Moving a spaceship fast is all about having
as much energy as you can for as little mass as possible. And when it comes to dense power sources,
not many things beat Plutonium. So today we return to the Upward Bound series
to look at atomic rockets. We are going to explore the basic concept
and talk about using it safely. Last time we talked about chemical rockets,
and how we could measure their efficiency in terms of their exhaust velocity or specific
impulse, the higher the better. Fundamentally those depend on the energy released
when you burn them, that accelerates the newly formed chemicals to high velocities and we
direct them the opposite direction we want our rocket to go. The energy stored inside particles that is
released by fission reactions is generally around a million times greater. Tapping that energy to provide thrust is a
lot trickier, but if you can do it safely, without adding too much weight, and in a fashion
fast enough to provide thrust, you have one heck of engine. This is easier said than done, but let’s
start by looking at the basic concept. I don’t want to spend too much time on fission
because we could do whole episodes on that, and maybe should at some point too, but not
today. The key thing is that lots of elements and
their isotopes are not very stable and prone to decay. Some like Uranium-235 take hundreds of millions
of years to do this on their own, but if we manage to get a neutron nearby and moving
slow enough to be captured, we can cause a reaction. Instead of decaying the nucleus will split
into Barium, Krypton, and 3 more Neutrons. If we can slow those down they can get absorbed
by other U-235 atoms, causing them to split, or fission, and emitting more neutrons, continuing
the chain of reactions and emitting a lot of energy. And it is a lot of energy, as mentioned, generally
speaking the energy being released by a kilogram of fissionable material is about a million
times greater than a kilogram of chemical fuel or what we can store in our densest batteries. Needless to say we would very much like a
power source that dense, because there are only a few more alternatives that are better. We’ve actually talked about those three,
fusion, antimatter, and black holes, on other occasions. They are in a similar range though, 10-1000
times better than fission in terms of energy density, but fission is still a million times
better than chemical power, and unlike those three we have entirely functional fission
reactors already. We also have entirely functional fission rockets
engines. Unlike a lot of the systems we have discussed
in this series, these aren’t just on paper. We’ve built and tested a few of these, even
before we landed on the moon, we’ve just never used them. Now as you might guess, with the space program
going on in the 1960’s and us already having nukes and fission power plants by then, some
folks considered using it for rocketry and both the US and Soviet Union experimented
with the concept. The big program for this for the US was Project
Rover. The main focus of that was Nuclear Thermal
Rockets, which is pretty simple as a concept. The name explains the concept, you are using
nuclear power to heat up some propellant, usually liquid hydrogen. We turn that nuclear energy into thermal energy
and let that superheated liquid hydrogen, now a vapor, go flying out the back. There’s nothing radioactive about what gets
sprayed out, you’re just using the reactor to heat the hydrogen up. It does a lot better than chemical rockets,
which tend to have specific impulses of a few hundred seconds, this atomic rocket hits
about a thousand. Which is a huge difference but nothing like
what you’d expect considering the energies involved. We are limited in this regard because all
we are doing is heating up the hydrogen and we can only heat it so much without melting
stuff. We call this specific design a solid-core
Nuclear Thermal Rocket. Or Solid Core NTR. Now with an effective Specific Impulse of
nearly a thousand, this design was quite capable of lifting objects to orbit better than chemical
rockets, much better. Where your typical rocket is virtually all
fuel, a solid core NTR can lift closer to an equal ratio of engine and payload mass
into orbit. But we have that concern of the thing blowing
up and throwing radioactive materials all over the place. This concern is not helped by us trying to
run everything as hot as possible. Those kinds of temperatures put all sorts
of stresses on the components, and even the fuel rods are going to be expanding and cracking
from all that heat. But the biggest thing working against this
engine early on was not the radiation issue though, it was the weight. Reactors and engines back in those days were
quite heavy. They still are, but we’ve learned some tricks
since. It doesn't matter how high something’s specific
impulse is, to be useful it has to be able to achieve a better than 1:1 Thrust Ratio. As mentioned last episode, specific impulse
is measured in time, but that’s not how long a rocket typically burns. In fact it has to burn in a shorter time than
that or it won’t get off the ground. A fuel with 400 seconds of specific impulse
could hold a rocket hovering off the ground in Earth gravity for about 400 seconds, less
since not all that rocket is fuel. That’s when it has a 1:1 thrust ratio, exactly
balancing gravity. Burn it faster and it will go up, burn it
slower and it won’t get off the ground. That’s the problem with things like ion
drives, they’ve got huge specific impulse, but they expend that over an even longer time. Not coincidentally most things melt at the
kind of energies chemical reactions generate, since they’re chemical bonds. So it isn’t too surprising that a lot of
the things that would let us heat a propellant up even hotter than burning rocket fuel also
tend to require either very slow usage or construction from some super material that
won’t melt under those energies. We have some clever solutions to help with
this, like cooling the rocket nozzles with the fuel, but fundamentally it is the same
sort energies binding materials together chemically as what we get from burning chemical fuels,
and nuclear energies are on an order of million times larger. In a jet engine we create a cool layer of
air around the walls of the burn chamber and most of fuel gets burnt in inside a smaller
tube of air in the middle, this keeps those components from heating up as much. Unfortunately this does not help the solid-core
which is producing all that heat in the Nuclear Thermal Rocket, so our limiting factor on
temperature is the core melting. The speed of exhaust particles is strongly
related to temperature, higher temperature, faster speeds of the particles, going with
the square root of temperature in Kelvin. Quadruple that temperature, double the speed. But it also relates to the mass of those particles
inversely, quadruple their mass, half their speed. If you take a sample of gas composed of various
chemicals but the same temperature, and measure their speed, you will notice a big difference. The oxygen will be moving a bit slower than
the nitrogen on average, as diatomic oxygen molecules are just a little heavier than diatomic
nitrogen. Carbon dioxide weighs more than both, and
will be slower. Helium, being monatomic and much less massive,
moves much faster. Hydrogen is even faster, though since it is
diatomic too, it isn’t too much faster than helium. Since diatomic oxygen weighs 16 times more
than diatomic hydrogen, the hydrogen will be moving four times faster at the same temperature. Again that speed also goes with the square
root of temperature, so at 4800 Kelvin, sixteen times higher than room temperature of about
300 Kelvin, that speed would be four times higher, meaning diatomic hydrogen is moving
about 16 times faster at those temperatures than oxygen is in the air you’re breathing
right now, 480 meters per second. Now that speed for diatomic hydrogen at 4800
kelvin is about 7700 meters per second, almost orbital velocity, and would be a specific
impulse of 785. Lone hydrogen atoms are even better. They would be moving at about 11,000 meter
per second, higher than orbital velocity and about Earth’s escape velocity. Of course just about every metal is melted
by then. Tungsten, the previous record holder, melts
at just 3700 Kelvin, and the Sun’s surface isn’t much higher at 5800 Kelvin. Though we do have a new material made of hafnium,
nitrogen, and carbon with a melting point of more than 4400 K.
Anyway, this is why we always talk about hydrogen offering the highest specific impulse or exhaust
velocity, as long as your main method of imparting energy to your propellant is heat, you will
always do best with the lowest mass particles you can find. To get around the issue of the core melting
we have some variations on the basic Nuclear Thermal Rocket besides the Solid-Core. These include the liquid core NTR, the vapor-core
NTR, or the gas-core NTR. I don’t want to go into the mechanics of
these but they let us get the propellant hotter without melting everything. The gas core is probably the best of these
options, potentially allowing between 1500-5000 seconds of specific impulse. In this configuration our uranium is in a
gas form and isn’t touching the reactor walls around it. We keep that gas spinning around in a toroidal
shape, a donut, and potentially even magnetically confined using a similar concept to what we
contemplate for the Tokamak Fusion reactor design. But if it isn’t touch the reactor walls,
how does the heat get to the propellant? In the open cycle version, which offers 3-5000
seconds of specific impulse, this uranium gas can get to about 55,000 Kelvin, 10 times
hotter than the surface of the sun, and this is very awesome except that when we say ‘open-cycle’
it does mean that the exhaust includes uranium. This means the use of an open-cycle gas-reactor
would probably qualify as a war crime. See a normal solid core rocket, if it blows
up, would scatter fragments of Uranium 235 through the area but that’s not too huge
a problem, relatively speaking. U-235 is an alpha emitter, making it pretty
safe to handle. You could juggle chunks of it without fear
of anything more than breaking your wrist, as the stuff is quite dense and hard. The alpha-particles can't penetrate your outer
layer of dead skin. It’s only dangerous if you ingest or inhale
it. Needless to say if we’re spitting out a
plume of propellant that contains uranium gas among the main hydrogen propellant, this
is going to result in folks inhaling it. So that makes it good for space-only applications,
far from Earth, but not for on or near Earth. Now we have an alternative form of this called
the closed-cycle gas core nuclear thermal rocket, which being a bit of mouthful is usually
called a Nuclear Light Bulb. In the closed-cycle version of this we line
the wall with quartz and that let the Ultraviolet light being produced by the very hot radioactive
gas to pass through into our propellant. This gives it the nickname of the Nuclear
Light Bulb. This achieves a higher specific impulse than
the normal solid core, 1500-2000 seconds, but not as high as the open-cycle. Still, since it can achieve a thrust-to-weight
ratio of better than one-to-one, this design should be buildable to get into orbit. That more or less exhaust the on-planet uses,
and with the concerns folks have with nuclear vehicles it isn’t too surprising none of
these have ever flown, and the solid core NTR was the only one ever built and tested. They do work though, obviously we’d need
some prototyping for anything but the solid-core design but the constraint here is all about
safety concerns, not engineering difficulties. For instance, the Nuclear Light Bulb is quite
safe unless it blows up on the way up, venting all that uranium gas into the air. Needless to say, while these things work and
even the solid core NTR is vastly better than chemical rockets, nobody’s been pushing
hard for more R&D and deployment of them. But you could absolutely build a spaceplane
that ran on these methods. People tend to be a lot more open to using
them away from Earth though, and we also have some more designs that work better there. Any of the designs we discussed so far work
off planet too, but they can work on-planet because they can achieve enough thrust to
get off the ground. Which is to say, they can burn their fuel
up faster than its specific impulse. Away from Earth this doesn’t matter. Gravity isn’t trying to drag you down. These things all have specific impulses measured
in minutes, but up there it doesn’t matter if you need to spend a whole day burning through
it, or even weeks, just so long as your burn time isn’t longer than your trip. This gives us our first option, which is just
a nuclear reactor providing power to run an ion drive. We discussed these more in the Spaceship Propulsion
Compendium, and it’s the same concept, you use a magnetic field to accelerate your propellant
up to high speed. The reactor provides the electricity for this. So can a solar panel though, at least near
the sun, though it takes a lot of them. So this isn’t really an atomic rocket any
more than the computer I make these videos on is an atomic computer, just because there’s
a nuclear power plant two towns over. Now I mentioned earlier that while we can
cause fission reactions, typically by hitting the material with a neutron, those materials
do decay on their own. Half-lives can range from less than a second
to longer than the universe is old. For U-235 it is about 700 million years, which
is why it makes up only a small portion of available uranium. Most of that is U-238, and we would normally
say that a sample of uranium that we’d removed the shorter half-life isotopes from was depleted
of them, while one where we removed a lot of the U-238 was enriched. Depleted Uranium is still pretty handy stuff,
partially because it is so dense it makes great bullets and armor. It has a second use though, which is making
plutonium. Add a neutron to U-238 and it transmutes into
Plutonium-239, which has a much shorter half-life. Reactors designed for doing that are called
breeder reactors, and we use the same idea for Thorium, adding to it and turning it into
shorter half-life isotopes of uranium. As a very loose rule of thumb, stuff tends
to give off about one-thousandth of its mass energy over the course of its half-life. So a handful of something like U-235 is going
to only be giving off milliwatts of heat energy, though it does it for around a billion years,
releasing a million times what an equal amount of gasoline would release on combustion. Pick some radioactive isotope with much shorter
half-life, in the hundreds of years instead of hundreds of millions, and that same handful
would give off kilowatts of power not milliwatts. Polunium-210 is the isotope I usually hear
considered for this, it is an alpha-emitter and without any gamma, meaning it is easily
shielded, and it decays directly to lead and has a half life of 138 days, meaning that
a single kilogram of the stuff generates about 140 kilowatts of heat. It’s crazy-toxic though, a microgram ingested
will kill you. This sort of rocket, a Radioisotope Rocket,
works much the same as the other kinds, you let it heat up a propellant, but it has no
moving parts itself. It’s just a big heater. We often use radioisotopes as a power source
in space vehicles. A Radioisotope Thermoelectric Generator, or
RTG, produces heat that we turn into electricity with thermocouples. Those are not terribly efficient, 3-7% heat
energy to electricity conversion tends to be the norm, but they have zero moving parts
so they are very durable. Also handy for keeping the object warm. We use RTGs in space vehicles fairly often,
and you just pick a radioisotope that’s fairly cheap and has a half-life similar to
intended mission duration. Or for a rocket, similar to how long it will
be in transit or for which you want to run the engines. The big disadvantage of course being that
you cannot throttle it. It produces a set amount of power, slowly
ebbing off before reaching half-power at the half-life. You wouldn’t want to use one for trying
to slow down after a long mission for instance since most of it would be gone when it came
time to decelerate, whereas you can keep U-235 around a long while and just add it to the
reactor like fuel when you need it, and you can throttle a fission reactor, though generally
nowhere near as quickly as a combustion engine. Now a more efficient but slower accelerating
version of this is the Fission Sail, where essentially you are directly harnessing the
decay particles rather than their heat. When a particle decays it always conserves
momentum doing so, meaning for a simple two product decay, one particles flies one direction
and the other flies the other direction with the same momentum. It does so totally randomly, but if you stuck
some on the back of a rocket ship behind some shielding, some would hit it, pushing it forward,
and others fly backward. Since direction is random, generally this
need to be a sail some kilometers wide, with basically two layers, one an absorber and
one behind it being the isotope. This idea comes to us from physicist and scifi
author Robert Forward, one of the folks I tend to consider a spiritual patron of the
channel, and he suggested it as an add-on for a solar sail, in that since it is already
so big we might as well let it reflect light and gain speed from that too. On a similar notion we also have the Nuclear
Photonic Rocket, a slow accelerating but high final-speed ship that just uses the heat to
push it along. Light has momentum, as we’ve discussed in
other episodes, so if you can get your waste heat on a ship to radiate a-symmetrically
you can get thrust. And of course this can be easily accomplished
with either natural decay or standard fission by just having a sphere of the stuff getting
hot, attached to the back of the ship inside a parabolic dish that is reflective to the
frequencies of light being given off, infrared presumably. This is a very easy type of interstellar probe
if you’re just using a sphere of some radioisotope wrapped in an absorber. It won’t get super-fast and it will take
its sweet time doing it, but it can get to speeds that will get to another star in centuries,
not millennia, and the design is so simple you could bang one out in your garage, assuming
you don’t irradiate yourself to death. We do have a few other designs like the Fission
Fragment Rocket and Robert Zubrin’s Nuclear Salt-Water Rocket, and these again are strictly
for off-planet use too. But the big one I wanted to spend a little
time on is Nuclear Pulse Propulsion. I discussed this once before way back in the
Interstellar Colonization episode and mostly bypassed it in the Spaceship Propulsion Compendium
as a result. This series is also mostly about getting off
Earth, not to other solar systems, and this system is beyond useless for the former so
it will still get only a short mention today. The reason it is useless for planetary launch
is that the propulsion utilizes nukes. Conceptually this is pretty straightforward. You shoot a nuke out behind you and it detonates,
shoving your ship forward. Sounds crazy but it’s not as bad as you’d
think in deep space. You stick a very large plate behind you made
of a sturdy substance and on a huge spring, it gets walloped by the blast and slams forward,
contracting the spring and then returning backwards, in much the same way the blast
from a bullet on the bolt of gun occurs, helping to distribute that sudden massive blast. When a nuke detonates in space there’s not
much of a shockwave, the atomized components of the bomb, and a ton of gamma radiation. You build your pusher plate thick enough to
handle the strikes and wide enough to catch most of the blast at whatever distance is
the minimum safe distance behind the ship to detonate the nuke. These are big ships, multi-megaton ones bigger
than an aircraft carrier. They can also use fusion bombs, not just fission
bombs, letting you achieve even higher speeds with even bigger ships. This is the only interstellar ship design
we have right now that could definitely take human passengers and deliver them to another
solar system with no new technology. The Low-tech version of it also allowed you
to get to Mars with quite a large ship in just 4 weeks, and it usually gets rated as
a specific impulse of 6000 seconds or more, one of the variations, the Medusa, having
an estimated 100,000 seconds, and 1% of light speed is a realistic option with them. Also a handy way to slow a ship down as it
approaches its destination if you are using lasers to push it up to speed, a technique
we’ve discussed before too in the Interstellar Highway episode. Of course no discussion of nuclear propulsion
is complete without at least mentioning fusion, we might figure out how to do that tomorrow
or ten minutes from doomsday, and it is something we have discussed at length elsewhere on the
channel, especially in the Interstellar Travel Challenges episode. This has a lot of the same problems fission
has though since you still have the problem of all that heat melting your equipment long
before you maximize its use in speeding up your propellant. It’s also one of the reasons I prefer photons
as a propellant, they’re already going quite fast. What’s more it is one of the reasons for
while I regularly discuss fusion for interplanetary and interstellar craft, but I don’t discuss
it for getting off planets much. No engine is useful for this purpose, no matter
how powerful, if you can’t produce a thrust to weight ratio of better than one-to-one. Some massive 500-gigawatt 100,000 ton fusion
reactor might make the space shuttle engines look weak but still be as useful for launching
itself into space as the Hoover Dam is. Needs to be powerful and light, and compact
too if it has to fight through an atmosphere. But for operations once in space, fusion would
be amazing. We see a bit of an emerging picture though
that the Nuclear Options aren’t great for on a planet. Now a few forms can be used there, and it
just comes down to if the engineers can make a design safe enough that folks would approve
its use. Off planet the biggest issue would be finding
fissionable material to operate them, since the radiation and disposal concerns are minimal. That makes them potentially very handy for
operations with asteroid mining, since atomic rockets are excellent for moving cargo at
those kinds of speeds and distances and there’s no gravity wells for them to fight. However asteroids generally will not have
concentrations of uranium or thorium ores, that’s more the product of geological processes
on larger bodies, so Earth has plenty readily available to offer in trade to asteroid miners,
and place like the Moon and Mars do too. Very little of the mass of nuclear thermal
rockets is the fission fuel so shipping that out of the gravity well of those bodies is
not a huge constraint. Normally at this stage of the episode I discuss
Safety and Cost aspects but that seems kind of pointless here. There’s nothing very expensive about Nuclear
Thermal Rockets though I’d imagine the R&D to fully develop them safely could get pricy. They should be a deal cheaper than chemical
rockets but estimating nuclear costs is always hard because of the safety & regulatory aspects. Those safety and regulatory concerns are the
same ones for normal nuclear power and it seems like a bad idea to discuss those here. If you are pro-nuclear power, you don’t
need any convincing these can probably be done safely and that we wouldn’t launch
them if they were not. If you are opposed to it, there’s nothing
I can say to change your mind. Atomics is always a tricky one in that regard
because it is genuinely dangerous stuff, so it’s hard for me to yell at people about
being overly afraid and cautious about the stuff, but I also won’t pretend that I think
fears of nuclear power are entirely reasonable either. Sadly I can’t say I expect those concerns
to diminish in the near future, so in the end I can’t say that I expect atomic rockets
to see much use in Earth or Low Orbit, and I suspect they’d only see a major role in
space travel if we developed a fairly large off-planet mining and refining operation before
we developed viable fusion. That why we so often see nuclear propulsion
designs as strictly space-based vehicles, not aerodynamic constructs. They mostly aren’t expected to take off
or land from planets, but rather move between them. Incidentally I wanted to thank animator and
fellow youtuber Fragomatik for providing the some examples for today’s episode. He does a lot of amazing animations of many
of the concepts we've discussed today as well as other spaceships and space habitats, and
I've included a link to his channel in the video description. Make sure to give him a like and subscribe. In this series we’ve mostly looked at getting
up into space. But once you get up in space, unless you just
want to hang around in orbit, you need to go somewhere and nuclear propulsion offers
us some of the best options for that with our current technology. Now even if it isn’t too suitable for launching
ships directly to orbit, we will be looking at how nuclear power, fission or fusion, can
help us get off the planet more as the series progresses. Indeed powering the mass drivers we discussed
a couple episodes back would be one such example. Most of the systems we will be discussing
after this require very large amounts of power to function, and so are ideal for ground-based
power plants where there’s no fear of atomic fuel crashing into the ground or vaporizing
over a huge swath of land to be inhaled. Indeed, since many of these launch systems
would be far from human habitation, they’re a lot safer for employing atomic energy. We will examine one such system, Launch Loops,
in the next episode of the series. However, next week it will back to Existential
Crisis Series for Infinite Improbability Issues, and we are going to explore some of the stranger
implications of concepts like the Many Worlds Interpretation of Quantum Mechanics and Alternate
Universes. For alerts when that and other episodes come
out, make sure to subscribe to the channel, and if you enjoyed this episode, hit the like
button and share it with others. Until Next Time, Thanks for Watching, and
Have a Great Week!
(not super-important)At 14:00 he says you use a magnetic field to accelerate, but the shown approach uses an electric field. Infact, F=qB×v the force is perpendicular to the velocity; so P=F⋅v=q(B×v)⋅v=0; (inproduct) you cannot add energy to a particle just using a magnetic field. (Note: you can add energy using "moving magnetic fields", but because those induce electric fields)
Hey! I made a science fair project over nuclear bombs or something like the solar sail ideas. It was in the seventh grade, two decades ago, before I had access to the internet. I didn't get a good grade, despite all of my research in libraries :( I was even trying to work around the EMP, by creating a magnetic field with the spinning living area to simulate gravity.
I'm not 100% happy with this. I expected you to look seriously at ways Orion could work, and evaluate the feasibility of plating the launch pad with graphite to minimise fallout.