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

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

If your lunavator is a sophisticated craft, rocket or ion
propulsion is needed for emergency only. You can increase
your orbital energy by picking up Moon dust, reeling it in
and dropping it backwards. With the help of an anchor you
can even change your orbital plane; just drop your anchor
on the Moon off your orbital plane and pull it gently.

================================================== ============

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

I believe the size of the transponder that can send live video
to the Earth determines the minimum mass of the lunavator craft.

VC At some smallness it gets hard to make fault tolerant
VC tethers, but I am not sure where that is exactly. I
VC don't think space junk is nearly the issue around Luna
VC as around Earth.

True. Ribbon-shaped tethers are better than rope-shaped
tethers. I do not have lunar meteoroid data, but I have
terrestrial data at: http://www.islandone.org/LEOBiblio/SPBI1RE1.GIF

VC You can make your tether shorter than 100 km, say 5 km,
VC since the non-human payload can tolerate high Gs.

True.

VC This also reduces the chance of collision.

False. The longer the lunavator, the safer it is. Make sure
the centrifugal acceleration (v*v/r) of the lunavator tip
is at least several times the gravity on the lunar surface.
If the centrifugal acceleration is too low, your lunavator
will vibrate (librations).

VC Very small diameter Spectra lines are available.

Plastic fibers are cheap. You can weave your own ribbon
shaped tether from the fibers. A much more serious problem
is space radiation. Inorganic (glass, carbon) fibers are
immune to space radiation.

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

You are on the right track.

VC ...there is no GPS system on the moon so far.

If your mini excavators are conspicuous, you do not need GPS
on the Moon. There are many ways to make them conspicuous:
radio beacons, laser retro-reflectors, microwave
retro-reflectors...

VC The rotational momentum of even 1 Kg at the end of a
VC 100,000 meter long tether is so huge that no momentum
VC wheel will have any impact on it.

Having a small momentum wheel is useful for fine adjustment
of the angular velocity of the lunavator. If you do the
adjustments with the winch only, the lunavator shakes
too much, and you have to use mini rocket thrusters.

VC The orbital momentum needs to be controlled by either
VC leaving something on the surface of the moon of equal
VC mass to what you are picking up, or using some kind of
VC thruster.

False!

Pick up Moon dust, reel it in and drop it backwards. You
will gain both orbital energy, and angular momentum about
the center of lunavator's mass. There are many ways to loose
the extra angular momentum. Probably the cheapest one is
dragging an anchor on the dusty Moon surface.

PS. You do not need rocket or ion thruster to fly your
lunavator back to the Earth. You can use the Moon dust
as reaction mass.

================================================== ============

Henry Spencer wrote:
HS The issue is not fuel cost -- xenon is one of the few fuels
HS whose cost actually *is* comparable to LEO launch costs, but
HS even so, that's a relatively minor issue here -- but hardware
HS costs and complexity and reliability.

The complexity and reliability must be vied in the context of
fail safe design. We are not talking about rocket launchers,
so we are not talking about catastrophic failure.

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

If your lunavator is a sophisticated craft, rocket or ion
propulsion is needed for emergency only. You can increase
your orbital energy by picking up Moon dust, reeling it in
and dropping it backwards. With the help of an anchor you
can even change your orbital plane; just drop your anchor
on the Moon off your orbital plane and pull it gently.

================================================== ============

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

I believe the size of the transponder that can send live video
to the Earth determines the minimum mass of the lunavator craft.

VC At some smallness it gets hard to make fault tolerant
VC tethers, but I am not sure where that is exactly. I
VC don't think space junk is nearly the issue around Luna
VC as around Earth.

True. Ribbon-shaped tethers are better than rope-shaped
tethers. I do not have lunar meteoroid data, but I have
terrestrial data at: http://www.islandone.org/LEOBiblio/SPBI1RE1.GIF

VC You can make your tether shorter than 100 km, say 5 km,
VC since the non-human payload can tolerate high Gs.

True.

VC This also reduces the chance of collision.

False. The longer the lunavator, the safer it is. Make sure
the centrifugal acceleration (v*v/r) of the lunavator tip
is at least several times the gravity on the lunar surface.
If the centrifugal acceleration is too low, your lunavator
will vibrate (librations).

VC Very small diameter Spectra lines are available.

Plastic fibers are cheap. You can weave your own ribbon
shaped tether from the fibers. A much more serious problem
is space radiation. Inorganic (glass, carbon) fibers are
immune to space radiation.

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

You are on the right track.

VC ...there is no GPS system on the moon so far.

If your mini excavators are conspicuous, you do not need GPS
on the Moon. There are many ways to make them conspicuous:
radio beacons, laser retro-reflectors, microwave
retro-reflectors...

VC The rotational momentum of even 1 Kg at the end of a
VC 100,000 meter long tether is so huge that no momentum
VC wheel will have any impact on it.

Having a small momentum wheel is useful for fine adjustment
of the angular velocity of the lunavator. If you do the
adjustments with the winch only, the lunavator shakes
too much, and you have to use mini rocket thrusters.

VC The orbital momentum needs to be controlled by either
VC leaving something on the surface of the moon of equal
VC mass to what you are picking up, or using some kind of
VC thruster.

False!

Pick up Moon dust, reel it in and drop it backwards. You
will gain both orbital energy, and angular momentum about
the center of lunavator's mass. There are many ways to loose
the extra angular momentum. Probably the cheapest one is
dragging an anchor on the dusty Moon surface.

PS. You do not need rocket or ion thruster to fly your
lunavator back to the Earth. You can use the Moon dust
as reaction mass.

================================================== ============

Henry Spencer wrote:
HS The issue is not fuel cost -- xenon is one of the few fuels
HS whose cost actually *is* comparable to LEO launch costs, but
HS even so, that's a relatively minor issue here -- but hardware
HS costs and complexity and reliability.

The complexity and reliability must be vied in the context of
fail safe design. We are not talking about rocket launchers,
so we are not talking about catastrophic failure.

Andrew Nowicki wrote in message ...
VC You can make your tether shorter than 100 km, say 5 km,
VC since the non-human payload can tolerate high Gs.
AN True.
VC This also reduces the chance of collision.
AN False. The longer the lunavator, the safer it is.

If you are going to say someone is wrong, it is nice to say why.
So, why do you think that a longer tether is safer from collision?
The odds of hitting a large object are not increased much by
the thickness of the tether and they increase linearly with the
length. Even on a simple swept area basis, a shorter cable is going
to be safer. If you make a cable twice as strong and half as long,
it is not twice as wide, only sqrt(2) as wide.

VC The rotational momentum of even 1 Kg at the end of a
VC 100,000 meter long tether is so huge that no momentum
VC wheel will have any impact on it.

Having a small momentum wheel is useful for fine adjustment
of the angular velocity of the lunavator. If you do the
adjustments with the winch only, the lunavator shakes
too much, and you have to use mini rocket thrusters.

Do you have anything to backup either of these claims?

VC The orbital momentum needs to be controlled by either
VC leaving something on the surface of the moon of equal
VC mass to what you are picking up, or using some kind of
VC thruster.

False!

Pick up Moon dust, reel it in and drop it backwards.

Ok, you can use the tether as a thruster with the regolith
as your reaction mass. And this is fun, for sure. There
are a couple issues, though it could be a reasonable
thing to do.

When you picked up the regolith it was going backwards
relative to your center of mass about 1.6 km/sec.
So you have to throw it backwards faster than this to get
any net thrust. If you want to throw it back twice as fast
then you are going to winch in like halfway first. So now
you have twice the tip speed and half the length. Given
that centrifugal force is v^2/r your tether now needs to
be 8 times as strong (2^2/0.5). This is an issue, but
it would not be as bad if you tossed slower.

The second issue is that you can easily make all kinds of
space junk that hits you or someone else on some future
orbit. By picking your velocity to be something less
than 3.2 km/sec backwards relative to the center of mass
of the tether, then it will be less than 1.6 km/sec relative
to the moon and fall to the moon. You might want to avoid
hitting Apollo sites, or any man made objects on the moon.
But this does not seem like killer problem either.

Ya, I think it could work. Both issues get easier if you
don't toss very fast. Since there is no shortage of mass,
that is ok.

Since you would not be limited by a fixed supply of xenon
reaction mass, you could bring back more lunar regolith
this way. It should take less in the way of solar power
since the energy put into the reaction mass goes up with
the velocity squared (1/2 m v^2). So you save mass on
solar and on the xenon. So you can put more mass into
making a bigger reentry capsule and bring back more regolith.
I can imaging starting with 4,200 Kg in LEO from a Falcon-5
and bringing back a few times this mass in regolith.

There are many ways to loose
the extra angular momentum. Probably the cheapest one is
dragging an anchor on the dusty Moon surface.

If you are rotating too fast, then you are going to be hitting
the moon as you rotate (dragging sounds too gentle). It seems
like it would be very hard on your anchor and tether. Winching
in and out to use the gravity of the moon to increase or
decrease you rotational momentum just has to make for a more
reliable system.

-- Vince

Andrew Nowicki wrote in message ...
VC You can make your tether shorter than 100 km, say 5 km,
VC since the non-human payload can tolerate high Gs.
AN True.
VC This also reduces the chance of collision.
AN False. The longer the lunavator, the safer it is.

If you are going to say someone is wrong, it is nice to say why.
So, why do you think that a longer tether is safer from collision?
The odds of hitting a large object are not increased much by
the thickness of the tether and they increase linearly with the
length. Even on a simple swept area basis, a shorter cable is going
to be safer. If you make a cable twice as strong and half as long,
it is not twice as wide, only sqrt(2) as wide.

VC The rotational momentum of even 1 Kg at the end of a
VC 100,000 meter long tether is so huge that no momentum
VC wheel will have any impact on it.

Having a small momentum wheel is useful for fine adjustment
of the angular velocity of the lunavator. If you do the
adjustments with the winch only, the lunavator shakes
too much, and you have to use mini rocket thrusters.

Do you have anything to backup either of these claims?

VC The orbital momentum needs to be controlled by either
VC leaving something on the surface of the moon of equal
VC mass to what you are picking up, or using some kind of
VC thruster.

False!

Pick up Moon dust, reel it in and drop it backwards.

Ok, you can use the tether as a thruster with the regolith
as your reaction mass. And this is fun, for sure. There
are a couple issues, though it could be a reasonable
thing to do.

When you picked up the regolith it was going backwards
relative to your center of mass about 1.6 km/sec.
So you have to throw it backwards faster than this to get
any net thrust. If you want to throw it back twice as fast
then you are going to winch in like halfway first. So now
you have twice the tip speed and half the length. Given
that centrifugal force is v^2/r your tether now needs to
be 8 times as strong (2^2/0.5). This is an issue, but
it would not be as bad if you tossed slower.

The second issue is that you can easily make all kinds of
space junk that hits you or someone else on some future
orbit. By picking your velocity to be something less
than 3.2 km/sec backwards relative to the center of mass
of the tether, then it will be less than 1.6 km/sec relative
to the moon and fall to the moon. You might want to avoid
hitting Apollo sites, or any man made objects on the moon.
But this does not seem like killer problem either.

Ya, I think it could work. Both issues get easier if you
don't toss very fast. Since there is no shortage of mass,
that is ok.

Since you would not be limited by a fixed supply of xenon
reaction mass, you could bring back more lunar regolith
this way. It should take less in the way of solar power
since the energy put into the reaction mass goes up with
the velocity squared (1/2 m v^2). So you save mass on
solar and on the xenon. So you can put more mass into
making a bigger reentry capsule and bring back more regolith.
I can imaging starting with 4,200 Kg in LEO from a Falcon-5
and bringing back a few times this mass in regolith.

There are many ways to loose
the extra angular momentum. Probably the cheapest one is
dragging an anchor on the dusty Moon surface.

If you are rotating too fast, then you are going to be hitting
the moon as you rotate (dragging sounds too gentle). It seems
like it would be very hard on your anchor and tether. Winching
in and out to use the gravity of the moon to increase or
decrease you rotational momentum just has to make for a more
reliable system.

-- Vince

Vincent Cate wrote:
VC So, why do you think that a longer tether is safer from
VC collision?

You mean collisions with the meteoroids.
I mean collision with the Moon.

If the lunavator (tether) is short, it must be in an elliptic
orbit to reduce probability of colliding with the Moon. The
elliptic orbit limits efficiency and flexibility of the
lunavator.

AN Having a small momentum wheel is useful for fine adjustment
AN of the angular velocity of the lunavator. If you do the
AN adjustments with the winch only, the lunavator shakes
AN too much, and you have to use mini rocket thrusters.

VC Do you have anything to backup either of these claims?

The real rotating tether is not a rigid straight line. Gravity
and reeling make it flex and shake. Finite element analysis
is the only way to model the rotating tether. The small
momentum wheel has the biggest influence on the inner end
of the tether, while gravity and reeling influence mostly
the outer end of the tether. This means that the wheel can
make corrections that cannot be easily done by other means.

By the way, having a small winch near the outer end of the
rotating tether is also useful, because it can be used as
a shock absorber and as a means to swing the outer tip in
a plane perpendicular to the orbital plane. Needless to say,
the winch does not waste rocket propellant.

VC When you picked up the regolith it was going backwards
VC relative to your center of mass about 1.6 km/sec.
VC So you have to throw it backwards faster than this to get
VC any net thrust. If you want to throw it back twice as fast
VC then you are going to winch in like halfway first. So now
VC you have twice the tip speed and half the length. Given
VC that centrifugal force is v^2/r your tether now needs to
VC be 8 times as strong (2^2/0.5). This is an issue, but
VC it would not be as bad if you tossed slower.

I agree as long as we ignore the tether mass.

VC The second issue is that you can easily make all kinds of
VC space junk that hits you or someone else on some future
VC orbit. By picking your velocity to be something less
VC than 3.2 km/sec backwards relative to the center of mass
VC of the tether, then it will be less than 1.6 km/sec relative
VC to the moon and fall to the moon. You might want to avoid
VC hitting Apollo sites, or any man made objects on the moon.
VC But this does not seem like killer problem either.

Being an environmentalist I do not like space junk, but it
seems to me that you exaggerate the danger. I would go as
far as launching sacks filled with Moon dust into Moon orbit
and using the sacks as the reaction mass. The lunavator grabs
the sacks one by one to climb out of the gravity well. It
flies by the Earth, drops its Moon rocks on the Earth and
returns to the Moon. The sacks are used again to climb down
to low Moon orbit.

This scheme has a flaw: the Moon rocks enter the atmosphere
at about 9.6 km/s. Will they survive? A simple terrestrial
bolo can reduce the velocity to 3.4 km/s. A complex system
of bolo and sling can reduce the velocity to zero. I am
talking about the lunavator bolo relay. It does much more
than reducing the Moon rock velocity: it is a two-way space
transportation system.

AN There are many ways to loose the extra angular momentum.
AN Probably the cheapest one is dragging an anchor on the
AN dusty Moon surface.

VC If you are rotating too fast, then you are going to be hitting
VC the moon as you rotate (dragging sounds too gentle). It seems
VC like it would be very hard on your anchor and tether. Winching
VC in and out to use the gravity of the moon to increase or
VC decrease you rotational momentum just has to make for a more
VC reliable system.

I like your method because of its novelty, but I have no idea
which method is better. The gravity method is useful only
when gravity is significant in comparison with the centrifugal
force. Unfortunately, this condition shakes the tether. The
anchor method has the advantage of increasing the orbital energy
of the tether, but it shakes the tether as well and the anchor
may hit a rock buried in the Moon dust. Maybe a chain would
be better than the anchor?

PS. I have been frequenting the sci.space.tech and a few other
newsgroups before the World Wide Web was invented. Until now I
have never seen any novel idea posted on the newsgroups. The
latest post by Ron Baalke "People Are Robots, Too. Almost"
hints at the plausible explanation of this lack of creativity...

Vincent Cate wrote:
VC So, why do you think that a longer tether is safer from
VC collision?

You mean collisions with the meteoroids.
I mean collision with the Moon.

If the lunavator (tether) is short, it must be in an elliptic
orbit to reduce probability of colliding with the Moon. The
elliptic orbit limits efficiency and flexibility of the
lunavator.

AN Having a small momentum wheel is useful for fine adjustment
AN of the angular velocity of the lunavator. If you do the
AN adjustments with the winch only, the lunavator shakes
AN too much, and you have to use mini rocket thrusters.

VC Do you have anything to backup either of these claims?

The real rotating tether is not a rigid straight line. Gravity
and reeling make it flex and shake. Finite element analysis
is the only way to model the rotating tether. The small
momentum wheel has the biggest influence on the inner end
of the tether, while gravity and reeling influence mostly
the outer end of the tether. This means that the wheel can
make corrections that cannot be easily done by other means.

By the way, having a small winch near the outer end of the
rotating tether is also useful, because it can be used as
a shock absorber and as a means to swing the outer tip in
a plane perpendicular to the orbital plane. Needless to say,
the winch does not waste rocket propellant.

VC When you picked up the regolith it was going backwards
VC relative to your center of mass about 1.6 km/sec.
VC So you have to throw it backwards faster than this to get
VC any net thrust. If you want to throw it back twice as fast
VC then you are going to winch in like halfway first. So now
VC you have twice the tip speed and half the length. Given
VC that centrifugal force is v^2/r your tether now needs to
VC be 8 times as strong (2^2/0.5). This is an issue, but
VC it would not be as bad if you tossed slower.

I agree as long as we ignore the tether mass.

VC The second issue is that you can easily make all kinds of
VC space junk that hits you or someone else on some future
VC orbit. By picking your velocity to be something less
VC than 3.2 km/sec backwards relative to the center of mass
VC of the tether, then it will be less than 1.6 km/sec relative
VC to the moon and fall to the moon. You might want to avoid
VC hitting Apollo sites, or any man made objects on the moon.
VC But this does not seem like killer problem either.

Being an environmentalist I do not like space junk, but it
seems to me that you exaggerate the danger. I would go as
far as launching sacks filled with Moon dust into Moon orbit
and using the sacks as the reaction mass. The lunavator grabs
the sacks one by one to climb out of the gravity well. It
flies by the Earth, drops its Moon rocks on the Earth and
returns to the Moon. The sacks are used again to climb down
to low Moon orbit.

This scheme has a flaw: the Moon rocks enter the atmosphere
at about 9.6 km/s. Will they survive? A simple terrestrial
bolo can reduce the velocity to 3.4 km/s. A complex system
of bolo and sling can reduce the velocity to zero. I am
talking about the lunavator bolo relay. It does much more
than reducing the Moon rock velocity: it is a two-way space
transportation system.

AN There are many ways to loose the extra angular momentum.
AN Probably the cheapest one is dragging an anchor on the
AN dusty Moon surface.

VC If you are rotating too fast, then you are going to be hitting
VC the moon as you rotate (dragging sounds too gentle). It seems
VC like it would be very hard on your anchor and tether. Winching
VC in and out to use the gravity of the moon to increase or
VC decrease you rotational momentum just has to make for a more
VC reliable system.

I like your method because of its novelty, but I have no idea
which method is better. The gravity method is useful only
when gravity is significant in comparison with the centrifugal
force. Unfortunately, this condition shakes the tether. The
anchor method has the advantage of increasing the orbital energy
of the tether, but it shakes the tether as well and the anchor
may hit a rock buried in the Moon dust. Maybe a chain would
be better than the anchor?

PS. I have been frequenting the sci.space.tech and a few other
newsgroups before the World Wide Web was invented. Until now I
have never seen any novel idea posted on the newsgroups. The
latest post by Ron Baalke "People Are Robots, Too. Almost"
hints at the plausible explanation of this lack of creativity...

"Andrew Nowicki" wrote in message
...

PS. I have been frequenting the sci.space.tech and a few other
newsgroups before the World Wide Web was invented. Until now I
have never seen any novel idea posted on the newsgroups. The
latest post by Ron Baalke "People Are Robots, Too. Almost"
hints at the plausible explanation of this lack of creativity...

I believe I can easily match your creativity if I choose to.
There are two problems though.

There is a difference between useful creativity and undisciplined
imagination. Most people find the latter boring if it does not
affect them personally. Me included. I do wonder about
Cherenkof radiation effects on the physical extremeties in
some of your posts.

Time is a resource best spent wisely. If transportation to LEO
is not made cost effective, projects beyond it won't happen.
This includes your current one.

I do question how hard you have been looking for creativity,
and your definition of same.

"Andrew Nowicki" wrote in message
...

PS. I have been frequenting the sci.space.tech and a few other
newsgroups before the World Wide Web was invented. Until now I
have never seen any novel idea posted on the newsgroups. The
latest post by Ron Baalke "People Are Robots, Too. Almost"
hints at the plausible explanation of this lack of creativity...

I believe I can easily match your creativity if I choose to.
There are two problems though.

There is a difference between useful creativity and undisciplined
imagination. Most people find the latter boring if it does not
affect them personally. Me included. I do wonder about
Cherenkof radiation effects on the physical extremeties in
some of your posts.

Time is a resource best spent wisely. If transportation to LEO
is not made cost effective, projects beyond it won't happen.
This includes your current one.

I do question how hard you have been looking for creativity,
and your definition of same.

In article ,
Vincent Cate wrote:
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.

Another option to consider: it could be better to use a smaller rotating
tether centered at the lower end of a non-rotating tether. Downsides are
a more complex system and higher G-forces on the rotating tether (because
at constant tip velocity, centrifugal force scales inversely with radius).
Upsides include independent control of rotation rate and altitude (by
independent length control of the two tethers).

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

Assuming you can make a solar-sail-based system work at all. Don't
forget that solar sails impose some troublesome constraints, like the
need for quite large surface areas and some sharp limitations on what
direction you can thrust in.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |