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?
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.
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.
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.
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.
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.
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.
Would the core just float off into space? Couldn't it make its way back to Earth? Or compromise potential life on Mars?
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.
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.
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?
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.
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.
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.
There are elements with far more favorable decay paths. Short decay + using that decay too = pretty much a clean nuclear reactor.
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.
"Mined" from where? How?
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?
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 ...
Theoretically, for reactors we don't have.
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.
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.
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.
>“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.
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.
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.
These things are tough! And also expensive, so you might as well reuse them once they shrug off the rocket exploding under them.
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.
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/
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 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?
Edit: available through outline https://outline.com/nbpE5n
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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).
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.
These types of engines have already been run: https://www.youtube.com/watch?v=eDNX65d-FBY
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.
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.
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).
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. ;)
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.)