Tethered Satellite (1994)

the show you're about to watch concerns a space shuttle mission unlike any other you've ever seen the tethered satellite Mission while the mission was successful in proving an idea that had only been a theory we had some Hardware problems and the tether did not extend as far as we want wanted it to go as often happens in research we learn from things when they go wrong here we learned enough to prove that this truly unique concept really does work in this program we do calculations based on the length we expected the tether to extend not how far it actually extended so keep this in mind and enjoy the show minus 10 9 8 we have a go for main engine start we hit main engine start 4 3 2 1 zero and lift off lift off of the Space Shuttle and it has cleared the tower is your touchdown [Music] an ordinary summer's day at the lake the Sun the water good fun and physics physics you might not think so but yes take this water skier for example there's a lot of basic physics involved in skiing there's the force of gravity the pull of the Rope the resistance of the water the skier balances these forces and a few more to keep a stable Center of mass gliding across the water but the physics is so basic he doesn't even have to think about it he just skis and if he doesn't get it right the first time he tries again until he does but in space we usually get only one shot at a task so before we do anything we analyze it to get the physics right the first time that's important because what you think should happen in space isn't necessarily what will happen let's say we want to speed up the Orbiter to rendevu with a spacecraft just ahead do we fire the Orbiter Jets to go faster no if we follow intuition and fire the Jets to speed up we'll actually be propelled into a higher orbit that will end up slowing us down so up here in space we can't always depend on what we think will happen we have to know what will happen and that takes a good understanding of physics physics whether we use it to Rendevous with satellites in space or describe water skiing on the Earth has unlimited applications and today we have a really unique one to show you welcome aboard the space shuttle Atlantis where we're about to launch a new type of spacecraft it's called the tethered satellite that's because the entire time it's deployed it remains Tethered to the Orbiter by a 2mm diameter strand of insulated copper this is one of the most complex missions ever attempted by a space shuttle crew what's amazing is that most of the physics that we use to fly the tethered satellite is the same physics that you study in high school it includes Concepts like gravity angular momentum and center of mass in the next few minutes we're going to see how these Concepts relate to the Orbiter the satellite and the tether We Begin The Experiment by deploying the tethered satellite we do this from the top of a 12 M boom to protect sensitive parts of the Orbiter small invisible nitrogen jets are used to get the satellite and tether in motion once they are far enough away from the Orbiter we can turn the Jets off and the satellite and tether will continue to deploy what makes the satellites continue to go up after all you can't push on the row well that's where the physics comes in we take advantage of a force that causes the tether to continue extending on its own away from the earth it's a force that cats stronger as the tether gets longer and what is this Force well it's gravity gravity but how can gravity make things go up well it's one of those things that doesn't make sense until you understand the physics so let's look at the physics of gravity all objects are pulled by Earth's gravity even objects in space but the further away from the earth the objects are the less the Earth gravity pulls on them Sir Isaac Newton described the force view to gravity as an inverse Square law that means that an object two times as far away from the center of the earth as another object of the same mass feels only a quarter of the gravitational pool it's a mouthful but next to the tether gravity is the most important influence on our teed satellite let's find out why we'll calculate using Newtons to measure the force when the 500 kg satellite is in the kago bay 6 ,700 km from the center of the earth it has a certain amount of force on it if we extend its 10 km upwards gravity pulls on it about 1 Newtons less extending it another 10 km reduces gravity's grip by another 13 Newtons for a total of 26 now we said that gravity is an inverse Square force and for our purposes the distance involv is from the center of the earth to our orbital andto the satellite as it moves away from the orbital but the distances the satellite is moving away from the Orbiter are much smaller than the distance to the Center of the Earth if we look at the force on these smaller distances we see that the change in force is linearly proportional to the change in distance we call this difference in the force of gravity with distance the gravity gradient Force gradiance being the mathematical term for difference when the tether length doubles the Gravity grent Force doubles now we have enough information to understand why the tether reels out and stays out so let's put it all together the acceleration caused by the Earth's gravity is greater for objects closer to the Earth's center this means the Orbiter is trying to accelerate toward WS the Earth faster than the satellite this extra acceleration causes the Orbiter to want to fall away from the satellite creating a stretching Force along the length of the tether and that's the force that keeps the tether taut and makes the tethered satellite go up but how can gravity make something go up doesn't that violate the law of conservation of energy could we be getting something for nothing to understand how gravity can make something go up we need to go a little deeper into Dynamics and look at the concept of center of mass the center of mass is the point in a system where the system is evenly distributed and in Balance balance or equilibrium is important because it means the system is stable we're all in the same orbit around the Earth Jeff the Orbiter and these two apples the center of mass between two objects such as these apples doesn't change just because they're in orbit their Center of of mass is between them here when I start them spinning their Center of mass continues to be in the same place if I cut the string so the apples are attached by the longer string the center of mass is still in the same place inside the Orbiter which means in the same orbit what does this have to do with the Orbiter and tethered satellite when the tethered satellite is in the Orbit's cargo bay the center of mass is inside the Orbiter when we deploy the satellite do you think the orbit of the center of mass changes if it did then we would be violating the law of the conservation of energy however like the apples the orbit of the shuttle satellite system doesn't change because no external Force has been applied to the system but the positions of the Orbiter and satellite do change to keep the system center of mass in the same orbit and continue its stable equilibrium around the Earth let's look at that concept and action on the ground to make a seesaw balance one person sits in the middle if we add another person the one in the middle has to move to keep the center of mass in place it's the same for our tethered system when the satellite is extended like the heavier person on the Seesaw the Orbiter stays closer to the center of mass let's see how much the 100,000 kg Orbiter moves away from the center of mass when the 500 kg satellite is deployed 20 km you could work this problem out for yourselves because the Orbiter is more massive it moves only about 100 met away from the center of mass while the satellite moves away almost 20 km and the center of mass of the system stays in its original orbit because the Orbiter moves so little it almost seems like the satellite alone is moving up but now we know both objects are moving one up and one down once you understand this principle you can also see the situation is symmetrical we could just as easily have made the satellite go down 20 km and the Orbiter go up 100 m here's another way of looking at it think of a pulley system a heavy weight moves a little to balance a lightweight that moves a lot the weights can move in either direction the center of mass stays constant but in space we don't need a pulley a tether will do now let's look at the forces that keep the tether system stable along a vertical axis when this toy is punched gravity makes it return to a ver vertical position the tethered system could also be perturbed off the vertical axis without any restoring Force the system would rotate like these tethered apples but the tethered system stable equilibrium keeps it aligned along a vertical axis what causes the tethered system's stable equilibrium and space well once again gravity is at work you see gravity pulls on both ends of the system the pull on the satellite at the upper end of the tether makes the system want to depart from the vertical while a pole on the Orbiter at the lower end makes the system return do you remember the gravity gradient the end closest to the earth feels the greatest acceleration so the system feels a net torque and the tether returns to the vertical it doesn't matter if the masses at the end of the system are equ or not the gravity gradient is stabilizing the system but gravity gradient is only part of the story there's another Force at work remember we saw that there is an approximate force of 26 Newtons for the gravity gradient over a 20 km tether well when the tether is extended 20 km our Force measuring device measures 40 Newtons not 26 Newtons now where did this extra Force come from here is a clue it's the same Force we feel when we will an object around on a string and the string stretches the faster the speed the harder the force tries to fling the object away from the center and this is what causes the extra force on the Tet as you have seen gravity helps keep the Tet vertical and you can actually calculate this extra force and turns out it's about half the gravity gradient force a significant effect therefore as it orbits the earth it is actually rotating once per orbit about its Center of mass creating this extra force that stretches the tether we can see it in action another way if we have one of our crews spin a bucket of water around in a circle the water will stay inside the bucket as long as the motion continues stop the motion the force disappears and the water obeys the law of gravitational attraction like the gravity gradient force it is also proportional to the length of the tether as long as the rate of rotation stays constant and speaking of constants we've always assumed that the tether remains at a constant length and this is not always the case when we lengthened or shortened the tether another physics principle that you've heard of comes into play and that's angular momentum you see it here in the motion of the skater angular momentum is the product of an object's rotational inertia and angular velocity about a particular axis when the object changes its mass distribution the angular momentum stays the same but the rotation rate changes we call this the conservation of angular momentum and you can see for yourself how this principle Works stand on a turntable with your arms outstretched and weights in your hands have someone start you spinning now bring your arms in you speed up that's because the more the mass is concentrated towards the center of mass the faster the mass spins now let's see how this part of physics relates to our spaceship and the tether satellite when the tether is pulled in the rotating mass of the system is concentrated into a smaller area and the system tries to speed up just like the skater when the tether is L out the mass is distributed in a bigger area and the Orbiter and tether system slow down just like the student slowed down when she extended her arms in the Orbiter we see this from a slightly different perspective because we are part of the rotating system remember during most of the mission for scientific reasons the Orbiter is moving with the tail forward do you notice how from our point of view the satellite moves forward with respect to our direction of motion and moves backward with respect to our direction of motion when it's let out and this is called the Coriolis effect and it occurs in all rotating systems as a consequence of the conservation of angular momentum it is the tendency of objects in rotating systems to move in a curve as opposed to a straight line we'll try to throw a ball from someone on one side of a merry go round to someone on the other side of the merry go round as we can see the ball is thrown in a straight line but not from this point of view here it appears the ball's trajectory is curving and this is what makes the satellite move backward or forward as we reel it out or in the coroli effect effect is what makes hurricanes spiral and other weather systems curve we won't go into the mathematics here except to say that the larger the tether reel out or Reelin rates the greater the backward or forward motion of the satellite from the vertical so let's review the aspects of tether dynamics that we've looked at so far gravity Center of mass and angular momentum all of these concepts are studied in your first year Physics course yet all of it is used in a way like nothing you've ever seen before and there are even more uses of tether technology on orbit we found we could use the tether to generate electricity as the copper cord tether flew through the Earth's magnetic field so the question left is is what can we use this tether technology for we could use tethers to directly study parts of the Earth's atmosphere that we can't reach by any present day technology tethers could reduce the need for chemical fuels in space boosting objects to stay in orbit and assisting others to deorbit and Engineers could use the open Wind Tunnel of the Earth's upper atmosphere for Tes ing the Hypersonic aerodynamics of re-entry vehicles and aerob breaking technology it's even possible that one day tether driven electric cable cars will f space travelers from earth to geostationary orbit what uses of tether technology can you think of the limits are not of physics but of the human mind [Music] the [Music] [Music] [Music]