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Lunar Sample Return via Tether



 
 
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  #21  
Old December 8th 03, 09:35 PM
Andrew Nowicki
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Default Lunar Sample Return via Tether

I believe there is more money to make on hands-on space experience than
on the Moon rocks. It would be fun for the overweight Internet generation
to control a low Earth orbit satellite and take pictures of a home town.
It would be even more fun to do some real work. For example, the lunavator
can deposit toy size, rock hunting excavators on the visible half of the
Moon so that relatively cheap terrestrial microwave dishes can communicate
directly with the excavators. The Internet users would control the excavators
with a mouse. They would hunt for rocks and scratch graffiti on the Moon
dust. A few years ago a businessman considered launching a remotely
controlled lunar buggy and renting it for "virtual" joy riders. Whatever
you do, keep in mind that novelty of such space hardware will wear off
quickly unless the hardware can do new things that have not been done before.

I would start with finding a massive piece of junk in low Earth orbit and
attaching a 100 kg Zylon bolo to the junk. The bolo has internal wires which
can be used as electrodynamic tethers and power supply for a remote
manipulator riding on the bolo. The wires can make lots of electric power
at night, and they can control the orbit of the junk/bolo satellite. Add
a 200 kg sounding rocket, a few Hall thrusters, and you can use the bolo
to launch your lunavator in 20 kg pieces assembled in space with another
remote manipulator. When the lunavator is assembled, you have your own,
dirt cheap, lunavator bolo relay, which is Earth-to-orbit, Earth-to-Moon,
and Moon-to-Earth space transportation system.
  #22  
Old December 9th 03, 04:38 AM
Vincent Cate
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Default Lunar Sample Return via Tether

(Henry Spencer) wrote in message ...
In article ,
Vincent Cate wrote:
...with the momentum
from 1 Kg of xenon we can pick up 98 kps / 1.6 kps or about 61 Kg
of regolith. If you simply deposited equal mass on the lunar surface
you would only pick up 1 Kg for every 1 Kg you put down. So this way
is cheaper for a probe on a sample return type mission.


Yes and no and maybe. It uses less fuel, but that does not necessarily
equate to "cheaper". The issue is not fuel cost -- xenon is one of the
few fuels whose cost actually *is* comparable to LEO launch costs, but
even so, that's a relatively minor issue here -- but hardware costs and
complexity and reliability.


I should not have said "cheaper", I really meant "a better business case".
If you did not use a high ISP thruster, xenon, or lots of solar cells, then
you could clearly make something that was cheaper.

However, without a high ISP thruster the deltaV from LEO to LLO (low
lunar orbit) and the deltaV from LLO back to Earth each reduce your mass
by some factor, maybe a factor of 10 together. So on top of the factor of
61 above we have reduced our return by another factor of 10 for a total
reduction factor of 610. So if we had 6,100 Kg of returned regolith in
the high ISP method, now we have 10 Kg of regolith to sell for profit.
It may not be exactly this bad because you save something on solar Kg too.
But whatever the exact number, but I don't think it would be better business.

If we have a high ISP thruster for getting to the moon and back, then we
might as well use it to reboost momentum after we do a tether pickup.

It may well be that a Hall Thruster makes more sense than an Ion Drive
because it is cheaper, needs less solar power, (and lasts longer?). I
can can not say what the ideal ISP or type of thruster is just yet. But
I am convinced that some kind of high ISP thruster makes sense for this
type of mission.

-- Vince
  #23  
Old December 9th 03, 04:38 AM
Vincent Cate
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Posts: n/a
Default Lunar Sample Return via Tether

(Henry Spencer) wrote in message ...
In article ,
Vincent Cate wrote:
...with the momentum
from 1 Kg of xenon we can pick up 98 kps / 1.6 kps or about 61 Kg
of regolith. If you simply deposited equal mass on the lunar surface
you would only pick up 1 Kg for every 1 Kg you put down. So this way
is cheaper for a probe on a sample return type mission.


Yes and no and maybe. It uses less fuel, but that does not necessarily
equate to "cheaper". The issue is not fuel cost -- xenon is one of the
few fuels whose cost actually *is* comparable to LEO launch costs, but
even so, that's a relatively minor issue here -- but hardware costs and
complexity and reliability.


I should not have said "cheaper", I really meant "a better business case".
If you did not use a high ISP thruster, xenon, or lots of solar cells, then
you could clearly make something that was cheaper.

However, without a high ISP thruster the deltaV from LEO to LLO (low
lunar orbit) and the deltaV from LLO back to Earth each reduce your mass
by some factor, maybe a factor of 10 together. So on top of the factor of
61 above we have reduced our return by another factor of 10 for a total
reduction factor of 610. So if we had 6,100 Kg of returned regolith in
the high ISP method, now we have 10 Kg of regolith to sell for profit.
It may not be exactly this bad because you save something on solar Kg too.
But whatever the exact number, but I don't think it would be better business.

If we have a high ISP thruster for getting to the moon and back, then we
might as well use it to reboost momentum after we do a tether pickup.

It may well be that a Hall Thruster makes more sense than an Ion Drive
because it is cheaper, needs less solar power, (and lasts longer?). I
can can not say what the ideal ISP or type of thruster is just yet. But
I am convinced that some kind of high ISP thruster makes sense for this
type of mission.

-- Vince
  #24  
Old December 9th 03, 04:58 AM
Vincent Cate
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Default Lunar Sample Return via Tether

(Charles F. Radley) wrote in message . com...
The Hoyt/Forward proposals do not need infrastructure on the Moon.
A bit of infrastructure might be nice to collect the specimens ahead
of time and make them ready for pick-up, but that is not essential.


It could be nice to have some tinny rovers collect some rocks and put
them in a basket for pickup. I really suspect customers would pay more for
lunar rocks than for lunar dust. But to pickup the basket you need
to be accurate about where the end of your tether goes, and there is
no GPS system on the moon so far. So there is added complexity and cost.

OK. Whether that is better than zero momentum exchange depends on

whether you are interested in putting payloads on the lunar surface.
Since soft landing on the Moon using rockets is extremely expensive,
the tether method of depositing payloads is much cheaper. It would
open up new markets for lunar development.


If there were people willing to pay big bucks to land small payloads
on the moon then it would be something to do. But if you give up
60 Kg of regolith that you can sell for $250+/gram in order to lower
1 Kg onto the moon you would have to charge at least 60*250*1000 or
$15 mil/Kg. But if their are customers, sure.

In the short term we can soft land lots of infrastructure on to the
Moon basically for free (except for the cost of launch from Earth),
and build hotels ready for the tourists when they come.


Also the cost of what we give up by having that payload. We only have
a limited total mass to work with. If we put on 1 Kg of something
going to the moon and we have to give up 1 Kg of xenon, it would costs us
a lot of valuable regolith.

-- Vince
  #25  
Old December 9th 03, 04:58 AM
Vincent Cate
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Posts: n/a
Default Lunar Sample Return via Tether

(Charles F. Radley) wrote in message . com...
The Hoyt/Forward proposals do not need infrastructure on the Moon.
A bit of infrastructure might be nice to collect the specimens ahead
of time and make them ready for pick-up, but that is not essential.


It could be nice to have some tinny rovers collect some rocks and put
them in a basket for pickup. I really suspect customers would pay more for
lunar rocks than for lunar dust. But to pickup the basket you need
to be accurate about where the end of your tether goes, and there is
no GPS system on the moon so far. So there is added complexity and cost.

OK. Whether that is better than zero momentum exchange depends on

whether you are interested in putting payloads on the lunar surface.
Since soft landing on the Moon using rockets is extremely expensive,
the tether method of depositing payloads is much cheaper. It would
open up new markets for lunar development.


If there were people willing to pay big bucks to land small payloads
on the moon then it would be something to do. But if you give up
60 Kg of regolith that you can sell for $250+/gram in order to lower
1 Kg onto the moon you would have to charge at least 60*250*1000 or
$15 mil/Kg. But if their are customers, sure.

In the short term we can soft land lots of infrastructure on to the
Moon basically for free (except for the cost of launch from Earth),
and build hotels ready for the tourists when they come.


Also the cost of what we give up by having that payload. We only have
a limited total mass to work with. If we put on 1 Kg of something
going to the moon and we have to give up 1 Kg of xenon, it would costs us
a lot of valuable regolith.

-- Vince
  #28  
Old December 10th 03, 06:49 PM
Vincent Cate
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Default Lunar Sample Return via Tether

(SkyeFire) wrote in message ...
Stupid question time: assuming that the cargo schedule isn't
very heavy, why would we necessarily need incoming cargo or propulsion
to keep the tether's momentum up? If we had a momentum wheel powered
by a solar-electric motor mounted in the hub of the tether, couldn't
we win back rotation that way with minimum effort?


There are 2 types of momentum you need to keep under control. One
is the rotational momentum around your own center of mass and the other
is orbital momentum around the moon.

The rotational momentum of even 1 Kg at the end of a 100,000 meter long
tether is so huge that no momentum wheel will have any impact on it.
However, you can easily control a tether's rotational momentum, when
near a gravitational body, by winching the tether in and out as it is
going up or down relative to the body. If you want to rotate faster
you let it out on the down side and winch in on the up side so it spends
more time going down (and pulled faster) than going up (and pulled slower).
So rotational momentum is an easily solved problem.

The orbital momentum needs to be controlled by either leaving something
on the surface of the moon of equal mass to what you are picking up,
or using some kind of thruster. The thruster could be a conventional
chemical rocket, an electric thruster, or a solar sail. All of these,
including leaving something on the surface, can be looked at in terms
of ISP or exhaust velocity.

ISP Exhaust velocity
Leaving mass on surface 163 1.6 km/sec
Chemical Rocket 400 3.9 km/sec
Hall Thruster 2,000 19.6 km/sec
Ion Drive 10,000 98 km/sec
Solar Sail infinite speed of light - solar photons

The Hall Thrusters and Ion Drives come in different ISPs, these are just
some sample values. Note that ISP times 9.8 equals the exhaust velocity
in meters/sec.

The ratio of lunar-pickup-mass/reaction-mass is the same as the
reaction-mass-exhaust-velocity/lunar-orbital-speed so that momentum
is conserved. The orbital speed you will be giving the regolith is
about 1.6 km/sec. The higher the exhaust velocity the less reaction
mass you need. In the solar sail case the reaction mass keeps coming
to you from the sun, so it is sort of an infinite ISP.

We are used to needing lots of rocket fuel to lift a small payload, since
launching from Earth you might use 30 to 100 times as much reaction mass
as you get payload to orbit. With a 10,000 second ISP ion drive and a
tether, we could lift 61 Kg of lunar regolith for every 1 Kg of reaction
mass (98/1.6=61). This is so amazingly good that it takes awhile to
sink in.

With a solar sail the only thing limiting how much you can lift is how
long your system keeps working.

-- Vince
  #29  
Old December 10th 03, 06:49 PM
Vincent Cate
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Posts: n/a
Default Lunar Sample Return via Tether

(SkyeFire) wrote in message ...
Stupid question time: assuming that the cargo schedule isn't
very heavy, why would we necessarily need incoming cargo or propulsion
to keep the tether's momentum up? If we had a momentum wheel powered
by a solar-electric motor mounted in the hub of the tether, couldn't
we win back rotation that way with minimum effort?


There are 2 types of momentum you need to keep under control. One
is the rotational momentum around your own center of mass and the other
is orbital momentum around the moon.

The rotational momentum of even 1 Kg at the end of a 100,000 meter long
tether is so huge that no momentum wheel will have any impact on it.
However, you can easily control a tether's rotational momentum, when
near a gravitational body, by winching the tether in and out as it is
going up or down relative to the body. If you want to rotate faster
you let it out on the down side and winch in on the up side so it spends
more time going down (and pulled faster) than going up (and pulled slower).
So rotational momentum is an easily solved problem.

The orbital momentum needs to be controlled by either leaving something
on the surface of the moon of equal mass to what you are picking up,
or using some kind of thruster. The thruster could be a conventional
chemical rocket, an electric thruster, or a solar sail. All of these,
including leaving something on the surface, can be looked at in terms
of ISP or exhaust velocity.

ISP Exhaust velocity
Leaving mass on surface 163 1.6 km/sec
Chemical Rocket 400 3.9 km/sec
Hall Thruster 2,000 19.6 km/sec
Ion Drive 10,000 98 km/sec
Solar Sail infinite speed of light - solar photons

The Hall Thrusters and Ion Drives come in different ISPs, these are just
some sample values. Note that ISP times 9.8 equals the exhaust velocity
in meters/sec.

The ratio of lunar-pickup-mass/reaction-mass is the same as the
reaction-mass-exhaust-velocity/lunar-orbital-speed so that momentum
is conserved. The orbital speed you will be giving the regolith is
about 1.6 km/sec. The higher the exhaust velocity the less reaction
mass you need. In the solar sail case the reaction mass keeps coming
to you from the sun, so it is sort of an infinite ISP.

We are used to needing lots of rocket fuel to lift a small payload, since
launching from Earth you might use 30 to 100 times as much reaction mass
as you get payload to orbit. With a 10,000 second ISP ion drive and a
tether, we could lift 61 Kg of lunar regolith for every 1 Kg of reaction
mass (98/1.6=61). This is so amazingly good that it takes awhile to
sink in.

With a solar sail the only thing limiting how much you can lift is how
long your system keeps working.

-- Vince
  #30  
Old December 10th 03, 10:09 PM
Vincent Cate
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Default Lunar Sample Return via Tether

I am not sure how far you could scale this project down. I don't think
it would be too hard to get by on the 1,300 lbs to LEO of Space-X.
I expect you could even get down to 500 lbs.

At some smallness it gets hard to make fault tolerant tethers, but I
am not sure where that is exactly. I don't think space junk
is nearly the issue around Luna as around Earth. There has not been
the human stuff smashing into each other and making lots of orbiting
junk. You can get fault tolerance by having spare tethers, as
the tethers you need for the moon are not really too heavy. You can
make your tether shorter than 100 km, say 5 km, since the non-human
payload can tolerate high Gs. This also reduces the chance of collision.
Very small diameter Spectra lines are available. And if your last
tether did happen to break, you just head back to Earth with whatever
regolith you have so far. So a very small tether is probably doable.

Scaling down solar power is easy. You can get small Hall Thrusters.
At busek.com they have one that is just 900 grams.

It might really be possible to do this project in under 100 lbs,
though that would be impressive.

-- Vince
 




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