The US government is taking a step toward space-based nuclear propulsion
arstechnica.com
arstechnica.com
It's possible that reliability/safety will improve as fast as the launch cadence does, in which case we could see the accident rate staying flat or decreasing.
Launches are less to be feared because they are oriented toward sea and the most critical moment are the first minutes. The device discussed is intended to travel away from earth so as you noted it won’t be dangerous after it quit orbit.
Disclaimer: not skilled in any related field, just a consumer of space science popularization videos for newbies.
(Not a direct comment on your opinion specifically, just building of the general idea)
Oceans are the cradle of life and are still populated by fishes, mammals, zoo and phytoplancton even away from coastlines.
https://www.rnz.co.nz/international/programmes/datelinepacif...
Abyssal zones of the ocean are only populated by creatures that feed on hydrothermal vents, or dead creatures falling down from the surface. But the mid pacific has no hydrothermal vents and the surface is a dead zone due to human activity. So there is no source of food for life to exist here, and outside of the occasional whale fall it is truly dead.
I'm kinda surprised the soviets even thought of the environment in that way.
Consequently, reactor payloads would be carried on a very small fraction of launches, then used in space for an extended amount of time.
> Or even if it broke up
> in the atmosphere.
It’s not the 99 rockets falling into the ocean shortly after launch that are problematic, it’s the 1 that almost works but then lands in the city we should be concerned with.
That's pretty close for what would be high level nuclear material.
A better way would just be shielding, make sure it stays in 1 piece through reentry and impact. And still doesn't leak radiation then.
Also you can presumably shield reactors to survive reentry or even ship the fuel in multiple such reentry proof containers. And of course longer term build these in space, given nuclear propulsion is basically in space only anyway given its characteristics.
Or it could go the way of the airline industry where failures become less regular as the industry develops ways of systematically reducing them.
Also, nuclear rockets should probably be launched to earth orbit in pieces, separately, so that if any one launch fails it won’t create a nuclear accident in the biosphere. Then assemble and power up the nuclear rocket as far away from earth as possible.
Cold war = rocket science, new fuels, materials science, major computing advancements, supply chain changes, innovation, science hype, broad optimism
I know most of this is happenstance and that peace time doesn't itself cause lackluster development, but it is interesting and palpable.
I was so down on 2000 - 2020, but the decade ahead is exciting, and I'm filled with motivation.
Next time the cold war cools down, we need to invent something to replace it.
Colonial wars in the asteroid belt and beyond. Interstellar wars. Maybe the first FTL drive could be on an autonomous space torpedo, and that could be lore explanation for the shape of warp nacelles in this timeline.
If America and China are dead set on having another dick measuring contest, we're all much better off if they do it with drones halfway between here and the moon, instead of near earthbound population centers. By all means let's make outer space the future of war, as far away from humans as possible.
The US declared war on the concept of terrorism in 2001, directly invaded two nations as part of that effort, and overthrew multiple governments before ultimately abandoning the war and handing a broken Afghanistan back to the Taliban in 2021.
We did an excellent job of making sure the war was fought on foreign soil and largely stayed out of our daily life back home, but we definitely were at war.
(I'm focusing on the US here, but there were quite a few other major conflicts and military coups around the world during the same time)
Its worth noting that Afghanistan may be less developed technologically but they have now successfully fended off invasions from both the Soviet Union and the US. Never underestimate your enemy in war, especially if you're Goliath invading David's home.
How much can a nation progress if its youth is pessimistic about everything? It doesn't matter whether the pessimism is based on actual problems (healthcare, housing prices etc.) or learned through terminally online reddit/twitter consumption.
If we're not gettin' out our (NASA-approved) Huarache sandals, "waxing down" our pusher plate, and surfing "the pocket" of those sweet nuclear detonations in the near future, then, I'm afraid our species is utterly consigned to the category of "posers"!!
See, Kilgore knew how to put this kind of (heavier) ordnance to good use:
https://youtu.be/8XJYnKhI_x0?t=1m06s
... and it don't get heavier than nuclear for us, currently!
* https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...
There were some paper studies showing that a thermonuclear version could theoretically work and reach speeds up to 10% the speed of light, which would be the Centauri system in a bit over 45 years. Of course you would have to slow down too which probably adds another few years.
If this can be theoretically done with technology contemporary with the Summer of Love, it really does make the Fermi paradox seem like something real.
Not particularly necessary, most of the starshot type plans made seem to be fly-bys.
https://ntrs.nasa.gov/api/citations/19920005899/downloads/19...
For this new initiative, I can see 2 big differences. On one hand they will use medium enriched uranium (up to 20%), while the original program used highly enriched one (85%), so it's going to be more challenging to extract as much power from a compact reactor. On the other hand, in 2023 we have computers and nuclear reactor simulation software that was not available in 1960, so that could provide a huge advantage.
The biggest challenge I see is how to power down the engine. You can't simply flip a switch a turn off a nuclear reactor. You can stick in all the control rods, and the fission stops in less than a second, but the fission products will continue to generate heat for a few hours, initially at about 10% of the full reactor power. During this cool-down period, you will use the same coolant (hydrogen) as during normal operation, but you won't get the same exhaust velocity. With a traditional chemical rocket you don't have this problem. If you get a specific impulse of 450 s, you always get that. If a nuclear thermal rocket can get 900 s running at full throttle, but it goes down to 100 seconds in cool down mode, the main advantage might not be so great after all.
That seems like a waste of propellant.
Once the reactor has cooled enough that you won't get the desired exhaust velocity, couldn't you allow it to finish cooling down by radiative cooling?
No-one wants the extra work that dealing in bomb materials would involve.
Now, imagine them with nuclear materials aboard.
(Could be worse, but the Orion drive is currently illegal by treaty and hopefully nobody is dumb enough to use that in Earth's ionosphere anyway).
Btw, the video I saw was on Veritasium.
But maybe I remember wrong.
Edit: the video in question is [1] and the conpany was Kodak.
The main reason that the US was secretive about it was that nuclear weapons were a secret, not because there was a shameful health problem to hide.
Incidentally, the best way to reduce your exposure to radiation is to move away from the mountains and go live at the beach. Mountains are mostly made of granite, and granite always contains small amounts of uranium and thorium which decay to radon gas. Also, live as far away from coal power plants as possible, those things are horrifying.
The claim, nuclear testing related increases in radiation exposure not being relevant for public health is probably false. There are only very few studies with dubious reliability. The lifetime increase due to that nuclear testing is estimated to be 4.4 Millisieverts. It would have risen continuously with more surface tests, which got abolished accordingly.
The idea, beaches were a sanctum against radiation is wrong again. Sand is ground down mountains in case you wondered. Some are highly radioactive. But of course, UV light is harmful radiation as well.
- from opening paragraph of https://en.wikipedia.org/wiki/List_of_nuclear_weapons_tests
But still, thousands of bombs were in fact tested, and the context here is testing a reactor that won't be on Earth at all via a diversion into "we can detect really small levels of radiation".
The fuel will only be needed for a lighter craft in deep space. Getting it up with a high reliability rocket (like Falcon 9) significantly decreases risk.
In principle yes — even just on the grounds that enriched fuel is expensive — but it's a hard thing to prove in advance, hence all the times humans died in a space mission one way or another.
https://en.m.wikipedia.org/wiki/List_of_spaceflight-related_....
Today's rockets are far more reliable, and anything holding nuclear fuel should be the best of the best.
Er, Challenger?
It also has well over 100 successful landings in a row, while the next closest rocket has 0. (Rocket, not capsule or shuttle, which are all still far lower)
"The rocket had a streak of 112 consecutive successful launches between 11 July 1990 and 5 May 1996"
It might be a good time to review the number of failed launches on crew rated rockets (the safety requirements on which are roughly equivalent to the requirements for carrying nuclear material). Particularly ones which led to a loss of crew, as those would correspond most closely to a possible nuclear material release.
It's far less scary, but alas not as convenient for FUD.
Fission reactors aren't appreciably more radioactive than rocks you can dig out of the ground until you turn them on. This is something that wouldn't be brought online until the riskiest part of the launch has past.
Any reasonable nuclear-powered craft would not start up the reactor until it's already in a nice, safe, high orbit.
The basic idea is straightforward: A nuclear reactor rapidly heats up a propellant, probably liquid hydrogen, and then this gas expands and is passed out a nozzle, creating thrust. But engineering all of this for in-space propulsion is challenging, and then there is the regulatory difficulty of building a nuclear reactor and safely launching it into space.
In a chemical rocket, the propellant provides both the bulk and the energy. As it happens, there isn't that much energy in chemical propellants per kg, but the rest of the engine is light enough that these things can fly to orbit.
In a nuclear rocket, the energy comes from a nuclear reaction and the gaseous fuel only provides the bulk. This is way more efficient. But such rockets likely cannot achieve orbit on their own because the whole set up is too heavy.
For atmospheric flight the closest thing is a jet engine which is "air breathing", i.e. requires air to run and works in part by sucking air in, using some of the Oxygen in it for combustion, and shoving the extra air out the back. This gives them much higher efficiency than chemical rocket engines, as they don't have to carry their own oxidizer or the mass to eject for propulsion - but only works because the air is dense enough where they operate. Which is not true for space flight. Commercial jets typically fly a bit under the speed of sound in air which is 767mph. Of course supersonic air craft exist - I think mach 5 is achievable by some military jets - but that's still a fraction of orbital velocity. Anyhow running in the atmosphere means jets don't "scoop up air" in any sense but rather just use it immediately - so any lost momentum can be immediately countered by the engine.
Another direction to think about it - a simple model for air resistance says that air resistance increases with the square of your velocity (and with the density of air, which exponentially decays with height though also depends on temperature. I think the height wins out though. Not sure if the equation works at extremely high altitude. But certainly this favors using air at lower altitudes and probably also lower speeds
https://en.wikipedia.org/wiki/Drag_(physics) https://en.wikipedia.org/wiki/Density_of_air
In an Ion jet, the energy comes from (usually) solar power and the gaseous fuel provides the bulk. These are even weaker because of the limited instantaneous power that can be generated to feed it, but they are very efficient in how much thrust they can provide per propellent weight. Some versions (like arcjet) are a combination chemical rocket/Ion jet.
That said, it’s hard to imagine that they’ll ever be used for launches from the Earth’s surface. Mars, maybe.
Either way I'm no expert.
Yes, that is mostly it. Just compare the explosion from some hydrogen with a nuclear bomb. It is all about energy for mass in space, because all the fuel you have, you have to bring up into space, which needs more fuel, so need more fuel to bring that fuel up ... nuclear could help with that mass ratio a lot. And I like the concept in theory - as long as none of them explodes halfway up to space.
A hydrogen+oxygen chemical rocket propels H₂O, while a nuclear thermal rocket can use the lightest propellant (H₂) because the heat comes from elsewhere.
Smaller molecule, faster exhaust, better rocket.
With constant mass, you increase the total momentum by increasing the exhaust velocity. With constant temperature, you increase the exhaust velocity by making the molecules smaller.
Specific impulse (Isp) is how much force you get from each mass unit of propellant you exhaust. So an engine with Isp = 300 gives 300lbs force for every pound of exhaust gas mass.
IIRC, Isp scales as the inverse square root of exhaust molecular weight. So a pure (molecular) hydrogen exhaust would have a molecular weight of 2. A pure H2/O2 engine would have an exhaust of H2O (assume it's running at stochiometric H2/O2 ratio to simplify things), or an average molecular weight of 18.
For everything else being equal (temperature, pressure, etc.), the H2 Isp would be sqrt(18/2), about 3x the H2/O2 Isp. That's mainly why rocketeers would like nuclear engines, more oomf from a given mass of propellant. It's also (conceptually) simpler because you only need to handle one propellant vs. two or more for conventional liquid fuel combustion.
It's more complex, of course. If the reactor is really hot, it can partially dissociate H2 into atomic hydrogen, lowering the exhaust molecular weight still more. But dissociation is an energetically expensive process, so there's bound to be a tradeoff between energy consumed by dissociation vs. increased Isp. I've no clue how that would work out.
Usually reactors run cooler by several orders of magnitude, as this is referred to as a ‘meltdown’ and people get snippy about the releases of radiation and expensive cleanup crews, etc.
Space is more forgiving and has fewer HOA types, so they can go closer to the limits - but it’s still the same underlying issue.
I feel like if designers would drop their effort to wring out the last increment of Isp from nuke engines and reduce temperatures to ~2000°C, it would give faster development. There are many materials that can live in 2000° hydrogen for >100K hours.
There's an old Rand study that showed H2 would give an Isp >1100 at 1 atm chamber pressure (!), temp ~2000°C, exhaust pressure 10e-4 atm. Seems worth exploring.
Part of the issue I suspect why reasonable isn’t being considered is that I don’t expect anyone to have a reason to really use them right now regardless - all the nuclear propulsion designs have a high ‘fixed’ weight cost in the reactor, so would only make sense overall in a ‘hundreds of tons to Uranus ASAP’ type situation, where the overall weight for thrust can be lower due to the better fuel efficiency.
So far we thankfully haven’t had a reason to really need to do that right now.
And if we’re doing theoretical designs, why not make it ‘interesting’?
On a side note, if we’re considering crazy ideas - my favorite is the one that makes Project Orion seem clean!
https://en.m.wikipedia.org/wiki/Nuclear_salt-water_rocket
Any rocket which calls for ‘salts of plutonium’ as fuel has to be cool.
You want to optimize for velocity gain (also known as delta-v) per unit mass of propellant. So the question is, should you eject something heavier at lower speed, or something lighter at high speed? Which is more efficient?
If you write out the equation for a rocket ejecting some propellant at some velocity, apply conservation of momentum, and solve for the ratio of delta-v to propellant mass (just a bit of high school physics), it turns out that a higher exhaust velocity makes a more efficient rocket.
As a sibling post points out, if you compare water vapor with hydrogen at the same temperature, hydrogen will have the higher velocity. Chemical rockets usually produce water and/or carbon dioxide, which are bigger molecules and therefore less efficient propellants than hydrogen. So if you have a way to heat hydrogen to the same kinds of temperatures you get in rocket exhaust — say, in a nuclear reactor — that’ll make a more efficient rocket.
There are of course considerations other than pure efficiency. Our most efficient rockets, ion engines, are indeed crazy efficient, but their thrust is measured in milli-newtons or newtons. They can’t be used to lift things off earth. They also use propellants like krypton and xenon, presumably because they’re easier to store than hydrogen.
[0] https://www.theregister.com/2009/11/15/zuppero_solar_system/
Voyager 2 is equipped with three Multihundred-Watt radioisotope thermoelectric generators (MHW RTG). Each RTG includes 24 pressed plutonium oxide spheres, and provided enough heat to generate approximately 157 W of electrical power at launch.
Or to put it another way, at what point should we throw a Hail Mary in the energy demand game?
There is and has been a anti-nuclear sentiment when it comes to energy written in media for quite a few years and seeing that sentiment be turned around is positive. I am convinced(and it's just an opinion) the negative portrayal of nuclear energy is directly related to the financial influence that oil and other energies have over many sectors of the economy and government.
„ In all cases, however, the primary safety issue would be the car's survivability (well the reactor) during a crash. Any serious crash could result in a very serious miniature nuclear contamination disaster. Not ideal to say the least, especially considering the number of serious crashes each year. Not to mention the need to protect nuclear fuel from abuse by potential terrorists.“
https://interestingengineering.com/lists/where-are-all-the-n...
> German engineer
Why does von Braun get such a pass from his Nazi past? Because he was useful?
Plus, people-controversy is a topic that's actively avoided in writing for a lot of audiences, especially if it invites fault-spreading, like if it's going to end up as a big whataboutism-fest.
Difficult topic in its way. Including of course the WW2-and-prior way involving terrible aspects like forced labor working under him, etc. So, less of a defense of a person here and more of a guess about rationale in authorship.
People who are extremely succesful in certain fields get an impressive amount of leeway to break every conventional rule, law or protocol.
Allowing nazis to clean up their past and work for NASA is not the only horrible things the US did during the war or immidietly after either...
Partly it's because his Operation Paperclip dossier was conveniently misplaced before it could be declassified.
Who knows what was in there? Not historians, that's who...
Source: https://www.archives.gov/iwg/declassified-records/rg-330-def...
Will the first launch be on PPV?
[1] https://en.m.wikipedia.org/wiki/Radioisotope_thermoelectric_...
So they're going to launch an untested reactor into orbit without even turning it on first? Getting a reactor up and running is hard enough on the ground. I would expect any reactor to have had thousands of hours of testing and tweaking before being placed on the launch pad; that is the process for military boats and we have 60 years of experience in that environment.
Totally different reactor designs.
After all, waste heat is much harder to get rid of in space than here on Earth.
For reference, other space nuclear reactor designs NASA has looked into have been in the lower kilowatts in terms of power output.
Space stuff doesn't yet need the power output relative to size of a modern military reactor, the bigger concern is being reliable and providing stable long term power.
Oh wait, helping the poor doesn't make those holding stocks in these companies any money.
Maybe they can propulse their rockets with their politicians hot air.