[HN Gopher] How do you power a rocket engine?
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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.
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(page generated 2022-04-30 23:01 UTC)