Lunar Sample Return via Tether

My father (Henry Cate) and I have come up with an initial tether project
that might be fun, affordable, and profitable. The idea is to use a
rotating tether to pickup some Lunar samples, bring them back to Earth,
and sell them.

The Apollo Lunar Orbit Rendezvous was a big win because they did not need
to land their return vehicle or fuel on the moon, just the lander. With a
rotating tether we could win even more by only having a small scoop at the
end of the tether touch the moon.

Since there is no landing vehicle, we can also use a high ISP ion drive the
whole time. By lifting a small scoop of regolith (probably under 10 Kg,
maybe under 1 Kg) many times we could lift a reasonable total mass of
lunar regolith using a small tether. For the 1.6 km/sec tip speed of a
tether for lunar pickup, the tether only needs to be like 3 to 10 times
the payload mass. The ion drive has to replace the momentum before the
next pickup.

The Dnepr at $10 to $13 mil for 9920 lbs (4500 Kg) to LEO seems like a
good deal.

http://www.spaceandtech.com/spacedat...nepr_sum.shtml

Starting with a 4500 Kg vehicle in LEO we use an ion-drive to go to the
moon, spin up a tether (probably winching in and out 2 tethers), and start
picking up samples when the end of the rotating tether touches the moon.
After picking up enough that it is running low on fuel, it could head back
to earth and have a capsule reenter with the samples.

The question of how many Kg of lunar sample you could bring back depends
on a lot of things, like ISP of thruster, watts/Kg for solar, ratio of
capsule mass to payload mass, how long the mission can be, etc. To really
get a detailed answer would take some real work. You can also trade off
time and mass returned to some extent (higher ISP can bring back more but
takes longer). Our initial guesstimate is that you could return between
1,000 Kg and 10,000 Kg in something like 1.5 to 4 years.

All of the Apollo missions combined returned 381.7 Kg. The Apollo costs
have been estimated at $100 billion in 1994 dollars (next URL). If this
project can be done for $50 million (not by NASA for sure) then this would
be like 2000 times cheaper and return 2.5 to 25 times as much. :-)

http://www.asi.org/adb/m/02/07/apollo-cost.html

In a more fair comparison, the Artemis people were going to start with
44,000 lbs in LEO (so 4.4 times as much as us). They would return between
200 lbs (95 Kg) and (227 Kg) of lunar material (we return ~4 to ~100 times
as much).

http://www.asi.org/adb/04/03/10/05/returned-mass.html

If you think of the investment cost as scaling with the lbs to LEO (a
reasonable first approximation) and the financial return value as the Kg
of Lunar material returned (not totally fair as the price goes down with a
bigger supply), then the tether method is 16 to 400 times better. Artemis
was sort of marginal as an investment, but this could be a reasonable
investment.

It is of course hard to estimate what people would pay for lunar regolith
once there was a real supply. If it was $1,000/gram and you had 2,000 Kg,
that would be $2 bil. Probably be hard to get that much, but it could
really be a good return on investment.

Additional missions would cost much less than the first, since you would
not have the development costs again. You could even design the vehicle
to be resupplied for a new mission (more xenon, new reentry capsule, etc).

This would be able to pick up samples from many parts of the Moon, any
part that passed under the orbit. If it was in a polar orbit it would even
be possible to get a sample from a dark crater at the North or South pole
to see if there was water ice.

Most of the regolith is very fine dust. You might get more money selling
lunar rocks. It might be possible to have a computer guided harpoon on
the end that could target small rocks.

Another interesting option is to use the tether to toss small reentry
capsules from the Moon in such a way that they fall back to Earth. One
nice thing about this is that you could start selling your product much
sooner. The other nice thing is that and if at some point their was a
catastrophic failure you would at least have what had been returned so
far. People bidding on what you had so far would not know how much more
would be coming. So concern that your vehicle might fail at any time
might keep the price of Lunar material high longer.

We have not seen this idea of using a rotating tether to pick up lunar
samples anyplace else and think it looks very promising. What do you
think?

-- Vince

Vincent,

Nice idea, but not original. Hoyt, Forward and Moravec have each proposed
using tethers for lunar landing and sample return some years ago.

It can be done even more cheaply than you propose.

You do not even need any ion drive or propulsion at all actually. You can
do it entirely with zero momentum exchange, you simply deposit payloads on
to the lunar surface whose mass equals that of the samples you remove.

For the details, take a look at these web links:

http://www.tethers.com/MXTethers3.html

http://www.tethers.com/MXTethers.html

Best regards,

Charles F. Radley AFAIAA

(Vincent Cate) wrote in message . com...
My father (Henry Cate) and I have come up with an initial tether project
that might be fun, affordable, and profitable. The idea is to use a
rotating tether to pickup some Lunar samples, bring them back to Earth,
and sell them.


[snip]


We have not seen this idea of using a rotating tether to pick up lunar
samples anyplace else and think it looks very promising. What do you
think?

-- Vince

Charles F. Radley wrote:
Vincent,

Nice idea, but not original. Hoyt, Forward and Moravec have each proposed
using tethers for lunar landing and sample return some years ago.

It can be done even more cheaply than you propose.

You do not even need any ion drive or propulsion at all actually. You can
do it entirely with zero momentum exchange, you simply deposit payloads on
to the lunar surface whose mass equals that of the samples you remove.

Assuming a spherical moon.
For non-spherical, non-ideal gravity moons, things get more interesting.

Ian Stirling wrote in message ...
Charles F. Radley wrote:
Vincent,

Nice idea, but not original. Hoyt, Forward and Moravec have each proposed
using tethers for lunar landing and sample return some years ago.

It can be done even more cheaply than you propose.

You do not even need any ion drive or propulsion at all actually. You can
do it entirely with zero momentum exchange, you simply deposit payloads on
to the lunar surface whose mass equals that of the samples you remove.

Assuming a spherical moon.
For non-spherical, non-ideal gravity moons, things get more interesting.

Maybe.

A lot of control can be gained by reeling the tethers in and out. A
small propulsion system might help, but it is too early to say whether
it is an essential component.

Some analysis is needed.

In article ,
(Charles F. Radley) writes:


Assuming a spherical moon.
For non-spherical, non-ideal gravity moons, things get more interesting.

Maybe.

A lot of control can be gained by reeling the tethers in and out. A
small propulsion system might help, but it is too early to say whether
it is an essential component.

Reeling in the tether between loads (just enough to miss terrain) seemed
like the obvious idea to me, too. Therefore, there must be something wrong
with it.
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? It *would* take quite a while, but if we have days or weeks
between loads, it should be within reach.
The first problem that comes to mind is the longevity of a mechanical
mechanism in hard vacuum for long periods without maintenance, but with
magnetic bearings that should be surmountable, surely? Actually, come to think
of it, the momentum wheel would probably be less of a mechanical problem than
reeling the tether in/out.

(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

(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

In article ,
(Charles F. Radley) writes:


Assuming a spherical moon.
For non-spherical, non-ideal gravity moons, things get more interesting.

Maybe.

A lot of control can be gained by reeling the tethers in and out. A
small propulsion system might help, but it is too early to say whether
it is an essential component.

Reeling in the tether between loads (just enough to miss terrain) seemed
like the obvious idea to me, too. Therefore, there must be something wrong
with it.
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? It *would* take quite a while, but if we have days or weeks
between loads, it should be within reach.
The first problem that comes to mind is the longevity of a mechanical
mechanism in hard vacuum for long periods without maintenance, but with
magnetic bearings that should be surmountable, surely? Actually, come to think
of it, the momentum wheel would probably be less of a mechanical problem than
reeling the tether in/out.

Ian Stirling wrote in message ...
Charles F. Radley wrote:
Vincent,

Nice idea, but not original. Hoyt, Forward and Moravec have each proposed
using tethers for lunar landing and sample return some years ago.

It can be done even more cheaply than you propose.

You do not even need any ion drive or propulsion at all actually. You can
do it entirely with zero momentum exchange, you simply deposit payloads on
to the lunar surface whose mass equals that of the samples you remove.

Assuming a spherical moon.
For non-spherical, non-ideal gravity moons, things get more interesting.

Maybe.

A lot of control can be gained by reeling the tethers in and out. A
small propulsion system might help, but it is too early to say whether
it is an essential component.

Some analysis is needed.

Charles F. Radley wrote:
Vincent,

Nice idea, but not original. Hoyt, Forward and Moravec have each proposed
using tethers for lunar landing and sample return some years ago.

It can be done even more cheaply than you propose.

You do not even need any ion drive or propulsion at all actually. You can
do it entirely with zero momentum exchange, you simply deposit payloads on
to the lunar surface whose mass equals that of the samples you remove.

Assuming a spherical moon.
For non-spherical, non-ideal gravity moons, things get more interesting.