[HN Gopher] DARPA moving forward with nuclear thermal engine design ___________________________________________________________________ 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] ___________________________________________________________________ (page generated 2022-05-25 23:00 UTC)