Why Are There Two Different Types Of Electric Space Engines, And How Do They Work?

foreign Scott Manley here last week SpaceX made news by launching their first batch of starlink version 2 satellites but for me perhaps the most interesting piece of technology on there were the new hall effect thrusters which were going to run on Argon making them vastly more cost effective than competing ion thrusters hall effect thrusters are one type of electrostatic propulsion system the other common type that's in use is the gridded ion thrusters but hall effect thrusters have come to dominate the higher thrust lore specific impulse end of the market there are other types of thrusters with other ideas but with hall effect thrusters on every single starlink satellite they're now the most common in use by a long way so electrostatic thrusters work on the principle of ionizing propellant atoms by knocking an electron off them and then you accelerate the ions through an electrostatic field to very high speeds and importantly after those ions are ejected they also need to be neutralized so you have an electron gun that's spraying electrons after them to you know neutralize the whole thing so exhaust velocities on these thrusters are many times higher than the exhaust from a chemical Thruster which means for the same fuel Mass they can accelerate a spacecraft to higher speeds the downside of course is that they need a lot of electrical power to do this and they don't generate much thrust because they're constrained by the power requirements even the most powerful ion thrusters in use generate only grams of thrust so it's like having a mouse push your spacecraft around and that means this spacecraft can take months of accelerating before they can really start to take advantage of the extra performance of these thrusters now while they're commonly called electric thrusters it is important to be clear that they are not pure electric they need a propellant that they can push against they need it for reaction Mass this isn't like the EM Drive Mark effect drives or whatever the quantized inertia people are thinking about I mean look a pure electric engine would revolutionize my science if one of these propellantless thrusters actually worked and I'd love to see that happen but for now we're working in the real world and engines generally need propellant to push against now historically the most popular propellant has been Xenon since it's the noble gas which requires the least amount of ionization energy per unit Mass SpaceX has already used a Krypton and their first generation Star Lincoln now they have argon in the new starlink other options that have been used include Mercury cesium iodine bismuth they actually have lower ionization energies which helps with the power use but there are also more chemically active and that means your changes to accommodate this reactivity in your Thruster Hardware the concept of electric thrusters actually dates back over a century to Constantine sulkowsky's work in the early 1900s it was all theoretical at that point but by the 1960s Harold Kaufman had developed a working gridded ion Thruster using mercury as to prepare as a propellant and this would actually fly in space and demonstrate itself on sounding Rockets as the solar electric rocket test you know basically demonstrating the Thruster the Soviet Union around the same time focused on developing the hall effect Thruster and they would actually begin flying those on operational satellites in the 1970s so the Soviet designs would be operational for two decades before the US started considering the electric thrusters to be not an experimental thing so in the 1990s after the fall of the Soviet Union the Soviet research on hall effect thrusters made its way to the U.S and now we have both hall effect thrusters and gridded iron thrusters in use on space missions the two designs have different performance characteristics so they actually complement each other quite well the gridded ion thrusters they get better specific impulse while the hall effect thrusters get higher thrusts so the choice of propulsion technology really depends upon the mission requirements I often talk about the tyranny of the rocket equation and you might think that the best engine will always be the one with the highest specific impulse but the thing is on an electric Thruster uh the specific impulse is basically the exhaust velocity and that exhaust velocity means kinetic energy and the kinetic energy goes as the velocity squared so if you double the specific impulse you quadruple the electrical energy requirements and that means in turn that the Power Systems the solar panels the batteries the electronics the power Regulators all that stuff are four times the size so you need more mass now to be Excel rated if you want to have this higher specific impulse engine you could build hypothetical spacecraft designs and you'll just run them through Mission scenarios and you can figure out that for a given uh duration of flight there is a specific impulse which maximizes your performance and for things like uh short-term missions and low earth orbit about 1500 seconds of performance is perfect if you are saying the Don spacecraft that is in deep space traveling between asteroids then higher specific impulses work better for you because you're accelerating for more time and have more time to exploit this okay so let's now talk about how these two different uh thrusters work in practice gridded ion thrusters they use a series of electrically charged grids to set up an electric field to drive the ions basically you have holes in the grid and as the ions go in you've got a positive charge plate a negative charge grid and as they go between them that means there's an electric field and so the ions get accelerated through but before that they need to go into an eye that you need to start within neutral atoms in an ionization chamber you basically inject the gas in and it gets ionized how does it get ionized well you tip what you do is you smash electrons into the atoms and that knocks electrons off so you can either have like an electron gun spraying electrons around and knocking them off or you can use like an electrodes electrodeless design which uses like oscillating magnetic a little electromagnetic fields and that will drive any free electrons in the neutral gas drive it hard enough you can set up a self-discharging uh you know Arc and that will ionize your plasma now the ionization chamber will also have uh like magnets around it to sort of help separate the ions and the electrons and also to sort of stop things hitting the walls so these ions will then drift towards the first of your grids and the first of your grids is like a screening Grid it's positively charged on the the exterior one is negatively charged so the positively charged ions flow through this and as soon as they go through they're like oh there's a big electric field here they accelerate through and shoot off into space and then of course you then have another electron gun spraying electrons after them so that the exhaust gets neutralized so this exhaust velocity is typically of the order of like 15 to 40 kilometers per second depending on the design now a number of designs also include a third grip this is actually a deceleration Grid it's positively charged and the idea is that this reduces the number of ions impacting the other grids and therefore reduces erosion because you can have the ions potentially flowing back and hitting the positively charged plate and this helps screen them from that and then there's a four grid design which has a pair of grids that are designed to pull the ions out of the ionization chamber and it has a much larger gap for the acceleration stage and that means that they can accelerate it much faster and this kind of four layer grid has achieved some of the highest specific impulses of any uh propulsion system something like over a hundred kilometers per second is pretty possible and yes as I said because of the power requirements this is all very interesting but there aren't really any missions that really fit with this so anyway that's the high performance one on to the hall effect thrusters and so on Surface hall effect thrusters they don't actually have these grids on the outside instead they emit the exhaust from like a circular Channel like a Groove an annulus which is uh where your your action happens the important thing to realize is in the hall thrusters they create a negatively charged cloud of electrons just inside this Channel and at the bottom of the channel is the positively charged anode where the propellant is introduced ionized and then because the the you've got the anode the positive anode and the electron cloud the ions get accelerated through this and again shoot off into space now how does this electron cloud get held in place well there's magnets involved I mean of course if you're a physicist you knew that magnets were involved somewhere because the whole effect very specifically describes what happens when you have an electric current meeting magnetic field lines it causes the electric current to go sideways and generates like a potential difference anyway in the center of the Ring there is a magnet and the poles of this Magna are basically pointed along the direction of thrust and then surrounding it there's a bunch of other magnets typically four and their poles will be aligned in the opposite direction that means the magnetic field lines cross neatly over the the annulus and outside of the Thruster you have an electron gun and that is spraying electrons out away from it but they then see the positively charged anode inside the ring and they're like I want to go there but as they fall down they hit the magnetic field and the magnetic field says Ah you're going to go sideways so they start swirling around and basically become this cloud of Trapped electrons are being that are slowly Falling Towards the anode but are mostly being held in place now it's important to realize that while the cloud itself is slow moving the electrons are actually moving really fast but they're just spinning around in circles all the time so at the anode you introduce the neutral gas and as it diffuses along the channel it starts to encounter these electrons are spinning around and those electrons are moving fast enough that when they collide with the ions they knock electrons off and the ions now see that there's this positively charged anode this negatively charged cloud of electrons and they get pulled along and shoot out of the Thruster and as they escape the thrust of that electron gun that started all this it is neutral it will neutralize those ions and you'll get a your Thrust out of this out of this thing so typically these hall effect thrusters they can't generate as higher electric Fields so the exhaust velocity is lower typically 10 to 20 kilometers per second one question the smart people might have is why the magnets can hold on to an electron cloud while then similarly allowing electrically charged ions to pass through why doesn't that affect the ions well there's a big difference in the mass between these two species is like factor of thousands and when you calculate the motion of a Charged particle in a magnetic field there's an important value called the larma radius that is for a given velocity mass and charge the particles will orbit in circles with a characteristic radius now a hall effect Thruster will be designed so this radius for the electrons is much smaller than the size of the channel allowing the electrons to basically move around inside the channel without hitting the walls but for the heavier ions they have a radius which is many many times the size of the Thruster so that they do curve but they don't curve significantly as they cross through it and they're effectively going in a straight line now as I said hall effect thrusters they don't get the higher exhaust velocities of the gridded ion thrusters but they do have better mass flow rates indeed there's actually something of an ongoing scientific question or mystery because they get higher flows than are predicted by all the model holes in the science and you know this is very nice to have the Thruster works better than you'd expect but because you can't do it analytically you pretty much need to build a test device and see how it works to actually quantify it so no uh one of the other important things to know about both types of electric Thruster is that they both suffer from erosion because of all those high energy ions and electrons flying around these can strike surfaces they can like embed themselves in the material they can break down they can modify Crystal structures and just or just knock atoms off eventually you erode things and the Thruster will start to degrade the in its performance until something goes wrong and the whole thing stops functioning you'll often see that the data sheets for these designs will have life limits guaranteed limits on how long they expect it to work in how long how much thrust it will generate over its lifetime and another important parameter for designers is the efficiency this is basically calculating the electrical energy that goes in versus the kinetic energy of the exhaust out now energy in these systems is typically lost as heat in the electrical systems there's also energy lost by the ions that strike the walls or the grids that also converts to heat but also importantly energy is required to ionize the propellants and this is basically like a participation tax you have to ionize the stuff you can't get away without it and this ionization tax is especially important if you change the propellant as I said Xenon is the most popular because ionization energy per unit mass is lower than the other noble gases but on top of this it's the heaviest noble gas at least amongst the ones that have stable non-radioactive isotope so that means you get fewer atoms per kilogram on top of less energy per atom so switching to Krypton or even argon has a double penalty changingly propellant in a hall effect Thruster may also need changes to the electric or magnetic field strength or the Thruster geometry as changing the atomic mass of the of the egg propellant means changing that air alarm or radius so I hope you appreciate that spacex's recent launch of starlink satellites using argon hall effect thrusters wasn't something they could have achieved just by buying off-the-shelf hardware and then switching up the propellant you know you had to develop a complete system and tune it and test it but it is it is something they did and it's a big step in cost savings for is your starlink satellites argon is something like one percent of the cost of xenon it's made as a side product from liquefying Air so you can get liquid oxygen and liquid nitrogen and yeah I've talked about the two main types of thrusters there are other electric Thruster designs out there and one is the field emission electric propulsion it uses a liquid metal propellant and like a blade or a needle that's apparently in use on the European space agency's uh Lysa Pathfinder Mission there's other types are like ion electrostatic electromagnetic plasma Dynamic thrusters lots of papers lots of test Hardware but as far as I know none of this stuff has really been flown in space and truthfully it's because existing Technologies are doing just fine for the mission requirements in hand and as you know space flight is rather risk-averse after all electric thrusters are an idea that is a century old and only now do we see the majority of satellites actually flying with them I'm Scott Manley fly safe [Music] thank you [Music]

Channel: Scott Manley

Views: 391,954

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Id: PmdDTOvLKC4

Length: 16min 41sec (1001 seconds)

Published: Sun Mar 12 2023