and liftoff liftoff of the 25th space
shuttle mission and it has cleared the
tower
this is just 73 seconds into the space
shuttle challenger's
final flight the cause of this accident
a thin rubber o-ring just a quarter of
an inch thick
just about the width of a pencil in the
months and years that followed
scientists engineers and even ethicists
have tried to learn from this disaster
so what actually happened and how did
this change the future of space travel
[Music]
for centuries humans have dreamt of
going to space
but it wasn't until 1961 that we first
left earth's atmosphere
that first trip was made by a russian
pilot named yuri gagarin
but the trip wasn't even that long just
one orbit around the earth
taking a mere hour and 48 minutes today
we have astronauts who spend
months at a time living hundreds of
miles above our planet
in fact for the past 20 years humans
have been continuously living aboard the
international space station
but before we had a home base above our
planet nasa sent astronauts to space on
the mercury gemini and apollo missions
and by the early 70s nasa had decided to
focus its efforts on a reusable space
plane that could bring astronauts into
orbit
and act as a laboratory and it paid off
because in 1981
astronauts left earth and soared into
orbit aboard the space shuttle columbia
and for the next 30 years shuttle
missions carried both astronauts and
equipment into space
even helping build the international
space station now
leaving our planet has always come with
risks and the scientists and engineers
do everything they can
to minimize those risks but when things
do go wrong
they do their best to learn from their
mistakes which brings us to january
1986.
barbara streisand has the number one
album in the country and haley's comet
was just starting its path of earth
which we won't see again until 2061.
sts-51l was a pretty routine mission as
far as flying into space can be
the crew included six astronauts and the
very first member of the teacher in
space program christa mcauliffe
the crew was prepared to study haley's
comet perform a number of science
experiments
and launch two satellites the entire
mission was only going to be
six days by the time the crew boarded
the shuttle schedule changes had already
moved their initial launch date
months behind and even by the time they
had a launch date they were another six
days behind it because of delays to
other shuttle missions
bad weather and equipment problems all
of these delays had nasa administrators
impatient to get the mission
underway which leads us to january 28
1986 and liftoff
liftoff of the 25th space shuttle
mission and it is cleared
high-speed photography later revealed
that the flight was in trouble from the
start
less than a second into the flight a
puff of dark smoke
burst through a joint on the right
rocket booster foreshadowing that in
just over a minute
a bigger disaster was coming
to understand what went wrong let's look
at one of the most dangerous parts of
any space mission
the rocket launch when you look at the
shuttle on the launch pad
there are a lot of different pieces all
strapped together to make up the launch
vehicle
first and probably the most recognizable
is the orbiter
this was the crew's home in space and
acted as a floating laboratory
it's attached to an enormous external
fuel tank along with two
solid rocket boosters and each of these
parts played its own important role in
the launch
the orbiter itself had three main
engines
the external fuel tank full of liquid
hydrogen and oxygen
powered these engines during launch but
to actually make it off the ground
you need a much bigger push than these
engines alone can provide
and that's where the solid rocket
boosters come in these provide
71 percent of the thrust needed to make
it off the ground and get to orbit
they're filled with a solid fuel which
if you were to touch feels kind of like
a rubber eraser
and unlike liquid fuel which can be shut
off with the flick of a switch in the
event of an emergency
once these solid fuel rockets are lit
that's it they can't be shut down or
stopped you are on your way to space
and these rockets are designed to get
the shuttle going really fast
over 3 500 miles per hour so they burn
through their fuel in no time
just two minutes once their fuel is gone
they're basically dead weight
that's when they separate from the
orbiter and parachute into the ocean
where they're recovered and reused by
nasa in the future
but back to our rocket launch the
massive external fuel tank continues to
provide power to the engines as the
orbiter climbs
before it too detaches and mostly
disintegrates in our atmosphere
so it takes a lot of energy to make it
to orbit
this is because you need to overcome
gravity which is holding you down to
earth
and the increasing potential energy it
creates the higher up you go
you see potential energy is energy
stored in a system
based on the position of objects in that
system it's like
a book high up on a bookshelf the book
on the top shelf
will release a lot more energy when it
falls than a book on the bottom shelf
but mass also affects that potential
energy imagine if we tossed
that book and a refrigerator out of a
plane at 3 000 meters
at the same time and for this let's
forget about air resistance let's get
that air out of there and now we can
just focus on the gravity
so both objects actually land at the
same time
because gravity causes them to fall at
the same rate
but the refrigerator is going to release
way more energy when it hits the ground
that's because it had a greater
potential energy because of its mass
we can calculate the potential energy of
each object by multiplying its mass
times the height it's at times gravity
so the bigger your mass and the higher
you go the greater your potential energy
and for a shuttle launched into space
that number can get
really big really fast so we need to
create enough
force which in our case is rocket thrust
to overcome
the gravitational potential energy of
our shuttle
otherwise the rocket wouldn't even make
it off the launch pad
on earth we could write out this change
in potential energy as you move between
two heights
as the change in gravitational potential
energy is equal to the mass
times gravity times the height to use
this equation
we will need the mass of the object
which for us is the shuttle in all of
its fuel at about 2
million kilograms though we should note
that a lot of this will burn up before
it actually gets to orbit
next we need the height it's traveling
to for low earth orbit this can be
around
400 kilometers or about 250 miles
and finally we need a constant for
gravity which is 9.8
newtons per kilogram and usually we can
assume that gravity's pull on the object
stays the same if you're moving between
the ground and the top of a tall
building
gravity's pull is pretty much unchanged
but if you're traveling
all the way to low earth orbit you have
to take into account the fact that
gravity's pull on an object
is going to be stronger down here on
earth than it is farther away
so we have to use a slightly different
gravitational constant
and an equation that takes into account
the mass of the earth
the mass of the shuttle and the change
in the distance between the center of
mass of both
objects this means our height isn't just
the height of the shuttle
it's the height of the shuttle plus the
radius of the earth
so if we plug in our new gravitational
constant which is 6.67
times 10 to the negative 11th meters
cubed over kilograms seconds squared
and the rest of our values we'll find
that the potential energy of the shuttle
all the way up at 400 kilometers is
negative 1.16 times 10 to the 14th
joules the energy here is negative
because when we're dealing with gravity
at these large distances
we set our zero point of height not on
the ground but rather infinitely far
from earth
but we have to find the difference
between this potential energy
and the potential energy that we started
at to find that we use the same equation
but now our starting height is just the
radius of the earth
this comes out to be negative 1.24 times
10 to the 14th joules
so the difference between the starting
and ending potential energies
is about 8 trillion joules and wow
that's a lot of energy just to get the
shuttle to orbiting height
a typical u.s household uses 3.2 billion
joules per month so this is like
powering a home
for 208 years and that's just the energy
needed to get it up there
never mind the energy needed to keep it
there it takes even more kinetic energy
to get the shuttle to a velocity where
it will stay in a circular orbit around
the earth
which also has to come from that initial
launch
now the solid rocket boosters were
structured a bit like stacked cans full
of corn
engineers knew that when the shuttle
lifted off the massive forces the
boosters were under
caused the segments to slightly pull
away from one another to compensate for
this
rubber o-rings were used to seal the
segments and prevent hot gases from
leaking out
and these o-rings were huge 12 feet
across
but surprisingly they were also really
thin only about a quarter of an inch
thick
to create a seal the o-rings had to be
both flexible and
resilient they needed to compress when
weight was applied to them but spring
back to their normal shape when that
pressure was released
but this responsiveness was affected by
temperature
and the challenger launch happened on an
unusually cold florida morning
only 36 degrees fahrenheit this was 15
degrees colder than
any shuttle launch in nasa history in
fact the makers of the o-rings were so
worried about the low temperatures
that they tried to delay the launch but
ultimately
they were ignored to show how
temperature affected the o-rings
let's recreate a demonstration performed
by physicist richard feynman
during a televised hearing after the
disaster feynman took a rubber o-ring
just like this one that is flexible and
resilient at room temperature
he then compressed it in a c-clamp and
placed it into a cup of ice water to
cool it down to 32 degrees fahrenheit
now when we remove it from the water you
can see that the o-ring keeps its shape
at least for a few seconds and that's
what ultimately caused the leak to occur
on the challenger at such cold
temperatures the o-rings simply weren't
flexible enough to change their shape
and create a seal
so temperature changes the way that
rubber behaves because of a property
called the glass transition temperature
this is the temperature at which certain
materials like polymers become
brittle and break just like glass
but the shuttle didn't break apart right
away scientists believe the chemicals
from the fuel
may have resealed the hole at least for
a few moments before strong winds
tore it back open to understand what
those chemicals were let's talk a bit
about the fuel inside the booster
the solid rocket boosters were filled
with a number of chemicals including
powdered aluminum fuel and ammonium
perchlorate which acts as an oxygen
source
because to have fire you need both fuel
and oxygen
alone this ammonium perchlorate is just
a colorless salt
but combined with combustible materials
it's used to make everything from
rockets to fireworks
these were held together by a binder
called p-ban or polybutadiene
acrylonitrile
which also acts as a fuel when mixed the
reaction creates products including
hydrochloric acid aluminum oxide water
gases like nitrogen carbon dioxide and
hydrogen and lots and lots of heat
this heat causes the water vapor and
gases to rapidly expand
and this expanding gas has nowhere to go
but down out the bottom of the rocket
which then propels the rocket up it's
like letting all the air out of a
balloon
the air streaming out the back of the
balloon pushes the balloon forward
but because the rocket's o-rings weren't
able to create a seal
this energy was able to force its way
out through the side of the joint
likely burning up grease insulation and
even the o-rings themselves
resulting in the black smoke we saw at
the beginning
the leak continued to vent hot
pressurized gases from the booster onto
the main external fuel tank which
contained
390 thousand gallons of liquid hydrogen
and 145 000 gallons of liquid oxygen
but the tank could only take so much
heat and it eventually ruptured in a
giant fireball
the solid rocket boosters broke away
beginning to veer off course
but were remotely detonated to prevent
them from landing in populated areas
the orbiter was subjected to 20 g's of
force
and broke apart its wings tearing off
in the process the reinforced cabin crew
however
remained intact plummeting to the ocean
below for almost
three minutes the astronauts likely died
upon
impact
the disaster launched a months-long
investigation into the cause of the
tragedy
it was determined that bad communication
was one of the main reasons why this
disaster
happened see engineers knew that the
rings could fail at such low
temperatures
but when they tried to raise concerns
they presented this graph
to management not exactly the easiest
thing to read
right now instead look at this chart
created by a yale professor
over a decade later you can see how much
easier this data is to read when it's
presented in a clear graph
the x-axis is temperature the y-axis is
the number of damaged o-rings
and that big red dot on the left is the
prediction of just how likely the
o-rings were to fail at those
cold florida morning temperatures
but nasa learned from their mistakes
when the next shuttle launched in
september of 1988
it was with redesigned rocket boosters
pressurized suits for the astronauts
new escape devices in case of disaster
and an emphasis on communication
since the disaster nasa went on to
launch over 100 more shuttle missions
however in 2003 another tragic accident
happened with the breakup of the orbiter
colombia
after a piece of foam hit the left wing
on ascent causing a piece of heat
shielding to come loose during re-entry
killing all seven astronauts aboard
after 30 years on july 8 2011
nasa sent up its final shuttle mission
on atlantis
the program flew 135 missions with two
ending in tragedy since then nasa's
astronaut program has focused on sending
scientists to the international space
station instead
on russian soyuz capsules and recently
on
private rockets like those built by
spacex and blue origin
leaving from american soil for the first
time since 2011.
and someday maybe in the not so distant
future
nasa's looking to send humans even
farther into space
traveling beyond earth's orbit and
eventually to mars
but for that we're gonna need a really
really big rocket
[Music]
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