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                                                             on Gopher (inofficial)
   URI Visit Hacker News on the Web
   URI   DARPA moving forward with nuclear thermal engine design
   DIR   text version
        TedDoesntTalk wrote 3 days ago:
        > One key challenge with nuclear reactors in space is the risk of
        contaminating Earth
          bobsmooth wrote 3 days ago:
          Should the rocket explode during launch.
            DennisP wrote 3 days ago:
            That's not much of a risk, if you haven't ground-tested the engine.
            Nuclear fuel is barely radioactive before fission. It's the waste
            products of fission that you have to watch out for.
          moffkalast wrote 3 days ago:
            doctor_eval wrote 3 days ago:
            Well, I laughed, anyway.
        marktangotango wrote 4 days ago:
        Presumably this is a solid core design and since these would never fly
        in the atmosphere anyway, I've always thought that going all in on
        nuclear salt water engines would be the way to go [1]. These things are
        so high performance, I bet even a small/micro one could enable tic tac
        levels of performance, buts that just a guess.
   URI  [1]: https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket
        Stevvo wrote 4 days ago:
        We have known how to build working NTRs since before the moon landings.
        They are a proven technology but we decided is was not worth the risk
        to fly them.
        What changed? Or will this rocket stay firmly on the ground?
          maccam94 wrote 3 days ago:
          From what I've read NERVA was actually killed by Nixon for political
          (cost) reasons.
   URI    [1]: https://en.wikipedia.org/wiki/NERVA#Cancellation
          Robotbeat wrote 4 days ago:
          It’s not risk but cost and also difficulty in ground testing
          safely. What changed is they may not test them on the ground but in
          orbit. Just design it very conservatively and launch to a safe orbit
          and test there.
          Technology can just progress, nothing massive needs to change. DARPA
          sees that the time is ready to advance this technology once again.
          They will test it first at very small scale. The purpose is deep
          space space force robotic vehicles being able to make lots of
          maneuvers (to avoid ASAT? To do multiple missions? Changing orbit to
          avoid detection?) with high thrust, ie quickly.
        jahabrewer wrote 4 days ago:
        This would be for a ship that stays in space, right? (as in, not using
        an NTR from ground to orbit)
          Robotbeat wrote 4 days ago:
          That’s right. NTR not that useful for launch to orbit anyway due to
          the really terrible thrust to weight ratio (compared to chemical) and
          the poor density.
        danans wrote 4 days ago:
        Darn,  I clicked on it hoping for a new terrestrial energy production
          ncmncm wrote 3 days ago:
          True, what we have certainly won't do.
          Fortunately, it doesn't need to.
        robonerd wrote 4 days ago:
        The PDFs here have a lot more information: [1] A few takeaways; they
        intend for such an engine to eventually support long duration human
        spaceflight (going to Mars.)  The propellant for the NTR engine to be
        liquid hydrogen.   One of the problems DARPA anticipates with using
        such an engine for such a mission is needing to store liquid hydrogen
        longer than the present state of the art.
        The PDF doesn't seem to mention it, but I think the Advanced Cryogenic
        Evolved Stage (ACES) is probably relevant to this project.  Does
        anybody know what kind of duration they expect to get from ACES?  I'm
        not sure but I think it's weeks, not months.
   URI  [1]: https://sam.gov/opp/af490b568d2a438498afa1e80bce63e5/view
          jhgb wrote 4 days ago:
          I don't see how an NTR helps you in any way to get to Mars or back.
          Heavy engine, voluminous tanks (~70 kg/m³), criminally wasted ISRU
          material (you have to throw away 88.9% of the water that you mine on
          site, whereas a hydrolox or methalox system uses almost all of it and
          the methalox system can even mix it with considerable amount of CO₂
          for better system performance). The performance figures for such a
          system will be terrible. At best a LANTR (not just an NTR) might be
          somewhat useful for cislunar uses. For Mars flights not even LANTR
          may be useful.
            credit_guy wrote 3 days ago:
            > you have to throw away 88.9% of the water that you mine on site
            Nobody cares about that. People care only about the end results,
            not about the efficiency (or inefficiency) of the intermediate
            steps. As long as you can travel to Mars and back in half the time
            it takes with a chemical rocket, exactly zero people will shed
            tears for all the oxygen wasted after splitting water on Mars.
            Besides that, chances are you will be able to find uses for the
            oxygen you produce. I don't need to remind you that people breathe
            oxygen, astronauts included.
              wumpus wrote 3 days ago:
              > People care only about the end results, not about the
              efficiency (or inefficiency) of the intermediate steps.
              Look up for the guy claiming hydrogen is best and most efficient,
              even though performance per dollar is poor.
                zardo wrote 3 days ago:
                Speaking of performance per dollar, what does the nuclear fuel
                for an NTR cost?
                  credit_guy wrote 2 days ago:
                  The NERVA engine [1], [2] used 60kg of 92.5% enriched
                  The online nuclear fuel cost calculator [3] shows that at
                  current Uranium market prices, the cost for 1kg of such
                  highly enriched Uraniums is about $50k, so the whole engine
                  core would come at about $3 MM.
                  This engine had a weight of about 2.5 metric tons, a thrust
                  of 75 kN and a specific impulse of 860s.
                  For comparison, the weight of a SpaceX Raptor engine is about
                  1.5 tons, it has a thrust of 1.8 MN (24 times higher than
                  Nerva) and a specific impulse of 360 s (2.4 times lower than
                  Nerva's). [1] [2]
   URI            [1]: https://en.wikipedia.org/wiki/NERVA
   URI            [2]: http://www.astronautix.com/n/nervaalphaengine.html
   URI            [3]: https://www.uxc.com/p/tools/FuelCalculator.aspx
            madaxe_again wrote 4 days ago:
            I don’t know that voluminous tanks and heavy engines are
            necessarily a problem for something that’s designed to
            permanently live in space - the tanks can essentially just be
            onion-layered gasbags, and could be km3 in volume if you wanted. As
            to fuel - don’t get it from heavy bodies. Mine asteroids, minor
            moons, whatever.
              jhgb wrote 4 days ago:
              > I don’t know that voluminous tanks and heavy engines are
              necessarily a problem for something that’s designed to
              permanently live in space - the tanks can essentially just be
              onion-layered gasbags, and could be km3 in volume if you wanted.
              It's the opportunity cost. At low and moderate speeds (we're
              talking delta-Vs of 10 km/s and less), the same tankage simply
              gives you higher performance with chemical propulsion, so for no
              size of tankage may it actually be advantageous to use an NTR
              instead of a chemical engine. Only at extreme delta V levels do
              NTRs actually get better performance, but that's not a
              mission-to-Mars territory. LANTRs could possibly lower the
              crossover point, especially with variable Isp, but properly
              estimating how much requires calculus of variations, as I already
              said elsewhere.
              > As to fuel - don’t get it from heavy bodies. Mine asteroids,
              minor moons, whatever.
              Same issue. Your supply may be limited and/or require effort to
              extract. NTRs throw oxygen away; hydrolox and methalox engines
              use it for propulsion. For every tonne of water extracted, you'll
              go MUCH further if you go chemical, or at least with LANTR
              instead of NTR.
                maccam94 wrote 3 days ago:
                >Only at extreme delta V levels do NTRs actually get better
                performance, but that's not a mission-to-Mars territory
                Why is that not mission-to-Mars territory? You can shave months
                off the transit time with >15km/s delta-v.
                  jhgb wrote 3 days ago:
                  1) The travel time benefits are degressive owing to
                  increasingly eccentric heliocentric trajectories - the
                  changes in trajectory length get smaller and smaller as your
                  velocity vector stops being colinear with the planet's orbit
                  upon intercept, so you don't really save a lot of additional
                  time. (But you get the most benefits with even small
                  increases above Hohmann transfer speed.)
                  2) Intercept velocities, on the other hand, are progressive
                  -- pretty much for the same reason, combined with Pythagoras'
                  theorem. At one point you stop being able to aerocapture,
                  even with exerting downward lift in Martian atmosphere to
                  prolong the braking phase.
                  Owing to these two things, I'm not quite sure that propelling
                  yourself from LEO to Mars at 15 km/s would be a good idea,
                  unless you intend to crash into the planet.
                    credit_guy wrote 1 day ago:
                    That's not quite right.
                    Using a Hohmann transfer orbit [1], you get from Earth to
                    Mars in about 9 months.
                    Using an Aldrin Mars cycler [2], you can get in as little
                    as 75 days. Of course, the Aldrin Mars cycler requires more
                    delta-v, but that's the point, if you have more delta-v you
                    get there sooner. [1]
   URI              [1]: https://en.wikipedia.org/wiki/Hohmann_transfer_orb...
   URI              [2]: https://en.wikipedia.org/wiki/Mars_cycler
                    maccam94 wrote 3 days ago:
                    I am assuming that you can spend the delta-v at both launch
                    and intercept, it sounds like you are assuming it is all
                    used at launch?
                      jhgb wrote 3 days ago:
                      This starts becoming disadvantageous even faster than
                      just trying to speed up. At that point a much better
                      improvement for you is the development of a magnetoshell
                      decelerator. Mass-wise, there's no situation where
                      propulsive intercept is better since a magnetoshell
                      decelerator will be much more lightweight than even an
                      NTR stage.
                        maccam94 wrote 1 day ago:
                        Why? Propellant is relatively light in an NTR system as
                        far as I'm aware. I am also assuming that making a
                        nuclear ship aerodynamic would be unfeasible (it will
                        likely be long and only structurally reinforced
            Symmetry wrote 4 days ago:
            This is more the sort of engine you develop if you're going for an
            Apollo style mission where there's a mother craft that goes into
            orbit and a separate lander goes down to the surface. A NTR's poor
            TWR compared to conventional combustion rockets means it would be a
            bad ascent stage.
            I wouldn't assume the plan relies on ISRU at all but if it do
            having to carry the resulting hydrogen up to orbit on the ascent
            stage will be a big limiting factor so not keeping the oxygen isn't
            so large a flaw.  And if you're carrying the fuel to orbit on
            another rocket you want to get as high an ISP as you can manage
            with what you bring up.
            All of which isn't to say this would be a good plan.  I've drunk
            the SpaceX koolaid on the topic.  But if it's a bad plan at least
            it isn't a stupid one and there are reasons behind things.
              ncmncm wrote 3 days ago:
              Yeah, the SpaceX Kool-aid, at least as regards Mars colonization,
              is about as lethal as the Jonestown variety.
              There will be no Mars colonization on Starships, whatever Elon
              says. Starship is just wholly inadequate to the task. Neither
              will there be any sub-orbital passenger or freight service on it.
              Starship should be adequate for lofting lots of  Starlinks, for
              getting to the moon, and for boosting just amazingly
              well-equipped 100t outer solar system probes and telescopes. It
              might suffice for a quick visit to Mars with a half-dozen crew.
              (BTW, 9, not 6 months, each way.)
              Probably the only way to make even that work would be to send two
              ships strung on a cable, nose to nose, spun for centrifugal gee
              force, so they could still walk when they got there. Maybe the
              second ship carries hydrogen (as ammonia?) to bond to ISR carbon
              to come home on. And solar panels, to crack the carbon.
              But the first thing any attempted colonist would transmit back is
              "Can I please come home?"
                pfdietz wrote 3 days ago:
                A Mars colony will involve living in small metal boxes,
                surrounded by danger, with ruinous prices on things, if they
                are available at all.
                We can get the same experience today, without going to Mars, in
                places called "prisons".
                  skissane wrote 2 days ago:
                  I think, in the medium term, all Musk is actually aiming at,
                  is a crewed research station on Mars, with a few dozen people
                  living in it. Rather similar to what we already have in
                  People will sign up to go. First person to step foot on Mars
                  gets their name in the history books, next to Neil Armstrong.
                  The rest get to join a very elite club. I suppose prison is
                  kind of a club too, but nothing elite about it.
                  And Musk will call it a "colony"–aspirational naming. And
                  maybe, one day, in centuries to come, it will actually evolve
                  into one. I don't think Musk has really thought a lot about
                  how to get from the "colony-in-name-only crewed research
                  station" to a genuine colony – that's too many steps ahead.
                  He just trusts he'll work it out when he gets there, or if he
                  doesn't live that long, somebody else will.
                    ncmncm wrote 2 days ago:
                    "Aspirational naming" is quite a curious synonym for lying.
                    And to say that Musk has not thought much about X, for any
                    X, is quite an understatement. The closer you look at
                    anything he says, the less evidence of thought you can
                    find. Today is a golden age for glib grifters.
                      skissane wrote 2 days ago:
                      Well, look at SpaceX: he founded it, he remains its
                      CEO&CTO, and in 10 years it has gone from 0% market share
                      to over 50% global market share–which was achieved, not
                      through anticompetitive subterfuge, but simply by
                      building a substantially better product at a
                      substantially lower cost (and whatever government
                      subsidies were involved, were made available in even
                      greater amounts to competing companies which failed to
                      leverage them into the same market success). Obviously he
                      must have some capacity for intelligent thought to be
                      able to pull that off. Of course, he employs many
                      brilliant engineers, without whom none of that would have
                      been possible–but, as founder/CEO, he created and
                      sustained the corporate environment which made it
                      possible for them to achieve that.
                        ncmncm wrote 1 day ago:
                        Yet, he does literally none of the work, and every
                        unscripted public statement shows he understands
                        nothing of the technical details beyond what he has
                        learned to parrot.
                        He did not found PayPal, Tesla, or Neuralink, although
                        he has often claimed to. Hyperloop is 100% grift.
                          pfdietz wrote 1 day ago:
                          > Yet, he does literally none of the work
                          This sounds like labor-theory-of-value BS. 
                          Management and leadership is work, and it's
                            ncmncm wrote 1 day ago:
                            I have perhaps naive expectations about somebody
                            styling himself Chief Technical Officer. I guess
                            Chief Grift Officer would be too revealing.
                              skissane wrote 1 day ago:
                              CTOs aren’t always super-technical-and even
                              those who are, while the CTO of a small startup
                              might realistically have an expert-level
                              understanding of all the business’s core
                              technologies, that is no longer a realistic
                              standard when talking about a multi-billion
                              dollar firm with a highly complex or diverse tech
                              stack. Arguably, one of the most important tasks
                              for a CTO, is to be able to tell the difference
                              between good engineering executives and bad ones.
                              And, judged by that standard, Musk actually has
                              done a very good job as SpaceX CTO, much better
                              than many of its major competitors. Doing that
                              requires understanding the technology well-enough
                              to distinguish engineers and engineering leaders
                              who really understand it from those who are just
                              pretending to do so-and I think it is obvious
                              Musk does understand the technologies at SpaceX
                              (and Tesla too) well-enough to successfully make
                              that distinction. People seem to be holding him
                              to an unrealistic standard, which I doubt
                              they’d actually apply to a CTO who wasn’t
                              named Elon Musk.
                                ncmncm wrote 1 day ago:
                                I would say he gets a free pass nobody else
                                does. If the CTO of Intel spouted things as
                                idiotic as he does routinely, they would have
                                to resign.
                                  skissane wrote 1 day ago:
                                  What about Larry Ellison, CTO of Oracle?
                                  Frankly I think Larry Ellison could say any
                                  crazy thing he liked, and no one would really
                                  care, and he'd stay CTO and chair of Oracle's
                                  board. Because C-suite executives get a "free
                                  pass" all the time–especially when they
                                  combine their C-suite role with a substantial
                                  ownership interest in the company (true of
                                  both Ellison and Musk). But most C-suite
                                  execs, the average person has never heard of
                                  them, and so they don't care what they say.
                                  Whereas, Musk is a controversial celebrity,
                                  so people judge him by rather different
                                  standards than the thousands of other
                                  near-anonymous CEOs, CTOs and billionaires in
                                  the world.
                            skissane wrote 1 day ago:
                            Compare SpaceX to Blue Origin - while SpaceX has
                            succeeded in conquering over 50% of the orbital
                            launch market, Blue Origin still hasn’t made it
                            to orbit - nor has ULA’s new rocket using
                            Blue’s engines. Is that the fault of Blue’s
                            engineers? I don’t think that’s fair - some of
                            them are just as brilliant as SpaceX’s. The real
                            blame, I think, is at the executive level. Bezos
                            has never invested anywhere near as much of his
                            time and energy and personal wealth into Blue as
                            Musk has invested into SpaceX. And Musk has made
                            much better choices of executive leadership (Gwynne
                            Shotwell vs Bob Smith). That’s just one example
                            of the massive difference the ability and
                            commitment of a founder can make to the success of
                            a business. (You’d think on a forum owned by a
                            Silicon Valley VC firm, that point would be
                            uncontested and accepted as obviously true.)
                              ncmncm wrote 1 day ago:
                              Shotwell anyway has to know that the whole Mars
                              colonization shtick is total BS.
                              He has been lucky in some of his hires.
              jhgb wrote 4 days ago:
              Yeah, I did notice that the original NTR plans arose from the
              wish to upgrade Saturn V with its limited "throw weight" at third
              stage separation ( [1] ). It doesn't seem to make a lot of sense
              to design a propulsion unit for a sixty year old mission
              architecture today, though.
   URI        [1]: http://www.astronautix.com/s/saturnc-5n.html
            robonerd wrote 4 days ago:
            I'm skeptical too, but DARPA is saying the DRACO program is for
            getting to/from Mars quickly:
            > The DRACO program intends to develop novel nuclear thermal
            propulsion (NTP) technology to
            enable time-critical missions over vast distances in cislunar
            space. Unlike propulsion
            technologies in use today, NTP can achieve high thrust-to-weights
            similar to chemical
            propulsion but with two to five times the efficiency. This enables
            NTP systems to be both faster
            and smaller than electric and chemical systems, respectively. The
            propulsive capabilities
            afforded by NTP will enable the United States to maintain its
            interests in space, and to expand
            possibilities for the National Aeronautics and Space Administration
            (NASA)’s long-duration
            human spaceflight missions (i.e., to Mars). Because of the ability
            to transit space faster than
            other propulsion systems, the NTR engine can return astronauts to
            Earth much faster in case of
            an emergency and similarly ensure reduction of overall trip time
            and exposure to deleterious
            impacts to astronaut health which come with long-term spaceflight.
              mlindner wrote 4 days ago:
              It's not for getting to/from Mars quickly. It's for giving money
              to congressional districts quickly.
              jhgb wrote 4 days ago:
              > but with two to five times the efficiency
              I suspect from the number that they're talking purely about Isp.
              Once one performs a whole system analysis, it's much less rosy
              for (non-LA)NTR.
                robonerd wrote 4 days ago:
                I agree.  Furthermore, besides the mention of Mars they're also
                talking about cislunar space in that same paragraph, but
                chemical propulsion seems sufficient in cislunar space.  It's
                only takes a days to return from the moon with chemical
                propulsion, which proved sufficient in the past.
                  jhgb wrote 4 days ago:
                  LANTR would improve performance of lunar landers/cislunar
                  shuttles, especially for variable specific impulse which is
                  what LANTR could plausibly do without much trouble -- start
                  with high oxygen flow for high thrust and high propellant
                  mixture density, decrease oxygen flow later in flight for
                  higher terminal Isp. This brings you the performance of a
                  multi-stage vehicle without staging, and LANTR can even with
                  high oxygen flow deliver Isp significantly higher than what
                  hydrolox has, with propellant density several times higher
                  than what pure-hydrogen NTR gives you.
                  I've thought about trying to optimize the performance of such
                  a variable Isp vehicle, but it requires calculus of
                  variations skills that I'm lacking at the moment. I guess I
                  need to take a look at that. But there's a decent chance that
                  with a such a vehicle, you could move from the "we need to
                  mine ice on the Moon" to the "we just need to extract oxygen
                  from lunar soil; we can bring hydrogen from LEO" territory,
                  which would be a win for lunar flights (for example you
                  wouldn't be limited to polar region bases where you'd need to
                  mine water to get back home).
            snek_case wrote 4 days ago:
            Seems like you could probably get more efficient by using a nuclear
            reactor to power an ion drive? Also wouldn't need to cool fuel down
            to cryogenic temperatures.
              credit_guy wrote 1 day ago:
              I think that's basically what the VASIMIR engine is.
   URI        [1]: https://en.wikipedia.org/wiki/Variable_Specific_Impulse_...
              Symmetry wrote 4 days ago:
              For a trip to Mars the time it takes an ion drive rocket to reach
              cruising speed isn't negligable compared to the overall flight
              time.  And missing out on the Oberth effect is fairly
              significant.  If this were a flight to, say, Jupiter though
              electric drives all the way.
              chipsa wrote 4 days ago:
              Ion drives don't scale up in thrust fast enough for it to be
              worthwhile for manned missions.
              jhgb wrote 4 days ago:
              At 1 AU from the Sun, and possibly all the way to Mars, advanced
              photovoltaics may very well be better than a nuclear reactor for
              powering ion engines: It has very high system-level power/weight
              ratio (in lab around 300 W/kg, currently in operation around
              150-200 W/kg), possibly could even power an ion engine without
              heavy power conditioning equipment ("direct drive electric
              thruster"), and also scales down for smaller probes. So for a
              trip to Pluto, a reactor would be useful, for a trip to Mars,
              it's hardly necessary.
          dotnet00 wrote 4 days ago:
          ACES is effectively dead, although a lot of its ideas ended up
          getting into Vulcan's upper stage. Long duration in its context means
          days. It was intended to do something similar to what SpaceX is doing
          with Starship, using the boiloff gas to pressurize the tanks.
          The problem with longer duration storage of hydrogen is that there
          really isn't any option besides going with a denser or thicker
          material, while modern rocket wall thicknesses are measured in
          millimeters of lightweight metals or composites.
          However, the convenient thing about NTR is it should be a lot easier
          to switch to something less prone to seeping through everything. It
          would be a matter of weighing the losses from needing a heavier tank
          against the losses from using heavier propellant.
            unchocked wrote 4 days ago:
            Long term hydrogen storage isn't that bad with the proper
            architecture. You need a cryocooler which can be powered by the
            nuke, and thermal shielding for the tank which in vacuum is thin
            film and of minuscule weight.
            Hydrogen leakage and structural embrittlement are overblown, i.e.
            the Space Shuttle tank is one of the most mass efficient
            architectures in history and it was full of liquid hydrogen.
            Terrestrially, you can buy a Toyota hydrogen car today. Materials
            matter, but people act like the thing needs to be made of 4" plate
            and will fall apart if you look at it. Scaling helps here too, as
            volume increases to the third power while wall area increases to
            the second.
            The thing will, if there is any sense in the architecture, be
            assembled in orbit so gossamer heat shields and the like won't be a
            problem, nor will an extended assembly program that makes with a
            separately launched nuclear reactor.
            For ISRU Mars return, water is incredibly abundant and there's no
            concern with "wasting" residual oxygen. For lunar applications,
            water may be scarce but oxygen is abundant in regolith.
            You can't beat hydrogen as a fuel. As the lightest molecule, you
            get the highest exhaust velocity for the least energy input.
              messe wrote 4 days ago:
              > pace Shuttle tank is one of the most mass efficient
              architectures in history and it was full of liquid hydrogen.
              That being said, it didn't have to last very long while filled
              with LH2/LOX; a few hours at most prior to launch, and a few
              minutes during launch. They were never reused, unlike the orbiter
              and SRB segments.
                ncmncm wrote 3 days ago:
                More's the pity. They actually used extra fuel to keep it from
                joining the shuttle in orbit. That much raw aluminum pressure
                vessel in orbit could have been so useful!
              namibj wrote 4 days ago:
              In vacuum, the required insulation is cheap and easy: more layers
              of the famous metallized crinkled plastic foil. The stuff that
              (with gold-colored metallization) is an iconic part of "the"
              satellite/space probe design.
              The hard part about that insulation is that on earth, you need to
              sustain a vacuum in the annular space while overall being light
              due to the LH2 itself being light. Ideas would be to get tension
              fibers bridging that annular space, the inner tank with the LH2
              being slightly pressurized, and thus the outer wall being kept
              from large-scale buckling (and small-scale buckling is cheap to
              reinforce for with an isogrid (triangle honeycomb) or other
              similar reinforcement structure on the outside of it). But in
              space, the outer wall isn't needed, because space is already a
                ncmncm wrote 3 days ago:
                That seems to be why GP suggested wrapping it after reaching
                foobarian wrote 4 days ago:
                Would that suggest a staged approach where the long-range
                vehicle is fueled up in orbit?
                  namibj wrote 3 days ago:
                  A pure vacuum shuttle should be fine with getting launched
                  and then wrapped with the insulation afterwards.
                  Launching it empty is likely not really easier than launching
                  it full, due to LH2's low density. But that's not important.
              jhgb wrote 4 days ago:
              > You can't beat hydrogen as a fuel. As the lightest molecule,
              you get the highest exhaust velocity for the least energy input.
              You can't beat it in terms of exhaust velocity, but you can often
              definitely beat it in terms of whole-system performance.
              trhway wrote 4 days ago:
              >Scaling helps here too, as volume increases to the third power
              while wall area increases to the second.
              Not really. As surface increases the wall tearing force at the
              given pressure is increases too, so you have to increase the wall
              thickness, and thus the mass of the tank also grows close to the
              third power.
                namibj wrote 4 days ago:
                The scaling benefit is that you save insulation.
              Filligree wrote 4 days ago:
              > You can't beat hydrogen as a fuel. As the lightest molecule,
              you get the highest exhaust velocity for the least energy input.
              This is probably right, but the way you said it made me wonder.
              Would it be possible to strip electrons from atoms, then use just
              the electrons as propellant? Or would the ensuing static charge
              of the spaceship render this infeasible? I imagine it'd pull in
              electrons from all around itself, but I don't know how the
              numbers come out.
                mLuby wrote 3 days ago:
                Or just photons. That's what the genie gives you when you ask
                for a torch drive—should've been more specific.
                perihelions wrote 3 days ago:
                - "Or would the ensuing static charge of the spaceship render
                this infeasible?"
                Back-of-the-envelope math says a large spaceship will reach 100
                kV potential at a charge imbalance of around 1e15 electrons
                (total mass: 1e-15 kg). So yeah, completely unfeasible.
                (It's asking the wrong question though. Electric thrusters
                aren't thermal systems, and aren't limited by molecular weight
                as severely as thermal engines are. You can get stupidly high
                Isp (>200 km/s) out of heavy ions, just by raising the
   URI          [1]: https://en.wikipedia.org/wiki/Ion_thruster#Comparisons
                  pfdietz wrote 3 days ago:
                  And, in fact, you WANT heavy ions, as the thrust/area of an
                  ion engine at a given exhaust velocity scales as the square
                  of the ion mass/charge ratio.  The thrust in an ion engine is
                  limited by space charge (where the charge of the ions between
                  the accelerating grids becomes similar to the charge on the
                  grids).  Using heavier ions also reduces the ionization
                  energy/mass.   There's been work on using molecules or small
                  droplets ("colloidal thrusters") to get even higher
                  mass/charge, but you need to totally avoid generation of
                  fragments with low mass/charge as they will dominate the
                e_y_ wrote 3 days ago:
                I'm assuming you're asking about ejecting the electrons and
                remaining positively-charge hydrogen ions separately, since
                keeping the hydrogen around would be a waste of mass.
                I'm no expert at space propulsion, but I think this would have
                a few issues:
                - Hydrogen has a pretty high ionization energy, even higher
                than Xenon
                - As you said, static charge buildup
                - Momentum = mass * velocity. Electrons have 1/1836 the mass of
                a proton, so for the same momentum you need a much higher
                - Imparting velocity is harder for low-mass particles because
                they tend to zoom off very quickly if not contained
                ben_w wrote 4 days ago:
                If you strip the electrons off some atoms and use just the
                electrons[0] as reaction mass, you will eventually get a large
                enough electric charge you can no longer throw the electrons
                away from you. Electric forces behave similarity to gravity, so
                while it wouldn’t normally be phrased like this, you could
                say your engine exhaust will eventually no longer have escape
                velocity from your ship.
                For this reason, ion drives do things to neutralise the net
                (If you meant using them as a power source rather than reaction
                mass, it’s technically possible but that’s called a
                capacitor and they have very low energy density).
                [0] or, by symmetry, just the nucleus.
                  tgflynn wrote 3 days ago:
                  You didn't address this part of the parent comment:
                  > I imagine it'd pull in electrons from all around itself,
                  but I don't know how the numbers come out.
                  I never thought of it before but it seems like that should
                  work.  "Space" is actually a neutral plasma, right, so it
                  should be full of free electrons.  Those should neutralize
                  the ship before any significant charge builds up.  It seems
                  like you should be able to use space itself (or more
                  accurately the interplanetary medium) as a massive ground
                  plane to complete the circuit for the charged exhaust beam.
                    e_y_ wrote 3 days ago:
                    Space is pretty empty. From what I can find, the
                    interplanetary medium is around 5 particles/cm^3 compared
                    to the exhaust from the ion thruster which seems to be
                    around 10^6 particles/cm^3 and disperses to 10^4
                    particles/cm^3 further away.
                    Source: [1] "As the charge-exchange plasma
                    density near spacecraft is at least about three orders of
                    magnitude larger than the solar wind plasma density, the
                    plasma environment of DS1 spacecraft is completely
                    dominated by the charge-exchange plasma in the plume."
   URI              [1]: https://trs.jpl.nasa.gov/handle/2014/15643
                toopok4k3 wrote 4 days ago:
                Funny that you mention this, Ion thrusters do exist. They are a
                thing but with very limited uses cases. They still need a kind
                of propellant gas like Xenon or Krypton that gets used.
   URI          [1]: https://en.wikipedia.org/wiki/Ion_thruster
            robonerd wrote 4 days ago:
            Apparently Bruno has been talking about two orders of magnitude
            improvement to Centaur V's duration.  Seems far fetched to me, but
            I think months of duration would be necessary to make this engine
            worthwhile (the PDF is talking about the value of this engine for
            getting astronauts home from Mars quickly in emergencies; that
            would only be possible with months of duration at least I think.)
            DARPA says they're expecting designs using liquid hydrogen, and as
            far as I understand liquid hydrogen would be the most efficient
            propellant for an NTR.    What might the best storable alternative
              dotnet00 wrote 4 days ago:
              From what I understand, Bruno didn't say that Centaur V has those
              two orders of magnitude improvements, rather that they're aiming
              to push improvements of that level over the next few years. That
              said, I don't think it's too far fetched, assuming that the long
              duration version is separate from the regular version (ie it can
              be heavier to support denser tanks).
              Liquid Hydrogen would be most efficient in a pure physics sense,
              but due to the mass tradeoffs with storage tech, there may be
              other propellants that are comparable in a practical sense. I'm
              not informed enough on the matter to say exactly which would be
              better, but for a somewhat comparable point of reference,
              Hydrogen+Oxygen is the most efficient propellant for chemical
              rockets but when accounting for the special tanks needed for
              storing hydrogen, methane can achieve pretty comparable
              performance due to being perfectly fine in a thin-walled
              stainless steel tank.
          ceejayoz wrote 4 days ago:
          Could you make the hydrogen en-route? Solar power to crack water, use
          the oxygen for breathing? [1] says this is done on the ISS currently.
   URI    [1]: https://www.nasa.gov/content/space-applications-of-hydrogen-...
            colechristensen wrote 4 days ago:
            The mass of the oxygen in water is 8x that of the hydrogen, and you
            just don't need all the much for humans, and what you do have after
            respiration (CO2) gets recycled through the Sabatier process (H2O
            -> O2, H2; CO2 + H2 -> CH4 + H2O)
            I.e. water is a quite inefficient storage medium for hydrogen and
            you're probably better of making heavier containment vessels for
            liquid hydrogen (of course a calculation could be shown to
            demonstrate the balance, but a tank weighing 8x the contents is a
            very long way from the extremely light tanks used in spaceflight)
            jandrese wrote 4 days ago:
            If you are lugging all of that water mass along you could store it
            in a jacket around the crew compartment, providing additional
            radiation protection for most of the trip.
            You're going to need humongous solar panels to support this, but
            since you are in space this isn't an intractable problem.  A small
            but constant acceleration would probably make life better in the
            spacecraft as well.
            robonerd wrote 4 days ago:
            Maybe?    I believe LH2 has about 35 mols of hydrogen per liter,
            while water is 55 mols per liter.   Storing hydrogen as water seems
            practical from that perspective, but what of the power needed to
            split that water?   I think you'd need quite a lot of power to
            split that much water fast (starting a few days before running the
            engine.)  Splitting it slowly over time using solar energy would
            seem to still leave you with a storage problem, but perhaps a more
            tractable one.
            Maybe instead of electrolysis, they could use heat from the
            reactor?   Thermolysis needs 2500 C though.
              chipsa wrote 4 days ago:
              NTRs are basically open-cycle gas cooled reactors. The thermal
              limit on the reactor temp is when does stuff start to melt.
              Project Rho[0] suggests that's the reactor temp anyways. But you
              need to be able to separate out the oxygen from the thermolysis
              stream, rather than just feeding the entire thing into your
              engine, both because your Isp would go to crap if you tossed the
              oxygen out too, and you'd have oxidizing your reactor problems.
              Though, you could just store it all as ammonia, and you get more
              hydrogen for your buck, and can probably just feed that all
              through the reactor.
   URI        [1]: http://www.projectrho.com/public_html/rocket/enginelist2...
                Symmetry wrote 4 days ago:
                Right.    In a NTR the nuclear fuel has to be hotter than the
                hydrogen (or ammonia or methane or whatever) propellant so that
                the heat energy from the first conducts to the second.    In a
                combustion energy the fuel and the propellant are the same
                substance so you try to limit conduction and can end up with
                propellant much hotter than the engine.
              jandrese wrote 4 days ago:
              Wouldn't you just size the engines small enough that they
              instantly burn off the H2 as you crack it?  The solar power
              should be even and constant so you can size the system to match. 
              It is going to require a very large solar array, especially since
              your spacecraft is going to be really heavy with all of that
                jhgb wrote 4 days ago:
                You could design a pulse detonation engine for this.
                Electrolyze water continuously; detonate it in a pipe every now
                and then. It's a very simple design that gives you quite a bit
                of performance for hopping in the asteroid belt. Specific
                impulse similar to a hydrolox engine, or slightly worse than
                regular hydrolox engines if operating stoichiometrically,
                although the detonation mode could compensate for that
                (detonation rocket engines can potentially get ~10% better Isp
                performance than "classical" rocket engines). However, you get
                triple the propellant density (water has ~1000 kg/m³; hydrolox
                is at around 340 kg/m³). This makes it much better compared to
                a classical hydrolox vehicle wherever gravity is near zero so
                that you don't need lift-off thrust.
            nicoburns wrote 4 days ago:
            Where are they going to get the water from?
              ceejayoz wrote 4 days ago:
              You'd be bringing oxygen and hydrogen along anyways. Why not
              bring it in water form?
              evgen wrote 4 days ago:
              Much easier to store water for long duration than to store
              hydrogen. It even serves a useful purpose as radiation shielding
              for some solar events.
        XorNot wrote 4 days ago:
        I really hope this gets off the ground - literally and figuratively.
        NTRs would be a game changer for Sol exploration and open up some real
        serious options for things like intercepting interstellar objects.
        hendler wrote 4 days ago:
        Link with anchor
   URI  [1]: https://orbitalindex.com/archive/2022-05-25-Issue-170/#darpa-m...
        _Microft wrote 4 days ago:
        If I remember correctly, SpaceX would be happy to experiment with
        nuclear-thermal propulsion but cited the lack of a engine test stand as
        reason why they aren't actively working on it. I'll see if I can find a
        quote for that. I am rather sure that it was by Gwynne Shotwell, COO of
        SpaceX. (Edit: progress! I think it's in a talk by her at MIT Road to
        Mars 2017. Too bad I cannot find a recording of that).
        NERVA is another term to search for if you are interested in
        nuclear-thermal propulsion.
   URI  [1]: https://en.wikipedia.org/wiki/NERVA
          dotnet00 wrote 4 days ago:
          Given the regulatory delays and uncertainty of somewhat safer things
          like approval for orbital launches from Starbase, I imagine that
          SpaceX would not be all too eager to experiment with NTR given the
          regulatory environment for anything nuclear and that they want to get
          Starship flying humans within this decade.
          The regulatory environment is bad enough that I still expect this to
          eventually get cancelled again, only to be taken seriously when
          eventually another country is close to catching up technologically.
            throwaway0a5e wrote 3 days ago:
            > imagine that SpaceX would not be all too eager to experiment with
            NTR given the regulatory environment for anything nuclear and that
            they want to get Starship flying humans within this decade.
            Or they might want to do it anyway knowing it would never be
            allowed to launch in order to drag the overton window in a more
            permissive direction.
          oldstrangers wrote 4 days ago:
          Whats the relevance of SpaceX here?
            ClumsyPilot wrote 4 days ago:
            As much as i like them, SpaceX approach of move fast with explosion
            is best kept away from nuclear :)
            Also they don't seem to have relevant experience
            mlindner wrote 4 days ago:
            There isn't one really other than SpaceX COO (or was it Musk?)
            making a single passing reference to them in response to a question
            at a conference keynote a few years ago.
            robonerd wrote 4 days ago:
            It's tangentially related insofar as SpaceX says they're planning
            to go to Mars, and this NTR engine is also for going to Mars.    But
            according to the DARPA announcement, DARPA determined that Falcon 9
            doesn't presently support this sort of liquid hydrogen payload.  
            They suggest Vulcan Centaur could do it with fairing modifications.
             (Vulcan Centaur hasn't flown yet.   Where are the engines, Jeff??)
              mlindner wrote 4 days ago:
              It's not really related as SpaceX has no plans to use them and
              isn't exactly interested in doing so as they don't see them as
              needed. Also NTR are kind of a tossup on efficiency as while you
              get somewhat better fuel efficiency, their mass is huge because
              you're lugging an entire nuclear reactor core along with with
              you. The thrust to weight ratio isn't great.
                leereeves wrote 4 days ago:
                Can you mount the nuclear reactor far away from the crew module
                and reduce the mass of shielding, as in old sci-fi?
                  mlindner wrote 4 days ago:
                  It's not just the shielding. An NTR engine needs a lot of the
                  same plumbing that other engines need, including turbopumps,
                  and they have additional cooling requirements because of the
                  much hotter fuel. Add on to that the already very heavy
                  Uranium and control rods.
                  Add on to that I'm not quite sure how you prevent the
                  engine's nuclear reactor from going into meltdown once it
                  shuts off. The residual heat from the decay products in the
                  seconds to minutes after shutdown will be substantial and
                  that heat needs to go somewhere or it'll cause a reactor
                  meltdown the instant you shut off the engine. So you need all
                  the hardware to dump heat somewhere (presumably radiators and
                  a cooling system that pumps hydrogen through the reactor
                  while it's shut off) so that's even more mass.
                  The only way NTR really makes sense to me is if your
                  spacecraft is truly massive, but literally no one has
                  anything like that even in planning stages.
                    Robotbeat wrote 4 days ago:
                    The fuel is actually cooler for NTR than chemical. With
                    chemical, the peak heat can occur in the gaseous state away
                    from anything solid, but for conventional nuclear thermal,
                    the peak heat is generated in solid material and needs to
                    conduct through to fluids, which are therefore at lower
                    And the way they handle shut down is they continue a small
                    flow of propellant through the engine until the core cools
                    off and the hottest, shortest lived stuff decays away. NTRs
                    usually run for a few hours at most, not years, so the
                    decay heat a few minutes after shutdown isn’t that bad.
                      mlindner wrote 4 days ago:
                      >  The fuel is actually cooler for NTR than chemical.
                      With chemical, the peak heat can occur in the gaseous
                      state away from anything solid, but for conventional
                      nuclear thermal, the peak heat is generated in solid
                      material and needs to conduct through to fluids, which
                      are therefore at lower temperatures.
                      Pretty sure this can't be true. In order to have a higher
                      exhaust velocity the fuel temperature needs to be higher
                      than chemical propulsion.
                        Robotbeat wrote 3 days ago:
                        It is true. Basic gas theory stuff, the average
                        molecular speed (close to the speed of sound) at a
                        given gas temperature is, to first order, higher for a
                        lower molecular mass.  Otherwise, why bother with such
                        a difficult to store propellant which you’re not even
                        extracting energy from (as the energy comes from the
                        reactor, not the propellant as in chemical rockets)?
                        Chemical rockets reach over 3500 Kelvin, but Nerva only
                        got to around 2300 Kelvin.
                        leereeves wrote 4 days ago:
                        > In order to have a higher exhaust velocity the fuel
                        temperature needs to be higher than chemical
                        Are you accounting for the fact that the NTR exhaust
                        (hydrogen) is lighter than chemical rocket exhaust?
                        At the same temperature, both propellants have the same
                        average kinetic energy per molecule, so the hydrogen
                        must be moving faster.
                          mlindner wrote 3 days ago:
                          I wasn't accounting for it, but I assumed it wouldn't
                          be significant. The efficiencies claimed are over 2x
                          better than chemical rockets. You don't get that much
                          just from changing gasses.
                            perihelions wrote 3 days ago:
                            Huge difference between the molecular weight of
                            H₂ (2) and its lightest combustion product OH
                            Robotbeat wrote 3 days ago:
                            Yes, you can. Hydrogen, for the same temperature,
                            has a far higher speed of sound (which is close to
                            the average speed of the gas molecules) than air or
                            water vapor. This is why your voice is higher
                            pitched when you breathe in helium (also a light
                            gas like hydrogen).
                            Basic kinetic gas theory stuff.
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