How the Space Station Moves In Orbit Like A Spaceship

hello it's scott manley here the international space station has been in orbit for over 20 years and in that time it's been visited by something like 200 people now it's easy to think of the international space station as a place and at the same time forget that it is very much a spacecraft in its own right free flying in space it can steer and use thrusters to change its orbit and we saw that in action this week as it performed a debris avoidance maneuver to steer clear of potential impact with space debris so i wanted to talk about the systems and techniques the international space station uses in orbit and guess what i have this lego model of the iss that i can use to actually point out some of the features so the international space station has thrusters which can burn propellant and for attitude control and orbit adjustment and it also has a set of control moment gyros it has four off the four of them which can be used for attitude control but these can do this without expanding propellant both of these systems need to work together to control the station so zarya here was the first part of the space station launched and it had to operate in space as a standalone spacecraft for quite a while so it came with its own thrusters and it has a propellant tank that's capable of holding something like six tons of propellant but these days it doesn't use its thrusters but that propellant is still used to feed the thrusters on the zvezda service module which is the russian module at the very very back now zvezda does do most of the thruster work it has 32 12.5 kilogram thrust reaction control thrusters and it has two 300 kilogram thrusters are used for maneuvering the space station now mission operations actually tries to avoid using the large thrusters on the back of zvezda as much as possible since they have limited operational life spans instead if there's a spacecraft docked on the back such as this progress here they can use the thrusters on that to perform maneuvers so yeah this is usually a progress cargo spacecraft but in the past it's also been the european atv that has performed some really major maneuvers in the life of the space station so for the control moment gyros there are four of those in this section here called the z1 truss and that has the two antennas sticking out so these reaction control or sorry cmg wheels are like 100 kilograms steel wheels that spin at 6600 rpm and they're mounted inside two axis gym gimbals so they can rotate the wheels to point their axis of rotation in any direction so normally when they start up they have the rotation vectors pointed out in different directions that when you add up the angular momentum of all these wheels it comes out to be zero but say there's some force on the space station such as some you know god like being pushing it down here well what will happen is those control moment gyros will have their axis of rotation rotated around to exactly counteract the torque that is being applied and this is great because it means you don't need to spend any propellant to counteract this the problem is there's a limit to this eventually you end up with all four wheels with their axis of rotation pointing the same direction and at that point you are forced to use thrusters to counteract the torque and also you need to use the thrusters to desaturate the reaction wheels or the cmgs and put their rotation vectors back into the zero position so this process is actually a perfect example of how both sections of the space station have to work together to provide full control now there's an interesting bit of history here if you look at the model here this along the backbone is called the integrated truss segment and it's made from segments that were launched by the space station about space shuttle now they're numbered and they're labeled based upon their position port and starboard so for example there's a section here that's called s1 for starboard 1. this section here is p6 for port 6. now if you look at the designs there's no p2 or s2 right no port 2 or s2 because this truss was originally designed for space station freedom and in space station freedom p2 and s2 contained the thrusters that were used but since this is the international space station we have the russian segment here which performs all those functions and so that's why those were eliminated so anyway normally in orbit the space station assumes the orientation called local vertical local horizontal that's with the coppola facing towards the earth and the international section pointed along the orbit but of course as it's orbiting the earth actually has to rotate to match the rate of its orbit that means it actually has to rotate all the way around once every 90 minutes but this is not free rotation there's still the control moment gyros doing their work to make sure it maintains this and as it goes around its orbit it experiences all sorts of forces and torque that will try to push things out of orientation there's things like your tidal effects with the earth there are electrical systems the magnetic field asymmetric drag from the solar panels um radiation pressure the thing is all these different contributing torques can be modeled and when they design the operations properly they actually make sure that these cancel out through an orbit for example as the sun rises for the station it might feel uh radiation pressure that causes it to torque one way but then as the sun is setting 45 minutes later the torque might be in the opposite direction so everything balances out and they don't need to desaturate the reaction wheels this is called like the momentum manage mode where they're trying to minimize the amount of thrust or descent you use but this isn't perfect there's many situations where they have to use other modes one example is when the space station is trying to grab an incoming cargo vehicle and we see it here now if you imagine a cargo vehicle floating in space you don't want the space station to be wobbling around and just you know going with flow to so to speak so in that situation they will use a very strict orientation mode to make sure that they can grab this with the canadarm the opposite happens right after docking now after docking right the spacecraft joined together using soft capture interfaces these are things that can absorb the shock and then they pull the two spacecraft together to perform a hard lock now during that pooling together they don't want any extra forces that might cause the docking thing to move out of alignment and make it harder so after docking the station goes into free drift mode where they turn off all the control systems and let the thing just get pulled together with nothing getting in its way so docking is also one of those moments where they might have to move the station to an orientation other than local vertical and local horizontal lvlh so in the past um they've moved into orientations like this and that's because some of the spacecraft wanting to dock along these ports they use guidance systems which really expect the docking port to be backwards now if you're docking to zvezda on the back that's fine but if you're docking to like one of the docking ports on the bottom the space station might take this orientation or it might take this orientation pointing along the orbit either way the station can reach this orientation to accommodate the incoming craft it might take a while to do this and also while it's in this orientation all that nice momentum managed mode may not work exactly as it's expected so it might need extra thruster firings now another situation where the space station might adjust from its normal orientation would be if it's making an orbital adjustment now as i said normally earlier the maneuvers are usually performed by either zvezda or spacecraft docked to the back of it and the most common maneuver is raising the orbit and this actually is a great orientation for it because the thrusters in the back will like push out this way and the space station will accelerate this way and go into a higher orbit so it works for the most common space station reboost maneuver but sometimes they actually need to slow the space station down and the main reason they do this is if they want to adjust the space station's orbit so that it synchronizes with the future launch of a soyuz the soyuz as you may know is able to do a rendezvous in six hours and they're hoping to demonstrate three hours and to do this the space station needs to be in exactly the right place at the right time and that might mean slowing the orbit down so it arrives at the launch site earlier than it would if they didn't do this of course this is taking fuel it's taking effort but if you've looked at the amount of leg room in the soyuz you would understand so anyway if they need to perform a braking maneuver they need to rotate the entire station they need to yaw the entire station around so that it slows down and this is a slow process this yaw can take like a day but hey the acceleration is pretty weak as well this would have like a 300 kilogram thruster and this is a 420 ton station so that's less than one thousandth of a g acceleration it's not uncommon for burns to take more than 10 minutes during these maneuvers incidentally the acceleration is really really low so astronauts can sort of move around the space station as if they are in free fall there are videos where you can see them playing with stuff to demonstrate the acceleration but it's not like the astronauts have to strap into acceleration couches to handle the stress there was an incident in 2009 which led to a really rough ride for the crew the attitude control thrusters on this had to start firing because when this fires you know there's a little bit of rotation so this has to cancel out and the sequencer software was firing these at in a pattern that exactly matched the natural vibration frequencies of the structure so it actually shook the entire structure because it all moved in resonance gave them a very rough ride it knocked some stuff off of the walls and things like that and they had to spend a lot of time inspecting the station afterwards to make sure that it was still it hadn't been damaged also performing any of these maneuvers does require a whole lot of setup one thing they have to do is rotate the solar panels out of the way for example they that means that they don't have enough power to operate their experiments they'll have to shut down a number of experiments they will have to orient in the correct rotation the reason why they have to move the panels incidentally is because the um the reaction control thrusters from here fire sideways and you don't want any panels in the area potentially taking damage so anyway this whole setup can take several hours to accommodate and that means the fastest that the space station could respond to an emergency maneuver is 5 hours and 20 minutes that's what it says it takes them three hours if they've got an emergency maneuver to avoid a piece of debris they get three hours to set the station up then the engines fire and then they have like 140 minutes of lead time so the effects of their velocity change move the station far enough away from the encounter they actually have these plans on the station at all times ready to go just in case something happens and they may have to change the plans because they might for example expect to be in a different attitude at a different time and different use different stuff so anyway other than zvezda and progress there are a few other spacecraft that have made orbit adjustments firstly there's the european atv which again docked to the back here and the atv actually did al the bulk of the orbit raising when they raised the orbit after the space station was constructed they raised it by like 30 kilometers or something the atv did most of that they burned more fuel than anything else this space shuttle has also performed orbit raising now the space shuttle of course it's an interesting one if you look at it because the thrusters are on the back so if you fire those thrusters well the station's just going to rotate around like that isn't it so for the space shuttle to fire they actually moved it into an orientation uh like this right so a little pointed down just a little and then they use the translation thrusters that would normally push the shuttle up fired those down and that accelerated the station along its orbit also uh more recently back in uh 2018 uh northrop grumman or orbital atk demonstrated that their cygnus spacecraft could actually perform again an orbit raising maneuver and and for that again they would have to rotate it around to like this orientation because it has engines on the bottom and would have to accelerate in this direction they also have contingency plans allow a progress docked here as well to perform like an orbit raising maneuver if that's necessary the cygnus one was of course very important because it showed that nasa was independently able to adjust the orbit of the space station which is kind of important since there have been problems with the progress and there have been problems with the zvezda you know after zvezda docked and they were using the station they didn't use the thrusters on it for a very long time they uh were using progress but when they did want to use it they actually encountered a problem the two main engines have covers on them to you know protect them so those had to open up before the engines fired and as they opened them up having not done this in six years the covers bumped into an antenna which had been added on an eva and therefore they had to cancel the burn that was kind of embarrassing i mean on a sort of more crazy political side nasa needs to know that they can do this to the space station because every now and then there's some you know political claim that hey maybe russia will just take its side of the space station and fly off and let nasa do it on its own that's probably that almost certainly never going to happen but nasa is always has its plans figured out and yeah they are able to now raise the orbit of the space station if necessary also you know other problems that you know to show that nasa has things planned the control moment gyros again in the z1 truss they have four of those now to get three axis of control you need three control moment gyro so they've already got one extra spare they can actually in an emergency operate with just two that gives them a much more limited control but it does give them some control but the truss actually has is stored inside its spare parts and they have two control moment gyros stored on the station at all times that can be installed using astronauts on an eva with the arm so that they have the stuff not only do they have one that's redundant a hot spare they have two other passive spares ready to go in the early days of the space station they actually had two of them fail and they eventually brought those back and checked them out you know in the lab they disassembled them and they found that the bearings had failed and the energy in those spinning wheels had actually melted the bearings down completely and that's just sort of great illustration of the amount of energy and force that is going on inside these cmgs but yeah i guess it's to be expected these little things are steering a 420 ton space station i'm scott manley fly safe [Music] you

Channel: Scott Manley

Views: 302,134

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Id: fDp8jbP_22c

Length: 16min 48sec (1008 seconds)

Published: Sun Sep 27 2020