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IsaacKuo
2009-Feb-03, 04:37 AM
I have this idea for a spacecraft to aeroscoop "air" from Earth's upper atmosphere, while periodically using Moon flyby maneuvers to regain lost apogee altitude (this also reverses the direction of the polar elliptical orbit).

But I'm not sure exactly what molecules are available in Earth's upper atmosphere, and in what proportions. Oxygen is obviously useful, and nitrogen may be useful also. Carbon monoxide or carbon dioxide might be useful for the carbon content. But the really big prize, if it's available, would be water molecules since it contains hydrogen.

Are there water molecules available in significant amounts at aerobraking altitudes? What about oxygen and other molecules?

Thanks!

m74z00219
2009-Feb-03, 05:06 AM
Hi IsaacKuo,

While I am not too sure of the answer to your question, I suspect that the relative concentrations are more or less the same (for the gases at least) because gases tend to spread evenly.

However, since we are talking about a system with a temperature gradient (colder as you get higher) I suspect concentrations of lighter gases increase somewhat as you get higher in the atmosphere. Lighter atoms of the same kinetic energy as heavier atoms will be "hotter" as in their temperature (which is directly related to its KE) and so will tend to exist in higher concentrations at higher altitudes.
Please someone correct me if I'm wrong.

Jeff Root
2009-Feb-03, 06:27 AM
If you want hydrogen, then you're a winner, because atomic hydrogen
is the dominant component of the atmosphere above 2,500 km. Note
that this is well above the altitude of the International Space Station.

Between 1,000 km and 2,500 km the dominant component is helium.
Between 200 km and 1,000 km it is atomic oxygen. Below 200 km the
dominant component is nitrogen. Below 100 km the atmosphere is
well mixed, and except for water vapor, the constituent gases are
pretty much the same throughout.

There is relatively little water above the troposphere (12 km) because
cold temperatures at high altitudes make water condense, freeze, and
precipitate.

-- Jeff, in Minneapolis

IsaacKuo
2009-Feb-03, 12:20 PM
Thanks! That hydrogen and helium seems too thin to plausibly harvest in decent quantities. I'm not positive, but I think the altitudes which will be of most interest to my scheme will be around 100km and below. So basically the contents will be nitrogen and oxygen, along with small amounts argon and carbon dioxide?

cjameshuff
2009-Feb-03, 06:11 PM
Nitrogen's important as an atmospheric gas, for plant growth, for fuels like hydrazine, and for several abrasives, refractory materials, ultra-hard coatings, and some polymers.

Cyanogen and dicyanoacetylene are potential rocket fuels for use in space, where their toxicity is less of an issue. They burn 1.5-1.7 times as hot as hydrogen-oxygen (though dicyanoacetylene is somewhat unstable)

Venus has more nitrogen than Earth, though highly diluted with CO2 to a few percent of the total. The atmosphere of Mars contains a slightly lower proportion of nitrogen. Titan has plenty, but is rather distant. Triton is covered in frozen nitrogen, but is extremely distant. I haven't found any information on the presence of ammonia on the gas giant moons, aside from Titan...

Another thing Earth has which is useful and rare in the rest of the system is argon. Nearly 1% of the atmosphere, and the only other reasonable source I see is Mars.

m74z00219
2009-Feb-03, 07:45 PM
If you're writing a story, you might want to consider the scramjet.
http://en.wikipedia.org/wiki/Scramjet

IsaacKuo
2009-Feb-03, 11:29 PM
Unfortunately, Venus and Mars lack a big moon to get free gravitational assists from. If you want to aeroscoop air from their atmospheres, you're going to have to regain lost altitude some other way.

Cyanogen looks like a potent possibility for chemical rocket fuel, perhaps using carbon from Phobos/Deimos. How complex/difficult would mining and synthesis be, I wonder?

I've decided that at least for the near term, the best propellant to develop with this technique will likely be nitrous oxide. It offers a mediocre performance of ~190s, but it has a bunch of advantages. Synthesis from atmospheric nitrogen and oxygen is pretty straightforward, and it makes for a convenient safe carrier compound for the nitrogen and oxygen (compared to liquid nitrogen and lox). Besides being an acceptable monopropellant, nitrous oxide is a pretty good oxidizer for bipropellant rockets.

(Oh--about the scramjet concept--it is almost ridiculously hard to implement a scramjet. You literally need mixing on a molecular level in order to burn the hydrogen fuel sufficiently fast. In contrast, the aeroramjet concept uses no combustion at all, and it technically doesn't require any mixing at all. Aeroramjet can use just about any fluid or slurry or powder or ablative for propellant.)

JustAFriend
2009-Feb-04, 12:41 AM
Go look back through some old fiction.

A.C. Clarke used similar ideas diving into Jupiter and/or other outer gas giants.....

IsaacKuo
2009-Feb-04, 01:49 AM
Diving in is easy. Getting back out? That's the challenge.

This concept, when applied to Jupiter/Io, can incredibly make it possible to economically extract pure hydrogen from Jupiter's atmosphere. You can then use an aeroramjet drive to sacrifice a large fraction of the collected hydrogen to return to Earth orbit. This may be a good option for bulk shipments of hydrogen into Earth orbit, if it turns out Phobos/Deimos lack easily exploitable water ice reserves.

It takes a LOT of energy to haul hydrogen out of Jupiter's immense gravity well. However, the energy available from Io's orbital energy is also immense.