The US military is getting serious about nuclear thermal propulsion
arstechnica.com
arstechnica.com
A few careers ago (NASA), I designed (not built) something called a duoplasmatron breeder reactor, which is a reactor driven remotely by a tritium ion beam. The nuclear radiation reflector would look something like a parabolic layered medium (gold, CVD diamond, Si3N4, carbon composite) to achieve a wide range of photon and neutron reflectivity with high heat tolerance. A second parabolic thermal reflector baffle may provide additional boost from anything that gets absorbed as heat.
So... if anybody is doing this sort of thing now, I’d be very interested to hear about it.
This reads like it's straight out of Star Trek ;).
Seriously though, one of the things I like about HN is the chance to hear from people with relevant backgrounds who don't otherwise have a major platform to share their perspective.
A common material is Tungsten Carbide, which was what caused the accidents with https://en.wikipedia.org/wiki/Demon_core
But here's an overview: https://en.wikipedia.org/wiki/Neutron_reflector
Reflectors are important because they allow you to have a quantity of radioactive material that's not supercritical on its own, but becomes supercritical when surrounded with the reflector. This allows controlling when things happen (unless you screw up like the accidents above)
1. If you jettison a 1kg rock at 99.999% the speed of light, it'll not really be 1kg anymore because of something with energy and mass being the same near the speed of light?
2. At that point you'd be jettisoning raw energy?
3. Could you make a single molecule act like a very heavy object this way?
4. What would happen if that single molecule hit a planet?
5. Would fusion (or fission) be capable of creating more mass through velocity than the weight of the reactor? As in, using a couple molecules to create enough energy to speed up a molecule so much that it acts massive.
(I’m less clear on how you go from energy to mass. I think you can knock a photon into an electron with enough force to escape its nucleus but you’re not going to get new matter out of it. I wonder whether further knowledge of the Higgs Boson would give us some ideas in this area.)
So photons can’t transmit forces dependent upon mass - but they can transmit momentum. I remember being told this is what keeps a stars gravity from collapsing the matter into a singularity or otherwise blowing apart.
I think you’re probably right that speeding matter up to significant fractions of c would increase its effect on your opposite velocity, but it’s probably hard to benefit from as an engineered system. Like having a particle accelerator the size of our orbit around the sun to make meaningful thrust-hard.
1. The relativistic mass is the rest mass / sqrt(1 - (v/c)^2) which in this case is 223.607356769625 kg. The kinetic energy is the change in mass, so 222.607356769625 kg which through the conversion E = mc^2 is about 2 * 10^19 Joules. The momentum is mass times velocity which is about 6.7 * 10^10.
2. No. You'd be jettisoning a very nasty rock. That will immediately explode on impact.
3. There is no limit to the mass it could have.
4. The first thing it hit, would convert into highly energized particles still moving in the same direction, which would hit new particles and so on. The result of the cascade will be to generate a shower of high energy particles with the same energy and momentum as the original. Or in short, BOOM!
5. Because of conservation of energy/mass, no. Even with 100% efficiency, the reactor would have to lose the same amount of mass that shows up as kinetic energy in the ejected particle.
Project Rho (the best source for this kind of stuff) is listing various nuclear thermal designs with high 3 digit Isps, e.g. 900s for this design: http://www.projectrho.com/public_html/rocket/enginelist2.php...
If that's realistic, it'ss certainly an improvement over the 348s of the Falcon 9 second stage (and that figure is representative generally of chemical rockets), but it's not a sea change.
You're thinking of nuclear-electric, which uses an ion engine just like solar does. It can burn continuously, but it has incredibly low thrust.
edit: (Note that I'm basically saying the same thing as baq, just in a different way)
edit 2: except for the part where I'm wrong.
Okay it's not normal "fuel" but you still have a limited supply that can be exhausted.
EDIT: The word is "propellant"! Thanks for the correction!
Convenient calculator: http://www.quantumg.net/rocketeq.html
Most Hall-effect thrusters provide millinewtons of thrust, with the best lab model producing 5.4N and the planned AEPS motor producing a theoretical max of 2.356N.
A single Merlin 1D motor produces 690kN of thrust at sea level. You would need over 290,000 AEPS engines to match one Merlin 1D engine. At 40kW per engine, this would use 11.7 gigawatts to run, so you’d also have to add in the weight for a dozen or so nuclear power plants.
Keep in mind that the falcon 9 uses nine Merlin 1D motors for the first stage.
Even worse, even an infinite number of AEPS engines would not match a single chemical rocket engine of any type, because Hall effect thrusters have a thrust-to-weight ratio of significantly less than 1. So it can't even lift itself off the pad, let alone anything else.
For a relevant comparison, the thrust-to-weight ratio of the Merlin 1D engine by itself is something like 180:1, but the overall thrust-to-weight ratio of the entire rocket at launch is only 1.4:1. So just in order to get a rocket that can lift off at all and deliver useful payload to orbit you need at least a 3-digit thrust-to-weight ratio on the engine itself. Ion thrusters don't remotely come close.
This is only a problem for take off from large bodies. Mass per mass, you'll get a lot more delta-v from a Hall engine than from a Merlin. It'll just take a lot longer to get to that point.
Which brings up an interesting option for NTRs: bimodal propulsion. From a single propellant tank you could drive both an NTR and a Hall thruster, depending on your acceleration needs. Hall thrusters aren't heavy (certainly not compared to fission reactors) and you could use your NTRs when you needed high acceleration (such as to reduce transit times or to take off and land from a big rock) and use the low-power extreme-efficiency Halls for most of the trip when you'd be coasting.
Yes, that's what we're talking about here.
> Mass per mass, you'll get a lot more delta-v from a Hall engine than from a Merlin.
Yes, that's what specific impulse measures (which is what most of the larger conversation has been about).
As for bimodal propulsion, it makes you wonder if that would be worth doing at all, or if it wouldn't be better to just have separate engines and fuel tanks so each can do what it's best at. I suspect it might end up being less weight and complexity just to use an ion thruster as designed using xenon propellant. Xenon fuel tanks aren't typically all that big anyway.
So, once you take into account these factors (rather than looking at the raw efficiency of just the engine alone), it ends up not being as much of an obvious win.
e.g. During the 1980 Damascus Titan missile accident the missile exploded underground in a bunker and the warhead was thrown a fair distance but was recovered relatively intact:
https://en.wikipedia.org/wiki/1980_Damascus_Titan_missile_ex...
And that was something that was intended to explode!
It needs to be able to survive any kind of catastrophic detonation/break-up/crash landing you can think of, because the alternative is spreading a large amount of radioactive material directly into the atmosphere/onto the Earth's surface. For example, the Challenger orbiter itself was fine, but when the SRB blew up it took up the orbiter with it. So if there'd been any nuclear materials in the orbiter, even if they weren't used at launch, they still would've needed serious shielding.
And it doesn't matter if the consequences of said radioactive material release aren't actually that bad in the grand scheme of things (like Fukushima looks not to have been) -- what matters is that the public reaction to such an event would preclude the possibility of ever launching it again.
Edit: Regarding the Challenger accident - the crew compartment survived the explosion and the crew were still alive for a while after :-(
https://en.wikipedia.org/wiki/Space_Shuttle_Challenger_disas...
https://en.wikipedia.org/wiki/USS_Thresher_(SSN-593)
https://en.wikipedia.org/wiki/Lenin_(1957_icebreaker)#Nuclea...
Rockets have gone the wrong way before and been intentionally self-exploded mid-air.
An awesome description of this is here: https://what-if.xkcd.com/29/
The linked article is talking about the radiation shielding properties of what essentially amounts to a large swimming pool's worth of water. You know how much that would weigh?? Water is not a good radiation shield, it's just cheap here on Earth, so we use large quantities of it in applications where weight doesn't matter.
We can use two things as shielding - propellant and distance. For the first, we would want to make sure we don't use all the propellant with the reactor running hot, but decrease power output as we run out of propellant. As for distance, we may want to add a foldable structure between the propulsion section and the habitable section that would be extended to its full length prior to starting the nuclear reactor.
* It needs to be strong enough to handle the full acceleration of the engines.
I don't think we can expect to have high accelerations with NTRs anyway, so I don't think it'd need to be particularly robust. Also, propellant tank walls can be structural elements here too. If a foldable structure isn't practical, we could just assemble it from rigid elements lofted on other launches.
DennisP was talking about SSTO, which implies the reactor is running during launch.
In reality you'd only use it in space. During launch you've just got uranium, which isn't very radioactive before you start getting fission products.
> You'd need heavy radiation shielding, heavy shielding around the reactor itself so that it wouldn't be blown to bits in the event of a launch explosion, etc.
Not nearly as much as you think. You need some shielding between the reactor and any radiation-sensitive payloads (including humans) but you don’t need anything around the reactor; radiating into outer space is kind of like pouring water into the ocean.
This requires the reactor not to be operating while on the ground, of course, and probably not even fueled until it reaches orbit. (Note that I said “fuel”—ie uranium—and not “propellant”.) But since the purpose of NTR’s is to enable deep space travel beyond cislunar space, that’s not a blocker.
> Plus, hydrogen is the least dense propellant so your fuel tank needs to be much bigger than e.g. the RP-1/LOX fuel tanks
NTR’s are competing with hydrolox upper stage engines for propulsion in space. They aren’t competing with high-thrust RP-1 or methane engines for launch. In fact RP-1 isn’t even in the picture after you reach LEO. SpaceX is going with methane over hydrolox for Starship largely because liftoff from Mars is a requirement and methane can be synthesized on Mars.
> You end up in a much worst place than even hydrolox, because at least in hydrolox the liquid oxygen fuel tank at least can be relatively small. With NERVA it's only hydrogen.
This makes zero sense. There’s no reason you couldn’t use oxygen as propellant in an NTR; it’s just that hydrogen works better.
Again I think you’re missing the point—density is important for launching from the ground because during launch, you need to be able to produce a TWR > 1. But once you’re in orbit, none of that matters anymore.
Upper stages—especially ones for going anywhere past LEO—are already predominantly hydrolox because the better specific impulse of hydrolox, combined with its low mass, more than compensates for the added dry mass of tankage.
So it's not overstating it to say that there are some problems with nuclear thermal rockets, otherwise they would be commonplace by now.
But heavy lifting to LEO and GEO us a well-paying market, drawing most of the investment. It excludes nuclear propulsion, though.
I'd much rather pin my hopes on chemical rockets in the form of SpaceX's Starship.
I didn't say it did. But it's one of the two most feasible options, the second involving lunar ISRU and orbital propellant depots. NASA's Mars Design Reference Mission has consistently included NTR studies.
Going past Mars and into the Belt, an NTR is practically essential.
> If you're going to pin all your hopes on NTRs I think it's still going to take decades.
Anything we try is going to take decades if you're talking about real time and not Elon time. But that's beside the point.
The point I was trying to make is that there has been approximately zero serious investment in human spaceflight beyond LEO once Apollo wrapped up. The Soviets decided they didn't want to land on the Moon after all, the US decided they would rather build a flying space truck that goes to LEO than build on Apollo, and everyone else spent decades just catching up. Since an NTR is only useful for interplanetary flight, no interplanetary flight means no need to develop NTR.
Interestingly you can inject the oxygen into the exhaust to act as a high-thrust, low-Isp afterburner. That’s the LANTR concept.
First is engine mass. (All numbers from memory)
A SpaceX Raptor weighs around 4,000 lbs, and generates over 400,000 lbs of thrust (100x). NERVA weighed around 40,000 lbs and only generated 50,000 lbs of thrust (1.2x).
A fully fueled orbital Starship in would weigh around 2.5M with about 25,000 lbs of engine, 150,000 lbs of ship and 2.3M lbs of fuel generating 1.2M lbs of thrust out of three vacuum Raptors.
For a similar level of thrust from NERVA would add nearly 1M lbs of dry mass. Newer designs can cut that engine mass, but at most in half. And your nuclear ship could get by with less thrust and just burn the engine longer. But that brings two new questions
Where does all the heat go? NTR send almost all of the heat out the back, but not all. Running that engine for hours on end in the vacuum of space (Instead of the very conductive earth NERVA was tested on) is going to heat stress your engine and everything it’s connected to. Do you need to add massive radiators and insulation to run the engines longer?
How do you take off on Mars? If you have a lot less thrust, you will only be able to lift a lot less fuel and cargo. And you can’t land or take off on Earth. Unlike Starship NTRs may only be usable in between planets, requiring carrying heavy and expensive landers.
Then there is propellant. A 900 ISP is only achievable with hydrogen (H2), using Methane or water or any other feasible propellant drops ISP below 600, making overall system performance far worse than burning methane in Starship Raptors.
OK, let’s use Hydrogen then. Well unfortunately Hydrogen is a shitty propellent in every other measure. It doesn’t compress well so yet requires larger, heavier tanks. Also H2 is a tiny slippery molecule, forcing you to freeze it near absolute zero in order to keep it from leaking, which makes those tanks even heavier. And guess what? It still leaks. Will you have enough fuel left to fire a retro burn to match orbits with Mars? Better leave with at least 30% more than you need.
Now, you probably don’t want to be killed by your engine on the way to Mars, so let’s add some radiation shielding between the crew and the NTR engines, or put the on a long spar sticking out behind the ship. Both add even more mass.
A NTR ship will likely require at least 2-3x more dead mass than a chemical rocket ship, so the question becomes, what’s the point? Why jump through political hoops when Starship VP can already fly crew to Mars in only 90 days and deposit over hundred tons of cargo to the surface of Mars, and cheaply?
The Dumbo / STNP engines were order of magnitude better than NERVA - way more than "half the mass" and I'm pretty sure we can do better than that these days. The quote is "Better than a conventional engine for any given amount of fuel", implying that even with balance-of-system mass issues they're still ahead of chemical rockets using virtually any fuel.
Also, I don't know whether anybody is suggesting crewed missions, or at least I'm not. But "Why bother", well, cost and efficiency and size, frankly. Starship is a bold move, but to me, it looks pretty small. I'd like them to add a zero onto the end of the mass budget. Think Gerald R. Ford class aircraft carrier going to space and back, with 5000 ton cargo capacity to LEO, or in that neighborhood.
My math on a NTR Starship: (1 ton = 1000kg throughout)
Two-stage Starship mass budget: 5000 tons, assuming max payload.
Dry mass: 180 tons
Current payload to LEO: 100 tons (my math says closer to 155t but whatever)
Even if you assume that an NTR version would be more than 5x the dry mass (quite a bit more than your estimate)
NTR mass budget (unchanged) 5000 tons
NTR dry mass: 1000 tons
Resulting payload to LEO: 800 tons
Plus, the NTR equivalent would be SSTO, straight up and back, only the tankage needing a refill, possibly for years. Isp really matters a ton and it outweighs a lot of the admittedly significant concerns you raise. Same thrust, more than 5x the dry mass, and it's still enough to 8x the cargo.
And of course this is without getting too sci-fi, no salt water Zubrin rockets or Project Orion or Project Pluto nonsense. SSTO using air as a working fluid until it gets up over ~30km is another nice concept.
Chemical rockets have had thousands of successful missions. NTRs not only have zero, they don't even have a single complete rocket that's ever been built. If you want to get to Mars any time soon, don't pin your hopes on NTRs. The people who are actually doing the thing certainly aren't.
https://spacenews.com/momentum-grows-for-nuclear-thermal-pro...
But of course, everything is up in the air globally right now anyway. A little Huntsville boondoggle can't hurt though.
So nuclear thermal would tend to only win over chemical propulsion for long duration low thrust large delta vee missions.
But then, at the top end, it's squeezed by solar electric propulsion which has much higher specific impulses (but even worse thrust to weight ratio). Solar cells and power electronics have improved a lot since the sixties...
Vac Isp: 841s
Sea Isp: 710s
Hydrogen being less dense causes a problem (not solves one) because the fuel tanks need to be much larger for the same propellant mass, and thus you're adding a lot of structural mass.
As for the shielding, you can't use solely propellant as shielding because you still need shielding once the propellant is exhausted (the engine remains highly radioactive after having been used). And it's not clear that hydrogen provides suitable shielding for all the kinds of radioactivity generated here anyway; you might actually just need lead. Plus you still need substantial shielding in the event of break-up/crash on launch.
My curiosity is essentially just whether the performance lost between an H2 and CH4 NTR would be gained back through reduced tank mass and boil-off. Shielding/engine mass will certainly be a big mass penalty versus chemical rockets.
Wouldn't an increase of a factor of 3 in ISP, result in a non-linear increase in performance (deltaV) due to less fuel requirements?
How important is it that nuclear thermal propulsion can use water as most of its reaction mass for in situ utilization (ISRU)?
The fuel to fill up a Falcon 9 costs only a few hundred thousand bucks. The fuel to fill up a nuclear thermal rocket ... would be astronomical in cost.
Perhaps I have it wrong, but my basic understanding of a nuclear thermal rocket is that it uses a reactor to energize a reaction mass like water. The water should be cheap, and the nuclear fuel should be expensive, but how much of the nuclear fuel would be reusable? Would the total lifetime cost be less than a Falcon 9 assuming you replaced the reaction mass?
The hydrogen propellant is cheap enough, sure. I'm specifically talking about the nuclear fuel itself as being expensive.
Let's say you are using a nuclear thermal rocket to ferry material from lunar orbit to mars orbit. You can just replace the propellant in Mars or Lunar orbit and reuse the reactor and reactor fuel. Processing H2O to H2 on the moon and pushing it the moon's gravity well shouldn't be too expensive.
I guess my specific question is: How often would you need to replace the nuclear fuel? Could you run this as a breeder reactor and breed your own fuel?
Land-based nuclear power plants, for what it's worth, are refueled every ~1.5 years. Nuclear-powered aircraft carriers go two decades, but they're not concerned about carrying around lots of extra mass.
Practically speaking I don't think you could refuel a nuclear-powered spacecraft in space. This is a very hard operation to do and everything involved is extremely radioactive. I don't think it could reasonably be done, either by robots or by people. Not until we have entire shipyards in space like we do on Earth, anyway. It takes 3 years to refuel a nuclear-powered aircraft carrier, that's how complicated the reactor tear-down process is (and you do need to tear the reactor down to replace the fuel).
Honestly it's probably just better to use a larger conventional rocket. Yes in theory the nuclear engine might be more mass-efficient, but it won't be more cost-efficient, and that's really what engineering optimizes for.
This is something SpaceX is proving to be really good at that others, e.g. Robert Zubrin, just don't get. He keeps proposing alternatives to the Starship design that, while more physically efficient, would actually cost a lot more money overall to do the same job.
Being able to reliably land chemical rocket first stages for reuse probably gained us a lot more than any realistic nuclear rocket design, at least as far as getting things from Earth surface into orbit.
Or maybe the reverse is true.
Chemical rockets (trad est): 8-9 months to Mars Chemical rockets (SpaceX est) : 6 months to Mars NERVA nuclear thermal rockets (NASA est): 3-4 months to Mars
If compressing the time to Mars becomes the more important mission parameter due concerns over radiation, 0-gravity effects on humans or just something going wrong. Then perhaps Nuclear Thermal are the only choice for people and chemical rocket makes sense for supplies.
I'd love to see any research people have done comparing these two options and seeing which one works best.
The actual nuclear fuel is not expensive at all. The online calculator [1] shows that to enrich uranium to 20% (HALEU grade, which is what DARPA is looking into), you pay about $6500/kg. At this concentration, Uranium has an energy density of about 800 GJ/kg [2]. The NERVA engine had a power of about 1.1 GW and provided about 246 kN of thrust, which is about 3.4 times the thrust envisioned for the Starship upper stage [3], so assuming you burn only 50% of the uranium at only 50% efficiency (that's very conservative), you can run the engine for 3 minutes. If you want to run this engine for 5 hours (how many trips to Mars and back would that be?), the fuel cost would be $650k. Most likely this would be well below the fuel cost for the Starship upper stage. The cost of the cryogenic hydrogen is a different story, but even that one should not be a dealbreaker.
[1] https://www.uxc.com/p/tools/FuelCalculator.aspx
AFAICT Raptor is planned to have ~2,000 kN thrust [0], and Starship has more than one.
And your metrics for fuel use are a bit off to me - better to assume that all of the fuel in the reactor is used with no possibility to recycle, because no one's going to be happy with you landing with a used engine. (And the value of a used engine is pretty significantly negative anyways - just look at the decommissioned naval reactors at Hanford, or the used fuel rods sitting in pools across the country) Similarly, a 50% burn rate is incredibly optimistic - commercial reactors normally manage around 6.5%, and while I believe naval reactors are higher, it's by a factor closer to two than ten. For NERVA I'd expect something more like 0.1%, because it'd be operating for such a short time. (minutes or hours instead of years) Combined, that puts the fuel cost in the $10,000,000-$100,000,000 range to operate for 5 hours, instead of $100,000-$1,000,000.
Or, coming from a completely different direction, from reading this [1], specifically page 99, NRX/EST had 176kg of highly enriched uranium for a 1,055MW reactor. At ~$30k/kg for 85% enrichment from your calculator, that means $5,280,000. Yes, modern NTR designs don't use HEU, but that means much lower power densities and thus more uranium.
0: https://en.wikipedia.org/wiki/SpaceX_Raptor
1: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/199200...
Your find [1] is fantastic. If it's $5.3 MM per engine, that would be about $250 MM for the 48 engines.
However, that could be used several times, at least 4.5 by my calculation: The NRX A6 was able to run for more than 1 hour [2]. The fuel flow was 32 kg/s, so that's about 1.5 tons/s for 48 engines, or about 800s to burn 1200 tons (fuel mass of Starship upper stage). That makes the uranium fuel cost per trip about $55MM.
Now if instead of 85% one uses 20% enriched uranium, the price per kg decreases by a factor of 4.6 (from 30k to 6.5k) but the burnup decreases too, but probably by less than that. Still, a ballpark figure of about $50MM per trip, only for the uranium fuel.
[1] https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/199200...
[2] https://en.wikipedia.org/wiki/NERVA#Reactor_test_summary
- nuclear thermal rockets can use different propellants. In [2] Nasa looked at some other propellants that could be available on Mars, such as CO2, H2O, CH4, CO, N2, Argon, NH3. They conclude that CO2 would be very convenient, H2O even more so (but were not sure at the time how easy it would be to source water on Mars). CH4 would be very appealing, but one would need to overcome some coking concerns. The other fuels were all inferior (except for H2, obviously).
- nuclear thermal rockets are not intended for takeoff from the Earth [3]. The intention is to be carried to space with chemical rockets, and use them for trips starting in LEO.
[1] https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/199100...
[2] https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/199100...
[3] https://www.energy.gov/ne/articles/6-things-you-should-know-...
Is insulation that much of a problem in the vacuum of space? I would think you could just cover it in a reflective material and call it a day.
So, yeah, it's fine for uses in stages to orbit, and it was even fine to use for the Apollo missions because it was only 3 days there, but anything longer than that and it's increasingly not suitable.
> In general it seems like density should matter less than mass
Mass is already incorporated into Isp. So, for the same total mass of propellant being exhausted at the same velocity, you definitely would rather have the denser fuel so that your fuel tank can be smaller and thus lighter.
Where the rocket equation helps is in the net to gross mass ratio. Starship is expected to have a 9% ratio. If you multiply the ISP by 2.2, you would increase the ratio to 34% (as someone else in this thread mentioned). The Starship proposed net mass is 120 tons, that could go to 450 tons. For comparison the largest (cargo) airplane in the world, the Antonov AN-225 has a maximum takeoff weight of 640T, but 300 of that is fuel.
It's so sad to listen to the documentaries and hear how NERVA "is on track to propel mankind to mars by the late 1970s or early 1980s." Ah well. At least we're starting to care about space again!
Many of the papers generated by UTC on this project were readily available a decade ago, though it seems many were reclassified since then.
Still, the fact that the article makes a big deal out of quartz being transparent in the UV makes me wonder if there isn't a clever trick up someone's sleeve. Ideas?
Like I said, UAC actually built functional prototypes that went just short of using fissile material. The non-nuclear parts of the design were validated experimentally. The only remaining theoretical parts are precise control of the fission reaction, which they didn't have the computational power for back then, and long term operation/maintenance, since imperfections in the SC beryllium oxide degrade the container and cause it to melt down eventually. There were plans to demonstrate a slow neutron plasma, which would significantly reduce material degradation, but they never got to it before Nixon canceled the Mars program.
Nuetron damage to container seemed to be the deal breaker to me, thinks for the additional information.
https://forum.kerbalspaceprogram.com/index.php?/topic/151286...
You're right that both are worrying until they're out of atmosphere.
As per the Wikipedia article there were numerous unresolved technical issues and really exciting failure modes. But a big 'no' was the massive nuclear fallout resulting from each (successful) launch, let alone the ones that failed.
Larry Niven's novel Footfall [0] (spoiler...) involves an interesting treatment of an Orion-style spaceship. The massive problems are tolerated in the face of an existential threat to Humanity unless a massive spacecraft can be launched into space. And very conveniently, it works perfectly first time, with no testing of the bomb engine mechanism, which seems a tad fanciful.
How would a spaceship handle effectively being shot with a similar device. It would be ripped apart (I'm not a physicist)
[1] https://en.wikipedia.org/wiki/Operation_Plumbbob#Missing_ste...
An aside, the strange craft reported recently with the famous jet fighter videos had "impossible performance" and were all filmed over the ocean. The ocean would be the only safe place to test nuclear drones. And their performance would be quite unmatched by anything else. The pilots even reported that they submerged, sounds like a great failsafe if your super secret black project gets seen. I doubt you would submerge a jet turbine, but nuclear propulsion could easily work under water
In the case of nuclear-powered engines, that isn't trivial (unless you mine them off world).
If we dragged in Psyche 16 into our neighborhood, it would make all gold here worthless. A NASA probe is already en route.
As to getting the stuff down, I imagine one could do that relatively low-tech; cut it into manageable chunks, wrap those in some kind of simple heat shielding, deorbit to some remote place and have some kind of parachute system slow it down enough to not disintegrate on impact; a chunk of solid gold should be able to take quite a beating.
Meanwhile, have a massive golden glider splashing down into the ocean somewhere near a cost, and have it towed by tug-boats towards the smelters, forges, metal works, catalyst producers, whatever...
I'm just spitballing here but.. crash it into the moon, mine it there, then use a mass driver to throw small pieces of it to earth?
Look up "rotating skyhooks" for one such scheme.
Let's hope nuclear rockets become a commercial reality before they become a military tool. There's a company in Seattle called Ultra Safe Nuclear Corp that develops the fuel and core for the NASA mission: https://www.usnc-tech.com/products/. They also do a terrestrial reactor for off-grid remote regions that is set to be demonstrated in Canada in the next few years: https://www.usnc.com.
I realize this is a very touchy subject, but military tools are not necessarily bad. GPS, for example, has been a great boon to society, despite being developed by and for military use.
Even weapons are not necessarily bad just because their purpose is to kill people. If we didn't develop newer and more precise weapons, we would still be using B-52s to carpet bomb cities hoping to take out the half dozen targets we care about instead of precisely excising the one bridge with a PGM with minimal casualties.
WMDs on the other hand... I don't see what good can come of those since we already have nuclear MAD.
The military has a goal, and it doesn't always line up with civilian interests. Unfortunately, their involvement could further drive dangerous variations of otherwise comparably safe and effective tools.
So what would the military even use such engines for? Maybe an X-37 successor, but why would that be particularly alarming?
https://en.wikipedia.org/wiki/Refractory_metals
The melting point of tungsten is astonishingly high (3422 deg C).
- Boating [1]
- Countertops [2]
The book recounts all sorts of crazy nuclear research, mostly from the 50s-70s, including using entirely unshielded reactors to test the effects of radiation on various materials, and attempts to build a nuclear-powered jet.
https://www.amazon.com/Atomic-Adventures-Islands-Forgotten-I...
The whole point of thorium is that it turns into uranium. The only real difference is that there are many fewer excess neutrons around, because they're used to convert thorium to uranium. To convert a uranium reactor to thorium you basically only need to change the fuel. The reasons to do that are scalability (thorium is more abundant than U-235) and the fact that fewer neutrons means less plutonium.
Molten salt reactors such as LFTR and molten metal reactors are not exclusive to thorium. You can even have a liquid flourine uranium salt reactor. Any submarine would be highly unlikely to use thorium due to it's much lower power density compared to highly enriched uranium. Thorium basically has to enrich itself over time, so you need a lot of it to get to a given power level.
2. The US is the only country that exclusively (except for one sub) uses pressurized water reactors. Russia uses liquid salt or metal mostly. Liquid salt reactors -like the ones most commonly proposed with thorium- still need conventional centrifugal pumps to operate. It's only certain liquid metal reactors that can use electromagnetic pumps, and I'm pretty sure they still need pumps for the secondary water loop.
3. The whole "nuclear water pumps are loud" thing is pretty much a myth anyway as far as I understand. Pound-for-pound, nuclear subs are much quieter than diesel, but the quietest submarines are obviously very small. Nuclear submarines are obviously very large, and so are loud simply by nature. That led to speculation that being nuclear meant submarines were louder, which led to people talking about pumps and turbines and whatnot being loud.
Google the patents for these.
I will admit I’m not sure what the timelines are; at this point of time even a lunar base seems beyond my lifetime.
If we say there's no life on the Moon, we still plan to be there. The fallout could affect the use of the Moon, which is far more important.
I guess the question is if it’s riskier to transport fissile material as cargo vs in an engine. I am not sure what the relative risk would be.
Consider the problem that the astronauts had when on the moon and the "prevent dust from getting in the lander". Not only do you have the worry of sharp, pointy dust now - but also radioactive dust.
While it isn't a science book... Seveneves has to deal with the problem of nuclear trust and dust and does so in a realistic way. Its a good book to read/listen to (if you acknowledge/expect the "Stephenson where did the ending go?!" problem)
Orion, by contrast, uses its bombs effectively as both fuel and propellant and achieves far greater temperatures/efficiencies and power. It's a stupendously efficient and effective engine. However, that does mean that you've got nuclear reaction byproducts coming out of the engine which are pretty radioactive and also will tend to fall out of the atmosphere onto us down below in a reasonable time. Each one only produces a little but you'll be setting off many of these and overall you're looking at the same level of fallout as a multi-megaton thermonuclear bomb airburst. And we banned open air test of those for a reason. Still, if you're launching from some place that's already exposed to cosmic radiation and the solar wind, like the surface of the Moon, it might be a useful means of transport there.
More accurate, possibly, to say you're not going to space in a controlled fashion today.
You have to add the weight of a pusher plate system, and account for blasts still happening quite a distance from it.
Your ISP will vary wildly depending how much "fuel" you will carry, and what yields.
To beat a nerva, it will definitely need to be patently gigantic
How practical is using railguns for launch compared to nuclear thermal propulsion and the other alternatives discussed here?
At the very least, it seems to be much safer than something that risks nuclear fallout.
Not sure if railguns in particular add much over chemical guns that justifies the complexity.
But as you say, a human-rated railgun has a much lower acceleration. And there's the problem of what happens when you hit the atmosphere at the end of the gun...
1. Storing the energy to accelerate the payload. 2. Not accelerating the payload so quickly that it breaks apart. 3. Thermal heating from atmospheric drag. 4. The energy required to overcome atmospheric drag itself. 5. Strength/size of materials to build the sled and contain a payload with that much momentum.
My understanding is that 3 & 4 on this list tend to make rail guns a non-starter on earth.
Rockets, OTOH, go up vertical for most of the atmosphere and then get horizontal in order to get to orbital speed, preferably where there is little or no atmosphere.
You'll also need to carry some form of propulsion to circularize your orbit, unless you are aiming for escape velocity, in which case your speed at the end of the rail will have to be much higher. Whatever engine you have will need to survive the acceleration.
And yet most launch systems have to throttle down the engines during the transition between the two, because the atmosphere is still just a bit too thick to safely plow through it at full tilt.
The benefit of solar power is that it’s almost limitless.
One big problem with solar energy is that it is a diffuse power source (i.e. energy per unit area is low), and gets exponentially more diffuse the further from the sun you go.
The second is that though solar energy is limitless, the components that need to use chemistry to turn solar energy into electricity, are not.
The advantages over solar power is thrust (which, within the solar system at least, is important if you want to get somewhere fast), and the fact that it works further out from the sun, which means you could use it to get mass to the outer planets (or mars) relatively quickly. Solar propulsion, yes, is limitless, but and can reach much faster speeds, but it can take so long to reach them, that by the time you're going faster than the NTR, the NTR has already arrived at its destination.
The advantage over chemical propulsion is efficiency.
And inside the orbit of Jupiter, solar + ion engines are way more efficient.
The Ars commenters are very well informed, they go back and forth on this a lot. I highly recommend reading the comments there.
But don't take my word for it, there are 12 pages of intelligent comments on the Ars article, comments mostly of much higher quality than any comment here, including and especially mine.
And those comments are fairly disparaging. Unless you're going past Jupiter, in which case Nuclear-Electric-Propulstion is your best bet. And nuclear-electric is very different than nuclear-thermal.
Nuclear-thermal's Achilles heel is that you can't use it to boost off Earth. So you need a high thrust chemical engine to get it into space. It's hard to make the numbers work -- nuclear-thermal may be more efficient than that chemical engine, but it's dead mass while you're boosting it out of the Earth's gravity well. If instead you can use the same chemical engine to get to Mars that you used to boost yourself out of Earth, you don't have that dead mass. If you want to get to Mars fast, just boost some extra fuel on an extra ship.
Nuclear-thermal looks nice on paper; it's probably the highest efficiency high thrust engine that's achievable with today's materials. But it's hard to find a good use case for it. A manned Earth->Mars run used to be it; but SpaceX's plan for in-space-refueling shortens the trip time just as much, if not more.
The questions here include:
- whether you can get more efficient propulsion overall when the heat comes from a reactor instead of from a reaction in the fuel
- whether switching from a fuel/oxidizer energy-source/mass combo to another working-mass-only substance might net you other engineering advantages (storage concerns, perhaps? though if you're switching to hydrogen like one often considers for nuclear rocket working mass, you will have plenty of storage problems)
- what sort of engineering disadvantages the nuclear reactor component brings (added mass, heat dispersal issues, etc)
Which means the solar array must be twice as large for same output on Mars alone, and solar propulsion is probably completely infeasible due to weight and volume constraints, if not structural as well, on Saturn orbit and beyond.
For missions further out or for Mars missions that cannot accommodate twice larger wings, we must “carry our own Sun” for power.
Which isn’t wrong at all, after all the Sun is a natural fusion reactor so solar panels aren’t like it’s free of stigmatic nuclear technology.
Oh and also nuclear propulsion aren’t considered chemical by the way...
Solar by itself doesn't provide reaction mass (outside of solar sails, which are very low thrust). You would probably be combining it with ion thursters, which have really high exhaust velocities, but low thrust to weight ratios.
The big difference there is thrust. Ion engines are efficient but have very low thrust, so it can take months to change your orbit. Chemical and nuclear propulsion is high thrust, so we're talking minutes instead. Nuclear has higher propellant (i.e. mass) efficiency than chemical as well.
Edit: reaction mass becomes the problem, say to move a ship 10^4kg, 2x10^9 m (the distance to mars) at 1g, I adapted an answer from [1]
So say the craft has a mass of 10^4 kg. To accelerate at 1 G you need F = ma = (10^4 kg)(10 m/s^2) = 10^5 N. To get a force of 10^5 N over 2x10^9 m (distance to mars), you would need (10^5 N)(2x10^9 m) = 2x10^14 J, lets say 10^14J. Antimatter is the most energy dense material we know. To get that from antimatter you would need m = E/c^2 = 10^14 J/10^17 m^2/s^2 = 1gram. So you'd only need 1gram of antimatter - is that right?
CERN has made around 1 nanogram so far, 1gram would cost around $25 billion in a 2006 estimate, its down from $62 trillion in a 1999 estimate [2], so it may be feasible one day...
Edit2: The moon is 4x10^8m, so you'd need 0.1gram of antimatter, you'd be there in an hour or so.
[1] https://forum.nasaspaceflight.com/index.php?topic=34996.0