Lex Fridman PodcastTim Dodd: SpaceX, Starship, Rocket Engines, and Future of Space Travel | Lex Fridman Podcast #356
EVERY SPOKEN WORD
150 min read · 30,069 words- 0:00 – 0:48
Introduction
- LFLex Fridman
And the nozzles, so as you're saying, there's a bunch of different design options, but it's a critical part of this.
- TDTim Dodd
Yeah.
- LFLex Fridman
How you do that conversion, which is what -
- TDTim Dodd
It's basically like how much can you convert is really like the ultimate game. How much pressure and heat can we convert into thrust? Like that's really, at the end of the day, that's what a rocket engine is.
- LFLex Fridman
The following is a conversation with Tim Dodd, host of The Everyday Astronaut YouTube channel, where he educates and inspires all of us with detailed but accessible explanations of rocket engines and all things space travel. This is a Lex Fridman podcast. To support it, please check out our sponsors in the description and now dear friends, here's Tim Dodd.
- 0:48 – 21:26
SpaceX rockets
- LFLex Fridman
Can you give a brief history of SpaceX rockets? So we got Falcon 1, Falcon 9, there's different versions of those. Falcon Heavy, uh, Starship, and also the, the Dragon capsules and so on.
- TDTim Dodd
Well, uh, yeah, Falcon 1 is where it all started. The original intent and the original idea of, of SpaceX was Elon wanted to try to get something to Mars. You know, um, he saw that NASA didn't have a current Mars plan and he wanted to go to Mars so he decided how do I best do this? Um, he literally wanted to at first purchase a rocket from, from Russia. Uh, then on the (laughs) after a foiled attempt at doing that, he decided that he was gonna try to develop his own rocket and the Falcon 1 is what came out of that process and he developed a pretty incredible team. Like I don't know how exactly he stumbled upon the team that he stumbled upon that quickly, but the people that he assembled were amazing and they built the Falcon 1 which was a single Merlin engine followed by an upper stage engine called the Kestrel engine. Um, pretty small compared to the things they're working on today, but that Merlin engine continued to evolve, uh, into being the power plant for the Falcon 9. They went from a small lift launch vehicle up into the medium class launch vehicle so they could provide services for NASA. Um, that's one of the big things they first kind of hung their hat up was they got the opportunity to fly cargo to the International Space Station under, um, originally it was called the COTS program, the Commercial Orbital Transportation Services, uh, for NASA which evolved into the commercial resupply contracts and that's when SpaceX developed both their Dragon capsule which is a, uh, uncrewed at first, uh, spacecraft that can dock to the ISS, and the Falcon 9 rocket that can take it to the International Space Station. And then-
- LFLex Fridman
The Dragon rides on... It's the thing up top that rides on the big booster thing that sh- uh, that launches it into orbit.
- TDTim Dodd
Exactly. Yep. The Falcon 9's the, the semi-truck, the Dragon capsule's the payload. You know, it's the thing being dropped off basically at its destination and in this case the destination is the International Space Station. Um, and, uh, yeah, so they developed those relatively quickly and, uh, became a commercial success before you know it. They're now the number one launch provider in the world, launching more mass to pa- to orbit than anybody else, launching more frequently than, um, countries (laughs) like the entire country of China who's going crazy right now with launches. Granted China beat them by two launches this last, in 2022, but prior, uh, prior year SpaceX beat the entire country of China. I mean it's, it's nuts what they're-
- LFLex Fridman
And, and, and just like you said, SpaceX still beats China even this year in terms of the amount of payload that was. So-
- TDTim Dodd
The mass to orbit.
- LFLex Fridman
Yeah, the mass to orbit, right. That China had like 60 something, a, a, a couple more, uh, launches-
- TDTim Dodd
Yeah.
- LFLex Fridman
... but the, there was just like small CubeSat type of launches.
- TDTim Dodd
Exactly. Some of them were literally like 100 kilograms or something, you know, like not, not large payloads.
- LFLex Fridman
And so SpaceX customers are different and different, so whoever wants to send payloads up into space?
- TDTim Dodd
Yes, but right now their biggest customer is actually themselves with Starlink.
- LFLex Fridman
With Starlink.
- TDTim Dodd
One of the biggest reasons they've launched so much mass to orbit is 'cause Starlink is designed around the payload fairing and the payload capabilities of the Falcon 9 rocket. So, you know, because they're vertically integrated, because they build their own satellites, because they're building their own rocket, they can literally design a system that's, you know, another manufacturer might have made a more square satellite that was heavier or something, but SpaceX looked at it from a blank slate and said, "Here's our constraints, our payload mass constraints, our volume constraints." And they made a funky looking satellite. The thing's like the size of a, you know, it's like a table folded up, which isn't anything I've, you know, really ever seen before. So but it's purpose built to fit as efficiently as possible inside their fairing and inside the capabilities of that rocket. So therefore, because they're launching those like an insane amount, you know, uh, dozens, you know, 40, 50 times a year or whatever, um, they're, uh, they're just putting up insane amounts of mass like we've never seen before.
- LFLex Fridman
What about the different versions of Falcon 9 so we can linger on that?
- TDTim Dodd
Yeah.
- LFLex Fridman
What are some interesting memories to you of the different developments in Falcon 9?
- TDTim Dodd
The very first Falcon 9s had a, a square, uh, array of engines. It had like a three by three by three grid of their Merlin 1 engines, the 1Ds. And, um, I think it only lasted, I don't remember if it was two or four flights before they went into this octaweb configuration where um there's eight, like a ring of eight engines with a center engine in the middle, um, still in the same diameter that the rocket was... The fuselage was more or less the same 3.7 meter wide diameter, but the, the actual thrust structure changed. And one of the big efficiency gains was you no longer have, you know, a corner engine and then like a edge engine and then another corner engine, you can just make eight of the same, you know, kind of part of the octaweb it's called, uh, you know, the same shape and then your interchangeability and your manufacturability becomes a lot simpler. So that was kind of one of the bigger upgrades at first and they kept stretching it. Every time they like touched this thing it got longer and like or taller and taller technically. Um, and then the next big feature that you saw, uh, in 2014 would have been they added landing legs to a Falcon 9 rocket which was- I was at- that was the first launch I ever went to was actually to see, um, it was CRS-3, so Commercial Resupply Mission 3, and it was probably their-... God, I don't remember what that was like, their 14th or 15th launch or something, like, pretty early on. Um, and people were literally laughing at the idea of them putting landing legs on it.
- LFLex Fridman
(laughs)
- TDTim Dodd
They just thought it was stupid. They were like, "Why are they wasting... Why is this billionaire, Elon Musk guy, wasting his time trying to land a rocket? It's not gonna work."
- LFLex Fridman
So you said the Mars plan was there in the beginning. Uh, what about the re- re-usability of rockets? Was that there in the beginning?
- TDTim Dodd
Uh, I think re-usability definitely, you know, it's, it's a necessary part of making any kind of interplanetary mission, you know. In order to actually do that just financially, you have to start reusing these things.
- LFLex Fridman
In terms of development of the Falcon 1 and Falcon 9, how early on did the, uh, goal of reusing the rocket, having the rocket actually land, how, how early did that goal creep in?
- TDTim Dodd
I, I can't speak for Elon and, and SpaceX, but it was pretty im- you know, immediate that they wanted to try to recover. And as a matter of fact, I think the very first two Falcon 9 rockets, and Falcon 1, I think, they even wanted to try to recover, uh, using parachutes to recover the first stage. Um, and it... Now, fast-forward, you know, almost 20 years later, and Rocket Lab's actually doing a concept like that where they're, uh, pulling a parachute after the first stage is reentering, and they actually are trying to recover it with a helicopter. It's gonna try to snatch it out of the air. They've actually done it. They've actually done it successfully once.
- LFLex Fridman
How does the helicopter grab the, the rocket?
- TDTim Dodd
With this giant, like, drag line and a hook.
- LFLex Fridman
Oh, wow.
- TDTim Dodd
And it literally just, like, grabs, snags onto the parachute.
- LFLex Fridman
Wow.
- TDTim Dodd
And it's pretty amazing. And it... But this is a small rocket. Their rocket's only about a metric ton. The, the booster is, empty.
- 21:26 – 25:37
Falcon 9
- TDTim Dodd
yeah.
- LFLex Fridman
Uh, the Falcon Heavy. So, what, what else is there? So, we, we talked about Falcon 9 and, uh, the, the rapid development there.
- TDTim Dodd
Yeah.
- LFLex Fridman
What other flavors of, of Falcon is there and, and how does that take us to Starship?
- TDTim Dodd
Yeah. Realistically, the, the Falcon 9 evolved more or less, kind of like s- ... Just got more powerful and a little bit longer and more capable. But, nowadays they fly what's called a Block 5, even though it's like the eighth or ninth iteration of the (laughs) -
- LFLex Fridman
Mm-hmm.
- TDTim Dodd
... of the Falcon 9, but they call it Block 5. It's the one that has the black landing legs, the black interstage. They have a fleet of roughly 10 or so that are doing the majority of the legwork these days. And they're flying, you know, up to 15 times I think right now is the current booster leader. They're also recovering the fairings, so the nose cone of the rockets are f- frequently, if not every time being recovered. Um, same with, yeah, same with the booster for the most part. And the only thing being expended is the upper stage, and that's kind of where the Falcon 9 is ending. It's really doesn't make sense to develop that infrastructure any longer, so they went with the next step which is go even bigger physically (laughs) so they have more margin for upper stage reusability. And that's what we see with Starship and Super Heavy. So, the Super Heavy booster, uh, the whole system is confusing. The whole system's kind of considered Starship, but it, technically the Starship is just the upper stage which is also like the spaceship which is also the upper stage.
- LFLex Fridman
Mm-hmm.
- TDTim Dodd
And then the booster itself is considered the Super Heavy booster. And that's what they've been working on, uh, publicly it came out in 2016 as the, uh, at the time it was the ITS, the Interplanetary Transportation System. Uh, later in I think about twenty s- ... By the end of that year, 2017, it kind of became known as the BFR, the Big, uh, Falcon Rocket.
- LFLex Fridman
Yes.
- TDTim Dodd
Yeah. Um, and then I think it was about end of 2018 they started calling it Starship. But that is the, that is where we're at today and that's what they're, uh, working full steam ahead on.
- LFLex Fridman
And, uh, what about Dragon? We mentioned Dragon, uh, Crew Dragon, Cargo Dragon.
- TDTim Dodd
Yeah. So, they went from the cargo version of Dragon that flew, uh, about 20 times, uh, successfully to the International Space Station, uh, except for that one CRS-7 where the rocket blew up and the capsule obviously didn't make it to the ISS. Um, then they went into the Dragon Tr- Dragon 2, which has two variants. It has a crew variant, so we just call it Cr- Crew Dragon, and then there's the cargo version of, of Dragon 2. Um, and that's just an updated, sleeker, sexier version of Dragon and it's, uh, ironically it's heavier altogether, so it, uh ... You'll never see those, those cool return to launch site landing, the boosters coming back to land for CRS missions anymore like we used to, but they landed on the drone ship anyway. And, um, and yeah, that's been flying successfully. That's kind of the ... So, there's, yeah, Starlink, Dragon, Falcon 9, Falcon Heavy and Starship system is kind of the whole, the whole SpaceX world really.
- LFLex Fridman
In ter- in terms of the, the, the spaceships involved, what, what, what to you are some of the major milestones in that history? We kind of mentioned the F-... few-
- TDTim Dodd
Yeah.
- LFLex Fridman
... sticking the landing. Is, is there something that kind of stands out?
- TDTim Dodd
Yeah. I would say definitely, um, the big ones obviously, like any of the firsts, like the first flight of Falcon 1, first flight of Falcon 9, first time they went to the International Space Station. Um, the first time they landed a booster. Um, the first time they reused a booster, which is, I think, about six months after... No. Oh, it was a year after. It was SES 10, um, 2017. It was the first time they, they reused one of those boosters, you know, and that was a big milestone. Like, can we even... Yeah, we recovered one. We, we caught one, you know. It's like we got one, now what?
- LFLex Fridman
Mm-hmm.
- TDTim Dodd
Um, that was the first time they re-flew one. Um, yeah. Then flying humans was a huge one, DM-2, Bob and Doug, um, for NASA.
- LFLex Fridman
(laughs) Bob and Doug, yep.
- TDTim Dodd
Bob and Doug.
- LFLex Fridman
(laughs)
- TDTim Dodd
That was incredible, you know. That was-
- LFLex Fridman
Yeah.
- TDTim Dodd
... that was a, a huge, huge step, I think, for SpaceX, was flying people.
- LFLex Fridman
So it's the first major commercial launching of humans out into space.
- TDTim Dodd
Yeah. And not just into space 'cause, you know, there's been people that have done, you know, space flights with, you know, like suborbital hops, but going into orbit-
- LFLex Fridman
Into orbit.
- TDTim Dodd
... and especially docking and rendezvous with the International Space Station is a, it's a big deal. It's a whole... It, it... Until you really understand the physics involved and the scale involved of, like, just crossing the Kármán line going straight up versus going into orbit, like, they're just completely different
- 25:37 – 29:57
Starship
- TDTim Dodd
things almost.
- LFLex Fridman
What about, uh, Starship? Are we, are, are we in a place where we can talk about milestones with Starship? Has there been, or has it just been an epic journey of failure-
- TDTim Dodd
(laughs)
- LFLex Fridman
... and successes of testing and, and so on?
- TDTim Dodd
Yeah.
- LFLex Fridman
Was there like... Y- yeah. W- what would you classify at this point as, as a, as a milestone that Starship or BFR, whatever the name is-
- TDTim Dodd
Yeah.
- LFLex Fridman
... was able to achieve?
- TDTim Dodd
Well, so far the milestones we've seen, th- I'd say the first one would be the hop of s- they call it Starhopper, and it's basically a very rudimentary rocket. But it was the first time they, they utilized their new Raptor engine to produce thrusted to fly something. It first flew like, literally like (laughs) three meters off the ground or something, like tethered to the ground. Then it flew like 15, and then finally it flew a 150 meters.
- LFLex Fridman
Mm-hmm.
- TDTim Dodd
Um, and that was in 2019. And that was the first big milestone of, of Starship. And then after that we saw, uh, SN5, SN6 kind of do the similar like 150 meter hops with a little bit more elegant systems, you know, proving out more of their, their tank building, proving out more their, you know, a lot of just subsystems. And then the big ones physically were in, uh, end, end of 2020 and early 2021 when they flew the, uh, SN8, 9, 10, 11, and 15.
- LFLex Fridman
What does the N stand for in SN?
- TDTim Dodd
I think just serial number or starting number.
- LFLex Fridman
Just ser- yeah.
- TDTim Dodd
Just number.
- LFLex Fridman
So SN, these are just la- names, numbers, numerical representations of the different testing efforts. They skip some numbers, right?
- TDTim Dodd
Yeah.
- LFLex Fridman
If, if they, if they scratch, uh, a test?
- TDTim Dodd
Yeah. And lots of times it'd be like literally that they're building... You know, 'cause at Starbase and what SpaceX is working on, like the one foot is always in front of someone else's foot, and like the arm is not knowing what the leg is doing sometimes
- NANarrator
(laughs)
- LFLex Fridman
Yeah.
- TDTim Dodd
They, they will have someone working on, you know, they'll just be like, "Hurry up and build 40 of these tank sections."
- LFLex Fridman
Yeah.
- TDTim Dodd
"And you build the bulkhead, and you build the downcomer, and you build the header tank," blah, blah, blah. And all of a sudden like, "Oh, we actually evolved that. We don't use that header tank now. So it's gonna go onto this one." So they'll have like parts of certain rockets built and just be like, "Ah, s- literally scrap it." Like, not scrap it like in the, you know, joke term, but like literally just go, "Scrap it." And they, uh... So they, yeah, they, they just evolve and iterate so quickly.
- LFLex Fridman
There were some epic explosions. Um, I think Starship, s- something about it, uh, probably just the amount of fuel, just leads to some epic, (laughs) epic failures.
- TDTim Dodd
Oh, yeah. Yeah.
- LFLex Fridman
Would you say Starship is the source of the most epic failures, uh, of in terms of size of explosion?
- TDTim Dodd
So you can literally measure in like a yield of explosive power, you know, like, like you could TNT.
- LFLex Fridman
Yeah.
- TDTim Dodd
Like, you can take a look at, uh, how much propellant is left over at the, at the time of the explosion. And, you know, Starship wha- what's flown so far, e- even though it's physically one of the largest flying objects ever, uh, just with the upper stage alone, they've not filled it more than like 10 or 20% full of propellant.
- 29:57 – 37:35
SpaceX rocket engines
- TDTim Dodd
- LFLex Fridman
Uh, so that was the story brilliantly told by you of the rockets, uh, for SpaceX. What about through the lens of engines? Uh, can you give a brief history of the SpaceX rocket engines that were, uh, used that we haven't covered? So you mentioned it all started with the Merlin engine and the Kestrel engine. What, uh, do you have for through th- that lens?
- TDTim Dodd
Yeah.
- LFLex Fridman
What's, what's there
- NANarrator
that you-
- TDTim Dodd
Engines are relatively small number, which is, which is easy for us. There's, yeah, the Merlin, and Merlin's evolved throughout time to be from like the Merlin to the Merlin 1C, to the Merlin 1D, to the Merlin 1D Full Thrust, and all these other kind of tweaks of the same architecture. Uh, Kestrel ended with Falcon 1.... um, they also have the Merlin Vacuum engine, which is the upper stage engine for Falcon 9. Same relative, uh, system, but just optimized for vacuum, so it has a much larger bell nozzle. There's the Draco thrusters which, you know, you kind of can consider engines. Well, they are rocket engines, but they're just small. They're not like the orbital engines. There's the SuperDraco engines, which are the abort thrusters on, on Crew Dragon capsule. And then, nowadays, they have the Raptor engine and the Raptor Vacuum variant. Um, but they've already had two versions of Raptor. We've already seen kind of the Raptor development engine. Um, we've kind of seen like a Raptor 1.5, where it was kind of taking hints of the future Raptor, but now we're well within the, well within what, you know, you'd consider a Raptor 2 variant.
- LFLex Fridman
(laughs) And-
- TDTim Dodd
That's really it.
- LFLex Fridman
Yeah, for the, uh, the Raptor. Maybe I'll ask you that separately. But I would like, in general, and people ... who doesn't know who Everday Astronaut is? But if you don't somehow know, go, go, go check his, uh, your YouTube channel, uh ... You're an incredible educator about the, the super technical and, uh, the more sort of ... even the, the philosophical, the actual like- the, the actual space travel. So you go down to the raw details of it, and there's just great videos on the Raptor engine. Um, I think you have one on Merlin, uh, and, and also actual tours with Elon, where he discusses some of those things. On one of the tours, he says, uh ... He's full of good lines, that guy.
- TDTim Dodd
(laughs)
- LFLex Fridman
Uh, (laughs) he says something about, uh, the number of fiddly bits.
- TDTim Dodd
(laughs)
- LFLex Fridman
And he's, um ... The, the number of fiddly bits was decreased between Raptor 2 and Raptor 1.
- TDTim Dodd
Yeah.
- LFLex Fridman
And I, I think that's actually a really beautiful representation of, um, the engineering efforts there, which is constantly trying to simplify.
- TDTim Dodd
Oh, yeah.
- LFLex Fridman
Uh, increase the efficiency of the engines but also, uh, simplify the design so you can manufacture it. And, in general, simplification leads to better performance and testing, you know, and everything. So the number of fiddly bits, I'm sure there's a Wikipedia page on that now.
- TDTim Dodd
(laughs)
- LFLex Fridman
Is an index. Is actually a really good one.
- TDTim Dodd
Well, and when you think about it, I, I don't know of any other company prior that had kind of tried to measure their performance of their engine not in like thrust to weight ratio or like how efficient it is, like in specific impulse, but literally in like dollar to thrust ratio. Like how much does this engine cost?
- LFLex Fridman
Yeah.
- TDTim Dodd
How much thrust can it produce? And like using that as a trade study instead of just like pure metrics of ... You know, because at the end of the day, like, okay, if it's, if it's cheaper and does, you know, X amount of work, even if it's less efficient, it can actually be better long term.
- LFLex Fridman
And so, I, I guess another way is not even just thrust. I don't know if that metric is used, but basically the cost of getting one kilogram of thing up into space.
- TDTim Dodd
Yeah.
- LFLex Fridman
That's basically what they're trying to minimize, right?
- TDTim Dodd
Especially, yeah.
- LFLex Fridman
So-
- TDTim Dodd
At the ... At the end of the day, that is definitely the ultimate metric, is how much (laughs) does one kilogram cost to orbit eventually? You know, and, and, but there's ... It's so funny 'cause space flight is just the ultimate, you know, it's the ultimate compromise. Every little thing, any variable can just change everything else. So you can tweak so many different things to get to different numbers and conclusions, you know. But even things like on your first stage when your, when your, the rocket's pointing straight up and the engines are pointing straight down, you're dealing more with the thrust to weight ratio of the rocket. So how much thrust is it producing versus how much is gravity pulling down on it is actually a more important metric than how raw efficient the engine is. So it's funny. Then in space, it's kind of the opposite. Thrust to weight ratio doesn't really matter. Uh, what really matters is the actual, the specific impulse it's called, or like the, the nozzle escape velocity of the ... or the ejection velocity of the ... How fast is the, the gas moving, is like the more important number (laughs) on orbit. But it's, it's just so crazy 'cause there's all these like ... I would just love to see the trade studies, you know, when you're like trying to figure out like is this more important than this or this or this? And it's like you change this one little thing and all of a sudden, fff, you know, like all, everything changes. It's just ... Even the profile, like the, the launch profile, the trajectory of it. The, I mean everything.
- LFLex Fridman
(laughs)
- TDTim Dodd
Everything.
- 37:35 – 53:10
Elon Musk
- TDTim Dodd
- LFLex Fridman
I wanted to talk to you a lot about engines, uh, and maybe about Starship, and maybe about your own, becoming an actual astronaut. But, like, let- let's just go there, uh, before all that and, and talk about th- the actual culture of SpaceX and, uh, your conversations with, uh, Elon. You've toured SpaceX facilities with him, you've interviewed him, you've interacted with him. Uh, what have you learned about rockets, about propulsion, about engineering, about design, about life from those interactions? Um, he's pretty transparent, uh, open human being a- as an engineer, as a, uh, as a leader, as a person.
- TDTim Dodd
I would definitely say the biggest takeaway I've had from my times with Elon at SpaceX is real- like, the, the idea of questioning your constraints, he says that a lot, but he also does it a lot (laughs) . Like, he, there'll, you know, there'll be times where, like, you'll see him change on a dime because he's, like, rethinking of something in a n- in a new or different way. And for me, you know, I, I, I think we all put constraints on ourselves, we, we think about our own limits, you know, on, on things that we can or cannot do, and I think it's made me kinda question, like, well, w- why am I, why did I say, "No, I can't do that?" Or, you know, uh, uh, you know, just off the top of my head. Uh, a good example, uh, I ... So in Iowa, I live in Iowa, or half the time or whatever. Uh, there's a bike ride across the state of Iowa called RAGBRAI, and every year you just, you know, like, thousands of people get together and they, they ride across Iowa. And (laughs) it was last summer, uh, I met up with some friends, they're like, "Hey, do you wanna go on RAGBRAI this year?" I'm like, "It's, like, a week away." They're like, "Yeah, you wanna go?" I'm like, "Yeah." (laughs) And so I did without ... and it was one of those moments where I was proud of myself 'cause it's like, I, it's easy to just be like, "No," you know, "I'm not ready," or "This is, my constraint is, like, I'm not in shape."
- LFLex Fridman
Mm-hmm.
- TDTim Dodd
But, like, just question that, you know? And, and so I think when it comes down to questioning your own constraints, it's yes, see, even to that level of like, why do you question yourself on what you can and cannot do?
- LFLex Fridman
So that's, for your personal life is really powerful, but a little bit more intuitive. I think what's really hard is to question constraints in a place like, uh, aeronautics or, or robotics or, uh, autonomous vehicles or vehicles, because there's people, there's experts everywhere that have done it for decades.
- TDTim Dodd
Yeah.
- LFLex Fridman
And everyone admires those experts and respects those experts-
- TDTim Dodd
Yeah.
- LFLex Fridman
... and you, for you to step into a room, uh, knowing not much more than just, uh, what's in a Wikipedia article-
- TDTim Dodd
Yeah.
- LFLex Fridman
... and to be, just use your intuition and first principles thinking to disagree with the experts, that takes, uh, that takes some, uh, guts I think. (laughs)
- TDTim Dodd
Well, and you can't have everyone doing that either, you know? Like, there has to be some humility of knowing that something is a hardened concept and a harden- you know, like, especially, I, I'm not an engineer. I don't, I don't do this stuff, (laughs) you know? But I can imagine you sitting there having spent six years on a type of valve that perfectly manages cryogenic propellants or whatever and someone walks in and says, "Why don't you just put a heater element in there," you know? Or something that's, you know, something like, be- ugh, because we've tried-
- LFLex Fridman
(laughs)
- TDTim Dodd
... you know, we've done that 40 times or whatever, whatever. You know, like, I'm sure there are things like that that are very frustrating. But-
- LFLex Fridman
But see, the thing is-
- TDTim Dodd
So I don't know what that's like, you know?
- LFLex Fridman
... the thing is, with the experts, they're always going to be frustrated when the newbie comes in with their first principles thinking, but sometimes that frustration is justified and sometimes it's not. Sometimes it's just stubbornness for failing to acknowledge a better way. And I've seen it both directions, which is really interesting. So you need, you need both, but that tension is always going to be there, and there has to be a, a, almost like a dictatorial, uh, imperative that breaks through the, the expertise of the way things have been, been done in the past to push forward, like, a new way of doing it. And Elon's done that, um, um, I've, I've seen a lot of great engineers, uh, do this in, in the machining, machine learning world 'cause there's been so much development, I've seen that happen. Usually when there's, like, rapid development, that starts to come into play.
- TDTim Dodd
Yeah.
- LFLex Fridman
Um, and, yeah, and I've seen that in autonomous vehicle space, um, brain-computer interfaces that Elon is involved with, all of it. It's kind of fascinating to watch. Um, what about the actual design and eng- engineering of the engine? Since you've learned about so many different kinds of engines over the past few years, just, like, what stands out to you about the way that engineering is done at SpaceX, or that Elon does engineering?
- TDTim Dodd
The hardest thing to kind of remember is, like, how much stuff was developed in the '50s and '60s. You know, the, the concepts finally being utilized today were already literally done in the '60s.
- LFLex Fridman
Yeah.
- TDTim Dodd
You know, so a lot of the things that SpaceX is doing isn't a novel concept per se. You know, they're, like, for instance, the Raptor engine utilizes the full flow stage combustion cycle engine, and that was already developed by the Soviets in the '60s.... um, for an engine called the RD-270. And it makes sense. Uh, like on paper, 100% it makes sense because you're basically extracting the absolute maximum potential of the chemical energy in both propellants and, you know, at the- at the end of the day, like, you have to be dumb enough to say, "We're gonna try using this thing," 'cause it's actually really complicated to- to do what they're doing. But at the same time, like, so are- so are rockets. Like, rocket engines are already stupid complicated, so adding, you know, 10, 20% more, you know, pain in the butt during the R&D, if it's, you know, in the long, long, long 20, 30 year existence or whatever, you know, like future of that engine, is that gonna be worth it? Obviously SpaceX said, "Yeah, I think we can actually develop this- this Raptor engine." So, it's just interesting to see the things that have been looked at. Or even reusability, you know, like the space shuttle was reusable. It was fully, uh, the upper stage, you know, the shuttle itself, the orbiter was, you know, I mean, that thing was, for all intents and purposes, a reusable rocket. Now, did it live up to its expectations? Not necessarily. So, it left a lot of bad taste in people's mouth on the- the ideas of reusability. So, for SpaceX to kind of come back into the room and on the table and say, "We're gonna use a reusable rocket. Specifically, we're gonna do a fully reusable rocket," you know, a lot of people are, even still today, a lot of people are going, "Yeah, you're not gonna be able to do that."
- LFLex Fridman
(laughs) Even today?
- TDTim Dodd
Even today. Oh, yeah.
- LFLex Fridman
So like, longterm, you're not gonna be able to reuse, scale...
- TDTim Dodd
(sighs) Yeah. But-
- LFLex Fridman
Yeah.
- TDTim Dodd
... definitely, I think the number of people that are saying that today is- is a small portion of those that were saying that type of thing five years ago. You know, when Elon did that announcement in 2016, um, for the ITS, it was very, very aspirational and people were just like, "Yeah, right." You know? And there's a large number of people that had, uh, factual reasons to- to think that and do that. You know, um, at the time, they'd only landed like two rockets or something, you know, when they did that. Or maybe three. It was very small number. Uh, when they announced that actually they had just lost (laughs) ... A couple of months prior, they had just lost, uh, MO6, so they like... They were still this young, blossoming company and to come in and be like, "We've figured out reusability and now we're gonna go full scale and make the world's biggest, most heaviest, most powerful rocket ever and we're going to fully reuse it and it's gonna go to Mars," was just pretty out there. Like it really was, and-
- LFLex Fridman
Yeah.
- 53:10 – 59:15
Twitter
- LFLex Fridman
years. Uh, what do you think about Elon buying Twitter? So, in this perfect balance, optimized reallocation of tornadoes-
- TDTim Dodd
(laughs)
- LFLex Fridman
... throughout, uh, the various efforts in human civilization, uh, do you think... Do you worry about his involvement with Twitter?
- TDTim Dodd
I mean, personally, I just... I, I see that as a lot less important than... And personally, for me, inspirational than Starship and, you know, the work done at SpaceX and Tesla. To me, those were two very, uh, impactful and really, really just generally like, you know, th- they're uniting, like, you know-
- LFLex Fridman
Yeah.
- TDTim Dodd
... something to rally around, get excited about, rally, and just like a future to look forward to.
- LFLex Fridman
Yep.
- TDTim Dodd
Um, you know, the idea of we're gonna be building the world's most powerful, biggest rocket ever, and it's eventually gonna be able to get humans on Mars for the first time, and we're gonna transition the world into fully sustainable, awesome, just totally badass cars that do all these cool things. To me, those were like, that brought a sense of unity and a sense of like, "We can do this." Personally, I just don't think that a social media, no matter what it is, I don't see that in a social media, and I don't, I don't see, um, any sort of politicking as ever anything that's really ever a uniting thing.
- LFLex Fridman
I understand that. I totally agree with you, especially with space, how inspiring it is. I have to push back. I, I do think the impact of social media, the basic level of, uh...... meaningful connections of this collective intelligence that w- we call human civilization, through the medium of, you know, digital communication, which is social media, I think that can have a huge impact. It could be-
- TDTim Dodd
Yeah.
- LFLex Fridman
... the very vehicle that increases the inspiration that SpaceX does and, and, uh-
- TDTim Dodd
Oh, 100%.
- LFLex Fridman
... all different ... Uh, the thing I've criticized them a bunch for is, like, why bring politics into this? Uh, so the politici- the political divisions that we see on Twitter, feeding them is tricky. Um, i- i- it's tricky to sort of understand what is the value of that, what is the contribution of that to, uh, to this whole e- effort we got going on. So th- that's been a, th- that's been a big challenge. But that said, like, again, this tornado, the number of tornados in social media I think is really important because social media h- has such a huge impact o- on us as a society.
- TDTim Dodd
Mm-hmm.
- LFLex Fridman
And to have a transparent, um ... Have a bit of turmoil, you know, it's like Tom Way says, "I like my town with a bit of drop of poison, with a little drop of poison." So, like, a, a little bit of that, um, shake things up, I think might be really healthy. I just worry about the lo- long-term impact on the whole Mars project through that. And so, but, you know what? (laughs)
- TDTim Dodd
(laughs)
- LFLex Fridman
Uh, this life, one of the reasons it's fun is, uh, through the chaos y- you, like, none of us know how it's gonna turn out and hope- hopefully we try to help each other to make sure it turns out well.
- TDTim Dodd
And, and this really isn't, like, anything about my personal, like, politics or anything like that, but really just generally-
- LFLex Fridman
Mm-hmm.
- TDTim Dodd
... any of my friends that are, like, the first thing you hear about them in their day is something that happened in politics or something that some world leader is doing or not doing or saying and not saying, I just don't find that to be the most important thing, really. Um, I, I know that obviously that can affect a lot of people, that has big real world consequences, politics do, but like, I just r- (sighs) and this is just me, I'm such a, like, "Oh, come together, you know, cheerio" kinda guy, that I just really-
- LFLex Fridman
(laughs)
- TDTim Dodd
... think, like, you need something bigger than bickering about what, you know, what people said and did and what they voted on and all the stuff to, to really push humanity forward. Like, I, you know, I know that politics and, and their, and, and by extracting that, social media can affect things like space flight and even our, like, planetary defense, like being able to defend ourselves against asteroids, like, if politics has their way and everything goes to crap and we don't even get to, you know ... Yeah, we're not gonna be able to pr- (laughs) you know, continue space flight and things like that. But, like, I don't know, I just think there's better ways to do it and more uniting ways to do it than, than, you know, what feels like immature name-calling sometimes, you know?
- LFLex Fridman
Yeah, I think the political bickering that most people talk about, th- that's on top of most people's minds, is the thing that'll be completely forgotten about history, it has actually very little impact. Yes, politics matters but, like, 1% of it. I think most of it is just political bickering, the push and pull of the right team-
- TDTim Dodd
Bicker.
- LFLex Fridman
... and the blue team and the-
- TDTim Dodd
Yeah.
- LFLex Fridman
... and then th- uh, th- the news media that feeds off the division for the attention-
- TDTim Dodd
Uh, 100%.
- LFLex Fridman
... and it's just, like, a fun athletic event, almost, with the, with the blue team and the red team. So-
- TDTim Dodd
100%.
- 59:15 – 1:04:07
How rocket engines work
- LFLex Fridman
uh, you have a, you have a few videos on this, but how does a rocket engine work?
- TDTim Dodd
(laughs)
- LFLex Fridman
Uh, you've, you're wearing some of the instruction manuals-
- TDTim Dodd
(laughs)
- LFLex Fridman
... but, uh, f- for one, one type of it. Like, what, what's the fuel, what are the kind-
- TDTim Dodd
Yeah.
- LFLex Fridman
... types of different rockets that you can kinda give an overview?
- TDTim Dodd
Yeah. Ultimately, a rocket engine converts, um, high pressure and heat into kinetic energy. Like, that's the only real job of a rocket engine is to take, uh, high pressure gas, hot high pressure gas, um, very energized, there's a lot of energy involved, and then literally turning that into molecules shooting in one direction, into kinetic energy. So, um, yeah, what you do, basically, you know, I mean, the simplest version of it is, of course, like famously a balloon. You take a balloon, you fill it up with air, you've got p- uh, pressure, you let go of it, uh, some of the air shoots out in a general direction-ish, you converted that pressure into kinetic energy. Now, if you start scaling that up, um, you know, you can continue to do something like that, like, um, cold gas thruster would be kind of the most simple and easiest rocket engine to make would be a cold gas thruster. And all that is is you literally just take air, or specifically nitrogen 'cause it's a little bit more dense than all the others, or, you know, and it's the majority of our atmosphere, um, you can, uh ... Or sorry, it's more sparse. Um, you can condense that down, store it in a really high pressure bottle, and then, uh, just literally shoot it through what's called a de Lavelle nozzle, which is something that chokes the flow a little bit, gets it to be, um-... takes it and gets it into supersonic speeds.
- LFLex Fridman
Mm-hmm.
- TDTim Dodd
Once it's at supersonic speed, you actually can't choke it down anymore. You'll just constrict the flow of, of mass flow. You'll constrict the airflow. So you actually go opposite, you start making it wider. And once it's already at supersonic speeds, if you expand it and make it wider, it actually gets faster and faster. So, at first, you know, when it's subsonic gas, you start shrinking, you constrict the flow, you know, and it's actually speeding up, just like, you know, a highway. If you go from, you know... Or any of these examples, like a, a water hose, you, you know, if you pinch it down and you want it to flow the same amount of water from point A to point B through a smaller pipe, you can, you can flow more water, or the same amount of water from point A to point B with a smaller pipe. It just has to go faster.
- LFLex Fridman
Mm-hmm.
- TDTim Dodd
So obviously, you can constrict it, but at some point, you actually get to a physical limitation, and that happens to be the speed of sound. Once it gets to the local speed of sound, um, you can then actually do the opposite. You actually expand it back out, and you're continuing to convert, um, the, the pressure into velocity at that point, but it's now supersonic. And what's interesting is while you're doing that, you're actually cooling it down too. Each, each bit of that pipe that you're making wider and wider and wider, you're cooling down. So the more heat energy you have to work with, the more work you can actually do. So at some point, a hot, high-pressure rocket engine is the best source of, like, that's the ultimate amount of work you can do.
- LFLex Fridman
And the nozzles, so, as you're saying, there's a bunch of different design options but it's a critical part of this-
- TDTim Dodd
Yeah.
- LFLex Fridman
... how you do that conversion, which is what the Inverted is.
- TDTim Dodd
It's basically, like, how much can you convert is really, like, the ultimate game. How much pressure and heat can we convert into thrust? Like, that's really, at the end of the day, that's what a rocket engine is. So you have to have a powerful enough rocket engine to actually lift the rocket and, well, a rocket is mostly just fuel. (laughs) It's like 90, 90-plus percent just the weight of fuel, so you just have to lift the fuel that's going to take it, uh, you know, into orbit.
- LFLex Fridman
And that's the thing, specifically for rockets, you're just saying generally rocket engines, but for the task of going to orbit, you're fighting gravity, earth gravity, which is fundamentally different than moon gravity or-
- TDTim Dodd
Yeah.
- LFLex Fridman
... Mars gravity-
- TDTim Dodd
Yeah.
- LFLex Fridman
... uh, or like you said, traveling out into space. Earth has a pretty intense gravity-
- TDTim Dodd
It's-
- LFLex Fridman
... to overcome.
- TDTim Dodd
(sighs) We're lucky 'cause if it, I think if it was 10% either way, like 10% harder, it'd be like, ugh. We could still do it, you know, we'd still be, with our current technology, we'd still be able to get stuff into orbit, but man, things like reusability and this, you know, commercialization and the success that we've seen in the last 10 years, we'd just be on too thin of margins, I think. 10% easier and pff, we would've been like, I mean, it's just, like, totally different. It's so much, so much easier. It's like this big, you know, sliding scale and, and 10% in either direction, we'd be either screwed or, or really happy, you know? (laughs) Um, as far as getting into space. So it's just hard enough that it, things like fully reusable becomes very, very, very difficult. I think it's completely achievable. Um, we have all the pieces to make it achievable. It does not disobey any laws of physics, it does not disobey any... There's no, like, hard stops to it, it's just very, very, very hard. And so ultimately, yeah, like, on earth for the first bit of, of launch, again, when, when the rocket's pointing straight up and it, and the engines are pointing straight down, pointy end up flaming and down, you're fighting gravity. And so that's kind of your biggest enemy, um, outside of the earth's atmosphere too.
- 1:04:07 – 1:07:33
Rocket fuel
- TDTim Dodd
- LFLex Fridman
So what kind of, uh, sources of fuel, uh, is there? So there's chemical rockets, liquid solid gas, hybrid, uh, there's electric. So w- what, what are the kinds of fuels we're talking about? What are oxidizers? What, what, what, can you just explain your shirt, I guess?
- TDTim Dodd
(laughs) Yeah.
- LFLex Fridman
Uh, the, the components in your shirt.
- TDTim Dodd
So, uh, so really, I mean, fuels, it's, there's kind of two terms. Well, you'll generally hear the word propellant-
- LFLex Fridman
Mm-hmm.
- TDTim Dodd
... being used as anything that is used to propel a spacecraft or used in a rocket engine. So, um, you have to have, you know, you can have a fuel, you have to have a fuel, you have to have an oxidizer, and you have to have a spark to actually get those things burning. And that's just a general law of like the universe. You have to have fuel and oxidizer and a spark. Um, now some fuels will, by themselves, spark, like hypergolic fuels, but ultimately, you're, you're always left with some kind of fuel, oxidizer, and a spark. So, um, the, the general ones used most often in rockets, liquid oxygen is kind of the king of, well, there's better oxidizers but they're extremely, extremely hard to work with, like fluorine. Um, but, uh, generally liquid oxygen, so you just chill oxygen down to its liquid state, minus 183 degrees Celsius, um, so it can be dense enough to store in tanks, you know, it's a thousand times more dense when it's in a liquid than it is as a gas. Um, RP-1, which is basically kerosene, um, is a very common fuel. Another common fuel nowadays is methane, liquid methane. Um, liquid hydrogen is another, it's the most efficient potential for the most efficient, since it's, uh, one of the lightest molecules.
- LFLex Fridman
So, I think, uh, correct me if I'm wrong, but Falcon 9 uses kerosene and then, uh, Starship uses methane?
- TDTim Dodd
Yep.
- LFLex Fridman
Liquid methane?
- TDTim Dodd
Yep, for fuel, and they both use li- liquid oxygen for their oxidizer.
- LFLex Fridman
Oh, for the oxidizer, okay.
- TDTim Dodd
Yep. Um, but then, you know, if you get into hypergolics, you'll normally have nitrogen tetroxide, um, which is your oxidizer, and some form of hydrazine for your fuel. Um, there's solid rocket propellants, like solid rocket boosters, and those are actually pre-mixed. Your oxidizer is inherently, like, baked literally, (laughs) like, kind of baked into the sludge of, of fuel.
- LFLex Fridman
So, like, for SpaceX, it's all, uh, chemical-
- TDTim Dodd
Liquid fuels.
- LFLex Fridman
... liquid fuels?
- TDTim Dodd
Yep, yep.
- LFLex Fridman
So how many solid based, uh, fuels are there? Is that, are they still being used today or is there most rockets they go, uh-
- TDTim Dodd
Yeah, and the United States really is the only ones that, well, the only ones, uh, I guess early on because it was really just the Soviet Union (laughs) versus the United States. The United States started to use solids pretty early on. Uh, they're simple and easy, but these days like, you know, you'll still see 'em kind of as traditionally like boosters, like they're used to just, uh, help get something off the ground or help give it a little extra boost. Um, so the space shuttle famously had those two huge white solid rocket boosters attached to the orange fuel tank. Those are solid rocket propellants. Um, things like the Atlas V can have up to five smaller solid rocket boosters. There's very few rockets that use a pure...... uh, at least these days that use a pure solid rocket motor for its first stage. Um, there still are, especially in China, there's a lot of startup rocket companies that kind of use just missile technology, you know, they might use, like, a, there might just be a variant of an ICBM, um, that just use solid rocket fuel 'cause it is very relatively easy to develop, you know, model rockets, use solid rocket motors and stuff like that. So they're, they're still around but they're just not as elegant and not as, uh, yeah, not as, as used these days, I'd say.
- 1:07:33 – 1:19:57
Rocket engine cycles
- TDTim Dodd
- LFLex Fridman
So, uh, what are rocket engines cycles? G- getting, I think getting more to your, towards your short question. You have a really good video called, that, uh, I mean, a lot of your videos that are technical are just exceptionally well done, so I just, I think, um, y- you deserve all the props you get. I mean, thank you for, for doing this work. Re- really, really, really well done. Uh, so it was called Rocket Engine Cycles, how do you power a rocket engine? A- and you go through all the different options. Is there something you could say about, uh, open cycle, closed cycle, full flow, um, all the different variants that you can use words to explain?
- TDTim Dodd
(laughs) Yeah, without all the pretty pictures.
- LFLex Fridman
Yeah, without the pretty pictures.
- TDTim Dodd
So ultimately, you know, like we said, we, your, your ultimate goal is you want to get heat and pressure into an engine. So obviously at some point you can either make really thick tanks of your rocket. You can, like, get it so thick that you store the propellants in really, really high pressures. But obviously, like, that doesn't scale very well. At some point your rocket's so heavy you can't even leave the ground or, you know, it's just so much of your mass is just literally the walls of the rocket. So at some point people realized, "Hey, we could actually just pump the fuels and the oxidizer into the engine at a high pressure and increase the pressure through a pump." Now obviously a pump's going to require energy. You have to get that energy from somewhere. Um, and again, at some point people were like, "Well, rockets are, there's already rocket fuel here, you know, we'll just use some of the energy from the rocket fuel to spin these pumps." So that, that would be considered, like, open cycle, closed cycle, full flow stage combustion cycles are, are ways to tap into the propellant, actually and then there's tap off (...) I mean all of them kind of do the same thing, but you end up, uh, at some point spinning a turbine, uh, you know, a turbine can take, uh, some of the heat energy and the, and the, and the pressure, uh, of an engine and then they can, that can be connected to a shaft to pumps. And those pumps can, you know, increase the pressure of the propellants and force it into the combustion chamber.
- LFLex Fridman
Right.
- TDTim Dodd
Now the difference between open cycle, closed cycle, full flow all those is what happens after the gas has flown through the turbine. So after you've used the turbine and spun up the energy, you know, spun up the engine what happens to that gas? So in an open cycle engine you basically have like a separate small rocket engine in a sense. It's, it's a gas generator they call it, and that will be used to create some of the, you know, take a little we'll say 10% of your, the propellant flowing to the engine and instead you reroute it to like a smaller rocket engine called the gas generator. You point that at your turbine and that will spin your turbine up to, you know, ridiculous speeds, 30,000 plus RPM. And then after it spins its wasted most of its, its energy, you know, 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, un- you know, unused fuel. A good amount of that fuel is just completely ... And especially because the turbine, uh, you, you have to keep it from melting. So you can't run it at, like, optimal ratios. Um, not necessarily stoichiometric, in a rocket engine you actually don't want it to be near stoichiometric where you're re- releasing all the energy, you actually want to release, um, you actually want to be throwing out the lighter molecules so it can be shot out faster generally in the engine. So, um, but in order to have the, a turbine survive you have to actually cool, you have to have the, the gas going through it, I- it can't be stupid, stupid hot or else you're just gonna melt your turbine. So they normally, um, 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 the temperatures at a reasonable, uh, you know, 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-
- LFLex Fridman
Oh, interesting.
- TDTim Dodd
... you know, like in the combustion chamber. It's not, it's not being used to propel the rocket, you know, it's just being used to cool down the, 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. So, you know, it doesn't take too long before, you know, 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 closed 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 gonna try pumping it back into the engine. Now you don't literally just pump that, that gas that's, you know, that hot, that gas into the engine 'cause it's actually way too low of pressure, uh, compared to the, the main combustion chamber. By that point, by the time it's gone through the turbine its 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, that (laughs) the much higher pressure hotter combustion chamber would just go backwards and it-
- LFLex Fridman
Mm-hmm. Right.
- TDTim Dodd
... would stall out the engine-
- LFLex Fridman
Yeah.
- TDTim Dodd
... and blow up the engine and whatever.
- LFLex Fridman
Yeah. Yeah.
- TDTim Dodd
What have you. So what they actually do is they normally will send, um, 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.
Episode duration: 5:15:44
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