YouSaid · the spoken record

Tim Dodd

lines on the record: 382
first: 2023-02-02
most recent: 2023-02-02
sittings or episodes: 1
sources: podcast

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“And remember how the bigger the nozzle is. It's continually just converting more and more. It's converting that high energy, hot, high pressure gas into a cooler and cooler, lower pressure, and faster gas. So each little millimeter along that nozzle is just getting it lower pressure and cooler, but faster. Now, if you take a big nozzle on Earth and at sea level and you fire it, you can actually get, even though we're going from say 300 bar of the Raptor engine, our atmosphere at sea level is about one bar is pretty much exactly one bar, depending on conditions. But you can actually get a nozzle to get way below one bar of pressure. So every little, you know, you can go from 300 bar in just two meters down to one bar or below one bar.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“Well, that would be like a traditional nozzle. Now, aerospike would be almost like squeezing an ice cube. You know, if you squeeze an ice cube, you can push in on it and kind of that wedge force will shoot that ice cube. So that's kind of what has happened. We have the high pressure gases on the outside of the spike squeezing in on that spike and then it's pushing up against the, you know, because it's equal on both sides, against the kind of the ramp is pushing up against the rocket. So that's where that force comes in. It's against the nozzle, against the chambers. The hard part with an aerospike. So the cool, okay, I guess the cool thing about an aerospike is that it can operate in space. You can have what's known as a really big expansion ratio. So that's your ratio between the throat, the area of the throat versus the area of the nozzle exit.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“You know what I love about this is that every question you've asked me is like a one hour video on my country. I was like, now boil it down to 45 seconds. Go. So, the aerospike engine basically is an inside out engine, more or less. So with a traditional engine, we've talked about the combustion chamber and the throat and then it expands out into the nozzle. Those walls are containing the pressure, right? Airspike is the opposite. It's basically the pressure of the engine is on the outside of it and it's pushing inward against a spike. So it's almost like the difference of if you were Let me think about this if you were standing in like a tent or a tepee right and you put your arms at the top and you pushed your arms out like into an iron cross or something, you know, you can physically lift the tent just by pushing outwards on the tent walls, right?”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“If your fixed cost is $5 million and you can put 300 kilograms in space versus you have a $5 million cost of operation and you can put 5,000 kilograms into space like it's the business case is going to send you in one direction pretty quickly”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“You can either launch something that size, or again, like a full big old Falcon 9 booster, the huge, huge thing. And that would be lucky to put 300 kilograms into orbit. So it's like, which one's going to be cheaper to build, you know, ship around all the stuff? And then you also look at, you have fixed cost. The idea of flying a, but this is again everything in rocket science is a compromise because now you have things like people on console time, all the people that are, you know, on comms and working on the rocket, going down to the pad, you know, filing paperwork, doing range control, making sure there's not planes and boats in the way, flight termination. You have all these fixed costs for any launch. I don't care how big the rocket is. There's a relatively fixed cost. So now you say like, okay, I'm going to be paying, well, let's just make a winner. I'm going to pay $5 million to fly a rocket between all the people going on site, all the propellant, all the licenses, blah, blah.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“That's what most convention tells you. You know, by the time, if your goal is cheap, then you're going to spend, you're going to have a physically larger rocket that has more engines, that has more propellant, blah, blah, blah, blah, to put the same amount of mass into orbit compared to something else. We're talking like rocket labs electron, a really small rocket. It's like, I think 1.3 meters wide and something like 18 meters tall or something. It's a small rocket. If you were to, you know, and it can put something like 300 or so kilograms into orbit.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“That's the dream becomes easier to reuse multiple stages because now, you know, like the booster doesn't have to survive orbital reentry temperatures and extreme environments. And you only have to make survivable the upper stage. You only have to put a big heat shield. I mean, starship's the perfect thing of this. The upper stage has a big giant heat shield. The booster doesn't need it because it's not going, the booster's not going to orbit. It's only going a fifth or a quarter of orbital velocity. So it's heat that it experiences is survivable just by the stainless steel. You don't need an additional heat shield. So all of a sudden, if you're trying to reuse, pretend that you just welded the two stages of Starship together, remove those engines on Starship, that whole vehicle, if you're trying to reuse it, the whole vehicle now has to have a heat shield on one side of it. The whole thing has to have these big heavy wings. By the time you come down to it, there's probably just zero payload capacity. You basically put your fuel tank in space, you know. Good job.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“Like, okay, apply that technology to a multi stage rocket, and it's going to do better, you know, like no matter where you end up, it's just always better to ditch that weight.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“So single stage orbit, your margins just become so small that it's border, it's not impossible, but it's just at the end of the day, like almost no matter who you are, you end up saying it's just simply not worth it. Like it'd be if you have two rockets that are using the same amount of propellant, you know, they're the same physical sizes and one of them is cutting, you know, and a third and has another little engine, it'll have a hundred or a thousand times more payload capacity than the one sitting right next to it. And now, so there's tricks you can do to like try to offset that, things like aerospike engines, which operate as efficiently at sea level, kind of optimized efficiency at sea level, and just by the way their design, the physics of them, they're also efficient in a vacuum too. You can do things like that. And at the end of the day, though, you just end up with a worse rocket than if you had just done stage, like no matter what. And people say, like, well, what if you developed a new technologies?”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“Why is it so hard? The payload fraction of a rocket is like three to five or six or seven percent would be the, you know, that's the amount of payload compared to the total mass of the rocket. Like you're lucky to get into be on 5%. So if you're now having to deal with the weight of the rocket by the time you're in orbit, like your payload fraction just, you're talking about margins, small amount of leftover if you have to take all of it with you. So the sooner you can ditch weight, the better. The sooner you can ditch weight, the better, the sooner you can, you know, and that's what you're doing, a rocket the whole time is actually ditching weight. All of that fuel, all that big giant flame you see is literally mass being thrown out the back of the rocket. But what typically isn't expended, you know, at least.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“Throw a second stage on that thing, and now you can put 17,000 kilograms into space. So it's just an order, you know, orders of magnitude more payload capacity because you did staging, because you ditched the visidual weight. So the other thing that's hard about that too is that the engines, again, that operate at sea level are often not great in space and vice versa. Like you physically can't most optimize for space engines, you can't even operate at sea level they'll they'll destroy themselves due to something called flow separation. So not only are you getting the benefit of ditching all the weight, but you're also able to use a much more efficient and less typically much less powerful engine in space.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“Big fuel tank, and what they did is they actually dropped off two of its three engines So it just ditched some of the engines, but if it hadn't done that, so kind of people were like, well, that was single stage. It's like it still had a staging event. It still had a ditch mass in order to even make it into orbit. Had it not done that. It would have not been able to get into orbit. So you pretty quickly look at your trade and say, okay, well, if I want to stick to single stage to orbit, my payload mass becomes tiny. You might be able to put a Falcon 9 booster on its own. Like if you just flew one of the sidecore boosters of a Falcon Heavy with a nose going on and everything to say, I'm just going to fly this on its own. You might be able to put 10 kilograms into space or something, you know, a very small amount.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“To push it around, it needs just one instead of nine. Its engine can be optimized for the vacuum of space as opposed to having to operate at sea level with all of our actually pretty thick atmosphere relatively. So staging is basically the idea that you get rid of things you don't need. On Earth, again, kind of that whole like 10% harder 10% easier. It was 10% easier, single stage orbit would be no big deal. And it probably would have been like the way to get to orbit by choice just because like it's not that hard. But with our Earth as it is, with physics as it is, it's just, it's doable. And we've had, and we almost kind of actually the first orbit to take humans or the first rocket to take humans into orbit from the United States, which was the Atlas rocket was kind of a stage and a half. It actually only had like one.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“Yeah, no, that's a good question. So ultimately, you know, like I said, you're kind of pushing about 90% the rocket's like basically just fuel with some skin on it. You know what I mean? And so that skin weighs a lot of, you know, skin and the engines do weigh a lot. Like I said, the Falcon 9 on someone is about 20 tons. Just the booster is about 20 metric tons. So it's not an insignificant amount of weight. So the idea is with staging is you ditch anything you don't need more or less. So Falcon 9 is a perfect rocket to think about this because you have an upper stage and you have a booster or first stage. And the first stage burns through all of its fuel once it's out of fuel. You let go of the second stage and ta-da, you actually just basically started and lit a brand new fresh rocket. This brand new fresh rocket now doesn't have all that 20 tons attached to it. So it's a lot lighter. It doesn't need as nearly as many.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“Then before they're testing, I think since last March or last April, they've tested a thousand after a thousand engine fires, I guess, not just raptors. But that's just an insane amount of data and an insane amount of edge cases to learn, oh my God, we found out that we're actually slightly overspinning our turbine in this degree and this frequency is harmonic at this blah, blah, blah. And all of a sudden we realize it's rattling and, you know, it did this. And then you can engineer around that. It's like ultimately I think Elon said something like high production rate solves many ills or something along those lines. And it's just true if you have an insane amount of engines and an insane amount of data and an insane amount of failures to learn from, you just know your system inside and out. You know those margins. You know where the failure points are. You know how to engineer around them.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“Well, and when you're pushing the boundaries, you want to know where and how it's going to fail. That's right. So you can engineer around them. So that's a luxury that SpaceX does have with the scale of Raptor. You know, they're building Raptor cheaper than probably almost any other engine. Maybe besides some of their own, at least at that scale.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“Even six inches too high on that nozzle. Yeah, it's just going to go upwards, you know. Pressure always wants to flow from high to low.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“Film cooling, an actual boundary layer of cooler gas against the hotter combustion chamber gas. So basically repurposing that gas that was normally wasted and they pump it back into the engine further down. So the trick there is it has to be far enough down that the pressure in the nozzle because remember as the nozzle gets bigger and bigger and bigger the pressure is getting lower and lower and the temperature is getting lower and lower. So you have to find this trade-off point where the pressure is lower than that gas from the turbine and then you pump it in and it's cooler than the gas still is in the nozzle and it can help not melt your nozzle. So you'll notice that the F1 is actually a good example of regen cooling. So the chamber walls, you can physically see the pipes actually on the F1 because it's so big and they just literally used pipes and bent them. And you can see the coolant channels all the way up and down the engine until you get to that manifold.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“Not melting your engine, but at the cost of performance usually. But you can also be smart and use film cooling. There's fun little clever tricks. For instance, you'll notice on the F1 engine that was on the Saturn V, the biggest rocket that had been built to date prior now to Starship. The F1 has this huge, huge, huge engines. There's five of them on the Saturn V'llice that the gas generator has a pipe that comes down and then it actually splits off in a manifold and wraps around part of the nozzle. And that manifold takes the hot gas from the turbine, which is actually, I mean, it's not hot. It's actually cold gas compared to the combustion chamber, but it's, you know, in human terms, you wouldn't want to put your hand in it, not live. And it actually pipes that gas into the nozzle so that it creates a”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“So that would be film cooling. So remember how a little bit ago we were talking about like keeping the turbine from melting, you can just run it off of off-nominal, basically, off, you know, typically fuel rich, just run more fuel through that. So it's cool enough. You can actually do that locally kind of in your engine. So you can keep it. So, you know, imagine a combustion chamber. And the top of it's just a flat, like imagine a shower head. And then you have like, you know, the combustion chamber attached to it. The outer perimeter there, the part where the flame front would be touching the walls, you can actually have just more fuel injectors. So you're injecting locally of more fuel-rich zone along the entire nozzle. And that would be called film cooling. So it's less efficient though. Again, you're kind of wasting fuel. There's fuel that's running, your mixture ratio is off, but only for a little portion of the big picture. So that's one of those compromises. Like you can do additional film cooling to make sure you're.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“Yeah, 100%, and at some point, some car guys literally become rocket guys and strap rockets to cars and try and break land speed records, you know, like it's the same universe here. And yeah.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“Exactly. So freaking amazing. Well, I think that's kind of part of my story arc is I just used to be a huge car and motorcycle guy. Like I just loved things that go fast and are loud and go fast and make lots of power. And at the end of the day, like at some point, you realize nothing goes faster and it's louder and makes more power in the rocket. I think that's kind of where I eventually just ended up wound up there just because there is nothing cooler than that.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“The same time, like we should be celebrating the people doing this crazy work, you know, clocking in countless hours just trying to figure out this one little thing that's going to help us further our understanding”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“Not to get too, I don't know what the word would be, introvertive or something or whatever, but that's actually kind of the whole point of everyday astronaut. Like that's almost the whole point of what I do each year from the beginning, I did a thing called the asteroids, trying to be like an award show hoping to, you know, lift up and celebrate and shine a spotlight on the people that are actually doing the hard work and try to treat them like the rock stars that they are that we don't know about. And I think that's one of the things that for sure I think Elon definitely helped make spaceflight cool, helped make that like a celebration thing where people are physically out cheering for rockets and science and space exploration. But I think that's just the beginning. I think like this should be a thing where the general public looks to these people as the coolest ones, as the coolest places to work, as the most important things. You know, sports are great and everything. I'm a big Formula One fan and things like that. But”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“Degrees that way and add four percent niobium, you know, like just things you were like. Is your life, and how do you know this”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“Dude, aerospace is full of that. You'll have people that are so niche in some thing that the average person has never even remotely thought of. Yet this person has done it 40,000 different ways in an environment being like, well,”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“You haven't lived. But that is something that people would calculate, and they find out, okay, copper does a better job of transferring the heat between the walls of it and into the propellant, blah, blah, blah, blah, compared to XYZ. So, you know”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“Heat flux, you know, you can literally see how much energy and how much heat is inside the combustion chamber, how much, you know, and that is a measurable thing even without a computer. Now, I'm not near smart enough to do any of this. Like I've never tried measuring the heat flux of anything. I barely even know what that means. I'm just smart enough.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“Well, especially because you're thinking about a conical thing or like a semiconical thing where the area is getting smaller and smaller and smaller or flowing the same amount of propellant through it as you are down. You know what I mean? Like the propellant has to, so they have all these unique things like, you know, sometimes different manifolds where they'll inject more, less fuel in certain areas.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“They'll actually still have a channel that goes down and U-turns and comes around and comes back. All the way down to the tip of the nozzle and everything. So it's just insane that, you know,”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“By doing that, you're taking heat out of the, you know, you're taking heat out of the metal of the walls and you're putting it into the propellant. So you're typically heating the propellant up, which is, remember when I said there's gas interaction versus a liquid, like liquid gas lots of times. Even if you pump them both at they are both being pumped as liquids, by the time it goes through the walls of the chamber, lots of times one of them is phase changed into a gas. So now you do have that gas liquid interaction. That's because they're using that the fuel or the oxidizer to cool the walls of the engine. So when you look at a rocket engine, although it looks like a nice, beautiful, uniform cylinder, you know, smooth thing, there's either there's oftentimes like channels actually milled into the walls that they run fuel through. And even though they're, you know, they can be like two, three millimeters thick.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“Dynamics that it's just. Which I can't imagine having to figure all that stuff out, honestly. So the more elegant thing to do, there's a couple other things you can do, but kind of the most common one, especially when we're dealing with liquid fuel rockets, is something called regeneratively cooling. And the idea is you basically just flow fuel or oxidizer through the walls of the nozzle in the chamber. Before they go through, like, into the injector or into the actual combustion chamber”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“This is part of the design seems so silly. So obviously you probably, you know, it's, again, it's not the most elegant. And the problem too, your geometry physically is changing too, because as you're eroding the walls, now things like your expansion ratio or the ratio between your throat and the nozzle exit is changing because the thickness, like the throat's diameters actually, like everything's changing. It's not great.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“Well, one of the ways is to let it be engine rich. There's actually, you can use a blade of cooling, you can literally let make the walls thicker than you normally make it, make it out of a material that will ablate away that will kind of chip away and take some of the heat away with it. It's very, again, primitive. And it's actually what SpaceX first used on their first Merlin engines. They used a blade of cooling. So it's basically a carbon nozzle, and you just let it get the inner layer of the engine was carbon. And you just let it get chewed away and eaten away. And that's just something you factor in. It's not very elegant and it's definitely not reusable in that sense.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“Because it's the best. I mean, it's just physics wise, if you're trying to extract as much energy out of your propellants. There just isn't another cycle type that is better than it. But of course, it's very, very hard to develop. So far to date, the RD-270 and the 60s was built. There is a Powerhead demonstrator built in the United States in the 90s and early 2000s, I think, maybe just the early 2000s. That was just the power, just the pumps and the turbines and the preburners, no chamber, no nothing. That was a big deal only the United States took millions of dollars to just develop that. And then there's SpaceX's raptor engine.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“Oxygen rich is generally very hard, but that is what the Soviet Union ended up doing with almost their entire line of engines was close cycle oxygen rich. But, you know, so those two are kind of generally hard but offer great performance benefits over OpenCycle. But at the end of the day, full flow is by far, it's the ultimate of all of them. It's the most difficult, but it also has the most potential to be the most efficient.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“In your engine, which just generally wants to react with everything. It's just a recipe like hot oxygen is just a recipe for things to catch on fire that shouldn't be on fire. So metals, you know, under those conditions, lots of times we'll just spontaneously start burning. You know, you'll actually turn your metal and it will now become fuel. You'll be engine rich before you know it because your hot oxygen is eating and using that engine as fuel, basically.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“Yeah, it's a pretty, it's kind of like a, what's the bears, the Goldilocks? You kind of generally, the easiest is open cycle because, you know, you're just expelling the exhaust gas, the gas generated exhaust. You're not having to worry about it. You just spin up that thing as much as you need. And deal with it, right? No big deal. Close cycles offers 10 to 15% greater performance generally because you're not wasting that propellant. But it's complicated. It's a lot more complicated, especially if you're doing oxygen-rich. Now you're having hot, gaseous oxygen.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“Still have enough pressure through the turbine to overcome the pressure inside the main combustion chamber. And they're both going to arrive, both your fuel and your oxidizer are going to arrive in the main combustion chamber as hot gases already. So what was liquid oxygen is now gaseous oxygen. What was liquid methane is now gaseous methane. And they're meeting this combustion chamber at still ridiculously high pressures again for SpaceX's Raptor engine. They're meeting at 300 bar insane amounts of pressure. And then they combust from there on and because they're already a gas gas interaction, they're happy to burn. They're ready to burn. They're ready to mingle as opposed to having a gas liquid interaction, which is what's a lot more normal. You know, you'll have two different states of matter and they just might not. They might take a little more, you know, coaxing to, what's that word?”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“So you'll have a gas, a fuel rich preburner, and you're going to have an oxygen rich preburner. Each one of those is going to get just, you know, they're going to heat it up just enough and get it up to just enough pressure to spin up that turbine as fast as they need to do to get the pumps up to the right pressure.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“So, in either of those situations, you're still actually just having the opposite. So, if you're fuel rich, you know, all the fuel is going through the turbine, but only a tiny bit of oxygen is actually being put into that pre-burner to spin the pumps. And the rest of the oxygen is actually going through the pump, the primary pump, and straight into the combustion chamber. Now full flow, the idea is you're going to actually pre-burn both your propellants. Both your propellants are going to go through a pre-burner, and they're both going to end up spinning one of the pumps.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“Many decades of trial and error, and many pieces of engines that you're trying to piece back together going, like, what the hell happened here? Yeah, what?”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“It can physically build up and turn into stalagmites and stalagtites of carbon. Really hard forged in a rocket engine carbon.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“Mostly just that carbon solid chunks of carbon and it can cake up and just literally like, you know, like ash almost, you know, like at some point, you know, especially under those high pressures and high temperatures,”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“Coming out of the gas generator, well, if you run soot through your engine like that and had to go through your injectors, like back into the engine, it'll clog the pores of the injectors and it'll end up blowing up the engine. The soot itself is so damaging that you can't really run a fuel-rich kerosene engine.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“Backward still. So, you really have to get that pre burner up to ridiculously high pressures, like at least 20% higher than your main combustion chamber. And these combustion chambers, you know, we're talking about engines that are at 200, 100 to 200, even in SpaceX's Raptor engine up to 300 bar in the main combustion chamber. So that's, what is that, 4,500 PSI, basically. Insane amounts of pressure inside these combustion chambers. So your turbine has to be even above that, or your gas generator or your pre-burner, sorry, has to be higher pressure than that even in order to have the flow going the right direction through the engine. So now you'll have those closed cycles. You'll have fuel rich, you have oxidizer rich. The tricks now, you start to get, it's crazy. There's just so many compromises. Every little decision you have of like, oh, I did this, now I know I'll now, crap, it's going to do this. For instance, fuel rich, if you ran kerosene, fuel rich, you know how I mentioned so.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“So, what they actually do is they normally will send, there might be some variations of this, but the general concept is you actually flow all of your fuel or all of your oxidizer through the turbine. So that would be closed cycle. So there's fuel rich closed cycle, which would be you're flowing all of the fuel through the turbine, or there's oxidizer closed cycle, which is where you're flowing all of the oxidizer that's going into the engine, through the turbine. Now, the trick here is you have to have that turbine after it's done its work. So after it's taken some of the potential energy, some of the heat energy from, we're not calling it a pre-burner, by the way, instead of it being a gas generator, you now call that device that's creating pressure to spin the turbine. You're now calling that a preburner because it's just going to pre-burn some of your fuel or some of your oxidizer. The trick is that has to be, by the time it's gone through the turbine, it has to be higher pressure than the combustion chamber. Because otherwise it's going to go back.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“Of pressure compared to the main combustion chamber. By that point, by the time it's gone through the turbine, it's lost most of its pressure and heat to the turbine. So if you tried pumping it into the engine, you know, just taking that pipe and sticking it right into the combustion chamber, the much higher pressure, hotter combustion chamber would just go backwards and it'd stall out the engine and blow up the engine and whatever, what have you.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“The temperatures that are reasonable at a reasonable temperature, so you end up with this like dark sooty smoke pouring out of that gas generator that's just unburned fuel. It's just wasted fuel. It never got a chance to be used. You know, like in the combustion chamber, it's not being used to propel the rocket. It's just being used to cool down the propellant that's being used to spin the turbine, that's being used to spin the pumps, to push a lot of propellant into the engine. You know, it doesn't take too long before you're a greedy rocket scientist being like, look at all this wasted propellant, all this potential energy that's just literally being spewed out the side of the rocket. So that's where the close cycle comes in. So now you have to get that propellant, take it from basically what was being wasted through the turbine, and you're going to try pumping it back into the engine. Now, you don't literally just pump that gas that's into the engine because it's actually way too low.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source
“And then after it spins, it's wasted most of its energy, and it's just dumped overboard. That would be open cycle. You're not worrying about it after that point, but you are left with a lot of unburnt unused fuel. A good amount of that fuel is just completely, especially because the turbine, you have to keep it from melting. So you can't run it at like optimal ratios. Not necessarily stoichiometric in a rocket engine you actually don't want it to be near stoichiometric where you're releasing all the energy. You actually want to release, you actually want to be throwing out the lighter molecule so it can be shot out faster generally in the engine. But in order to have a turbine survey, you have to actually cool. You have to have the gas going through it. It can't be stupid, stupid, hotter. Also, you're just going to melt your turbine. So they normally, especially in the open cycle, you just run it really fuel rich. So there's a lot of extra fuel being pumped into it that will keep.”
2023-02-02 · Lex Fridman Podcast · #356 – Tim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel · IDENTIFIED FROM THE TRANSCRIPT · source