The Satellites Are Falling - EWTS #004

Joe: Hello and welcome to Enough with the Science, I'm Joe.

Senan: And I am Senan and this week we have an out of the world episode for you.

Joe: We're heading deep, deep well maybe not deep, maybe shallow, shallow space we're going to this week.

Senan: We're going to go deep up into space.

Joe: Aha but space is in the news this week? Yes do we have that rocket launch with all that famous singer Katy, who was her name?

Senan: Katy Perry? Yeah. Now people know when we recorded this.

Joe: That could have been this week or last week or last month. We're terrible with time.

Senan: Mind you which one of Katy Perry's flights are we talking about is the question.

Joe: In the future they will look back and they will never be able to date this episode. It'll be timeless.

Senan: Anyway Joe I have a bit of a confession to make.

Joe: Okay. No. Here we go. I like you but not in that way.

Senan: This episode is complete and utter self-indulgence for me because I'm a bit of a rocket nerd.

Joe: So it is actually rocket science?

Senan: Yeah. We are talking this week about rockets, what they are, why they work the way they do, why we need to use them, all that good stuff.

Joe: Do I get to probe? Do I get to probe and like not a space probe but like just to try and figure out what drives Senan Mahon like what is it about rockets why when did you become a rocket nut?

Senan: Oh it's a combination of factors isn't it. I mean first it's the sheer amount of physical power that's on display when a rocket engine is running that's just awesome, there's nothing else we do that concentrates so much power in one place.

Joe: Just love burning fuel. That's what you just love like burning tons and tons.

Senan: Ruining the atmosphere. Then of course there's that whole business of the excitement of maybe the human race expanding out into space because like, let's face it if we could take a little analogy if the Earth that we all live on is like one grain of sand, well then the rest of the universe is far more than all the other grains of sand on all the beaches everywhere on Earth. In fact if you were to take all the grains of sand on Earth and all the grains of sand on Mars you still wouldn't have a suitable analogy for the size of the rest of the universe. So it's a big space out there and it's kind of exciting to think that we have these machines that might one day let us get out there.

Joe: Imperialist expansion. You have notions of imperialist expansionism.

Senan: Try saying that after a few pints.

Joe: I know yeah. I can't even say it now.

Senan: So anyway I suppose we start off with the basics. What is a rocket? Essentially if you throw stuff out the back at high speed, thanks to...

Joe: You're going to have to be more specific now come on.

Senan: We're getting there we're getting there. We gotta start at the absolute basics.

Joe: Okay. There's a machine. There's a machine of some sort.

Senan: Yeah. No but if you throw stuff out the back of a thing at high speed, thanks to Isaac Newton and his third law, you get an equal and opposite reaction that pushes you forward. So that's essentially what a rocket is doing, it's throwing stuff in this case hot gas at very high speed out the back and lots of it in a very short period of time and that pushes you forward with the opposite reaction.

Joe: And is there anything in nature that somebody looked at and went ooh, that's how that works and then they decided to copy it?

Senan: Yeah it's funny you should say that, I mean there are underwater creatures like squid and octopus have an escape mechanism and actually scallops do it too, where they literally squirt a jet of water if they're under threat out of a some kind of squirty thing in their body and...

Joe: There is a word for it I'm sure there is a word for it.

Senan: And they can suddenly dart off in the opposite direction to get away from a predator. So yeah I mean there is there is some precedent for it in the natural world. Now the interesting thing about rockets of course is that we're using them to go up into space. And gravity pulls everything back down to the Earth and to get into space you need to throw an awful lot of stuff out the back awfully quickly to get that forward motion from Mr. Newton. And rockets are really good at that, that's kind of why we're using them.

Joe: And so what initially fueled human innovation in rockets, see fueled? Look at that I got in there look at that.

Senan: Well of course now that you've mentioned fuel which is going to form a large part of this discussion, the rocket fuels have got to be highly potent they've got to be able to produce a lot of thrust and you know we'll be talking in extensive nauseating detail later about rocket fuel and what it is and why we use it.

Joe: All innovations who was at it first who who were...

Senan: Yeah so the earliest records I think that we have go back to the Chinese in the 10th Century. They had these fire arrows so they were tubes full of gunpowder that they could literally launch they were probably bamboo poles full of gunpowder.

Joe: Right.

Senan: And they'd light one end of it and the thing would take off like the clappers and land on your enemy and set his stuff on fire.

Joe: Oh so it wasn't like just for fireworks it was...

Senan: Probably that as well but yeah no I think they used them in battle as well. And then the Mongols who of course had battles with the Chinese in the 13th century they learned about those techniques and they started using them the Mongol Empire spread all over the place.

Joe: So Mongols and Chinese were firing rockets at each other the guts of 800 years ago.

Senan: Yeah well pretty basic ones. Yeah. But yeah yeah. So then we move a bit forward into the modern age. In the United States around 1926 guy called Robert Goddard a scientist, he launched the first known liquid fuel rocket. Now there's all kinds of reasons why that's a good why you want to use liquid fuels in your rockets but his was the first successful one that we know of and it was petrol and liquid oxygen that he used. And it lifted up only a paltry 41 feet but at least it did get into the air and didn't kill him.

Joe: That's always good when you're doing experiments.

Senan: Yeah because history unfortunately is littered with examples of experimental rockets that did kill people.

Joe: Right.

Senan: But then coming a little bit further after him the Soviet military in the 30s in the 1930s they experimented with rockets and they actually successfully shot down a couple of planes with their experimental rockets.

Joe: So this is only 10 years after the first person put one up we were shooting down planes with it.

Senan: Yeah well I mean I don't think they were reliably shooting down every plane they could try they tried to shoot down but they did hit a couple of them.

Joe: They actually probably weren't trying to shoot down the planes they just randomly oh uh oh.

Senan: That may very well have been the case yeah. But those none of those rockets really looked anything like the kind of rocket that we think of today when we think of a space rocket so to get something that looks like a modern rocket we've got to move into World War II and regrettably it was the Nazis who who came up with the next big innovation there, V2 missile which was used to bomb Britain. It was looked like a modern rocket was pretty big thing was 14 meters tall and again liquid fuels, ethanol which is pure alcohol and cryogenic oxygen, I'll come back to what the cryogenic bit means in a while.

Joe: Oh yes you will.

Senan: But that was even though it didn't go fast enough to actually get away from the Earth or even to go into orbit, it did get into space briefly because it was launched on a what's known as a high arc ballistic trajectory so essentially it went up at a high angle almost straight up well not quite because if you put it straight up it'd come back down on top of you. But only a very slight angle so almost straight up and it went into a very high arc out into space beyond the atmosphere and then the engine cut off and it began falling back down and because of the way they carefully aimed it, it would come down in Britain somewhere you know that they were aiming it at.

Joe: I wonder is there like someone forgotten in the history of rocketry who discovered that you shouldn't point the rocket directly up.

Senan: Yeah perhaps and forgotten fairly quickly I'd say. But like so that was as far as we know the first man-made object that went into space even though it was only in space briefly and it was used for very dark purposes.

Joe: So we use rockets to fire these things up into space but now we use jet engines to sort of propel us up into the sky. Why not use jet engines for rockets?

Senan: Yeah and and it's a very good question because of course jet engines are massively reliable they're used like right now as we're speaking there are thousands of planes in the air around the world using jet engines, they're very fuel efficient and they get us places very fast. But the interesting thing is right so we need to talk about what is combustion which is essentially what's going on or burning for a simple simple word for combustion.

Joe: Good let's use simple words I like simple words.

Senan: So burning is essentially a chemical reaction between usually between oxygen and some other fuel. Now the reason I say usually between oxygen is because technically it's an oxidizer, there are other chemicals like fluorine and compounds of oxygen and stuff that you can use as an oxidizer in a burning reaction it doesn't always have to be oxygen but it's usually oxygen that that you're using. And essentially you oxygen is one of those chemicals that wants to react with absolutely everything you bring it into contact with and some things you bring it into contact with it reacts so violently with them, those things we call fuel, that it burns produces this massive amount of heat and massive amount of expanding hot gas when you burn. So that's essentially what's going on in both a jet engine and a rocket engine. The difference is of course the jets don't carry their own oxidizer. They they don't have a tank of oxygen in the jet so they're they're relying on the oxygen in the air that we're breathing to use so they're only carrying they're carrying a tank full of kerosene or something that's very like kerosene and but they're relying that's the fuel part but the oxygen part is coming from the atmosphere. So there's a couple of problems with that if you want to use it for launching a rocket. One is like once you go above an altitude of about 30 kilometers the density of the air in our atmosphere has reduced so much that there just isn't enough oxygen there to support the jet engine to run the jet engine.

Joe: Right.

Senan: So that's the first problem. And obviously most rockets we know go up into space and there's no oxygen at all up there so the jet engine couldn't possibly run up there.

Joe: That would be a problem all right.

Senan: But the other thing is the amount of thrust you get from a jet engine in other words how much stuff it throws out the back to go back to Mr. Newton from earlier on, is not really enough to launch into space. A rocket engine by virtue of the fact that it's using really energy dense fuels and liquid oxygen which is much denser than and and more reactive than gaseous oxygen that's in the atmosphere, you get just much higher thrust out of a rocket so that's kind of why we're using rockets to get into space instead of instead of jets. Now having said that it's funny you should you should ask, there has been some hybrid approaches some companies have experimented with putting a smallish rocket under the wing of an airplane flying that airplane up to you know nearly 30 kilometers above the ground to to give the rocket a head start as it were and then launching the rocket from underneath the wing of the plane. They're having mixed resuts making it economically viable you see it's going to have to be a small rocket because planes can't carry great big heavy rockets so that means whatever kind of a payload you're putting into space is going to be small too and you mightn't be able to get it you might only be able to get it into a low orbit because you can't put enough fuel on the rocket because it would make it too heavy etc etc. So it's not met with huge amount of success but people have tried it yeah.

Joe: So like these are lightweight rockets essentially.

Senan: Yeah lightweight rockets yeah.

Joe: But non-lightweight rockets there certainly aren't lightweights when it comes to drinking fuel. A bit like your drinking habits.

Senan: Ah no I've gotten over that now I'm on the wagon or nearly. I'm certainly got one finger on the wagon anyway.

Joe: I've seen the wagon. I've seen the wagon when it passes.

Senan: So I suppose as as I mentioned earlier we need to talk about rocket fuel because it's a big part of the rocket story. So you need fuel that's powerful to get a useful amount of payload into orbit, a you know a satellite of a reasonable size or a capsule with a couple of astronauts in it, you actually need an awful lot of power an awful lot of energy to overcome the Earth's gravity to get up to sufficient speed to actually stay up there. So that's why we need these really energy dense fuels to give the rocket enough power to actually achieve its goal of getting up into orbit or whatever. Because remember you've got to lift the weight of the fuel as well and for a lot most rockets just before they take off about 70 percent of the entire weight of the vehicle is fuel and oxidizer. So I mean all that has to be lifted as well at least in the early stages obviously as time goes along it's burning it and the weight of the fuel is reducing. But most rockets are using what's called cryogenic liquefied gas. So the oxygen in practically every case is is cryogenic liquefied and other fuels like methane or hydrogen would also be cryogenic liquefied. Now what does that mean?

Joe: Thank God. Thank God.

Senan: I'll explain.

Joe: I was just smiling and nodding there I was going yeah cryogenic liquefied I know them. I know what that means.

Senan: Cryogenic essentially means really cold. That's what it means. And the thing is right these at room temperature things like hydrogen or methane or other gases that we might use as a as a rocket fuel say propane or whatever, they want to be a gas at room temperature. So in order to persuade them to be a liquid which concentrates them and gives us more energy if we can we can burn they'll burn more vigorously if we burn them from their liquid form versus their gas form, we actually have to make them cold. That's how you persuade them to go from becoming a gas to a liquid.

Joe: I like that description you persuade them. Pssst come here. Come here. Yeah. We don't need you as a gas we need you as a liquid.

Senan: Take off your coat there you're too warm.

Joe: Yeah.

Senan: So like if you think of it in terms of steam and water, you know so water as it happens is liquid at room temperature but if you raise its temperature above 100 degrees Celsius it becomes steam and it turns into a gas so it's the same thing in reverse going on with these we just have to lower the temperature. But we're talking about really seriously cold here like liquid oxygen the temperature of it has to be maintained around minus 185 degrees Celsius and you know similar kind of temperature for the cryogenic fuels.

Joe: Right. That's cold. That's colder. That's colder than winter.

Senan: That that that's pretty cold and we in a minute we'll talk about what the challenges are to do with dealing with that kind of cold fuel. But we might as well talk about some of the other fuels. There is also sometimes solid fuel rockets. One of the most famous ones of course was the the Space Shuttle, the NASA Space Shuttle it had...

Joe: Turf?

Senan: Coal?

Joe: Coal, turf?

Senan: They're solid fuels.

Joe: Potentially.

Senan: Potentially I'm not sure if they're energy dense enough or if they'll burn fast enough for our purposes. So the space shuttle had a mixture actually the two strap-on boosters on the side that were made of solid fuels and then the central the three engines in the middle that were actually part of the vehicle itself used liquid fuels. So it was a mixture but you're talking about pretty exotic chemicals to make solid fuel rockets and they're extremely powerful like the Space Shuttle crews used to report that as soon as the solid rocket boosters powered up when they were launching the shuddering that went through the vehicle was just shocking like that the force of them was incredible. Also they have no off switch. Once you turn on a solid rocket booster that's it, it's on until all the fuel is gone.

Joe: Right. So you don't want to change your mind or forgot the keys.

Senan: Yeah. But there's much simpler engineering generally speaking involved because you know there's no fuel pumps or pipes or valves involved. It's literally you if you can imagine it's like a a hollow candle so they just set fire to the hole in the middle of the candle and it burns from the inside out so you're actually burning in the fuel tank as it were there isn't like a separate combustion chamber. And then getting even more exotic actually I should probably talk about briefly the first space shuttle disaster people might remember that there was two space shuttle disasters separated by many years but the first one was was caused by a problem in the solid rocket boosters. They launched the vehicle at a much colder temperature air temperature than they normally would and the solid rocket boosters were made in sections in cylindrical sections and they were joined together with little rubber O-rings to seal the the outside of them so that the combustion would stay inside. And but the cold weather the frosty weather made those O-rings brittle and they didn't seal and so the combustion was able to leak out the side of the rocket and burn the vehicle.

Joe: Oh right someone must have gotten in trouble for that.

Senan: Oh god yeah there was a long investigation actually fascinating at the time because the every couple of days the investigation team gave a really detailed briefing about what they had discovered so it was for nerds like me it was kind of fascinating. Anyway moving along there's these things called hypergolic fuels.

Joe: I've been like that. I think sometimes when I eat prawns I feel very hypergolic.

Senan: So these are like ridiculously reactive chemicals. So again we're talking about complex chemistry here so you have a some kind of complicated oxidizer and some kind of really complicated fuel and as soon as you bring them into contact with each other they just literally spontaneously ignite. They're not used for large rocket engines apart from anything else the fuels are ridiculously toxic so they're really difficult for anybody to handle and you know if you get a leak or anything you're going to kill half the employees on the pad.

Joe: That wouldn't be good.

Senan: But the other thing about them is of course they're expensive to make and what they have been used for is what's called maneuvering thrusters so after a rocket gets into space if you want to steer it make it go left or right a bit or up or down or whatever you need these small little rocket engines on the side of it that that squirt out little jets of thrust in one direction or another. They have used these hypergolic fuels for those because they're reliable and simple you just literally need to get the two of them into the same space and they'll ignite. So that's hypergolic fuels but they're not really used very much. And the two other kinds of interesting rocket propulsion, thing called electric ion thrusters so these aren't these are not powerful enough to launch a rocket off of the surface of the Earth but they use very little fuel. What what it is is they take a gas typically Xenon and they're able to set up two electric fields at opposite ends of the engine and with an opposite charge so there's negative at one end positive at the other and the static electricity charge in the chamber accelerates these Xenon gas ions out the back and you get a small bit of thrust. So you might say why would you bother with a rocket engine that only pushes a small bit doesn't have big thrust. And the thing is right once you're in space if you want your satellite to be able to maneuver a little bit maybe it's drifted slightly out of position and you want to get it to go back into position you don't need to to push very hard. You know.

Joe: So you could use like a leaf blower.

Senan: You could if there was air in space for the leaf blower to push.

Joe: Have your leaf blower with Xenon gas in it.

Senan: So the good thing about these ion thrusters is they operate at very low thrust but they use very little fuel so you could run one for days with you know you could carry a few liters of Xenon gas compressed and you could run one of these thrusters for days just based on so they're grand for like long-term slow acceleration or maneuvering. And finally my favorite type of alternate propulsion that thankfully nobody has actually tried yet but it's a staple in science fiction novels and it is theoretically possible somebody tells us and that is nuclear bomb propulsion.

Joe: So not it's they don't even go with the fusion or fission they're just going straight to the bomb. It's a bomb. It's a nuclear bomb.

Senan: No it's not about making energy, well it is about making energy but not electricity. So the basic idea is that you build this massive big spaceship and you couldn't use this to get a spacecraft off the ground on Earth into space because you would destroy the ground underneath the spacecraft and everything within a 20 mile radius.

Joe: Cape Canaveral would be toast.

Senan: But maybe you would maybe you would build this massive spaceship in orbit so you would send it up in pieces using ordinary rockets you'd build it in orbit around Earth and then when you're ready to go off to Andromeda or wherever the hell you're going there would be at the bottom of it this massive thick metal plate that might be you know 100 or 200 meters thick and and much wider than that. And there'd be a little hatch in the middle of it somewhere where you would chuck out a nuclear bomb close the hatch quickly the bomb explodes and pushes against this metal plate and pushes the ship forward really quickly and then a few minutes later you throw another one out the hatch and you explode it and you just basically keep exploding nuclear bombs behind your ship to push you forward.

Joe: But would you not just keep going even after the first one? Do you have to keep...

Senan: You would but if you want to go somewhere really far remember space is absolutely enormous so if you want to reach really high speeds you know the amount of force produced by a nuclear bomb is far in excess of what an ordinary rocket engine produces so you're going to keep accelerating yourself faster, faster, faster because that's the only way you're going to get away to say another solar system that's like our nearest solar system is uh I believe four light years away somebody's going to correct me on that but I think it's four light years. That means that it would take light four years to get there.

Joe: Yes.

Senan: And and that's an enormous like the speed of light is an absolutely enormous speed which chances are we'll never be able to accelerate any vehicle anywhere near the speed of light but even if you wanted to get to a quarter of the speed of light so you could get there in 16 years for example the only way to do it would be something like this nuclear bomb propulsion that would accelerate you up to those massive speeds.

Joe: And then the problem is when you get there how do you slow down?

Senan: (laughter)

Joe: Well that mean that's not going to keep me awake at night but I'm sure somebody is thinking about it. But just the idea that like the best theoretical answer we have to traveling through space is to lob nuclear bombs out behind the ship whenever we need to speed up. I mean is that it? Is that is that where we're at science wise?

Senan: It does have a fringe benefit in that you know at the moment we have our our military people have thousands of these nuclear bombs in stock and at least if we could persuade them to send them off into space there'd be less of a risk that they're going to blow up us with them.

Joe: Well so I wouldn't mind just shooting them into just let them go like not set them off just send them into space. Let them drift gently. Shoot them at the sun.

Senan: And let some aliens find them and set them off. Oh no we want the sun we don't really want to blow that up either.

Joe: I don't think we've the capacity to blow up the sun though. Would we?

Senan: I don't know. I don't know. I mean I mean if we put a lot of nuclear bombs in one part of the sun would we damage it enough who knows.

Joe: Push push it off course.

Senan: I don't know I don't know.

Joe: Yeah.

Senan: Anyway let's not let's not explore that one.

Joe: Let's not yes let's not dwell on that. That's I think that could be another episode.

Senan: Perhaps it could. Let's get back to cryogenic fuels because I just couldn't wait they're the most common ones. So to recap that's gases that have been liquefied by making them really cold. But that of course brings some challenges. Of course as I said the reason...

Joe: Gloves everybody has to wear gloves.

Senan: Everybody has to wear awfully thick gloves. As I said the reason why you're doing it is of course it's very energy dense so you get the most the most vigorous burning in your rocket engine. And they only remain liquid if they you keep them really cold so in the case of liquid oxygen you're talking about minus 185 and for most of the other fuels the cryogenic fuels is similar. So the tanks have got to be able to handle something called off-gassing.

Joe: Oh I've been there. I've been blamed for that sometimes. I've been accused of off-gassing in very social situations.

Senan: I know and the cinema is like the worst place of all because you're doing your best to look around say who did that knowing damn well you did it yourself. Anyway the thing is right at the tank of cryogenic fuel of liquefied oxygen or whatever it is at the top there's going to be a small gap at the top of of where the liquid stops and the gas starts and that's a little warmer than the liquid part. So what's essentially happening is the surface of that liquid is constantly boiling because of the fact that that gas space at the top of the tank is a little warmer and as a result gas what was liquid is becoming gas at the top in that space and the pressure of the gas in there is rising all the time because more of the liquid is gradually turning into into the gas. And if you don't deal with that essentially your tank is going to burst from that increase in gas pressure at the top of the liquid.

Joe: So you shouldn't what like the message is that you shouldn't hold it in.

Senan: Yes let it rip.

Joe: You shouldn't keep it in you should always like let ou,t let it go, let it go, okay let it go.

Senan: So so that's the first problem they have to deal with they have to have vents in the fuel tanks and if you see a rocket sitting on the launch pad about to launch you will usually see some kind of gases drifting away from it from little valves in the side of it and that's because they're venting the gas out of the fuel tanks to stop the fuel tanks from bursting. That's the first thing then the next thing you've got to talk about is that all that fuel and oxidizer it's going to be traveling through pipes and valves and pumps and various other pieces of plumbing that are part of the rocket and all of those have got to be able to handle this cold extreme cold. So you know most materials become brittle when you make them cold like that so if you subject them to a lot of stress a lot of physical stress they they're likely to crack.

Joe: Human.

Senan: It's actually a bit like it's a bit like people yeah a lot of stress and they're going to crack. And then often these materials when they get cold will contract they'll get slightly smaller so if you've got say two pipes that are joined together the gap the seal between the two pipes mightn't seal awfully well anymore if the pipes have shrunk a bit because they're so cold. And then at the other end of it when you light the engine the pipes that are down at near the engine are going to suddenly get really hot because the engine gets awfully awfully hot. So you it's really difficult to make pipes and valves and and pumps and stuff that can handle both things that can handle the cold and the heat. Then you need obviously your tanks and your pipes and all that have to be insulated to try and keep the liquid from getting too hot that you want it to stay as liquid not turn back into gas. And then another insidious problem is it's difficult to remove every last bit of of moisture that's dissolved into the fuel and when you make the fuel that cold that moisture that's in it will decide it wants to turn into ice so you can end up with bits of...

Joe: Slush.

Senan: Bits of slush floating around in your fuel and suddenly the pumps start running sucking fuel out of the tanks and that slush gets sucked into the pipes and then it blocks the valves or it blocks the pipes or so there's all these problems that have to be dealt with so...

Joe: I'm reading completely different into this but yes rocketry we're talking about rocketry.

Senan: So what's your insight into that?

Joe: Well just it sounds like my last medical examination.

Senan: And did they find any slush in the pipes?

Joe: Well they didn't want to tell me.

Senan: But yeah so it's it is rocket science.

Joe: It is actually rocket science.

Senan: So I suppose that's the that's the fuels but of course there's other extreme engineering challenges involved in rocket engines. So let's...

Joe: I don't believe it. I don't believe that. Surely gets easier after this.

Senan: No we're only getting started. So there's there's like the combustion chamber so essentially you've got like a bell-shaped thing at the bottom of the rocket that we're all familiar with seeing you know kind of a cone shape thing that sticks out of the bottom of the rocket. So the top of that cone is is the combustion chamber where all the burning takes place inside. And like the metal the heat of the temperature you remember we're using really energy dense fuels and they're burning really vigorously so the temperature in there is way above what's necessary to melt that metal so in seconds your rocket engine could be reduced to a pile of molten metal. And so to deal with that they have a real innovative method of dealing with it I don't know if you've ever noticed if you look closely at some rocket engines you'll see these...

Joe: I think I can say no already to this question I think I can already jump ahead and say no.

Senan: What you have not looked closely at rocket engines? I'll send you a couple of hundred pictures on WhatsApp for you to study. But so you might notice these these circular ridges going around the outside of the rocket bell and what's actually going on there is they are pumping the cryogenic fuel through the little pipes that are running around the outside of the rocket bell to cool it down to stop it from melting. So they're actually using the cold fuel as a coolant to cool the body of the rocket engine to stop the engine from melting it's real innovative.

Joe: So essentially if they didn't do that then they'd be...

Senan: Oh they'd melt yeah they'd literally melt into liquid yeah the engine would basically come apart and you'd have a very big explosion then. So that's one challenge then of course there's the pure physical pressure I mean when the rocket engine starts firing a lot of hot stuff out the back and Mr. Newton's law comes into effect there's a lot of a huge amount of forward pressure coming from the engine on the structure that's holding it in place and all the pipes that are connected to it from the fuel tanks and so on. So they've all got to be able to cope with that pressure. Typically you get a lot of strong vibration because the burning is not perfectly smooth so the engines are vibrating shaking violently and so that can result in in causing cracks to the support structure that's holding the engine in place or the pipes or whatever. And a whole we could probably do a whole episode on rocket fuel pumps because they are...

Joe: No let's not do that.

Senan: They are the most incredible pumps you have ever heard of. They have to pump huge quantities of fuel to satisfy the appetite of the rocket engine. I mean the Saturn V moon rocket that was the first stage of that, we'll talk about stages in a minute, of the Saturn V moon rocket which is the rocket that brought men to the moon at the end of the 1960s start of the 70s, it burned at takeoff it burned 12,000 liters per second. 12,000 imagine the kind of pumps you need to pump 12,000 liters of fuel every second.

Joe: I can't actually. My imagination stops short of that.

Senan: Yeah so like they have to run at massive high speed and when you run any pump at high speed you've got a risk of cavitation which we mentioned in another podcast is essentially little gas bubbles forming and damaging your pump blades when they collapse again and it's due to the high speed agitation of a liquid so cavitation is a risk. And then because they're running so fast and they're freezing cold fuel going through them you need bearings that you know the shafts that the rocket blades are that the pump blades are mounted on have got to have bearings to allow them rotate and those bearings have to be able to cope with high speed and extreme cold so like the engineering of those pumps is something else. Then you're talking about I mentioned earlier on cryogenic fuels it's hard to get pipes to seal because the pipes are expanding and contracting and there's all the vibration going on. And then a problem which is really only become a problem in recent years when SpaceX have started reusing some of their rockets and something we're going to discuss in more detail later on is sloshing of fuel so if you say you've used up three quarters of your fuel and you turn off your engine and now you want to turn the rocket around and make it go in a different direction and then you want to turn on your engine again, by the act of turning your rocket around might cause the fuel to slosh up to the other end of the tank and suddenly your pipes and your pumps are sucking gas or sucking air, sucking gas, they're not they're not sucking liquid fuel anymore out of the tank so there's all those problems that have to be dealt with. Then you've got it's very easy to have uneven like if there's turbulence in the combustion chamber where bursts of fuel or bursts of oxidizer are coming in blobs you can get this uneven combustion and that can cause the engine to literally explode so you need to have your combustion nice and smooth there's all these problems that have to be dealt with. And finally, finally...

Joe: Oh thank god.

Senan: We're going to come to the sound wave produced by the launch of a rocket.

Joe: Right.

Senan: The the noise is so loud and so intense you know confined into a small area that those pressure waves in the air that are the sound they actually cause physical damage to the launch pad, to the rocket, they hit the ground underneath the rocket and they can reflect back up and damage the mechanism of the rocket engines, the fuel tanks, it's another huge problem that has to be dealt with and often it's sometimes you see when they're launching a rocket you see this massive spray of water turns on in the last few seconds before they light the engine and that water is to absorb sound so that water is to stop the sound wave from actually causing damage. So there's like it there's all this...

Joe: So they're shooting water at sound waves.

Senan: Yeah yeah.

Joe: You know what just disgusts me about this whole situation. Like they've looked at all these engineering problems and they have found solutions like through years of graft and hard work and study and understanding physics and chemistry and the sciences and engineering and at the same time when I bring the car into the mechanic he can't find out what is wrong without it costing like 10 grand. Like it's I mean I don't understand like they can do all these things and surely at the combustion engine and all the bits in the car surely they can just it's like playing with Lego for people now for these guys.

Senan: Yeah I know I mean like the we both of us know somebody whose car has spent three months in a garage until they figured out what was wrong with it and you would think with all modern electronics and the fact that every car engine has about three computers in it now that they could just figure out that stuff in 10 minutes with a probe of some kind, we're getting back to probes again.

Joe: Yes. Here we go. Back to the probes.

Senan: And I'm sure it has absolutely nothing to do with the desire of the garages to maximize the amount of money they make from repairs and diagnostics.

Joe: Should send all the mechanics to NASA for six months and like they come back.

Senan: Actually preferably put them on top of a rocket.

Joe: No no we need them. I love mechanics. Don't listen to him.

Senan: So stages we spoke about stages earlier on.

Joe: Oh you should be on stage. Or in prison. Special wing, special wing in the prison for you.

Senan: I don't know I mean prison life I just don't know if I could cope with that.

Joe: Well you'd have a lot of time to read.

Senan: I suppose I would yeah and I could teach all the my fellow inmates how to how to make rockets.

Joe: Absolutely. Do like a workshop, workshop on ballistic trajectories I'm sure the prison authorities would like that.

Senan: I'd say some of the inmates might be familiar with ballistic trajectories. Anyway multiple stages. So most modern rockets have what they call multiple stages and what that means is actually it's two or three rockets piled up on top of each other. So the one at the bottom lights up first and gets you away from the Earth and after a while that one has used up all its fuel so you throw it away and you turn on the second one in the stack and it runs for a while and then it uses up all its fuel and if you have another one then you throw away the second one and you light up the third one. That's called staging. Their first stage would be the lowest one second stage third stage etc. So most rockets have at least two stages some have three.

Joe: Like all together now.

Senan: Oh yeah like Lollapalooza or what was the one Glastonbury in England yeah. A lot of stages. So what's the point in doing that? Well for starters you can reduce the amount of weight that you're having to lift so by burning all the fuel for the first stage and then throwing the empty fuel tanks and the engines for that first stage away you're saving all that weight. So how much might you be saving? If we go back to our friend the Saturn V rocket that brought people to the moon the empty first stage was 130 metric tons. So that was 130 tons they did not have to bring any further with them. So the other thing is that you need most of your power earlier on in the flight to get off Earth and to build up your initial speed. So later on you can you can get by with less power once you've reached a certain amount of speed and you've gotten above the densest part of our atmosphere. So you can throw away your big engines throw away your big fuel tanks and just turn on smaller engines and you now have less weight to lift and the result of it all is that you can get more stuff into orbit you can get whatever your payload is be it a satellite or a capsule with people in it you can get bigger ones into orbit by using this multi-stage method.

Joe: And they're still doing so that's that's the standard for currently.

Senan: That is the standard because if the theoretical other way of doing it is is single stage to orbit or SSTO it's called and the thing is...

Joe: Wow. You're obsessed. You are obsessed.

Joe: I'd say there's about eight people in Ireland who know what that acronym means.

Senan: I'll be asking you tomorrow what that is and I'm sure you'll remember.

Senan: Anyway the physics of the whole thing because of the strength of Earth's gravity and the amount of power you need to get away from that, the physics of using a single stage rocket means that you have this enormous rocket that might be you know 60 or 70 meters high and eventually you're able to put something into space that's the size of a dinner plate. You know. So if you want to get a usable decent amount of stuff into space you've got to use multiple stages because just the physics, I mean that one stage is just not powerful enough.

Joe: Yeah. I suppose it's just good to know that if you're having a dinner party like on the space station and you're just one plate short at least they don't have to use multi-stage rockets to get it up there.

Senan: Send up the plate send up the orbital plate. So there's different kinds of, what is your rocket going to do when it gets up into space when it gets away from Earth? There's different...

Joe: I could think of several things but I'm going to remain quiet on those.

Senan: Ideally it's not going to blow up although some of the some of the more recent SpaceX ones have been have been blowing up. I'm sure they'll solve that problem soon. So there's three basic things your rocket might do when it gets into space. We'll talk about the biggest one first that's escape velocity. That's where you actually want to get away from Earth altogether and go off to deep space you want to completely leave Earth's gravity behind so you're not going to get pulled back down by Earth's gravity after you switch off your engines. And escape velocity is 40,000 kilometers, slightly more than 40,000 kilometers an hour.

Joe: Regardless what size you are?

Senan: Yeah it's just to do with the the amount of strength of the gravity of the Earth. So that's used rarely enough obviously if people are going to the moon they need to go at escape velocity or if they're going to Mars or the outer solar system but an awful lot of the rockets we launch are on an orbital trajectory. And what that means is you don't go fast enough to get away from the Earth but as soon as you launch your rocket you turn, you point it sideways like more parallel to the surface of the Earth. You what happens is you reach a certain speed where you begin falling you turn off your engines it's not escape velocity it's typically half of that say a little bit more than 20,000 kilometers an hour, you switch off your engines and you start falling back towards Earth. But because you're going sideways you miss the Earth. You fall down the side of it.

Joe: Okay this is hurting my head now. So the rockets are falling but missing hitting the Earth.

Senan: Yeah. So they just go down the side and keep falling and keep missing it. That's what orbit is.

Joe: But then like so if they fall and then they fall in a different direction or is it just because the Earth is spinning that they just are falling in one direction?

Senan: Remember you pointed your rocket sideways right and you then accelerated like mad. So you're going sideways at a huge speed right? But at the same time you're also falling downwards so if you like you're going diagonally. Your initial sideways direction then you you pull that down a bit from the gravity of the Earth. But you don't pull it down far enough to actually hit the surface of the Earth instead you miss the Earth and you keep falling around it.

Joe: But so you don't need to use any more fuel you don't switch anything else on so once you hit that orbital velocity and then turn your engines off that just goes forever then?

Senan: Yeah. Except the lower down ones there's very tenuous wisps of atmosphere in the air where they're orbiting and that gradually begins to slow them down like really, so months later they might actually fall back into the atmosphere because they have slowed down enough.

Joe: I thought you were going to say many decades later they might fall down but no a couple of months after they put them up there.

Senan: Well it depends on how high they started up, it could be decades if and but for example the Space Station which has been in orbit there for I don't know about 15 or 20 years, they have to boost up the speed of that they have so...

Joe: So the Space Station is falling now it's not just the satellites everything is falling.

Senan: Everything that's in orbit is falling. But they send up you know rockets with supplies for the astronauts or with new astronaut crews and they use the engines of those rockets periodically to speed up the Space Station a little bit so it doesn't fall.

Joe: Give it a nudge.

Senan: Yeah just speed it back up because it's getting dragged a little bit by the tenuous atmosphere. So the other kind of trajectory which we spoke briefly about earlier is ballistic. So that's where you don't go fast enough for um for escape velocity and you don't go sideways enough for orbit so you you're kind of going not quite straight up but quite high up so you you go up into space then you turn off your engines and you start falling back down in an arc. And that's typically used by missiles.

Joe: The only good use is for this one though the really happy uses for this is it just purely to strike fear into the hearts of children?

Senan: Well it's funny enough you mentioned Katy Perry's little adventure um with um Jeff Bezos' space company um the other day and that they use a ballistic trajectory. So they launch their like essentially they're bringing space tourists for a very brief jaunt into space so they literally barely go out of the edge of the atmosphere on a ballistic trajectory and then they fall back down again and land on the Earth so they're only up there for a few minutes. But no mostly the thing about ballistic trajectory for rockets is when it's coming back down it's, by the time it gets into the atmosphere, it's going at enormous speed it's coming more or less straight down at you it's really hard to shoot that down. Like the speed that it comes down at means that most anti-aircraft or defense missile systems can can't target it because where you thought it was going to be it's now 50 yards further down in a matter of a second you know. So that's what ballistic trajectories are.

Joe: So ballistic trajectories essentially are evil.

Senan: Yeah well not because they're missiles just for letting Katy Perry into space.

Joe: You realize now we've lost half our listenership because you're slagging off poor old Katy.

Senan: I'm not 100 percent sure the old Katy Perry listenership is is like tuning in to Enough with the Science. I'm not 100 percent sure.

Joe: At this point I have to step in we're going to stop insulting people and move along. Right let's talk about everybody's favorite SpaceX rocket company. We're going to forget about the elephant in the room.

Senan: Your favorite.

Senan: We're going to forget about the elephant in the room of the certain individual who's in charge of that company.

Joe: Yeah we focus on the science.

Senan: Anyway they have developed a unique ability to land the first stage of their rockets and reuse them. Nobody else has been able to do this. SpaceX have been doing it now for several years. Every other rocket company or even government that's launching rockets the first stage of the rocket is allowed to dump into the sea and and is demolished basically when they throw it away. Whereas SpaceX have figured out after a lot of trial and error and a lot of explosions where their rockets didn't land successfully, what happens is so the first and second stage, the bottom half of the rocket separates from the top half and then after a gap is allowed to you know builds up between the two halves it flips itself over uses tiny little rocket engines to, thrusters they're called, to flip itself the other direction. Then it lights its engines again and it starts heading back the way it came. And when it gets close to the landing area it essentially falls back down so it has reduced its speed a bit by going backwards and it essentially falls back down and uses a really precise set of steering fins and little jets to glide it back to the landing pad so the real magic here is is the ability to navigate something that's essentially a flying skyscraper falling through the atmosphere with no wings or anything just these tiny little fins and jets to be able to use those so precisely with computers, this is all controlled by computers, to let it glide back down to the exact right place for the landing pad. And it's amazing they've been doing it for several years now and nobody else has managed to repeat that process. Jeff Bezos we mentioned him a moment ago he has his Blue Origin space company they're trying to do it they've only tried once with their new rocket which is called New Glenn and they it crashed they didn't succeed on the first attempt. But it makes SpaceX much cheaper than everybody else for launching stuff into orbit because they're reusing like a single booster they call the first stage another name for the first stage is a booster, a single booster they've used some of them up to 15 times. So instead of manufacturing 15 new boosters they use the same one and so it's much cheaper to, they're absolutely killing the launch market at the moment because nobody else can compete with them. So much so that they're launching about two rockets a week every week SpaceX now because...

Joe: To bring stuff into space.

Senan: To bring stuff into space yeah.

Joe: Who knew the market was so big who is like we want our own satellites this is very Bond bad guys stuff.

Senan: Well a lot of it is their own they have this thing called Starlink their internet system and they have thousands of those satellites up there so a lot of those launches are for their own purposes. Now you didn't say nefarious there. I'm not saying nefarious either because we don't want to get sued but just there is a word nefarious and that exists.

Joe: Nefarious purposes. I need to look that one up in the dictionary. I don't use words I don't know. Is that something to do with fairies?

Senan: Anyway we we're going to talk about one more thing which is also a SpaceX topic and that is they're developing this new Starship system and some people call it a big leap forward in rocket capabilities and there's a few reasons for that. First of all it's by far the biggest and most powerful rocket that anybody has ever built which that means that it can either put really big things into space or lots of people...

Joe: Or really big people.

Senan: Yeah small amount of big people or a huge amount of small people. But yeah this thing is enormous it's 123 meters tall you know when it's standing on the launch pad that's 26 meters higher than Big Ben in London and it's also wider it's about nine meters wide I think so it's much wider than other rockets as well so it can carry a hell of a lot more fuel. It's got like the first stage has like 33 engines and even that in itself is an engineering miracle because remember the space race between the Americans and the Soviets to see who could get to the moon first? Who could forget. The reason the Soviets failed was their rocket, the N1, I think it had 26 or 27 engines and they could not sort out the vibration problems so when all those engines were running together all the vibration harmonics in the vehicle just broke it up. So like for SpaceX to even get a rocket with 33 engines on it to work is a miracle in its own right. But the second stage then the upper half of it has has six but one of the key innovations apart from the sheer size of it is they're going to try and make the second stage reusable as well. So nobody, including SpaceX, up till now has managed to soft land a second stage of a rocket and reuse it. And there the way they're doing it is really cool they're actually gliding it down sideways it has wings this thing the second stage and they're gliding it down sideways and at the last moment just before it's going to land they flip...

Joe: Or crash.

Senan: Well it has there's been some spectacular crashes they've test launched it about 10 times now but at the last minute they just flip it upright and land it on its base it's really cool. So if they can reuse both the first stage and the second stage it means the cost of getting stuff into orbit per kilogram is even much cheaper again than it is with...

Joe: So it would be cheaper say to get rid of the body of a co-host in space.

Senan: Yeah but then then this podcast would become a monologue and you know people have only a certain amount of tolerance for that kind of thing. But one other interesting thing, it's still in the test phase and they've got a lot of it to work, they've successfully landed the first stage a few times now they in fact they use these really cool robot arms to grab it out of the sky so they don't let it go all the way to the ground, they have these massive robot arms that grab it while it's on the way down. But the the second stage they've had a problem with that's been blown up a bit a couple of times in some of their test flights but they have actually got it down to sea level and just softly dropped it into the sea a couple of times so they're nearly there with getting the second stage reused as well. So a lot of the the scientific exploration robots that we send to Mars or Saturn or Jupiter or whatever they're extremely weight constrained because the rockets we have the traditional rockets we have, if I can use that word with rockets, are so they're only capable of launching small amounts of weight into escape velocity away from Earth. So that means that the scientific instruments on those robots are really really weight constrained and they can only put really lightweight simple instruments...

Joe: Dinner plate. A dinner plate.

Senan: On those robots. This thing will allow them to will remove those constraints and we'll be able to send big heavy robots to other planets with lots of sophisticated scientific instruments on them.

Joe: You are saying this with the glee of someone who thinks it's a good thing.

Senan: Yeah let the robot invasion commence. Some other things they're going to do with it they are they're using methane cryogenic methane as the fuel and they believe it should be possible to manufacture that on the moon or on Mars from from locally sourced materials that are already there so it brings the option of actually refueling the vehicle and using it to fly back to Earth from the moon or Mars or wherever. They're also looking at in in-space refueling where they like launch one of them and it's in orbit and then they launch a second one only use half the fuel in that one and fill up the first one from the second one again it adds extra flexibility. So this is like...

Joe: This is where we're going. It's a big leap forward yeah.

Senan: It's a big leap forward yeah. If if they can get it to work I mean they have they have massive factory like they have a production line for these things they're building like they have they have dozens of them being built at any one time so if they can get them to work it's it's going to be a new exciting age in space exploration.

Joe: And I think...

Joe: Like no I just I mean you've really hidden your love of rocketry under a bushel for this podcast I I don't think people really like I'm staring across at the gleeful eyes of a madman when he's talking about rocketry.

Senan: Well you can pick the next topic so you you can be completely self-indulgent about the next topic for next week.

Joe: No that was very, I don't think I could come up with a more interesting topic Senan, that was brilliant and an awful amount of information but I do think after that amount of information we do have to say enough with the science for another episode.

Senan: Yeah have we? don't have any rocketry limericks do we?

Joe: We have no rocketry limericks because um yeah I've lost the poetic urge.

Senan: The the the limerick AI is on holidays this week.

Joe: Yes absolutely.

Senan: Anyway that is enough with the science I'm Senan and until the next time.

Joe: Catch you next time take it easy.