An Orbiting Atmospheric Gatherer

[nexium]

Until recently, ion engines have been very inefficient, as have other devices such as electrodynamic tethers. As you suggest, it may soon be practical to counter friction losses in orbits that dip as low as 90 kilometers. A net energy gain however may still be beyond reach. Compressor efficiency is not high. Neil

 

[eniac]

"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."<br /><br />That could work. A liquid film also comes to mind, which could permit continuous operation. However, you would have to ensure that 1) the material does not by itself outgas more than it takes in, and 2) that impacting molecules get absorbed without knocking any other molecules loose. Either of these two things could well not be possible.<br /><br />Andreas<br /> <div class="Discussion_UserSignature"> </div>

 

[richalex]

If it turns out that incoming particles would bounce off the cryogenic plates instead of sticking to them (as I think they would do), I think another type of entrance would work. It would be shaped like a long, tapered cone, going down to a hole that then opens into a wider space. The shape of the cone is such that particles striking the sides would bounce down along the length of the cone, until they reached the hole at the end. The particles would enter the hole into a wider space, to reduce the chances of their exiting back out the hole. The UHV pumps would collect the particles from there.

 

[eniac]

"It would be shaped like a long, tapered cone, going down to a hole that then opens into a wider space. The shape of the cone is such that particles striking the sides would bounce down along the length of the cone, until they reached the hole at the end. The particles would enter the hole into a wider space, to reduce the chances of their exiting back out the hole. The UHV pumps would collect the particles from there."<br /><br />That cone would be the parabolic reflector I had in mind. A long thin parabola with a hole where the point would be. A large cavity could work, but I think it would be even better if there was a lot of stuff to bounce off of inside, which made me think aerogel.<br /><br />Andreas <div class="Discussion_UserSignature"> </div>

 

[richalex]

<blockquote><font class="small">In reply to:</font><hr /><p>That cone would be the parabolic reflector I had in mind. A long thin parabola with a hole where the point would be.<p><hr /></p></p></blockquote>When you described it previously, you said the particles would be gathered at the focus, which is not the same location as the point. It sounded like you would be attempting to reflect the particles similar to reflecting light in a solar collector, instead of funneling particles down to the end. <br /><br /><blockquote><font class="small">In reply to:</font><hr /><p>A large cavity could work, but I think it would be even better if there was a lot of stuff to bounce off of inside, which made me think aerogel.<p><hr /></p></p></blockquote>I really don't see that as necessary, because these particles don't have very much momentum, despite their velocity. As I wrote previously, air particles at sea level travel at speeds similar to what the collector would encounter in orbit. The air you are breathing consists of particles traveling at an average velocity of about 1800 kmh (lighter atoms, such as helium, travel at faster velocities, and hotter particles travel faster than colder particles). They don't travel very far in one direction at sea level, because they bump into each other frequently. Up at high altitude, above 500 km, some of these particles are traveling in the right direction and with enough velocity that they escape Earth's gravitational influence. So, you really aren't going to slow them down appreciably by bouncing them around inside a container. You just want to get them in an area where they can be cooled (which does slow them).

 

[eniac]

<blockquote><font class="small">In reply to:</font><hr /><p>When you described it previously, you said the particles would be gathered at the focus, which is not the same location as the point.<br /><p><hr /></p></p></blockquote>With a paraboloid, particles will always be reflected to the focus. Usually people envision satellite dishes when they hear paraboloid, but the upper rim of a paraboloid will focus just as well, with glancing reflections, and with the focus behind the reflector rather than in front of it. X-ray telescopes work that way. It looks and works much like a funnel, so we are really talking the same thing, except I am suggesting a more specific optimal shape.<br /><br /><blockquote><font class="small">In reply to:</font><hr /><p>I really don't see that as necessary, because these particles don't have very much momentum, despite their velocity. As I wrote previously, air particles at sea level travel at speeds similar to what the collector would encounter in orbit. The air you are breathing consists of particles traveling at an average velocity of about 1800 kmh (lighter atoms, such as helium, travel at faster velocities, and hotter particles travel faster than colder particles). They don't travel very far in one direction at sea level, because they bump into each other frequently. Up at high altitude, above 500 km, some of these particles are traveling in the right direction and with enough velocity that they escape Earth's gravitational influence. So, you really aren't going to slow them down appreciably by bouncing them around inside a container. You just want to get them in an area where they can be cooled (which does slow them).<br /><p><hr /></p></p></blockquote>Orbital velocity is WAY faster than thermal, as you know, judging from the number you cite. The temperature that corresponds to orbital velocity ranges around 100,000 K, if I recall correctly. The only way to slow down/cool the particles is to have them bounce into something relatively colder (1000 K will do fine. <div class="Discussion_UserSignature"> </div>

 

[kelvinzero]

<font color="yellow">The only way to slow down/cool the particles is to have them bounce into something relatively colder </font><br /><br />An important exception to this is the MHD generator<br />

 

[richalex]

<blockquote><font class="small">In reply to:</font><hr /><p>With a paraboloid, particles will always be reflected to the focus. Usually people envision satellite dishes when they hear paraboloid, but the upper rim of a paraboloid will focus just as well, with glancing reflections, and with the focus behind the reflector rather than in front of it.<p><hr /></p></p></blockquote>What would be the point of focusing the incoming particles? The shape I suggest is elongated enough to ensure that particles go through the hole at the end, but it does not matter if the particles reach a focal point or not. Once the particles have passed through the hole and into the chamber at the end, they can bounce around until they are captured; if the hole is much smaller than the chamber, the particles probably would not bounce out. <br /><br />Actually, I expect that any energetic particles striking any parts of the orbiter would embed themselves in the skin of the surface they strike. And, perhaps a better pump than the cryogenic pump would be an ion pump, which uses this very mechanism to collect particles in an ultra high vacuum.

 

[eniac]

<blockquote><font class="small">In reply to:</font><hr /><p><br />What would be the point of focusing the incoming particles? The shape I suggest is elongated enough to ensure that particles go through the hole at the end, but it does not matter if the particles reach a focal point or not. <br /><p><hr /></p></p></blockquote>The point is to ensure the particles actually all reach the hole at the end. A conical shape, for example, would have many particles bounce multiple times and eventually send them back out at the front, depending on the size of the hole. Try catching a rubber ball with a funnel, and you will see what I mean.<br /><blockquote><font class="small">In reply to:</font><hr /><p><br />Once the particles have passed through the hole and into the chamber at the end, they can bounce around until they are captured; if the hole is much smaller than the chamber, the particles probably would not bounce out.<br /><p><hr /></p></p></blockquote>Sure, but if the hole is too small it will be hard to get the particles in it in the first place, and if the chamber is as large as the funnel opening, you haven't really concentrated them at all and might as well pick them up directly without funnel. And why use a large chamber if a small piece of foam can do a better job.<br /><blockquote><font class="small">In reply to:</font><hr /><p>I expect that any energetic particles striking any parts of the orbiter would embed themselves in the skin of the surface they strike.<br /><p><hr /></p></p></blockquote>If that were true, the whole funnel idea would be moot. However, regular air molecules here on Earth bounce off surfaces quite reliably, and it is not clear to me that that should change just because they are faster in orbit. <br /><br />I'd be more worried that each impacting particle might knock loose one or more atoms from the surface. That is called sputtering, and it would obviously destroy the economics of gathering. I once looked at that a while ago and found that at orbital velocity sputtering is avoidable w <div class="Discussion_UserSignature"> </div>

 

[eniac]

<blockquote><font class="small">In reply to:</font><hr /><p><br />And, perhaps a better pump than the cryogenic pump would be an ion pump, which uses this very mechanism to collect particles in an ultra high vacuum.<br /><p><hr /></p></p></blockquote>Both the cryogenic and the ion pumps work by capturing molecules, and have no outlet. That is clearly a disadvantage if the pumped gas is what you are interested in. The ion pump in particular buries the pumped gas in titanium ( http://www.thermionics.com/ip_too.htm ), from which it would be difficult to extract without wasting more titanium than there is gas to be harvested.<br /><br />Wikipedia ( http://en.wikipedia.org/wiki/Vacuum_pump ) lists three classes of pumps: displacement, molecular, and entrapment. I think only the molecular pumps are useful in this context. These are again classified into diffusion and turbomolecular pumps, of which the former will be problematic because working fluid losses would tend to offset any gain from the gathering.<br /><br />Thus, my bet is on turbomolecular pumps.<br /><br />Andreas<br /> <div class="Discussion_UserSignature"> </div>

 

[eniac]

<blockquote><font class="small">In reply to:</font><hr /><p><br />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.<br /><p><hr /></p></p></blockquote><br />This is a good point. After being cooled down to the thermal range (whether chamber, gel, foam, or whatever, let's call it the stopper), the particles will tend to scatter in all directions, and only some will make their way into the vacuum pump.<br /><br />Perhaps a stopper can be designed that will let the particles penetrate for, say, 1 cm, before they are equilibrated. If the stopper is just over 1 cm thick, most particles will emerge on the other side, and very few will make it back all the way to escape, now that they no longer have the orbital momentum. Alternatively, perhaps the particles can be focussed by a reflector/funnel directly onto the blades of a turbomolecular pump, to be equilibrated and compressed at the same time.<br /><br />Another thought: Liquefaction could possibly be achieved cheaply by radiative cooling. Take a very long, thin pipe, and arrange it to meander back and forth many times behind the solar cells. Make sure it is shaded from all heat sources (sun, Earth, solar cells, other spacecraft parts) by reflectors. Make it black. It should then tend towards radiation equilibrium with empty space, which would be very cold indeed. Cold enough to condense the scooped up air. True, radiation cooling is slow at low temperatures, but the gas flow is low, too, so it just might work out. The different components (e.g. Oxygen and Nitrogen) would condense at different positions in the pipe and could be readily extracted in pure form.<br /><br />Andreas<br /> <div class="Discussion_UserSignature"> </div>

 

[keermalec]

<blockquote><font class="small">In reply to:</font><hr /><p>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 required, I do not know how much. <p><hr /></p></p></blockquote><br /><br />I would guess that precession can be easilly accomplished using rudders to divert the flow of high-atmosphere particlers around the vehicle. Solar panels must be perpendicular to the direction of sunlight, ie in the orbital plane. Therefore giving a slight, controlled angle to the solar panels could accomplish this. No need for propulsion. <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>