This episode of Real Engineering is brought to you by CuriosityStream, watch over 2,400
documentaries for free for 31 days at curiositystream.com/realengineering.
Some crazy things happened during the Cold War. Dogs were put into orbit, bears were
fired out of supersonic jets, and humans landed on the moon. Needless to say, humans were
going through a rough break-up and were really trying to find themselves. All of these events
were centered around one technology that changed the game for every human on this planet, nuclear
The bombing of Nagasaki and Hiroshima woke the world to a nuclear future. Changing the
political and cultural landscape of the world forever. Infecting the imaginations of people
all over the world. Spawning monster stories like Godzilla, in Japan, and heroes like the
Incredible Hulk and Spiderman in the United States. People dreamed of a future where electricity
was free, while simultaneously fearing the ever-present threat of nuclear annihilation.
These fears and dreams came together to form perhaps one of the most interesting technologies
conceived during the Cold War. Nuclear powered planes.
During World War 2 research and development of nuclear energy had been focused on its
weaponization, but with the wars conclusion, the United States began to seek out ways of
utilizing the power of the atom to fuel their energy needs. The Atomic Energy Commission
was created in 1946 with the express purpose of commercialising this new technology, and
just one year later the US Air Force invested 10 million dollars  into studying the feasibility
of utilizing this energy to power their long-range bombers. In an era before in-flight refueling
and ICBMs had been perfected, the technology was appealing. With just a small amount of
fuel, a bomber could fly indefinitely. It would be capable of reaching anywhere in enemy
territory. An omnipotent threat to any advisory. Despite the obvious dangers of combining these
two technologies, the possibilities proved too tantalizing.
From 1948 to 1951 the brunt of the research centered around a means to transfer the energy
generated by nuclear fission to propulsion.
Heat energy is gained through nuclear fission. When uranium is bombarded with a neutron it
absorbs that neutron into its nucleus, which causes severe vibrations ripping the atom
apart, producing heat, additional neutrons and new lighter atoms. But the sum of products
of that split are lighter than the original atom. Experimental proof of Einstein's
ground breaking 1905 paper teaching the world of the energy and mass relationship through
the equation of E equals m c squared. The energy released by a single uranium fission
reaction like this is tiny, at 200 million electron volts , but crucially uranium
produces additional neutrons when it splits allowing for a chain reaction to occur. So,
for very little input energy we can get a tremendous amount of kinetic energy as an
output in the form of heat. When controlled this reaction can give us energy to heat water
and power our steam turbines when uncontrolled this reaction gives us the atomic bomb.
Experiments began here in the Idaho National Engineering and Environment Laboratory. Dubbed
the HTRE, standing for Heat Transfer Reactor Experiment, these engines sought to find the
most efficient solution for transforming this thermal energy into thrust. 
They eventually came to the HTRE-3. Consisting of 2 modified general electric J47 engines,
that would perform both propulsion and cooling functions. Air would be ducted from the low-pressure
compressor through the reactor core, where it would gain heat and expand, this air would
then pass through the high-pressure turbine and exhaust to provide power and thrust.
As the air was needed for cooling, the engine had to be started using traditional fuel sources,
allowing the air to pass through the cool reactor. Once sufficient airflow was achieved
the reactor could then be brought up to power. The engine contained a temperature control
thermocouple , which fed data to a control module that would automatically close the
chemical fuel valve as the heat of the nuclear reactor began to be added until the valve
was completely closed.
The HTRE-3 was successfully run multiple times, but there were still several problems to be
solved. Perhaps chief among these was the energy transfer method’s propensity for
spewing radioactive air out of its exhaust.
This was an open or direct cycle configuration, meaning the air is directly used to cool the
reactor core.  This was a simpler set-up, requiring no additional pumps within the nuclear
reactor, that the program managers at General Electric preferred, but resulted in air passing
through the highly radioactive core and thus being contaminated, and subsequently exhausted
Obviously not an ideal situation, and this program actually spurred the creation of the
very first molten salt nuclear reactor through the ARE, or Aircraft Reactor Experiment.
This was instead a closed cycle system, where a molten uranium tetrafluoride salt is used
a fuel, while a secondary closed loop containing molten salt with no uranium was used as a
coolant. This coolant would then pass through a liquid-to-air heat exchanger to power the
turbine. This would result in radically reduced radioactivity in the exhaust, but required
more plumbing to circulate the liquids in the two inner closed loops, and resulted in
a lower efficiency as we are introducing an additional heat transfer step that allows
more heat to the be lost to the plumbing and environment.
This method was never tested with a jet engine, but this is the earliest ancestor of the thorium
reactors which are easily the most requested topic on this channel, as a result of their
potential to provide cheap and safe nuclear energy, but research and funding for this
technology was gradually dropped.
Had the program developed further, these molten salt reactors likely would have gone on to
power any eventually bomber, had the power to weight issues been overcome.
These power to weight issues were one of the primary roads blocks facing designers. While
nuclear energy can provide extremely long lasting energy, its power output, the energy
provided per unit time, is not infinite. Nuclear reactors like this have maximum power settings,
limited by the heat the cooling system can transport away before it can begin to melt
and damage the reactor and it’s control mechanisms. This made it difficult to design
a reactor capable of providing enough energy to power jet engines with enough thrust to
get the gargantuan weight of the reactor off the ground.
The HTRE-3 is estimated to have weighed 45 metric tonnes and could produce up to 35 megawatts
of thermal output. Far too large and heavy, and short of the 50 megawatts of thermal output
targeted for a flight worthy power source. 
The HTRE-3 did include removable radiation shielding, consisting of a stainless steel
shell with a lead core surrounded by water [R1-4], but further testing was needed to
design and assess shielding for an aircraft.
To do this Convair modified a B-36, renaming it the NB-36H Crusader. The B-36 was the only
aircraft in the United State’s arsenal capable of taking off with a massive nuclear reactor,
and it’s associated shielding and coolant systems. Fitting it with a small 1 megawatt
reactor, dubbed the ASTR, or aircraft shield test reactor, which was lifted from a shielded
underground vault and mounted in the B-36s bomb bay moments before take-off. 
A plane could never carry the amount of lead needed to shield every facet of a reactor,
so this plane would test shadow shielding, which would primarily shield the crew and
instruments in the cockpit. Shielding for the reactor was achieved with water tanks
which could be filled or drained to vary the shielding and allow the nuclear engineers
on board to assess the minimum volume of water needed to protect the planes crew and instruments,
with a 5 tonne 13-centimetre lead shield mounted between the reactor and the crew compartment.
On top of this, the crew compartment was an 11 tonne lead and rubber shielded removable
section. The leaded glass windows were a foot thick, and a closed circuit tv system was
used to monitor the reactor.
The plane was further modified with air scoops to funnel air to the coolant system that would
trade heat with the internal closed loop water coolant, and then exhaust the heat to atmosphere.
The Crusader made its maiden flight on September 17th 1955. The reactor provided no power to
the engines, but the plane would make a total of 47 flights, which occured only over remote
land far from human populations. The plane was escorted at all times by a B-50, which
contained sensors to measure any air scattered radiation emitted from the reactor and air
coolant system. It also contained a team of marines ready to parachute and secure a crash
location if the need arose.
At this point, the United States was at the forefront of aircraft nuclear propulsion technology
and likely could have developed the technology far enough to produce an nuclear-powered plane.
But, for the best, it never came to fruition.
On November 18th, 1958 the HTRE-3 engine suffered a meltdown when temperature sensors malfunctioned,
recording a lower temperature and withdrawing control rods. 
This may have been the impetus to shake sense into the US Government, to remind them that
a mobile nuclear meltdown was simply not something they wanted to contend with. On top of all
this, with the advancement of aerial refueling and intercontinental ballistic missiles, the
technology was made completely redundant.
President Kennedy ultimately cancelled the program in 1961, just 1 month into his presidency.
Putting an end to the insane idea. This hysteria and fear of aerial bombing has made humans
do some astounding evil and stupid things. If you would like to learn more about this
gradual descent into madness that the advent of aerial bombing fostered, I highly recommend
watching this documentary titled the Bombing War: From Guernica to Hiroshima. It takes
you from the first small grenade dropped from a plane in World War 1 all the way to the
monstrous and unnecessary bombing campaigns that took place in Europe and Asia in World
War 2. It’s a fascinating insight into the minds of the civilians that lived through
those times and the madness that gripped the military leaders that approved them.
You can watch it now, for free, by signing up to curiositystream using the code realengineering,
or using the link the description. This will give you a month of completely free access
to over 2,400 documentaries and nonfiction titles from some of the world’s best filmmakers.
After that first free month you can continue you access for just two dollars ninety nine
As usual thanks for watching and thank you to all my Patreon supporters. If you would
like to see more from me, the links to my instagram, twitter, discord server and subreddit