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NASA Wants to Send Nuclear Rockets to the Moon and Mars (wired.com)
98 points by n0pe_p0pe on Sept 27, 2019 | hide | past | favorite | 83 comments

what happens at the end of such a trip, when the propellant is exhausted, and can no longer evaporatively cool the core?

it has to contain enough propellant to come back to Earth orbit to refuel the propellant coolant?

even if the core is throttled to its lowest levels, it will still produce heat in the vacuum of space? or will radiative cooling balance the remaining production of heat?

You do what you would on a terrestrial reactor, and shut down the reactor e.g. by using control rods, or pulling out fuel rods to shut down the chain reaction. These wouldn't be simple radiothermal devices, but proper actively managed reactor cores.

Having said that you absolutely would need active cooling even in a 'shut down' state, probably by circulating a cooling fluid to radiator fins. You might use the same fluid as the propellant or might not. In the former case sure, you wouldn't be able to use all of it as propellant.

Absolutely not, in space you have the option of jettisoning the core, carrying coolant for a controlled shut down is a waste of weight. You might even go as far as having 4 disposable cores, one for each acceleration stage.

Not sure why this is being downvoted jettison the core would likely be the go-to solution you can put into a stable orbit or even set on course with the sun and let it burn off...

There is no much need for a complex control mechanism in fact keeping those core as orbital modules on mars would likely allow you to skip many of the complications of having a complex control system to prevent a run away effect and modulate power output no one really cares if the core would go critical even at low orbit on Mars as there is so much radiation there from natural sources anyhow.

If the core goes critical mid way then either have a fallback core or just jettison it and wait for a rescue mission.

The vessels would likely have enough supplies for such occasions and as far as risk to the crew goes nuclear propulsion in space is much less dangerous than chemical propellants that can actually explode.

With a nuclear core you can safely vent hydrogen and xenon to space not worrying that much about radiation or long term contamination the cores can be more or less exposed with large fin stacks for radiative cooling.

"set on course with the sun and let it burn off" - https://laughingsquid.com/why-it-is-more-difficult-to-crash-...

Brilliant solution man.

Additional nuclear fuel, unlike the popular belief says, will not be taking any much significant weight. A critical mass of plutonium is only 10kg, and you will likely have it reduced with moderation and reflectors.

If you have to keep disposing heat away from a powered down, but still residually active core, you will have to keep a no joke mass of radiators just for that.

So you would not build in mechanisms to moderate the heat output of the core, and have it generating at full power constantly? Ok, what state is the core in before you want to fire the engines? Suppose you want to burn to leave Earth, and agin to slow down at Mars, what happens to the engine core on the coasting period of many months in between? If you're carrying several disposable cores, if there's no way to moderate their output how do you store them safely when not in use and switch them on? Also, whole propulsion cores are lighter than the cooling system for a single core? Really?

A core is just nuclear fuel with enough other parts that it can be handled. Of course there needs to be a control mechanism for adjusting the output, you just can't use that for eliminating heat output. Once the core has been turned on there is decaying fission products producing heat, that is why you can't turn off a nuclear power plant in an instant, and why you might want to get rid of the core. Unused cores can be stored with few special precautions as the unspent fuel decay too slowly to be a major hazard.

The ship would carry one small nuclear reactor, and several tons of coolant/propellant. The only way the cooling system is actually going to keep up is by ejecting the hot coolant into space, which is also what creates thrust. You can't recycle the coolant as there is no efficient way to cool the coolant.

In effect, if you don't want to dispose of the core you have to keep the engine on consuming coolant/propellant, depending on a lot of factors that may or may not be viable.

>You can't recycle the coolant as there is no efficient way to cool the coolant.

Alright, this is the crux of the problem. Yes you can, the ISS has huge radiator panels in it's shadow for thermal management. The shuttle also had radiator panels in the bay door structure, they were so crucial the vehicle had to re-enter soon after closing the doors to avoid excessive thermal buildup. This is a known solved problem. And that's on vehicles that aren't carrying any nuclear power systems, and just have to contend with solar radiative heating.

The ISS doesn't have a heat budget anywhere close to that of a reactor core, you'd need to carry a much larger radiator construction, just for this job. Extra cores cost way less weight.

But if you don’t have mechanisms to moderate those cores output, they’ll be blazing away all the time. You can’t get away from the fact you need a way to moderate the reaction, including For ‘spare cores, which you at least need a mechanism to activate, and you need an active cooling system.

And on top of all that, even with reaction mass cooling the core when it’s active, that not going to be enough to capture all the generated heat.

Serious questions...

Would the core just float off into space? Couldn't it make its way back to Earth? Or compromise potential life on Mars?

It's pretty straightforward to put it in a safe, permanent orbit.


your option of coolant without loss implies radiative cooling, how large a surface would be needed for how large a rocket for radiative cooling in the off shut down state? would the sunny side need to be reflective while the dark side needs to be emissive?

Nuclear reactors can stand quite high temperatures, so it will be much easier than you would assume if you looked at other rocket projects. Remember that radiating power increases with the 4rth power of the temperature.

Also, a shut down reactor quickly gets to ~1% of the energy of an active one, so, again, it's easier than it seems. Generated energy goes down exponentially with time.

Of all the problems of creating a nuclear rocket, I don't think cooling stands up.

but what cools the radiator fins, in the absence of air?

If you look at diagrams of the International Space Station you'll see they have rather large radiators, so it does work in space. They're always positioned in shade to radiate heat most efficiently. It's the two long pieces labeled 11 in this diagram: https://www.nasa.gov/sites/default/files/thumbnails/image/it...

Infrared radiation from the hot fins to outer space should be taking energy out (cooling it) a bit, not sure how much though.

I think he is wondering about the utility of fins geometrically speaking in steradians when it comes to radiative cooling?

It's radiative cooling. Same way the sun heats the earth.

With a nuclear thermal rocket generally you do 90% of your burn all at once to maximize the Oberth effect then you shut down the reactor. But you've still got secondary decays going on for a while so you slowly use the remaining 10% of the propellant to cool the engine while getting a bit more thrust.

this is getting close to the answer, but still not there, the secondary decays decay for a long long time, what do we do with the rocket or core after use? how much propellant is needed in theory given radiative cooling? is 10% enough?

10% is enough for the fast decaying parts that give off most of the energy. I'm sure you still need a small radiator for the longer half-lived parts but any spacecraft is going to need radiators anyways.

Most likely it would be a very simple core without power throttling or control at all and will be used as a disposable booster and jettisoned once the acceleration phase is complete.

Another core can then be used for deceleration and disposed off as well before entering earth orbit where for the final orbital insertion a much smaller chemical rocket can be used.

Alternatively you don’t get back to earth at all but rather back to the moon where you could jettison the core even on the surface during final approach without giving much thought to radiation.

Then use moon to earth transit via chemical rockets.

Why do you believe they haven't/aren't already thought/thinking about these things and solving them? Or are you just genuinely curious?

> which will almost certainly be used on any crewed mission to Mars.

This is interesting. I hadn't seen anything about NASA plans to get humans to Mars requiring nuclear-thermal propulsion. Does NASA even currently have a serious plan for Mars missions with the whole Artemis thing going on?

They branded Artemis as part of a larger "Moon to Mars" plan, with logo and all. https://www.nasa.gov/moontomars/

Interesting. I'm kinda down on Artemis overall, as it seems like the whole thing is getting bogged down in political maneuvering, as usual.

NASA keeps saying it's more than boots on the ground, but all the plans they announce seem to be boots on the ground or overly complicated paths to the surface meant to please contractors.

This gem of a line from this week's Orbital Index kinda sums up why I'm preparing for disappointment:

>"Meanwhile, contractors (cough Boeing? cough) are pushing for the Gateway plan to be nixed in favor of… The Exploration Upper Stage, a large interplanetary upper stage (launched on SLS Block 1B) in development by (wait for it) Boeing."

Boeing and the SLS have taken forever already, and the version that may launch soonish isn't even the "real" SLS.

I'm skeptical of any human exploration plans NASA has beyond 2-4 years or so. Anything more and the next President winds up nixing it to put their own stamp on things.

My money's on a commercial program like they one they've been doing for ISS resupply.

> My money's on a commercial program like they one they've been doing for ISS resupply.

Fully agree. If I had to bet right now, I would guess that SpaceX will get humans to the moon before NASA does. They’re moving so quickly on Starship and seem extremely determined to prove their new rocket.

How big is the added risk of toxic waste? I'm not sure how much waste is produced in relation to propulsion energy given but it must be quite small? And I imagine that during travel in space you could just dump that waste out into space considering the vastness of it all.

Whenever you're splitting Uranium atoms the results will tend to be radioactive. The results will build up in the fuel over time and eventually make the reactor stop working. Conventional reactors breed a bit of plutonium too as U238 captures neutrons but most aerospace reactors want to be as light as possible and so use highly enriched Uranium. So after your trip the engine will be quite radioactive but, as you point out, there's a lot of space and outside Earth's atmosphere and Van Allen belts it's moderately radioactive anyways.

Thankfully nuclear reactors aren't particularly radioactive until you turn them on, which is a big improvement on the radiothermal generators, RTGs, that we sometimes use in probes headed for the outer solar system where solar panels don't work. It's during launch, before this part gets turned on, that you have a risk of crashing and losing the reactor somewhere on Earth.

Who said anything about uranium?

There are elements with far more favorable decay paths. Short decay + using that decay too = pretty much a clean nuclear reactor.

Well, I'm no nuclear engineer but I'd hope that if you could make one of those that was light enough NASA would use that instead.

It's called an RTG, and they've been using them for decades.

I'm not sure what you're talking about? RTGs don't have short decay paths, they have to have long decay paths to last through a mission. P238 is what we use for most probes and has a half life of 88 years. It decays to U234 which has a half life of 200,000 years, short enough to be dangerous but long enough to almost never go away. RTGs tend to produce on the order of 100 watts of electricity from 500 watts of heat. A good nuclear engine will want to use 100+ megawatts when in use.

And more importantly RTGs don't put out nearly enough heat to make a usable nuclear thermal rocket. The important thing is being able to turn them on when you're doing a burn but then turn them off when you're coasting to your destination then turn them on again to stop there. RTGs can't do that.

What was asked for sounded like an RTG to me. I wasn't saying it was a nuclear thermal rocket or that it could be used as one. As you point out, it's a constant power source.

I’m increasingly of the opinion that nuclear for space should be mined and built in space. Just launch the infrastructure needed to bootstrap the process.

I'm all for that in the long run, but that's in the long run. Especially with Orion Drives they're too dangerous to fly themselves off the Earth but too heavy to launch on something else so they're entirely impractical right now. But it would be cool if we could be launching them from the Moon in the 2080s.

Um, uh, um, uh...where to start?

"Mined" from where? How?

The moon? Asteroids? Or even just ship up unrefined ore such that a catastrophic failure can’t threaten the population.

Neither of the first two are options... at all realistically, nither uranium nor thorium exist in anything below a large planetary size body in any quantity: https://www.quora.com/Do-uranium-and-thorium-exist-in-signif...

And shipping up unrefined ore is also a bit of a ludicrous idea for mass reasons and the rocket equation alone. You do realize you can isolate a nuclear reactor core from explosions on rockets right? What catastrophic failures are you attempting to design your solution of avoiding a nuclear reactor around?

What risks make that extraordinary cost worthwhile?

The moon's dust contains huge amounts of helium-3.

Which apparently is an amazing power source.

Also, why fission? We do have working fusion reactors. They are called hydrogen bombs. (The outer part, at least.) As long as you can keep the G forces low ...

> Which apparently is an amazing power source

Theoretically, for reactors we don't have.


i was about the type the same thing, have my upvote friend

I don't know for certain but I'm fairly sure that the idea is that you don't activate the reactor until it's in space. Before a reactor is turned on the fuels are less radioactive. It's once you turn it on that radioactivity increases dramatically and you get all the nasty decay products and such.

So not zero but not as much as you might think.

Personally I don't like the idea. Environmental concerns are real, but those aside it's likely more expensive than multiple refueling flights with big conventional rockets. These would be expendable and very costly to research, develop, fuel, and launch, whereas for the same cost you could probably put stages in orbit and send fuel up to them with reusable tankers. Like hydrogen this is another example of NASA chasing the sexiness of high performance in a pure sense (high iSP etc.) without doing a total cost analysis.

In general SpaceX and Blue Origin have the right approach.

Why is this downvoted? This is informative and (in my opinion) basically correct.

And while I agree that in the near term, refueling via chemical rockets is a far cheaper (and even higher performance) way of solving this problem, I do support the research because someday we'll want to go even beyond refueling of chemical rockets. When you get REALLY high transfer times between Earth and Mars, the higher Isp makes a significant difference.

To explain: Conventionally, it takes about 6-8 months to get to Mars. Nuclear thermal rockets can shorten this time for the same mass in LEO to like 3 or 4 months. HOWEVER, agreeing with what api said, you can get the same exact speedup by using refueling with conventional rockets (and aerocapture/braking/direct-entry). It increases the required mass in LEO, but if you have cheap (especially reusable) rockets, then cost to launch more mass to LEO is not a major factor compared to the cost of a nuclear thermal rocket. And this is exactly what SpaceX has proposed: (see slides 19 through 22) http://www.spacex.com/sites/spacex/files/making_life_multipl...

But the Isp (exhaust velocity) advantage is maintained. The rocket equation is exponential: mass full = (empty mass)*e^((mission delta-v)/(exhaust velocity))

So eventually, when mission delta v is much higher than exhaust velocity, the mass ratio explodes. So a factor of 2 improvement in Isp is worth the extra cost, even if you have reusable rockets. The exponential curve eventually beats even the cheap, brute-force approach, if you want transfer times of on the order of 1 month.

It's also the kind of work NASA should be doing. Private industry is doing a really good job reducing the cost to orbit, so NASA can focus on these longer-term problems.

> Why is this downvoted? This is informative and (in my opinion) basically correct.

I once wrote that Chernobyl had no chance to explode in a nuclear explosion in rebuke to some guy called Moxie Marlinspike. I had -4 for the next few days on all my posts, and somebody even bothered to find my work email, and futilely tried to troll me and my colleagues into deleting my rebuke for a week.

"That" demographic is definitely there, and working in a "tech" occupation does not preclude a person from being a part to it these days.

> "The massive amount of energy produced by these reactors could be used to sustain human outposts on other worlds and cut the travel time to Mars in half.

>“Many space exploration problems require that high-density power be available at all times, and there is a class of such problems for which nuclear power is the preferred—if not the only— option,”

It seems that nuclear reactors has more utility than simple power to weight ratio.

Nuclear reactors will be needed if you're going much further out than Earth/Mars. Solar power falls off rapidly as you go further out.

The risk is that in a catastrophic launch failure (read: exploded rocket), the radioactive materials could be dispersed downrange.

The solution--if that's really a problem--is to use the same escape systems used for crewed launches to eject the nuclear fuel with a parachute and emergency beacon, and keep it all inside a durable shielded container until the craft needs to start up the nuclear engine.

Effectively, they already do that. Although the United States doesn't launch reactors, we do on occasion launch radioisotope thermoelectric generators (RTGs). These use a core of sub-critical plutonium surrounded by thermocouples, which turn the heat into electricity. These are used for probes going to the outer solar system, where solar panels aren't effective enough.

Anyway - there is certainly a concern with the plutonium in RTGs being dispersed by a launch failure. The engineering that goes into designing the protective system for RTGs is extensive; they each have their own miniature heat shield, and are surrounded by iridium and carbon blocks. Tests show that they can indeed survive the explosion of the launch vehicle.

IIRC long ago a US RTG ended up in the ocean due to launch failure, only to be recovered and sussessfully re-launched on a new satellite.

These things are tough! And also expensive, so you might as well reuse them once they shrug off the rocket exploding under them.

Except that's not a big problem. A non-activated reactor just contains enriched uranium. Uranium is dug out of the ground and you can buy it on amazon and chemically concentrate it yourself. It's safe to hold and handle (wash your hands afterwards so you don't eat particles) and store in your house even. (In the US this is all legal.)

Reactors only become dangerous after you activate them and short lived isotopes are created that also happen to be types that are bioavailable, like cesium-137 and strontium-90 which the body will take up and store inside the body.

> Uranium is dug out of the ground and you can buy it on amazon and chemically concentrate it yourself.

Well now I know what I'm putting in all my nieces' and nephews' stockings this year: https://www.amazon.com/Images-SI-Uranium-Ore/dp/B000796XXM/

Reactor fuels or RTG cores are a bit more dangerous than ore.

A properly designed reactor requires the fuel to be in the core to sustain a chain reaction, and neutron activation of other elements in the reactor does not occur until the reaction has started. Thus, a rocket explosion would not cause a criticality event. The worst that would happen would be dispersion of nuclear fuel to a place where someone might handle it without its transport-safety shielding. Which still wouldn't be that bad.

I would imagine the engine would be fairly inert if it's not activated until in orbit.

I could see issues if the craft all of a sudden loses its orbit with an radioactive engine burning up in the atmosphere spewing radiation (although I'm sure we get bombarded with way more from the sun potentially?)

Maybe if during launch something goes catastrophically wrong and blows up mid-air like a bomb of sorts?

How does one convert nuclear heat energy into directional energy for propulsion purposes?

Links dead.

Edit: available through outline https://outline.com/nbpE5n

so things got really serious and we need to get out there as fast as we can

Anyone else just get a JSON 404 page from this link?

        statusCode: 404,
        error: "Not Found",
        message: "Not Found"

I got that error from another article linked to Wired from HN the other day. After a short time, I clicked again and all was fine.

Something wonky is going on there, since it probably shouldn't even be 404 unless they keep deleting their articles and then resurrecting them shortly thereafter (which would be just a little weird).

I tried it just now and was able to see the article. On another note, why do they include the HTTP status code in the JSON body? Isn't it already present in the status line of the HTTP response?

People using funky software stacks don't always have access to the status line information, so some APIs include the status in json.


They admitted to encountering the literal definition of UFO -- "Unidentified Flying Object", which in no way at all proves or even suggests alien activity. If you want to get annoyed with the media, direct it at them convincing people that "UFO" = aliens

By unidentified they probably meant unidentified to anyone without a clearance.

Unidentified != Alien

UFO = ( classified human-built experimental aircraft | meteorological oddity | optical illusion | camera malfunction | intentional hoax ) & !( extraterrestrial artifact )

For the first time, period. Doesn’t look like we had one 60 years ago...

We did: https://en.wikipedia.org/wiki/NERVA

Tested successfully (on the ground) in the 60s.

Real pity about its cancellation, too. It was considered for the "Grand Tour" that the Voyager probes wound up doing; they could've sent nearly 30x the spacecraft mass with NERVA rockets.

I don't have time to verify the video at the moment, but I believe this is them testing said engine 60 years ago: https://www.youtube.com/watch?v=GmxPRCyR-Co

It's much more likely to look like NERVA than Orion.


These types of engines have already been run: https://www.youtube.com/watch?v=eDNX65d-FBY

I’m disappointed.

I expected a project-Orion-type interstellar solution. Not a measly ”twice as fast as conventional“ water kettle.

Also, no word about what they will acually use. Because classic uranium is a quite limited resource actually. It has been said to run out even before fossil fuels.

Also, why not a fusion rocket? Given that we know how to make fusion bombs. Because until we find a massive amount of anti-matter, this will be the next best thing for a loong time. The only limiting factor would be a human body's ability to withstand G forces.

All of those things are harder than a nuclear thermal rocket. Got to walk before you run. And a fusion reactor is actually likely to be much heavier than a fission one (at least in the near term).

They're using Low Enriched Uranium for this design. We have plenty of uranium (resources are huge, but no one bothers to prove them into reserves until the price is right), and not much is required for this project.

Don't be disappointed by the first step in a journey not taking you immediately to the destination.

> Because until we find a massive amount of anti-matter

I honestly hope this never occurs, or we never are able to contain/store such a mass for any real length of time.

Because if we can do it, it will be used for a weapon.

Seriously - I can't even imagine what - for instance - one kilogram of anti-matter coming in contact with regular matter - the amount of energy that would be released...it staggers the imagination. Today's fusion weapons release only a fraction of their potential energy; anti-matter conversion would be 100% (roughly):


"Using the convention that 1 kiloton TNT equivalent = 4.184×1012 joules (or one trillion calories of energy), one gram of antimatter reacting with one gram of ordinary matter results in 42.96 kilotons-equivalent of energy (though there is considerable "loss" by production of neutrinos)."

So...one kilo of anti-matter would be equivalent to 42 megatons - which is close to yield of the Tsar Bomba:


...but in a much more compact package. 50 kg of antimatter - which would be feasible for current launch systems, and comparable in size to current warheads:


Well - that's a 2 GT weapon...while I'm sure such a thing has been considered as to it's effects...I honestly don't know what that would be. Best guess might be that one such warhead could easily take out a good portion of say, the west coast (of the United States)?

Ultimately - we are not ready in any manner - socially, morally, politically - as a species to wield that kind of power responsibly. Honestly, even nuclear weapons fall into that assessment, despite recent history - I'm honestly not sure how we have gotten this far without a major nuclear war occurring.

Sadly, though, I know that my conjecture (in which I am not alone, I hope) will not do anything to stop the research - right now, though, the cost to produce anti-matter (let alone contain it) is so high as to make even a small mass cost an exorbitant amount of money. I sincerely hope there isn't any breakthrough on that front.

I honestly think we, as a species, are not ready for it (that isn't to say none of us are - but those who would be responsible with such "stuff" are likely very few - I know I am not one of them).

Well, if we found it, it would already be contained unless accessed by definition.

Also, magnetic fields could definitely contain it, as is already done.

We can already make anti-matter, as it't essentially the process of making matter bounce off, using a photon, in such a way that it reverts its time direction. Or, in classical view: Turn a photon into a particle/antiparticle pair. The problem is, of course, that it first takes those shitloads of energy, that it would release later.

And to actually find anti-matter in nature, you would most likely have to turn into anti-matter yourself, travel back in time, and somehow survive the big bang without touching anything, to come out the hypothetized other side where time is reversed and anti-matter expanded to. Or try to get inside a black hole, and revert your direction of motion (as time and space are reversed in there). Both not yet technically available, to say the least. ;)

Who cares about weapon power? We already have _ridiculously_ and _maximally_ powerful weapons now. Our urge to kill one another is no longer constrained by the limits of our tools. It doesn't matter whether we're able to make even more powerful weapons.

Well, the Tsar Bomba was made as a single of a kind, ust for testing, and at half of its designed size because a bomb of that size is basically useless.

So I don't think we would get larger warheads. It's more likely we would get smaller ones, at power levels that can be used, and use their small weight as a feature. (Not something great, but I don't think we will ever use an Earth Crust removing bomb.)

Antimatter bombs emit mostly gamma radiation and so are unlikely to efficiently couple it to air. You would need a tungsten or lead (or even uranium) coupler to absorb the gamma radiation and heat up to drive the explosion.

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