[HN Gopher] DARPA moving forward with nuclear thermal engine design
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DARPA moving forward with nuclear thermal engine design
Author : tectonic
Score : 164 points
Date : 2022-05-25 15:24 UTC (7 hours ago)
(HTM) web link (orbitalindex.com)
(TXT) w3m dump (orbitalindex.com)
| _Microft wrote:
| 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.
|
| https://en.wikipedia.org/wiki/NERVA
| dotnet00 wrote:
| 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:
| > 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:
| Whats the relevance of SpaceX here?
| ClumsyPilot wrote:
| 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
| robonerd wrote:
| 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:
| 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:
| 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:
| 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:
| 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.
|
| 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:
| > 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:
| 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:
| > In order to have a higher exhaust velocity the fuel
| temperature needs to be higher than chemical propulsion.
|
| 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:
| 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.
| Robotbeat wrote:
| 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.
| mlindner wrote:
| 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.
| timcavel wrote:
| hendler wrote:
| Link with anchor
| https://orbitalindex.com/archive/2022-05-25-Issue-170/#darpa...
| XorNot wrote:
| 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.
| robonerd wrote:
| The PDFs here have a lot more information:
| https://sam.gov/opp/af490b568d2a438498afa1e80bce63e5/view
|
| 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.
| ceejayoz wrote:
| Could you _make_ the hydrogen en-route? Solar power to crack
| water, use the oxygen for breathing?
| https://www.nasa.gov/content/space-applications-of-hydrogen-...
| says this is done on the ISS currently.
| jandrese wrote:
| 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:
| 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.
| jandrese wrote:
| 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 water.
| jhgb wrote:
| 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.
| chipsa wrote:
| 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.
|
| [0]:http://www.projectrho.com/public_html/rocket/enginelist
| 2.php
| Symmetry wrote:
| 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.
| nicoburns wrote:
| Where are they going to get the water from?
| evgen wrote:
| 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.
| ceejayoz wrote:
| You'd be bringing oxygen and hydrogen along anyways. Why
| not bring it in water form?
| colechristensen wrote:
| 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)
| dotnet00 wrote:
| 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.
| robonerd wrote:
| 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 be?
| dotnet00 wrote:
| 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.
| silencedogood3 wrote:
| unchocked wrote:
| 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.
| trhway wrote:
| >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:
| The scaling benefit is that you save insulation.
| jhgb wrote:
| > 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.
| namibj wrote:
| 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 vacuum.
| foobarian wrote:
| Would that suggest a staged approach where the long-range
| vehicle is fueled up in orbit?
| messe wrote:
| > 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.
| Filligree wrote:
| > 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.
| ben_w wrote:
| 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 charge.
|
| (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:
| 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.
| toopok4k3 wrote:
| 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.
|
| https://en.wikipedia.org/wiki/Ion_thruster
| jhgb wrote:
| 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.
| Symmetry wrote:
| 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.
| jhgb wrote:
| 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
| (http://www.astronautix.com/s/saturnc-5n.html). It doesn't
| seem to make a lot of sense to design a propulsion unit for
| a sixty year old mission architecture today, though.
| robonerd wrote:
| 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._
| jhgb wrote:
| > 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:
| 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:
| 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).
| mlindner wrote:
| It's not for getting to/from Mars quickly. It's for giving
| money to congressional districts quickly.
| madaxe_again wrote:
| 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:
| > 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:
| >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:
| 1) The travel time benefits are degressive owing to
| increasingly hyperbolic trajectories - the changes in
| trajectory length get smaller and smaller as your
| velocity vector stops being colinear with the planet's
| orbit, 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 downwards 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.
| snek_case wrote:
| 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.
| jhgb wrote:
| 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.
| Symmetry wrote:
| 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:
| Ion drives don't scale up in thrust fast enough for it to
| be worthwhile for manned missions.
| Stevvo wrote:
| 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?
| Robotbeat wrote:
| 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.
| maccam94 wrote:
| From what I've read NERVA was actually killed by Nixon for
| political (cost) reasons.
| https://en.wikipedia.org/wiki/NERVA#Cancellation
| danans wrote:
| Darn, I clicked on it hoping for a new terrestrial energy
| production technology.
| jahabrewer wrote:
| This would be for a ship that stays in space, right? (as in, not
| using an NTR from ground to orbit)
| Robotbeat wrote:
| 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.
| marktangotango wrote:
| 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.
|
| [1] https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket
| TedDoesntTalk wrote:
| > One key challenge with nuclear reactors in space is the risk of
| contaminating Earth
|
| What?
| [deleted]
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