There is still a propellant problem. The other possibility for nuclear radiation drives is reflection. It’s much less efficient per watt because the momentum to energy ratio decreases as energy per mass increases, but the critical metric for long-term (nonrelativistic) space travel is momentum transfer per mass ejected, which is just the speed of the propellant medium, maximum c for photons, so reflection is preferred over expelled gas.
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.
> duoplasmatron breeder reactor, which is a reactor driven remotely by a tritium ion beam.
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.
My physics isn't great, but I have questions based off what I have (mis) learned:
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.
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.
c is the speed energy travels at and less than c is the speed matter travels at in the universe. You’re right that ultimately matter is energy, but you typically have to introduce it to it’s antiparticle to turn it into energy.
(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.
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)
The original intent of the duoplasmatron breeder was to eliminate the dangers of going supercritical, actually to eliminate HE fissile material entirely from the bulk, as the plutonium would have to be neutron-bred by the fusion reactions triggered by the tritium beam. Also short pulse control. This was mainly to make it safe and cheap. You could also just use a subcritical mass of HE-fissile material surrounded by a neutron reflector with aperture in the direction of thrust. Recommended only if you are certain to be far enough away from humans to avoid recovery and conversion to a weapon. Aliens too; they might send one back. There is a design for a safer and simpler cylindrical slow-burn wave reactor that may also work for low levels of neutron propulsion.
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.
Maybe not but it's pretty significant. From 350 to 900 takes the payload mass fraction to LEO from 6% to 34%, so SSTO would be practical (though I wouldn't expect to see this for launch). Or, with a 10% mass fraction, the delta-v goes from 7.9 km/s to 20 km/s.
Yes. The difference is definitely a sea change. To put it in concrete terms, a nuclear-powered rocket with sufficient thrust could be single-stage to mars and back for the same propellant budget and mass fraction of a first stage chemical engine which doesn't even get to orbit.
A nuclear thermal engine is not remotely a drop-in replacement for a chemical engine. There are substantial other changes required to the rest of the rocket that add lots of mass and expense, that work to substantially negate a lot of the benefits.
That's true from a political perspective but it's definitely not true from an engineering perspective. Got enough energy to hoist almost anything you like into orbit if you choose your thrust and scale appropriately. As to what happens to the ground underneath, that's a different problem :)
No, extra mass is needed to make them operational, not politically acceptable.
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?
A lot of your math is way off, brother. Sure, NERVA was close to 1.0 thrust to weight ratio, but that says a lot more about the state of technology at the time than about the capabilities of nuclear thermal rockets.
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.
This is all so theoretical though, and many decades in the future in the best case scenario. Someone with your opinions arguing about this in the 1970s would've argued we'd have had all this long before now, yet we don't. It's not clear that any progress will actually be made in the coming decades either; no one is seriously working on it like SpaceX and Blue Origin are working on their Mars rockets.
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.
This isn't taking into effect the added mass of the engine itself, though. 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. 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 used by the Falcon 9, Saturn V first stage, etc. 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.
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.
> 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.
Oxygen in an NTR provides an ISP barely better than chemical rockets, and how reactive would 1500K oxygen be with your reactor? with the additional mass requirements of massively heavy NTR engines, shielding and cooling oxygen doesnt make sense.
We'll see ... personally I think the proof is in the pudding that despite having had these designs on the drawing board for 60+ years, there hasn't even been a single complete rocket that's ever materialized for any of them, let alone a successful mission. Contrast that with the thousands of successful missions using chemical rockets over that time period.
So it's not overstating it to say that there are some problems with nuclear thermal rockets, otherwise they would be commonplace by now.
It doesn't take an NTR to land on Mars. If you're going to pin all your hopes on NTRs I think it's still going to take decades. No one is even seriously working on them.
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.
The reactor isn't going to be running during launch - so it's not going to be highly radioactive. Also I'd expect the reactor to be fairly small, dense and robust - I doubt if it would be "blown to bit" in the event of a problem with the rest of the launcher.
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:
It is going to be highly radioactive at some point (when it's in use), so it does need shielding for that point.
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.
The nuclear fuel doesn't really need to be shielded any better during launch than the existing RTGs used on space missions. Scaremongering notwithstanding, those are pretty much impossible for a mere launch accident to "blow to bits" in a way that exposes the PuO, and so would the fuel rods of a proper reactor. At least unless the reactor itself suffers a catastrophic excursion and blows itself to bits à la Chernobyl No. 4, but we've become pretty good at building reactors that don't do that.
And the RTGs use an isotope that provides significant energy just from radioactive decay. By contrast, uranium in a reactor is barely radioactive at all, before the reactor has been turned on.
If they're launching it over the ocean (they would be, if they were launching from Kennedy or Vandenberg) they could just say "YOLO" and let the reactor fall into the ocean. It certainly wouldn't be the first time a nuclear reactor was dumped into the ocean (either by accident or deliberately.) It's obviously not a popular thing to do, but ocean water does provide a lot of shielding..
While true, a reactor blown up over land would also be simpler to clean up; while a reactor scattered over the ocean bed would be easier to leave in place.
"Simpler" here is relative. It would probably still cost billions of dollars and would create enough of a PR disaster to permanently forestall an NTR from ever launching again.
Water is not remotely as good as lead is by mass as a radiation shield. If you aren't already hauling many cubic meters of water for other reasons (and you wouldn't be), it absolutely wouldn't make sense to bring it along solely for that purpose. Water also has the severe problem of being liquid at habitable temperatures, so you either have to continuously spend energy to freeze it or deal with absolutely ruinous slosh.
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.
> It is going to be highly radioactive at some point (when it's in use), so it does need shielding for that point.
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.
The long foldable structure would add considerable weight* and complexity (and thus risk), and would also be unsuitable for use as a first or second stage. So, this would maaaaybe be acceptable to handle the Hohmann transfers on a crewed mission to Mars, but not for much else.
* It needs to be strong enough to handle the full acceleration of the engines.
> 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.
The problem is that Hall-effect thrusters provide basically no thrust.
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.
> You would need over 290,000 AEPS engines to match one Merlin 1D engine.
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.
> So it can't even lift itself off the pad, let alone anything else.
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.
> This is only a problem for take off from large bodies.
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.
If I’m doing my math right, the 25kg AEPS engine would need to produce 245N to even hover without any payload. So it’s theoretical max is about 0.9% of what’s needed to even lift itself.
And of course these figures get much worse when you consider the other components that are a necessary part of the engine working, namely, the xenon fuel tank and massive solar panel array.
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.
We're talking about orbit raising, not launch, which the nuclear thermal can't do. I can't believe I've been downvoted this much when I probably know more about electric propulsion and orbital dynamics than anyone in this thread.
Also, downvoted for not wanting to accidentally create radioactive fallout? Do the cost benefit analysis when there's plenty of other technologies for moving mass around off planet and to other planets.
Do we have any concrete figures on an entire rocket designed to use this engine? Like, mass fraction, etc.? Because if you look at the engine in isolation it seems good, but that's ignoring all the shielding you'll need and the huge fuel tanks because your propellant density is so low.
I'm just an armchair space enthusiast, but both of those problems seem like they have possible workarounds or solutions. Using methane instead of hydrogen could significantly reduce the required tank volume since CH4 is denser, and you don't lose that much performance with it (based on Project Rho's table linked above). For shielding, if you put the fuel mass between the engine and the humans aboard, you should be able to get a decent amount for "free".
It's not clear that methane would be as workable; that's a large change from hydrogen to that. We at least know that hydrogen is workable, because an engine was built and tested.
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.
At what distance is radioactivity no longer an issue? The engine could be attached to payload at a longer distance with relatively lightweight structures. It doesn't have to be designed to stand up to the stress of launch/land with a payload. Just to ferry between points in space.
It's definitely worse than H2's ISP in an NTR, but ~600s is still a solid boost over the ~450s that's about the best a practical LOX/LH2 chemical engine can do.
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.
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.
Well, there was the Saturn C-5N [0] - basically a Saturn V with a nuclear upper stage instead of another J-2. That increased the payload to LEO from 118,000kg to 155,000kg. (and an even larger increase in mass to TLI/TMI/etc)
It may not be something we know yet -- based on reading the available materials it doesn't seem like we got past the stage of designing the engine on its own.
Not the OP, but the way the rocket equation works is that for the same ratio of gross to net mass if you double the exhaust velocity, you double the delta-v. So, it's exactly linear. It's actually a little sublinear, as you add the delta -v to the original v (which is the speed of the planet Earth plus the orbital speed of the vehicle in the LEO that starts the trip to Mars). So, if you double the delta-v, you don't actually double the deep space velocity.
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.
And they use H2 for reaction mass. Which means incredibly massive tanks because H2 is so much less dense and requires a lot of insulation to stay liquid.
Shouldn't that scale well in space as the shell of the H2 tank scales at the square whereas the volume scales at the cube? In general it seems like density should matter less than mass, but I've never attempted to contain and accelerate large quantities of liquid H2.
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.
Another big problem is leaking and boil-off. There's a reason that no space mission longer than days uses LH2 as its propellant; it just doesn't last long enough, because of boil-off caused by solar heating and the hydrogen actually leaking out through the fuel tank. Insulation isn't 100% effective and it's hot in space near the Sun where we are, certainly much hotter than the 20K boiling temperature of LH2. Even if the insulation is 99% effective that's still significant heat flow at a temperature higher than 20K, and you have boil-off.
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.
The mass-fraction actually doesn't scale like that. While yes if you just wanted to cover your H2 tank it would scale with the square of side length, it's not just about covering it but mostly about structural matters. If you were to keep the same wall thickness your rocket would not be structurally sound.
Yup, that's exactly the problem. Once you have it incorporated into an overall rocket (and aren't just looking at the engine in isolation), the gains are smaller. And the engine and fuel are significantly more costly and risky.
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.
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.
> 246 kN of thrust, which is about 3.4 times the thrust envisioned for the Starship upper stage
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.
You are right, I made quite a number of mistakes in my calculations. In particular, the Raptor engine indeed provides 2000 kN of thrust, and the upper stage has 6 of those, for a total of 12000 kN. The Nerva engine is only about 250 kN, so you'd need 48 of them to match the 6 Raptors.
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.
Some updates after some further googling:
- the NRX A6 engine was tested for 1 hour, but based on the observed corrosion, NASA extrapolated that the lifetime of the engine could be 2-3 hours ([1] page 10). That would reduce the fuel cost that I estimated above from $50MM per trip to Mars to $20MM per trip.
- 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.
>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?
Nuclear fuel is very expensive to process and manufacture, and is not reusable. In particular, for these nuclear rockets, you'd never even be returning it to the Earth, so it is definitely not reusable for that reason alone. The reactor itself is going to be very expensive too; doing anything nuclear just fundamentally ramps up the cost a lot vs building e.g. a Merlin engine.
The hydrogen propellant is cheap enough, sure. I'm specifically talking about the nuclear fuel itself as being expensive.
>In particular, for these nuclear rockets, you'd never even be returning it to the Earth, so it is definitely not reusable for that reason alone
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?
It's an interesting question. The more fuel you send up there, the longer it'll last, but also the more mass you'll be wasting by constantly having to carry it around. I suspect that it would not be practical to bring along enough nuclear fuel to make many Earth->Mars round-trips on every one of those round trips.
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).
IIRC, Naval reactors usually use Uranium enriched to much higher levels that civilian nuclear plants, both to get higher power output for smaller reactor size, and to not need refueling as often. Reactors designed to operate in space may do the same sort of thing. I wonder what other kind of optimizations and/or workarounds might be required for a real space-capable reactor.
Whatever it is, it's going to be insanely expensive. That's just guaranteed.
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.
It certainly would be. As cool as the tech sounds, I do feel more inclined to let Elon Musk and those types see how far they can run with conventional space tech and some non-nuclear efficiency optimization work.
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.
I'd be curious to see a side by side comparison. It might make sense to use Starship for humans and initial supplies and a nuclear thermal design for much heavier loads that would require many back and forth runs by SpaceX Starship rockets.
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.
It's true. Specific impulse is only one part of the equation. Tankage mass is really bad with pure hydrogen, which is usually the reaction mass of choice. Hydroden's density is abysmally low. And the reactor weighs a lot too, compared to chemical rocket engines which have very high power densities and thrust to weight ratios.
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...
Nuclear-thermal rockets still require a "fuel tank". They heat up a reaction mass (typically LH2) and then throw it out the back. So they can't burn continuously, they'd run out of reaction mass.
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)
The word you're looking for is propellant. In chemical rockets, the fuel and propellant are the same thing (to be precise, the combustion byproducts of the former comprise the latter). In nuclear fusion and ion thrusters, they're different. In all cases, Newton's second law means that you are using up propellant to generate an equal and opposite reaction in the direction you want to go, and thus the amount of "going" (delta-v) you can do is limited.
"Propellant" - Thank you! I knew it wasn't technically fuel, but it's still being expelled (as you mention, if you don't expel mass with a different velocity relative to the rocket, then you won't be able to accelerate a rocket)
this requires Isp in the high 10-100k, likely sub-1M range (think particle accelerators) instead of sub-1k (think very high temperature nuclear gas heaters).
Some of the better ones that are actually in use get in the range of low thousands, like ~3k or so, but keep in mind that the thrust is so low that even just getting to the Moon takes months. They're only suitable for unmanned craft, and even then, only after you get into orbit and if you don't care about taking potentially many years to get somewhere outside the local SoI.
The idea was considered before and showed great promise:
https://en.m.wikipedia.org/wiki/Project_Orion_(nuclear_propu...
Of course there is always the backside, imagine a rocket exploding in the upper atmosphere leaving nuclear waste and rocket parts over vast areas of land.
NERVA https://en.wikipedia.org/wiki/NERVA was way more practical. They actually built these things and experimentally confirmed specific impulse, they just never launched them.
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!
Even more promising was the nuclear lightbulb reactor [1], which was developed by United Aircraft Corporation (now UTC) for NASA under the Mars program in the 70s. They got just short of testing the engine with nuclear fuel and all of the remaining obstacles were with material science and computation which would have been very solvable given the tech developed independently in the 90s. The design could also have changed the trajectory of the entire nuclear power industry because it uses compressed uranium hexafluoride plasma to reach criticality so it has the benefit of using tens of kilograms of fuel instead of tens of thousands and it's an actively maintained magnetohydrodynamic system so if anything breaks or power is lost, the reactor slows down and fission stops. Large centrifuges are needed to recycle fuel from the buffer gas (when it's used as a terrestrial power plant instead of a rocket engine) so this reactor can also be self-breeding and recycle nuclear waste.
Many of the papers generated by UTC on this project were readily available a decade ago, though it seems many were reclassified since then.
That reaches temperatures of 25,000C. AFAICT we don't have any materials that can withstand that, so that reactor remains theoretical rather than practical.
The genius of the nuclear lightbulb design is the irrotational vortex of neon gas that contains the plasma - this design separates the hot stuff from any solid materials that might melt. This vortex provides the pressure required to reach criticality and separates the plasma from the single crystal beryllium oxide walls, which in turn separates the neon vortex from the reaction mass (hydrogen gas seeded with tungsten nanoparticles when used as a rocket engine and water when used as a power plant). At that temperature, the plasma emits most of its energy as a black body radiator and the SC BeO walls are designed to pass through all of that radiation. The constant flow of neon cools down the entire system.
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.
> 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.
Rocket engines have a long history of obtaining combustion temperatures far in excess of what their combustion chamber would tolerate under static conditions, but yeah, 25kK vs 2kK seems excessive. Also, chemical rockets have an easier time with this trick because the heat is generated inside the propellant, so you just have to stop it from getting out quickly, and the fuel itself is cryogenic and makes for a very convenient coolant.
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?
Nuclear pulse propulsion (Project Orion) is a different concept to nuclear thermal. Pulse works by using bombs to generate your energetic reaction mass in bursts, whereas thermal uses a more normal fission reactor to superheat some reaction mass that then passes through an ordinary rocket nozzle.
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.
It's continued development afterwards in different forms. Project TIMBER WIND was a pebble-bed reactor upper stage for SDI boost phase intercepts. The reactor wouldn't be spun up until after the missile had cleared the atmosphere, so there's no fallout to worry about.
I always wondered how this was supposed to work. The 'pusher plate' is going to have to be made of strong stuff. A nuclear explosion launched a steel cap to incredible speeds [1]
How would a spaceship handle effectively being shot with a similar device. It would be ripped apart (I'm not a physicist)
This gist is that by controlling the size of the explosion you control the energy, and thus can keep it within a range that the materials you're using can handle. I realize that's a hand-wavy statement, but a lot of actual research has gone into this, and it seems like it should work without exotic materials.
Conspiracy, but I don't think the US ever stopped researching nuclear propulsion. There's too many advantages. They built a few working and miniaturized test engines in the 60's but never put them on planes? I don't believe that. They put nuclear weapons on planes all the time and thats not much safer. More likely, they never told anybody they put them on planes, for obvious radioactive reasons.
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
I can't find it, but I saw a comment on HN back in April I believe (when the Pentagon released videos of UFOs) explaining exactly your theory with some interesting sources.
One of the most exciting aspects of the renewed interest in space these days is that we have once again the chance of start building things in space quite soon (unless the world economy collapses 100% finally and war is waged). Once we can get to space cheaply people will start experimenting with all forms of propulsion up there because fuck-the-government and that is not easy to do on Earth or even just launch from down here.
Which is why, space mining will likely never benefit Earth. Any material 'up there' is so much more valuable in space than dropping it down to Earth to make Coke cans or whatever.
There are designs to couple the dropping of one thing to the lifting of another, like an elevator (though not necessarily at the same time). This makes delivering payloads to Earth necessary for your launch system. They might as well be valuable payloads.
Yes I can imagine Earth financial markets will speculate in space-based industry. Grounders may become 'wealthy' from off-world activities, but without ever actually benefitting from them directly.
I imagine the amount of stuff floating around up there is so vast, once you can build all the heavy, bulky parts of a propulsion system in space, produce fuel in space, and automate the mining, you could truck absurd amounts of all kinds of raw materials to Earth, easily more than you could realistically use in orbit short-term. But I guess people could be made to pay a pretty penny for Space Gold accessories – or space diamond, space platin, space whatever. Say, an Apple Watch milled from a solid chunk of Space Platin or even Space Diamond, I think that might sell pretty well.
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.
Have it mined optically in space and then molded/vacuum deposited/3-d printed up there into (passive) lifting bodies of appropriate proportions out of comb like structures, put a jetpack for steering onto it which brings it down into the atmosphere in a controlled way, detach that (reusable) jetpack before burning up, reroute to some orbiting space dock for refueling.
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...
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.
> Let's hope nuclear rockets become a commercial reality before they become a military tool
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.
One factor to consider is the US military wanted fissile material so lobbied for US nuclear plants to be made in a way that created weapons grade material, even if it needed refining. If the US had opted for alternative reactor types we may have had a boom of very safe, non weapons grade material creating reactors today. Instead, we have many nations with the technology to create weapons grade material as that was the reactor type they were sold or were trying to copy.
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.
It's not clear to me what the modern concern would be with nuclear powered military rockets. ICBMs don't have particularly high performance demands by modern rocket standards, and modern ICBMs are built with solid fuel boosters because they're simpler to deal with in every way. A nuclear powered ICBM doesn't make much sense; I don't think they'd make those and if they did, I don't think they'd be more alarming than the ICBMs which already exist.
So what would the military even use such engines for? Maybe an X-37 successor, but why would that be particularly alarming?
A minor nit, the us military created the nuclear industry to begin with, it did not lobby and co-opt it into building nuclear weapon material, that was a major goal from day one.
If you enjoy learning more about this sort of stuff, I'd highly recommend "Atomic Adventures" by James Mahaffey.
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.
I always wondered why the navy didn't push for thorium reactors. It was my understanding that they would not need the water pumps that current sub reactors need thus being quieter.
1. Thorium reactors are almost identical to uranium reactors and it is a recent but common misconception that they are so fundamentally different.
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.
Nuclear fallout is pretty dangerous on earth, what about the Moon? Would the risk be lower, since there is no life on the lunar surface? So, for the ultra long range missions that are being considered, could fissile material be transported first as cargo to a lunar base with chemical rockets, assembled there and then used to launch longer range missions?
I will admit I’m not sure what the timelines are; at this point of time even a lunar base seems beyond my lifetime.
While there is no life on the surface, when we get to the "colonize the moon", the fallout becomes a problem.
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)
It's not a dirty bomb! The nuclear thermal rocket has not operated yet which means the fuel is not radioactive and contains no fission products. The surface to space chemical rocket could explore and there would be no fall out.
Transporting fissile material with chemical rockets is dangerous, it's making a dirty bomb and hoping it explodes in one direction. That hasn't always worked for transporting people.
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.
Darn, it seems nuclear thermal propulsion is not the same as the Project Orion "spacecraft intended to be directly propelled by a series of explosions of atomic bombs behind the craft (nuclear pulse propulsion)". https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...
Yeah, this is nuclear thermal where uranium is burned as fuel to heat a hydrogen propellant. That limits the heat and energy the propellant can absorb so while the efficiency end up being high due to using all-hydrogen fuel with a very low molecular weight the thrust isn't that great. These are good for second stages or transfer once in orbit but they don't make very good booster engines. Well, not unless you're going to do something really exotic like a gas phase nuclear lightbulb design[1]. But thankfully it won't be releasing any radiation into the atmosphere if it's working properly, the hydrogen fuel has to absorb two neutrons on its pass through to turn radioactive and it really doesn't want to absorb that second neutron. And if something goes wrong before its turned on uranium, even enriched uranium, isn't that radioactive and honestly there are scarier heavy metals that aren't radioactive at all.
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.
Just to be clear, uranium is not literally burned in a nuclear thermal rocket. Unless things are very very wrong and you’re not going to space today. Nuclear thermal propulsion is simply using a fission (or more speculatively fusion) reactor to heat hydrogen gas which is then allowed to escape at high velocities from the business end of the rocket. In theory the heat source could be anything, but very few things offer remotely as much W/kg as nuclear.
Saying that nuclear fuels "burn" when you cause them to undergo fission in a reactor is a common colloquialism. Hence standard terms like "burnup rates" when describing the efficiency of reactors.
Project Orion is outdated anyway, we have technology like laser detonation and computer coordination such that you can detonate tiny fuel pellets continuously rather than bouncing off of entire bombs with a looney tune pogo stick arrangement.
The standards for propulsion are lower than that for electric power; you don't really need a continuous nuclear explosion flung out the back to be net-positive electrically, you just need it to happen.
Yeah I dont think any of these technologies are good enough for travelling in the solar system. Unless some breakthrough in propulsion happens like maybe bending space time to travel.
May be with this one would reach Mars in 4 months instead of 1 yr.
These are still long timelines, Not good enough.
Starship can get to Mars in 4 months, which is plenty fast enough. It took what was left of Magellan’s crew many times longer than that to circle the globe.
All gun-style launches have the problem that they have to do the entire acceleration in a very short amount of time, which means very high g-forces. Also happens at the ground, so needs even more speed to compensate for loss in the atmosphere. For small & robust payloads it might be an option, but for large satellites? E.g. Gerald Bull did high-atmosphere studies using payloads fired from converted artillery guns in the 60s.
Not sure if railguns in particular add much over chemical guns that justifies the complexity.
I knew a futurist who proposed using a railgun on Kilimanjaro as stage 1 of a rocket launch system. A lot of fuel is burned just trying to shove a lot of fuel through the lower atmosphere.
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...
Rail guns have a lot of problems for orbital insertion from an atmosphere:
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.
On a railgun you'll have to pass orbital speed by the end of the rail, because, from that point on, you will only lose speed to the atmosphere and you have most of it still above the end of the rail.
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.
> 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.
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.
I’d like to see them explain the benefits of this over solar power. They write in the article that it’s much faster, but that’s true for all chemical propulsion.
The benefit of solar power is that it’s almost limitless.
Solar power is not a method of propulsion, it's a method of collecting energy. Perhaps you mean ion engines paired with solar panels? If so, the difference in thrust is massive, as in several orders of magnitude - to the point where getting anything larger than a satellite moving with electric-based propulsion would take many orbits of acceleration. You also run into issues of not being able to collect power when in shadow, needing power storage banks (batteries are very heavy), and diminishing power output as you move away from the sun (ex. at Mars, solar energy density is 40% less than at Earth).
You have to have some pretty big engines before the mass of the reactor is a small enough portion of the mass that the Thrust to Weight ratio of a nuclear thermal engine + tank + reaction mass is better than that of a chemical engine + tank + fuel. The only place that kind of thrust is needed is getting off of Earth, and a nuclear engine doing that is a non-starter.
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.
The implied goal is to send humans to places in the solar system. That means life support, food, water, etc. So a lot of mass anyway. Worse, the longer the trip the more mass you need to bring onboard. Nuclear would allow for the shortest trip durations while ion engines would be the longest. With ion engines you have the issues of micro-gravity health risks on the crew, psychological risks and the need for a lot more supplies.
For super long trips, efficiency is king. Coupled with the ability to ability to burn continuously, solar-electric and nuclear-electric are way more efficient than nuclear-thermal. Ion engines may have low thrust -- just use a lot of them.
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.
That's still several orders of magnitude more massive than the chemical and ion thrusters used on small satellites.
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.
If you can get 1g of acceleration, the trip to mars is a couple of days, I don't think you could ever get that with solar.
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.
Only if really all the mass of the antimatter is usable energy for propulsion. A large chunk this energy would probably be in the form of gamma ray photons: they are not usable for thrust and irradiate the materials of the spaceship, another large chunk would be heat. And I assume you would also need quite a bit of energy for whatever vessel is holding the antimatter in place (magnetic fields maybe?), to avoid that the entirety of the 10^14J is produced instantly by annihilation with normal matter.
Nuclear thermal isn't chemical propulsion. It heads a propellant to a high temperature using an onboard reactor, and then expels it.
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.
You can also have solar thermal propulsion where you reflect large amounts of sunlight onto some hydrogen to heat it up before using it as propellant. The efficiency is roughly the same since in both cases you're basically limited by the melting point of your engine. Solar thermal designs tend to have lower thrust than nuclear thermal but much more thrust than solar electric.
>I’d like to see them explain the benefits of this over solar power.
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.
I assume you're talking about solar-powered ion engines, as solar panels cannot on their own move a spacecraft appreciably.
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.
The average solar irradiance on Mercury is over 9000W, on Earth about 1400W, on Mars 600W, on Saturn 15W, Pluto 0.87W, per square meter(or per 10ft^2, or per 14 medium sized Pizza). Farther you’re out the sunlight must disperse to a broader area thus gets weaker. Inverse square law you know.
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...
Combination of both high thrust-to-weight ration and a high exhaust velocity. The first ensures you can accelerate quickly when not loaded with too much fuel, the second ensures you don't need to bring that much fuel.
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.
Well, now that I think about it, our chemical propulsion requires an oxidizer, and judging by how this works, this doesn't need one. So by using this method they cut down on the oxidizer mass, which is something fairly significant. Not sure if it'll beat an ion thruster powered by the sun when it comes to convenience, though.
It's wrong to think of ditching oxidizer and keeping fuel. They both contribute as components of the rocket's energy source and its working mass. If you get rid of the chemical reaction, the "fuel" is not fuel any more, and you should replace your concept of it as such with a narrower concept of "working mass."
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)
The article says it is about getting to "close" destinations, like Mars, as fast as possible. I am not sure how solar would achieve that over short distances.
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.