I thought ion engines could come to their own if they had the capability to collect gases from planets like the gas giants and rocky planets with an atmosphere.<br /><br />But I think that should be a different post. <div class="Discussion_UserSignature"> <p> </p><p><font color="#0000ff"><em>"SCE to AUX" - John Aaron, curiosity pays off</em></font></p> </div>
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<font color="yellow">"...capability to collect gases from planets like the gas giants and rocky planets with an atmosphere."</font><br /><br />I'll ignore the engineering problems of the probe getting close enough to the atmosphere to 'collect' the gas without also getting close enough to aerobrake and lose more velocity than would be gained by refilling the (ha ha) 'gas tank'.<br /><br />Ion engines need a *lot* of power. By te time they get to the gas giants, their problem is less a matter of running out of propellant as it is running out of energy with which to accelerate said propellant.<br /><br />Mars supposedly *does* have Xenon (the most common propellant used by ion drives) in its atmosphere. According to the figure I Googled, it's about 0.08 ppm of the atmosphere. This would mean that assuming 100% efficiency in the gas separation process, to get about 50kg of Xenon for propellant, the craft would have to process about 625,000,000 kg of Mars' atmosphere. Of course, since Xenon is one of the heaviest of gasses (one of the reasons it's used as a propellant), I would <b>assume</b> that it would be more prevalent at low altitudes... complicating the task of collecting it from orbit.<br /><br />Oh -- and the processing equipment would have to mass considerably less than the amount of gas to be collected, of course. Otherwise -- you'd just increase the amount of gas onboard at launch.
not to get too far off topic, but any gass should work for an ion engine correct? I am not an expert on planets, but to combine thoughts from several different threads. There was recently a thread about autonomous balloons in Juvian atmosphere. If said balloons collected gas and periodically sent tanks of compressed gas to orbit it would maybe be possible to resupply a ship. I believe the biggest hurdle there would be getting from upper Juvian Atmosphere to orbit. Same idea would probably be workable on venus too.<br /><br />
<font color="yellow">"not to get too far off topic, but any gass should work for an ion engine correct? "</font><br /><br />Are you basing this hopeful question on anything? In all of the reading I've done on ion engines (which is considerable, but not exhaustive) -- Xenon has always been used as the propellant. I know that grid erosion is one of the problems with ion drives, and I have always assumed that a noble gas was selected specifically because they are extremely non-reactive. Googling, I found links indicating Cesium and Mercury were used in early experiments: <br /><br /><i>"Ion drives have been tested extensively in the laboratory. Early versions employed mercury or cesium rather then xenon, but these materials caused problems because they tended to condense on the outside of the engine. Xenon, a gas at normal temperatures, does not react chemically, so it is much cleaner."</i><br /><br />I can find nothing on ion drives using any other gas than Xenon. You'll also note that all three elements used have fairly large atomic weights. Selecting a propellant that does *not* have a large atomic weight would tend to make for a very inefficient ion drive. This would include most gasses readily available in the upper atmosphere of planets.<br /><br />And again... whatever scheme you come up with has to be *more* efficient than simply adding more Xenon at launch. The propellant in an ion drive is used ***so*** efficiently that this mass is a very small subset of the spacecraft mass itself and particularly of the 'ion drive' portion of that mass. A much larger fraction is spent in the solar arrays and the engine itself. Adding additional propellant makes the ion drive more efficient overall, as it marginalizes the mass of the hardware. It's difficult to imagine it being reasonable to *increase* the amount of hardware required in an attempt to avoid adding more propellant at launch.
I wasn't implying that additional hardware be added to every spacecraft to collect its return mission fuel. I was simply putting forth an idea that in my opinion would only have marginal merit of a system that could reuse gasses collected by something that would be in the atmosphere (lower not orbiting, I was thinking something blimplike) that could collect propellent and boost the propellent to orbit to be used by whatever ion powered craft would come into the vicinity. This intra-atmosphere craft would be reuseable with multiple tanks that could be sent to orbit. <br /><br />I don't see a project of this nature really being justifiable unless you are transporting humans or have plans on reusing probs. It would only be worth while if you had very large masses that you are moving in and out of planetary orbits where the savings in mass for the return trip would be significant.
Maybe we need a new thread on ion engines? They do use space storable propellants, but . . . <br /><br />My understanding is that the preferred atomic mass of an ion engine's propellant is as light as possible. Xenon is the lightest element that does not cause severe grid erosion.<br /><br />I could be wrong here, but my understanding is that the limit on current systems is how much energy can be converted from the source to the kinetic energy of the exhaust, whatever the propellant may be. Given that, and the same quantity of ions, each little ion has its own little share of the overall kinetic energy, the same for whatever propellant is used.<br /><br />Now kinetic energy is given by<br />KE = 1/2 * m * v^2<br />so you see that if KE is known, if the mass is smaller, the velocity must be bigger. And exhaust velocity is what we're looking for, it's basically the same thing as specific impulse (divide by g). <br /><br />So we want lighter particles because they will go faster.<br /><br />My understanding goes a little further out on a limb in that the number of ions is in fact limited as well, so that loophole is not available.<br /><br />A limiting factor with ion engines, in terms of the High Thrust / High Isp we seek, is the grid erosion, not just the power available. I was pretty much up to speed on the technology 4 years ago but have not kept up as well as I'd like, but then again I don't recall any huge breakthroughs either.<br /><br /> <div class="Discussion_UserSignature"> </div>
<font color="yellow">"My understanding is that the preferred atomic mass of an ion engine's propellant is as light as possible. Xenon is the lightest element that does not cause severe grid erosion."</font><br /><br />My understanding would be the exact opposite. That's not to say I'm right... <img src="/images/icons/smile.gif" /><br /><br />I tried a Google on the subject and came up with this interesting link. An extract: <br /><br /><i>"However, some studies propose using oxygen as a source of propellant for ion drive as well. This is questionable. The main issue is the effects of ionized oxygen with the thruster materials (cathode, neutralizer, filament grids) of the ion drive engine. The thruster would need to either be made of different materials than today's ion drive engines and/or have easily replacable parts, neither problem being easily solved. A thruster design such as the German RIT10, which uses RF energy instead of a cathode to ionize the gas, may be the best present day design to start to consider for oxygen use. A second issue is that oxygen is not easily ionized, which means lower electrical efficiency. Thirdly, ion drive works best with heavier elements. Overall, oxygen does not look attractive for ion drive interorbital propulsion. (For satellite stationkeeping, the effects of ionized oxygen on the satellite would preclude its use.) <br /><br />Xenon is the most popular propellant for ion drive today since it is a heavy gas (high atomic mass) that is easily ionized. Argon and mercury have also been used. Xenon and argon are inert gases which are not expected to be recoverable in useful quantities from the Moon and asteroids. The best candidate fuel for ion drive using lunar or asteroidal materials may be sodium, which is fairly abundant in some nonterrestrial materials, extractable without very much effort, easy to store and handle, and would work well with ion drive -- easily</i>
Argon is fairly cheap. It's lower in efficiency than xenon and krypton, but when (or if) LEO costs start coming down trades may indicate it a good budget alternative for LEO stationkeeping and cislunar long haul of bulk non time-critical cargo like water and storable cryogens like lox/methane. <br /><br />Being more mass efficient, xenon is probably going to be much better for GEO stationkeeping and single use probes making earth escape. Then again, if BDBs become cheap enough, maybe not.
This would be the second or third time this question went unresolved. I know that I had seen that permanent link before drawing my conclusion, and that I concluded they were wrong. It's an old article and the technology was very young, esp. in terms of the quest for high thrust. I love permanent.com, all due respect, but I also dug into the available academic literature as part of the research.<br /><br />The math I gave seems sound enough. It depends on my assumptions: that the system is power limited anyway, as is the ion density by total charge.<br /><br />I would not be shocked to be wrong and would be delighted in having some certainty. I know that the 'heavier the better' maxim is published elsewhere, but I suspected that the same source was used and thus an error propagated. <div class="Discussion_UserSignature"> </div>
propylene glycol dinitrate (PGDN), 2-nitrodiphenylamine, and dibutyl sebacate would make great candidates for a store able propellant (combined). I of course don't know what the ISP would be when combined (Otto Fuell II) but I do know that it could make one hell of a bang if the temperature is just above 265 degrees F.<br /><br />I have no idea if the Navy ever tested Otto Fuel II as a rocket propellant but I do know it works great in torpedoes. Perhaps if they haven't they should give it a go and see what it can do.<br /><br />But I also know the by-products are deadly (Hydrogen Cyanide) gas being one and other by products as well. Like that would matter in space right?<br /><br /><br /><img src="/images/icons/smile.gif" /><br /><br /><br /><br /><br /><br />
Oh no I don't mean for launches into space I mean as an on orbit stored propellant for in space use.<br /><br />No way on God's green earth I'd ask them use it for manned launches even if it was better than LOX/LH2 or kerosene blends. That stuff would kill all the birds and animals on the cape not including the ground control teams. Oh Goodness No!!!!!!!<br /><br /><br /><img src="/images/icons/smile.gif" />
How about a pad disaster? Just as bad -- maybe worse. <div class="Discussion_UserSignature"> <hr style="margin-top:0.5em;margin-bottom:0.5em" />Will Pittenger<hr style="margin-top:0.5em;margin-bottom:0.5em" />Add this user box to your Wikipedia User Page to show your support for the SDC forums: <div style="margin-left:1em">{{User:Will Pittenger/User Boxes/Space.com Account}}</div> </div>
<font color="yellow">"The math I gave seems sound enough. It depends on my assumptions: that the system is power limited anyway, as is the ion density by total charge."</font><br /><br />Yes -- well you're well aware that I tend to rely heavily on math myself. However, I've written several formulas with sound assumptions, lots of cool looking numbers, and results that really bit the big one. <img src="/images/icons/smile.gif" /><br /><br />Without finding a trusted source that truly spells things out, my problem is with your statement/assumption: <i>"Xenon is the lightest element that does not cause severe grid erosion."</i><br /><br />I'm <b>reasonably</b> confident that Xenon does not cause erosion of the grid because it's a noble gas and as a class, they're extremely unreactive. Given that -- if lighter is better, I'd have expected to find extensive experimentation with the lighter gasses in that family. They're more common, and much cheaper. However, I can't find a single article talking about experiments with Neon for example. Neon is less than 1/5th the mass of Xenon, so presumably could be expelled >5 times faster based on your assumptions. For that much of an iSP increase, I would expect that there would be experiments at least attempting to control grid erosion if that were the only barrier to such a performance boost. On the other side of the coin -- the only noble gas <b>heavier</b> than Xenon is Radon... which is not only rarer, but poisonous and therefore introduces hazards.<br /><br />I can't offer proof-positive, but Occam's Razor points to Xenon being the propellant of choice because it's the heaviest safe Noble gas possible rather than the lightest one that doesn't cause grid erosion.
You must balance Isp with thrust requirements. If you don't have sufficient thrust from the engine. You many hinder mission requirements. <br /><br />Below is my rational from things I have learned of ion engines. Please point out areas where I may be mistaken.<br /><br />Unless my memory of chemistry is fading all noble gasses have the same charge potential. Therefore the same amount of power is required to strip one electron from Neon as it is Xenon. This means that the same volume of ions was ionized by <br /><br />Moving to the accelleration of the ions. The given constraints of an ion system is the maximum power capable of being sent through the grids without burning them out. We have the same number (Mols) of neon and xenon ions being sent though the grids. Everyone would agree that the Neon will accellerate to a higher velocity than Xenon. But if the product of mass and velocity is lower for neon than xenon, you'll have greater thrust from the xenon. The lower exhaust velocity will therefore have a lower ISP. <br /><br />
<font color="yellow">"Everyone would agree that the Neon will accellerate to a higher velocity than Xenon."</font><br /><br />Not everyone. I haven't actually <b>agreed</b> with that. I simply haven't <b>challenged</b> it. I haven't challenged it because I don't know enough about the process involved, but I can envision the possibility that this is not the case. If the magnetic 'launcher' has a top speed, and Xenon is being accelerated *to* that speed, then putting a lighter atom through the launcher won't result in a higher speed.<br /><br />Going with an analogy that is so far off that it may have zero relevance: I have a slingshot. I put a 10gram BB in the cup, pull it back and let fly. I then put in a 100gram marble and do the same. Did the BB launch at 10-times the velocity of the BB? Heck no! The tension supplied by the slingshot band accelerated the two at almost the same velocity because their mass difference was negligible compared to the energy potential of the stretched bands. Mind you, if I put a 4500-gram bowling ball in the cup and try to launch *it*... then the velocity is going to be extremely low. However, that's because now the mass is no longer negligible when compared to the energy potential of the bands.<br /><br />So -- it may be that a Neon atom will be accelerated to five+ times the velocity of a Xenon atom. Or it may not be. I acknowledge both possibilities. I just can't say which is the case. My **feeling** is that if Neon in the same position as Xenon were to accelerate to 5.4 times the velocity that Xenon attains (i.e. making their thrusts equivalent given the same power input), then NASA would not be fiddling with Xenon but would instead be using the much more common and inexpensive gas.
Whether or not Neon would be a better propellant I agree with Spacester that ion propulsion would be usable only in long term situations, just as it is being used now. What the focus should be on is propellant to make timely orbital changes as well as allow vehicle control. An ion engine would be dead weight on a manned mission to the moon, unless you wanted to take two or three years, and would be of questionable value for a Mars mission. <br /><br />Equally questionable is accessing lunar Oxygen, at least until it becomes cost effective to put equipment and the people needed to run and service it in place. I would say we will need to bring propellant and oxidizer from the surface to orbit for quite some time and to my conclusion water outweighs other candidates in direct cost, ease of handling and long term storage economics. <div class="Discussion_UserSignature"> </div>
Should I infer that your thoughts on water's superiority for long storage assumes it will take less mass in equipment to split the water and pressurize the products than it will to keep cryo's cool? <br /><br />Maybe someone with a little more expirence in chemical rocketry could verify this but running a rocket motor on GOX and GH2 wouldn't be the best idea? If so you'll need to liquify the products too, thus negating mass benefits. <br /><br />My other thought on you wouldn't want gaseous propellants is their low density. To use small tanks at low pressures you'll either need to split the water as the rocket is burning, have multiple burns with propellant production in between or you'll need to liquify the propellants.<br /><br />Finally my thoughts on LOX an LH2 long term storage. It seems NASA wants to launch cryocoolers (is this the correct term) on ever EDS. wouldn't it be just as easy for the EDS to dock with a module permanently on orbit that could provide the propellant refrigeration needed? This would probably lower the mass needed on every launch substatially. Again I don't know the anticipated mass of the coolers. <br /><br />The major techincal hurdle I see in this is the connection of fuel lines with the on orbit cryo cooler. I envision the cryo cooler being solar powered. Ion engines for station keeping. Ion engines could provide a long orbital life. Since the mass of a single cooler is likely less than capable lifting mass of the rocket it will go on. I was thinking Ares IV or V you could add redundency of a second backup cooler sharing power supplies and electronics.
I very much apologize for arguing from memory. It is not my custom, but I know without trying that digging into this would take way more time than I'm willing to invest. (As it turns out, I caught a clue while writing this . . .)<br /><br />This is weak, but here it is anyway: I remember that there was some special problem with neon. Do I remember what? Not really, but it all seems to do with grid erosion. IIRC all grids are eroded by all elements, it's just a matter of the rate. Neon caused higher erosion than Xenon IIRC.<br /><br />I can do this much to clarify my (shaky?) contention:<br />Atomic wt:<br />XE = 54<br />Ne = 10<br />My contention is that for a single particle<br />KE(neon) = KE(xenon)<br />1/2 * m(neon) * [V(neon)^2] = 1/2 * m(xenon) * [V(xenon)^2]<br /><br />[V(neon)^2] / [V(xenon)^2] = m(xenon)/m(neon)<br />[V(neon)] / [V(xenon)] = sqrt[m(xenon)/m(neon)]<br />V(neon) = sqrt[m(xenon)/m(neon)] * V(xenon)<br />V(neon) = sqrt[54/10] * V(xenon)<br />V(neon) = 2.324 * V(xenon)<br /><br />The Specific Impulse being directly proportional, if I'm right there would be a big advantage to neon <b>if we are trying to maximize specific impulse.</b> But the challenge with the technology is to increase the thrust, so the work is being done with the heavier element.<br /><br />AHA! That's it! I started this off by saying that "the preferred mass is as light as possible" but I should have specified <b>in terms of more specific impulse.</b><br /><br />So my error I guess, I wasn't wrong per se, but my focus was off the target, i.e. NASA uses Xenon in order to increase thrust. I'm not sure if I realized that before, actually. 
Sorry, I guess that's the part I was forgetting.<br /><br />Further, let's compare momentum between these identical setups with different gases, setting V(neon) equal to unity:<br />momentum = m * V<br />m(neon) * V(neon) = 10 * 1 = 10<br />and<br />m(xenon) * V(xenon) = 54 * [V(neon)/2.324] <br />= 54 / 2.324 = 23.24<br /><br />momentum(xenon) = 2.324 momentum(neon) <div class="Discussion_UserSignature"> </div>
Sorry, I guess that's the part I was forgetting.<br /><br />Further, let's compare momentum between these identical setups with different gases, setting V(neon) equal to unity:<br />momentum = m * V<br />m(neon) * V(neon) = 10 * 1 = 10<br />and<br />m(xenon) * V(xenon) = 54 * [V(neon)/2.324] <br />= 54 / 2.324 = 23.24<br /><br />momentum(xenon) = 2.324 momentum(neon) <div class="Discussion_UserSignature"> </div>much more techincally precise arguement of what I was attempting to state above, well done. My logic agrees with what you are saying.
xenon has a lower ionization energy than argon or krypton. Rember you have to ionize it AND accelerate it, so if the ionization takes more energy, you're thrust/watt goes down.
I think, when you consider other uses, not just as a propellant, the usefulness of water outweighs other substances.<br /><br />I also agree that gaseous Hydrogen and Oxygen would be unsuitable for propulsion needs and cryocoolers would be needed. I think in the long run being able to produce Hydrogen and Oxygen as needed would be more efficient and the cost of putting the equipment in LEO would be balanced out by the mass of cryogenic containers for long term storage as well as containers simply to get the products into orbit to begin with for other possible propellants.<br /><br />On the other side I can see it being a lot cheaper and requiring less weight to orbit to use fuel-cells and water instead of batteries to provide power to facilities. This type of system could easily use gaseous Hydrogen and Oxygen and the same supply could be drawn on to produce propellant in relatively small quantities as it is needed, eliminating the long term storage needs of cryogenic materials. The same scenerio would hold true for longer range transports, rather than contain LH2 and LOX to Mars and back the required amounts could be produced in-transit, reducing the storage requirements considerably.<br /><br />Water would also be needed for crew needs, the toilet thread here getting into that. Treating waste water to remove solids, breaking it down to Hydrogen and Oxygen and running it through fuel cells would be an ideal way to purify it for re-use.<br /><br />Since a fuel cell and hydrolizer are the same thing, just operating in reverse, they could be combined in stacks to deal with temperature considerations and built cheaply in high volumes. Add the need for radiation shielding during long missions, and that water provides an ideal medium, water is going to be essential anyway, and it would be more cost effective to use it in as many ways as possible.<br /><br />Other propellants being dicussed, before the ion digression, have draw-backs of their own, hydrocarbons would require LOX, pe <div class="Discussion_UserSignature"> </div>
Will, about pad disasters:<br /><br />Look up the...erm...colorful launch history of the Proton. <div class="Discussion_UserSignature"> </div>
Good example. The point, however, is to avoid one at KSC. <div class="Discussion_UserSignature"> <hr style="margin-top:0.5em;margin-bottom:0.5em" />Will Pittenger<hr style="margin-top:0.5em;margin-bottom:0.5em" />Add this user box to your Wikipedia User Page to show your support for the SDC forums: <div style="margin-left:1em">{{User:Will Pittenger/User Boxes/Space.com Account}}</div> </div>
"Although not as common as N2O4/UDMH, Hydrogen Peroxide/Kerosene is another storable propellant combo. While the Isp is lower than N2O4/UDMH, H2O2/Kerosene is non-toxic and therefore easier to handle."<br /><br />Sort of. The US Navy has investigated using various HTHP combinations as a 'green' subsitute for hypergolic systems, but hydrogen peroxide is not that good a choice for a long term space storable mono or bi-propellant. Aside from the mediocre ISP hydrogen peroxide provides, it has two other problems for use as a storable propellant.<br /><br />One problem is the high freezing temperature of hydrogen peroxide, sort of the opposite problem of liquid hydrogen storage. Keeping hydrogen peroxide tanks from freezing in a space environment could be a problem, particularly in the feed lines. Powered heating elements could be a necessity.<br /><br />The other problem is hydrogen peroxide will naturally break down over time, which is why stabilizers are sometimes added. The warmer the hydrogen peroxide the faster it breaks down.<br /><br />Hydrogen peroxide is interesting stuff, but I think it's future lies more as a 'green' propellant for ground launched boosters than as a long duration storable propellant for deep space spacecraft.
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