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Engine cooling – why rocket engines don’t melt (everydayastronaut.com)
559 points by wolfram74 13 days ago | hide | past | favorite | 164 comments

One of my favorite bits from Tim's videos was during a tour of Firefly Aerospace's facility when they talk about engine cooling. They discuss EDM machining small holes into the coolant channels just before the throat, which lets a small amount of cryogenic coolant out to cool the interior. The funny part is that you can purposefully undersize the holes and they will melt larger until they are big enough to adequately cool the engine. You basically pre-season the engine with a test-fire and let it choose how much internal cooling it needs.


This thing I can not comprehend about rocket engines is how the turbopump manages to hold together.

A turbine blade in the SSME about the size of your thumb makes 600 horsepower.


I the power level of these engines is difficult to comprehend. The fuel pump has thousands of horsepower. Compare this to the fuel pump on a car engine, which is a tiny little electric thing. The combustion power must be in the gigawatt range.

Edit: the SSME high pressure fuel pump turbine produces 63000 hp (46 MW). There's also one for the oxygen, and a pair of low pressure pumps as well. Crazy...

Edit edit: the fuel pump transfers 155 lb/sec of liquid hydrogen. If fully combusted (142 MJ/kg), that would release 10.0 GW of heat per engine.

And then on top of it all, throttling ability.

People don't grasp what an unbelievably complex engineering problem that is. It's at least an addition of difficulty at the same magnitude as building a steady-state 63000 hp turbine pump in the first place.

Mechanically implementing it in the inherently steady-state design rocket of most rocket cycles. Having variable controls able to work at those pressures. Testing structural dynamics for a range of harmonic conditions instead of one. And do all of that with materials that need to tolerate temperatures going from cryogenic to white hot, without allowing thermal expansion to affect the mechanical tolerances of parts running at thousands of RPMs. And now you have varying flow rates and negative pressures in the lines coming from the external tanks, so have to design such that cryogenic liquids (that normally would require immense positive pressure to keep liquid) don't spontaneously boil or cavitate or cause a shock-like wave (think water hammer turning off your bath faucet) under changing negative pressures.

It's really difficult even for seasoned engineers to grasp the scale of difficulty involved.

It's really difficult even for seasoned engineers to grasp the scale of difficulty involved.

Its rocket science

To be fair much of rocket science is essentially extreme plumbing

True :) .

Turbopumps are finicky, but at least heat and cryogenics are separated with the turbine shaft. In combustion chambers, you have both mechanical stress from high pressures (and thin walled constructions) and thermal stress - difference of a few thousands K, so chambers aren't that simple either.

Still - now we have enough knowledge to repeatedly design flyable rockets from scratch, different teams, periods of time, countries. A lot of work still remains - and the plumbing is a good manifestation for that.


The engineering of the seals that can operate at those speeds and pressures is a whole specialised field. If I remember correctly, the SSME design uses a labyrinth seal pressurised with Helium.

In other words, the various gases are kept separate only by more gas.

Similarly, the Saturn V main engines were unlubricated because no lubricant could be found that could tolerate the extreme conditions. They were just designed to wear out slow enough to keep operating over their operating lifetime, which was measured in hundreds of seconds (including static test firing).

The turbopumps for the main engines were unlubricated? No fluid bearings?

What do today's reusable engines use?

The bearings of something running that fast, under that much stress, would 100% need to be lubricated. Probably the seals are what would have no lubrication and gradually wear out. Look up "labyrinth seal".

Edit: They used carbon seals. The turbopump uses ball bearings cooled by the fuel. Detailed description here: http://web.archive.org/web/20150509173846/http://agentdc.uah...

At those conditions, "lubricant" is probably the wrong way to think about it.

Those pressures, rates of fluid flow, and shaft rpms is going to result in the mechanical surfaces being separated by hydrodynamic forces and/or boundary layer flow.

Technically, lubrication is the same physical principles, but viscosity, weight, chemical stability/ durability and other factors become more important for lubricants that are recirculating.

(Oil in eg, a 4-stroke engine operates on these principles, with the rotating shaft causing pressure differences in the oil that "lift" the shaft from contact with the surrounding metal, until pressure (and therefore spacing) is roughly equal on all sides.

A ping-pong ball floating on a column of air self-stabilizes in the same way.)

The primary concern for these turbopumps becomes heat. So heat transfer and temperature of the "lubricating" fluid become more important than its other nominative qualities as a lubricant. Fluids being pumped at cryogenic temperatures can obviously help here.

So instead of a normal "lubricant", most of these turbopump designs just run a portion of the fuel/oxidizer fluid through the critical areas to provide the surface separation and cooling required.

The documents in the nasa links below don't seem to agree witht this. Actually they are not fluid bearings, they're ball bearings and mechanical surface wear was a big problem for the oxidizer pump bearings in particular because of the lack of good oxidizer compatible lubricants.

I'm not sure how you see this as incompatible.

Ball bearings require "lubrication", and the lack of good lubricants is exactly why they use fluids that end up behaving as I described, and why "lubricant" is the wrong way to think of it in this case.

If anything, the existence of a good lubricant would prevent the surface wear. Temp control and fluid dynamics are what they have to work with, and yes it results in surface wear because it's suboptimal.

But so does running your 4-stroke too hot.

If I remember correctly, on the fuel side they use the fuel itself (kerosene) as a lubricant where possible. On the other side I believe they ran dry, but I'm not an expert in the topic.

Interesting: https://ntrs.nasa.gov/citations/19950008234

"Liquid oxygen is [the high pressure oxidizer turbopump's] only lubricant and a poor one at that."

So it's some lubricant and I imagine also significantly a coolant.

This https://ntrs.nasa.gov/citations/20100023061 goes into a lot more detail. But exotic materials and designs bathed in LOX which provides cooling (and minimal lubrication) seems to be how the space shuttle engines achieved their long bearing lifetimes.

As I understand, rocket engineering makes rocket science look pretty simple by comparison!

A joke you hear unceasingly when you take tours of engineering schools with a heavy aerospace focus.

That's nice but I hope my calcs on the geotechnics of the ground that your school or house is situated on are correct. Obviously, I didn't do the calcs myself and probably no calcs were done at all either. I'm sure everything will be fine.

Don't forget the foundations!

If you cut open a rocket engine, is it rocket surgery? Does that require even more knowledge?

I suppose overhauling a reusable rocket engine would count as surgery.

According to Elon it is engineering actually, because science discovers the existing whereby engineering _engineers_ something that isn't now.

Appreciation of engineers for the difficulty of an assignment like this seems bimodal.

On the one hand, you have https://xkcd.com/793/ responses:

> You're trying to predict the behavior of <complicated system>? Just model it as a <simple object>, and then add some secondary terms to account for <complications I just thought of>. Easy, right? So, why does <your field> need a whole journal, anyway?

Just pressurize the tanks, and meter the flow with some valves. Easy, right?

They move to the other side of the distribution after a little more thought, when they they realize it's simply infeasible to put thousands of horsepower in a pump that size, and declare the whole endeavor completely nonsensical and impossible.

Ran through this on a recent project involving an automated sewing machine. At first, it seems ludicrous that you could tie knots thousands of times per second. Oh wait, it's a single motor and old cam-driven tech from the 1800s, available off the shelf for a couple hundred dollars?

They also move to the "simple" side as well. The phrase "the rocket science is not a rocket science" has reasons to exist.

Yes, it is difficult and it is stunning. On the other hand, in theory, you can go to an engineering school and learn the calculations and design involved in making sure it works. Also, it has been done like 80 years ago with the V2 with at best mechanical calculators, without most of the materials we can use today, without 3D printing, without simulations etc.

Honestly, I think building micrometer/nanometer scale stuff like a nanopore sequencer is a lot more impressive. That's the true "rocket science" for me.

To be fair, that turbo pump and throttling control only has to work for about 5 minutes, that does make it a bit simpler by just engineering it to work for 15 minutes without breaking (safety margin included then).

You're perhaps overstating how easy it is target a certain design life when constrained by other parameters. It's entirely possible that targetting '15 minutes' is actually meaningless for many of the parts because all of the other constraints are far tighter. Once you're through the stress of getting up and running, then chugging along in the steady-state is often less stressful (for something non-ablative, and of course start-stop cycles are another matter entirely).

To give a far less glamourous example, I am a mech eng who works in rail. We needed some more orecars to complement an existing fleet that only had about 10 years' operational life left in them. Thus, they wanted me to design for a shorter life than the 25-30 year standard that we target, to save cost. However, trying to thin out the structure so it only had 10-15 years' of fatigue life in it meant that it fell well short of the proof load requirements needed to stop it ripping in half in a worst-case shock load. Put differently, the constraint around peak loads effectively baked about 20 years of operational life into the structure, and in turn made it difficult to save money on a shorter-life design.

It would not surprise me if many of the non-ablative parts in a rocket are in fact fairly durable without the stop-start cycles. So whilst a launch may only take 5-8 minutes, I could totally believe that a 15-20 minute launch wouldn't demand heaps more from the parts.

Of course, this absolutely doesn't translate to ablative parts, or items that undergo stop-start cycles. The latter of course is where the devil is for reusable equipment. Depending on the failure modes in question for turbo pumps and throttling controls, those may or may not apply.

Assuming single-use. Reusable rocket engines are different beasts.

Engineer it for 30 minutes and swap it regularly (SpaceX swaps turbines and pumps often enough because they do break down after only one or three launches)

Saturn V at take off had an equivalent power of 166GW. If that was electricity it would be around 2x the total capacity of all the power stations in the UK.

To nit-pick, that's comparing heat production with electrical power. If we assume the power stations are 50% thermally efficient (which is a bit optimistic) the power in "heat" terms of the power grid would then be about the same as the Saturn V.

Rocket engines are extremely efficient. Especially in vacuum, they can convert almost all the heat into kinetic energy of the exhaust jet. They're the most efficient heat engines we have.

I wonder if anyone's experimented with using rockets to charge something like a flywheel battery or reservoir for hydro power. A 777's engine puts out tens of megawatts using a less refined fuel than rockets use. What if you hooked a rocket full of RP-1 to a pump to haul water from a river to a dam? This is probably nonsense.

You should keep in mind that, sure, the rocket produces 166GW of power(which is wild!) but it does so for all of 5 minutes (or less).

Whereas "all the power stations in the UK" keep on trucking, day in, day out, year after year.

A 777 engine is actually far more fuel efficient than a rocket engine. Take a look at specific impulse [0] for different engine types. A rocket is optimised to be very light and to use its own oxygen supply. A normal jet engine can be heavier and use the air as an oxidizer. Much more efficient for power production in a CCGT.

Rocket engines are actually very efficient, especially in vacuum. Very high expansion ratios can be achieved that converts almost all the heat into jet kinetic energy.

The turbopumps on the Saturn V first stage make about the same power as an aircraft carrier, over 200MW. Civilian electric power plants that make this much power are called "medium sized" and can power about 100,000 homes.

That's not the power output of the rocket (which someone quoted below). That's the power required to move the fuel and oxidizer into the rocket engines. Pumping the same amount of liquid can move a 100,000 ton floating city across the ocean at 40mph.

I shouldn't say the same amount of liquid, rather the same effort to pump water.

As I understand it, a huge factor in the power requirement of the pumps is raising the pressure to the required level which is very high (combustion chamber pressure might be hundreds of atmospheres).

Reminds me of fuel injectors of funny cars. Watch it go from just idling, to full throttle. Then remember there's 8 of them on the engine. https://www.youtube.com/watch?v=xGTbQuhhluY

I got to stand next to one of these dragsters during a launch and suffice to say it was rocket like.

I got pretty close to the jet powered big rig in norwalk. It blew the flags off the sign and then the sign eventually fell over. https://www.youtube.com/watch?v=VBdvooWanJw Seems like it happens often.

This is why electric turbopumps (like those used on RocketLab's Electron rocket) don't scale up to larger rockets well - the power draw is just infeasible to support with current battery technology.

Not quite. Electric pumps scale just fine (that is, linearly), but turbopumps scale better. It's hard to build a very small turbopump but not much harder to build a larger one, and turbopumps improve in efficiency as they get larger. BTW, the largest electropump (4 times that of RocketLab’s Rutherford electropump engine) for a rocket engine is the electropump for the 100kN (10 ton) thrust rocket engine for the reusable crewed suborbital Spica space rocket by the volunteer-run Copenhagen Suborbitals group, which more people ought to know about:


The major issue is the energy storage, not the power plant itself. The penalty of the battery mass scales far worse than a tank holding very energy dense rocket propellant.

Again, the energy storage scales just fine: linearly. Rocket propellant tends to scale better than linear, but it is not in every case better than lithium ion batteries! For example, the R7/Soyuz rocket family is the most-launched orbital rocket ever, and it uses a hydrogen peroxide gas generator to drive the turbopump. That has a concentration of 82.5% peroxide. Pure peroxide has a heat of decomposition of 2.84MJ/kg, and turbine that is typically 30% efficient (actually, it might be much less than that… I think the V-2 turbine was only like 10% efficient, maybe worse… so 30% is optimistic) gives you a usable energy density of only 700kJ/kg, or about 194Wh/kg. The best lithium batteries available are about twice that, up to 400-500Wh/kg (with those in the lab even better still), and electric motors can have 90-95% efficiency.

And gas generators using main propellants are better, certainly, but less than you might think because they have to haul all their oxidizer with them (unlike aircraft) and are also usually run very far from stoichiometric (maybe just 0.3 O:F ratio compared to a stoichiometric 3.4) to keep the temperature down. So unless you have a pretty high temperature turbine, you might not beat peroxide by much!

So the easiest gas generators have worse energy density (keep in mind RocketLab does stage off batteries if necessary…), and the next easiest, while better, aren’t MASSIVELY better without careful efficiency improvements. The real efficiencies come when you use like an expander cycle or a staged combustion cycle or you feed the gas generator exhaust back into the nozzle like Merlin Vacuum or F-1. And those are all much more complicated. A level of complication that is not worth it for small rockets but is for larger.

So it’s really not about electric scaling poorly (electric scales just fine) but about the greater complexity of better engine cycles being worth it at larger scales.

> The real efficiencies come when you use like an expander cycle or a staged combustion cycle or you feed the gas generator exhaust back into the nozzle like Merlin Vacuum or F-1.

If I remember correctly, with F-1 the gas generator exhaust was sent to the nozzle to cool the nozzle, not to add efficiency to the engine main cycle.

We're still trying to get more efficiency from isochoric combustion, but the expected wins aren't too big. It's good that full-flow combustion becomes more of a norm.

That as well. But the extra mass accelerated by entrainment (instead of dumped overboard at low velocity) is still helpful for increasing efficiency.

Oh no, CopSub… in hindsight they should’ve just launched it that time. Then everyone would have been largely happy.

Yeah, it's pretty amazing. Doing this in a controlled fashion is the hard part; a largish wooden building on fire can also dissipate 10 GW.

A .22 LR rifle bullet might acquire 200 J in 2 ms, which means the firing gun is producing 100 kW mechanical, plus probably another 300 kW thermal. So another way of thinking of this is that an engine dissipating 10 GW is equivalent to something like 25000 handguns firing at once, without ever stopping.

It sounded incomprehensible to me too but as I did some more learning about the process of building rocket engines I learned some interesting details. First, remember that people have been making high strength metals through careful processing for thousands of years. Second, the parts of engines are not made as part of large-scale industrialized manufacturing. Almost all the parts are made as few-offs, with far more energy, time, and effort put into making sure that a single instance of something is extremely reliable. Third, we got damn good at materials science in the past 100 year, and metals can be absurdly resistant to deformation under heat.

The alloy on the oxygen side pump in the Raptor engine is a work of magic and or art. An oxygen rich turbopump runs so hot it was thought impossible to create because the turbine would fail before the burn was over. SpaceX had to invent the alloy before the first Raptor full duration fire. That's a very low level, never done before, breakthrough required just to get to the real hard stuff.

edit: if you watch gas generator tests on youtube the gas coming out is dark because it's very fuel rich which keeps it cool (in a relative sense)

> SpaceX had to invent the alloy before the first Raptor full duration fire. That's a very low level, never done before, breakthrough required just to get to the real hard stuff.

Russians had oxydizer-rich gas generators in large engines since e.g. early 1960-s (see Proton 1st stage engines). Oxygen-rich gas generators are at least since mid-1980-s (RD-170). So not exactly never done before.

Bet the Russians are not revealing their alloy...

Russians sold RD-180 with technology to reproduce them in USA. It was just too expensive to make them in America, so eventually Energomash got all orders to make them for Atlases. And now Falcons are pretty good too, so less need in RD-180 - Orbital still buys RD-181, single chamber engine though. And Raptors could be in use soon. And BE-4 from Blue Origin hopefully will power new ULA Vulcan rockets soon too... and that oxygen-resisting metallurgy is likely used at least in some of them (RD-181 for sure).

The Russians cold war era rockets was so far ahead of anything USA produced that old rockets from that time is still being bought up used today.

bet they even sell it to you

They sell it and they deliver it but they’ll make extradite it’ll be manufactured by their laborers I bet.

It's because of the liquid oxidizer.

A jet engine can't have such power density because it uses gaseous air which is about 1000 times less dense than liquid oxygen and only contains 20% oxygen.

Everything follows from that.

The pumps are not challenging temperature wise since they pump cold liquids.

The turbine is challenging, but the temperature can be limited by varying the ratio of propellants in the preburner (very lean or very rich means lower temperature). If you use lean, then it's a very oxidizing environment. If you go very rich, there's soot (if you use fuels with carbon).

And the chamber is not so challenging because there is so much cool liquid available for cooling.

You can boil water with a candle and a paper cup.

A high performance jet engine is a harder problem than a medium performance rocket engine.

They also only run for a few minutes.

SSMEs are reusable.

Designing for thermal cycles and serviceability[0] is at least as difficult a problem as running a hypothetical rocket engine an equal amount of time in one longer, hotter burn.

(Such a design isn't needed and wouldn't be practical, but then again multiple aspects of SSMEs being reusable turned out not very practical either, depending on what version of design criterea you evaluate and how the expected vs actual usage changed over the lifetime of the program.)

[0]In both the engineering sense, as durability of the various loading cycles (ie lifetime turbine rotations or number of thermal cycles before eol or failure), and as being constructed as able to undergo maintenance and refurbishment between launches.

> SSMEs are reusable.

Not as much as RL-10s. SSME you can disassemble - because you should do that, as thermal stresses on turbine blades are too dangerous, so you have to periodically replace the parts which are nearing the fault.

Engines in dragsters are also know to have very high power comparing to engine size, but they also make less than 10k rotations at full power before they fail. That is enough to last one drag race which is several seconds.

The top fuel record is around 3.5 seconds and at 9500rpm is around 600 revolutions for the whole race!

If that interests you then this series of videos is definitely worth watching. It details several parts of the German V2 missile, the grand father of all modern liquid propellant rockets. This one is about the turbopump which is one of the most interesting parts.


I had no idea they individually manufactured those blades on the steam turbine... the number of parts that little factory had to crank out was astounding. I can see why the Russians simplified it to a bent piece of sheet instead.

Considering all the manufacturing, precision manufacturing involved the V2 effort, it's astounding they could do it at all.

Don't think of it as "making horsepower", think of it as "resisting the forces upon it".

A gear tooth the size of your thumb pulls a semi truck up a mountain.

Also try to imagine the thermal stresses when you have cryogenic propellants on the pump side and hot exhaust gases (gas generator or staged combustion) on the turbine side!

With "fire wall" of the engine, over the distance of a millimeter you have full temperature of the chamber - that's thousands of Kelvins, not just gas generator temperatures which are lower because otherwise turbine blades will melt - and relatively low temperature of the coolant, and also mechanical stress of high pressure in the cooling channels and lower pressure in the thrust chamber.

You don't have the full temperature of the chamber b/c of the film cooling discussed in the article. Also, the chamber doesn't spin at tens of thousands of rpm.

Film cooling could be absent, though the temperature distribution is still such that layer next to the wall is cooler than the center of the flow. Chamber doesn't spin, yes, but gas on the turbine blades has mostly dynamic effect - that's how the turbine works - while at the chamber wall the pressure difference is mostly static - relatively low liquid flow speed in the channels and relatively low (tens of meters per second, compared to hundred meters per second at the blades) gas flow speed at the chamber, so the wall has to withstand the difference without moving.

Agree that these are rather different stress modes, just wanted to point to another place in the engine with pretty extreme stresses.

Sure. But another key feature of how the turbine works is -- that it spins. That means that you need bearings and seals that can not only withstand but actually work effectively under those conditions.

The turbo pump is the main barrier to entry when making a large rocket engine. There's so much energy per unit volume that there is no failure mode that does not result in an explosion. Also keep in mind, you have very hot gas on one side and a few inches over cryogenic liquid oxygen. Liquid oxygen turns anything in to fuel so think about how to lubricate that shaft let alone seal it.

Another interesting thing to think about, it's the turbopump keeping the combustion from running back up the injectors. So, the turbopump has to outperform the combustion chamber in terms of pressure and flow rate.

> Liquid oxygen turns anything in to fuel

Not quite literally, in e.g. Soviet engine designs some gas generators are oxygen-rich, they produce oxidized exhaust of high temperature, and the engine still doesn't burn (usually).

> So, the turbopump has to outperform the combustion chamber in terms of pressure and flow rate.

Yes, the injector pressure drop can't be too small.

Robert Goddard had terrible difficulties with fuel/oxidizer pumps. It wasn't because he was not smart enough. Most of what was needed hadn't been invented yet.

It is pretty mind-boggling. Makes the average turbofan's turbine assembly look like child's play, and those are also pretty ridiculous in terms of power-to-weight ratio.

It's mostly because the working fluid is at very high pressure (much denser than the air coming through an aero engine). The turbine side works with hot gas at 100+ bar, and the pump side is dealing with liquid. Therefore small parts can exert a lot of force (large pressure differences) and do a lot of work (high speeds).

The most similar technology is a boiler feed pump (used to feed the boilers in a steam turbine system, e.g. in a ship or power station) - these work in a similar way, high pressure steam is bled off from the system to drive a small turbine, which drives a high pressure pump to feed the boiler, which feeds the 'main' steam turbine. Similar pressure (>100 bar), but the turbine on the rocket engine needs to handle much higher temperatures (hot gas rather than steam).

Turbofans can be more complex than rocket engines - in the rocket engine thrust chamber you don't have moving parts, but with turbofans the whole engine is like a big turbopump, with air compressor instead of centrifugal pump.

Sure, and in the gas turbine combustor there are also no moving parts. But there’s lots going on outside.

Well, 63 blades together make 600 horsepower. But as that paper notes, each blade is subject to 50,000 psi which is wild.

It is wild, but not as wild as you might think.

The fuel rail in a modern diesel engine is operating at 25-30,000 psi all the time.

I know, and that is wild too!

600 horsepower each. The whole turbine has tens of megawatts of mechanical power output.

Yep, you are correct. I misread.

Minor nitpick: fluids are not either liquids or gases, this is particularly true for rocket engines where many of the discussed processes (injection, compression, regenerative coolant flow) actually occur at super- or transcritical conditions.

Also, I don't think you can say the faceplate is heat sink-cooled. Remember that just behind it is the propellant manifold, so it's rather some form of regenerative cooling.

Everyday Astronaut is such an impressive dude, I don't understand why he doesn't just work for SpaceX at this point. He knows more about rocket engineering and can explain it 10x better than most aerospace new grads.

There's a big difference between researching a topic to a standard good enough for 'popular rocket science' and 'understanding the maths well enough to work in the field'.

Has he built prototype rocket engines like Integza, Ben Krasnow, or Tech Ingredients? They're still primarily divulgadores (there isn't a good word for this in English) but they have a certain amount of practical experience getting things to work.

OTOH at the point that you're EDM-drilling thousands of micron-scale holes in your combustion chamber for film cooling, you may start to need practical experience with different things.

I've never heard the term _divulgadores_ (I don't speak Spanish, so that's no surprise), but it sounds maybe like "science communicator" in this context? There's something a bit more to the people you've listed in that they are also entertainers, not straight educators, I don't know if that's wrapped up in divulgadores as well?

It comes from the word "vulgar", meaning "common", like vulgus, the common people, for whom Jerome wrote the Vulgate (since they didn't read Greek or Hebrew). English "divulge" is a cognate, but as in many cases, the English word has experienced larger meaning shifts than Spanish. (Pidgins commonly have high rates of change.) The image I have is something like someone coming down from the ivory tower to bestow the blessings of their wisdom upon the jostling masses teeming without.

Yes, straight educators are not generally considered divulgadores; if someone is teaching a university class on linear algebra, that doesn't make them a divulgador. Unfortunately https://dle.rae.es/divulgar is not very helpful, but https://www.etymonline.com/word/divulge gives a bit of the flavor.

The Italian divulgatore is translated as 'popularizer', which works pretty well.

Hmm interesting, thank you! It sounds a bit like "vernacular design" used to kind of mean "design and manufacturing done by the common people" to meet simple needs rather than overly fancy mass-manufactured objects. "Vernacular" kind of means "the local language".


He is definitely knowledgeable, but strikes me as too much of a fanboy to be taken seriously. I have a hard time taking his videos all that seriously. And it's not about his knowledge, it's about his presentation.

I take him seriously (although frankly there's no requirement that it all be serious) and enjoy some of his stuff but it's true that sometimes the screaming glee level and emotional outbursts are way beyond my comfort zone, landing somewhere between cringe and get me out of here.

Reminds me of the launches I have seen where I really wanted to soak in the thumping sound of the rocket engines, but the people nearby are just screaming at the top of their lungs… "America!!!" as if that's helping anything at all.

He's also friends with a previously Tesla-focused YouTuber (name redacted, but rhymes with "Ken Mullins") who imho sold out to affiliate and sponsorship deals to the point where I lost all respect for him (the friend) and can no longer trust a word he says. So I have a suspicious eye on Tim's fire hose of cash harvesting mechanisms, but he seems to be staying legit so far. And he does have fantastic content. Also I'm not saying he doesn't deserve the cash he's earning; he puts it to good use and has a growing staff, as I understand it.

Classic SpaceX fanboys, always making 90 minute documentaries about the history of every Russian rocket engine.

If you can pick your video to base your assessment of my comment on, then so can I:


And by the way, I never mentioned SpaceX

I watched the first 5 minutes of that video and it just seems like someone talking about something they’re interested in, in a relatively normal way. He’s definitely a spaceflight fan but I didn’t see anything that unusual in the part I watched

Did you link the wrong video? You linked a top 10 list that SpaceX did not win.

Take seriously? Ok let’s use his last Russian rocket program history piece….what exactly did you feel was factually inaccurate about it?

Weren't you aware? learning is #SeriousBusiness, no fun allowed.

It's not about fun it's about getting past the "ZOMG SPACEX!!!!!" stage.

I know you are getting a lot of snarky comments, but honestly I know what you are saying. I can't watch his channel for whatever that gushiness thing is.

Worth checking it out again, he toned it down a lot recently IMO. Still prefer Scott Manley, but only astronaut gets the level of access like that. I mean Elon showed him basically everything in front of a camera.

There's definitely reason to watch his channel, and that particular two part series was one I consumed in whole. Loved it. HUGE takeaways from it.

I appreciate that. Thanks.

It is a different type of learning. Eg overview and fun facts vs deep understanding.

Different strokes for different folks.

> And it's not about his knowledge, it's about his presentation.

They're not quibbling with the factual content.

I never said anything about his accuracy.

I guess some people's exuberance can turn people off, but honestly its great in my opinion. Why should someone's joyful exuberance for something make his educational content not worth watching? The guy is obviously just very passionate about spaceflight and people who are pushing the boundaries of science/technology. I think 99% percent of the people I talk to approach this kind of stuff with mundane indifference and cynicism, so I think seeing someone genuinely passionate and excited by it is a breath of fresh air.

I sincerely appreciate his passion. It just goes over the top for my personality and keeps me from really getting into it.

Yeah fair enough, I totally get that.

I find he toned down the fanboy aspect a bit. A few years ago, he could have been part of SpaceX marketing department.

He still loves SpaceX, and to be fair, who doesn't. You may not like Elon Musk, his fanboys, the outrageous claims, and the way the company is run, but most of the exciting news in rocketry for the last decade are about SpaceX. But for the last few years, it is clear that he makes some efforts to be impartial and focus on the technical aspects.

And you should watch the videos he made with Elon Musk when he visited SpaceX. I expected little more than an ad for SpaceX, and it turned out surprisingly technical and hype-free.

I did watch both those videos, and they were excellent! Well, Elon's parts were excellent. I still found the questions to be more on the fanboi side than the engineering side, and that's okay. It's his brand, he can do what he likes.

I prefer Scott Manley's approach to things, and so I watch him instead. And when he says that EverydayAstronaut has a great video, I go watch it :)

I understand the sentiment: he comes across as one of those "hype guys" who's always talking about get-rich-quick schemes, but he's honestly incredibly knowledgeable about rocketry. He toured Starbase with Elon and asked questions that belayed a deep understanding of the mechanics involved.

> too much of a fanboy to be taken seriously

I would be really interested to know why "caring too much" is a disqualification.

Based on what he's said elsewhere, it isn't "caring too much" that's the issue, it's just his style. Scott Manley isn't any less enthusiastic about space or SpaceX, but his style is a lot less 'hyper'.

Maybe a better comparison would be that Everyday Astronaut's style is a lot like 'new YouTube', fast paced, with a strong emphasis on being exciting etc, including the stereotypical big YouTuber slick studio setup etc.

Scott Manley's style is more of 'old YouTube' (fits given how long he's been doing things), a more casual and slow paced but still informative style in a sort of standard framework that isn't all too flashy, just casually recorded from his room.

Personally I too prefer Scott Manley's style over EA's, although I'll still watch EA for interesting enough content. With Scott's videos I can watch them essentially whenever I see them, while with EA's videos I feel like I can only enjoy them when I'm feeling energetic, as otherwise the style feels kind of draining.

In the end though, it's down to personal preference.

I completely agree, but the fanboy aspect isn't really my main sticking point, it's his tone that feels like he's teaching elementary schoolers something.

He's teaching literal rocket science to the masses.

Isn't that the point of the channel? Making astronomy accessible to "Everyday" people?

I've seen this kind of comment before about his presentation, and I don't really understand it. It's true that he's a fanboy and that's especially visible in his video interviews/visits with Elon Musk, but the style of those videos is markedly different from the more educational ones which are often trying to cover a topic with a lot of detail while remaining relatively accessible.

I watched the video version of the article linked here this morning, and don't recall any part that was fawning over anyone or anything in particular.

What is it about his presentation that puts you off?

I don't know what the right word is for it. I used "fanboy" but maybe there's some other better word. Somebody else above used "gushiness" I think and that sounds about right.

It's like having a friend who won't shut up about a new sushi place they found. Yes, the sushi is amazing. Yes, the staff is nice. Yes, the atmosphere is great. And I could enjoy it more if my friend would stop making a big deal about every little thing about it.

Edit: Just skipped through the video this thread is about and that vibe just won't go away. I don't feel like I'm being informed, I feel like I'm being sold. The information itself is very, very good!

For what it's worth, the Professor of Rock on YT has the same vibe for me.

Having a YouTube channel with 1M+ subscribers and videos that get 5M+ views each is a much more profitable and generally better gig than SpaceX employee #10,000+ working 60-80 hours a week in a high pressure culture.

Also... even aside from the $/year, having 1M+ fans (many quite enthusiastic) is a far stronger position to be in than having one boss whom you must keep happy at all times.

Small pedantic note: this article is authored by Claire Percival, not Tim Dodd.

I'm not familiar with the working dynamic - Tim very well could been involved in authoring this piece - but, credit where credit is due.

Edit: I partially recant this. The article includes a video by Tim.

As I understand (as a Patreon), in general, Tim writes the video and then one of his team write an article that covers what the video says only with more text to substitute for the lack of visuals.

For Pre-Launch Previews they are written first by a team member.

There's a distinction between wanting to do something and being something.

Finding joy in cooking does not mean you'll be happy as a cook. And some people like being a cook where the cooking part is tedious.

SpaceX sounds pretty nasty to work for. Long hours, machismo/sexist culture, aggressive management, and certainly not the best pay you can find.

He's probably happy where he is.

I worked there for a few years, on flight software. The culture depends quite a bit on what department you're in. I learned how to push back on schedule pressure and was able to strike a reasonable work-life balance, and the head of my department was very good at buffering us from management. My departure was uncommon in that I left on good terms while I was happy. There were other departments, though, that seemed to operate more like a fraternity, and plenty of burnt-out people.

He's probably making more money with his media that he would working there.

I don't actually think he's that impressive and I don't get people's praise for him. His videos have a lot of mistakes in them and while they're written in a way that dumbs down a topic well for an audience who doesn't understand the subject very well, that's his only major skill that's involved here. He's not especially smart or ingenious, he's just a good communicator. If he was to work somewhere it would be in a communications department, but he probably makes more from Youtube than such a job would provide.

That's why he doesn't work for somewhere in the industry, youtube makes him more. (He makes enough money to hire other people to write for him, as we can see in the linked article.)

Tim spends countless amount of time going through the scripts, read throughs, first recordings, and so on, with Patreons. I've been on many of these read throughs, usually with several engineers in the aerospace industry present, and it's meticulous how Tim makes sure any possible mistake is identified and rectified. Even then, once you upload, there's no editing of a video.

The goal is to bring the subject down to a level where everyday people can still follow. It's not meant to be a college course, so of course there will be some dumbing down.

That's just wrong and I think you've been sucked into the "distortion field" that surround him. I jumped to a random spot in this video and found several inaccuracies in seconds in how he described how the cooling process worked.

Making rocket engines by metal 3D printing has become popular. A rocket engine bell and combustion chamber has one big rigid part with lots of channels and voids inside. That's the ideal case for 3D printing. Much simpler than building the thing up by machining and welding together many individual parts.

Maybe the main problem is that you want different materials in different parts of the structure?

E.g., the inner wall of the cooling channel in the bell is better made of something weaker but more heat-conductive, as noted in TFA. And, different materials in the chamber, throat, and bell.

I suppose you could have multiple feeders, on an additive system. But there are various reasons to want to use laser sintering of powdered metal, instead. And bonding the different metals is its own challenge. You could vary the mix of powder dropped at different levels, and rate of motion of the laser to match. But the combustion chamber probably wants concentric layers.

Plenty of hard choices and problems to solve. Engineers earn their keep.

I remember reading in an article that without 3D printing they could simply not get the cooling channel geometry they needed on the Super Dracos on Dragon 2.

Without 3D printing the combustion chamber they would need to use other cooling channel geometry, makin ghr engine heavier, bigger and less efficient.

An alternate scheme is to stack thin metal layers into which holes have been cut, then diffusion bond them together.

There are different problems with 3D printing than with classical subtractive manufacturing. E.g. you need to get the supporting powder out of long thin passages in the cooling chamber - you don't have such problem with milling machine approach.

Tim's videos are always so well done! Even if you're not a rocket enthusiast there's tons to get out of this video.

Agreed. Tim was an old friend of mine in university, but we lost touch over the years. He was always a good, quirky, creative, and talented guy. He bought an old cosmonaut suit on ebay on a lark and started doing funny/silly photo shoots with it and then photo shoots at rocket launches and then real educational stuff. It's been wild to see it take off (pun intended).

Ignorant question: are solid-fueled rockets at all interesting, anymore? Do they have any advantages (e.g. simplicity of design) over the fancy throttle-able liquid-fueled engines?

Extremely simple (basically just one big rocket combustion chamber with the propellant already inside, no pumps or plumbing) and shelf storable and potentially extremely high thrust to weight ratio.

Useful for munitions and for one-time-use. I’d like to think we’re going to reusable rockets and not as much war, so I’d LIKE to think they have fewer uses, but…

Biggest drawback is Americans don’t like them built in volume and ITAR-free. They didn’t like it in free hands so much that they once did a civilian “military aid” to let an ally build a clone of then top of the line Delta II.

Looking back at history, maybe what goes on top actually do keep the peace in the world, so maybe we could safely go back to building solids at scale for peaceful space transportation. But not likely without an American nod, I would imagine.

Solids are not ideal for space exploration as they are difficult to control and reusing them is expensive (& thus usually isn’t worth it). I am fully on board to reusable liquid rockets, which can use extremely cheap propellants liquid liquid methane or liquid hydrogen fuel and (70-90% of the mass of propellant) liquid oxygen. These propellants are extremely cheap, about 25¢/kg of propellant on average, enabling very cheap launch costs, compared to around $5/kg in solid rocket motor propellant costs (and greater difficulty in reusing them and much higher dry mass and lower Isp). And the exhaust is really bad for the environment. The high acceleration and sometimes high vibration and inability to turn off in an emergency make them non-ideal for human spaceflight. And they’re basically use-once (with possibly of remanufacture) so you can’t acceptance-test the same motor like you can for liquids, and refueling them is a complicated process that can’t easily be done off-planet (unlike liquid rockets which can be and are refueled such as the ISS’s propulsion systems). So while there are some niche uses of them for space exploration (interplanetary kick stages for probes), from an energy, performance, safety, reusable cost, and ride comfort standpoint, I’m rooting for liquids.

I suspect their primary advantage remains shelf-stability at room temperatures, which will make them stay relevant for military applications, e.g. you don't want a cryogenics facility in your submarine or cruise missile launch platform.

Historically, I think they're cheaper than an equivalent disposable liquid fueled engine but don't hold up to the fully reusable designs of today, and from a reliability perspective there's not a lot of room between working-as-intended and "activate the flight termination system" at a total loss.

> I suspect their primary advantage remains shelf-stability at room temperatures, which will make them stay relevant for military applications

I while ago I read--but barely understood--a book that went into a lot of this: "Ignition! An Informal History of Liquid Rocket Propellants" by John D. Clark.


IIRC there were some cases where a fuel was not militarily-acceptable because you would need to warm/thaw the mobile missile prior to firing in a Russian winter, or other cases where a permanent missile-silo meant it was cost-effective to run heating/refrigeration all the time, etc.

> [I]n applications which do not require a low freezing point, hydrazine itself is fused, either straight or mixed with one of its derivatives. The fuel of the Titan II ICBM doesn't have to have a low freezing point, since Titan II lives in a steam-heated hole in the ground, but it does need the highest possible performance, and hydrazine was the first candidate for the job.


Another fuel-choice issue involves how badly it might self-destruct if anything unusual happened:

> [I]n the summer of 1960, we tried to fire a 10,000-pound thrust Cavea B motor. [...] Well, through a combination of this and that, the motor blew on startup. We never discovered whether or not the [detonation] traps worked—we couldn't find enough fragments to find out.

> The fragments from the injector just short-circuited the traps, smashed into the tank, and set off the 200 pounds of propellant in that. (Each pound of propellant had more available energy than two pounds of TNT.) I never saw such a mess. The walls of the test cell—two feet of concrete—went out, and the roof came in. The motor itself—a heavy, workhorse job of solid copper— went about 600 feet down range. And a six-foot square of armor plate sailed into the woods, cutting off a few trees at the root, smashing a granite boulder, bouncing into the air and slicing off a few treetops, and finally coming to rest some 1400 feet from where it started. The woods looked as though a stampeding herd of wild elephants had been through.

> As may be imagined, this incident tended to give monopropellants something of a bad name. Even if you could fire them safely—and we soon saw what had gone wrong with the ignition process—how could you use them in the field?

> Here you have a rocket set up on the launching stand, under battlefield conditions; and what happens if it gets hit by a piece of shrapnel? LRPL came up with the answer to that. You keep your monoprop in the missile in two compartments: one full of fuel-rich propellant made up to A. = 2.2 or 2.4, and the other containing enough acid to dilute it to X = 1.2. Just before you fire, a can-opener arrangement inside the missile slits open the barrier separating the two liquids, you allow a few seconds for them to mix, and then push the button.

They are useful when you need a rocket that can take off at a moments notice but sit idle for decades at a time. For typical rocket launches they don't make much sense.

I suspect the Space Shuttle SRBs were chosen because they were a handout to ICBM manufacturers.

Depends on the purpose. E.g. for military uses like ICBMs they're quite important...

Interesting enough to still be of use. Japan has a three-stage solid-fuel rocket[1] for example, with several launches planned.

[1]: https://en.wikipedia.org/wiki/Epsilon_(rocket)

Two of them just helped get JWST into space.

Depends on your definition of "interesting". If you mean do people still use them for practical purposes, then yes they are. If you mean are people still researching them for use in exploring space, then probably not.

Not really. Gel propellants are a new direction.

This is just a really clearly written introduction of a lot of rocket engine concepts.

It was a super good read.

Anybody interested in this topic might want to read the NASA Saturn V Owners' Workshop Manual: 1967–1973 (Apollo 4 to Apollo 17 & Skylab) by David W. Woods. It's an incredible collection of information, tables, and drawings relating to the Saturn V's development and operations.

a bit related - modern turbofan engines are running extremely hot and to avoid damage/melting to the turbine blades the blades are cooled from inside and with the boundary layer by the cooling air flowing out of the holes in the blade



In fuel-rich engines can't the fuel be harvested after it does its function of cooling the chamber walls at the end of the nozzle. There are cases where fuel is boiling before it's ignited so I don't think the heat will be an issue. If it's not possible to have a pipe that transfers the fuel from the end of the nozzle back to the tip of the engine because of re-transfer of heat can't there be a secondary small engine near the end of the main engine that directly gets its fuel from the excess fuel used to cool the main engine?

All the fuel goes through the cooling channels before injection into the thrust chamber, typically. So what you're talking about there is standard practice.

Not in those type of engines. I meant the type of engine that creates a layer of fuel on the outside walls that gets pushed out with the exhaust gases.

Serious question: why is it bad to run your rocket engine-rich? If you know you're not going to use the engine and it's nearing its end of life why not get more power out of your fuel/oxidizer by hitting the stoichiometric point? Also, if engine is melting maybe allow it to continue to melt, but at a lower rate and rebuild it later.

The optimal (in the sense of maximum specific impulse) operating point of a liquid propellant rocket engine is typically fuel rich. This is because with stoichiometric fuel/oxidizer ratio, the temperature is so high that there's significant dissociation, which robs the gas of some of the heat of combustion. Going a bit fuel rich wins back some of this, enough that the reduction in average molecular weight of the gas from inclusion of more hydrogen atoms is a net win.

An exception to this is if your oxidizer is hydrogen peroxide. Peroxide/hydrocarbon engines typically optimize to stoichiometric, because the flame temperature is lower. If you look at photos of that British launch vehicle, the Black Arrow, that used peroxide, the rocket exhaust jet is radiating very little visible light, because most of the carbon has been oxidized.


The turbopump, regenerative cooling, and boundary layer cooling by drilling holes, were innovations from the V2.

All of that was greatly improved over time. Centrifugal pump was taken by von Braun from fire engines, boundary level cooling wasn't as good as transpirational cooling, and regenerative cooling was with little supports, which required thick and strong fire wall, so they only could use 70% ethanol.

Still - hats off to the pioneers. It's always pretty hard.

I would hope that it would have improved over the last 80 years :-/

The V2 had other foundational inventions - the ballistic missile shape, and the gyroscopic guidance system.

I forgot to add that they also added baffles to prevent pogoing, a technique that persisted at least into the Saturn V engines.

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