| Topic: |
Science > Physics |
| User: |
"Robert Clark" |
| Date: |
05 Apr 2006 10:04:51 AM |
| Object: |
Harvesting fuel from the Saturn system. |
Nasa scientists are investigating possibilities for follow-up missions
to the Cassini mission to the Saturnian moons Titan and Enceladus:
Encore For Enceladus! Saturn Moon Ripe For Astrobiology Exploration
http://www.space.com/scienceastronomy/060403_mystery_monday.html
Ideal would be sample return missions. A problem would be carrying
enough fuel for landing and assent. It may be we can get the required
fuel for the return from the Saturn system itself.
This article in Science gives the estimated amount of molecular oxygen
above the Saturn A ring:
Oxygen Ions Observed Near Saturn's A Ring.
J. H. Waite, Jr., T. E. Cravens, W.-H. Ip, W. T. Kasprzak, J. G.
Luhmann, R. L. McNutt, H. B. Niemann, R. V. Yelle, I. Mueller-Wodarg,
S. A. Ledvina, S. Scherer
Science, 25 February 2005: Vol. 307. no. 5713, pp. 1260 - 1262
http://www.sciencemag.org/cgi/content/abstract/307/5713/1260
It estimates the number of neutral O2 molecules as 10^4 to 10^5 cm^-3.
However, they note it could be much higher than this range because of
the limitations of the measurements. I'll take the upper number, 10^5
cm^-3. There are 10^15 cubic centimenters in a cubic kilometer so this
amounts to 10^20 molecules per km^3.
The article gives the orbital velocity around Saturn at the radial
distance of the A ring as in the range of 15 km/s. Actually the article
explains there are magnetic effects that accelerate the various ionized
molecules even faster which when exchanging momentum with the neutral
molecules accelerate these faster as well. I'll use the 15 km/s number
for simplicity. Then if we orbit the spacecraft in the opposite
direction we would have a relative velocity with respect to these
molecules in the range of 30 km/s. So if we had a scoop with a 1km x
1km opening we could collect 30 x 10^20 molecules of O2 per second. To
calculate the mass of this oxygen, Avogadro's number of O2 molecules,
6.0 x 10^23, amounts to 32 grams. So 30 x 10^20 molecules is 5 x 10^-3
moles or (5 x 10^-3) x 32 grams = 160 x 10^-3 grams. This is the mass
collected every second. There are 31,536,000 seconds in a year, so
after a year we would have 31,536,000 x 160 x 10^-3 grams = 5,045,760
grams, or 5045.76 kilos of O2.
This article gives the density of water molecules around Enceladus:
Enceladus Eruptions.
Larry W. Esposito Larry W. Esposito
Principal Investigator Principal Investigator
UV imaging Spectrograph UV imaging Spectrograph
http://saturn.jpl.nasa.gov/multimedia/products/pdfs/20050830_CHARM_Esposito.pdf
On page 21 is given the density of water molecules versus altitude. The
greatest density shown is about 10^7 molecules per cubic centimeter at
200 km altitude. The methane is 1.6 percent of this amount, so 1.6 x
10^5 molecules/cc. However, you couldn't move at high velocity around
Enceladus such as 15 km/sec because the orbital velocity around it is
so low.
One possibility would be to put it at one of the two Lagrange points
that is close to the satellite. The distance from the surface would be
higher than 200km so the density of the methane would be lower. You
would need pumps then to draw in the methane. You would also have to
separate out the more prevalent water from the methane.
Getting back to the molecular oxygen found around the rings, the
researchers also found monoatomic hydrogen and monoatomic oxygen. They
give the amounts of the ionized versions, but not the larger neutral
amounts.
Both of these are known to be very efficient for propulsion: pure
monoatomic hydrogen has about 3 to 4 times the ISP (specific impulse)
as H2 with O2 as an oxidizer. A problem though that still has not been
solved is how to store the monoatomic propellants stably within a
rocket.
If all you wanted to do was to accelerate the rocket from Saturn then
you could just use the monoatomic hydrogen as it normally combusts by
bringing the single atoms together. That is, you would not need to
store it. You couldn't use it though as an onboard propellant to engage
in trajectory changes or landings and launches form satellites.
Bob Clark
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| User: "Henry Spencer" |
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| Title: Re: Harvesting fuel from the Saturn system. |
12 Apr 2006 12:54:14 PM |
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In article <1144249490.936492.148290@j33g2000cwa.googlegroups.com>,
Robert Clark <rgregoryclark@yahoo.com> wrote:
If all you wanted to do was to accelerate the rocket from Saturn then
you could just use the monoatomic hydrogen as it normally combusts by
bringing the single atoms together...
Unfortunately, it's hard to make such a "recombination ramjet" work well.
(The idea got studied in some depth in the late 1950s, because there is
monoatomic oxygen in Earth's upper atmosphere.)
Recombination on a solid catalyst is straightforward, but that requires
decelerating the incoming gas, which heats it, and thermal dissociation
ruins things unless the "airspeed" is quite low.
If you don't want to decelerate the gas, you need some magic way of
inducing recombination without contact... and nobody was ever able to find
a good one. It doesn't help that recombination is a three-body process:
when two atoms come together, they always have enough energy to come apart
again, unless there's a third atom which can carry off some of the energy.
The probability of three-body interactions is very low in a rarefied gas,
which is why such dissociated gases can persist in the first place.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |
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| User: "Robert Clark" |
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| Title: Re: Harvesting fuel from the Saturn system. |
29 Apr 2006 12:19:59 PM |
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Henry Spencer wrote:
In article <1144249490.936492.148290@j33g2000cwa.googlegroups.com>,
Robert Clark <rgregoryclark@yahoo.com> wrote:
If all you wanted to do was to accelerate the rocket from Saturn then
you could just use the monoatomic hydrogen as it normally combusts by
bringing the single atoms together...
Unfortunately, it's hard to make such a "recombination ramjet" work well.
(The idea got studied in some depth in the late 1950s, because there is
monoatomic oxygen in Earth's upper atmosphere.)
Recombination on a solid catalyst is straightforward, but that requires
decelerating the incoming gas, which heats it, and thermal dissociation
ruins things unless the "airspeed" is quite low.
If you don't want to decelerate the gas, you need some magic way of
inducing recombination without contact... and nobody was ever able to find
a good one. It doesn't help that recombination is a three-body process:
when two atoms come together, they always have enough energy to come apart
again, unless there's a third atom which can carry off some of the energy.
The probability of three-body interactions is very low in a rarefied gas,
which is why such dissociated gases can persist in the first place.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |
It seems to me the hydrogen would be brought together if you used a
converging scoop like proposed for the Bussard ramjet. Here the
collected material is automatically compressed by the scoop. You might
have the problem with the monoatomic hydrogen or oxygen combining with
the scoop walls.
Perhaps the walls could be covered with an inert element such as
argon, krypton, etc. that has a melting point significantly higher than
the temperature of interplanetary space, about 40 K. But you have the
problem of the high temperatures due to the combustion heating the
walls to thousands of degrees. Perhaps the method of regenerative
cooling could be used where cryogenic liquid is circulated around the
outside of the engine. However I don't know if this would work for when
the temperature of the walls have to be kept above say -100 C, rather
than the melting point of steel, about 1500 C.
Are there compounds that are stable solids at high combustion
temperatures that do not react with monoatomic oxygen or monoatomic
hydrogen?
Bob Clark
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| User: "Henry Spencer" |
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| Title: Re: Harvesting fuel from the Saturn system. |
29 Apr 2006 04:01:07 PM |
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In article <1146331199.125140.221360@y43g2000cwc.googlegroups.com>,
Robert Clark <rgregoryclark@yahoo.com> wrote:
Unfortunately, it's hard to make such a "recombination ramjet" work well.
(The idea got studied in some depth in the late 1950s...
It seems to me the hydrogen would be brought together if you used a
converging scoop like proposed for the Bussard ramjet.
There are unfortunately some practical difficulties.
One is that the scoop has to be *very* large. This is very thin gas. The
hypothetical Bussard ramscoop used magnetic fields, somehow... but they
won't work on neutral atoms. And while Bussard-ramjet designers can
handwave about trickery to ionize the incoming atoms, that takes far more
energy than you're hoping to recover from the recombination ramjet.
Another is that the scoop, be it material or magnetic, creates quite a bit
of drag. (In fact, the magsail was born when Andrews&Zubrin discovered
that Andrews's concept for a Bussard ramjet was generating much more drag
than thrust.)
The big one is that if your scoop has physical walls, the incoming gas
atoms don't just bounce off them. What usually happens instead is that
the gas atom hits the wall and *sticks* -- its kinetic energy turns into
heat, in the atom and the wall -- and then a moment later pulls itself
loose and wanders off in a random direction. The result is exactly what
you're trying to avoid: the gas gets decelerated and thereby heated.
As I noted, people looked at this quite thoroughly in the 1950s. The USAF
was very interested and was willing to part with quite large sums of money
if somebody could make an oxygen-recombination ramjet work. Nobody came
up with anything that even *looked* like it might work.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |
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| User: "Robert Clark" |
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| Title: Re: Harvesting fuel from the Saturn system. |
30 Apr 2006 12:48:49 AM |
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Henry Spencer wrote:
In article <1146331199.125140.221360@y43g2000cwc.googlegroups.com>,
Robert Clark <rgregoryclark@yahoo.com> wrote:
Unfortunately, it's hard to make such a "recombination ramjet" work well.
(The idea got studied in some depth in the late 1950s...
It seems to me the hydrogen would be brought together if you used a
converging scoop like proposed for the Bussard ramjet.
There are unfortunately some practical difficulties.
One is that the scoop has to be *very* large. This is very thin gas. The
hypothetical Bussard ramscoop used magnetic fields, somehow... but they
won't work on neutral atoms. And while Bussard-ramjet designers can
handwave about trickery to ionize the incoming atoms, that takes far more
energy than you're hoping to recover from the recombination ramjet.
Another is that the scoop, be it material or magnetic, creates quite a bit
of drag. (In fact, the magsail was born when Andrews&Zubrin discovered
that Andrews's concept for a Bussard ramjet was generating much more drag
than thrust.)
The big one is that if your scoop has physical walls, the incoming gas
atoms don't just bounce off them. What usually happens instead is that
the gas atom hits the wall and *sticks* -- its kinetic energy turns into
heat, in the atom and the wall -- and then a moment later pulls itself
loose and wanders off in a random direction. The result is exactly what
you're trying to avoid: the gas gets decelerated and thereby heated.
As I noted, people looked at this quite thoroughly in the 1950s. The USAF
was very interested and was willing to part with quite large sums of money
if somebody could make an oxygen-recombination ramjet work. Nobody came
up with anything that even *looked* like it might work.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |
I just saw this link on sci.astro:
Bumpy Dust Makes Molecular Hydrogen.
"For two hydrogen atoms to have enough energy to bond in the cold
reaches of space, they first have to meet on a surface, explained Eric
Herbst, Distinguished University Professor of physics at Ohio State.
"Though scientists suspected that space dust provided the necessary
surface for such chemical reactions, laboratory simulations of the
process never worked. At least, they didn't work well enough to explain
the full abundance of molecular hydrogen that scientists see in space."
....
"Given the amount of dust that scientists think is floating in space,
the Ohio State researchers were able to simulate the creation of the
right amount of hydrogen, but only on bumpy surfaces.
"When it comes to making molecular hydrogen, the ideal microscopic host
surface is "less like the flatness of Ohio and more like a Manhattan
skyline," Herbst said."
http://www.universetoday.com/am/publish/printer_bumpy_dust_hydrogen.html
Apparently, this was shown only through simulations, but assuming this
holds up in experiments perhaps this might provide a means of combining
the monoatomic hydrogen into H2 by providing very bumpy dust for the
hydrogen to adhere to.
Conversely, this might provide a means of preventing the monoatomic
from adhering to the walls by making them atomically smooth.
- Bob Clark
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| User: "Henry Spencer" |
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| Title: Re: Harvesting fuel from the Saturn system. |
30 Apr 2006 04:09:44 PM |
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In article <1146376129.692428.107550@y43g2000cwc.googlegroups.com>,
Robert Clark <rgregoryclark@yahoo.com> wrote:
I just saw this link on sci.astro...
Bumpy Dust Makes Molecular Hydrogen...
...perhaps this might provide a means of combining
the monoatomic hydrogen into H2 by providing very bumpy dust for the
hydrogen to adhere to.
Unfortunately, it's a solid catalyst and therefore requires decelerating
the incoming atoms, bringing them more or less to rest with respect to the
engine. Same old heating problem.
Conversely, this might provide a means of preventing the monoatomic
from adhering to the walls by making them atomically smooth.
Smooth surfaces don't catalyze hydrogen recombination well, it would
appear, but the atoms still stick momentarily. You have to work quite
hard, and set up quite unusual conditions, to get atoms of a rarefied gas
to just bounce off a solid surface.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |
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| User: "ma1ibu" |
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| Title: Re: Harvesting fuel from the Saturn system. |
05 Apr 2006 10:49:15 AM |
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Rockets ain't gonna do it, Bob.
Think 'magnetic effects'.
John
Galaxy Model for the Atom
(or how we can make matter have
virtually no inertia)
http://users.accesscomm.ca/john
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