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  • This episode of Real Engineering is brought to you by Brilliant, a problem solving website

  • that teaches you think like an engineer.

  • If you have been on the internet in the past month, you have probably seen a picture of

  • Elon Musk’s latest project.

  • A rocket that looks like the brainchild of a H. G Well’s fever dream of the future.

  • It doesn’t look like any current generation rocket by any shape or measure.

  • It’s shorter and fatter than your typical Space X rocket, and most strange of all, it’s

  • made of stainless steel.

  • A material that has largely fallen out of use for propellant tanks since the 60s.

  • Steel is strong, but it’s pretty heavy.

  • Making it unsuitable for flight structures.

  • Reducing the weight of the launch vehicle is an art form in rocket science.

  • Every kilogram matters, and engineers have come up with some innovative ways to reduce

  • weight.

  • WD-40, was originally developed to displace water, which is where its name comes from,

  • to protect the metal tanks of the Atlas rockets from rusting, because they weren’t painted

  • to save weight.

  • [4]

  • And those Atlas rockets were made of stainless steel.

  • In those days aluminium alloying material science hadn’t quite developed far enough,

  • and the engineers of the Atlas rockets instead opted to use extremely thin stainless steel

  • for their propellant tanks, varying from 2.5 millimetres to about 10 millimetres.

  • These were essentially metal balloons.

  • As they were structurally unstable when unpressurised.

  • In one infamous case on May 11th 1963, an Atlas Agena D lost pressurisation on the launch

  • pad, allowing the weight of the upper stage to buckle the thin steel.

  • Pressurisation adds strength to pressure vessels as the pressure provides a restoring force

  • for small deformations, so if the metal attempts to bend inwards the internal pressure pushes

  • it back out.

  • This strengthens all rocket tanks allowing their thickness to be minimised, but this

  • application took it to the extreme to make up for steels density.

  • Our choice of material for aviation and aerospace applications has evolved with our mastery

  • of material science.

  • Specifically with the materials available to us that have the highest strength to weight

  • ratios.

  • We can visualise these strength to weight ratios on graph like this.

  • Plotting the strength of the material against its density.[6]

  • Looking at this it’s pretty clear that steel adds a significant amount of weight, while

  • not adding a proportional amount of strength.

  • Steel is typically 2.5 times heavier than aluminium, but it is not 2.5 times stronger.

  • So why use stainless steel?

  • Well, strength to weight ratios are not the only factor engineers have to consider.

  • Something you may not consider are things like thermal conductivity.

  • Aluminium has a much higher thermal conductivity than steel, and thus can conduct heat from

  • its surroundings into the cryogenic fuel much faster.

  • This can vaporise the fuel, which requires boil-off valves to vent the vaporised fuel.

  • To minimise this problem, rocket fuel tanks are often sprayed with foam insulation, that’s

  • what gave the external tank of the space shuttle it’s distinctive orange colour, but this

  • adds a substantial amount of mass itself, which in turn decreases the weight saving

  • benefits aluminium provides.

  • [2]

  • However, the Falcon-9 fuel tanks are not insulated.

  • To prevent major boil off of the fuel, the fuel is loaded as late as possible.

  • This reduces the amount of fuel that will be vaporised, but also makes the job of getting

  • the Falcon 9 certified for human payloads a bit of a nightmare.

  • NASA did not want Space X to fuel the rocket with passengers on board, because as we saw

  • earlier things can go wrong during this phase.

  • In August 2018, they finally approved the Falcon 9 for thisload and gostyle

  • of fueling for human flight.

  • [3]

  • The aluminium-lithium alloys used in the Falcon-9 were not developed until the late 50s and

  • early 60s, which increased their strength to weight ratios, allowing the introduction

  • to aerospace applications.

  • [4]

  • The stainless steel balloon tanks of the Atlas rockets were eventually made with this aluminium

  • alloy metal, and their strength to weight ratio were boosted by using a unique stringer

  • pattern called an isogrid, which boosted the aluminiums ability to resist buckling, like

  • that of the Atlas Agena D.

  • NASA performed these huge compressive buckling tests on the aluminium lithium tanks of the

  • SLS rocket.

  • Typically you use little strain gauges, whos electrical resistance change as you stretch

  • them forcing the electrons along a longer path to keep track of the strain in the material,

  • but for something this big they would have needed thousands.

  • Instead they painted dots all over the structure to allow computer imaging software to keep

  • track of the strain.

  • That isogrid structure is excellent for maximising strength while minimising the material needed.

  • It is essentially an inter woven pattern of I beams that increase the stiffness of the

  • overall structure.

  • You will see this pattern everywhere in aerospace.

  • From these sixties era rockets to Space X's new dragon 2 capsule.

  • Space X, to date, has used aluminium-lithium alloys in their propellant tanks.

  • But they opted not to use this isogrid structure, even though it provides fantastic strength

  • to weight performance, it is absurdly expensive to manufacture.

  • To manufacture isogrids you start off with a thicker piece of aluminium and machine it

  • down using a CNC machine.

  • This results in about 95% of the material going to waste.

  • Instead Spacex opted for a thin skin of aluminium-lithium alloy and then stir welded strengthening stringers

  • in place.

  • We are constantly balancing a huge number of factors.

  • Here the cost of manufacturing the rocket influenced it’s design.

  • Typically the cost of launching an extra kilogram of material to space far outweighs the cost

  • of material, but in cases like this the waste in the manufacturing process can influence

  • our material choice.

  • For example Musk attributed the cost of carbon fibre composites as one of the primary reasons

  • he abandoned it as a material for the Starhopper.

  • Carbon fibre composites cost about 135 dollars per kilogram, and a significant amount of

  • it is thrown away in the lay-up process.

  • The manufacturing process for carbon fibre composites is extraordinarily expensive and

  • difficult.

  • As explained in my carbon fibre video.

  • Carbon fibre composites gain all of their strength from the long and thin carbon fibres

  • inside the plastic resin that holds them together.

  • This means that their strength is not the same in all directions, and in order to ensure

  • the material can be strong in all directions you have to layer your carbon fibre composite

  • in a very specific way.

  • You then have to cure it in a pressurised oven.

  • This was one of the major flaws I pointed out in predicting the failure of the early

  • prototypes of the BFR carbon composite tanks, which were made in two parts presumably because

  • they couldn’t find tooling and an autoclave big enough to cure a full sized tank.

  • Being perfectly honest this is the only subject area where I have enough expertise to make

  • comments on other peoples designs, and I was surprised Space X were pursuing the material

  • at all, for the reasons stated above, and as it’s unsuitable for a vehicle that not

  • only has to withstand the freezing temperatures from the cryogenic fuel on assent, but the

  • scorching temperatures of re-entry.

  • Not once, but twice.

  • As this will be the first vehicle in history expected to visit Mars AND return.

  • Here we really start to see where stainless steel shines, and why Musk is opting for a

  • stainless steel vehicle.

  • Let’s plot another graph, this time plotting strength against maximum operating temperature.

  • Here we can see that stainless steel outperforms both aluminium alloys and carbon fibre composites

  • by a significant margin.

  • [6]

  • The Falcon 9 first stage rocket serves only to boost the second stage to about 65 to 75

  • km in altitude and between 6,000 to 8,300 km/h, before flipping over and performing

  • re-entry burns to slow down before entering the thicker atmosphere at relatively slow

  • speeds.

  • Even then, the engine nozzles, which are designed to tolerate massive temperatures take the

  • brunt of the re-entry heating, allowing the aluminium tanks to avoid any major reentry

  • heat.

  • This is not how the Starhopper is intended to work, because it is being built as an interplanetary

  • vehicle.

  • The starhopper can expect to enter into the Martian atmosphere at speeds of up to 21,000

  • km/h and experience temperatures up to 1,700 degrees.

  • Well above the maximum service temperature of both aluminium and stainless steel, but

  • we have ways of leaching some of that heat away before it can heat the metal.

  • The curiosity rover utilised a phenolic impregnated carbon ablator, which is extremely extremely

  • light, has a low thermal conductivity, and can resist extreme temperatures of up to 1,930

  • degrees.

  • [5]

  • But nothing this heavy has ever entered the Martian atmosphere before, and it’s not

  • going to be any easy task for it to slow it down.

  • It’s going to have to enter the martian atmosphere at an extremely high angle of attack

  • to allow the thin martian atmosphere to sap away speed through drag for an extended period,

  • but drag comes with heat.

  • Stainless steel may be heavy, but it will require significantly less heat shielding

  • that an aluminium or carbon fibre composites.

  • Once again closing that weight advantage gap of these alternate materials . In fact Musk

  • has stated that the rear side of the Starhopper will require no heat shielding at all, and

  • he plans to use a strange technique to cool the wind facing side of the vehicle.

  • Using the same method humans use to cool down, by sweating.

  • Musk plans to pump liquid methane between two steel panels on the windward facing side

  • of the Space X rocket, where it will gain heat, vaporise and evaporate through small

  • holes in the rockets surface.

  • This is pretty weird way of cooling a ship, and I wondered why you would not just opt

  • to use the tried and true method of ablative tiles.

  • Then I remembered that this ship needs to make a return journey, and the entry into

  • the Martian atmosphere will damage the tiles and require maintenance.

  • There is no oil on mars to manufacture new phenolic resin or the carbon needed for ablatives.

  • So, using methane, the fuel the new Raptor engines that Space X will use for the Starhopper,

  • makes a lot sense.

  • It reduces the equipment the rocket will need to carry to Mars, making the rocket significantly

  • lighter.

  • They can just use the equipment they already needed for refueling, making it double purpose.

  • They just need to mine water and extract carbon dioxide from the atmosphere, and then do some

  • fancy chemistry to produce methane and oxygen.

  • The prototype they are building at the moment is likely just to test the manufacturing techniques

  • needed to build it, and test it’s flight capabilities.

  • This ship does not need to be space worthy, it just needs to have the same weight, centre

  • of gravity and shape to allow space x to test it.

  • On the surface though the whole operation looks like a bit of a shitshow, and I really

  • try to be positive about engineering advancements, but the thing literally fell over in the wind

  • last week.

  • I’m really curious on how this whole thing is going to unfold.

  • Sometimes you just need to make mistakes to learn, which is why you should sign up to

  • Brilliant.

  • Brilliant recently introduced a new feature, calledDaily Problems”, which will present

  • with you with interesting scientific problems to test your brain.

  • Like this one that will teach you how solar sails allow spacecraft to gain speed without

  • rocket fuel.

  • If you answer a question wrong like I did here, it teaches you exactly why.

  • Allowing you to learn from your mistakes.

  • Brilliant even have an app that you can download to play these brain teasers on your morning

  • commute.

  • Each Daily Problem provides you with the context and framework that you need to tackle it,

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  • If you like the problem and want to learn more, there’s a course quiz that explores

  • the same concept in greater detail.

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  • As always thanks for watching and thank you to all my patreon supporters.

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This episode of Real Engineering is brought to you by Brilliant, a problem solving website

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Why SpaceX Built A Stainless Steel Starship

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    joey joey に公開 2021 年 06 月 09 日
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