
The US military is getting serious about nuclear thermal propulsion - rbanffy
https://arstechnica.com/science/2020/06/the-us-military-is-getting-serious-about-nuclear-thermal-propulsion/
======
jl2718
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.

~~~
seph-reed
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.

~~~
btilly
Answers.

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.

------
CydeWeys
Show me the Isp figures on these proposed nuclear thermal propulsion designs.

Project Rho (the best source for this kind of stuff) is listing various
nuclear thermal designs with high 3 digit Isps, e.g. 900s for this design:
[http://www.projectrho.com/public_html/rocket/enginelist2.php...](http://www.projectrho.com/public_html/rocket/enginelist2.php#ntrsolidcore)

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.

~~~
DennisP
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.

Convenient calculator:
[http://www.quantumg.net/rocketeq.html](http://www.quantumg.net/rocketeq.html)

~~~
CydeWeys
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.

~~~
arethuza
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:

[https://en.wikipedia.org/wiki/1980_Damascus_Titan_missile_ex...](https://en.wikipedia.org/wiki/1980_Damascus_Titan_missile_explosion)

And that was something that was intended to explode!

~~~
CydeWeys
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.

~~~
Sharlin
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.

~~~
DennisP
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.

------
Knufen
The idea was considered before and showed great promise:
[https://en.m.wikipedia.org/wiki/Project_Orion_(nuclear_propu...](https://en.m.wikipedia.org/wiki/Project_Orion_\(nuclear_propulsion\))
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.

~~~
jjoonathan
NERVA
[https://en.wikipedia.org/wiki/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!

~~~
akiselev
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.

[1]
[https://en.wikipedia.org/wiki/Nuclear_lightbulb](https://en.wikipedia.org/wiki/Nuclear_lightbulb)

~~~
bryanlarsen
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.

~~~
akiselev
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.

~~~
marktangotango
> 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.

------
randombytes6869
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

~~~
Zealotux
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.

------
caiobegotti
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.

~~~
blhack
You still have to get the materials for your experiments up there.

In the case of nuclear-powered engines, that isn't trivial (unless you mine
them off world).

~~~
JoeAltmaier
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.

~~~
nix23
Not earth but belters ;)

~~~
2StepsOutOfLine
I, for one, will keep a fish tank on earth in case any better descendents
swing by to visit.

~~~
nix23
Yesyes...family-meal is a bit more complicated that normal, but my surprise
gifts are massive ice-cubes.

------
hairytrog
NASA is developing its own system:
[https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/201700...](https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20170003378.pdf).
It's expected to follow a similar industry bidding process as the commercial
crew launch etc. It's too bad the military had to get involved.

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/](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](https://www.usnc.com).

~~~
umvi
> 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.

~~~
charwalker
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.

~~~
catalogia
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?

------
Gatsky
> One advancement has come in the ability to manufacture refractory metals,
> which are extraordinarily resistant to heating.

[https://en.wikipedia.org/wiki/Refractory_metals](https://en.wikipedia.org/wiki/Refractory_metals)

The melting point of tungsten is astonishingly high (3422 deg C).

~~~
cletus
Tungsten. A popular XKCD What-If? topic:

\- Boating [1]

\- Countertops [2]

[1]: [https://what-if.xkcd.com/50/](https://what-if.xkcd.com/50/)

[2]: [https://what-if.xkcd.com/89/](https://what-if.xkcd.com/89/)

------
sparker72678
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.

[https://www.amazon.com/Atomic-Adventures-Islands-
Forgotten-I...](https://www.amazon.com/Atomic-Adventures-Islands-Forgotten-
Isotopic-ebook-
dp-B01MYNQLZX/dp/B01MYNQLZX/ref=mt_other?_encoding=UTF8&me=&qid=)

------
protomyth
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.

~~~
hwillis
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.

------
samstave
My favorite is nuclear thermal tunnel boring machines.

Google the patents for these.

~~~
LargoLasskhyfv
HyperNuke, err ...Loop

------
pm90
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.

~~~
tobylane
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.

~~~
pm90
Sure, it’s question of relative risk. It’s likely that people living on the
moon would be wearing protective suits/inside lunar facilities anyways.

I guess the question is if it’s riskier to transport fissile material as cargo
vs in an engine. I am not sure what the relative risk would be.

------
waterhouse
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...](https://en.wikipedia.org/wiki/Project_Orion_\(nuclear_propulsion\))

~~~
Symmetry
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.

[1][https://en.wikipedia.org/wiki/Nuclear_lightbulb](https://en.wikipedia.org/wiki/Nuclear_lightbulb)

~~~
Sharlin
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.

~~~
Symmetry
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.

[https://en.wikipedia.org/wiki/Burnup](https://en.wikipedia.org/wiki/Burnup)

~~~
catalogia
Nuclear fusion is also sometimes described as "burning plasma."

------
jeffreyrogers
In the 50's and 60's similar technology was in the early stages of development
as an alternative delivery mechanism to ICBMs:
[https://scottlocklin.wordpress.com/2015/12/31/putins-
nuclear...](https://scottlocklin.wordpress.com/2015/12/31/putins-nuclear-
torpedo-and-project-pluto/)

~~~
JoeAltmaier
And in the 70's, project Rover:

[https://en.wikipedia.org/wiki/Project_Rover](https://en.wikipedia.org/wiki/Project_Rover)

~~~
aerostable_slug
And later, Project Timberwind / TIMBER WIND

[https://en.wikipedia.org/wiki/Project_Timberwind](https://en.wikipedia.org/wiki/Project_Timberwind)

------
pankajdoharey
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.

~~~
valuearb
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.

------
pmoriarty
An alternative I somehow never hear discussed in these threads is railguns.

How practical is using railguns for launch compared to nuclear thermal
propulsion and the other alternatives discussed here?

At the very least, it seems to be much safer than something that risks nuclear
fallout.

~~~
detaro
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.

~~~
pmoriarty
I have read of very long railgun designs (on the order of many kilometers),
where the acceleration should be more gradual.

~~~
magicalhippo
Aka a very fast maglev train.

------
Aeolun
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.

~~~
mdorazio
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).

~~~
bryanlarsen
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.

~~~
marcinzm
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.

~~~
bryanlarsen
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.

------
petermcneeley
Research in the previous century:
[https://youtu.be/Zm7PNlK5Aco](https://youtu.be/Zm7PNlK5Aco)

