[HN Gopher] How do you power a rocket engine? ___________________________________________________________________ How do you power a rocket engine? Author : Jarlakxen Score : 154 points Date : 2022-04-30 11:24 UTC (11 hours ago) (HTM) web link (everydayastronaut.com) (TXT) w3m dump (everydayastronaut.com) | pontifier wrote: | I've often wondered if "gravity feed" might be an option. In a | tall liquid fueled rocket, Gravity, then thrust, could create | arbitrarily high pressures at the nozzle if the column of fuel is | tall enough. | simne wrote: | Very good article, but have few shortcomings. | | 1. Only first generations of "Soyuz" used H2O2 propellant, | because it have very limited time before use (for "Soyuz", | guaranteed 6 months), and because it have relatively high melting | temperature. | | First chose H2O2, because "Soyuz" planned as independent ship, to | fly relatively short missions around Moon (it is near impossible | to withstand even 6 months in so small volume). | | When "Soyuz" primary role become companion ship for space | station, it switched to hydrazine type propellant. | | 2. In pressure fed engines used almost all possible propellants | and oxidizers. This is not error just clarification. | | 3. Exists three-propellant engines. For example, exists soviet | engine for "spiral" system, which used kerosene+LOX+LH. First it | run on mostly kerosene+LOX, with small percent of LH, than | switched to pure LOX+LH (sure, LOX share also other). | | 4. Exists simpler bipropellant engines than mentioned, | unfortunately with worse efficiency. - First Britain satellite | flown on H2O2+RP1 Black Arrow rocket. | avmich wrote: | Soyuz rocket uses 82% H2O2 for all first and second stage | engines. | | Soyuz spacecraft uses H2O2 for capsule control during reentry. | | Soyuz spacecraft uses a variant of hydrazine fuel for orbital | maneuvers. | | All of this is true for all versions of Soyuz rocket (except | Soyuz 2-1v) and Soyuz spacecraft, since 1950-s to today. | phkahler wrote: | Has anyone ever tried to capture outside air and mix it in the | rocket exhaust? The reason would that more mass expelled at lower | velocity can produce the same thrust with less energy. | | Momentum is MV while kinetic energy is 1/2 MV*2. what a rocket | really needs is change in momentum, so high energy exhaust. | NickNameNick wrote: | Are you thinking of Air augmented rockets? Which capture and | entrain surrounding air into their exhaust. | | https://en.wikipedia.org/wiki/Air-augmented_rocket | | I have the impression that they see periodic use in various | categories of mid-size military missiles. | phkahler wrote: | Missed the edit window. Does not need high energy exhaust, it | needs high momentum exhaust. | panick21_ wrote: | Yes. See: | | https://en.wikipedia.org/wiki/SABRE_(rocket_engine) | | https://en.wikipedia.org/wiki/Skylon_(spacecraft) | | But there are lot of problems with this and it adds a huge | amount of complexity. | phkahler wrote: | Maybe too small an effect, but what if they put 8 NACA scoops | on the sides of Falcon 9 and just fed the air in between the | center and outer engines. It would get accelerated out with | all the regular exhaust, and I suppose would cause a small | back pressure on the center engine in particular, but they | could throttle it slightly if that were a problem. | gameswithgo wrote: | xbmcuser wrote: | If you are interested in rockets I would recommend this youtube | channel. https://www.youtube.com/c/Integza | | It is a great resource to learn about all different kind of | rockets and rocket engines. And quite amazing to see how 3d | printing can allowto build so many of miniature versions. | SCNP wrote: | +1 for Integza. You should probably include a disclaimer that | it is backyard engineering at its finest. He doesn't really do | a lot of hard science in his videos. He has great ideas and the | persistence to get them at least semi-functional. | speedylight wrote: | Integza is one of my favorite channels of all time, his videos | are always really informative and fun to watch. The only | problem is he's completely careless about safety--mainly by not | wearing any protective gear for the exhaust fumes which include | burned plastic. | showerst wrote: | I'm really torn on Integza. It's a great channel and I love | his spirit, but guys like that (and the teenagers that follow | them) end up getting hurt or burning down their houses, which | makes the news and gets new restrictions passed on all of us. | | Just a _little_ more nodding to safety would make him an | awesome resource. | | Maybe safety culture in Portugal is different, but in the US | he does tons of stuff that violates the model rocket safety | code, or is outright stupid. | speedylight wrote: | You're exactly right it would go a long way. Thankfully | people in the comments do call him out on it so hopefully | he'll listen. | Robotbeat wrote: | I wouldn't consider the rocketry code itself a good | argument. The model rocketry code, frankly, is biased hard | against any kind of liquid (especially non-hybrid) rocket | engines, so there's not even really a way he could follow | it with a liquid in any reasonable way. | | (Which isn't to say he couldn't follow better safety common | sense.) | numpad0 wrote: | There is something that separates Integza, colinfurze et | al, and Applied Science, Huygens Optics et al, and another | in guys making thesis/paper worthy developments like | [0],[1]. They are technically all amateurs, in the sense | that their performances do not involve organizational | fundings or decisions, and I have to admit that | entertainment value decreases in the order I mentioned | these channels, but it seems to me as if there is a sloped | ceiling for technical excellence of YouTubers, proportional | to their popularity, and it isn't calming to think about | it. | | 0: "My recreation for Yamanote line | trains(E231-500/E235-Series) using self made VVVF inverter" | https://www.youtube.com/watch?v=i0B2bvd9rFQ | | 1: "I put a biped robot on a bicycle" | https://www.youtube.com/watch?v=SqBw7XapJKk | kortex wrote: | Likewise. I love his channel, but I'm kind of amazed he | hasn't lit his dwelling on fire by now. | ncmncm wrote: | I wonder about screwing a solid slug of aluminum down through the | top of the combustion chamber during the first few seconds of | launch, to get more massive exhaust particles (Al2O3) when you | need that most. The top of the slug would be something refractory | to seal the hole when the top end hits the stop. | | The energetics of burning aluminum are pretty favorable, enough | so that there are solid rocket boosters that use it. 1500 C is | hot enough to liquify aluminum pretty vigorously, but not boil | it. | melony wrote: | Doable, but one mistake in modeling and the whole thing goes | boom. I like the idea, it reminds me of detonation engines. | ncmncm wrote: | Yeah, but they always want to go boom. That's what makes it | rocket science. | | I was taken by the the idea of a lead screw as a fuel pump. | questiondev wrote: | it's going to be interesting to see personal rockets one day | complete with regulations and all the ins and outs of that | industry becoming mainstream | Synaesthesia wrote: | How will we ever afford the colossal amounts of fuel that will | use, and think of the pollution! | alangibson wrote: | > think of the pollution! | | Hydrolox engines only emit water | Toutouxc wrote: | Can someone ELI5 to me the physics behind the divergent part of a | de Laval nozzle? I know that in a straight pipe subsonic flow | tends to accelerate towards M=1 and supersonic flow tends to slow | down towards M=1, I know what choked flow is and generally how | the convergent-divergent design works, but for the life of me I | can't find anywhere an understandable, non-hand-wavy explanation | of WHY supersonic flow in a divergent nozzle does what it does. | Cerium wrote: | I have never really looked into it before myself, but this | explanation seems to make sense: | https://www.grc.nasa.gov/www/k-12/airplane/nozzled.html | | Quote: On the other hand, if the converging section is small | enough so that the flow chokes in the throat, then a slight | increase in area causes the flow to go supersonic. For a | supersonic flow (M > 1) the term multiplying velocity change is | negative (1 - M^2 < 0). Then an increase in the area (dA > 0) | produces an increase in the velocity (dV > 0). This effect is | exactly the opposite of what happens subsonically. Why the big | difference? Because, to conserve mass in a supersonic | (compressible) flow, both the density and the velocity are | changing as we change the area. For subsonic (incompressible) | flows, the density remains fairly constant, so the increase in | area produces only a change in velocity. But in supersonic | flows, there are two changes; the velocity and the density. The | equation: | | - (M^2) * dV / V = dr / r | | tells us that for M > 1, the change in density is much greater | than the change in velocity. To conserve both mass and momentum | in a supersonic flow, the velocity increases and the density | decreases as the area is increased. | Toutouxc wrote: | That's why I was asking for an ELI5 explanation. I know that | the equation holds and that compressibility turns everything | upside down, I just haven't been able to figure out an | intuitive explanation. | | I have this idea that in supersonic flow, a pressure wave | can't move backwards against the flow, right? Which would | mean that any single molecule inside the flow has no way of | knowing what's in front of it (because the information simply | can't get there), but it feels the pressure of the molecules | behind it, so it accelerates towards the void. | | The "Fanno flow" article on Wikipedia says that "... For a | flow with an upstream Mach number greater than 1.0 in a | sufficiently long enough duct, deceleration occurs and the | flow can become choked ... Conversely, the Mach number of a | supersonic flow will decrease until the flow is choked.", | which means that supersonic flow behaves differently in a | diverging nozzle than in a simple straight pipe. This is the | part that I don't understand. Is the friction inside the | nozzle somehow inhibited by the walls of the nozzle gradually | moving out of the flow's way or something? | namibj wrote: | Basically, it would already work if you left out the diverging | part. However, the exhaust has still excess pressure. You want | to make it do work while expanding to ambient pressure, so you | provide a ring around the throat where the exhaust pressure | pushes against to direct the exhaust momentum vector for every | part of the exhaust plume to be as close to retrograde as | possible. Sideways momentum is wasted momentum. | | To actually get it to push against your ring, the diameter | can't increase too fast, because the sideways flow velocity of | the exhaust inside your bell needs to be subsonic. So you first | increase steeply, because it's still high pressure and thus hot | and thus has a high speed-of-sound, and gradually reduce how | fast you increase (gradually reducing the taper). | | Eventually you have expanded it to a pressure equalling ambient | pressure, and won't get more thrust from further expansion | (doing more also causes flow separation at the edge where | ambient air pushes the plume radially inwards and separates it | from the inside surface of the bell. The supersonic shock | effects can break your bell if you're not careful.). | | Also at low pressure you get little thrust per additional | nozzle area, while the low temperature requires a low taper | that requires a lot of surface for the additional nozzle area. | That is why even vacuum engines don't go down to extremely low | exhaust pressure. | | IIRC you can only ever double your momentum with an expanding | nozzle, even in the asymptotic limit of an infinite bell in | perfect vacuum and compared to a knife edge throat. | | Fundamentally, rocket nozzles are weird open-cycle heat engines | subject to the Carnot limit. Thrust times (exhaust) velocity is | power, and the chamber (combustion) temperature your hot side. | If your exhaust has a different molecular mix than the | atmosphere, there is some additional energy to theoretically | gain from there. Otherwise, you should have IIUC reached | outside temperature when you reach outside pressure. Or maybe | you would need to expand to outside temperature regardless of | pressure to scratch at Carnot-efficiency... | bombcar wrote: | This made something I never considered thinking about | perfectly clear - thank you! | rasengan wrote: | A rocket engine is more properly referred to as a chemical rocket | engine. | McLaren_Ferrari wrote: | panick21_ wrote: | People don't use GPS and google maps at all. Didn't know that. | Toutouxc wrote: | And satellite TV, satellite phones, satellite internet | connection. | McLaren_Ferrari wrote: | db48x wrote: | I think that you are wrong in many ways, but I will point | out just one of them: GPS was developed in the 1970s, and | was made available for public use in the early 1990s. Quite | a lot of people whose taxes paid for the development of GPS | in the 70s were still alive in the 90s to make use of it. | In fact, many of them are still alive today, 30 years | later. They have benefited greatly from that investment. | McLaren_Ferrari wrote: | The conversation is about GPS for the general public and | google maps. | | Those things emerged for the general public circa 2010. | | So 1970 to 2010 that's 40 years not 20. Also those who | paid the lionshare of that were of course not young but | in their 50 or 60, because that's the demographic which | pays more taxes. | | Highly unlikely they got to see it. | | That's the reason why every person talking about space | should know that the state goes out for them to take | money from people's pockets. | | A brief stint into an insurance company, pension fund, | Sovreign wealth fund or hedge funds would render them | much more pragmatic about costs and tradeoffs. | dlsa wrote: | I've planted many seeds to grow trees I'll never sit | under. As did many many many others before me. This is | the true nature of this world. | | Perhaps you might reconsider your limiting beliefs before | reality catches up with you. Maybe not. At least you | might still yet have fun on roads and highways you didn't | pay for. Schools you didn't build. Parks your money never | constructed. | | I cannot imagine being so limited and short-sighted. But, | as they say, You be you. | Stratoscope wrote: | You're off by a decade. | | Garmin introduced their StreetPilot in 1998. I didn't buy | the first model, but when the StreetPilot III came out in | early 2002 I jumped on it. That GPS served me well for | many years. | | Sure, GPS wasn't ubiquitous like it is today where | everyone has one in their pocket, but it certainly was | available to the general public. | McLaren_Ferrari wrote: | > Garmin introduced their StreetPilot in 1998. I didn't | buy the first model, but when the StreetPilot III came | out in early 2002 I jumped on it. That GPS served me well | for many years. | | https://www.cnet.com/roadshow/reviews/garmin-street- | pilot-ii... | | "the manufacturer lists $1,272" | | 1,272$ in 2002 = 2120$ in 2022. | | This is what people mean when they say techno- | utopianists, space fans and SV types are completely | detached from reality. | | Living in the world of fairy tales and unicorns. | | In the end when things are not done with enthusiastic | consent from all parties involved, it's only a matter of | time before the chicken come home to roost and people | will look to get their money back from the organizations | that took them | | Rockets and Mars helicopters are cool but the vast | majority of the population has other priorities. The | companies providing actual quality of life to Americans | are Walmart, Costco, Google, Microsoft, Exxon, IBM, | Shell, McDonalds, Berkshire...not spaceX | panick21_ wrote: | > All that stuff was financed by people who didn't live to | see such applications, and without their consent. | | How can a collective group of people give consent? | Basically everything any state does is without consent. | | Do you only consider direct democracy where people can | directly decide on each choice the government makes as | legit? | | By your definition it seems social security is also without | consent. | | So by that logic no fundamental research should ever by | financed by governments. good to know. | | Not really interested in further discussion. | McLaren_Ferrari wrote: | > Do you only consider direct democracy where people can | directly decide on each choice the government makes as | legit? | | Humanity is like a plane at some point there is enough | lift that unless somethnig traumatic happens the plane is | in the air and would still be . The analogy is that | people would still get interested in stuff on their own | and they'd finance those things even without extorting | them. | | And it would be a much better distribution of capital and | human resources. | | This model where we extort people was valid maybe in the | 1800s. | | Speaking of today's situation NGOs, foundations, | endowments are much better than governemnts because they | can't use violence, hence they have to convince and only | those who they manage to convince end up giving them | money. | panick21_ wrote: | One thing I always wanted to see was a close expander cycle | areospike. The reason is that expander cycles are heat limited. | Areospikes biggest negative is that they require more cooling but | if you are heat limited that is an advantage. | | You could make the highest possible thrust expander cycle and it | would be high efficiency upper stage for early staging as you | don't lose efficiency by staging early. | | ------- | | Another thing that jumps out at me is that its really sad | development on the F-1 and J-2 stopped. The mentioned J-2S never | flew. The US had this amazing technology stack, F-1 + variants, | J-2 + variants and Apollo stack. | | Instead of keeping around a Saturn 1B type vehicle with the | Apollo on top they threw literally all of that stuff in the trash | and started new with Shuttle. | | But non of that new Shuttle stuff is better when thinking about | it end to end. Because of Shuttle SkyLab could not be saved and | because of Shuttle SkyLab 2 wasn't launched. | | The US with the Apollo stack and incremental updates could have | dominated in space. | | A complete and utter mismanagement of the investment that was | made during Appollo. | cubefox wrote: | Regarding the first point, I'm not sure whether aerospike | engines can still be considered heat limited. Even in the 90s | with the X-33 aerospike spacecraft, heat was not cited as the | reason for cancellation. And nowadays material science as | progressed by a lot, which resulted in more heat resistant | materials, e.g. this one: | https://www.nasa.gov/feature/glenn/2022/nasa-s-new-material-... | panick21_ wrote: | Maybe I wrote it wrong. Closed expander cycles are heat | limited. | | Areospike require more cooling then traditional bell designs. | | So an areospike epxnader cycle could produce more thurst. | | See here for some research: | | https://www.phoenix-int.com/wp- | content/uploads/2017/05/Param... | JKCalhoun wrote: | Perhaps, being pragmatic, tweaking existing designs was not | going to put more money into the R&D of Yoyodyne [1]. | | [1] https://en.wikipedia.org/wiki/Yoyodyne | panick21_ wrote: | I assure you, the same amount of money could have been spent. | Just for far better results. | oxplot wrote: | Tim Dodd is a world treasure when it comes to space education. | The material he's produced are timeless. Thanks Tim for the | quality and down to earth content you've made and all the behind | the scenes interviews with Musk and other rocket companies. | master_crab wrote: | For all the talk of SpaceX and a new race to Mars, no one seems | to remark on the fact that rockets haven't remarkably changed | much since Goddard's day. | | I remember asking one of my profs in college (an early researcher | of the ramjet) what's holding jet and rocket technology back. He | said: melting point temps. | panick21_ wrote: | Well, they are all chemical engines. We have no chemical | engines that would work well for launching. | | In space we do use a lot of solar electric propulsion and lots | other things that Goddard knew nothing about. | | Nuclear thermal propulsion could potentially be used but that | has a whole host of issues where its not clear that its | actually worth it compared to chemical. | | SpaceX Raptor is approaching pretty much the peak of what is | doable with chemical and if its fully and rapidly reusable it | can bring the price to orbit down. | | What really matters is not what method you use, but how much | does it cost to go to orbit, or from LEO to Mars. From that | perspective something like Starship is on a totally different | level then anything that came before. | master_crab wrote: | "Well, they are all chemical engines. We have no chemical | engines that would work well for launching." | | I mean that's what chemical engines are for. You aren't | launching off the ground with any of the electric based | systems, and accidental radiation concerns have always | hobbled nuclear engines. | | But, yes point taken on using them outside the atmosphere. | However, they don't improve our ability to launch manned | missions greatly (at least not for the foreseeable decades). | WalterBright wrote: | Haven't changed since Goddard? Oh my! Here's a list of the | innovations the Germans made to Goddard engines in order to | scale them up for the V2: | | 1. turbo-pump | | 2. building a jacket into the nozzle to both cool the nozzle | and pre-heat the fuel | | 3. putting tiny holes in the jacket so the leaking fuel would | form a boundary layer that would protect the nozzle from heat | | 4. baffles in the combustion chamber to damp out pogo-ing | | There's a picture of Goddard looking at a captured V2 engine | with his mouth hanging open in astonishment. | | The Saturn V engines were scaled up V2 engines. | gameswithgo wrote: | jacksonkmarley wrote: | Wouldn't the pressurant/backfill gas dilute the propellant as it | was added? | Torkel wrote: | I think it's liquid oxygen/methane that goes into the engine. | So backfilling with some other gas in the gas part of that | should be ok. | | No expert though :) | hydrogen7800 wrote: | Expert here. I'm a longtime lurker on HN, but registered just | for this comment. I design aerospace pressure vessels for a | living. As showerst alluded to, there are often things called | "propellant management devices" or PMDs. There are a few | reasons these might be used. In zero G, you need the propellant | sump to remain wetted with propellant; it would be bad to | ingest the pressurant/ullage gas into your engines. Some PMDs | allow the pressurant and propellant to occupy the same volume, | and will use surface tension devices inside the tank to direct | liquid to the sump. These can be screens, vanes, channels, etc. | In other cases, the pressurant and propellant are kept separate | by a bladder. These bladders can be rubber, or even metal. The | propellant mass is usually a large proportion of a space | vehicle's mass, and you can't have that much sloshing around | when you need dynamic control. A metal diaphragm keeps the | propellant more or less static, and its center of mass in a | predictable location. | | As for the pressurant gas dissolving into the propellant in | non-PMD tanks, I don't know enough about that. I imagine the | solubility of He (it is usually helium) in these propellants is | either accounted for, or negligible. | | Edit: P.S. Software engineering is mostly foreign to me and | much of HN content is over my head, but I like he level of | discourse here. So, when a topic came up that I could actually | contribute to, I jumped. | mLuby wrote: | Love how much expertise there is here; thanks for adding | yours! | | When you say "metal bladder/diaphragm" is that a sliding wall | with propellant on one side and pressurant on the other? (I | can't imagine how the seals in that would work.) Or do you | mean the metal actually deforms in place? | jacksonkmarley wrote: | Awesome detailed answer! Much appreciated. | kortex wrote: | Nice writeup! Former chemist here. Helium is indeed poorly | soluble in most liquids. It's actually used to sparge (purge | by bubbling through) solvents to drive out other dissolved | gasses. | | This is because Helium has very weak intermolecular forces | due to its electronic symmetry. For that same reason, it's | also as close to an ideal gas, giving you the most | pressure/volume bang for your mass buck (only hydrogen is | better, and that's bad for oxidizer tanks for obvious | reasons). | | But this also limits the ability to cryogenically condense | helium, which would improve storage density. But you really | don't need much in turbopump fed engines. | bumlebi wrote: | Only if the pressurant gas dissolves into the liquid | propellant. Even then, it doesn't even dilute it per se, it | just makes something akin to rocket-fuel-soda which causes all | sorts of flow problems in your feed lines. Naturally, if your | propellant is stored as a gas, adding more gas to pressurize | will of course dilute it as you say, but I've never heard of | something like this. | | So the short answer here is: no, it won't. | simne wrote: | Sure, if the pressurant gas dissolves into the liquid | propellant. | | But mostly chosen pressurant which is not dissolves, or used | some type of separating membrane (or piston), for example, in | Soviet space stations used | https://en.wikipedia.org/wiki/Metal_bellows | CraftThatBlock wrote: | Not an expert, but fuel and oxidizer is stored as liquid (at | cryogenic temperatures), so adding gas creates pressure at the | "top" of the tank, pushing the liquid down into the engine (in | addition to gravity). | showerst wrote: | In some cases they use a bladder or a push plate. ___________________________________________________________________ (page generated 2022-04-30 23:01 UTC)