WEBVTT

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Mark Kushner:  but before we do that it's honor you with the mimsy mode.

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Mark Kushner: Maybe you know they know and everybody knows

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Mark Kushner: you.

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Mark Kushner: Okay. Thank you, Scott, for that introduction, and thank you, Mark, for the invitation today.

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Mark Kushner: I'd like to begin by immediately acknowledging my collaborators just very quickly. And these include my former colleagues at trust me! And he called Polytechnic, my colleague Oliver, who's actually a assistant professor here at University of Michigan. My colleague Javier, who's an ex engineer at trust me, and also some of my colleagues at the Australian National University.

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Mark Kushner: So today I'm going to take a pretty broad definition of the term plasma propulsion. and for us, propulsion will imply

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Mark Kushner: any force that we produce. To move a spacecraft or change against trajectory was orbit

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Mark Kushner: plasma in this context means that we're using a plasma in some way to generate this force. And so it's critical to the process.

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Mark Kushner: I'm going to split my talk up into 3 main parts, and I'm going to start by looking at electrostatic propulsion. And I'm going to talk to you a little bit about some alternative propellants and their need and and some emerging alternatives, and why they're needed in the industry.

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Mark Kushner: I'm then going to talk about an innovative electro thermal propulsion concept that is emerging

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Mark Kushner: and also connect that with alternative propellants.

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Mark Kushner: And finally, I will end by talking about how we can use plasma, propulsion even without a propellant while exploiting the background ionosphere around the earth.

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Mark Kushner: So I'm going to start electrostatic propulsion.

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Mark Kushner: And a very important thing when we design a proportion assist system is to select the correct type of propellant.

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Mark Kushner: And you can imagine there's a wide range of different substances that exist. And so, you know, a good place to start is to look at sort of the periodic table of elements and try and pick something from there, and you can really immediately eliminate a whole bunch, such as the the heavy elements, very rare elements on the bottom.

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Mark Kushner: But for historical reasons, the very first electric propulsion system made use of Mercury. Explain a bit why, that is later. So that's a picture there of the certain one spacecraft which was launched in, I think, 1,964.

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Mark Kushner: But Mercury was quickly abandoned because of its toxicity, and the dominant propentive choice has since become pretty much Xenon for the high performance. Electric propulsion systems

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Mark Kushner: in general, though the best propellant to use depends on your technology and your mission. and I hope to try and provide you with a bit of a holistic view of why that is today by looking at different propulsion technologies.

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Mark Kushner: So if we take as an example the gridded iron thruster, and I've given a little schematic of one of those in top left there

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Mark Kushner: in the system we inject propellant from the left-hand side, for example. And then we create a plasma using one of several possible mechanisms. And yeah, this uses a radio frequency coil.

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Mark Kushner: We can then extract ions from this plasma, and accelerate them with a pair of grids to generate thrusts, and then to make sure that we have a charge neutral or a current balance in our propulsion system. We need to have a neutralizer to eject electrons back into the plume.

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Mark Kushner: Now, from simple theory, the thrust of a of a grid line thruster is related to the iron beam current

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Mark Kushner: and the voltage across the bridge.

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Mark Kushner: and you can also find the power consumption which is just, essentially the sum of the power needed to accelerate your ions, and whatever energy costs you had to actually produce the plasma in the first place, and that's usually quantified by something called the energy cost per ions.

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Mark Kushner: And if you divide these 2 equations together, you find that the thrust-to-power ratio is proportional to the square root of the mass to charge.

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Mark Kushner: and since satellites are power-limited systems. the thrust that you generate is going to be connected to the mass of your propellant, the molecular mass, atomic mass.

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Mark Kushner: And so, if you take this argument further, you find that if you have a low mass propendant.

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Mark Kushner: this ends up being a low voltage, high current thruster.

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Mark Kushner: and if you have a high mass propellant, you have a high voltage, low current thruster, and that's the one you want.

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Mark Kushner: Some of you might have already encountered a plasma physics, space charge limit or child Langar law.

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Mark Kushner: And that basically says we can only extract so much current between a pair of grids.

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Mark Kushner: and so we'd rather have high voltage low current. Otherwise, to get a certain thrust we need a very wide, very large thrusting, and so that leads to a larger thrust the size.

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Mark Kushner: and so a higher dependent mass would leave a smaller, thruster, and smaller mass.

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Mark Kushner: So at least for gridded Ion thrusters. A good propellant to choose is some convenient heavy propellant. and so that's why Mercury was originally peaked.

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Mark Kushner: But, as I said, Mercury was abandoned fairly soon afterwards because of toxicity concerns, and Xenon became the propellant of choice.

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Mark Kushner: Xenon is quite rare in the atmosphere, although it's not rare in in a like. We're going to use it up very soon. Sense. It's more just got a low concentration.

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Mark Kushner: and we usually produce it through fractional distillation of of the air.

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Mark Kushner: And because this is quite an energy intensive process. Xenon can be quite expensive.

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Mark Kushner: So that's the number one problem with X, you know, it's expensive.

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Mark Kushner: But I've got a second problem, too, and that is the Xenon market who's very susceptible to fluctuations. And this has been particularly true in recent years with the war in Ukraine, because both Ukraine and Russia have very important Xenon and other rare gas production facilities.

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Mark Kushner: And indeed, when NASA does some of their large interplanetary missions. They have to acquire Xenon many years in advance and stockpilate, otherwise they can disrupt the Xenon market.

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Mark Kushner: So, in the short term, the solution as an alternative parent is to use some other noble gases

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Mark Kushner: which have systems have been well pissed on them, and these include things like Krypton and Argon.

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Mark Kushner: Argon's an interesting one, because on paper it's not a very good propellant for most propulsion systems, but it's very low-cost and widely available. And Spacex, some of you might know, have recently used argon on their styling constellation satellites, which I think has covered the surprise of many propulsion engineers in the community.

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Mark Kushner: But there's a broader problem with Xenon that goes above those issues I've just mentioned. If you look at an industry perspective, there could be between 17,000 300,000 satellites launched in the next 10 years, depending on what estimates you want to believe.

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Mark Kushner: And if you look at the mass breakdown of those satellites, the average mass is probably going to sit around 300 kg.

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Mark Kushner: And so if you do some simple sums, if we take kind of the lower end of the prediction scale. There you can estimate how much propellant you would need per year, and you get a number of roughly about 60 metric tons.

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Mark Kushner: And that's interesting because global production of xelons is also about 60 metric tons.

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Mark Kushner: and Xenon also has to be used for other applications, such as lighting in semiconductor industry and various other things. So there's a bit of a problem where there could be

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Mark Kushner: space industry demand, that is outpacing production.

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Mark Kushner: So that's a challenge.

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Mark Kushner: If we go back to the periodic table, and you sort of zoom in near Xenon.

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Mark Kushner: You can see that sitting right next to it is iodine and iodine has a similar atomic mass.

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Mark Kushner: And it's actually got some really interesting properties. So this photograph here shows that iodine can be stored as a solid under ambient conditions.

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Mark Kushner: And so that's very interesting because you don't need a high pressure storage tank.

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Mark Kushner: Unlike Xenon, iodine, is actually mine from Kalichi ore, or it's extracted from underground brines. And what's very interesting is that global production is about 500 times higher.

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Mark Kushner: Xenon. So it's able to meet industry needs in the short and potentially long terms. It's also much cheaper than than Xenon.

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Mark Kushner: So these are some strong advantages. It's lower cost. There's high production output.

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Mark Kushner: and because it can be stored as a solid, you can have much easier, easier handling and storage.

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Mark Kushner: I should point out just briefly that iodine is not the only choice you could have potentially made. You know, a couple of others that have been investigated. These include things such as zinc, bismuth, and magnesium. But I think, iodine, it's fair to say is the most mature alternative propellant, and I'll try and give you some perspective on that as we go through.

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Mark Kushner: I want to touch a little bit on on storage density. And this is where I want to take a bit of a holistic view. Oftentimes in research, we get focused just on one aspect of the system, such as the thruster. But we need to consider the whole propulsion system.

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Mark Kushner: And so what I've shown in this graph is the storage density of typical noble gases as a function of pressure.

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Mark Kushner: And as you can imagine, the higher the pressure, the higher the storage densities. I've also compared. Yeah, compared here at the top.

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Mark Kushner: the density of iodine when it's in its solid form. And so the take home message here is that iodine has a much higher storage density than all of the other noble gases, and if we put this in kind of a more visual chart.

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Mark Kushner: the the relative storage density is, is about 2 or 3 times higher than Xenon.

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Mark Kushner: 5 times higher than Krypton, and almost 14 times higher than armor.

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Mark Kushner: So this just gives you a perspective of if you're using iodine as pennant

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Mark Kushner: compared to say, Argon, your propellant tank is so much smaller, and that has a knock-on effect to the rest of the propulsion system.

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Mark Kushner: Not only is the tank smaller, though, but because you don't need to store it under high pressure. Your tank doesn't need to be as thick because it's not a pressure vessel anymore.

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Mark Kushner: And smaller tank, and not being a pressure vessel means your tank is also lighter.

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Mark Kushner: so you've almost saved doubly because your whole process propulsion system, mass is much lower than it would have been.

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Mark Kushner: Course, there are some challenges with iodine, and one of them is the fact that it is a solid. So if it's a solid, how do you actually get it to flow into your into your thruster?

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Mark Kushner:  Well, the graph I've shown here. On the right is the vapor pressure curve for iodine. and you can see the vapor pressure is a function of temperature.

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Mark Kushner: And so what you can do is you can slap on a bunch of heaters to the propellant tank.

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Mark Kushner: and if you have a heater, a temperature sensor and some sort of feedback control system, you can control the temperature of the iodine propellant. You then shift along this vapor pressure curve, and you can cause the iod to sublimate directly to a gas.

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Mark Kushner: and so you can control your mass flow rate by controlling your temperature.

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Mark Kushner: Of course you do have to be a bit careful. This little inset figure I've shown here is the phase diagram for iodine.

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Mark Kushner: and you need to make sure that you choose a temperature that doesn't cause a phase transition to a liquid.

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Mark Kushner: because if you form into a liquid, then you don't have as much control as you thought you did, and the propellant might flow out of the tank something like this. So you have to be very careful about where you operate in the phase diagram, and I think this tries to illustrate how multi-disciplinary space engineering sometimes has to be.

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But

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Mark Kushner: so for iodine flow control. We can solve that by control sublimation, and that has been demonstrated on the ground, and a space

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Mark Kushner: when I was making this presentation, I thought it would be quite useful to give you a bit of perspective of how iodine has developed in space industry, and the earliest known use of iodine that we could find was in 62

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Mark Kushner: where iodine was tested as a substance for generating negative ions

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Mark Kushner: to try and neutralize a positive ion beam as opposed to using electrons, which is usually that today

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Mark Kushner: you fast forward to. About 40 years later several papers noted that with space industry growth. if we continued to use Xenon, we were probably going to run into supply problems, and at this time they proposed that iodine might be an attractive alternative propellant.

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Mark Kushner: Just over 10 years later.

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Mark Kushner: the first electric propulsion system running on iodine was tested on the ground, at least, that we could find.

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Mark Kushner: And this was a hall thruster developed by the Us. Company. Busack put the question mark there on first, because you never really know if it is the first, but that's first that we found, and they claimed it also to be the first, so it might well be the first.

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Mark Kushner: About 7 years later French Company thrust me, which is why I used to work. They tested the world's first Id and propulsion system in space.

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Mark Kushner: and that was just a warm gas thruster which is in the previous slide. You sublimate the iodine, and then you expand that iodine vapor through a conventional rocket puzzle.

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Mark Kushner: So it's not really a plasma thrust to that one

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Mark Kushner: about a year later

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Mark Kushner: tested, thrust me also, tested a gridded Ion Thruster, which I'll talk a little bit more about later.

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Mark Kushner: and that was the first demonstration of I mean in space.

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Mark Kushner: And then, in subsequent years, there have been a number of further launches, both by thrust me also now by the Us. Company Busec and the Italian Company, T. 4, I.

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Mark Kushner: So this photograph shows the first propulsion system, which is named Mp. 30,

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Mark Kushner: in a Cubesat. and it was launched into a sun synchronous orbit of about 480 kilometers. and it was tested in 2020 and 2021.

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Mark Kushner: And this graph on the bottom shows effectively a measure of the satellite altitude with date.

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Mark Kushner: and I've highlighted with the arrows and the numbers.

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Mark Kushner: different thrust of firings that were performed.

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Mark Kushner: and the rest of the data points there represent GPS, measurements or measurements from the space Surveillance network managed by the Us. Space Force.

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Mark Kushner: and also the black line represents what the planned manoeuvre profile was supposed to be.

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Mark Kushner: So you can see that with each known date of firing there was a distinct change in thruster in satellite altitude. So the system worked.

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Mark Kushner: We recently went further than this, and together with my colleague, Oliver Geo. Richards, here, who's assistant professor at Michigan.

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Mark Kushner: we performed one of the world's first thrust audits of a micro propulsion system and a thrust audit is really looking at a whole series of different techniques for determining and measuring thrust.

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Mark Kushner: checking the consistency between them all, and and making sure that there's a kind of traceable history that everything matches and lines up.

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Mark Kushner: So for each of those firing maneuvers in the previous slide

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Mark Kushner: we calculated the or we. We looked at the thrust from different methods, and this includes a plasma discharge model that I'll explain a bit later.

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Mark Kushner: Laboratory measurements with thrust balance and diagnostic probes

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Mark Kushner: in space testing, using telemetry on board the satellite, and it telling us what it thought thrust was based on certain measurements

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Mark Kushner: as well as in space testing using GPS data.

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Mark Kushner: and you can infer the thrust by seeing what thrust value would explain the orbit that was produced.

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Mark Kushner: And you can see. The agreement here is pretty good between all the methods and the horizontal sorry. The vertical horizontal dashed line

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Mark Kushner: represents the set point of the thrust for each maneuver. And so you can see that's fairly consistent with that set point as well.

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Mark Kushner: So the first Id thruster was a grid iron thruster from thrust me. There was also the cold, warm gas thruster.

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Mark Kushner: The company boozek now also has a gridded iron thruster on iodine.

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Mark Kushner: and they've also developed a neutralizer that runs on iodine.

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Mark Kushner: They also have a hall thruster on iodine, and there is an Italian company, T. 4. I. That has what's called an ambipolar thruster that runs on iodine and an ambipolar thruster. Some of you may know it by sort of

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Mark Kushner: other name which is called a Helicon thruster. It basically has a magnetic nozzle.

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Mark Kushner: and it accelerates and generates thrust in that way.

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Mark Kushner:  let's see, there's a question in the chat here.

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Mark Kushner: Don't have this.

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Mark Kushner: Wait a moment, but So I want to talk a little about the iodine plasmas now.

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Mark Kushner: So if you look at Xenon, what I've shown here in this graph

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Mark Kushner: are rate coefficients for certain reaction processes in Xenon.

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Mark Kushner: And we have certain things like this blue curve which is elastic scattering between electrons and neutral Xenon atoms. You also have excitation and things like ionization.

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Mark Kushner: and for Xenon you can use a fairly simple chemistry and get a pretty accurate model of of the system. If you want to help design a grid thruster or understand how it's working

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Mark Kushner: with iodine. However, we have a much more complicated situation.

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Mark Kushner: Firstly, iodine is a molecule. So it's a diatomic molecule of 2 iodine atoms.

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Mark Kushner: And so if you look on the right-hand side. in addition to elastic scattering and ionization and excitation.

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Mark Kushner: We now also have vibrational excitation, so we can excite vibrational modes of the of the molecule. We can also dissociate the molecule into iodine atoms.

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Mark Kushner: and we can have alternative dissociation reactions. Where, when we dissociate that molecule, we also ionize one of them at the same time. or we produce a negative ion, which is when the electron attaches to one of those atoms.

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Mark Kushner: The atomic iodine, then, has similar reactions to to Xenon. There's excitation, ionization and elastic scattering.

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Mark Kushner: And we found that in order to accurately model iodine plasmids, we have to consider many of these different reaction processes.

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Mark Kushner: So it means there's a bit more of a burden on the modeling compared with Xenon.

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Mark Kushner: But with the reaction set that we have now, we have quite good understanding of how the iodine custom was.

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Mark Kushner: One thing I want to. Highlight, though, is something about excitation.

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Mark Kushner: So if you look at Xenon and you look at the first few excitation levels, you can see that the threshold energy is about 8 or 9 electron volts.

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Mark Kushner: But if you look at atomic iodine, the very first excitation level is just below one electron volt. This turns out to be quite significant, because

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Mark Kushner: at higher pressures you can have much higher energy losses, collisional energy losses. And when compared to Xenon. So this is an important difference.

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Mark Kushner: One way of quantifying these energy losses is something known as the collisional energy loss.

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Mark Kushner: which is essentially how much average energy do you expend to produce a single electron aisle? Pen

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Mark Kushner: Xenon is here in in green, and I've also plotted iodine and molecular iodine in in blue and red.

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Mark Kushner: And the point is, if you're operating in conditions where your electron temperature

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Mark Kushner: is below about 3 or 4 electron volts.

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Mark Kushner: The energy losses in iodine are orders of magnitude higher than Xenon, and so it would be a very inefficient plasma discharge.

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Mark Kushner: Conversely, though, if you operate above this threshold temperature, the energy losses are actually lower than Xenon.

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Mark Kushner: and this has been something that has been confirmed independently by other researchers. So this is kind of a similar plot of the collisional energy loss. And again, there's a very similar threshold of about 3 or 4 eb.

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Mark Kushner: And so this is important for us, because many high performance electric propulsion systems operated low low pressure and higher temperatures.

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Mark Kushner: And so this means I iodine. Aside from having these, let's say, engineering advances. Advantages

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Mark Kushner: also has a plasma discharge advantage

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Mark Kushner: in terms of modeling. There have been a number of groups that have developed plasma models for Ig, and most of them have centered around this kind of geometry, which is inductively coupled plasma.

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Mark Kushner: I'll talk a little bit more later.

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Mark Kushner: They've also included various positive and negative ion species, and they may include as well ground states or even excited states of some of the neutral species. These models have both been 0 dimensional as well as one or 2 dimensional and just. Very briefly, they include things like some of the conservation laws for the different species.

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Mark Kushner: a sort of power balance to connect with how much power. The propulsion system is using

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Mark Kushner: gas heating. So iodine can be heated in various ways. And this is important for a plasma discharge perspective, because for the same pressure.

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Mark Kushner: A higher gas temperature means a lower gas density, and this can affect all your collision rates.

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Mark Kushner: And finally, there's also usually some model describing the power transfer between the antenna and the plasma.

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Mark Kushner: See that there.

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Mark Kushner: So we look at some of these results for the trust me, propulsion system. This graph shows the iron beam current

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Mark Kushner: for both xenon and iodine as a function of power. and we have fairly good agreement

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Mark Kushner: between measurements and models.

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Mark Kushner: and if you extend that to other operating conditions like mass flow rate.

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Mark Kushner: and also look at some easily measurable performance. Metrics like the propellant utilization efficiency

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Mark Kushner: which I just remind you is basically, how much of the propellant we input into the thruster actually gets ionized to generate an ion bin. Obviously want that as high as possible.

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Mark Kushner: as you can see, the agreement is pretty good, qualitatively and quantitatively.

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Mark Kushner: and you can also use these models to predict the thruster performance like thrust values and and specific impulse and things like that.

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Mark Kushner: And so, for example, this graph shows predicted and measured thrusts.

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Mark Kushner: You can see the agreement is very good, the kind of shaded region for the model is there because the

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Mark Kushner: power is an input parameter to the model. And in the experiments there's a certain uncertainty in what that power value is, and so that propagates into the model predictions.

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Mark Kushner: So the Takeo message is with the reaction set for iodine that has been developed in recent years we have a pretty good understanding of iodine plasmers, and we're able to understand, describe them quite well.

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Mark Kushner: Problem with iodine. And this is one of the major technical challenges that remains still is that it's very reactive with some materials.

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Mark Kushner: Some papers that have listed at the bottom. There have done some studies on on compatibility with different materials. But there's still a lot of work, I think, needs to be done.

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Mark Kushner: But, in short, anything

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Mark Kushner: like an aluminium or stainless steel alloy reacts very strongly with iodine, and those are unfortunately some common materials.

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Mark Kushner: Materials with a high resistance are things like tungsten and molybdenum, which are refractory metals, also technical ceramics like aluminum

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Mark Kushner: and certain polymer coatings. And so the point here is when we design

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Mark Kushner: a new propulsion system that uses alternative propellants. We need to make sure that we have the correct material to use

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Mark Kushner: one of the challenges today, which I think is the most

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Mark Kushner: prevalent challenge with widespread adoption of iodine in electric propulsion communities is its compatibility with hollow cathodes, which are widely used for as a neutralizer in hall thrusters and and some other systems. And that's because there's certain material inside a hole thruster inside a hollow cathode

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Mark Kushner: that has to be a certain material to to operate correctly. And iodine so far has not been so compatible with that material.

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Mark Kushner: All right, I'm going to move on to the second part of the talk. and I'm going to change tact a little bit

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Mark Kushner: and talk about electro thermal propulsion, which is a different type of electro propulsion technology.

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Mark Kushner: So during my time in industry, we did a white paper study with a unnamed space agency

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Mark Kushner: and looking at the future propulsion needs and different propulsion technologies that were existing, and what might be interesting for upcoming missions or commercial evolving commercial needs.

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Mark Kushner: And so we did an analysis of different propulsion systems.

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Mark Kushner: And there are different ways to compare these propulsion systems. And what I want to do for today's purposes is is use what's called a kind of thrust and specific impulse map and thrust is really a measure of how long it takes to perform a maneuver

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Mark Kushner: and specific impulses is is a measure of how much propellant you need to do that maneuver. and you can then populate this map with different technologies that exist today.

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Mark Kushner: And we noted that there was a gap in the middle

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Mark Kushner: which was of interest to a lot of missions because they wanted a balance between high thrust. Well, let's say moderate thrust and moderate efficiency, because time is money for many commercial satellites, and so the faster you can be placed in your mission. Orbit.

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Mark Kushner: the faster you can start performing services, and many companies are willing to accept a lower fuel efficiency in order to do that.

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Mark Kushner: This gap is sits in the realm of electro thermal propulsion. And I'll explain in the next slide certain systems that exist there.

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Mark Kushner: And what we're going to look at is a kind of new type of electro thermal propulsion system that uses radio frequency inductive fields. and the target here is trying to get a thrust-to-power ratio that's about 2 or 3 times higher than a horse thruster and a specific impulse. That's about 2 or 3 times higher than a chemical thrust. So we're kind of in between.

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Mark Kushner: The typical example of of the most mature electro thermal thruster is known as arc Jed, which is basically a glorified arc welder. You have a cathode and an anode.

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Mark Kushner: you strike an arc, and when the propellant gas flows through the arc. You you it gets heated up. And so the system's got a lot of maturity. But because these electrodes are immersed.

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Mark Kushner: they can get eroded or or damaged.

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Mark Kushner: There's also a microwave electro thermal thruster, not so mature yet. I think it's flown, maybe once

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Mark Kushner: and had some challenges. But essentially, it's kind of like a microwave oven in the sense that you've got a discharge, a resonant discharge cavity, and if you inject microwaves into it, and you design the cavity correctly, you can get the pellant gas inside to break down, form a plasma

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Mark Kushner: that plasma then superheats the remaining propellant gas to high temperatures, and you generate thrusts by expanding

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Mark Kushner: propellant out of a nozzle.

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Mark Kushner: What we're going to look at is a variation on that which is a radio frequency inductively coupled plasma. And here we use a radio frequency antenna in very similar, actually to what I showed you with the gridded Ion thruster.

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Mark Kushner: So it's the same plasma generation technology. But this system operates at a pressure that's about 5 orders of magnitude higher.

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Mark Kushner: So what that means is the plasma hat does not generate any thrust.

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Mark Kushner: it only serves as a mechanism to heat the propellant gas which is unionized.

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Mark Kushner: That hot gas, then, is expanded through a conventional rocket nozzle to generate thrust.

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Mark Kushner: and the reason that you would do that is because the temperatures you can get are much higher than combustion temperatures with chemical propulsion systems.

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Mark Kushner: This little schematic shows an inductively coupled plasma. You usually inject gas.

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Mark Kushner: Well, typically from, let's say, the upstream end. You have some sort of insulating dielectric tube. You have a radio frequency coil that's wrapped around, and that's what

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Mark Kushner: essentially sustains the plasma.

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Mark Kushner: And then the plasma heats the propellant gas and you expand that out

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Mark Kushner: the open end, which might be terminated with a nozzle.

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Mark Kushner: This is a device that's very well used in industry for ground-based applications.

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Mark Kushner: sometimes also called a plasma torch.

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Mark Kushner: and it's widely used for analytical chemistry.

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Mark Kushner: So because the temperature is so hot inside, any sample that you inject is quickly dissociated, and then, if you perform optical or mass spectrometry, you can determine what the composition of that sample was.

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Mark Kushner: It's also used for advanced manufacturing. So you can do some deposition.

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Mark Kushner: You can manufacture nanopoulos and certain things like this.

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Mark Kushner: and it can be used to generate a high enthalpy flow

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Mark Kushner: for testing materials, high high temperature materials

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Mark Kushner: for simulating re-entry of hypersonic or

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Mark Kushner: one of the challenges, though particularly from a propulsion perspective, is that because the the gas is so hot inside, and there can be quite strong heat losses to the walls.

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Mark Kushner: What we've been looking at to try and reduce those heat losses and to improve performance is something known as a bi-directional vortex.

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Mark Kushner: whereas before you would inject the gas conventionally from the upstream end cow, we're actually injecting the gas from the downstream end just before the nozzle.

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Mark Kushner: And if you take this, this schematic and you look at it sort of head on. We're injecting the gas tangentially.

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Mark Kushner: And so this imparts a certain angular momentum initially. And so what you have is this gas that first spirals along the outside of the tube

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Mark Kushner: or the inner surface of the aren't you? Reaches at the top, reflects, and then spirals back down. So you've got 2 counter-propagating vortices. The outer vortex is cooler, and so it helps to shield walls

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Mark Kushner: and the inner vortex passes through the hot plasma region, and so can enhance heating.

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Mark Kushner: And we were kind of inspired to use this idea from something that's been used in liquid chemical rocket engines known as a vortex rocket engine, and it's shown to have a very market improvement in performance, because the vortices improve mixing.

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Mark Kushner: So you improve combustion. In that case you can shield the wall so you can use different materials than you could otherwise use previously.

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Mark Kushner: and because the vortex essentially doubles the length of the tube, because it's going up one way and then coming back down. You can shorten your combustion chamber. And so we wanted to apply this method to inductively coupled plasmids.

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Mark Kushner:  before I go further, I'd like to just talk a little bit about the power transfer process in these systems, because it's kind of multi-step. And it's important because it influences where we need to study and and optimize.

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Mark Kushner: You have one power generator, which is your rf. Power generator.

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Mark Kushner: and immediately you lose some power in your external circuit. So it's always unavoidable. Some of it's lost as omic heating in the actual Rf antenna itself

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Mark Kushner: the remaining power gets coupled into the plasma.

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Mark Kushner: and then you have certain plasma. Losses like plasma might be lost to the walls.

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Mark Kushner: You have ionization, excitation, and things like this. although I should just note that at the high pressures used

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Mark Kushner: well, it's below an atmosphere, but it's still relatively high for potent

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Mark Kushner:  Some of these energy losses can be recoverable because you can have what's called collisional quenching where you de excite excited species, and then that can be used to heat.

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Mark Kushner: He'd guessed.

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Mark Kushner: After these losses you get the power that's transferred to the gas.

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Mark Kushner: You then have some conductive heat losses to the walls. Maybe some radiative heat losses as well. If radiation trapping is curved them.

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Mark Kushner: and the power that leaves is is kind of a useful power that you can do, generate thrusts or process of materials, or something like that.

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Mark Kushner: Obviously, if you've got a nozzle, you might have some losses in the nozzle, such as divergence, and so on.

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Mark Kushner: And so from our perspective, there's kind of 3 powers that are interesting, and there's kind of 2 efficiencies that are interesting, and that which we need to maximize.

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Mark Kushner: maximize, the power transfer from the antenna to the plasma. That's step one.

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Mark Kushner: And then you want to maximize what's called the thermal efficiency, which is the power you put in basically

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Mark Kushner: to how much power leaves.

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Mark Kushner: And so what we wanted to do with this project that we still ongoing was to firstly, test the feasibility of this concept. which is to say, an inductive electro thermal.

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Mark Kushner: and also to test this bi-directional vortex, to see if it really has any impact on these systems. And so we made a number of experimental prototypes.

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Mark Kushner: I'm just going to give a little schematic here.

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Mark Kushner: We basically have a vacuum chamber. We attach a prototype to one end.

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Mark Kushner: We have a Rf. Power generator.

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Mark Kushner: We have sort of a gas bottle which provides the propellant.

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Mark Kushner: And then we have a series of different diagnostic probes that I'll explain next.

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Mark Kushner: Because this was a first stage

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Mark Kushner: low Trl low technology readiness level system we didn't place thrust inside the vacuum chamber. We wanted a more controlled environment that we could easily study some of the physics and and make some good measurements.

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Mark Kushner: If you zoom with zoom in on the blue region. This is a photograph of the system operating, and you get this very nice kind of light, saber-looking plume.

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Mark Kushner: And what I'd like to do now is just talk a little bit about some of the diagnostics. So because the temperature's so hot.

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Mark Kushner: We can't just stick a probe in there because probable melt. And we had that problem beginning.

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Mark Kushner: The other thing is the vortices, the the bi-directional vortex. We don't want to disrupt that vortex flow, so we don't want to stick something in that's going to cause some asymmetry then.

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Mark Kushner: So we've used only non-invasive diagnostics.

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Mark Kushner: and one of the prime lines has been optical emission spectroscopy. which is to say, the plasma is emitting light, and we can measure the emission spectrum.

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Mark Kushner: and we do 2 things we measure, we measure

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Mark Kushner: discrete emission lines of of different excited states that are radiative radiatively to returning to a lower state.

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Mark Kushner: and we also look at. We were sort of surprised to see this. But there's a continuum background that occurs because of radiative recombination. So that's when an electron and ion recombined to form a neutral atom

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Mark Kushner: and also brainst drowling due to electron neutral and electron ion

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Mark Kushner: our collisions.

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Mark Kushner: You take the spectra and then you can determine. I won't go into the analysis details, but you can determine certain properties about plasma. And so, for example, since our systems, quite high pressure is sort of near local thermal thermodynamic equilibrium. And so, if you measure the transitions of all the excited species, you can fit a kind of temperature to that, because they roughly follow kind of Boltzmann distribution.

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Mark Kushner: We also have tested, confirmed this by injecting a bit of nitrogen into the discharge and nitrogen is interesting because it's got some very well known emission bands.

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Mark Kushner: And there's some software that exists

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Mark Kushner: that can generate a synthetic spectrum. And if you compare the measured and synthetic spectrum, you can determine the temperatures that must have been present.

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Mark Kushner: Just to give you an idea in in a plasma you might have the following temperature ordering where the electron temperature is usually the highest and that's larger than the excitation temperature of gas.

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Mark Kushner: which is higher than the vibrational and the rotational temperature. If you've got a molecular gas, and that's usually higher than the translational temperature, which is what we kind of want to determine to estimate trusted performance.

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Mark Kushner: If you're at in local thermodynamic equilibrium, then all these temperatures are pretty much the same.

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and in this system we've measured peak temperatures of about 10,000 kelvin.

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Mark Kushner: which is close to what the electron temperature is. and in at least in Argon, some very high densities, 10 to the 2010, and 21

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Mark Kushner:  power transfer efficiency. I said to you, this was one of the first things that is is most important to to guarantee and ensure that is high, and if we put a current probe around one of the Rf. Antenna legs, we can measure the Rf. Current, and then from that we can infer what the resistance of the plasma is.

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Mark Kushner: and how much power is transpuding into the passenger.

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Mark Kushner: And this just gives you a graph of that where the power transfer efficiency that we've been able to obtain is is about 90 95%.

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Mark Kushner: I told you before that there are 2 vortices, an inner and out of vortex.

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Mark Kushner: We realized early on that the dimensions of that inner vortex are probably going to be quite critical, and so we wanted to test the geometry of the muzzle. and see if that influenced certain properties of the system. And so that's what all these different markers are. This is the size of the nozzle throat

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Mark Kushner: in terms of the rf. Power of transfer efficiency. Things don't change too much. but if we look at some other measurements, I'll show you soon. Does have a strong effect.

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Mark Kushner: Another interesting diagnostic that we use, which is kind of old school now, because it's not really used so often for low temperature plasmas, but it's simple water calorometry.

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Mark Kushner: When we set up the experiment we had a water flowing along the outside of the tube just to cool it and maintain a safe environment and a stable environment have a well-defined boundary condition. But we realized if we measured the flow rate of that water

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Mark Kushner: and the temperature of the inland and outlet, we could determine how much power was being lost to the water. And that's an extra diagnostic that gives a sort of global perspective.

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Mark Kushner: And so that's what I've shown in this graph power dissipation as a function of mass flow rate, propellant mass flow rate for those different nozzles.

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Mark Kushner: and the total power from the generator in all these examples is 800 watts. At low mass flow rates. There's almost no power which leaves

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Mark Kushner: the thruster because the flow rate is too low, and so all the power has to be lost on the walls. In some way

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Mark Kushner: you'll notice that the maximum power is not equal to 800 watts.

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Mark Kushner: It's actually about 5 to 10% lower. And that's because the missing power is what I just discussed in the previous slide is lost in the circuit.

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Mark Kushner: And so the the numbers are actually very consistent between different diagnostics.

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Mark Kushner: and the point here is at higher flow rates, and particularly for the larger nozzle sizes, you can significantly reduce heat losses to the walls.

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Mark Kushner: and so this helps to try and funnel, more of the plasma, more of the power that you put into the plasma out.

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Mark Kushner: and this has an influence on your performance. Metrics. So this is thermal efficiency of the different nozzles. And we're able to get our thermal efficiencies close to about 70%.

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Mark Kushner: And a related metric is the specific enthalpy which is essentially a measure of how hot the gas is, the higher the specific enthalpy, the higher the temperature. In a sense.

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Mark Kushner: and we're able to get quite high specific enthalpies and quite high thermal efficiencies.

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Mark Kushner: Okay, so I've

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Mark Kushner: given this kind of perspective

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Mark Kushner: more from general physics. But I want to. Just before I end the section. Just talk a little bit of how this connects with propulsion

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Mark Kushner: from the simple nozzle theory.

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Mark Kushner: The way a nozzle works. Is, it converts thermal energy into directed kinetic energy.

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Mark Kushner: In a nozzle theory. There's certain performance characteristics which help characterise the performance of the nozzle and also the kind of

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Mark Kushner: system in the in the let's call it the combustion chamber.

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Mark Kushner: And these are things known as the characteristic velocity and also the thrust coefficient.

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Mark Kushner: It's not really so important the mathematical details. But the point is, they both connect to the specific impulse of the system. So the fuel efficiency of the system.

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Mark Kushner: and if you study the equations, you come up with 3 conclusions on what you should do to pick a propellant. Well, firstly, you want a temperature as high as possible.

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Mark Kushner: Of course.

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Mark Kushner: conversely, to the gridded Ion thruster, you actually want a propellant that's got a low mass now.

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Mark Kushner: because that leads to a higher exhaust velocity.

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Mark Kushner: And you kind of want to minimize your specific heat ratio, because that means you've got more degrees of freedom to store internal energy.

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Mark Kushner: But, as I said to. I wanted to take more of a holistic view

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Mark Kushner: propulsion. That would be the conventional view where we should pick a propellant that has a low molecular mass.

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Mark Kushner: What is the one with the lowest molecular mass? Well, that would be hydrogen.

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Mark Kushner: But.

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Mark Kushner:  we need to think about the whole propulsion system, including the propellant tag and all the other subsystems. And so there's kind of a new metric that can be introduced.

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Mark Kushner: which is a modification of the conventional

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Mark Kushner: specific impulse, and that looks not just at the propellant mass, but the mass of all of the other subsystems.

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Mark Kushner: And so we performed a study that looked at almost 5,000 substances.

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Mark Kushner: We didn't quite go to as high temperatures as electro thermal thruster because of availability of data. But we went to high temperatures anyway.

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Mark Kushner:  to try and get a perspective of what kind of interesting propens would be suited. Hydrogen is one of the worst, because its storage density is so low that you need a very large tank.

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Mark Kushner: And so if you kind of look at the graph at the left, which is a mass breakdown of certain subsystems, even though it's got, requires the lowest amount of propellant for a given mission.

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Mark Kushner: That advantage is completely lost

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Mark Kushner: by the extra mass of the propane tank. So you you don't gain anything by having picked hydrogen. In that case

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Mark Kushner: the best propenets looked to be things like hydro water and ammonia.

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Mark Kushner: and that's because they both have relatively low molecular mass Stor. but they can be stored in liquid form with a high storage density.

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Mark Kushner: Okay, I'm gonna end that section. I'm gonna start the last section which is actually now removing parents altogether.

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Mark Kushner: In the space environment.

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Mark Kushner: we have a number of things that can generate a force on our spacecraft.

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Mark Kushner: One of them is solar radiation pressure.

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Mark Kushner: because photons all carry momentum. And so

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Mark Kushner: you can use this momentum for solar sailing and and generating a weak force like this

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Mark Kushner: around the earth and some of the other planets. There is a weak atmosphere still, and this generates a drag force which is well known in space industry, because this slowly causes orbital decay satellites in lower form.

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Mark Kushner: But there's another mechanism which has been little studied, and that is drag from the ionosphere.

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Mark Kushner: Now the ionosphere is a environment that stretches from about 60 kilometers to 1,000 kilometers in altitude. And it's a low-density plasma environment.

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Mark Kushner: And this graph here shows the plasma density compared with the atmospheric density

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Mark Kushner: as a function of altitude.

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Mark Kushner: And there is a low density plasma that's about one or 2 orders of magnitude lower than the atmospheric density. So from that alone, you might say, Oh, okay, who cares? It's it's there. But it's not so important.

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Mark Kushner: But I'll show you the next few slides. Well how it can become important. just as an indication. The electoral temperature's very low.

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Mark Kushner: relatively speaking in the atmosphere.

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Mark Kushner: and there are various Ionic species. And so the average mass changes a little bit.

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Mark Kushner: What is well known from the onosphere is that it leads to leads to charging of spacecraft

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Mark Kushner: and if you're not careful with this charging, you can get arcing or discharges occurring that can damage some components, and that has to be managed very carefully.

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Mark Kushner: But what's been found as well is that the spacecraft potential relative to the plasma

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Mark Kushner: can be much higher than expected, because certain parts of a satellite are biased relative to each other or

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Mark Kushner: relative to the chassis of the space crime, and particularly things like solar panels can be almost sitting at like a thousand minus 100 volts.

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Mark Kushner: So why is this important? Well. it can lead to something known as sheath expansion around the spacecraft.

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Mark Kushner: If you look at this, what I've just called a toy model just to kind of introduce the subject. If you've got a a spherical satellite, and you immerse it in the ionospheric plasma

348
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Mark Kushner: it's going to charge up, and it's going to generate an electric field, and that electric field is going to influence

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Mark Kushner: a plasma particles around.

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Mark Kushner: So in orbit the satellite is moving at orbital velocities, but relative to the satellite, you can see it is stationary and the plasma flowing around it. So it's almost like like a wind tunnel. If you, if you can imagine that

351
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Mark Kushner: in this perspective you can kind of model the system from binary collision theory, and depending on the what you might be familiar with the impact parameter.

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Mark Kushner: Some ions will strike the object, the the spacecraft, and some will be deflected away or just pass through.

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Mark Kushner: And the objective quickly in this little slide is to try and calculate this

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Mark Kushner: threshold impact parameter, because anything below with an impact parameter below that value will be intercepted by the object.

355
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Mark Kushner: So if you apply simple energy conservation to the system and conservation of angular momentum.

356
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Mark Kushner: You can solve for this

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Mark Kushner: a threshold parameter.

358
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Mark Kushner: then the force that you generate will simply be the area

359
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Mark Kushner: of this presented by this sort of effective threshold parameter multiplied by the momentum flow

360
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Mark Kushner: of the plasma.

361
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Mark Kushner: And the point I want to make here is that because the threshold parameter depends on the potential relative to the plasma.

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Mark Kushner: Your potential changes. You have, you can influence a much larger volume of plasma and increase your effective collecting area

363
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Mark Kushner: that turns out to be very, very significant.

364
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Mark Kushner: So that was just a point model. But if you go into much more to sort of find a detail, I won't do that too much here, but I'll just say a couple of points.

365
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Mark Kushner: You can simulate Ion trajectories sort of kind of like a pick simulation if any of you familiar with that, or a direct simulation like Carla.

366
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Mark Kushner: from binary collision theory, you can also find a analytical solution.

367
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Mark Kushner: and both of those require

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Mark Kushner: some potential. It's actually quite difficult to model the potential self consistently. And you need some sort of particle in cell or kinetic model. But if you use an effective potential, which is something that's often done in plasma kinetic theory, particularly when

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Mark Kushner: trying to describe Coulomb collisions, you can get a pretty good description of the system.

370
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Mark Kushner: And so if you use a spherically symmetric potential.

371
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Mark Kushner: you can solve the system, and you can calculate the drag force that the plasma has on the satellite.

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Mark Kushner: I'm not going to go into the maths. It's not really important, but I just want to highlight 2 points.

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Mark Kushner: There's one part that are ions which directly strike the spacecraft

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Mark Kushner: and momentum is lost that way. And some of those ions can then grab an electron recombine to form a neutral and then get ejected off it. So there's a little extra momentum change there.

375
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Mark Kushner: and there's a second big term, which is ions that don't get collected, but whose trajectories are still deflected. And so there's a transfer of momentum that way.

376
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Mark Kushner: If we look at some of these trajectories just as an example to illustrate the main point. So this shows kind of a small satellite, spherical shape.

377
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Mark Kushner: with with a bias of minus one volt relative to the atmosphere.

378
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Mark Kushner: and the dashed circle represents the sheath edge that forms around the object. and the blue lines are ion trajectories that intercept the sphere.

379
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Mark Kushner: and the red lines are lines that either don't intercept or just deflect it away.

380
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Mark Kushner: And as you increase the bias, so the object size is staying the same. But we're just changing potential.

381
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Mark Kushner: You can see that the sheath width increases significantly.

382
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Mark Kushner: and the the fraction of ions that are intercepted also increases.

383
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Mark Kushner: And so the point is for the same environment.

384
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Mark Kushner: the same object, but a different bias. You can control the force.

385
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Mark Kushner: you're effectively increasing your collecting area. and you can do so orders of magnitude more.

386
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Mark Kushner: then you could potentially have done with just atmospheric drag where you can't control the area at all. Well, almost notable.

387
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Mark Kushner: So just as a illustrative example, if you calculate the force for different sized objects. So I've given yeah, something that's representative of Cubesat and something at the bottom which is representative of Smallsat, small satellite

388
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Mark Kushner: or different biases.

389
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Mark Kushner: You can see that as the

390
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Mark Kushner: voltage increases.

391
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Mark Kushner: the the drag force relative to the atmospheric drag increases.

392
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Mark Kushner: and particularly for smaller objects.

393
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Mark Kushner: the drag force can become equal to, or even higher than that from the atmosphere alone.

394
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Mark Kushner: That's kind of interesting. For 2 reasons. There have been some proposals of using what are called plasma brakes.

395
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Mark Kushner: electrostatic tethers, which are very long, thin pieces of wire which are then biased relative to the spacecraft.

396
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Mark Kushner: and because they're so so thin, you get a very large relative increase in sheath area. And so collecting area is much larger than their physical area.

397
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Mark Kushner: And that can be interesting for some of these higher altitudes here, because you can Deorbit set light faster than it would have done

398
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Mark Kushner: through atmospheric drag alone. And there have also been some proposals

399
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Mark Kushner: actually at the university I'm at now. They launched the satellite. But unfortunately there was an electrical wiring problem, so they didn't get to test it. But

400
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Mark Kushner: they basically mounted 2 charged plates onto these large solar panels, and the idea was to by controlling the voltage on each of these panels relative to the spacecraft chassis.

401
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Mark Kushner: you could generate a force to change the attitude satellite.

402
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Mark Kushner: You could do formation flying to do some precise maneuvers to keep satellites together, and you could also potentially do

403
00:51:00.220 --> 00:51:06.600
Mark Kushner: some small collision avoidance maneuvers if you needed to. And what's interesting about that is, it requires no propellant.

404
00:51:06.740 --> 00:51:10.580
Mark Kushner: And there's no moving parts. It's essentially just a solid-state system.

405
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Mark Kushner: So okay, I will leave it there. And just kind of summarize a few points. So Id is really an important emerging propellant. There's a few challenges that need to be addressed, but it's been demonstrated in space. And it's it's come a long way since its first proposal.

406
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Mark Kushner: I've talked about a new electro thermal propulsion system.

407
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Mark Kushner: Also uses some interesting alternative propellants.

408
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Mark Kushner: What wealth in the future.

409
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Mark Kushner: And it's it's kind of a you know, interesting performance range for what has been identified as a gap in market. and then, finally, propellant system can potentially offer some additional ways to maneuver a spacecraft without requiring a propellant.

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Mark Kushner: I will also take this opportunity to unashamedly advertise Ph. D. Position. If there's anyone here who wants or knows of someone who'd like to come to Australia and do a Ph. D.

411
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Mark Kushner: We're going to be doing some plasma Cfd simulations on the new electro thermal propulsion concept, together with our colleagues at the Von Karman Institute in Belgium.

412
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Mark Kushner: So you might get to go to Belgium for about 3 months, if you like. Belgian chocolates or Belgium. Yeah, I don't know. All right. Thank you.

413
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Mark Kushner: When conducting your study for the specific impulse as a function of

414
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Mark Kushner: mass

415
00:52:40.070 --> 00:52:43.820
Mark Kushner: did you take into account for

416
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Mark Kushner: tank mass for iodine or all the reactive propellants that you might need a different type of storage method.

417
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Mark Kushner: Yep, so so we looked at some of those things. It's obviously it's difficult to do it for every substance. But there's a few things that need to be noted

418
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Mark Kushner: iodine. You usually don't just store it. You don't just pour iodine into a tank. It usually comes in like a pellet form, or like crystals. And so what's been done in industry so far is there's you. You

419
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Mark Kushner: try to embed it in a certain rigid matrix to keep its shape. That's obviously, if you're using that matrix that's not a propellant, so that changes the mass fraction of the propellant tank some of the liquid propellants. You wouldn't just pour liquid into a tank. You you need to have maybe a flexible bladder. It might need to be a little bit of a pressureant to force it out. And things like this. Yeah.

420
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Mark Kushner: no. So this was because of the lack of data for very high temperatures. We used available data. We could get chemical databases. So this was a first step and looked at so below 1,000 degrees

421
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Mark Kushner: and we're trying to push this further now, because main element that's missing here is the association of the different substances. And so obviously, you can imagine that becomes far more complicated when you've got 5,000 different

422
00:54:03.020 --> 00:54:11.139
Mark Kushner: basic substances you're looking at. And then they can you should associate. So that's kind of the next step. But this serves as kind of a useful guide for seeing what substances might be interesting.

423
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Yep.

424
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Mark Kushner: great, thank you. I had a question about your iodine. Is it? Is it even feasible to use iodine cause like it's so hard to test right like are you gonna do a vacuum test? Stainless steel aluminum, your metals or your thrust are, gonna get destroyed, and soon as you open it up solidifies.

425
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Mark Kushner: Is it? Is it feasible to do it in that regards? Yeah. So I'll just come back to the slide. So you, referring to it's reactivity with stainless steel aluminium. Right? Yeah, this is an excellent question. The first is, it's been done.

426
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Mark Kushner: it's been tested. The system has flown.

427
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Mark Kushner: The second answer is, yes, it's a problem, and you have to be very careful with how you operate your vacuum chamber. It's not as simple as if you use Xenon.

428
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Mark Kushner: There's various ways of doing it. You can install cryotaps, cryotraps that trap the iodine, and then you have to periodically clean the chamber.

429
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Mark Kushner: You need to make sure that it doesn't get into your vacuum pumps. So you need certain filters.

430
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Mark Kushner: So there's certain like, let's say, practical overheads that become a challenge. But it can be done absolutely.

431
00:55:25.280 --> 00:55:27.399
Mark Kushner: Just follow the

432
00:55:28.730 --> 00:55:39.670
Mark Kushner: for, like the the discharge chamber with the iodine is, there issues with lifetime limitations due to, you know, the iodine depositing on the chamber wall or the discharge chamber walls.

433
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Mark Kushner: Yeah, so this is one of the reasons why a radio frequency antenna is used. Particularly in the Us. Gridded Ion thrusters have tended to be DC. Thrusters that use a holocaust discharge, whereas in Europe they favored more Rf. Discharges, and one of the advantages of Rf. Is, there's no electrodes, so there's nothing immersed in the plasma, and that means the walls of the chamber can be any convenient material that doesn't react with iodine, such as a technical ceramic. And that's what's been used. And there's been no problems there.

434
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Mark Kushner:  when operating, the temperature is high enough that if there was going to be any Iid striking walls it sublimates back out.

435
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Mark Kushner: and it doesn't react with any of the surfaces it's in contact with.

436
00:56:23.480 --> 00:56:26.629
Mark Kushner: But that's that's a design and choice that has to be made very carefully.

437
00:56:26.990 --> 00:56:28.540
Yeah, thank you.

438
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Mark Kushner: Yeah. You mentioned in comparing to Xenon the excitation kind of inversion? Does that ultimately make up for the dissociation losses?

439
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Mark Kushner: Or are those so more significant than this immersion?

440
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Mark Kushner: So these these calculations include the dissociation losses. And so, because the dissociation energy is much lower than the ionization or excitation threshold, it turns out to be not so much of a problem. But I should also mention that at least from the measurements we've been able to make.

441
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Mark Kushner: the system is quite highly dissociated already.

442
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Mark Kushner: And so the performance we've obtained is definitely higher than Xenon at these conditions. But we're concerned that if you try to use iodine, for, like a hollow cathode discharge, which is usually much higher pressure and lower temperature. You might run into these kind of problems, even if you solve

443
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Mark Kushner: material compatibility. And so that's project. We're trying to study and get some funding for. But

444
00:57:29.420 --> 00:57:31.620
Mark Kushner: yeah, we don't. We don't really know the answer to that question.

445
00:57:33.310 --> 00:57:37.640
Mark Kushner: Scott. I see there's some questions online. I don't know.

446
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Mark Kushner: How does the performance? Efficiency is. Efficiency of iodine stack up against other propellants

447
00:57:47.480 --> 00:57:50.720
Mark Kushner: may be interested in later.

448
00:57:51.400 --> 00:57:58.259
Mark Kushner: Yeah. So so I'll just quickly add a few comments, so far at least, for gridded iron thrusters. The performance we've obtained in the lab

449
00:57:58.330 --> 00:58:01.960
Mark Kushner: has been slightly higher with iodine competency points.

450
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Mark Kushner: and this is on top of all the other advantage, like the like. We originally had developed a Xenon version

451
00:58:08.300 --> 00:58:17.940
Mark Kushner: that was twice as large as the iodine version, because of the high pressure storage tank and some of the fluidix components, and so, moving to iodine, we were able to cut that thruster size almost in half

452
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Mark Kushner: and improve the performance

453
00:58:22.470 --> 00:58:23.170
percent.

454
00:58:24.760 --> 00:58:35.499
Mark Kushner: And then is iodine condensation on spacecraft surfaces like solar panels, radiators? Yeah, that's an excellent question. That's one that often came up with industry, satellite operators.

455
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Mark Kushner: So far on, all of the satellites tested. There's been no evidence or indication that it's a problem. Okay, that doesn't mean that that might always be the case. But one thing we've noted is that

456
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Mark Kushner: if you exploit the vapor pressure curve of iodine and point your spacecraft in certain direction. You can sublimate that iodine layer if it did exist. And that's maybe what's happening anyway. And maybe that's why there hasn't been a problem. But of course you might need to be careful that you don't place any payload equipment close to the plume, because if there's any chemical reaction occurring that's obviously different from just pure sublimation.

457
00:59:12.910 --> 00:59:13.570
But

458
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Mark Kushner: yeah, testing of these items. Sort of what is the maximum lifetime?  yes, there have been lifetime tested Icon. For

459
00:59:27.680 --> 00:59:39.449
Mark Kushner: commercial reasons. I can't tell you how long it's been tested, but it's been tested for the life. It's been tested long enough that it meets the stated performance goals and propellant storage capabilities.

460
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Mark Kushner: Yeah, but but I would say it's an ongoing thing.

461
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Mark Kushner: partly because of the challenges with

462
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Mark Kushner: the vacuum facility

463
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Mark Kushner: and also because of challenges with the neutralizer. So the the trust me system uses a kind of old-school filament neutralizer.

464
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Mark Kushner: and those are known to have some challenges, but they've managed to design in a way that's preserved a lifetime. But for hollow Catholic neutralizers there's still some challenges

465
01:00:06.050 --> 01:00:17.709
Mark Kushner: given the difficulties with practical testing and some discussion about how lower masses for them like for the propellant are better and different types of stressors

466
01:00:17.750 --> 01:00:21.029
Mark Kushner: outside of grid ion thrusters. Do you recommend iodine?

467
01:00:21.300 --> 01:00:32.379
Mark Kushner: Yep, so iodine will be. Will be a good choice for electrostatic systems. such as gridded Ion thrusters and hall thrusters, and maybe the one I briefly touched on Helicon thruster

468
01:00:32.630 --> 01:00:41.709
Mark Kushner: for electro thermal thrusters. You wouldn't. You wouldn't need to. You would need to use it. There's other propellants that give better performance because of them. It's actually it's kind of like a trade-off. If

469
01:00:42.100 --> 01:00:45.590
Mark Kushner: hydrogen with the lowest molecular mass gives the best

470
01:00:45.660 --> 01:01:02.069
Mark Kushner: rocket nozzle performance. But it's got a worse storage density. So your propellant tank is much larger. Iodine is got a much better storage density, but it's got a very high molecular mass, so your rocket. Your your nozzle performance is poor, so you need to be somewhere kind of in the middle to get the best of both. Let's say

471
01:01:02.590 --> 01:01:03.360
I don't think.

472
01:01:05.580 --> 01:01:09.319
Mark Kushner: But the Morgan.

473
01:01:19.270 --> 01:01:20.020
Mark Kushner: what's

474
01:01:23.150 --> 01:01:23.850
okay?

475
01:01:25.910 --> 01:01:33.940
Mark Kushner: No, that's the