00:00:00
in 1965 a young Caltech graduate student
00:00:03
by the name of Gary flandro was pouring
00:00:06
over his notes in building 180 of the
00:00:09
jet propulsion laboratory studying the
00:00:11
intricacies of using planets
00:00:13
gravitational fields to slingshot
00:00:16
spacecraft into the outer reaches of
00:00:18
space with no need for additional
00:00:20
propellant the gravity assist a method
00:00:23
of propulsion that uses the gravity of a
00:00:26
gravitational body to literally pull the
00:00:28
spacecraft into a new trajectory and
00:00:30
velocity as he examined the possible
00:00:33
trajectories Gary noticed something
00:00:35
incredible in the late 70s a window
00:00:38
would open up where the alignment of the
00:00:41
planet would allow for a single
00:00:42
satellite to pinball its way between
00:00:45
Jupiter Saturn Uranus and Neptune before
00:00:48
eventually being spit out to the outer
00:00:50
reaches of our solar system and beyond
00:00:53
with this knowledge a flurry of activity
00:00:56
began within NASA and JPL a once in a
00:00:59
lifetime Opera opportunity had been
00:01:01
found that would not come about again
00:01:03
for another
00:01:05
175 years and it was imperative that the
00:01:09
opportunity was not squandered planning
00:01:12
began and in 1977 two satellites Voyager
00:01:16
1 and 2 would launch a board that
00:01:18
three-staged Titan 3E rocket Voyager 2
00:01:22
would utilize a gravity assist with
00:01:24
Jupiter Saturn and Uranus before racing
00:01:27
by Neptune before it eventually exited
00:01:30
the solar system while Voyager 1
00:01:33
completed a more direct path whipping by
00:01:36
Jupiter and Saturn before venturing into
00:01:38
deep space moving away from Earth faster
00:01:41
than any other spacecraft
00:01:43
Voyager 1 is now over 22.8 billion
00:01:47
kilometers away from Earth that's so far
00:01:50
away that Voyager 1 is now outside the
00:01:53
influence of the sun's constant stream
00:01:55
of solar wind meaning it is now an
00:01:58
Interstellar space officially reaching
00:02:01
the region in August 2012.
00:02:04
we as a species have now left our Mark
00:02:06
in interstellar space after a 35-year
00:02:09
long journey a ponderously long time in
00:02:12
the scale of human life and that was
00:02:14
with the help of multiple gravity
00:02:16
assists while we can build and deploy
00:02:19
these technical Marvels to investigate
00:02:21
other worlds sending ourselves into
00:02:24
space is another thing entirely and
00:02:27
using gravity assists to visit our
00:02:29
closest neighbors makes little sense to
00:02:32
date the furthest humans have ventured
00:02:34
into space is to the dark side of the
00:02:36
moon a mere 400 000 kilometers away if
00:02:40
our Ambitions to create colonies on
00:02:43
Maris are to be realized humans are
00:02:45
going to need a spacecraft that are
00:02:47
faster and more efficient than the ones
00:02:49
we currently have at our disposal Maris
00:02:52
is on average about 64 million
00:02:54
kilometers away the fastest and most
00:02:57
efficient method we have to reach Mars
00:02:59
using the Homan transfer method with
00:03:01
launch Windows every 26 months takes
00:03:04
about nine months to complete faster
00:03:07
transfer times are possible but
00:03:09
Engineers are stuck in a catch-22 it
00:03:12
takes fuel not only to accelerate the
00:03:14
spacecraft but to decelerate it too
00:03:16
there are no disc brakes in Space the
00:03:19
more we accelerate the more we have to
00:03:21
decelerate too in order to carry all
00:03:24
that additional propellant Engineers
00:03:27
will be forced to sacrifice payload ways
00:03:29
reducing the space available for food
00:03:31
water and other vital supplies for a
00:03:34
crude Mission to Mars we can take some
00:03:37
solace in the fact faster transfer times
00:03:39
will reduce the supplies needed and will
00:03:42
reduce the cruise exposure to the high
00:03:44
radiation of space but what if we could
00:03:47
achieve faster transfer times without
00:03:49
sacrificing payload what if we could
00:03:52
achieve faster transfer times with even
00:03:55
more payload let's first examine current
00:03:57
technology to see where things could be
00:04:00
improved the atlas V rocket that brought
00:04:03
perseverance to Mars utilized chemical
00:04:05
combustion to propel itself a method
00:04:08
where a fuel and an oxidizer are
00:04:10
combined in a combustion chamber and
00:04:12
ignite it the resulting exothermic
00:04:15
reaction causes the combustion products
00:04:17
to rapidly heat up and expand the nozzle
00:04:20
design then directs the expanding gas in
00:04:23
one direction to achieve thrust
00:04:25
getting the most out of her Fuel and
00:04:27
oxidizer is the first step to maximizing
00:04:30
our thrust per unit weight of fuel there
00:04:32
is a useful quantity Engineers use to
00:04:35
describe this property of fuels and
00:04:37
oxidizes specific impulse we described
00:04:40
in detail what this value represents in
00:04:43
our last video on the X-15 explaining
00:04:46
how it represents the total energy we
00:04:48
can extract from our propellants per
00:04:50
unit waste in that video we used this
00:04:53
equation where specific impulse is
00:04:55
defined by thrust Force divided by Fuel
00:04:58
flow race this is an extremely important
00:05:01
metric for our Maris transfer vehicle
00:05:04
the higher we can push our specific
00:05:06
impulse the less fuel we need to bring
00:05:09
which frees up space for payload or we
00:05:12
can bring the same amount of fuel and
00:05:15
increase our velocity to reach Maris
00:05:17
sooner or even be able to leave outside
00:05:20
that ideal Omen transfer window
00:05:23
so how do we improve specific impulse
00:05:26
the technology with the best specific
00:05:29
impulse currently is Ion propulsion take
00:05:32
the n-star ion drive aboard the now
00:05:34
retired Dawn spacecraft this engine used
00:05:37
electric power to propel ions and
00:05:40
Achieve astronomical specific impulses
00:05:43
the engine releases Xenon atoms into an
00:05:46
ionization chamber and then bombards
00:05:49
them with high energy electrons the
00:05:51
collisions produce a positive Xenon atom
00:05:54
and more electrons these electrons are
00:05:56
then collected by a positively charged
00:05:59
chamber wall while the positive Xenon
00:06:01
atoms migrate towards the chamber exit
00:06:04
which contains two grits a positive grid
00:06:07
called the screen grid and a negative
00:06:09
grid called the accelerator grid the
00:06:11
high electrical potential between these
00:06:14
grids causes the positive ions to
00:06:16
accelerate and shoot out of the engine
00:06:18
at speeds of up to 40 kilometers per
00:06:21
second Which is vastly higher higher
00:06:23
than what chemical combustion can
00:06:25
provide which has a typical exhaust
00:06:27
velocity of about three to four
00:06:29
kilometers per second this exhaust
00:06:32
velocity is Paramount to achieving
00:06:34
higher specific impulses if we play
00:06:37
around with the specific impulse
00:06:38
equation we can see why specific impulse
00:06:42
equals the thrust Force divided by Fuel
00:06:44
flow rate thrust force is equal to the
00:06:47
mass flow rate times the velocity while
00:06:50
the fuel flow rate is the weight of the
00:06:52
fuel on Earth and thus this value
00:06:54
changes to mass flow rate times
00:06:56
acceleration due to gravity and as we
00:07:00
see the mass flow rates cancel
00:07:02
themselves out leaving us with only
00:07:04
exhaust velocity divided by gravity so
00:07:08
it's rather obvious to maximize specific
00:07:10
impulse we need to maximize exhaust
00:07:13
velocity and ion propulsion is the best
00:07:16
technology we have to do that right now
00:07:19
with 10 times the exhaust velocity the
00:07:22
ion Drive can achieve 10 times the
00:07:25
specific impulse
00:07:27
that's a phenomenal increase so why
00:07:30
aren't we using this technology for
00:07:32
interplanetary Missions mass flow rate
00:07:34
may not be important to specific impulse
00:07:37
but it is massively important to thrust
00:07:40
as we saw just now thrust force is equal
00:07:43
to mass flow rate times the velocity
00:07:45
chemical combustion happens extremely
00:07:48
quickly it is after all a controlled
00:07:51
explosion and so it is capable of
00:07:54
accelerating Millions upon millions of
00:07:57
molecules out in a very short space of
00:08:00
time leading to a high mass flow race
00:08:03
High thrust is very important for
00:08:06
particular Maneuvers like capture Birds
00:08:08
where a rocket will fire to slow its
00:08:10
speed down enough to be captured by a
00:08:13
planet's Gravity the window for this
00:08:15
deceleration may only be a few hours
00:08:17
where ion propulsion simply cannot
00:08:20
provide enough trust in a short enough
00:08:22
time to successfully complete the
00:08:25
maneuver ion propulsion simply does not
00:08:28
have the mass flow rate necessary to
00:08:30
achieve High thrust it took the dawn
00:08:32
spacecraft four days to change its
00:08:35
velocity by just 94 kilometers per hour
00:08:37
to increase our ability to change
00:08:40
velocity quickly we need to increase the
00:08:43
mass flow rate to do this we need to
00:08:45
increase our input power for ION
00:08:47
propulsion that power comes in the form
00:08:49
of electricity which provides the energy
00:08:52
to ionize our propellant and accelerate
00:08:55
it using an electric or magnetic field
00:08:57
current generation ion propelled
00:09:00
spacecraft use solar panels to provide
00:09:02
that electricity the dawn spacecraft has
00:09:05
panels capable of producing 10 kilowatts
00:09:07
of power when orbiting Earth which
00:09:10
diminished to 1.3 kilowatts by the time
00:09:13
it reached its destination in the
00:09:15
asteroid belt three times further away
00:09:17
from the Sun scaling that solar power
00:09:20
becomes impractical very quickly NASA
00:09:23
estimates that Ameris transport vehicle
00:09:25
would need at least 400 to 2 000
00:09:28
kilowatts of power to carry astronauts
00:09:30
and cargo to and from Mars
00:09:33
so how do we power something like that
00:09:36
nuclear power is the only thing that can
00:09:39
give the power density needed to make
00:09:41
this viable this isn't a New Concept in
00:09:45
1961 the atomic energy commission and
00:09:48
NASA launched the nuclear engine for
00:09:50
Rocket vehicle applications program or
00:09:53
nerva for short this program developed
00:09:56
and ground-tested 20 reactors before it
00:09:59
was disbanded in 1973 due to budget
00:10:02
constraints but was recently revitalized
00:10:05
when the U.S Congress approved 125
00:10:08
million dollars in research funding for
00:10:11
nuclear propulsion there are two primary
00:10:14
types of nuclear space propulsion
00:10:15
nuclear electric which would power an
00:10:18
ion drive like we saw above and nuclear
00:10:21
thermal which was the focus of the nerva
00:10:23
program so let's start there nuclear
00:10:26
thermal propulsion works by harnessing
00:10:28
the heat created during nuclear fission
00:10:31
to provide the energy needed to expand
00:10:33
and accelerate a propellant through an
00:10:35
exhaust nozzle here the nuclear reactors
00:10:38
work in much the same way as a nuclear
00:10:41
reactor here on earthwood where a chain
00:10:43
reaction of neutrons colliding with
00:10:45
uranium atoms splits them and creates
00:10:48
more neutrons and a tremendous amount of
00:10:50
heat to capture this heat a propellant
00:10:53
typically liquid hydrogen is pumped
00:10:56
through the Reactor Core which will cool
00:10:58
the reactor core and pass the heat to
00:11:00
the liquid hydrogen which rapidly
00:11:02
expands and accelerates out of the
00:11:05
Thruster nozzle at high speeds typically
00:11:07
around 8 kilometers per second twice as
00:11:10
fast as chemical combustion and thus
00:11:12
about twice the specific impulse at
00:11:15
around 887 seconds however it's not all
00:11:19
sunshine and rainbows using hydrogen as
00:11:22
a propellant comes with some issues it
00:11:25
can attack the fuel rods if they are not
00:11:27
adequately protected with a material
00:11:29
that is resistant to hydrogen's
00:11:31
destructive Tendencies liquid hydrogen
00:11:34
also has to be stored at extremely low
00:11:36
cryogenic temperatures and if it is
00:11:39
allowed to rise in temperature it needs
00:11:41
to be vented to prevent an explosion and
00:11:44
on top of this the tiny molecule is so
00:11:47
small it can slip through seemingly
00:11:50
solid materials as it can fit between
00:11:52
the spaces of larger molecules this
00:11:55
makes it unsuitable for long storage
00:11:57
periods and ideally we want a Maris
00:12:00
transfer vehicle that can sit in orbit
00:12:02
around the Earth or Mars for extended
00:12:05
periods waiting for the crew to arrive
00:12:08
and begin its Journey between the
00:12:09
planets then when it arrives the crew
00:12:12
can sense in a separate vehicle leaving
00:12:15
the transfer vehicle parked in orbit
00:12:17
once again liquid hydrogen is just a
00:12:20
pain to use in this application so why
00:12:23
use it because when it comes to
00:12:25
maximizing exhaust velocities and thus
00:12:28
specific impulse low molecular weight
00:12:31
exhaust products are important assume
00:12:35
for a moment that all the heat energy we
00:12:37
input into the system is converted to
00:12:39
kinetic energy in the exhaust products
00:12:42
kinetic energy equals a half times the
00:12:45
mass times the velocity squared to find
00:12:48
the velocity we can rearrange this
00:12:50
equation so now we see that velocity
00:12:53
equals the square root of two times the
00:12:55
energy divided by the mass it's clear
00:12:57
here that increasing the mass of the
00:13:00
exhaust particles will decrease the
00:13:02
velocity of our exhaust hydrogen is the
00:13:05
lightest element and thus maximizes
00:13:08
specific impulse if we were to use
00:13:10
another propellant it would be extremely
00:13:13
difficult to make a nuclear thermal
00:13:15
propelled spacecraft with a high enough
00:13:17
specific impulse to justify its use the
00:13:20
next lightest gas is helium which is
00:13:22
twice as heavy as hydrogen in and thus
00:13:25
will reduce our specific impulse by the
00:13:27
square root of 2 nearly negating all the
00:13:31
advantage nuclear thermal propulsion can
00:13:33
provide the next slightest element which
00:13:36
isn't a solid at temperatures we require
00:13:38
is nitrogen which is 14 times heavier
00:13:41
and thus would decrease our specific
00:13:43
impulse by the square root of 14 which
00:13:46
is 3.7 times worse making a nitrogen
00:13:49
nuclear thermal engine worse than a
00:13:51
traditional combustion engine
00:13:53
so we can't get away from this hydrogen
00:13:56
storage problem and if we hope to use
00:13:58
nuclear thermal propulsion we are going
00:14:01
to need to figure out how to keep
00:14:02
hydrogen cryogenically stored for
00:14:05
extended periods
00:14:06
if this problem could be solved the
00:14:09
higher specific impulse and higher
00:14:10
thrust could cut our transfer times to
00:14:13
Mars by half or potentially open launch
00:14:16
Windows outside of the ideal home and
00:14:18
transfer window
00:14:20
so can we get around this hydrogen
00:14:22
storage problem while using nuclear
00:14:25
power to achieve higher specific
00:14:26
impulses this is where ion propulsion
00:14:30
becomes really attractive again one
00:14:32
massive advantage in ion propulsions
00:14:35
favor is its ability to use heavier
00:14:37
inert and easily storable noble gases as
00:14:41
propellants like Xenon or Krypton this
00:14:44
goes against our previous understanding
00:14:46
where low exhaust molecular masses are
00:14:49
beneficial to higher exhaust velocities
00:14:51
this is possible because we are using
00:14:54
electric power to launch these atoms at
00:14:57
tremendous speeds the ion exhaust
00:14:59
velocity is defined by the charge of the
00:15:01
ion the voltage that it has been
00:15:03
accelerated by and the mass of the Ion
00:15:06
at the charge and mass of the ion are
00:15:09
defined by the propellant choice but we
00:15:11
can scale that voltage very high before
00:15:14
we hit a limit in performance due to
00:15:16
material Properties or some other
00:15:18
physical limit for or combustion or
00:15:21
nuclear thermal engines we are
00:15:22
converting thermal power to kinetic
00:15:24
energy that thermal power is difficult
00:15:27
to scale chemical combustion is limited
00:15:30
by the energy we can liberate from the
00:15:32
chemical bonds of the propellants and by
00:15:34
the temperature our engine can operate
00:15:36
at before it melts this is a problem for
00:15:39
nuclear thermal power 2 which has to run
00:15:42
extremely high Reactor Core temperatures
00:15:44
of 2500 degrees Celsius to achieve
00:15:48
exhaust velocities high enough to
00:15:50
justify its use specialized nuclear fuel
00:15:53
designs are needed to survive these
00:15:55
temperatures and any higher would
00:15:57
destroy the reactor
00:15:59
for reference this is an order of
00:16:01
magnitude higher than nuclear reactors
00:16:03
here on Earth need to achieve which
00:16:05
typically operates at about 300 degrees
00:16:07
Celsius as they are in effect just
00:16:10
boiling high pressure water ion
00:16:12
thrusters do not come close to the
00:16:14
operating temperatures that thermally
00:16:16
driven engines do and we can crank that
00:16:19
voltage up high enough that the added
00:16:21
mass of the ion barely matters we are
00:16:24
still achieving 10 times the specific
00:16:26
impulse of traditional engines could we
00:16:28
use a lighter propellant to increase
00:16:30
specific impulse of course but the
00:16:32
advantages of using propellants like
00:16:34
Xenon and Krypton are so good that the
00:16:37
drop in exhaust velocity and specific
00:16:39
impulse are worth it being inert they
00:16:43
can easily be stored over the long
00:16:44
thrust Cycles iron propulsion needs
00:16:47
making them the ideal propellant for
00:16:49
long duration interplanetary missions
00:16:52
larger atoms like Xenon also hold on to
00:16:55
the electrons in their electron cloud
00:16:57
much looser than smaller atoms like
00:17:00
hydrogen so it takes less energy to
00:17:03
ionize Xenon than it takes to ionize
00:17:05
hydrogen so this reduces the electrical
00:17:08
power needed for the first step in our
00:17:11
ion propulsion process
00:17:12
and most critically higher Mass exhaust
00:17:15
improves thrust this equation defines
00:17:19
the thrust and iron propulsion engine
00:17:21
can generate where iron Mass forms the
00:17:23
denominator of our specific impulse
00:17:25
equation it forms the numerator for our
00:17:27
thrust equation meaning an increase in
00:17:30
Iron mass will increase our thrust which
00:17:33
is the spec that iron propulsion
00:17:35
struggles with most a worthy trade-off
00:17:38
to use nuclear power to generate
00:17:40
electricity in space and power our ion
00:17:43
drive we will need to figure out a way
00:17:45
to cool the reactor core
00:17:47
for the nuclear thermal engine the
00:17:49
propellant acts as a coolant for the
00:17:51
nuclear electric engine we will need a
00:17:53
closed loop coolant system where we do
00:17:55
not expend the coolant but keep it in a
00:17:58
cycle between the hot engine and a heat
00:18:01
exchanger the only method we have to
00:18:03
dump heat overboard in space is through
00:18:06
radiative cooling so a nuclear electric
00:18:09
propelled spacecraft will need massive
00:18:12
radiator fins where this coolant can
00:18:15
pass through this is feasible but we
00:18:18
have a long way to go with developing
00:18:20
nuclear engines for space
00:18:23
and even with this added power
00:18:25
nuclear-powered ion propulsion would
00:18:27
still be on the low end of thrust in all
00:18:30
likelihood these ion propelled engines
00:18:32
will need to be a Hybrid engine that can
00:18:35
use chemical combustion for high thrust
00:18:37
Maneuvers or if the problem of long-term
00:18:40
hydrogen storage can be addressed a
00:18:43
nuclear Hybrid engine is extremely
00:18:45
attractive where our high thrust Burns
00:18:47
can be produced by the nuclear thermal
00:18:50
engine and then through Neutron
00:18:52
absorbing control mechanisms like these
00:18:54
rotating drums where one side is coated
00:18:57
in a neutron reflector and the other is
00:18:59
coded in a neutron absorber by simply
00:19:02
rotating these drums the engine
00:19:04
temperature could be lowered and
00:19:06
switched to a closed-loop coolant system
00:19:08
that could power our electric generator
00:19:11
and provide extremely high specific
00:19:13
impulse and a gradual increase in
00:19:16
velocity that could drastically cut our
00:19:18
travel times to Mars or perhaps allow
00:19:21
humans to venture even further further
00:19:23
into our solar system and begin our
00:19:26
gradual exploration and settlement over
00:19:29
Cosmic neighborhood
00:19:30
this is an incredibly complex topic with
00:19:33
many nuanced and complicated ideas that
00:19:36
I struggled to grasp until I found the
00:19:38
right equations I struggle to understand
00:19:40
how thrust would require high mass flow
00:19:43
rate yes specific impulse is much higher
00:19:45
when your exhaust molecular weight is
00:19:47
low the Eureka moments all came from
00:19:50
dimensional analysis of the equations
00:19:52
and their derivations being able to
00:19:56
understand the language of the universe
00:19:57
is a vital tool in decrypting the world
00:20:00
of physics that's why brilliant is such
00:20:03
a great partner to these videos
00:20:04
brilliant has curated two learning paths
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that would be perfect for both newcomers
00:20:09
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00:20:12
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00:20:16
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00:20:19
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00:20:21
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00:20:23
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00:20:28
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00:20:30
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00:20:32
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00:21:14
or like me find fulfillment in continual
00:21:17
lifelong learning then get brilliant
00:21:19
premium to learn something new every day
00:21:22
if you are looking for something else to
00:21:24
watch right now why not watch my
00:21:26
previous video about the insane
00:21:28
engineering of the fastest aircraft in
00:21:30
history the X-15 or watch real Sciences
00:21:33
latest video about the insane biology of
00:21:36
the hammerhead shark
