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For all intents and purposes, a working rocket is basically a controlled explosion.
So the idea of sticking a nuclear reactor inside one
might seem like a…questionable idea…at best.
And yet, a nuclear-powered rocket was exactly
what scientists and engineers were trying to build in the 1950s.
And 60s.
And 70s.
Because despite all the risk, it could literally
launch humanity to a new era of space exploration.
The NERVA program, short for Nuclear Engine for Rocket Vehicle Application,
got further than you might think.
Before the funding got cut in the early 70s, they’d built multiple versions
of the engine, and had a bunch of successful tests.
And while NERVA’s nuclear engine never got off the ground,
the idea of one never quite disappeared, either.
Just this year, NASA proposed bringing it back. Again.
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Now, a rocket powered by splitting the atom
might be the most 1950s thing I can imagine.
It probably lived in the suburbs and danced to Elvis too.
But the fifties weren’t just one big uranium party.
Every rocket, no matter its appearance, is based on the third law of motion.
That’s the one that says if you push something, it pushes back.
Rockets push exhaust one way,
while the exhaust pushes back on the rocket the other way.
The harder the rocket pushes the exhaust,
the harder the exhaust pushes the rocket.
That’s rocket science in a nutshell.
The hard part is figuring out how to give the exhaust as big of a kick as possible.
The best exhaust is lightweight, so that it doesn’t take much energy
to make it go super fast out the back end of your rocket.
The push it provides in return gets the best bang for your energetic buck.
So as the lightest known molecule in the universe,
hydrogen makes for great exhaust.
But you’ve also got to make sure you have
enough energy to shove that exhaust out the back.
In traditional rockets, that energy comes from cracking apart
different molecules in your rocket fuel and smushing them back together
in new ways that leaves a bunch of leftover energy.
In other words, you’re both making the exhaust and the energy
to move that exhaust with the same chemical reaction.
Hydrogen is pretty awful at releasing extra energy.
So instead, rocket fuel usually relies on bigger molecules
that can release more energy, but produce exhaust that’s heavier than hydrogen,
and therefore less efficient in providing that oomph.
Ultimately, in the choice between lots of energy with
heavier exhaust or less energy with an ideal exhaust, engineers chose the former.
But here’s the thing: the laws of physics don’t say the exhaust needs to be
pushed out of the rocket by the same process that makes the exhaust.
You just need a supply of ready-to-go, super light hydrogen,
and something that can make a bunch of energy for that hydrogen to absorb.
I think you know where I’m going with this.
According to the math, a rocket powered by a nuclear fission engine
could produce twice the thrust of a chemical rocket on its best day.
Plus, it would need much less fuel for the same-sized trip.
The rocket would be both lighter and more powerful,
getting probes or astronauts where they were going a lot faster.
So in 1955, not that long after the first nuclear power plant
was opened, Project Rover was born.
Research progressed quickly,
with successful reactor prototypes starting tests in 1959.
NASA formed around the same time, and soon it consolidated Project Rover
and other related research under the NERVA banner.
Over the years, NERVA scientists developed small,
powerful nuclear reactors that could travel aboard a rocket and activate for
specific parts of the mission whenever a big push would be needed.
One of the challenges they had to overcome was those reactors creating
such high temperatures that some of the rockets’
components would have degraded and fallen apart.
For the record, that’s also a problem that comes up with chemical rockets.
So as a fix, they designed the engine to circulate some of the super
cold liquid hydrogen through a bunch of tubes to keep everything cool.
So the hydrogen actually served two jobs!
Oh, and don’t worry, they also knew they would need to
shield whatever cargo or humans were aboard.
They weren’t about to risk anyone turning into the Hulk.
Or just, you know, getting radiation poisoning.
Soon, people were imagining nuclear-powered shuttles to the Moon or even Mars.
The plan still involved chemical rockets for launch,
since launches have a high enough chance of going wrong.
And you definitely don’t want to have to worry about the literal fallout
from an active nuclear reactor blowing up a few kilometers above the ground.
But once outside the atmosphere,
the chemical piece would fall away and the nuclear engine would start up.
Now, a flying container of uranium is still not ideal, even if the reactor is turned off.
So in 1966, scientists blew up a model of the reactor with explosives
to see how bits and pieces would spread if the worst did happen during launch.
These kinds of safety and technology tests were proceeding so well that,
with NASA planning for crewed missions after the Apollo missions,
a full-scale NERVA rocket seemed inevitable.
But NASA’s budget was shrinking even before astronauts reached the Moon.
Long-term, long-distance human spaceflight wasn’t a priority.
By 1973, NASA had to choose between NERVA
and what would become the Voyager missions.
They chose the Voyager.
Which, honestly, not a bad choice.
Every ten years or so, though, scientists glance back at
nuclear-powered propulsion, with flurries of interest in the mid-eighties,
the early nineties, and the mid-2000s.
But even as satellites have employed all sorts of other non-chemical thrusters
over the last few decades, nuclear rocket engines still haven’t reached space.
At least, not yet.
In early 2023, NASA and the Department of Defense announced
a plan to use nuclear propulsion to take humans to Mars.
Which is…awesome!?
But NASA also proposed something similar way back in 1969,
soon after NERVA’s funding was first cut.
… and then again in 1991.
So for now, nuclear-powered propulsion remains a well-verified laboratory curiosity.
But soon, maybe it’ll have its day in the Sun.
Or, even better, its day in the blackness of space.
Scientists might dream of a future spacecraft flying through the final frontier
using the power of nuclear fission.
And to bring those dreams a little closer to reality, we here at SciShow
have created our very own tiny nuclear-powered rocket engine.
Or at least the image of one.
Our new pin of the month celebrates NERVA and the wild dreams
scientists have had over the years to help humans explore the universe.
If you’d like to celebrate with us, head on over to
DFTBA.com/SciShow and pick one up.
Thanks for watching.
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