An Orbiting Atmospheric Gatherer

[keermalec]

Hm I work out the following characteristics for a 100 m2 atmospheric gatherer orbiting at 150 km altitude at 7820 m/s:<br /><br />Air liquifier (2.8 kg/hr. capacity, 310 W): 18 kg<br />LOX tank (200 kg capacity): 5 kg<br />LN tank: (800 kg capacity): 20 kg<br />Radiators: 29 kg<br />Solar panels (4000 W): 44 kg<br />Batteries (4000 W): 5 kg<br />Ion thruster (1.5 N thrust, 1800 W): 37 kg<br />Vacuum pump (2.8 kg/hr): 13 kg<br />Structure, avionics: 19 kg<br /><br />Total mass: 190 kg<br />Liquid air capacity: 1000 kg<br />Time to fill capacity: 360 hours<br />Nitrogen expenditure: 50 kg <br /><br />Production after 360 hours: 200 kg LOX, 750 kg LN (assuming the atmosphere is composed of 80% N2 and 20% O2 at this altitude).<br /> <div class="Discussion_UserSignature"> <p><em>“An error does not become a mistake until you refuse to correct it.” John F. Kennedy</em></p> </div>

 

[kelvinzero]

Whos back again?<br /><br />I didnt follow all your working keermalec but thought I would make a few comments.<br /><br />Probably most of the details of whether this can work depend on the vacuum pump you use. Paul Klinkman mentioned a mercury diffusion pump I think. The particles might mainly be charged, which could help.<br /><br />Drag factor and streamlining are not important. You are aiming for pretty much maximum drag. What is important is the ratio between the particles that impact and you collect, and the particles that impact and escape... also particles that escape by bouncing forwards may actually rob you of more forwards momentum than ones that you stop dead.<br /><br />Simplisticaly, it doesnt matter if what lies between the collector and the thruster weighs a gram or a ton. If the thruster provides more thrust than the collector creates drag, and expends less propellent than the collector collects, then it has a chance. Mass only matters when you start considering mass of launch compared to gas collected before it wears out.<br /><br />Also there might possibly be some way to cheat on the drag vs thrust equation. Maybe you could somehow collect only the slower moving (relative to the gatherer) particles. I dont think this exactly violates the second law of thermodynamics but I might be wrong.

 

[keermalec]

<blockquote><font class="small">In reply to:</font><hr /><p>If the thruster provides more thrust than the collector creates drag, and expends less propellent than the collector collects, then it has a chance.<p><hr /></p></p></blockquote><br />Exactly, that is what has to be demonstrated. The reason I threw some numbers together above is precisely to get a feel about where this might go. It looks promising but I just realised I forgot the power consumption of the vacuum pump: solar panels and batteries may have to be a bit more massive. Also, in order to have a 100 m2 absorption area, my 19 kg mass contingency for structure seems a bit weak.<br /><br />The numbers just a give a general impression that the project is feasible, but a lot of study and experimentation needs to be carried out, specifically in the following two fields:<br /><br />1. Nitrogen-propelled ion drive<br />2. Low density vacuum pump <div class="Discussion_UserSignature"> <p><em>“An error does not become a mistake until you refuse to correct it.” John F. Kennedy</em></p> </div>

 

[richalex]

<blockquote><font class="small">In reply to:</font><hr /><p>Probably most of the details of whether this can work depend on the vacuum pump you use. Paul Klinkman mentioned a mercury diffusion pump I think. The particles might mainly be charged, which could help.<p><hr /></p></p></blockquote>I am not an atmospheric scientist, but a bit of googling (and my class in vacuum technology) tells me a few things that might be of interest. <br /><br />Our atmosphere is homogeneous up to about 100 km, due to natural mixing. Past that point comes the turbopause, where atmospheric turbulent mixing ends, and then the thermosphere. This region is known as the heterosphere, because the composition of the atmosphere is stratified based on the molar weights of the components. The occurrence of nitrogen and oxygen decreases rapidly with altitude in the thermosphere. At some point, the diatomic molecules break down into atomic components, which are more corrosive than their diatomic forms. Helium and hydrogen become the dominant components of the atmosphere. So, if you are going to collect oxygen from Earth's atmosphere, you are probably going have to do it at a somewhat low altitude for an orbiting spacecraft, probably pretty close to 100 km altitude. Incidentally, ultra-high altitude balloons (zero-pressure design) have reached about 50 km altitude, with payloads up to 1000 kg. <br /><br />There are a few types of molecular pumps that can collect atmospheric components in the thermosphere. Three broad categories that I recall are cryogenic, diffusion and turbomolecular. I don't remember exactly the pressure regions in which they operate. <br /><br />A cryogenic vacuum pump operates by freezing any molecules that touch it. Schematically, it is a really cold block of metal to which molecules in a vacuum stick. <br /><br />A diffusion pump is like a jet that entrains molecules along its route, similar in operation to an eductor. One of the first used a jet that consisted of the heavy molecule mercury. A

 

[richalex]

I went to YouTube to see what they might have on vacuum pumps. I didn't have much luck with turbomolecular or diffusion pumps, but I found an interesting video on a cryogenic pump. <br /><br />Sicera Cryopump - Sumitomo Heavy Industries : DigInfo - I can't believe the specs on this thing; "The Sicera can pump 4,000 liters of water, 1,500 liters of air or 2,200 liters of hydrogen per second, and has a gas capacity for hydrogen of 12 liters." Wow! Things have sure changed since I took that vacuum technology class! <br /><br />

 

[nexium]

I re-read most of this thread. Most seems off the original topic and over my head. The Basard ramjet proposes to gather it's own fuel even in deep space, so at least one great mind thinks this is possible with nuclear fusion occuring in the ram jet. My guess is the compression of one microbar gas to several bar pressure takes considerable energy which would decay the orbit. Correction might produce negative net energy, instead of a useful result. Also much of what is gathered would be nitrogen, which is chalanging to use as a fuel. I have not heard of nitrogen being used as reaction mass for ion engines. Does nitrogen have a disavantage for use in ion engines and simular propulsion systems? Neil

 

[billslugg]

<font color="yellow">"The Sicera can pump 4,000 liters of water, 1,500 liters of air or 2,200 liters of hydrogen per second, and has a gas capacity for hydrogen of 12 liters."</font><br /><br />I think they might be referring to water vapor, not water. <div class="Discussion_UserSignature"> <p> </p><p> </p> </div>

 

[richalex]

<blockquote><font class="small">In reply to:</font><hr /><p>The Basard ramjet proposes to gather it's own fuel even in deep space, so at least one great mind thinks this is possible with nuclear fusion occuring in the ram jet. <p><hr /></p></p></blockquote>The Bussard ramjet has to travel at a large fraction of light speed, with a collector several square kilometers, to collect enough interstellar hydrogen to work, and it is really a different environment completely (besides that it might not even be a workable idea). <br /><br /><blockquote><font class="small">In reply to:</font><hr /><p>My guess is the compression of one microbar gas to several bar pressure takes considerable energy which would decay the orbit. <p><hr /></p></p></blockquote>The vehicle's orbit would decay only if the vehicle's orbital kinetic energy were used to compress (or deflect) the atoms or molecules in its path. But, that is not how this device would work, and I doubt it could work that way (because the atoms or molecules are too diffuse for the conversion to work on a device of human scale, and it would lose so much energy it would soon crash back to Earth if anyone did get it to work). Instead, the device must first collect the atoms (during which it would lose some kinetic energy), then compress with onboard pumps and compressors (which would get their energy from electricity, not kinetic energy). This is not a ramjet. <br /><br /><blockquote><font class="small">In reply to:</font><hr /><p>Correction might produce negative net fuel, instead of a useful result. Also much of what is gathered would be nitrogen, which is chalanging to use as a fuel. I have not heard of nitrogen being used as reaction mass for ion engines. Does nirogen have a disavantage for use in ion engines and simular propulsion systems?<p><hr /></p></p></blockquote>I would suppose that nitrogen would chemically react with the metal of the vehicle during the ionization process. <br /><br />As of now, the proposal is not to collect fuel, just oxidant. If they were

 

[richalex]

<blockquote><font class="small">In reply to:</font><hr /><p>I think they might be referring to water vapor, not water.<p><hr /></p></p></blockquote>They might be, but even that is a much higher rate than I learned in class, long ago. A cryogenic pump is classified as an ultra-high vacuum pump, but it is about the slowest pump invented. It takes (or took) days to reduce a high vacuum to an ultra-high vacuum, just for a small chamber. What is more, I was taught that water is a poison in this system; you want the chamber as dry as possible, because water will simply clog up the pump. Times have changed, technology has improved, demand forced advancement.

 

[billslugg]

When considering the volumes pumped by high vac pumps, it is always amazing to ponder the incredible volumes pumped until one considers that those volumes are mostly empty space. I was being knid in my reference to water. It most certainly, beyond any doubt, does not refer to water. More than a few tiny droplets of water in a rotor running at the speed a high vac turbine pump runs at would mean instant destruction. As for the water droplets being bad for a pump, not because they "clog" it, they simply out pace it. The pumps wants to deal with a bit of gas. As the pressure is lowered, the liquid droplets produce more and more and more gas. No end to it. Takes forever to pump down a vacuum system with any moisture in it. That is why the smart designer wraps the system with heat tape and runs it up to the highest possible temperature consistent with the lowest temp component in the system, for as long as possible, under as high a vacuum as possible. <div class="Discussion_UserSignature"> <p> </p><p> </p> </div>

 

[richalex]

IIRC, the cryogenic pumps would clog on water vapor because the water would form ice around the cold sink, which would insulate the sink and kill its efficiency.

 

[billslugg]

Well, yes. Then the cold sinks would be doing what they are supposed to do and they should be valved off, baked out, reinstalled and fired up again! <div class="Discussion_UserSignature"> <p> </p><p> </p> </div>

 

[richalex]

So, what do you suppose would happen if someone put one of these pumps (turbomolecular, diffusion or cryogenic) into LOE orbit, somewhere between 100 - 200 km altitude, and ran it with the intake opened to the outside? I suppose that single particles traveling at 28000 kph is not that big a deal, as single particles do that on Earth's surface.

 

[billslugg]

In low Earth orbit, at an altitude commensurate with an inlet pressure within the pump's rating, the output would be air at atmospheric pressure. Not much, but at atmospheric pressure. Erosion by wayward gas particles would be a problem as would the desire to de-orbit. <div class="Discussion_UserSignature"> <p> </p><p> </p> </div>

 

[kelvinzero]

One difference between the pump problem and the collector problem is that for the pump it is not too serious if particles often bounce back out of the pump. If 9 out of 10 bounce straight out it will just make the pump ten times slower.<br /><br />For the collector this is very serious because if it bounces off you, you have already payed its momentum penalty. if 9 out of ten bounce off you then you need to put ten times the momentum into the propellent, which implies a hundred times the energy.<br /><br />

 

[richalex]

<blockquote><font class="small">In reply to:</font><hr /><p>One difference between the pump problem and the collector problem is that for the pump it is not too serious if particles often bounce back out of the pump. If 9 out of 10 bounce straight out it will just make the pump ten times slower.<p><hr /></p></p></blockquote>People who want an UHV usually care about time, too, particularly in the semiconductor industry. <br /><br /><blockquote><font class="small">In reply to:</font><hr /><p>For the collector this is very serious because if it bounces off you, you have already payed its momentum penalty. if 9 out of ten bounce off you then you need to put ten times the momentum into the propellent, which implies a hundred times the energy.<p><hr /></p></p></blockquote>I doubt that very much momentum would be lost if ALL the particles bounce off the orbiter. There simply aren't that many particles. <br /><br />Anyway, another reason that I doubt that bounce would be a problem is that we capture particles at this speed in our vacuum chambers on Earth. Even if we did not, it should not be a very big design change to ensure that bounced particles did not exit.

 

[kelvinzero]

The penalty is huge. It isnt merely an efficiency factor but is key to whether it can work at all.<br /><br />To be of any use the gather must collect many kilograms of gas <i>and bring it up to orbital velocity</i>. That means it must put something like 8km/s velocity into whatever it collects. Ideally it would collect many many times its own mass over its lifetime. The only way it can do this and stay in orbit is some sort of electric propulsion. Most importantly it must expend less propellent than it collects. The propulsion does not need to be high thrust, just enough to counter the tiny drag of collecting the gas.<br /><br />You may be right about the second point.

 

[richalex]

<blockquote><font class="small">In reply to:</font><hr /><p>To be of any use the gather must collect many kilograms of gas and bring it up to orbital velocity. That means it must put something like 8km/s velocity into whatever it collects.<p><hr /></p></p></blockquote>That's a good point. But, it isn't going to accelerate all that mass at one time. It could take it days or weeks, maybe even months, to collect several kilograms of mass. <br /><br />F = m*a<br /><br />F = 1000 kg @ ((8 km/s)/86400 s) ~ 93 newtons<br /><br />So, assuming that the device collects 1000 kg of particles in a day, it will need to expend about 93 newtons to maintain its velocity. I'm too tired (and bad at math) to calculate how much current would have to flow through magnetic wires to supply that much force from Earth's geomagnetic field. I suppose that if the nitrogen collected were not needed, it could be heated into a gas and vented out the back; that might provide 93 newtons over the course of a day. The point is, the collector would not need a lot of power at one time for it to be successful.

 

[kelvinzero]

That is true. It doesnt necessarily need a lot of power.<br /><br />What is vital is the balance between momentum lost through impacts and momentum gained through thrust, and gas collected vs gas expelled.<br /><br />For every gram (moving at relative speed of 8km/s) that we collect, we have regain the lost momentum using thrust. We could expell 1 gram moving at 8km/s, but then we have wasted what we collected. Better would be to expell 0.5 grams at 16km/s. Thus we have maintained our momentum, but gained half a gram of mass, total.<br /><br />If any significant proportion of particles bounce off you have to expend just as much energy and propellent as if you collected them in just in order to stay in orbit. However their loss will cut into the propellent you have to do this with. <br /><br />Actually, particles that bounce forwards could in fact rob you of twice as much momentum as particles you collect because you have changed their velocity by twice the amount as if you had stopped them.

 

[richalex]

I came back to this post, wondering if anyone was going to take my calculations to task. If nothing else, they are only good to a first approximation; we would not be accelerating all 1,000 kg at one time. The actual force needed probably would require a solution from calculus, but if the collection were linear, simply taking half of the first approximation would be close to the correct value, I think. So, the actual force needed would only be 47 newtons, sustained during collection. <br /><br />The orbit also need not be circular, and it would probably be beneficial if it were not. I read on a website that orbits under 183 km are not stable, undoubtedly due to atmospheric drag. But, a collector would want to have as much of that atmosphere as it could get. So, maybe a good orbit would be a parigee of 100 km, with an apogee of 200 km? The collector could coast during the higher parts of its flight, recharge its batteries and whatever other housekeeping chores it needs to do, then take another dive through the thicker atmosphere, perhaps with its solar collectors (if it uses solar collectors) folded for lower drag. So, it could assume a low-drag/high thrust profile while collecting and then recharge its batteries the rest of the orbit. Of course, this would require more thrust than a simple circular orbit would, more than the 47 newtons I just provided, but probably less than the 93 newtons from the first approximation.

 

[eniac]

I agree with KelvinZero that what counts is the momentum balance between particles gathered and rejected. In fact, if your method of propulsion is an ion drive, your Isp needs to be much higher than orbital velocity, and even higher if there are losses. For example, if for every particle gathered you reflect 3 forward, you need an Isp at least seven times higher than orbital velocity, which is possible, but not easy. Electrodynamic tether propulsion is probably the better option.<br /><br />A few more thoughts:<br /><br />Since the gas to be pumped comes at you with super-thermal velocity, you have to really think about 1) the shape of your inlet, and 2) the temperature. All molecules come essentially from the same direction, so I would think a parabolic reflector (perhaps made from mylar) could be used to concentrate incoming particles in the focus area. There, the particles need to be captured and cooled, perhaps by an aerogel of some sort. Once they equilibrate to thermal velocities at a reasonable temperature, they can then by sucked into a pump inlet and the rest would be standard vacuum pumping. I think it is inevitable that most particles will escape during the cooling process, exacerbating the propulsion issue.<br /><br />Presumably the propulsion required for gathering would also be more than sufficient to maintain a dawn/dusk sun-synchronous orbit at ~100 km altitude, which would afford permanent sunlight, making batteries unnecessary. The solar panels would be arranged in a long line behind the reflector, which will shield them from the atmosphere. They would permanently face sideways towards the sun and would not need to be turned mechanically.<br /><br />The dawn/dusk solar synchronous orbit is one that is on a plane perpendicular to the direction of sunlight, and it avoids the Earth's shadow completely. It needs to precess once a year to keep facing the sun, which can be achieved through natural precession at 600-800 km altitude. At 100 km, some propulsion is <div class="Discussion_UserSignature"> </div>

 

[kelvinzero]

Hi Alex, sorry about that.<br /><br />I dug up an old physics book. I think I will do the calculation using impulse.<br /><br />note:<br />v = v0 + at<br />== /> m*v - m*v0 = (m*a)*t<br />== /> Change in momentum = F*t = impulse = J<br /><br />The total change in momentum (ie impulse) of bringing 1000kg up to velocity 8000m/s is m*v1 - m*v0<br />J = 1000kg * (8000m/s - 0km/s)<br />J = 8,000,000 newton seconds. <br /><br />Using impulse J = F*t and given the time given was one day,<br />F = J/t = 8,000,000Ns/86400s = 92.6 Newtons.<br /><br />So we agree there.<br /><br />I would have to think a bit more about power requirements but the main factor in this is how much propellent you have to throw away since momentum is propotional to velocity but energy is proportional to velocity squared. So it still comes down to the efficiency of your collector.<br /><br />One thing that occured to me is that your collector could be as simple as a waxy material that particles embed themselves in. Every now and again you could take the panels inside and purify them. Even without the purifying stage there could be an immediate benifit if the material prevented elasitic collisions that sap up to twice as much momentum. <br /><br />(edit)<br />I wonder if something like the 'ionocraft' could be used to keep something like our gatherer in orbit without any propellent cost. You apply force directly to the ions as they flow by. <br />http://en.wikipedia.org/wiki/Ionocraft

 

[richalex]

Hi, KelvinZero (would you mind if I called you KZ?), <br /><br />I was thinking more about this design as I took my shower this "morning" (which is actually afternoon, as I work nights). I am fairly certain that this device would only collect a few grams an orbit at most, and the orbital period would be about 90 minutes, give or take, so it would complete about 16 orbits each day. That doesn't matter terribly much regarding the collection itself, as we could let the collector orbit for years. I don't know how fast it must gather oxygen to reach economic break-even. As far as our design is concerned, it is easier to compensate for deceleration if the collection at a low rate. Of course, we probably want the highest rate we can get, as that is our ROI!<br /><br />So, we launch this into orbit. It probably can collect more particles at 100 km than at a stable orbit of 183 km, so we would like to find a way for it to orbit at 100 km, at least some of the time. But, the craft that is going to use the bottled particles probably isn't going to stop at a 100 km orbit to p/u supplies. I'm guessing that the customer is going want to pick up supplies inside a stable orbit, meaning, above 183 km. This is another reason for the collector to have an elliptical orbit. <br /><br />So, the collector spends 15 minutes or so of each orbit at 100 km altitude, firing its engines to maintain velocity. This is still a high vacuum region, so gas laws don't exactly apply; the atmosphere acts like particles. Then, the collector swings up to, say, 200 km altitude, where a waiting craft wants its supplies. The collector could either fire its engines to come into a circular orbit at 200 km, or the waiting craft could grab the collector and bring it up to velocity (quite a jolt, I would expect), or both. Either that, or the waiting craft would have to grab the supplies from the collector in the brief moment the two meet. Timing would be important in any event. <br /><br />I imagine that very strong grap

 

[richalex]

<blockquote><font class="small">In reply to:</font><hr /><p>Once they equilibrate to thermal velocities at a reasonable temperature, they can then by sucked into a pump inlet and the rest would be standard vacuum pumping.<p><hr /></p></p></blockquote>I doubt there would be enough particles or enough ambient pressure for a pump to suck particles that collected on their own from the outside. This entire design is probably going to operate at ultra-high to high vacuum until nearly the final stage of compression/liquification. That means that all the collecting activity is going to deal with particles, not fluids.

 

[kelvinzero]

You could be right about the cryogenic thing. I was imaginining a very thin deep grill so that any particle entering would bounce thousands of times before it could escape. An elliptical orbit could help if you have two stages: gathering and then sealing and warming to extract the frost?<br /><br />I dont know why the ISS orbits at the height it does. I imagine it is a compromise between being too high making it too expensive to shuttle to, and being too low making it too hard to keep in orbit. It occurs to me that if you can keep something in a very low orbit indefinitely because it collects enough propellent and power to sustain itself AND has the additional benifit of storing propellent... in that case perhaps there is no reason to orbit anything at the ISS height.<br /><br />Instead. craft launched from earth could dock with a large gatherer/station in very low orbit. In its wake they would not need to pay any additional drag penalty. If they want to go higher, eg geostationary or the moon they tank up at the gatherer/station and off they go. <br /><br />There was also some sort of DoD experiment with refueling drones of some kind.