Scooping atmospheric air (PROFAC revisited)

[alexterrell]

Following a discussion here (http://forum.nasaspaceflight.com/index.php?topic=17776.75) , I was wondering if anyone was revisiting the PROFAC concept of the 1960s.

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

Profac required a 10MW nuclear reactor, which is probably not feasible in a 120km orbit. So how about we revisit it using tethers.

I propose:
An accumulator module at an altitude of 120km, consisting of:
- a 4m diameter scoop, imposing a drag of 500N, and collecting 3 tons of air per day (assuming 50% capture)
- A compressor, and heat rejection radiators. I estimate this needs about 50 KW.
- Two tanks for liquid air.

A mother ship, orbiting at about 320km, consisting of:
- Solar power
- Electrodynamic reboost facility
- Tether deployment mechanism, for raising and lowering both the accumulator module, and the air tanks.
- Liquid air storage and separation facilities
- A housing for the accumulator stage when it's reeled in so it can be maintained in a shirt sleeve environment
- Optionally, a crew visiting module.
- Tether repair facility
- A counter balance to keep the main elements of the mother ship at zero g.
- A HVDC connection to the Accumulator to power the compressor. (Can you send DC over 200km through the Earth's magnetic field - or do we need a laser?).

The whole mission would be launched by an Ares V or a Jupiter 232. The mother ship would then deploy the accumulator and lower it 200km, where it would scoop up about 3 tons per day of air (about 20 tons of oxygen per month).

The accumulator would be linked by a number of hoy-tethers, so it can be reeled in.

When one of the air tanks is full, it would be reeled up to the mother ship over a day or so (<10km/hr).

A repair unit can move between the accumulator unit and the mother ship to replace broken stands of fibre. (Probably spectra 2000).

Once every few years, the accumulator can be reeled into the bay of the mother ship, the doors closed and the bay pressurised. A visiting Orion crew will carry out maintenance and replace the tether reels.

Question: 1. I can't get a figure for electrodynamic thrust in N/KW. I assume the accumulator has a 500N counter force. The tethers and mother station will bring this to below 1KN. Given day time operation only, probably want to scale for 2KN of thrust. What power is needed for this?

2. Would this concept be worthwhile, in the medium term for supporting constellation, and in the long term for supplying nitrogen to space. Apart from breathing air in lava tubes etc, does nitrogen have a use?

3. Any other improvements or major issues?

[alexterrell]

Question: 1. I can't get a figure for electrodynamic thrust in N/KW. I assume the accumulator has a 500N counter force. The tethers and mother station will bring this to below 1KN. Given day time operation only, probably want to scale for 2KN of thrust. What power is needed for this?

Thinking this through, an electrodynamic thruster is an electric motor and Power = FV, which is about 3.5MW. Taking into account inefficiencies, the fact that the thrust is aligned with the equator, the power for the compressor, and the fact that the craft is almost 50% in shadow, 8-10MW is needed. That's about 40 tons, and 2-3 Hectares of solar cells. Quite a deployment problem!

(Discuss deployment of large solar arrays here http://forum.nasaspaceflight.com/index.php?topic=18022.new#new)

It should also be possible to power the compressor with a small electrodynamic generator.

Or, can the compressor be powered by generating power from the incoming air stream? Either by a turbine, or a thermo electric generator / heat pump between the 900K scoop and the radiators?

[khallow]

Hmmm, it's an interesting idea. Would be a good way to pull in the usual volatiles from Earth's atmosphere as well as some hydrogen and helium. The mass of the scoop would be moving at orbital velocity of the mothership which is a bit slower than orbital velocity 200 km lower, meaning it would have some gravity pulling it down as well. Finally, you'd be pulling that 3 tons of air up 200 km of gravity well. It's possible that a 10 MW power source won't be sufficient (though my calculate indicates that even on Earth's surface, moving 3 tons up 200 km would take, in terms of energy, about 10 minutes of 10 MW power, if there are no inefficiencies in the system.

The big uncertainty for me is the amount of tension the cable needs to support. It appears to need to support the scoop (which experiences a relatively modest gravity force) plus 3 tons of atmosphere, and the force of pulling the harvest (3 or more tons) up 200 km of cable.

Finally, there's a typo in your first post. At 3 tons per day, you're harvesting 60 90 tons roughly per month not 20. That's quite a bit of mass in orbit for a project that appears to be doable with near future technology. If you can harvest 60 90 tons per month, that's 12 18 tons of oxygen (plus some small amount of hydrogen) or roughly 140 210 tons of LOX propellant per year which in turn is more than an Ares V launch per year. If a high value use for the nitrogen is found, then that's a lot more launches saved (you'd have the equivalent of around 6 Ares V launches in nitrogen and oxygen per year).

[A_M_Swallow]

There are plenty of possible uses for Nitrogen - non-flammable atmospheric
filler, flushing of rocket engines, filler for Stirling engines, component of fertilizer and other organic chemicals.

[gospacex]

Why do you need to have a tether?

Put power source (solar cells?) and engines on the free-flying collector module. Make it use these to balance out aerodynamic slowdown. Can ion engines with nitrogen be used for this?

I figure out it would be impossible to collect 3 tons per day this way (the drag is too high). How much is possible?

Pop out of atmosphere when your tanks are full, and transfer collected stuff into a depot.

[alexterrell]

Khallow: some numbers:

The accumulator described experiences 0.09g. I guess the accumulator weighs 10 tons empty, so 900kg force (9KN).

The drag is required to accelerate 6 tons / day from zero to 7,025m/s, = 503N.

I've guessed < half is lost, > half captured. So > 3t /day =~ 100t/month. But that's only about 20 tons of LOX. The rest is Nitrogen with traces of water, CO2 and noble gases.

I'd assume a transfer tank mass of 10t, so the tank is sent up and back twice per week. (Effective total weight 1,800kg).

The energy/power to pull it up the last 200km is fairly trivial. (10 tons @ 0.09g =~10KN, @3m/s = 30KW.

Basically, the energy required (average power about 4MW) is the energy used to accelerate 6t/day from rest to 7,025m/s. In short, it's a way of getting material from Earth to Orbit using electric energy.

I think the biggest uncertainty is the drag on the tether(s). They need to support say 20,000N. Assuming a working strength of 1GPa, that's 20mm2 cross section. This could be 4 tethers of 5 strands front on, so I assume the front cross section is 4mm wide. Over 10km length (assuming the atmosphere at all points is equivalent to 10km at base pressure), that's 40m2, which is more than the scoop. However, most air molecules that hit the tether will bounce off at an angle and not pick up 7,000m/s of velocity, so it's a lot lower than this.

gospacex: What you propose is the original PROFAC. However, you can't put several hectares of solar power in orbit at 120km, only. The original PROFAC concept used a 10MW nuclear reactor but then you're talking a 100 ton vehicle with a large nuclear reactor in low Earth Orbit.

[gospacex]

gospacex: What you propose is the original PROFAC. However, you can't put several hectares of solar power in orbit at 120km, only. The original PROFAC concept used a 10MW nuclear reactor but then you're talking a 100 ton vehicle with a large nuclear reactor in low Earth Orbit.

And if we put a solar-powered PROFAC into 160 km orbit, where drag is ~200 times lower? 50kW is less than half of ISS power output.

[khallow]

gospacex: What you propose is the original PROFAC. However, you can't put several hectares of solar power in orbit at 120km, only. The original PROFAC concept used a 10MW nuclear reactor but then you're talking a 100 ton vehicle with a large nuclear reactor in low Earth Orbit.

And if we put a solar-powered PROFAC into 160 km orbit, where drag is ~200 times lower? 50kW is less than half of ISS power output.

Drag is necessary because you're scooping up almost stationary air relative to orbital velocities. 200 times less drag means 200 times less air scooped. Instead of 3 tons a day, you'd scoop 15 kg a day.

[alexterrell]

gospacex: What you propose is the original PROFAC. However, you can't put several hectares of solar power in orbit at 120km, only. The original PROFAC concept used a 10MW nuclear reactor but then you're talking a 100 ton vehicle with a large nuclear reactor in low Earth Orbit.

And if we put a solar-powered PROFAC into 160 km orbit, where drag is ~200 times lower? 50kW is less than half of ISS power output.

Drag is necessary because you're scooping up almost stationary air relative to orbital velocities. 200 times less drag means 200 times less air scooped. Instead of 3 tons a day, you'd scoop 15 kg a day.

Though there is some optimisation to be done.

Raise the orbit by 10km (to 130km), you halve the atmospheric density. If you double the area of the scoop (5.6m diameter), in theory, you should get the same yield, but you've halved the drag on the tethers.

Raise it by another 10km, and you need an 8m diameter scoop. At this point the scoop mass starts to dominate and you need thicker tethers. Another 10km and you need an 11.2m diameter scoop which would need in-orbit assembly.

Up to about 5.6m scoop diameter, a replacement accumulator unit could be launched by EELV. (Consisting of simple compressors, heat exchangers and radiators, they should be quite cheap.)

Also, no one is sure whether the efficiency of the scoop falls off with increasing size. I suspect this question needs a lot of computing power. The original PROFAC concept was at 120km, so I've stuck with that.

Ultimately, you can't cheat nature. The power generated at the mother ship is being used to accelerate the gas to orbital velocity. It's just a lot more efficient than a rocket.

[alexterrell]

There are plenty of possible uses for Nitrogen - non-flammable atmospheric
filler, flushing of rocket engines, filler for Stirling engines, component of fertilizer and other organic chemicals.

OK - but if we have a system providing 20t of O2 and 80t of N2 per month, the uses above are not going to demand 80 tons. Granted, O'Neil cylinders will need silly amounts (~15 million tons), but near term needs are trivial.

So, question for the rocket scientists:

How about basing transport on Hydrazine + Nitrous Oxide?
N2H4 + N2O4 -> 2 H2O + 2 N2.

(http://www.astronautix.com/props/n2oazine.htm - Isp 339)

With 20 tons of O2 per month, you'd need 35 tons of N2 and 2.5t of H2, giving 57.5 tons of fuel.

Alternatively; 2 H2 + O2 -> 2 H2O (Isp ~ 450)

With 20 ton O2 per month, you'd need 2.5t of H2 and get 22.5 tons of fuel. (At 8:1 - I know Earth launch uses about 6.5:1, but in this case the O2 is cheap and the H2 has to come from Earth.

Would you rather have 57.5 tons of storable hydrazine / nitrous oxide, or 22.5 tons of LH2 / LOX fuel?

You still have 45 tons of N2 remaining. Uses:
- Boil off to keep the LOX cold in transit.
- Electric arc thrusters for inter-orbit tugs

[cgrunska]

what does this do to earth's atmosphere? Well, actually no, it's trivial.

What would supplying o niel colonies with oxygen/nitrogen with a scoop do to the atmosphere? 15 million tons a month? Also nothing/trivial?

[GI-Thruster]

Would be interesting to see figures for making this work on a gas giant.  Something that scoops H2 off Jupiter or Saturn and lifts it to a depot. . .

[alexterrell]

Would be interesting to see figures for making this work on a gas giant.  Something that scoops H2 off Jupiter or Saturn and lifts it to a depot. . .

Zubrin did some analysis for nuclear ramjets scooping up He3 from the gas giants. (In Entering Space)

He concluded that for ramjets with an exhaust velocity of 9km/s, the gravity well of Jupiter made it infeasible (Mass ratio 23.7), but Saturn was feasible (Mass ratio 4.6).

[khallow]

GI, it gets tricky due to the depth of the gravity well. Something like Jupiter would be very hard to pull things out. Saturn, Uranus, and Neptune are better, but they are of course, further away from the Sun which means less solar energy and having to move the product further down the Sun's gravity well in order to deliver it to inner system markets.

Scooping from Venus would be particularly interesting since one could harvest both carbon and oxygen (plus small amounts of sulphur and hydrogen) and there's plentiful solar power (about double the intensity of Earth). That's the only really significant source of atmospheric carbon, as far as I can tell. Mars also has a CO2 atmosphere too, but it's density is at least thousands of times less.

[jongoff]

Hmmm, it's an interesting idea. Would be a good way to pull in the usual volatiles from Earth's atmosphere as well as some hydrogen and helium. The mass of the scoop would be moving at orbital velocity of the mothership which is a bit slower than orbital velocity 200 km lower, meaning it would have some gravity pulling it down as well. Finally, you'd be pulling that 3 tons of air up 200 km of gravity well. It's possible that a 10 MW power source won't be sufficient (though my calculate indicates that even on Earth's surface, moving 3 tons up 200 km would take, in terms of energy, about 10 minutes of 10 MW power, if there are no inefficiencies in the system.

The big uncertainty for me is the amount of tension the cable needs to support. It appears to need to support the scoop (which experiences a relatively modest gravity force) plus 3 tons of atmosphere, and the force of pulling the harvest (3 or more tons) up 200 km of cable.

Finally, there's a typo in your first post. At 3 tons per day, you're harvesting 60 90 tons roughly per month not 20. That's quite a bit of mass in orbit for a project that appears to be doable with near future technology. If you can harvest 60 90 tons per month, that's 12 18 tons of oxygen (plus some small amount of hydrogen) or roughly 140 210 tons of LOX propellant per year which in turn is more than an Ares V launch per year. If a high value use for the nitrogen is found, then that's a lot more launches saved (you'd have the equivalent of around 6 Ares V launches in nitrogen and oxygen per year).

Karl,
Remember that at orbital altitudes the atmospheric composition is different from here on the surface.  It's actually predominately Oxygen at that altitude (something like 70-80%), not the 20-something percent it is down here on the ground...

Edit:  Correction, while the concentration of O2 is higher at 120km than on the ground, it doesn't become the dominant constituent until at higher altitudes (in the 150-180km range).  So one benefit of having the scoop a bit higher (and going with a bigger scoop but less tether drag) is that you will get a higher percentage of your collected fluid in the form of useable propellant.

http://www.phys.uu.nl/~reijmer/ns154b/class_05.pdf has a chart (on slide 7) showing how the relative concentrations vary with altitude.

~Jon

[yinzer]

The predominant constituent at 150-180km is not O2 but O.  That'll be interesting to scoop up.

[jongoff]

The predominant constituent at 150-180km is not O2 but O.  That'll be interesting to scoop up.

I think that Klinkman et al's current approach uses a technique that has the incoming oxygen oxidize a thin film of mercury liquid, which is then collected and degassed...so atomic oxygen might actually make the job even easier....

BTW, I'm really liking the idea now of splitting the collector into something down at the end of a tether.  That allows the collector to be something small and compact, with the depot/power generation mothership being the bigger, bulkier, but at much higher altitude part....

Still not entirely sure if the numbers add up, but it looks pretty clever so far.

~Jon

[A_M_Swallow]

When doing the number you need to know how long the machine lasts.  Delta-v surface to LEO is enormous.

[Proponent]

I'm trying to understand why O has a greater scale height despite its larger atomic weight. Does N remain predominantly molecular at altitudes where O has become atomic?

[yinzer]

N2 has a triple bond, O2 has a double bond.  The dissociation energy of N2 is more than twice as high as that of O2.  It's presumably high enough that solar ultraviolet light can't cause the formation of monatomic N the way it can form monatomic O.