Reaction Wheels - Things Kerbal Space Program Doesn't Teach

hallo its scott manley here with another episode of things Kerbal space program doesn't teach you and today i would like to talk to you about reaction wheels now in Kerbal space program reaction wheels are parts that you attach to your spacecraft they take an electrical power and they provide rotational torque so you can turn your spacecraft and control it in flight without using any of that pesky monopropellant that you might find required for reaction control systems now it turns out that in real life devices like this actually exist in fact there are multiple schemes for generating torque out of nothing more than the electrical power or rather than doing so without expanding fuel Kerbal space program is perhaps confusing things by calling all of these schemes reaction wheels when in fact they could be reaction wheels control moment gyros or maybe something like a magnet or cur so one of the fundamental laws of physics is that a momentum must be conserved at all times and that's how Rockets work if you accept you apply a force to something in one direction you will feel an equal and opposite force and that ensures that momentum in a closed system is conserved now there's a derived version of this for rotational system where angular momentum must be conserved so reaction wheels in their purest form also known as momentum wheels they work on this principle whereby they have a heavy wheel attached to a motor and if the motor spins the wheel in one direction then the spacecraft has to rotate in the other direction now generally the reaction wheels are smaller than the spacecraft obviously because they're part of a larger vehicle and that means that the reaction wheels have to rotate much faster to provide any rotation to the spacecraft also wheels will rotate in only one axis so if you want full three axis control you're going to need at least three reaction wheels at 90 degrees or at least at an angle to each other now reaction wheels tend to be found on spacecraft that are like Space Telescope's we're pointing accuracy is very very important so we find reaction wheels on the Hubble Space Telescope and on Kepler for example larger and heavier spacecraft tend to prefer control moment gyros these use the gyroscopic effect so what you have is a heavy wheel there is spinning continuously even when no force is being applied now this wheel is sitting inside a cage which is attached to Gimbels and a gimbal is essentially the motor is attached at 90 degrees to this thing so it can rotate the gyroscopes in one direction or the other and because of the way the gyroscopic effect works if you apply a torque in one direction and you have a rotation vector in another direction then you will get an output torque at 90 degrees to both of these so by putting force by trying to rotate these gyroscopes you will get a torque applied back out to the rest of the vehicle and with control moment gyros you can actually have two axes gimbals so in theory you only need to control moment gyros to control a an entire spacecraft or at least to get three axes of rotation so there's actually some improvements on that front they tend to be better in terms of you know force because the momentum the angular momentum is already baked into the wheel continuously now of course control moment gyros scopes you shouldn't confuse those widths answer gyroscopes which are essentially they're freely rotating gimbals and the idea that these are used for sensors they can sense the rotation of the spacecraft and of course a spacecraft that is trying to maintain an orientation will have gyroscopes to sense the rotation and then in turn it may use other gyroscopes or momentum wheels to control the rotation these are two separate devices now both classes of devices and omentum wheels and control moment gyros have to deal with a problem called saturation now if you imagine a Space Telescope trying to hold a specific orientation then it's possible that there's a number of exterior forces that are perhaps trying to change this perhaps it's sitting with its solar panels at an angle to the Sun and this the radiation pressure against these solar panels may be applying a force now that means that the control moment gyros are the reaction wheels are up having to apply an opposite force and that means if this force is maintained over time then the reaction wheels will spin up faster and faster and faster and eventually you can't spend these any faster and at that point you have to bring in the reaction control system and burn some thruster fuel so that you can desaturate the wheels similarly with control moment gyros you can apply forces in various directions but if you end up in a situation where you keep applying forces in one direction then the axis of rotation will tend to end up aligning and you can end up with a situation for all your gyroscopes your control moment gyros Scopes all have their axes of rotation apply I aligned and suddenly there's no way to apply a rotation force in that direction and then at that point the switch the system is once again saturated and you need to desaturate using fuel now while we're on the subject of forces due to radiation pressure it's a good time to talk about other schemes for applying rotation to a spacecraft if you're in the Earth's magnetic field there is something called a magnet or cur a magnet or cur is nothing more than a set of electromagnets that you can apply a current to they will generate a magnetic field and because you're in the Earth's magnetic field the spacecraft's magnetic field will tend to want to align with this and therefore you can control it these are great for low Earth orbit satellites they are fantastic for things like cube SATs square or you don't need any moving parts and they're very simple to build they aren't particularly useful once you've moved beyond the Earth's magnetic field and the magnetic field of the Sun is there but it's very very weak another possibility is to use light control to essentially use radiation pressure now a good example of this our Mariner 3 & 4 the spacecraft included little air reflect or vanes at the end of the solar panels and depending upon what the spacecraft wanted to do it could either present a large area to the Sun or it could fold these away and present a small area and that meant that the differential torque arising from these vanes could be used to control the attitude of the spacecraft again without using any reaction control thrusters and interestingly enough Kepler the Kepler space telescope after it was reduced to two working gyroscopes it was able to maintain stability in the third axis by exploiting radiation pressure this brings me to another point I mentioned that previously for for reaction wheels you need three control axes and four control moment gyros you can get away with two generally spacecraft builders like some margin for error and so they will send up hot spares that are ready to replace malfunctioning parts the Hubble Space Telescope was launched with four reaction wheels so it could handle a single failure and thanks to the servicing missions two of the wheels have been replaced by new ones the Kepler space telescope on the other hand had four wheels to start with and it was unfortunately too far away in deep space so after two wheels failed they had to rely on this radiation pressure stability scheme to keep it working and meant they could no longer point at its original set of targets but a new mission was proposed and it has now generating decent data in Kerbal space program you can generally build your spacecraft without paying too much attention to the exact mechanics of attitude control the generic reaction wheels in the game are wonderfully powerful and they don't suffer from problems like Sacher nation but in the real world of course you have to trade off all the complexities of these systems and come up with whatever will suit your mission it can make the difference between success and failure of your mission and that's why it's worth going into these details I'm Scott Manley fly safe

Channel: Scott Manley

Views: 664,190

Rating: 4.9523344 out of 5

Keywords: kerbal space program, astronautics, hubble, kepler, iss

Id: 7Js5x4NhUxU

Length: 9min 32sec (572 seconds)

Published: Sat Jul 09 2016

Reddit Comments

[u/[deleted]]

Where are the other comments here? Also I didn't know Scott was an actual Astronomer, that's cool!

[u/Exxmorphing]

Mistake at 7:30

Kepler was reduced to two working reaction wheels, not gyros.