Atmospheric Scooping

Terraformer · 2015-04-17 08:30:21

Currently, the ISS has spare hydrocarbons (waste plastic), but they aren't much use without oxygen.

So I looked up atmospheric scooping and found this, and also this.

I'm also wondering, would it be feasible to beam solar power to a scoop in low orbit, without needing a tether, since there's very little atmosphere to get in the way?

What we might have then, is a system in which a scoopcraft operates in a very low orbit, sustaining its orbit using beamed power and waste nitrogen. When its tanks are full, it boosts to a higher orbit and rendezvous with a higher orbiting tanks, into which it offloads its product. This done, it drops back to a lower orbit and begins the process anew.

...and now I'm wondering if it would be possible to do a demonstration mission using beamed power from the ISS and a small scoopcraft...

JoshNH4H · 2015-04-17 10:55:08

At 150 km, the atmospheric density is 2e-9 kg/m^3 (using a matlab function one of my professors gave me; I would be glad to send it to anyone if they're interested).

I'm going to assume a drag coefficient of 2*, a speed of 8 km/s, and assume that the v^2 equation for drag holds. Call it an order of magnitude guess, or maybe a +/- 100% guess.

The equation for drag is:

$52694ea39410aa410b12afef2746d8a4.png$

Where rho is density, u is speed, Cd is drag coefficient, and A is area.

Given these numbers, the drag on the craft will be .128 N/m^2.

Assuming you collect approximately 20% of the incoming air and use the rest as propellant, you need an exhaust velocity of 10 km/s. Assuming an engine efficiency of 60%, that's about 8.5 kW/N. At this atmospheric density and speed, the 20% of the atmosphere that you retain will be 3.2e-6 kg per m^2 per second. Using the magic of dimensional analysis, I calculate that the energy required will therefore be at least 333 MJ/kg of Oxygen recovered, plus the energy required to change orbits back up to the ISS. Another figure of merit is that it would require 1.07 kW per m^2 of frontal area.

That's not a proposal killer, because there are no other options that work in quite this way - So although launching a rocket from Earth with the Oxygen on it might use less energy, it's not really comparable because we're comparing on cost, not energy.

Let's say the ISS wanted 1 kg/day of oxygen. This would require a continuous power level of under 4 kW, which isn't so bad. What I find really exciting about this is that the numbers work out pretty well for a solar thermal propulsion rig. It should even be possible to get the orbit to precess in such a way as that the spacecraft is constantly in sunlight.

*Given that you're actually going to be collecting most/all of the incoming fluid, it's actually quite easy to calculate a value for the drag coefficient. Force=change in momentum/change in time, or the mass flow rate times the speed. Mass flow rate is A*rho*u. Speed is u. Therefore drag is rho*A*u^2, giving a drag coefficient of 2, by setting the above equation equal to that one and solving for Cd. The actual value of the drag might be somewhat higher because of the effects on the atmosphere surrounding the craft, but I suspect that clever placement of thrusters can reduce this.

GW Johnson · 2015-04-17 14:59:08

So, if light gas gun launch of gee-tolerant materials (plus a small solid to circularize) costs around $100-300 / pound, and you need something to go retrieve it on-orbit, would that be an even cheaper option?

GW

Terraformer · 2015-04-17 15:17:07

Hang on - Josh, are you basically suggesting that Tom Jolly's old Solar Thermal Airship, if it was already in orbit, could remain on orbit?

Also, that the orbit could be arranged such that it would remain in sunlight?

Which then opens up the possibility of orbiting gas mining rigs in funny low altitude orbits, filling up electric tankers which then make the plane change required to rendezvous with a depot. At first mining oxygen, but once the Lunar colony is established, they should probably switch to nitrogen.

So my question is, how much for a demonstration vehicle?

JoshNH4H · 2015-04-18 15:35:46

Hmmm...

The relevant equations are the first two under heading 28 on this page. Nodal Regression speaks to the rate of change of the line of nodes, and Arg of Perigee Change refers to the rate of change of the Argument of Periapsis for a given orbit. Wikipedia has a good explanation of both of these terms, which are used to describe an orbit in 3D space.

The basic requirement is that the craft always be exposed to direct sunlight. The secondary requirement is that it always have at least some desired minimum altitude, let's say 150 km, with as small an eccentricity as possible so that it remains there.

The basic problem is that line of nodes needs to rotate by 360 degrees per year so that the orbit remains in sunlight. Using the first equation under heading 28, we see that the inclination cannot be 90 degrees. The atmospheric drag will change the orbit from how you'd normally expect it to be (Presumably at some point atmospheric force will exceed engine force and it will average out over the orbit - this also makes it possible to go lower in the atmosphere). I don't think the calculation is worth doing since we already know what the orbit will look like, more or less.

The bigger question I have is whether it's possible to get 1000 s out of Nitrogen gas. My guess would have to be that it isn't.

SpaceNut · 2015-04-18 16:05:15

The way that a scooping function would work is on the initial aerobreaking, aerocapture of an incoming craft into orbit as we are using the thicker atmosphere to get into a more circular orbit, If we could make use of that we would definitely have a plus.
As far as the circle orbit lowering into the atmospher and then return with a payload of gasses Its going to depend on energy to over come the pull of gravity and drag...

Terraformer · 2015-04-19 11:20:35

Scooping nitrogen would be a boon to a Lunar settlement. Trading ice and oxygen for nitrogen?

I wonder about electric propulsion using nitrogen...

Terraformer · 2015-04-20 04:15:04

I am also wondering about the difficulty of making a small demonstration craft, using solar electric propulsion and atmospheric scooping to maintain a very low orbit. No need to retain any atmosphere at this point, just prove that such a thing is feasible. I'm envisioning a craft consisting of flat "wings" of solar panels, and a "beak" which opens up to scoop atmosphere. During the night, the beak would shut to significantly reduce drag. Maybe there would be enough power that, if the beak was closed during the day, it would be able to raise itself to a higher orbit and eventually rendezvous with a station to transfer it's payload?

SpaceNut · 2015-04-20 17:40:35

The design of what ever method and craft are determined by where we will use the system as to what planet we are attempting to demonstrate the princile of scooping atmosphere for use as an insitu resource for vehicle fueling and for crewed use depending on quantity achieved.

JoshNH4H · 2015-04-20 22:33:36

Based on research I did for my most recent blog post, you're looking at probably at most 300 W/kg for your panels, and probably about 800 W/kg (At about 50% efficiency) for your engine. Let's say the all-in power system is 200 W/kg. At 1500 s, for each Newton you're looking at about 10 kg of engine and about 40 kg of power systems. At .128 N/m^2, you would need at least 13 kg of "stuff" per meter squared of frontal area, but more importantly about 7500 W. At 25% efficiency and averaging over the sunlit half of an orbit (e.g. mean insolation is 1366/pi W/sq. m because your solar panels aren't always pointing at the Sun) that means that either the craft is 70 m long or has some extremely impressive solar panel "wings".

Actually rather the same case holds for the solar thermal craft, but its efficiency would be somewhat higher due firstly to the direct solar-to-hot-gas conversion (instead of solar-to-electric-to-microwave-to-very-very-hot-gas like in VASIMR and other electric propulsion schemes), and because it seems pretty feasible to recoup a significant portion of your input energy in the sense that fast-moving gases normally become very hot gases and can therefore be used to reheat the gas after it's cooled before passing through the solar concentrator. This might not sound like it makes sense but it could be the only way (Assuming that 100% of the kinetic energy is converted to thermal energy, which is not too far from true, the gases striking the craft at 8 km/s would reach a temperature of 75,000 K)

Terraformer · 2015-04-21 04:21:53

Hmmm. What are the prospects for beamed power? Existing proposals have power transmitted down a tether.

JoshNH4H · 2015-04-21 07:51:01

Depends how you do the beaming. A tether sounds heavy and draggy to me. There's nothing particularly wrong with a long aspect ratio.

It occurs to me that 200 W/kg is for panels in direct sunlight. For this we're looking more at 65 W/kg, so roughly 125 kg of power systems. That's actually not so much of a problem, although less than 2 kg of panel per square meter of panel is not a whole lot.

SpaceNut · 2015-04-21 19:00:31

Power beaming technology is still sort of under developed and needs work to make it a reality but here are some of what is happening.

Power Beaming

NASA Armstrong Fact Sheet: Beamed Laser Power for UAVs

LASER POWER BEAMING FACT SHEET

Got to thinking about the scoop and the beaming power unit an having them as seperate vehicles one in a higher orbit traveling in the same path at a geo synchronous speed to the scoop diving into the atmosphere would allow for maximum power transfer to the scoop and if the sychronous vehicle has tracking panels it keeps contant power being obsorbed that keeps the scoop at full power for use.

SpaceNut · 2015-04-22 18:41:40

I am thinking that the orbiting ships panels and beaming system can be made as large as we want as the ship recieving the power needs to stay manageable in mass as we want the mass of the atmosphere not to drag it down as it takes it in. We also want the duration that its in the atmosphere to be kept to a minimal time frame so as to allow for as many trips that are possible to happen for the days time.

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#1 2015-04-17 08:30:21

Atmospheric Scooping

#2 2015-04-17 10:55:08

Re: Atmospheric Scooping

#3 2015-04-17 14:59:08

Re: Atmospheric Scooping

#4 2015-04-17 15:17:07

Re: Atmospheric Scooping

#5 2015-04-18 15:35:46

Re: Atmospheric Scooping

#6 2015-04-18 16:05:15

Re: Atmospheric Scooping

#7 2015-04-19 11:20:35

Re: Atmospheric Scooping

#8 2015-04-20 04:15:04

Re: Atmospheric Scooping

#9 2015-04-20 17:40:35

Re: Atmospheric Scooping

#10 2015-04-20 22:33:36

Re: Atmospheric Scooping

#11 2015-04-21 04:21:53

Re: Atmospheric Scooping

#12 2015-04-21 07:51:01

Re: Atmospheric Scooping

#13 2015-04-21 19:00:31

Re: Atmospheric Scooping

#14 2015-04-22 18:41:40

Re: Atmospheric Scooping

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