Apollo's Forgotten Computer - The LVDC

watching a saturn v launch is still an or inspiring sight but there's far more going on than meets the eye the Saturn 5 was far too complicated to fly by hand but contrary to popular belief it wasn't the Apollo guidance computer or AGC in the command module doing the job it was a completely different computer and one which would probably have a bigger impact on modern computers than the AGC so what was flying the Saturn 5 rocket [Music] [Applause] [Music] now as we know the computers used in the Apollo missions were just a fraction of the power of modern smartphones and too many they just can't believe that you could get to the moon and back with so little computing power but they did in fact you don't need a massive computer to fly a rocket in fact you don't need a computer at all well not in the way that we think the v2 the ancestor to the Saturn 5 you used gyroscopes linked to graphite veins but sat in the Rockets exhaust to steer it and accelerometers that cut the power when it reached a predetermine height as the distance and heading to the target was already known the rest is pretty much physics following a trajectory curve the same as firing artillery shell but of course for Saturn 5 was in a completely different league and much much more complicated so whilst it's doing a similar thing of going from its place of launch to a predetermined point above the earth at a certain speed there's a lot of extra details and ultimately the lives of the astronauts were also at stake it's not just the flying of a rocket that had to be done but also things like monitoring of all the systems telemetry between the rocket and the ground and the emergency detection system to name a few this was all housed in a special section called the instrument uni or viu this was a ring-shaped section located between the third stage and the section but held the command module and the lunar lander this ring measured 91 centimeters high in six point six meters in diameter in here was the launch vehicle digital computer or the lvdc then the analog flight control computer that would take the signals from the lvdc and control the engines and flight controls a stable gyroscopic platform accelerometers radar telemetry and for silver zinc batteries to power the electrical systems when the Saturn 5 launched it would switch control from the ground computer to the lvdc to run a pre-programmed routine that would put it in the correct position to roll the rocket to point in the desired direction once in orbit ground stations would track its position and speed so that the correct coordinates an engine burn duration time could be transmitted from Mission Control to the lvdc for the translunar injection the lvdc also controlled and kept of a third stage steady whilst of a command module performed the transposition and docking procedure of the lunar module once the vehicle was on course for the moon during the launch over 200 parameters were transmitted back to monitor vital systems as well as communications with the crew the Rockets attitude and speed were also monitored so that any slight deviations in the flight path could be detected and corrections sent back to the lvdc now when we see the Saturn 5 it looks like one large sturdy structure but in reality a Saturn is really a collection of fuel tanks held in three stages stacked on top of each other with powerful rockets at the base of each all held together by an external framework which also had to be as light as possible the combined total of these tanks held thousands of tons of liquid fuel and oxidizer all subject to the forces of gravity and acceleration they'd also change their mass considerably from their launch to the separation just a few minutes later this made the vehicle aerodynamically unstable as the fuel was used up and the center of pressure and mass changed during the flight the body of a Saturn was also like a flexible tube that would Bend at frequencies from about naught point three to four Hertz as it reacted to the sloshing of the fuels and the thrust from the engines the flight control system had to take this into account when adjusting and gimble in the engines so that positive feedback loops didn't build up which could lead to the bending oscillations becoming so strong that they could damage the framework and ultimately break up the rocket to help avoid this happening the lvdc ran a major computational loop for vehicles guidance once every two seconds and a minor computational loop 25 times a second to control its attitude these timings were chosen so that they wouldn't coincide with the slosh and bending mode resonant frequencies to highlight why this was so important you only have to see another structural but unrelated issue that occurred called the pogo effect and resonant vibrations in the fuel piping this is something which was well known about in rocket design and was particularly bad on the unmanned test flight of Apollo 6 the phenomenon is called pogoing because the whole rocket starts to vibrate longitudinally or up and down the length of its body like a pogo stick on the Apollo 6 the vibrations caused some damage to the lunar lander docking structure even though want wasn't on board and were so strong but it very nearly aborted the mission a j2 engine on the second stage also failed due to ruptured fuel pipe caused by resonant vibrations and the wrong engine was shut down because of a mix-up in the electrical wiring the pogo effect occurs if there are sudden changes in acceleration due to variations in the engine's thrust the fuel acting under the effective gravity can move up or down either decreasing or increasing the pressure in the fuel lines and thus affecting the thrust of the engines this can become caught in a cycle of slowing and surging acceleration which flexes the entire rocket body if the frequency of this surging matches the resonant frequency of a rocket a positive feedback loop can occur and the amplitude of the vibrations increases so dramatically that they can severely damage or even destroy a rocket the main cause of it in the Apollo 6 incident was tracked down to the flexing of the x-shaped cross member which held the f1 engines as well as the fuel pipes resonating and causing fuel pressure variations which started the whole process because the power of the computers was nothing like we have things that would be done in software now had to be performed by discrete electronic circuits like resistor capacitor networks the leylines when processing the signals that will receive from the gyroscopes accelerometers and other sensors before their values were processed by the computer again this was done to remove the build-up of resonant frequencies between the Rockets structure and the computer's processing loops the lvdc computer itself was also a major step forward and operated in an entirely different way to the Apollo guidance computer which was built by MIT the lvdc was built by IBM and not dissimilar to one of their commercial computers but with many architectural changes whereas the AGC was designed to be crash proof by rebooting back to the same state it was just before the crash the lvdc had built in triple redundancy basically every computation was done on three identical logic systems split into seven consecutive pipelines the three results from each stage would be compared against each other by a voting system if all three were the same then the result would be passed on to the next part of the pipeline however if there was an error and one of the three results were not the same as the others then that one would be ignored by the voting system and the results from the other two agreeing logic units would be used instead by using this method the likelihood of an error in the calculations due to hardware was greatly reduced in fact one third of a computer could file and it would still operate correctly something else that the lvdc did was to compare the current results of a calculation to the last known good value or what IBM called the zero and reasonableness test if the result of a sensor or a critical calculation suddenly went to zero or it jumped to a much larger value than the previous one in the computational loop it would ignore this new data and use the last known good value instead this was done so a false data from faulty sensor for example or a computational error wouldn't be applied to the flight controls as a sudden large change could cause the Rockets to become unstable the stated reliability of the calculations done by the lvdc was ninety nine point six percent for a two hundred and fifty our mission or ninety-nine point nine nine six six percent for a six-hour mission something which helped this was the way it was built it used newly developed integrated circuit or logic units as they were called these contained a transistor or two diodes and a resistor and were surface mounted onto through-hole plated multi-layer printed circuit boards in fact the boards from 1966 looked remarkably like modern PCBs with surface mount components and twelve layers in the PCB something that wouldn't become commonplace in modern computers for decades and shows just how far ahead this system really was one thing that was common between the lvdc any AGC was the memory they still used magnetic cores VL VDC having 32 kilobytes of RAM and a clock speed of two megahertz given it a speed of twelve thousand one hundred and ninety instructions per second modern PC by comparison is about a million times faster a total of forty thousand silicon semiconductors and resistors were contained in the simple ICS which made up the main processor the PCBs that made up the lvdc were mounted into a magnesium frame and the chassis had a liquid-cooled wall to help dissipate 138 watts that the computer used should there have been a problem with the first stage in some way there was the emergency detection system of the EDS this could do one of two things if it detected an imminent breakup of a craft it would trigger the automatic abort sequence if it was less serious it would notify the crew for them to take the appropriate action but once the abort sequence was triggered it was irrevocable and would run to completion the last procedure of every lvdc performed was once the vehicle had reached this was to keep the undocked third-stage and the instrument unit away from become our module on Apollo's 13 14 15 16 and 17 they were also directed to crash onto the surface of a moon not only to keep them from colliding with the command module as it orbited the moon but also to act as a seismic event that would be monitored by seismometers that had been left by previous missions so whilst the lvdc was the unglamorous counterpart to the Apollo guidance computer its job was probably even more essential so thanks for watching and don't forget to check out some of our other videos if you get the chance and also don't forget to subscribe 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Channel: Curious Droid

Views: 463,728

Keywords: nasa, lvdc, launch vehicle digital computer, agc, apollo guidence comuter, saturn v, rocket, computer, ibm, mit, paul shillito, curious droid, apollo 6, apollo, apollo 11, apollo 17, whats flying the saturn v, guidence computer

Id: p12CinjFh1I

Length: 12min 28sec (748 seconds)

Published: Tue Aug 06 2019