Random Thoughts: Atmospheric Scooper?

I only have a few minutes before bedtime, but I got a lot of thesis stuff today, and since I was a good boy, I’m going to blog.

This was a quick idea mentioned by a commenter on one of my previous posts. It’s crazy, and has a good chance of not working, but was rather clever nonetheless. Basically the idea is to have an atmospheric collector mounted on a station in LEO. Basically, even though the atmosphere at say 400km is really thin, a 1 m^2 collector area travelling at a relative velocity of 29000km/hr will have a flow rate of roughly .03 km^3 / hr flowing into it. At that rate even a minuscule density can add up. The inherent drag at that altitude is also low enough to be possibly handleable by an electrodynamic tether.

The commenter claims that over the course of a year, such a system could collect nearly 7.8 tons of oxygen, as well as ~1 ton of water, 0.7 tons of nitrogen, and 0.5 tons of helium. It would need to overcome a constant drag of ~2N, which is well within what an ED tether could handle. For comparison, the frontal area of a Sundancer module is going to be about 28m^2 by my estimation. If this commenter’s claims are even remotely close, that would amount to nearly 220mT of LOX per year. That’s enough to refill the LOX tanks on a Centaur stage once per month. It would need a decent ED tether to counteract the drag, but the gravity gradient stabilization it would provide would also help with fluid settling. Interesting no?

An interesting point the commenter missed is that with how hard helium is to get, it might actually end up being almost as valuable as the LOX. Many stages use helium for lox pressurization and purges. Being able to top up on that in orbit would be helpful as well.

Anyhow, I have to wrap up in the next minute or so, so I want to mention the potential problems I see with this idea before I go. All of those numbers assume that you’re getting 100% efficiency of collection–ie that none of the molecules that hit your collector can escape. I’m not sure how real that is. And if turns out that you can only get a relatively low efficiency, it might not make the idea worthwhile. Also molecular pumps like that are rather power hungry. I’m not sure if a Sundancer module would actually be able to pump out enough power to run both the pumps as well as the ED tether. And who knows how well the thing will hold up in orbit.

But there’s something rather nifty about the idea of on-orbit ISRU propellant gathering isn’t there?

[Update: the volume flow error was mine, the original commenter had only specified the velocity. So I fixed and clarified it. That’s what I get for trying to write a post in 10 minutes before bed time. Also the group working on the idea is at Worcester Polytechnic Institute, under a Dr John Wilkes–supposedly they’ll be presenting more info in the near future at some conferences]

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Jonathan Goff

Jonathan Goff

President/CEO at Altius Space Machines
Jonathan Goff is a space technologist, inventor, and serial space entrepreneur who created the Selenian Boondocks blog. Jon was a co-founder of Masten Space Systems, and is the founder and CEO of Altius Space Machines, a space robotics startup in Broomfield, CO. His family includes his wife, Tiffany, and five boys: Jarom (deceased), Jonathan, James, Peter, and Andrew. Jon has a BS in Manufacturing Engineering (1999) and an MS in Mechanical Engineering (2007) from Brigham Young University, and served an LDS proselytizing mission in Olongapo, Philippines from 2000-2002.
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28 Responses to Random Thoughts: Atmospheric Scooper?

  1. Karl says:

    Sounds like a cool idea.

    I was surprised at the amount of helium available. Hmmm, I find that helium is supposedly set at about $7 per kilogram. So you have a process that in theory harvests $3,500 worth of helium (if it somehow found its way to Earth) per year. Could be interesting if for some reason the price of helium went up and launch costs went way down. You might even be able to make some sort of inflatable tank that you could drop from orbit without parachutes (just make it modestly denser than air).

    Of course, the in situ resource utilization is by far the more important use. But that is a potential value-add for this technology.

  2. john hare says:

    >>This was a quick idea mentioned by a commenter on one of my previous posts. It’s crazy, and has a good chance of not working, but was rather clever nonetheless. Basically the idea is to have an atmospheric collector mounted on a station in LEO. Basically, even though the atmosphere at say 400km is really thin, a 1 m^2 collector area will have a flow rate of roughly 29 km^3 / hr flowing into it. At that rate even a minuscule density can add up. The inherent drag at that altitude is also low enough to be possibly handleable by an electrodynamic tether.< <<< My BOTE gives flow rate 1/1,000 of the orriginal commenter. Could somebody check me on that?

  3. Anonymous says:

    I can only check it if you post your working.

  4. Christine says:

    I get 1m^2 * 8000m/s * 3600s / 1000^3
    = 0.0288km^3/h.

  5. Jon Goff says:

    John, Christine,
    The volume flow error was mine, not his. He had stated a velocity, and I screwed up the calculation. Guess it happens when you’re trying to race to get a post in before bedtime. I’m not sure if his other numbers are correct though. I’m getting closer to 26000km/hr at that altitude, which would be ~.026km^3/hr, or about 230km^3 per year. Is there 10 tonnes of gas in 230km^3 at that altitude? That would imply 1 tonne per 23km^3, or about 43kg/km^3. Which would yield a density of about 0.000000043kg/m^3. Is that even in the right order of magnitude?

    ~Jon

  6. Josh says:

    someone’s been running part of this discussion on Uplink:

    http://uplink.space.com/showflat.php?Cat=&Board=businesstech&Number=704268&page=0&view=collapsed&sb=5&o=0&fpart=3#Post711341

    My take on it is somewhat limited interest. If we could put gas-collectors on space stations (or depots), that would be excellent. I’m looking forward to better numbers about how much material could be harvested from operating LEO altitudes. For a tether-powered, dedicated harvester, it seems like a craft that flew ellipses (maybe HEEO then ED tethering to L1) to dip the collector into denser atmosphere would provide more material, much faster. If 400km produces X amount of material, shouldn’t a pass through 100km produce about x^8 gas collected? The harvester would make regular passes through denser atmosphere, then spend the outward parts of the orbit compressing/refining the gas? The craft would ED tether to L1 when full for servicing and de-tanking. Starting on a Bigelow module is an interesting idea, especially for OX collection. One last question, is there a way to make the gas self-compressing at these speeds, via intake geometry?

    If it works in some form, it will be a wonderful enabling technology.

  7. Jon Goff says:

    Josh,
    I don’t know enough about ED tether technology to say for sure, but I think that the ED tether is most effective at lower altitudes. Which means that I’m not sure that your elliptical idea would actually work. But maybe someone here has more info?

    I doubt you could make the gas “self-compressing”. It’s going to be a non-continuum (is that the right term?) fluid due to the really low density, so it doesn’t behave like you would think a gas behaves. It behaves like a collection of particles, not like a continuous fluid.

    But any way you slice it, if this idea pans out, you could be talking about being able to support dozens or hundreds of exo-LEO flights per year…

    ~Jon

  8. Josh says:

    Thrusting tethers need an external power source, solar or nuclear, so you’ll have extra power sometimes.

    Thanks for the info on low density gases, it almost sounds similar to a plasma. My base assumption is that the relative velocity difference would cause the gas to bounce away from the collector.

    I’m somewhat skeptical of the concept, but open to it.

  9. Jon Goff says:

    Josh,
    My main concern is what you mentioned–that the gas particles would just bounce away before they could be collected. I’m not sure how he plans to deal with that.

    ~Jon

  10. Anonymous says:

    This is not a wholly new idea. A variant of the concept was called PROFAC, and was I believe dropped after a fair amount of study. Using tethers to counteract drag may be a new twist that addresses previous issues. For more information see:-

    http://www.bisbos.com/rocketscience/spacecraft/profac/profac.html

  11. Simon H says:

    Congratulations on getting your thesis in!!!

  12. Jon Goff says:

    Simon H,
    Alas, the thesis isn’t done yet. All that post was showing was that I finished one of the ugliest models in the thesis. I still have a lot to wrap up in the next 2-4 weeks.

    ~Jon

  13. Mike Puckett says:

    http://www.bisbos.com/rocketscience/spacecraft/profac/profac.html
    Profac, with enough juice, this could make a depot work well. Problem is, solar woud be too much of a drag. Nuke would scare the sheep.

    Could beamed power work?

  14. Anonymous says:

    Try a Bussard fusion power reactor.

    http://www.askmar.com/Fusion.html

  15. Stellvia says:

    … as featured in the Niven/Pournelle novel ‘Fallen Angels’, which postulates using spaceplanes equipped with ramscoops ‘dipping’ into the upper atmosphere to obtain volatiles.

  16. BD says:

    PROFAC once again: http://www.bisbos.com/rocketscience
    /spacecraft/profac/profac.html

  17. mz says:

    Please, don’t you start using milliteslas for mass too.
    SI system is internally rigorously consistent and that breaks it.

    (You could use t which means 1000 kg in SI associated units instead.)

  18. Bob Steinke says:

    This reminded me of an idea I heard about a while ago called plasma windows. The idea is that you want to have a pressurized volume with a hole in it open to vacuum. So you put a magnetically confined plasma over the hole. You make the magnetic confinement leaky on the inside and as leak-proof as you can on the outside. Any neutral gas that tries to escape through the window gets thermalized with the plasma, which ionizes it, and then it’s trapped in the magnetic confinement. Eventually it leaks back inside where you want it.

    You could use a plasma window as a molecule collector. It would probably be limited to small sizes. maybe a very long narrow angle scoop with a plasma window at the apex could increase the collecting area without bouncing the molecules out. And I don’t know if the energy costs would make it uneconomical.

  19. Rick Boozer says:

    The problem with particles bouncing off is easy to solve. Use the bounce to your advantage! Just have a flat smooth hard surface (it needs to be dense as well to reflect atoms) that is tilted 45 degrees relative to the direction that the station is moving. This will cause the particles to bounce off at a ninety degree angle toward whatever receptical you wish to direct it to.

  20. Rick Boozer says:

    The URL associated with my name in the above comment did not register properly. Hopefully it is on this one.

  21. Rick Boozer says:

    I think I should have supplied more details in my “bounce off” scheme. The bouncing of the atoms and/or molecules off of the plate would rob these particles of a lot of their kinetic energy, energy that the plate would radiate away as heat. Since the particles would be going at a slower speed into the recepticle, there would be less of the particles bouncing out. There are solids such as metal hydrides that can absorb enormous amounts of gas into their crystal matrix. This gas can be released via the application of heat to the solid. Such metal hydrides are being investigated as fuel tank reservoirs for hydrogen powered vehicles that would alleviate the need to store hydrogen as a pressurized gas. So you could have the space station’s gas receptical tank filled with a similar substance.

  22. Zaydana says:

    If an ED Tether wouldn’t work for an elliptical orbit, maybe you could feed the collected helium into some sort of electric propulsion system?

  23. Christine says:

    Please, don’t you start using milliteslas for mass too.

    Reading engineering forums and seeing yanks use milliteslas (mT) makes me wish that the powers that be had included a bitchslap function into the tcp/ip specification. What’s even worse is when I read people giving temperatures in Rankine units without specification.

    Blatant abuse of SI has destroyed spaceprobes, and it’s only a matter of time before this idiocy destroys a manned vehicle.

  24. Christine says:

    I think I should have supplied more details in my “bounce off” scheme. The bouncing of the atoms and/or molecules off of the plate would rob these particles of a lot of their kinetic energy

    The problem is that the collisions are elastic. Each mv that bounces off the collector is two mv that is busy trying to crash your collector into Perth.

  25. Rick Boozer says:

    It is my understanding that collisions between gas molecules (such as air molecules) are almost perfectly elastic, but collisions between gas molecules and solids are less so. After all, hot air can heat up a solid object and would not do so if some of the gas molecules’ kinetic energy was not being tranferred to the solid and manifesting itself as heat.

    The collector need not “crash into Perth” because the gas is very rarified in near-earth orbit, indeed a better vaccuum than we can achieve anywhere on Earth, and so the relatively miniscule drag forces on the collector could be counteracted by a tether altitude control system as mentioned in John Hare’s post.

  26. Rick Boozer says:

    Oh, sorry, I forgot to address the issue of the second of the “two mv” mentioned, which would be the cumulative force of the bouncing of the particles in a perpendicular direction. This force should also result in an extremely small delta V (smaller than the first because of the K.E. loss at the plate) and thus should not add a great amount of extra work for the tether control system.

  27. john hare says:

    >>>The collector need not “crash into Perth” because the gas is very rarified in near-earth orbit, indeed a better vaccuum than we can achieve anywhere on Earth, and so the relatively miniscule drag forces on the collector could be counteracted by a tether altitude control system as mentioned in John Hare’s post.< <<<< Josh’s post. I wish it was mine.

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