but I would say it is still on thetable for now. That is true of a lot of the systems we havediscussed today, and hopefully at least oneof those will turn out to be both possibleand practical so we can replace chemical rockets. I think we are justified in being optimisticand putting some faith in human ingenuity,but we also always want to keep our exuberancedampened and scientific. As I’ve said before, there is a big differencebetween proper skepticism and naysaying, aswell as just trusting to science as a magicwand. It can often seem like propulsion technologyis proceeding at a snail’s pace and sensationalistjournalism constantly reporting every newtheory like it was proven fact can make aperson pretty jaded, but we are making goodand constant progress. For my part I am very optimistic about thatprogress, if properly skeptical too, and whilewe could only cover the basics here you shouldhave a good place to continue your own researchinto these. I’ll be placing several linksin the episode’s description to good placeslike Project Rho to either follow up on theseor get more specific data, but this is wherewe end for today. Some quick announcements. First I wanted tothank Drew McTygue, he was the first winnerof our Patreon subject selection and chosetoday’s topic. We spent a lot of time onthe phone discussing what to cover and whatto bypass for time constraints and we workedon the script together which was a lot offun, and I’m looking forward to future collaborationswith folks on such topics. Next, we have our second winner for a topic. That topic is going to be Star Lifting, andthat winner was Bill Mains, so congratulationsto Bill and that was an excellent topic pickand one I have been considering covering fora while. Star lifting is term given to various hypotheticalprocesses for lifting matter of stars, asthe name implies. This has quite a few possibilitiesand uses, the two most obvious being that99% of the matter in our solar system is inour star, so it is create source for matter,and secondly, that you can use this processto extend the lifetime of a star. We willlook at that and some other awesome thingsyou can do with star lifting when that episodecomes out on October 6th, 2016. That will be after we take a look at DarkEnergy next week, and that will be followedby a look at Cryptocurrency and BlockChain,then get followed by a look at the KardashevScale on the 20th, that topic was selectedby a poll over on our new Facebook group,Science & Futurism with Isaac Arthur whichDrew is now a moderator for too. I want tothank everyone who came over and joined thatlast week and everyone who volunteered tomoderate and administrate it, and we willbe selecting a lot of our topics by pollsconducted over there and topics selected therefrom now on, as it is a better place to discussthe ideas. This week though, while I’d encourage youto join the Facebook group, we are also launchingthe identically titled Sub-Reddit Science& Futurism with Isaac Arthur so I will beover there after this episode comes out answeringquestions and I hope you will join us on both. We will have one more Patreon Topic selectionin a few more weeks so it is not too lateto join and submit topic suggestions overthere, and indeed I am considering doing itmore in the future. You can opt to submitanonymously if you do not want to me sayingwho you were during the videos but I preferto be able to say who’s idea it was andto be able to sit down on the phone and discussthe idea with the person who submitted it,that won’t affect whether or not the topicgets selected but do let me know if you donot want your name used, or want a pen nameused, or are not comfortably chatting aboutthe idea on the phone. That third episodewill probably come out in Early November. Again, next week we will look at Dark Energy,and clear up some of the confusion about thismysterious force that seems to be shovingthe Universe apart. Until then you can tryout some of the other episodes on the channelor at the website, IsaacArthur. net, or followme on facebook or twitter or reddit, and ifyou enjoyed this episode, make sure to likeit and share it with others. Thanks for joining me today and until nexttime, have a great day!