Why SpaceX is Using a New Fuel

"WEBVTTKind: captionsLanguage: enWe have been given the scientific knowledgethe technical ability and the materials topursue the exploration of the universe, toignore this great resources would be a corruptionof a god given ability.It’s estimated that about 650 million peoplewatched the first moon landing in 1969 - nearlya quarter of the world’s population.As the world watched those powerful SaturnV rocket engines burst into life, they witnessedplumes of sooty exhaust billow from its 5shuddering nozzles as it pushed itself offthe ground, burning through a colossal volumeof kerosene to lift the rocket’s massive3 million kilogram weight off the ground.Soon the first stage shut off and separated,allowing the second stage to roar into life.This time powered by a different fuel, liquidhydrogen.A fuel capable of providing better performance,but it occupied too much space.Liquid hydrogen is considerably less densethan kerosene, so needs to be combined ina much higher ratio with liquid oxygen.For every litre of liquid oxygen burned inthe Saturn V first stage, it required 0.64litres of kerosene, while its second stageneeded 3.25 litres of liquid hydrogen forevery litre of oxygen.NASA engineers couldn’t feasibly make theSaturn V first stage fuel tanks any larger,so liquid hydrogen was not an option.The science of rocket fuel is a fascinatingand complicated field.Combining not just physics, chemistry, andengineering, but also logistics.That’s the challenge facing Space X as itdevelops the next generation of heavy liftrockets, designed to take us not just to theMoon, but further, to Mars.Starship’s Raptor engines will not use keroseneor liquid hydrogen.It will use methane.A fuel that was considered frequently overthe past century of rocket fuel research,with a few honourable mentions in John D Clarks’seminal book, “Ignition!”, but it hasnever seen widespread use.So, why is SpaceX using it now?Putting people on the moon in the 1960s wasone of the greatest technological challengeswe’d ever faced, but getting humans to Marsis a considerably more difficult task, especiallywhen you factor in the enormous challengeof keeping humans there - of creating andsustaining a human settlement on the red planet.How do you reduce the cost of launches?How do you make the oxygen needed to stayalive, how do you provide water for growingfood and for drinking, and how do you makethe fuel to power a return trip to Earth?Kerosene and Hydrogen are not perfect.Kerosene is extracted from crude oil via fractionaldistillation and is made up of a mixture oflong-chain hydrocarbons, reaching up to around20 carbons in length.The longer the hydrocarbon, the harder itis for it to burn completely in oxygen, asthey require more oxygen per gram of fuelto be completely oxidised into carbon dioxideand water.And so, even in its refined form, keroseneoften burns incompletely, decomposing insteadinto smaller, reactive radicals.The result is coking - the production of sootycarbon particulates that we saw in the SaturnVs launch.This soot can easily clog up the intricatemechanism of a rocket engine, which is a problemfor SpaceX and its goal to make its enginesreusable with minimal maintenance.Especially on Mars, where the facilities tofix these issues will not be available.Liquid hydrogen, of course, doesn’t havethis problem, and it has the advantage ofburning more efficiently than kerosene.We can quantify this efficiency with specificimpulse.Specific impulse describes how efficientlya fuel can convert its mass into thrust.To understand this let’s first look at totalimpulse, which describes the thrust forcegenerated over the entire burn period of theengine.We can graph this rather easily, by plottingthe thrust the engine is providing in eachsecond of its flight, that may look somethinglike this.The total impulse is found by finding thearea under this graph, which gives us thetotal energy the rocket released.This is a useful metric in itself, but specificimpulse is better, because not all propellantsare born equal.Two different fuel and oxidiser combinationscould provide the same total impulse, butwe need to consider the weight of the fueland oxidisers themselves, after all, the initialweight of rockets is always dominated by theweight of their own fuel.To find the average specific impulse we dividethe total impulse by total propellant weightthe rocket expelled.Going by this metric, a liquid hydrogen andliquid oxygen fuel mixture is by far the best.(Table from )Hydrogen has a much higher specific impulsethan kerosene - around 390 seconds, againstkerosenes’s 285 seconds.However, as mentioned earlier, Hydrogen ismuch less dense than kerosene.Requiring much larger fuel tanks.Hydrogen also has an exceptionally low boilingpoint at minus 252.8 degrees celsius - andso the tanks need to be heavily insulatedto avoid expansion of the liquid hydrogen,but thermodynamic equilibrium is a war ofattrition that will the universe will alwayswin, and so it also requires boil off valvesto release gaseous hydrogen to prevent anexplosion.This all adds mass and complexity to the rocket.Hydrogen also degrades and weakens metalsin a process known as hydrogen embrittlement.This is a massive issue for SpaceX’s reusabilitydesign ethos.Combining two parameters, the density andthe specific heat of combustion, we can getan idea of the difference between these 3fuels.If we want to release 100 Megajoules of energyfrom each of these three fuels, we would need11.9 liters of hydrogen, 2.2 liters of kerosene,or 5.5 liters of methane.Methane is much closer to kerosene than hydrogen.Allowing fuel tanks to be smaller than liquidhydrogen fuel tanks, but not small enoughto offer much performance benefits over kerosene.When the necessary design changes are madeto switch from kerosene to liquid methane,like increasing the fuel tank volume, theincrease in specific impulse is all but negated.This is why Methane hasn’t seen use yet.Methane simply sits in an awkward middle groundbetween the two most popular fuels.It provides better performance than kerosene,but not as good as hydrogen.And it’s easier to store than hydrogen,but not as easy as kerosene.Its benefits are only now becoming usefulas SpaceX works to unlock the magic of reusablerockets..Methane is a single-carbon hydrocarbon, sounlike the long-chain molecules found in kerosene,it produces significantly less soot when burnt,leading to less damage to the engine overtime.Its boiling point is actually higher thanliquid oxygens.Allowing much of the necessary infrastructureto liquify and use oxygen to be also usedfor liquid methane.Important when working with limited infrastructureon Mars.But, most importantly, the real reason methanehas suddenly become very attractive to SpaceXis that it can be synthesised from the carbondioxide rich atmosphere of Mars.Mars’ atmosphere is almost entirely carbondioxide.95% of the Martian atmosphere is CO2, withthe remaining 5% being made up from gaseslike nitrogen, argon and a trace amount ofoxygen.Whilst this carbon dioxide-rich atmospheremay be a disadvantage when it comes to establishinga city on Mars, it provides huge advantageswhen it comes to creating rocket fuel.We have come to see carbon dioxide as a wastegas - something produced as a by-product ofcombustion, instead of as a raw material,but it has the enormous potential to act asa feedstock for the production of methane.Over 100 years ago, a chemist called PaulSabatier came up with a process of convertingcarbon dioxide into methane by passing itthrough a catalyst, usually Nickel, with hydrogengas at an elevated temperature and pressure.The reaction takes one mole of carbon dioxideand reacts it with four moles of hydrogento produce one mole of methane and two ofwater.When combined with our electrolysis process,this produces one mole of methane to two molesof oxygen.The ratio of moles - the stoichiometry - isgoing to be important soon.But the first question is, how do you getyour chemical reagents on Mars?Well for this, we need In Situ Resource Utilisation.Getting carbon dioxide is a relatively easytask.With an atmosphere made up from 95% CO2, extractinga pure sample of the compound is straightforwardenough, but we do need to get rid of thatother 5% and condense the carbon dioxide.Currently, cryofreezing is the most viableoption, carbon dioxide has the highest freezingpoint of the gases present in Mars’ atmosphere.So, in a process that is essentially the oppositeof distillation, we can cool the air to separatethe carbon dioxide, which will freeze intoa solid while the other gases remain as gas.This also naturally compresses the gas.Then, when we need to use it in our sabatierreactor it can be simply warmed up to createa high pressure stream of gaseous CO2. .However, getting the necessary hydrogen ismuch more difficult.The first option is to import it directlyfrom Earth, but given how much is required,and the difficulty storing it for long periods,this isn’t a great option.So, long term, we will want to extract itfrom resources available on Mars.Water is contained within Martian soil, but,most significantly, it’s found in the formof ice in the polar regions of the planet.If we can find an efficient way of miningthe water, we can then convert it into oxygenand hydrogen using an electric current, andthen the hydrogen can be combined with carbondioxide to produce methane.Remember when we said the molar ratios wouldbe important.Here’s why.That 2:1 molar ratio of oxygen to methane,gives us a mass ratio of 4:1 of oxygen tomethane.The propellant mixture employed by SpaceX’sraptor engine uses a 3.4 : 1 ratio, so thiswhole process gives us an excess of oxygen,which can be put towards the life supportsystems in your Martian city .A win win.But extracting large quantities of water fromMars, either from ice in the polar regionsor from the small quantities of liquid waterin the soil, would not be easy, and the technologyto do this on a large scale doesn’t existyet.Over the short term, if we brought our ownhydrogen to Mars, we end up with a 1:1 moleratio of oxygen and methane, which isn’tenough to burn our methane completely.So, if we are going this route, we need away to produce additional oxygen.The best way is to use another process whichwould benefit from Mars’ carbon dioxiderich atmosphere - the Reverse Water Gas Shiftreaction (RWGS).This looks very similar to the Sabatier process,and involves the reaction of carbon dioxidewith hydrogen, but instead of producing methaneand water, it produces carbon monoxide andwater.The water that’s formed in this reactioncan then be electrolysed, and the hydrogenrecycled back into the reactorAgain, when the equations of the processesare combined, it gives us the 2 : 1 molarratio between oxygen and methane that’sneeded for the propellant mixture.This gives us options if mining water on Marsproves difficult.But the additional advantage of this approachis that these two reactions - Sabatier andReverse Water Gas Shift - can be done in thesame reactor, as demonstrated by Pioneer Astronauticsin 2005 .This combination of these two reactions hasan enormous thermal advantage.Because the Sabatier reaction is exothermic- it releases heat energy - and the ReverseWater Gas Shift reaction is endothermic - itabsorbs heat - it leads to an overall reductionof 37% in heat generation, improving the efficiencyof the whole process .But despite how long the Sabatier reactionhas been around, the research into optimisingthe process is only a couple of decades old.For most of its life, the Sabatier reactionhas been used to remove carbon dioxide fromhydrogen in the production of ammonia, asit damages the catalysts used in the processof making ammonia, but in the last twentyyears, scientists have begun to realise itspotential in helping mitigate the effectsof climate change.Instead of letting the carbon dioxide formedfrom burning fossil fuels into the atmosphere,what if we could collect it, and turn it backinto fuel using the Sabatier process ?This would require a method of not only efficientlycapturing carbon dioxide from the air, buta method of creating hydrogen.These processes are currently expensive, butwork is being done to reduce the cost in bothenergy and money.This could create a realistic economy forcarbon capture to help mitigate climate change,but we are a long way off from it being cheapenough.If the technology advances, it could offera method of storing renewable energy overextremely long periods of time as methane.There are already a handful of Power to Gasplants and projects in operation, like theAudi e-gas plant in Germany , but it hasnot yet reached wide-scale use.In December of last year, Elon Musk tweetedthat SpaceX is funding programmes into carboncapture and storage, adding that the researchwould also be important for SpaceX’s Marsmissions.It’s not yet clear exactly what they aredeveloping, but it will likely be very similarto the Power to Gas projects already in development,based on the century-old Sabatier reaction.People often criticise NASA and other spaceinitiatives as a waste of time, why work onscientific research to help us live in spaceor on other planets when we have so many issuesthat need research here on earth, but it’sclear for anyone paying attention.Working on difficult problems to make Marshabitable, will directly lead to helping solvethe greatest problem facing Earth today.This problem needs all hands on deck.We need as many talented engineers and scientistsworking on climate change as possible.That’s part of the mission of this channel,to inspire the next generation of engineers,which is why I work with Brilliant.Where I can inspire, Brilliant can educate.Brilliant has created the perfect learningplatform for science and mathematics.Focusing on interactivity that helps you trulyunderstand complicated subjects, and continuallytests your knowledge as you go along.If you are stuck on a particular question,Brilliant does not punish you and impede yourprogress, instead they give you an in depthexplanation to guide you on your learningpath.Encouraging you to learn from your mistakes,rather than building a barrier to furtherlearning.You can start your learning journey on anynumber of courses, from neutral networks andalgorithms to logic and probability.You can learn at your own pace, learn on thego with their mobile app, and most importantlyof all enjoy the process.You can try Brilliant for free with the linkin the comments and description or by clickingthe button on screen now, and the first 200people to sign up will get 20% off BrilliantPremium.If you are looking for something else to watchright now you could watch our last video onthe insane engineering of the javelin missile,or you could watch Real Science’s recentvideo on the insane biology of the Harpy Eagle.\n"