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Engine cooling – why rocket engines don’t melt
- jamesmunns 5y agoThis is just a really clearly written introduction of a lot of rocket engine concepts. It was a super good read.
- opwieurposiu 5y agoThis 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. https://www.enginehistory.org/Rockets/SSME/SSME6.pdf https://www.enginehistory.org/Rockets/SSME/SSME6.pdf
- bernulli 5y agoAlso 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!
- avmich 5y agoWith "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.
- bernulli 5y agoYou 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.
- avmich 5y agoFilm 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.
- bernulli 5y agoSure. 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.
- jaywalk 5y agoWell, 63 blades together make 600 horsepower. But as that paper notes, each blade is subject to 50,000 psi which is wild.
- TylerE 5y agoIt 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.
- BenjiWiebe 5y agoI know, and that is wild too!
- jhgb 5y ago600 horsepower each. The whole turbine has tens of megawatts of mechanical power output.
- jaywalk 5y agoYep, you are correct. I misread.
- HPsquared 5y agoI 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.
- 7952 5y agoSaturn 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.
- Kye 5y agoI 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.
- adewinter 5y agoYou 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.
- 7952 5y agoA 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.
- throwaway0a5e 5y agoDon'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.
- dekhn 5y agoIt 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.
- dokem 5y agoThey also only run for a few minutes.
- yetihehe 5y agoEngines 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.
- snypher 5y agoThe top fuel record is around 3.5 seconds and at 9500rpm is around 600 revolutions for the whole race!
- beerandt 5y agoSSMEs 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.
- VBprogrammer 5y agoIf 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. https://youtu.be/EgiMu8A3pi0 https://youtu.be/EgiMu8A3pi0
- mikewarot 5y agoI 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.
- kunai 5y agoIt 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.
- HPsquared 5y agoIt'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).
- avmich 5y agoTurbofans 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.
- bernulli 5y agoSure, and in the gas turbine combustor there are also no moving parts. But there’s lots going on outside.
- chasd00 5y agoThe 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.
- avmich 5y ago> 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.
- ncmncm 5y agoRobert 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.
- deleted 5y ago[deleted]
- mountainwalker 5y agoTim's videos are always so well done! Even if you're not a rocket enthusiast there's tons to get out of this video.
- throwaway894345 5y agoAgreed. 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).
- bernulli 5y agoMinor 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.
- Dig1t 5y agoEveryday 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.
- criley2 5y agoSpaceX 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.
- sgtnoodle 5y agoI 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.
- geocrasher 5y agoHe 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.
- texasbigdata 5y agoTake seriously? Ok let’s use his last Russian rocket program history piece….what exactly did you feel was factually inaccurate about it?
- ceejayoz 5y ago> And it's not about his knowledge, it's about his presentation. They're not quibbling with the factual content.
- Animats 5y agoMaking 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.
- m4rtink 5y agoI 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.
- avmich 5y agoThere 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.
- pfdietz 5y agoAn alternate scheme is to stack thin metal layers into which holes have been cut, then diffusion bond them together.
- ncmncm 5y agoMaybe 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.
- ortusdux 5y agoOne 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. https://youtu.be/ac-V8mO0lWo?t=2203 https://youtu.be/ac-V8mO0lWo?t=2203
- jpm_sd 5y agoIgnorant 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?
- anarazel 5y agoDepends on the purpose. E.g. for military uses like ICBMs they're quite important...
- j8asic 5y agoNot really. Gel propellants are a new direction.
- evo 5y agoI 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.
- Terr_ 5y ago> 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.
- WalterBright 5y agoThe turbopump, regenerative cooling, and boundary layer cooling by drilling holes, were innovations from the V2.
- avmich 5y agoAll 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.
- WalterBright 5y agoI 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.
- WalterBright 5y agoI forgot to add that they also added baffles to prevent pogoing, a technique that persisted at least into the Saturn V engines.
- trhway 5y agoa 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 https://www.researchgate.net/figure/Evolution-of-turbine-entry-temperature-and-future-trend_fig4_221919089 https://www.researchgate.net/figure/Evolution-of-turbine-ent... https://www.youtube.com/watch?v=4FqMLyo7fyU https://www.youtube.com/watch?v=4FqMLyo7fyU
- soheil 5y agoSerious 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.
- pfdietz 5y agoThe 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. https://ichef.bbci.co.uk/news/976/mcs/media/images/55155000/jpg/_55155523_s2000072-launch_of_a_british_black_arrow_rocket.jpg https://ichef.bbci.co.uk/news/976/mcs/media/images/55155000/...
- soheil 5y agoIn 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?
- pfdietz 5y agoAll the fuel goes through the cooling channels before injection into the thrust chamber, typically. So what you're talking about there is standard practice.
- soheil 5y agoNot 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.
- shmeeed 5y agoAnybody 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.