This is the Saturn V rocket moments away from lifting off and taking the Apollo 11 astronauts to the Moon.
With 15 seconds to go NASA commentator Jack King says this
15 seconds guidance is internal.
It may not seem very significant, but it meant that the Saturn V was now relying on its own guidance system
the one that would steer it towards the moon. This incredible system was completely independent from the outside world
and worked by measuring how the Rocket's velocity and orientation had changed relative to its starting position.
Because of this the rocket needed to be absolutely sure of its starting position
or else it would end up on a completely different trajectory.
In this video we're going to look at the Saturn V's incredible guidance system and how a hidden bunker below the launch
pad helped to keep it pointing in the right direction. We'll also be giving away this awesome Falcon 9 framed print
so stick around to the end of the video to see how you could win.
The Saturn V had a predetermined starting position with its y-axis facing North and South, the z-axis
facing East and West, and its x-axis pointing directly up but as the Earth rotated these axes would no longer
reflect the Rocket's true orientation. So how did NASA correct this in time for launch?
The entire Saturn V rocket was controlled by the instrument unit located at the top of the third stage
It contained the inertial guidance platform and advanced mechanical gyroscope that could measure the rocket's orientation with Incredible accuracy.
The basic principles of a gyroscope involves a disc mounted on a three-axis gimbal
the disc is free to rotate in every direction but once the disc starts to spin
its angular momentum keeps it perfectly stable even if the gimbals are rotated.
The Saturn V's gyroscope worked on the same principle, but was much more advanced.
Instead of a single spinning disc the guidance unit had a central platform containing three spinning
gyros placed 90 degrees apart from each other. If the the Rocket's orientation changed these
gyros would sense a slight change in torque which was turned into an electrical signal.
That signal was then sent to motors on each gimbal which would reproduce the change in torque
but in the opposite direction canceling out the rotation and keeping the platform completely stable.
On the opposite side of the gimbal a device would measure how much the gimbals had rotated relative to the platform
telling the rocket exactly what direction it was pointing in. This along with the acceleration
data would allow the flight computers to plot the Rocket's trajectory. But if the gyroscope
wasn't perfectly aligned to the Rocket's axes during launch, it would completely throw off the Rocket's trajectory.
Say you have a car that's programmed to follow this trajectory through a Crossroads.
Its guidance system only knows that it accelerated and took a right turn.
If we change the starting position of the car, the guidance system will see the exact same thing
but the car will end up in the wrong place. In the five hours leading up to the launch, the Saturn V's
gyroscope would be powered on and trying to keep itself stable. However the Earth was constantly rotating
which meant the Rocket's orientation was also slowly changing. Without any external input
the gyroscope would become so misaligned that the rocket would think it was pointing sideways
and so in order to fix this, the gyroscope had to be constantly reset by a special piece of Machinery on the ground.
That's where this tank came in. Just kidding. This has nothing to do with the Saturn V
but it does have something to do with the sponsor of this video. World of Tanks is a fun free-to-play
PC game, which allows users to compete against more than 100 million players worldwide.
With more than 600 tanks in their collection, there are always new ways to play. You can drive your
tanks across open fields, deserts, climb steep hills and sneak through forests. Choose from destroyers,
artillery vehicles, light, medium and heavy tanks, and play matches in more than 40 Arenas.
All of the tanks are historically accurate and each model has a huge amount of attention to detail with
World of Tanks you can earn, experience, modify, and upgrade your tank creating a steel beast
ready for any challenge. Download World of Tanks today using the link in the description and use
code TANKMANIAwhen you sign up to get a free 7-Day premium, account along with 250,000 credits,
the premium tank excelsior, and three rental tanks for 10 battles each.
In order to reset the Saturn V's gyroscopes to the desired axes it used highly accurate reference points. The Y and Z
axes were reset by two pendulums located inside the guidance unit. These were weighted iron rods
that floated on a pocket of air. As the Earth rotated, gravity would pull the mass downwards
keeping the rod perfectly level. The gimbals would then rotate the stable platform to match this.
But in order to reset the x-axis, NASA came up with a much more impressive solution. 200 meters away
from the base of the Saturn V was a bunker built into the ramp of the launch pad. Inside there was a
device called a theodolite which would emit a beam of light and control its direction with incredible accuracy.
During the launch countdown, infrared light was emitted from the theodolite and up to
the rocket at precisely 25 degrees where it would enter via a small window on the Saturn V's instrument unit.
From there the light would reach one of two poro prisms. When light enters a poro prism
it's reflected twice, causing it to leave the prism at the same angle it entered. One prism
was fixed to the stable platform and the other was motorized. The theodolite in the bunker would
communicate with the rocket and slowly rotate the motorized prism. Once the prism was in the
correct orientation, it would reflect the beam of light back to the theodolite where a set of
mirrors were directed onto a detector. Each prism was coated with a special material that would only
reflect a specific wavelength of infrared light. This allowed the detector to plot exactly where
each prism was relative to where it should be. Since the light left the prism at the same angle
it entered, the orientation could be determined by how offset the outgoing and returning beams
were on the detector. The Theon light system was so accurate that it could align the gimbal to within
one thousandth of a degree. Once the motorized prism was correctly positioned, a device would
measure how much it had rotated relative to the stable platform. The motor on the inner
gimbal would then rotate the stable platform by this exact amount. If everything was correct,
the second prism would then line up perfectly and reflect the light back to the theodolite.
This system worked perfectly, but it relied on the Saturn V being completely still. That wasn't always the case.
When a rocket sits on the launch pad, strong winds can cause it to sway back and forth.
This kind of movement would completely throw off the accuracy of the theodolite. And so in order
to get a reference for how much the rocket was swaying, the theodolite would focus on a third
prism mounted to the outside of the rocket. This would shine the light back to the theodolite's detector
which would see it moving back and forth as the rocket swayed. The target points for the
other two prisms could use this as a reference point. This system could account for the top of
the rocket swaying more than 30 centimeters in either direction. All of these ingenious systems
worked autonomously to reset the gyroscope until 15 seconds before launch. From there the rocket
was on its own, with the task of guiding it and three astronauts all the way from the launch pad to the translunar trajectory.
and now it's time for something really special. The winner of this
awesome space shuttle canvas is Evan Larson. Sorry if you didn't win this time, but don't
worry we give away an awesome space prize in every video. Next time we'll be giving away this Falcon 9
framed print. All you have to do is sign up at the link below and leave a comment about your most
controversial space opinion. Thank you very much for watching and I'll see you in the next video.