[HN Gopher] Nuclear Conversion for Starship
___________________________________________________________________
Nuclear Conversion for Starship
Author : weird_user
Score : 121 points
Date : 2023-11-03 16:52 UTC (6 hours ago)
(HTM) web link (toughsf.blogspot.com)
(TXT) w3m dump (toughsf.blogspot.com)
| Skunkleton wrote:
| First I am reading about nuclear engines applied to Starship. I
| wonder if it would be more practical to launch the reactor, fuel,
| and propellant into orbit using conventional rockets. Later a
| mission could pick up this Nuclear rocket engine and use it as a
| sort of booster while not inside a strong gravitational field.
| Ofc that doesn't sound like Starship at all.
| ianburrell wrote:
| Nuclear rockets are useless for launching things into orbit.
| They don't have the thrust-to-weight ratio to get off the
| ground. Nuclear rockets are useful because they have high
| specific impulse which means using less fuel for thrust.
| zare_st wrote:
| Exactly opposite of what the person above you is talking
| about.
| sonicanatidae wrote:
| shhh...cage match.
| therein wrote:
| One of the core teachings of Kerbal Space Program.
| Kim_Bruning wrote:
| However, in KSPI [1]: Consider the combination of a nuclear
| lightbulb with compressed air as the reaction mass:
| Surprisingly good midpoint qua TWR and ISP! If you install
| a compressor you can get some pretty interesting SSTOs out
| of it. Whether these would work IRL is a different
| question, of course. [2]
|
| Nuclear lightbulb is of course highly theoretical, but
| scientifically/engineeringly plausible at the least. [3]
|
| [1] https://forum.kerbalspaceprogram.com/topic/173818-181-1
| 122-k...
|
| [2] https://xkcd.com/1244/
|
| [3] https://en.wikipedia.org/wiki/Nuclear_lightbulb
| matheusmoreira wrote:
| It's that close flyby of the sun that always gets me.
| scythe wrote:
| Did you read to the end? That's the idea he ends up with: only
| use nuclear propulsion for space-to-space travel where you
| don't care as much about shielding.
| Skunkleton wrote:
| I make no claims about my reading comprehension. That last
| bit seemed too narrow to me. Once you have all the bits in
| orbit, shielding doesn't seem too onerous. Is the claim that
| even space-space shielding + reactor is too heavy to be worth
| the advantage of the nuclear drive?
| marsokod wrote:
| No, they are saying that the nuclear drive shall be
| reserved to a shuttle that remains in space. At that point,
| it makes much more sense as you can really use the nuclear
| drive to its full capabilities.
|
| > If we need the full performance advantage of nuclear
| propulsion, we should design a spaceship that is intended
| for it from the get-go. It never lands, only going from
| orbit to orbit, so there is no need for heat shielding,
| flaps, high thrust engines, thick steel structure or
| aerodynamic shaping requirements.
| simonh wrote:
| In space if you're careful you only need a plate of
| shielding between the drive and the crew/cargo section.
| Kim_Bruning wrote:
| That was (part of) the original plan, post Apollo program.
| Somehow it got stripped down to to just a space shuttle. :-(
|
| https://en.wikipedia.org/wiki/Space_Transportation_System
| psunavy03 wrote:
| The "we haven't solved all of our problems on Earth yet"
| crowd . . . as if we ever will.
| lifeisstillgood wrote:
| Hell we put humans on another planet, we can fix the stuff
| down here if we really want
| Kim_Bruning wrote:
| It's a different kind of problem, unfortunately.
| jerrysievert wrote:
| which other planet have humans been to? I can only
| remember humans making it to earth, which isn't that far
| of a journey ... from earth.
|
| we put humans on a moon, and we've put robots on another
| planet.
| matheusmoreira wrote:
| That's exactly what I do in Kerbal Space Program.
| datadrivenangel wrote:
| The summary of this methodical and enlightening article from an
| excellent source for mathematically backed science fiction /
| space travel is:
|
| No. Nuclear is not worth it for starship, as the added engine
| weight limits the additional delta V and the additional
| challenges of handling nuclear material.
| jdblair wrote:
| The no accompanies the short explanation of a nuclear planetary
| transfer vehicle that never lands on earth. A lot of conceptual
| work has been done on these designs, and Andy Weir's The
| Martian features one.
|
| Nuclear powered space tugs is a particular rabbit hole I've
| spent too much time on.
|
| See, for example, this Boeing design:
| https://www.projectrho.com/public_html/rocket/spacetug.php#b...
| chris_va wrote:
| Since this is presumably for interplanetary, and not heavy lift,
| I'm not sure why you'd use this over just having thin-film solar
| + pulsed plasma thruster (which can do anything from 1km/s to
| 3000km/s exhaust velocity).
| idlewords wrote:
| Because nuclear thermal is high thrust, and sometimes you need
| high thrust.
| politician wrote:
| I think we'll see nuclear propulsion for Starship variants that
| receive fuel rods in orbit and are not designed for reentry.
| Interplanetary logistics between Earth/Moon/Mars orbits.
| idlewords wrote:
| Name me a country willing to give Elon Musk control of fissile
| material, however indirect.
| euroderf wrote:
| NK, "if" he has enough cold hard cash.
| idlewords wrote:
| "I'll trade you Twitter for six critical masses" doesn't
| have the appeal of a year ago.
| euroderf wrote:
| Yeah rilly. I mean, what can you possibly accomplish with
| a measly two critical masses.
| justrealist wrote:
| Niger Central African Republic Cambodia Bolivia Palau Laos
|
| There are in fact a lot of countries that would be thrilled
| to give Elon fissile material in exchange for investment.
| idlewords wrote:
| I'm going to go out on a limb and say Niger, CAR, Palau,
| etc don't have access to highly enriched uranium.
| justrealist wrote:
| Who says it has to be highly enriched to start?
|
| https://en.wikipedia.org/wiki/List_of_countries_by_uraniu
| m_r...
| idlewords wrote:
| Well, there's a reason Niger hasn't nuked us yet and
| isn't flying rockets to Mars.
| jaggederest wrote:
| Starship and the booster already collectively carry 70
| kiloton-tnt-equivalent masses of liquid methane and oxygen.
| As a weapon of mass destruction, it's hard to beat
| relativistic impact.
| idlewords wrote:
| That's like saying they carry 4400 tons of mass-energy,
| which could destroy the planet.
| renewiltord wrote:
| Would you not realize a lot of damage by just pointing
| Starship somewhere instead of at the sky?
| ben_w wrote:
| Surprisingly little, as the thing is mostly fuel -- by
| the time it hits anything[0] it's almost empty and
| therefore very light.
|
| It wouldn't even be like the effect of the planes on the
| WTC in 9/11, as planes use fuel continuously during
| flight, whereas spacecraft use almost all of it just to
| take off.
|
| [0] which implicitly assumes the self-destruct still
| isn't working right
| rsynnott wrote:
| > As a weapon of mass destruction, it's hard to beat
| relativistic impact.
|
| > relativistic impact
|
| You... er, realise they're not actually starships, right?
| At the speeds they go at (or would go at if they could take
| off without exploding) there is no significant relativistic
| effect.
| dgacmu wrote:
| How do you calculate this? Starship has a propellant
| capacity of 1200 metric tons - 1.2 kilotons. Super heavy
| booster has 3400 tons, total of 4.6 kilotons, per wikipedia
| [1]. By weight, you need about a 4:1 ratio of O2:Methane
| for complete combustion (Starship is rumored to carry 22%
| methane). Which means you're getting about 0.92 kilotons of
| methane completely combusted. At 50-55MJ/kg, that gives you
| - roughly - 50 thousand gigajoules. A ton of TNT is 4.184
| thousand gigajoules[2], which means that Starship+Super
| Heavy carries the energy-equivalent of about 12 kilotons of
| TNT [3][4].
|
| [1] https://en.wikipedia.org/wiki/SpaceX_Starship
|
| [2] https://en.wikipedia.org/wiki/TNT_equivalent
|
| [3] It can't explode all at once unless pre-mixed in
| perfect proportion, of course. What you'd really expect is
| an awfully big fireball + fire but not the same massive
| force all at once of a nuclear detonation.
|
| [4] Or 50 terajoules (the ship) and .184 terajoules (TNT),
| which come out to the same thing, but I find gigajoules
| both more intuitive and more a more fun BTTF reference.
| ben_w wrote:
| If he _really_ wanted to, he 's rich enough to build the
| enrichment facilities himself. Could hide it underground in
| one of those tunnels, claiming the power consumption is a
| cryptocurrency mining rig or an AI server farm.
|
| I suspect at least ten government agencies from various
| nations all have him/his assets under 24/7 surveillance just
| to make sure he doesn't do something at least this unwise.
| nickik wrote:
| If SpaceX was willing to do the work and put in the
| investment they could absolutely get access to fissile
| material. The US government evaluates things based on
| regulations and not who is popular on twitter.
| maxcan wrote:
| AFAIK, the only nuclear rocket that makes sense is landing a
| reactor on an icy comet and using it to shoot super heated steam
| wherever you want thrust.
|
| #seveneves
| darknavi wrote:
| Just don't let it leak.
|
| RIP New Caird
| idlewords wrote:
| The Project Orion design (shoot hundreds of fission bombs out
| the back and ride the shock wave on a reaction plate) is
| surprisingly practical, and probably our only way to get humans
| past Mars. It's just kind of terrifying.
| lostlogin wrote:
| Yup, it's scary.
|
| "A moderate-sized nuclear device was estimated, at the time,
| to produce about 5 or 10 billion horsepower."
|
| https://en.m.wikipedia.org/wiki/Project_Orion_(nuclear_propu.
| ..
| idlewords wrote:
| George Dyson (Freeman's brother) wrote a book on it that is
| fantastic reading.
| whycome wrote:
| > Freeman's brother
|
| Son. https://en.wikipedia.org/wiki/George_Dyson_(science_
| historia...
|
| Relatives Sir George Dyson (grandfather) Freeman Dyson
| (father) Verena Huber-Dyson (mother)[1] Esther Dyson
| (sister)
| idlewords wrote:
| My bad; I should have checked. Thank you for the
| correction!
| moffkalast wrote:
| Finally, the gigahorse.
| throwaway4aday wrote:
| The energy required to accelerate a useful amount of mass to
| a high enough velocity to travel interplanetary (or
| interstellar) distances in a timely manner is always going to
| be terrifying. Even if it runs on rainbows and hugs the
| kinetic energy imparted to the payload would turn it into a
| planet killer if you ever rammed it into one. Managing that
| risk so you get the payoff without any accidents along the
| way is the only sensible thing to do.
| woopsn wrote:
| Not building the ship of thousands of little nukelets IS
| managing risk.
| xwdv wrote:
| Not really any more terrifying than driving around in
| vehicles powered by thousands of tiny explosions per minute.
| idlewords wrote:
| If someone siphons your gas tank, they're not going to be
| able to nuke Paris. But a nuclear pulse rocket needs
| thousands of bomblets the size of coke cans. That's the
| scary part.
| marcinzm wrote:
| > size of coke cans.
|
| Not really, each one would be 6 inches in diameter and
| weight over 300lbs.
| idlewords wrote:
| Dyson's book hints that they ended up far smaller, but
| the details of that are classified to the gills.
| p1mrx wrote:
| Sounds impossible. A 6 inch diameter uranium sphere is
| only 80 lbs, and there are no materials significantly
| denser than that.
| postalrat wrote:
| They sell coke cans in the shape of a sphere?
| p1mrx wrote:
| https://www.google.com/search?q=spherical+coke+bottle&tbm
| =is...
| api wrote:
| Ahh the old "devil's pogo stick."
| tomatotomato37 wrote:
| Should also note that the Orion design most people are
| familiar with is from the 60s, when the latest in engineering
| used vacuum tubes. Modern innovations in material science,
| computer assisted design, and just technology that just
| simply didn't exist yet such as laser ignition would permit
| ditching the ridiculous pogo stick pusher plate arrangement
| with something that's more efficient, runs smoother, and
| looks a lot more traditional too. I wouldn't be surprised if
| a modern variant looks like that one Sea Dragon concept with
| beefier nozzle suspension
| SECProto wrote:
| > The Project Orion design [...] is surprisingly practical,
| and probably our only way to get humans past Mars.
|
| I don't think this is the general consensus. It is seen as
| one of the only ways to get humans out of the solar system,
| but other nuclear rockets[1], and even chemical options[2],
| could get humans to eg the Jovian moons[3] with TRL's much
| higher than Project Orion.
|
| [1] https://en.wikipedia.org/wiki/NERVA
|
| [2] https://en.wikipedia.org/wiki/Interplanetary_spaceflight#
| Cyc...
|
| [3] https://en.wikipedia.org/wiki/Exploration_of_Jupiter#Huma
| n_e...
| idlewords wrote:
| It's not just about propulsion, but radiation. The
| radiation environment in space is awful and you need
| thousands of tons of shielding to effectively screen a crew
| from cosmic rays. Moreover, because of secondary radiation,
| partial shielding from energetic cosmic rays is worse than
| none.
|
| A unique property of nuclear pulse propulsion is that the
| engineering gets easier the more massive the spacecraft.
| That leaves lots of room for radiation shielding and
| consumables, and makes it the only halfway practical choice
| for multi-year missions into the outer solar system.
| fsloth wrote:
| Project orion was super cool and I warmly recommend the book by
| similar name by George Dyson (son of Freeman Dyson!) who did a
| tremendous job of putting everything back together as well as
| humanly possible for an ops with very top secret components.
|
| Nukes can totally function as the energy for rocket's impulse.
| Very _small_ and very _directed_ nukes. The directed explosion
| plasma wave hits a pusher plate, where _very_ specific plasma
| physics play out - the plate is not vaporized but merely
| receives an impulse - and then there are dampers to smoothen
| the blow.
|
| Half of the interviewed experts were positive it would work and
| half were skeptical.
|
| So, it's sort of sad they had to wrap it up due to nuclear test
| ban before actually being able to run actual tests.
|
| The best part? The _bigger_ it is, the better. We could totally
| have gigantic nuclear space cruisers that go to Jupiter in like
| a week (I exaggerate because I forget the exact orbits but with
| ISP from nukes that bastard goes _fast_ )
| whycome wrote:
| https://www.gatesnotes.com/Seveneves
| ragnot wrote:
| Might be a stupid question: would this result in radioactivity
| just flying through space as we try to propel spacecraft with it?
| tantony wrote:
| There's already plenty of it in space.
| idlewords wrote:
| It's not a stupid question. Space is already highly
| radioactive, with lots of protons and bigger nuclei from
| outside the solar system flying around in all directions, along
| with a ton of proton radiation from the Sun.
| marcosdumay wrote:
| Yes, but with very little long-lived decaying material that
| you could carry inside a survivable habitat.
| idlewords wrote:
| Most nuclear spaceship designs have a long stick with the
| reactor at one end and the crew on the other, and some kind
| of shielding in between. Since you only need to shield a
| cone, it can be quite effective.
| devmor wrote:
| Yes, but space is already full of radioactivity. An incredibly
| large amount of it. The average range (outside of the upper
| Earth's atmosphere) would be equivalent to getting somewhere
| around 10 chest x-rays per day, every day.
| starbase wrote:
| More or less, yes. Space is so large and so rarified and so
| riddled with energetic particles and photons that the
| contribution of a few measly rockets is of no concern.
| misterbishop wrote:
| Aren't SpaceX launch explosions big enough already?
| daveslash wrote:
| I don't know why you're downvoted. I think it's a legitimate
| question. When I was in high school I asked " _Why don 't we
| just shoot our nuclear waste into the sun?_".
|
| As an adult, Kerbal Space Program has taught me that it's
| incredibly difficult to lose the orbital velocity and get
| things into the Sun!
|
| But the answer my teachers told me was _" Well, even if rockets
| have a 99.5% success rate and a 0.5% failure rate, and only 1
| out of every 200 rockets explodes during launch... then
| eventually a rocket carrying nuclear waste is going to explode
| and spread it_ everywhere"
|
| As others have pointed out: Nuclear rockets don't have the
| thrust ratio necessary for take off, so presumably they'd be
| assembled and used in space. My question is: What level of risk
| does launching nuclear rocket parts/fuel into space pose?
| rsynnott wrote:
| > Nuclear rockets don't have the thrust ratio necessary for
| take off, so presumably they'd be assembled and used in
| space.
|
| Or just used as an upper stage! See
| https://en.wikipedia.org/wiki/Saturn_C-5N, for instance, and
| a bunch of even more ridiculous Soviet designs.
|
| Amazingly, this project (nuclear third stage for Saturn V)
| was actually under serious development. This to be used on a
| rocket stack which had had two major incidents in 13
| launches; it really was a very different safety culture back
| then.
| UniverseHacker wrote:
| This article addresses that. They claim that, like you said,
| they are only used at very high altitude or in space, and the
| raw materials, before ignition are not dangerous. They say
| that the low enrichment uranium is even safe to touch with
| your bare hands.
| starbase wrote:
| We can store the fuel pellets in a crash can during transit.
| That way if the rocket blows up nothing hot gets dispersed.
|
| But even without a crash can, it's of less concern to public
| health than the radioisotopes in coal.
| mcpackieh wrote:
| A rocket explosion is unlikely to pulverize solid nuclear
| fuel or vitrified nuclear waste. It would most likely fall,
| intact or in large pieces, into the ocean which is a pretty
| good place for it anyway. If it blows up over Florida that's
| worse, but the worst case scenario is that it reenters
| earth's atmosphere at near-orbital speeds over a populated
| area and burns/breaks up. Then you have a situation like
| Kosmos 954, where the radioactive material is spread over a
| large area and is consequently much harder to clean up.
|
| The biggest issue for the nuclear waste disposal idea is that
| it doesn't make any economic sense. It's better to dump it
| into the ocean, or down a mineshaft, or even just to let it
| sit in storage near the power plant for decades. But with
| nuclear fuel for rovers, satellites or rockets the rewards
| could outweigh the risks, and so this is sometimes done.
|
| Here's a NASA doc that goes over the risks of launching
| plutonium into space for their Mars rovers. To summarize,
| it's not really a big deal:
| https://rps.nasa.gov/resources/81/mars-2020-launch-
| nuclear-s...
|
| > _The General Purpose Heat Sources (GPHS) inside the MMRTG
| is designed specifically to prevent such an occurrence. The
| fuel inside each GPHS is surrounded by several layers of
| protective materials, including the type of tough material
| used in the nose cones of missiles designed to survive fiery
| conditions during re-entry into Earth's atmosphere. In
| addition, the radioisotope fuel is manufactured in a ceramic
| form (similar to the mate- rial in a coffee mug) that resists
| being broken into fine pieces, reducing the chance that
| hazardous material could become airborne or ingested._
| nickik wrote:
| > The biggest issue for the nuclear waste disposal idea is
| that it doesn't make any economic sense.
|
| No the biggest issue is that its a stupid idea in the first
| place. Nuclear waste is fine and perfectly reasonable to
| handle here on earth with close to zero chance of actual
| danger.
| agent327 wrote:
| Why would you dispose of such a useful material by attempting
| to shoot it into the sun? Sure, there is some danger
| associated with storing it, but it's not as if nuclear
| material is the only hazardous substance on the planet, and
| with the rest of it we seem perfectly ok with just sticking
| it in a hole somewhere. Nor does nuclear material, by itself,
| represent a danger of civilisation-ending proportions (only
| if you build a bomb out of it, and that is quite a
| significant operation that can only be conducted by people
| that know what they are doing. In other words, it won't
| explode if left by itself.).
| Symmetry wrote:
| Yes, but nuclear reactors carried up to space for rockets or
| electricity can still make sense. Before you turn on your
| reactor it's not very radioactive, to survive to the present
| day uranium has to have a very long half-life which means low
| radioactivity. But as soon as you turn it on you start
| transmuting the uranium into all sorts of things with much
| shorter half-lives and so much higher radioactivity.
|
| So its safe to fly a new reactor into a stable orbit, if
| something bad happens you'll scatter some uranium but that's
| just toxic heavy metal bad, not radiation hazard bad. Just
| make sure the orbit is high enough to be stable, not like
| Kosmos 1402.
|
| https://en.wikipedia.org/wiki/Kosmos_1402
| andrewla wrote:
| I only skimmed the article; but as I understood it the liquid
| hydrogen here is being used strictly as ejecta. It is not being
| combusted, the way that it is used in the space shuttle boosters.
|
| If that is the case, liquid hydrogen seems like an awful
| substance -- hard to produce and transport, hard to store and the
| mass required to support keeping it is not insubstantialy, all
| for a mass density of ~70g/L.
|
| Why not just use water? Water is 1000g/L and can be stored
| without any real effort at all. If all you're doing is shooting
| it out the rear end of the rocket with as much energy as
| possible, it's mysterious why you would even consider liquid H2.
| CarVac wrote:
| For specific impulse. When you have unlimited power, you want
| the lightest possible molecular mass exhaust for the best total
| delta V.
| andrewla wrote:
| I need to play Kerbel Space Program. I have absolutely no
| intuition about this sort of thing. Some intuition around
| orbital mechanics from my physics days but rocketry is a bit
| of a mystery.
| idlewords wrote:
| For me it helped to remember that the term that matters in
| the rocket equation is exhaust _velocity_ , not momentum.
| You want to be shooting the lightest things you can out the
| back of the rocket, at the highest speed.
|
| Unfortunately with chemical rockets, the energy source and
| reaction mass are the same thing, so you're kind of stuck
| with whatever burning your fuel gives you. But when you can
| separate reaction mass from energy source (like in nuclear
| or electric rockets), hydrogen is always the best bet.
| Kim_Bruning wrote:
| Mostly true. However, there's tradeoffs qua thrust and
| specific impulse for different propellants. For pure
| vacuum work hydrogen is a good candidate. For operating
| in a gravity field (eg. the moon), you might be able to
| use a smaller nuclear engine with a denser propellant.
| Obviously you ARE trading in some range when doing this.
| But if the question is whether you can get off the ground
| in the first place, you may have less of a choice. It
| might also depend on what you can actually lay your hands
| on qua ISRU (if/when relevant)
| Kim_Bruning wrote:
| Yes, absolutely: https://xkcd.com/1356/
| matheusmoreira wrote:
| Do it! You'll know you got it when you're maneuvering the
| rocket on pure intuition without even using maneuver nodes.
|
| Just steer clear of the planet Eve. It's been years and I
| still haven't been able to leave it.
| klyrs wrote:
| So, since beta radiation is even lower mass... just put a
| block of nickel-63 into a magnetic lens?
| abecedarius wrote:
| https://en.wikipedia.org/wiki/Nuclear_electric_rocket
| klyrs wrote:
| That's not quite what I'm picturing (and let's be honest,
| the idea is silly). That page describes capturing the
| heat of a nuclear reactor, converting it to electricity,
| and then using that electricity to force ions out of a
| thruster. I'm saying, the radiation itself has momentum;
| don't bother with the contraption.
| ben_w wrote:
| Shooting _just_ electrons out the back of your ship is fine
| for the brief period before your ship gains a positive
| electric potential comparable to the net potential those
| electrons experience due to the inside of the particle
| accelerator you were using, after which your ship 's own
| field is a major source of drag.
|
| If your ship has a positive electric potential of over
| 1.044 MeV[0], you also start getting positron-electron
| pairs forming on your hull.
|
| [0] less in practice, because any electrons you're going to
| encounter were already moving
| klyrs wrote:
| Twin beta+ and beta- engines?
| ben_w wrote:
| First, you have a way to store an absurd quantity of
| positrons. For a sense of scale, without shielding the
| electric fields, 1 picogram of positrons (or electrons)
| confined within a 10 cm radius is going to trigger
| positron-electron pairs formation, thanks to free
| electrons in the area responding to the surface
| potential.
|
| Second, AFAICT if you can do that then you're either
| going to want to use them as an energy source to propel
| your cheap reaction mass even harder than a fission
| rocket would, or you're going to want to react them with
| electrons to make a photon rocket.
| klyrs wrote:
| I proposed nickel-63 for my electron source. Let's say,
| aluminum-26?
|
| And, yes. The idea is patently absurd. But I'm waiting on
| a ridiculously long compile.
| ben_w wrote:
| If you're getting them from radioactive decay, it _works_
| , but those examples are 63 and 26 times heavier than
| just using hydrogen as your reaction mass in the first
| place :)
| moffkalast wrote:
| Don't ionic thrusters typically solve this problem by
| also emitting oppositely charged particles from a thin
| rod behind the motor? Maybe that still causes drag, but
| overall the main problem is that while the ISP is high,
| the thrust is almost non-existent.
| ben_w wrote:
| They do indeed; the oppositely charged particles being
| electrons. But there's no charged particles lighter than
| an electron, and the only one with the same mass is the
| positron. If you try to balance it with protons, then
| what you've really got is a slightly over-complicated
| hydrogen-based ion drive.
| perihelions wrote:
| - You'd need to account the mass of the copper-63 waste, in
| addition to the electron
|
| - This math is specifically about thermal propulsion; your
| 1 MeV electron wasn't in thermodynamic equilibrium with
| anything, it's a different sort of problem
| rsynnott wrote:
| Some explanation here:
| https://en.wikipedia.org/wiki/Nuclear_thermal_rocket#Princip...
|
| I think all real-life actually-tested nuclear thermal rockets
| have used hydrogen as propellant, though use of other
| substances has definitely been proposed.
| Kim_Bruning wrote:
| Water is annoying: when you heat it, some of the energy goes
| into dissociating it rather than accelerating it. In the end
| you apparently get pretty bad performance out of it.
|
| In deep space you don't really care about the volume of your
| fuel tank, so then it does make sense to use something light
| like hydrogen.
| paulsutter wrote:
| The article covers this. Virtually all nuclear engine proposals
| have used H2 as propellant because of the specific impulse, but
| this article mentions methane as an alternative. Of course,
| Starship today uses methane and Elon plans to create methane on
| mars.
|
| > Most significant for our purposes is methane as propellant.
| It is six times denser than liquid hydrogen, can be stored at
| 100K, which is compatible with liquid oxygen, and it can be
| produced using water and carbon dioxide. At high temperatures,
| it breaks down into hydrogen and carbon, turning it from a 16
| g/mol molecule into a 3.25 g/mol plasma. That is how it
| achieves a specific impulse only mildly lower than what is
| achievable using liquid hydrogen. Zubrin lists its specific
| impulse as
|
| >Previous calculations using hydrogen propellant revealed how
| volume-limited the Starship design was. There was no room for
| the bulky liquid hydrogen, and getting to orbit meant
| sacrificing the payload mass and volume advantages that the
| Starship is built around.
|
| >These could be solved by using denser liquid methane as
| propellant for the nuclear propulsion system. The Isp will be
| lower, but the mass ratios become so much better that more
| deltaV is available overall.
| jhallenworld wrote:
| The design is not advanced enough, they should be thinking of
|
| https://en.wikipedia.org/wiki/Fission-fragment_rocket
|
| So here is my science fiction theory: fusion will never be
| possible, but fission already is. So in the galactic
| civilization, rare uranium is the most valuable resource since it
| enables interstellar travel. Uranium is only produced in the
| collisions of neutron stars, so we are lucky to have any.
|
| We should not squander our uranium for power generation when we
| can use solar for that. It's like not using oil or gasoline for
| heating.. we need it for transportation.
| m463 wrote:
| > We should not squander our uranium for power generation.
|
| I think we might have enough.
|
| "He noted that fast breeder reactors, fueled by naturally-
| replenished uranium extracted from seawater, could supply
| energy at least as long as the sun's expected remaining
| lifespan of five billion years"
|
| https://en.wikipedia.org/wiki/Uranium_market#Available_suppl...
| Animats wrote:
| That's a useful analysis.
|
| Nuclear power gives you more energy, but you're still limited by
| how much reaction mass you can carry.
|
| The plans that work look like early 1960s NASA plans - build
| infrastructure in orbit, assemble nuclear power interplanetary
| craft in orbit, nuclear power is from planetary orbit to
| planetary orbit only. That was Wernher von Braun's "Man Will
| Conquer Space Soon" plan.[1]
|
| The Apollo program, at US$20 billion, was the _economy_ version.
| To the moon and back, no space station, no permanent
| infrastructure in space.
|
| [1] https://www.youtube.com/watch?v=Sz7njI0wEIw
| asdff wrote:
| It makes you wonder though, should you set up such space based
| infrastructure, is it even worth publicizing lest you trigger
| an arms race?
| bobthespaceguy wrote:
| I think advancing science sometimes comes with risks. I would
| welcome a friendly arms race for better access, but who knows
| what would actually happen.
| sandworm101 wrote:
| You couldnt do it without publicizing. Space stuff is big and
| loud. Space itself is transparent. Doing big secret stuff in
| space is extrodinarily difficult.
| asdff wrote:
| There are spy satellites that we know nothing about in
| orbit, only until someone like Trump leaks an image on
| twitter. Who is to say where that line of development
| ended, if ever, given so little is spoken publicly of these
| efforts? Perhaps others could detect some of your efforts,
| but you hold them to secrecy like you do other state
| secrets among allies out of mutual benefit. I'd expect
| there to at least be a well worked out plan for such an
| infrastructure along these lines by now, if not
| implementation. It's one of those things where the losing
| move is not to engage with it, for fear someone else is and
| making strides.
| sandworm101 wrote:
| Except we know when they were launched, thier general
| orbital parameters, and what they are generally doing. We
| dont know the specifics but their existance as spy sats
| is well understood. There is even an online community of
| enthusiasts looking for and tracking them. Wikipedia is
| full of data.
|
| https://en.wikipedia.org/wiki/Orion_(satellite)
|
| https://en.wikipedia.org/wiki/Lacrosse_(satellite)
|
| https://en.wikipedia.org/wiki/National_Reconnaissance_Off
| ice
|
| https://www.nro.gov/launches/
| dylan604 wrote:
| You can't hide that your about to do something in space
| because launches are loud and obvious. However, once in
| space, you can absolutely hide what you are actually doing.
| We still don't really know what the Air Force Space Plane
| does while in orbit.
| paulsutter wrote:
| >So will going Nuclear be worth it?
|
| >The short answer is no
|
| >If we need the full performance advantage of nuclear propulsion,
| we should design a spaceship that is intended for it from the
| get-go. It never lands, only going from orbit to orbit, so there
| is no need for heat shielding, flaps, high thrust engines, thick
| steel structure or aerodynamic shaping requirements.
| larrydag wrote:
| What are the safety ramifications? Is there not a radiation
| fallout from the exhaust?
| moffkalast wrote:
| The ram-ifications of open cycle nuclear propulson are ill-
| advised.
|
| https://en.wikipedia.org/wiki/Project_Pluto
| MrTortoise wrote:
| yay radioactive shit falling from space again
| davedx wrote:
| Yes, there's a lot of radioactive shit in space :P
| ben_w wrote:
| The sun is a giant fusion reactor known to cause cancer on a
| good day, and emit radiation bursts that can damage or disrupt
| power grids often enough that people can reasonably expect to
| read reports of them more than once in their lifetimes.
| flashback2199 wrote:
| > The sun is a giant fusion reactor
|
| And chernobyl was a giant fission reactor.
|
| What's your point?
| ben_w wrote:
| The comparison between the two is almost exactly the same
| as the comparison between a fart and a hurricane: 16 orders
| of magnitude, give or take.
|
| Space is _really_ hostile.
| flashback2199 wrote:
| Ok if it's not taking off or landing from earth then sure
| bluescrn wrote:
| Nuclear doesn't work with Elon's approach to development. You
| can't blow up multiple nuclear rockets within the atmosphere just
| to learn what needs fixing for the next attempt.
|
| Then again, it's looking like the whole 'fail fast' approach is
| over anyway, with Starship grounded by red tape.
|
| A full-stack Starship failure is seemingly too big/dramatic/risky
| to get away with doing repeatedly, especially admidst the
| politics of Twitter/X.
| amluto wrote:
| I assume some of this is that the many early Falcon failures
| merely blew up the test vehicles. The starship test kind of
| blew up the launch site.
| bunabhucan wrote:
| You can. You just can't legally do it in Texas. Maybe SpaceY
| will be based in North Korea or somewhere to escape the
| burdensome regulatory regime in the USA.
| nradov wrote:
| Countries with less burdensome rules are subject to ITAR
| restrictions.
| smegger001 wrote:
| Or just launch from a barge in the middle of the ocean or
| some uninhabited atoll.
| numpad0 wrote:
| Maximum punishment for a launch failure in NK is a firing
| squad.
| perihelions wrote:
| - _" You can't blow up multiple nuclear rockets within the
| atmosphere just to learn what needs fixing for the next
| attempt."_
|
| Sure you can. Russia has blown up multiple experimental
| nuclear-powered cruise missiles in the last few years [0,1].
| It's a political question of tradeoffs, of how much you value
| technological progress on a particular front.
|
| America hasn't blown up any atmospheric nuclear reactors in
| this century. America is also unlikely to learn how to build
| aerospace nuclear reactors in this century.
|
| [0] https://en.wikipedia.org/wiki/Nyonoksa_radiation_accident
|
| [1] https://news.ycombinator.com/item?id=16509396 ( _" U.S. Has
| Been Watching Russia's Nuclear-Powered Cruise Missiles Crash
| and Burn (thedrive.com)"_)
| rockemsockem wrote:
| Nuclear rockets are good in space, not in atmosphere and they
| aren't radioactive until you turn them on. You test in space.
| Also that isn't "Elon's approach to development" that's just
| how everything good has ever been made. Also if you plan to
| fail then you usually wind up failing more safely than if you
| don't plan to fail.
| INGSOCIALITE wrote:
| why aren't they just researching the creation of dilithium
| crystals?
| rockemsockem wrote:
| Interesting thought experiment, not shocking at all that a craft
| not designed to use NTP wouldn't work well with it.
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