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Space Forum / Space Flight / July 2005Why Xenon?
The header of the message pretty much says what my question is: Why is Xenon the propellant of choice for an ion thruster? What makes it a better choice than for instance nitrogen or oxygen? TIA Roland. > The header of the message pretty much says what my question is: Why is > Xenon the propellant of choice for an ion thruster? > What makes it a better choice than for instance nitrogen or oxygen? Xenon is an atom, oxygen and nitrogen are molecules, though they (IIRC) have a total atomic weight about the same as xenon. An electric thruster basically wants to singly ionise the fuel (for reasons of saving power) before throwing it out the back at speed. More massive fuel atoms/molecules are better, as the amount of ionisation you need to do per unit mass goes down, reducing power use. But, with diatomic (and more) molecules, you often end up splitting the molecule instead, which takes more power. Xenon is used, because it's inert, monatomic, fairly readily available in the quantities needed, as it's a byproduct of liquid gas making, and non-radioactive. Hmm. Wacky idea of the day. For long duration missions, has anyone considered radium as a source? Power source, and reactant source in one handy lump. > Xenon is an atom, oxygen and nitrogen are molecules, though they (IIRC) > have a total atomic weight about the same as xenon. Not even close. Molecular weights: O2: ~32 N2: ~28 Xe (atmospheric): 131.30 Paul > The header of the message pretty much says what my question is: Why is > Xenon the propellant of choice for an ion thruster? > What makes it a better choice than for instance nitrogen or oxygen? It requires less energy to ionize per unit mass. Also, for a fixed exhaust velocity and grid spacing, the space charge limit on thrust density goes as the square of the ion mass/charge ratio. Paul >> The header of the message pretty much says what my question is: Why is >> Xenon the propellant of choice for an ion thruster? >> What makes it a better choice than for instance nitrogen or oxygen? > > It requires less energy to ionize per unit mass. Is this because it's monoatomic instead of diatomic molecules? > Also, for a fixed exhaust velocity and grid spacing, > the space charge limit on thrust density goes as > the square of the ion mass/charge ratio. > > Paul A noble gas doesn't cause chemical corrosion. Was this a factor in the choice?  SignatureHop David http://clowder.net/hop/index.html 1. Chemically neutral 2. Heavy > The header of the message pretty much says what my question is: Why is > Xenon the propellant of choice for an ion thruster? > What makes it a better choice than for instance nitrogen or oxygen? > > TIA Roland. > 1. Chemically neutral > 2. Heavy What are the reasons why mercury isn't used nowadays? Point 1. ? Bruno >> 1. Chemically neutral >> 2. Heavy > > What are the reasons why mercury isn't used nowadays? Point 1. ? And it's toxic; some organomercury compounds extremely so. Paul >The header of the message pretty much says what my question is: Why is >Xenon the propellant of choice for an ion thruster? >What makes it a better choice than for instance nitrogen or oxygen? As others have noted, you want a monoatomic material -- something that will produce ions of only one mass, rather than a molecule that might break up and might not -- and you want it heavy. Easy ionization is also nice, although the ionization process in most ion thrusters is grossly inefficient anyway and it probably doesn't make a big difference. Finally, for practical reasons, it should be something that is neither corrosive nor poisonous. Cesium, often cited as a likely propellant back before people actually tried to build ion thrusters, is both. Mercury was used for some time, and has advantages, but it's quite poisonous, and as safety standards were tightened, the cost of ground testing of mercury thrusters became prohibitive. Xenon is scarce and expensive, and the high-pressure tanks for storing it are a bit heavy, but otherwise it's a pretty good choice. The only real problem is that there simply isn't *enough* of it for really large-scale space activity. Getting a ton of it should not be difficult, if you've got the better part of a million bucks to spend. But you simply cannot buy a thousand tons of it for delivery next year, no matter how much money you have; the production capacity simply isn't there. You'd have to build your own extraction plants -- big ones. Signature"Think outside the box -- the box isn't our friend." | Henry Spencer -- George Herbert | henry@spsystems.net Roland> Why is Xenon the propellant of choice for an ion thruster? Henry> As others have noted, you want a monoatomic material -- something Henry> that will produce ions of only one mass, rather than a molecule Henry> that might break up and might not -- and you want it heavy. I don't think I fully understand either of these points. Care to check? 1. The problem with a molecule that might break up is... that a population with two very different charge/mass ratios will deliver worse overall Isp and thrust to power than a population with the same average charge/ mass but a single ion species? An example: Thruster1: one ion species, 40 km/s exhaust velocity Thruster2: two ion species, 30 and 50 km/s exhaust velocity, half of each kind of species. If both thrusters flow 1g/s of propellant, both produce 40 newtons of thrust. At perfect efficiency, Thruster1 uses 800 kW, and Thruster2 uses 850 kW. Clearly thruster1 is better. 2. Large ion mass. ? I assume any electric field you set up is going to deliver a fixed number of electron-volts to each ion on the way out. Heavy ions would appear to need more eV to reach the same exhaust velocity as lighter ions. Why is this good? > 2. Large ion mass. ? I assume any electric field you set up is > going to deliver a fixed number of electron-volts to each ion on > the way out. Heavy ions would appear to need more eV to reach > the same exhaust velocity as lighter ions. Why is this good? For a fixed grid spacing, there is a limit to the current/area that a single stage electrostatic acceleration can achieve, due to the 'space charge' of the ions in transit across the gap between the grids (basically, the charge on the ions cannot exceed the charge on the grids). For a given exhaust velocity, the voltage must be higher for the heavy ions. This is a *good thing*, since the current will also be higher. And since the mass/charge is also going up, we see that the thrust density of the engine is proportional to the square of the ion mass/charge ratio (for fixed exhaust velocity and grid spacing.) Paul > 1. The problem with a molecule that might break up is... that a > population [quoted text clipped - 10 lines] > thrust. At perfect efficiency, Thruster1 uses 800 kW, and Thruster2 > uses 850 kW. Clearly thruster1 is better. I was under the impression that energy is used in breaking up molecules and contributes nothing to the thrust. >Roland> Why is Xenon the propellant of choice for an ion thruster? > [quoted text clipped - 10 lines] >overall Isp and thrust to power than a population with the same average >charge mass but a single ion species? No. It's because energy is needed to separate the ions. This energy is not thrust-producing. Imagine it like this: Pick up (ionize) and throw a baseball (our ion). Now imagine that the baseball is glued to another baseball. So it becomes: Pick up (ionize), separate (break the bond), and throw a baseball (our ion). The energy to break the bond is wasted as part of the process. You *could* try and throw the whole ion... But as you pump energy into it to accelerate it, at some point you will put in enough to break the bond and that energy gets wasted anyway. >2. Large ion mass. ? I assume any electric field you set up is > going to deliver a fixed number of electron-volts to each ion on > the way out. Heavy ions would appear to need more eV to reach > the same exhaust velocity as lighter ions. Why is this good? You must store your propellant in a storage tank, yes? A mole of Xenon is not going to take up significantly more space than a mole of a lighter gas. (Perhaps even less... but I don't know; I have a vague memory that the noble gasses have more compact electron shells because they are fully filled/balanced.) Since the mole of Xenon stores more mass in the same space, it is a more space efficient reaction mass. You can get more thrust from it per unit of fuel VOLUME. While mass is a traditional limiting factor, increasing the cubic volume per unit mass of a reaction mass *is* a mass issue since you must increase the size of the storage tanks... and thereby increase mass in the form of extra tankage. Use one mole of Xenon or several multiples of a mole of something lighter. In terms of storage efficiency the Xenon wins hands down. Gene P. Slidell LA SignatureAlcore Nilth - The Mad Alchemist of Gevbeck alcore@uurth.com > Mercury was used for some time, and has advantages, but it's > quite poisonous, and as safety standards were tightened, the > cost of ground testing of mercury thrusters became prohibitive. I ask because I don't know: If we're talking about elemental, metallic mercury, is there a source for the toxic risk of the stuff? Or are we talking about some compound of mercury that might result from squirting mercury plasma into the atmosphere? I also ask because, when I was a kid in the 1950s, nobody knew it was a risk and we all played around with the little shiny blobs, used larger quantities in science experiments, etc. AFAIK, nobody from those days has suffered identifiable consequences, and indeed most of the cohort is impressively healthy for their years, have kids and grandkids that don't look more weird than one would normally expect. > If we're talking about elemental, metallic mercury, is there a > source for the toxic risk of the stuff? Or are we talking about > some compound of mercury that might result from squirting > mercury plasma into the atmosphere? Everyone interested in such questions should know about _Sax's dangerous properties of industrial materials_, now edited by Richard J. Lewis, Sr. The latest edition in our library is the 10th, 2000, ISBN 0471354074. Any decent library should have a copy, but it might be difficult to get on interlibrary loan because it will probably be on permanent reserve in most libraries. The book lists dangerous properties of just about everything and makes fascinating browsing, if you like that sort of thing. > >The header of the message pretty much says what my question is: Why is > >Xenon the propellant of choice for an ion thruster? > >What makes it a better choice than for instance nitrogen or oxygen? <Snip> > Finally, for practical reasons, it should be something that is neither > corrosive nor poisonous. Cesium, often cited as a likely propellant back [quoted text clipped - 5 lines] > Xenon is scarce and expensive, and the high-pressure tanks for storing it > are a bit heavy, Look at George Dysons book about the Orion project, page 229, then substitute the word tungsten for mercury, cesium or xenon. Xenon is used because it is a constituent of the Earth atmosphere, and it will return there by way of the magnetic field. Outside the Earth magnetic field you could no doubt use mercury, which is mainly poisonous as compounds (mercury chloride as in the mad hatters). Can anybody say hydrazine ? Regards Carsten Nielsen Denmark PS If you see George Dysons book, turn to pg 113 and look at that propulsion unit, then fix a note saying 'Add space probe below' |
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