Author Topic: Scooping atmospheric air (PROFAC revisited)  (Read 66348 times)

Offline Robotbeat

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Re: Scooping atmospheric air (PROFAC revisited)
« Reply #120 on: 01/06/2015 03:40 am »
I think that a power cable (or possibly beamed power, as much as I think it's usually the completely wrong answer) from higher orbit could improve this significantly, as others have stated.
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Offline Asteroza

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Re: Scooping atmospheric air (PROFAC revisited)
« Reply #121 on: 01/06/2015 07:34 am »
The study seemed to go after monolithic designs like the original PROFAC, so tethered scoop air trawlers where the power system mass itself is up high changes the parameters a lot (tether drag becomes a non-trivial part of the problem too). Offloading power, via beamed sources (terrestrial beaming from ground stations, as well as higher orbit SPS) and whether you go microwave or laser-PV will matter a fair bit too.

I still have a soft spot for a solar cell tape tether style scoop in SSO though for a self contained system, especially if the primary storage at the scoop is small and much larger secondary storage (the actual depot) is maintained at the other end of the tether, for use as tether counterweight mass, to improve scoop dipping depth.

Offline RobLynn

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Re: Scooping atmospheric air (PROFAC revisited)
« Reply #122 on: 01/06/2015 10:14 am »
Tether drag reduces quickly with altitude/density (scale height at low altitudes is 5-6km is that also true at 120km?), and can be mitigated by increasing scoop area and increasing altitude.

2 tethers is optimal for redundancy and drag minimisation and surface erosion (3 or more has greater frontal area for same weight capacity with 1 strand failed), or perhaps even 1 with an emergency boost system on the scoop.  Though at higher altitude where drag isn't an issue the optimal number increases (to save mass).

The debris/micrometeoroid flux probably reduces greatly at the altitudes a scoop operates at due to drag deorbiting it.

A tethered scoop might also serve as a docking and release station for LEO transports to reduce launch+de-orbit deltaV requirements by up to a few hundred m/s.  Accumulated stored liquids at the top station can be used to create the necessary counterbalance mass to gradually raise the centre of mass and so reduce the scoop speed as well as improving the radiation shielding for those onboard.

Could use collected gases as part of structural materials for space exploration.  Oxygen is half of mass of alumina that can also be used monolithically or to make high strength fibers for aluminum alumina MMCs.

Nitrogen is almost half of mass of Silicon Nitride which can make extremely strong monolithic and fiber materials.  Nitrogen is also 67% by mass component of melamine - that can be used as a matrix for the ceramic fibers.
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Offline AlanSE

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Re: Scooping atmospheric air (PROFAC revisited)
« Reply #123 on: 01/06/2015 04:08 pm »
Correct about the density effect over the tether length. The characteristic length is more like 8.4 km, and this holds strong until about 150 to 200 km in altitude. After that, the drop is much slower. Using the data I posted on Space Stack Exchange about this, the characteristic height is more like 40 km in that regime. As long as you're dangling the tether below that elbow in density, almost all of the drag will be from the very bottom of it.

However, trying to characterize the main effect is still tremendously frustrating. The author claims to get exponentially poor performance in payoff time for the investment. Many of the designs could only collect micrograms over an entire year. It would take 1,000s of years to collect enough propellant to make up for the payload the system took up initially. But why?

The explanation given is that the minimum altitude the system can operate at is very high, and the low density there accounts for the poor mass collection rate. But then why is the altitude constrained so much? Getting the answer to this was frustrating. Fig 82, for instance mentions the area ratio for closure. That area ratio is the ratio between the collection area and the solar panel area. In the case of the nuclear engine, it's the radiator area. The "closure" condition is clearly a matter of sufficiency of power and thrust for the drag under those conditions. But this doesn't answer the question. What causes the drag to blow up? If it was the collection area, the nuclear and solar cases would be the same aside from different required areas.

The answer appears to be skin friction, but this isn't clearly offered in quantitative terms anywhere in the paper. In the equations, we have a separation of the two drag factors, the body versus "planform", where planform is the solar panels or radiator. Figure 28 breaks down these two components of the drag coefficient, but only for a specific case. I don't even know what this case is.

You would think that it would be resolved by Fig 30, as it plots a frontal-to-skin drag ratio, but this "front" isn't the right front. This is the front of the solar panels (not collector area), and it's merely setting the scale at which some assumptions are valid. Area ratio (of the panel area to collector area) is still referenced in later analysis, but this is highly derivative work giving sensitivity ratios, leaving the underlying mechanism unclear.

The closest to a direct acknowledgement of the limiting mechanism I can find is:

Quote
At one, the only contribution to the body drag coefficient is from skin friction.

I think this should read "At one point". This is to say that when we push the area ratio to the reasonable limits, almost all of the drag is from skin friction. If you want to get to the heart of the design problem, this is what we should say. Any reasonable person would reject a design off-hand when almost none of the losses are going to head-on collisions with the molecules, or to collection of those molecules. All skin friction is wasted momentum.

But as I understand the optimization process, they essentially place the collector at a location where you're bottoming out due to skin friction. In order to reduce the area of the solar panels (relative to the collection area), you have to decrease the flux of molecules into the collector by using a higher altitude. This, in turn, runs up on the mass constraint because its mass is proportional to its area.

Ways around this
- Use some hyper-light materials for the collector to get more area per unit mass (at a relatively high altitude)
- Use the tether for power delivery (also fixes most issues with heat rejection)
- Configure the solar panels to not have their own independent source of skin friction

The 3rd one is actually the most obvious solution from the approach the paper comes at it IMO. He mentions once that the panels could be placed inside of the collector's mouth, and waves it off as only slightly changing the y-intercept of one of his graph.

But that's not the most obvious solution to counter the problem of skin friction, which is far and away the limiting constraint in the paper. Skin friction comes from the area demands of the solar panels combined with molecular movement perpendicular to the direction of motion. If this was the problem you were trying to solve, you would align the gas collimators to pick up those molecules, instead of having them bouncing off and sucking up your momentum.

Actually, you might even be able to do that. Put the collection area on the side of the craft so that the upward-facing surface hosts the solar panels and also collects gas. These collimators would look somewhat like a sawtooth pattern from the side. This would be slightly worse than the front-facing collimators because the gas molecule velocity distribution is more random, since the flux comes from thermal movement.

Or if you didn't want to bother with a two-function surface, you could angle the solar panels so that you pull nearly a complete vacuum on it. In that case, there is effectively no skin friction from the solar panels. As I understand it, this possibility isn't included in the concept. This would be like a big right-triangle wedge.

That would get rid of the skin friction within a certain ratio of collection to panel area. This is possible because orbital velocity is so much higher than molecular velocity, and the slope would be low. Since it only buys you a constant geometrical factor (maybe 10, maybe 15, I'm not sure), it might still not be enough to get the payoff time to within a human life.

Actually, it might make more sense to have an isosceles triangle shape (side profile) where the two long legs are the solar panel surface and the downward-facing surface (no function). Then the tiny side, facing in the direction of motion, is the collector. This would solidly reduce the skin friction, and it would improve the numbers in the paper by some amount. Exactly how much I'm not sure.

Offline Danderman

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Re: Scooping atmospheric air (PROFAC revisited)
« Reply #124 on: 01/06/2015 04:15 pm »
As long as this is the Advanced Concepts section, I should interject with my dream project, cycling a ship that would scoop atmosphere at Venus and then dump it at Mars. It probably would require magic, but that would be a very useful thing indeed.


Offline Asteroza

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Re: Scooping atmospheric air (PROFAC revisited)
« Reply #125 on: 01/06/2015 10:20 pm »
For reference, Daniel Eder (AKA danielravennest) on his big space infrastructure wikibook https://en.wikibooks.org/wiki/Space_Transport_and_Engineering_Methods/Resource_Extraction has a interesting design for a 400Km orbit type monolithic ramscoop. Notably, the solar panels remain in the collector frontal area drag shadow.
« Last Edit: 01/06/2015 10:28 pm by Asteroza »

Offline MP99

Re: Scooping atmospheric air (PROFAC revisited)
« Reply #126 on: 01/07/2015 06:25 am »

Just doing some thread necro...

I've been trying to figure out the best way to crack CO2 for CO/LOX in LMO. So far, the three best options I've arrived at are electrolysis, direct reduction via zirconia, and reverse water-gas shift. They are as follows:

<snip>

2. Direct reduction via zirconia... heat CO2 from -250C (ambient mars@100km, guesstimate) to 1100C, which dissociates ~30% of CO2 into CO + O2, electrochemically pump O2 across zirconia for collection. Heating requirement reduced by exchanging heat from hot CO/O2 output exhaust with cold CO2 input.
   -Advantages   : relatively low power requirements
   -Disadvantages: zirconia tubes are brittle, have low output, requiring large numbers. least mature.

I'm surprised to see that "ambient" temperature mentioned.

I had expected that gas collected via collisions at multiple km/s would end up at high temperatures at the throat of the nozzle.

A craft doing entry needs a massive heatshield to cope with high temperatures from gas collisions.

PROFAC will operate at similar orbital velocities, just at lower pressures. ISTM the collected gas will be heated via the same effect? Is the nozzle that efficient at bleeding away collision energy that the gas ends up very cold?

Cheers, Martin

PS can that -250C really be correct? That's apparently colder than the surface of Pluto out around 40 AU.

Offline RobLynn

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Re: Scooping atmospheric air (PROFAC revisited)
« Reply #127 on: 01/07/2015 10:48 am »
For reference, Daniel Eder (AKA danielravennest) on his big space infrastructure wikibook https://en.wikibooks.org/wiki/Space_Transport_and_Engineering_Methods/Resource_Extraction has a interesting design for a 400Km orbit type monolithic ramscoop. Notably, the solar panels remain in the collector frontal area drag shadow.

I am surprised that they do not use a turbomolecular pump type design (crude looking fan rotor on front like original Star Trek Enterprise) to prevent incident gas from exiting the inlet (1 stage of turbomolecular pump can have a pressure ratio of >100).  Could be made from aluminium foil on a magnetic bearing, almost zero power consumption (rotation is at least partially driven by incident gas molecules).

With orbital speed ~7.5km/s and thermosphere gas molecular velocities of probably 300-500m/s average the flanks would need to be tapered at about 3-4° to prevent molecules hitting the sides of the scoop ie length to width of only about 6.  That inevitably limits the available surface area for solar panels - making beamed power, tether power or nukes very attractive.

It would be possible to add trailing scoops in wake of primary one to collect any laterally incident molecules (you are already paying drag price for them hitting as I would intuit that they bounce off at speeds of only a few hundred m/s) so that would increase area available for solar panels to an almost arbitrary degree by reducing their drag penalty.

Another problem is that a long skinny scoop design is unstable in pitch due to tidal forces.   A pair of scoops coupled together by a vertical tether so that they fly in formation a few hundred vertical meters apart would be a nice solution to do alignment, these tethers could be tiny in cross section, and could maybe also be used for propulsion.  But the turbomolecular pump approach can also greatly shorten the scoop.  Alternatively a vertical array of scoops could work.
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Offline AlanSE

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Re: Scooping atmospheric air (PROFAC revisited)
« Reply #128 on: 01/07/2015 02:49 pm »
For reference, Daniel Eder (AKA danielravennest) on his big space infrastructure wikibook https://en.wikibooks.org/wiki/Space_Transport_and_Engineering_Methods/Resource_Extraction has a interesting design for a 400Km orbit type monolithic ramscoop. Notably, the solar panels remain in the collector frontal area drag shadow.

I am surprised that they do not use a turbomolecular pump type design (crude looking fan rotor on front like original Star Trek Enterprise) to prevent incident gas from exiting the inlet (1 stage of turbomolecular pump can have a pressure ratio of >100).  Could be made from aluminium foil on a magnetic bearing, almost zero power consumption (rotation is at least partially driven by incident gas molecules).

With orbital speed ~7.5km/s and thermosphere gas molecular velocities of probably 300-500m/s average the flanks would need to be tapered at about 3-4° to prevent molecules hitting the sides of the scoop ie length to width of only about 6.  That inevitably limits the available surface area for solar panels - making beamed power, tether power or nukes very attractive.

It would be possible to add trailing scoops in wake of primary one to collect any laterally incident molecules (you are already paying drag price for them hitting as I would intuit that they bounce off at speeds of only a few hundred m/s) so that would increase area available for solar panels to an almost arbitrary degree by reducing their drag penalty.

Another problem is that a long skinny scoop design is unstable in pitch due to tidal forces.   A pair of scoops coupled together by a vertical tether so that they fly in formation a few hundred vertical meters apart would be a nice solution to do alignment, these tethers could be tiny in cross section, and could maybe also be used for propulsion.  But the turbomolecular pump approach can also greatly shorten the scoop.  Alternatively a vertical array of scoops could work.

Turbomolecular pumps have low efficiency compared to the theoretical minimum needed to do the compression. That imposes a major penalty on designs that use these. I also imagine that the gas cooling will involve a lot of energy. However, electrostatic propulsion without any kind of confinement or compression of the gas also has huge problems (I can't remember specifically right now). The most promising looked to be the plasma confinement, where the gases are mostly self-compressed, they are not cooled, and they are heated by microwave to get good specific impulse from your thruster. The funnel idea also seems to be bunk. The logic is that these could work, but only below the Karman line. Otherwise the densities are too low. The air molecules are primarily not hitting each other. Unless you can get them to a high compression ratio just by wall ricochet geometry, you'll have almost all molecules bounce back out. That's why the promising proposal is a honeycomb series of tubes oriented in the direction of motion. The high degree of collimation makes it easy to get in, but after a single collision the angles are random and it's very hard to get back out because there's only a few degree window.

I strongly agree with the statement that "flanks would need to be tapered at about 3-4°". In fact, I would like to see analysis of this done, and I'm thinking about returning to that myself. You have the Gaussian thermal angular distribution, and with this you can find the fraction of molecules that would hit the flank at any degree of tapering. With that angle, you can find an acceptable ratio of solar panel area to collector area, which then gives the power per area you can get, which then gives the altitude you can operate at. That "triangular" collector seems to be the best alternative to the tethered concept.

Stability should be dominated by aerodynamic forces (at least for the altitudes I have in mind), not tidal forces. That's a big difference between VLEO and LEO. In the triangular design, I'm mildly hopeful because any rotation will expose a surface to more air pressure, which would give a stabilizing torque. However, this depends on the relative locations of the center of mass versus the center of pressure. If the heavy equipment was all located near the mouth (at the front), then the outlook for stability looks good.

Heck, you could even have control surfaces, making the concept weirdly resemble aircraft while being totally not at all the same physical thing. The problem is that these surfaces would impose a major energetic penalty, but they could still be used for quick attitude adjustments. You would just need some way to either fold them up when not in use, or have them integral to the sides of the craft.

Offline Hershey

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Re: Scooping atmospheric air (PROFAC revisited)
« Reply #129 on: 01/07/2015 05:49 pm »

Just doing some thread necro...

I've been trying to figure out the best way to crack CO2 for CO/LOX in LMO. So far, the three best options I've arrived at are electrolysis, direct reduction via zirconia, and reverse water-gas shift. They are as follows:

<snip>

2. Direct reduction via zirconia... heat CO2 from -250C (ambient mars@100km, guesstimate) to 1100C, which dissociates ~30% of CO2 into CO + O2, electrochemically pump O2 across zirconia for collection. Heating requirement reduced by exchanging heat from hot CO/O2 output exhaust with cold CO2 input.
   -Advantages   : relatively low power requirements
   -Disadvantages: zirconia tubes are brittle, have low output, requiring large numbers. least mature.

I'm surprised to see that "ambient" temperature mentioned.

I had expected that gas collected via collisions at multiple km/s would end up at high temperatures at the throat of the nozzle.

A craft doing entry needs a massive heatshield to cope with high temperatures from gas collisions.

PROFAC will operate at similar orbital velocities, just at lower pressures. ISTM the collected gas will be heated via the same effect? Is the nozzle that efficient at bleeding away collision energy that the gas ends up very cold?

Cheers, Martin

PS can that -250C really be correct? That's apparently colder than the surface of Pluto out around 40 AU.


-250C was just to give an upper bound for heating requirements, accounting for only "ambient" temp. It didn't consider other factors. I don't recall how I obtained that figure, but it was probably before I even found a temperature profile for Mars. Apparently around -120C @100km.


Hershey



Offline AlanSE

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Re: Scooping atmospheric air (PROFAC revisited)
« Reply #130 on: 01/07/2015 09:38 pm »
Interesting, I had not looked into the role of temperature. This is very important for the discussion of the taper ratio. Here are some quick numbers using the tool:

http://omniweb.gsfc.nasa.gov/

I calculate v = sqrt( 3 k T / m ). The tool can give you composition, so it's easy to get this. Here are some example points:

h(km)   v(m/s)
0   484
50   471
100   409
150   852
200   1013
250   1102
300   1164
400   1248
500   1345
750   2064
1000   2526

So for 150 to 200 km altitude orbit, the molecular speed is at around 10% of the orbital velocity. You could then imagine the tapered surfaces of having a rise-to-run of about 1-to-10. But if you operated at 100 km, this would be closer to 5%, for a rise-to-run of 20. That could allow for an extremely narrow wedge, but that altitude is unlikely to work under any circumstances.

Offline Asteroza

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Re: Scooping atmospheric air (PROFAC revisited)
« Reply #131 on: 01/07/2015 11:11 pm »
For reference, Daniel Eder (AKA danielravennest) on his big space infrastructure wikibook https://en.wikibooks.org/wiki/Space_Transport_and_Engineering_Methods/Resource_Extraction has a interesting design for a 400Km orbit type monolithic ramscoop. Notably, the solar panels remain in the collector frontal area drag shadow.

I am surprised that they do not use a turbomolecular pump type design (crude looking fan rotor on front like original Star Trek Enterprise) to prevent incident gas from exiting the inlet (1 stage of turbomolecular pump can have a pressure ratio of >100).  Could be made from aluminium foil on a magnetic bearing, almost zero power consumption (rotation is at least partially driven by incident gas molecules).

With orbital speed ~7.5km/s and thermosphere gas molecular velocities of probably 300-500m/s average the flanks would need to be tapered at about 3-4° to prevent molecules hitting the sides of the scoop ie length to width of only about 6.  That inevitably limits the available surface area for solar panels - making beamed power, tether power or nukes very attractive.

It would be possible to add trailing scoops in wake of primary one to collect any laterally incident molecules (you are already paying drag price for them hitting as I would intuit that they bounce off at speeds of only a few hundred m/s) so that would increase area available for solar panels to an almost arbitrary degree by reducing their drag penalty.

Another problem is that a long skinny scoop design is unstable in pitch due to tidal forces.   A pair of scoops coupled together by a vertical tether so that they fly in formation a few hundred vertical meters apart would be a nice solution to do alignment, these tethers could be tiny in cross section, and could maybe also be used for propulsion.  But the turbomolecular pump approach can also greatly shorten the scoop.  Alternatively a vertical array of scoops could work.

Turbomolecular pumps have low efficiency compared to the theoretical minimum needed to do the compression. That imposes a major penalty on designs that use these. I also imagine that the gas cooling will involve a lot of energy. However, electrostatic propulsion without any kind of confinement or compression of the gas also has huge problems (I can't remember specifically right now). The most promising looked to be the plasma confinement, where the gases are mostly self-compressed, they are not cooled, and they are heated by microwave to get good specific impulse from your thruster. The funnel idea also seems to be bunk. The logic is that these could work, but only below the Karman line. Otherwise the densities are too low. The air molecules are primarily not hitting each other. Unless you can get them to a high compression ratio just by wall ricochet geometry, you'll have almost all molecules bounce back out. That's why the promising proposal is a honeycomb series of tubes oriented in the direction of motion. The high degree of collimation makes it easy to get in, but after a single collision the angles are random and it's very hard to get back out because there's only a few degree window.

I strongly agree with the statement that "flanks would need to be tapered at about 3-4°". In fact, I would like to see analysis of this done, and I'm thinking about returning to that myself. You have the Gaussian thermal angular distribution, and with this you can find the fraction of molecules that would hit the flank at any degree of tapering. With that angle, you can find an acceptable ratio of solar panel area to collector area, which then gives the power per area you can get, which then gives the altitude you can operate at. That "triangular" collector seems to be the best alternative to the tethered concept.

Stability should be dominated by aerodynamic forces (at least for the altitudes I have in mind), not tidal forces. That's a big difference between VLEO and LEO. In the triangular design, I'm mildly hopeful because any rotation will expose a surface to more air pressure, which would give a stabilizing torque. However, this depends on the relative locations of the center of mass versus the center of pressure. If the heavy equipment was all located near the mouth (at the front), then the outlook for stability looks good.

Heck, you could even have control surfaces, making the concept weirdly resemble aircraft while being totally not at all the same physical thing. The problem is that these surfaces would impose a major energetic penalty, but they could still be used for quick attitude adjustments. You would just need some way to either fold them up when not in use, or have them integral to the sides of the craft.

Honeycomb collimator idea made me think, the point is to have a a lot of skinny tubes, but not necessarily in a single lump of tubes. If so, then a hex honeycomb block should be equivalent to a vertical stack of tubes, right? Could an essentially edge on 2D tether tape be expanded to 3D, such that it looks like a helicopter blade with an enormous number of imbedded leading edge inlets? Basically a soft sollector with internal tubing/routing paths to send collected gas up/down the tape to the initial collector tank? If you are already doing flexible solar cell fabric 2D tether tapes, then you are essentially just adding more intermediary layers to the tape to add the transverse fabric to make up the squarish tubes. Then the tether does potentially triple duty (collector, solar array, electrodynamic tether propulsion). If the transfer plumbing in the tape is fairly long, you might get a natural radiator effect too.

Offline RobLynn

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Re: Scooping atmospheric air (PROFAC revisited)
« Reply #132 on: 01/08/2015 02:07 am »
Turbomolecular pumps have low efficiency compared to the theoretical minimum needed to do the compression. That imposes a major penalty on designs that use these. I also imagine that the gas cooling will involve a lot of energy. However, electrostatic propulsion without any kind of confinement or compression of the gas also has huge problems (I can't remember specifically right now). The most promising looked to be the plasma confinement, where the gases are mostly self-compressed, they are not cooled, and they are heated by microwave to get good specific impulse from your thruster. The funnel idea also seems to be bunk. The logic is that these could work, but only below the Karman line. Otherwise the densities are too low. The air molecules are primarily not hitting each other. Unless you can get them to a high compression ratio just by wall ricochet geometry, you'll have almost all molecules bounce back out. That's why the promising proposal is a honeycomb series of tubes oriented in the direction of motion. The high degree of collimation makes it easy to get in, but after a single collision the angles are random and it's very hard to get back out because there's only a few degree window.

Unless there is a sufficiently large flux of incident gas molecules to collide with and push gas molecules inwards I don't see how the honeycomb collimation would work to prevent molecules leaking out (unless mean free path << honeycomb length).  My guess is would depend on the ambient gas density and would only work below some cutoff altitude.  But as already established the power requirements per frontal area set limits on how low you can operate - and solar panels cannot provide enough power below a certain altitude.

I think turbomolecular pump inlets could work at any altitude (particularly higher orbits with lower densities) - providing a definite one-way-valve effect at inlet.
http://www.pfeiffer-vacuum.com/know-how/vacuum-generation/turbomolecular-pumps/design-operating-principle/turbomolecular-pump-operating-principle/technology.action?chapter=tec2.8.1.1

I can see how efficiency of the turbo molecular pump is an issue (though I would guess it increases with blade speed and so stage pressure ratio), but at least it is possible to make an operable solution as can raise the altitude until the panel area is sufficient for the inlet area, and tech is reasonably conventional - perhaps even off-the-shelf.
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Offline savuporo

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Re: Scooping atmospheric air (PROFAC revisited)
« Reply #133 on: 01/08/2015 07:01 am »
Atmospheric Mining in the Outer Solar System: Design Options and Observational Systems
Quote
Uninhabited Aerial Vehicle (UAV) and drone options may use nuclear ramjets or rockets
..
Let’s go to the stars, as quickly as possible.

 :o

( from http://aspw.jpl.nasa.gov/workshop-proceedings )
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Offline AlanSE

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Re: Scooping atmospheric air (PROFAC revisited)
« Reply #134 on: 01/08/2015 03:02 pm »
Turbomolecular pumps have low efficiency compared to the theoretical minimum needed to do the compression. That imposes a major penalty on designs that use these. I also imagine that the gas cooling will involve a lot of energy. However, electrostatic propulsion without any kind of confinement or compression of the gas also has huge problems (I can't remember specifically right now). The most promising looked to be the plasma confinement, where the gases are mostly self-compressed, they are not cooled, and they are heated by microwave to get good specific impulse from your thruster. The funnel idea also seems to be bunk. The logic is that these could work, but only below the Karman line. Otherwise the densities are too low. The air molecules are primarily not hitting each other. Unless you can get them to a high compression ratio just by wall ricochet geometry, you'll have almost all molecules bounce back out. That's why the promising proposal is a honeycomb series of tubes oriented in the direction of motion. The high degree of collimation makes it easy to get in, but after a single collision the angles are random and it's very hard to get back out because there's only a few degree window.

Unless there is a sufficiently large flux of incident gas molecules to collide with and push gas molecules inwards I don't see how the honeycomb collimation would work to prevent molecules leaking out (unless mean free path << honeycomb length).  My guess is would depend on the ambient gas density and would only work below some cutoff altitude.  But as already established the power requirements per frontal area set limits on how low you can operate - and solar panels cannot provide enough power below a certain altitude.

I think turbomolecular pump inlets could work at any altitude (particularly higher orbits with lower densities) - providing a definite one-way-valve effect at inlet.
http://www.pfeiffer-vacuum.com/know-how/vacuum-generation/turbomolecular-pumps/design-operating-principle/turbomolecular-pump-operating-principle/technology.action?chapter=tec2.8.1.1

I can see how efficiency of the turbo molecular pump is an issue (though I would guess it increases with blade speed and so stage pressure ratio), but at least it is possible to make an operable solution as can raise the altitude until the panel area is sufficient for the inlet area, and tech is reasonably conventional - perhaps even off-the-shelf.

At your first sentence, you can just stop right there, because that's exactly how it works. The mean free path isn't only longer than the honeycomb tubes, it's longer than any dimension of the entire satellite. In order to change this assumption, you have to get close to the Karman line - the paper mentioned altitudes on the order of 120 km, and this should be too low to work. Maybe you could have a Karman line scoop with a power cable that was juicing that sucker up like a Texas pig. But anyway, it's not the design we're talking about. The power limitations will generally put it 50 to 100 km above the limit where mean free path becomes smaller than the craft. The difference between those altitudes are like comparing an anvil to a helium balloon.

That makes it a fun problem, because the fluid dynamics are entirely about lines of sight. It's not even a fluid. The molecules are collimated to about 10 degrees angular spread, but as a fraction of all solid angles, that's only around 2%, so you could only loose that fraction of your gas due to random scatter out. I don't know of any other engineering problem that designs for this type of flow regime. It's strange, but we expect it to be strange because it's not otherwise done.

I'm not saying that the turbomolecular pumps won't be used. It's just that you'll use some collector geometry to pre-compress it by the velocity alone. The honeycomb pipes do this very well, and the density/pressure increase compared to ambient is huge. But you use the turbomolecular pump after that. You're trying to go from near vacuum to a propellant tank, so there are a lot of orders of magnitude to climb.

Actually, I think you would want a hybrid approach where you use 2 scoop technologies at the same time. One of them is to generate thrust by minimizing compression, cooling, etc. The other technology diverts some of the flow into permanent storage. I'm not yet sure what my favorite technologies for both of these are. I don't picture a small system. To get economic throughput, this will be a large folded and assembled system from multiple launches.

Offline Solman

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Re: Scooping atmospheric air (PROFAC revisited)
« Reply #135 on: 01/10/2015 12:56 am »
Couldn't a rotating tether allow air to be collected at a low speed relative to the collector?
Assuming an appropriately sized tether rotating in a perpendicular plane to the surface such that the lower end is traveling opposite to the orbital direction?
The collector would open at the low end when the relative speed would be less.
Would need solar that is deployed on the high end and retracted and stored before the arm dips back into the atmosphere perhaps.
Wouldn't need to have a long pipe to transport liquid air up to the higher end of a non rotating tether.
Could release tanks, if attached to an appropriate vehicle, to higher orbit propellant depot without much propulsion needed using the tether's rotational energy. Replacement tanks would have to be moved along the tether though, I suppose, since docking anywhere but the center of rotation seems difficult.
Just want to add that SEP using nitrogen as propellant might be a good way to keep the thing spinning. :)


 
« Last Edit: 01/10/2015 01:11 am by Solman »

Offline Robotbeat

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Re: Scooping atmospheric air (PROFAC revisited)
« Reply #136 on: 01/10/2015 03:54 am »
...rotovator atmospheric collection. I like it.


BTW, Jon Goff, isn't there some nice closed-form equation for the specific strength needed for a rotating tether of a certain velocity?
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Offline RobLynn

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Re: Scooping atmospheric air (PROFAC revisited)
« Reply #137 on: 01/10/2015 02:23 pm »
...rotovator atmospheric collection. I like it.


BTW, Jon Goff, isn't there some nice closed-form equation for the specific strength needed for a rotating tether of a certain velocity?
here:
http://en.wikipedia.org/wiki/Space_tether
http://upload.wikimedia.org/math/2/d/b/2db81384da1695a4081e5bed26a33bd5.png
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Offline Hanelyp

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Re: Scooping atmospheric air (PROFAC revisited)
« Reply #138 on: 01/10/2015 07:38 pm »
The rotovator air scoop appears to address some difficulties with hypersonic heating and parasitic drag.  The system still needs thrust to accelerate the collected air into orbit.

Offline Robotbeat

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Re: Scooping atmospheric air (PROFAC revisited)
« Reply #139 on: 01/10/2015 07:44 pm »
The rotovator air scoop appears to address some difficulties with hypersonic heating and parasitic drag.  The system still needs thrust to accelerate the collected air into orbit.
...electrodynamic tether can produce thrust without needing to use propellant.
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To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

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