Lunar Sample Return via Tether

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. |

In article ,
Henry Spencer wrote:
In article ,
Vincent Cate wrote:
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.

...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.

How much worse would argon be? Argon is easily obtained
in large quantities, being on the order of 1% of the
atmosphere. It probably does not need to be highly
purified for this purpose.
--
This address is for information only. I do not claim that these views
are those of the Statistics Department or of Purdue University.
Herman Rubin, Department of Statistics, Purdue University
Phone: (765)494-6054 FAX: (765)494-0558

In article ,
Henry Spencer wrote:
In article ,
Vincent Cate wrote:
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.

...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.

How much worse would argon be? Argon is easily obtained
in large quantities, being on the order of 1% of the
atmosphere. It probably does not need to be highly
purified for this purpose.
--
This address is for information only. I do not claim that these views
are those of the Statistics Department or of Purdue University.
Herman Rubin, Department of Statistics, Purdue University
Phone: (765)494-6054 FAX: (765)494-0558

I added the ability to simulate a winch pulling in a tether
to my simulator. You can have the winch limited by any
combination of power, force, speed, distance, and time.

I simulate picking up 2 Kg, winching in for a bit to get
higher tip speed, then tossing 1 kg backwards (and by
Newton's equal and opposite reaction we must get some
thrust from the 1 kg that we tossed). So we are
simulating a tether flinging regolith for thrust.

Also found one big problem with my applet. It does not work
under some microsoft JVMs. Hope to have this fixed soon.

See example 89 for winching in applet at:

http://spacetethers.com/spacetethers.html

-- Vince

I added the ability to simulate a winch pulling in a tether
to my simulator. You can have the winch limited by any
combination of power, force, speed, distance, and time.

I simulate picking up 2 Kg, winching in for a bit to get
higher tip speed, then tossing 1 kg backwards (and by
Newton's equal and opposite reaction we must get some
thrust from the 1 kg that we tossed). So we are
simulating a tether flinging regolith for thrust.

Also found one big problem with my applet. It does not work
under some microsoft JVMs. Hope to have this fixed soon.

See example 89 for winching in applet at:

http://spacetethers.com/spacetethers.html

-- Vince

Andrew Nowicki wrote in message ...
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.

Yes, I understand. This is how my tether simulator works.
While a winch can cause waves, it can also help get rid of
them. Also you can use it in such a way as to not cause a
problem wave. So I don't think that using a winch means you
need to be firing rockets.

I am not convinced that a momentum wheel is any real use for a
rotating tether. But I am happy to agree to disagree.

I agree as long as we ignore the tether mass.

I can now simulate a winch, so we can put real numbers to
the winch stuff.

The gravity method is useful only
when gravity is significant in comparison with the centrifugal
force.

I will have to simulate this winching in and out to get some
real numbers on how fast you can gain or loose angular momentum
at what distances from the moon. But I think you could do it
at any realistic centrifugal force level, just that it could take
a bigger winch motor.

Unfortunately, this condition shakes the tether.

But I don't think "shaking the tether" is going
to be a big problem. Winches with simple things like a
"maximum force limit" or a "power limit" will reduce waves.
You want to pull in during the weak parts of the wave and
not so much when the tension is high, and these can do some
of this. A real computer control algorithm could do very well
at getting rid of waves. But with even simple thing, it has not
been a real problem on my initial tests.

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...

Just checking, you are saying that this thread is the first
with some novel ideas?

--- Vince

Andrew Nowicki wrote in message ...
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.

Yes, I understand. This is how my tether simulator works.
While a winch can cause waves, it can also help get rid of
them. Also you can use it in such a way as to not cause a
problem wave. So I don't think that using a winch means you
need to be firing rockets.

I am not convinced that a momentum wheel is any real use for a
rotating tether. But I am happy to agree to disagree.

I agree as long as we ignore the tether mass.

I can now simulate a winch, so we can put real numbers to
the winch stuff.

The gravity method is useful only
when gravity is significant in comparison with the centrifugal
force.

I will have to simulate this winching in and out to get some
real numbers on how fast you can gain or loose angular momentum
at what distances from the moon. But I think you could do it
at any realistic centrifugal force level, just that it could take
a bigger winch motor.

Unfortunately, this condition shakes the tether.

But I don't think "shaking the tether" is going
to be a big problem. Winches with simple things like a
"maximum force limit" or a "power limit" will reduce waves.
You want to pull in during the weak parts of the wave and
not so much when the tension is high, and these can do some
of this. A real computer control algorithm could do very well
at getting rid of waves. But with even simple thing, it has not
been a real problem on my initial tests.

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...

Just checking, you are saying that this thread is the first
with some novel ideas?

--- Vince

In article ,
Herman Rubin wrote:
...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...

How much worse would argon be?

With current thrusters, you do take a significant performance hit, but it
may be acceptable, depending on what you're doing.

(Also worthy of note is that ion thrusters are not necessarily the optimal
solution, when delta-V requirements are modest. Ion rockets need a lot of
power, which means rather heavy solar arrays and related equipment.
Something like an arcjet, with lower Isp but also lower power demands,
may well be preferable.)

...It probably does not need to be highly purified for this purpose.

You want ion-thruster propellants fairly pure, to keep various annoying
secondary effects under control. In any case, purifying gases is not
particularly hard.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

In article ,
Herman Rubin wrote:
...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...

How much worse would argon be?

With current thrusters, you do take a significant performance hit, but it
may be acceptable, depending on what you're doing.

(Also worthy of note is that ion thrusters are not necessarily the optimal
solution, when delta-V requirements are modest. Ion rockets need a lot of
power, which means rather heavy solar arrays and related equipment.
Something like an arcjet, with lower Isp but also lower power demands,
may well be preferable.)

...It probably does not need to be highly purified for this purpose.

You want ion-thruster propellants fairly pure, to keep various annoying
secondary effects under control. In any case, purifying gases is not
particularly hard.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

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

Interesting idea. The independent control of rotation
rate and altitude is very useful. The main problem is
control of the non-rotating tether. Perhaps winches and
weights riding on the non-rotating tether can dampen
its vibrations.

There are many other options, for example guns or
catapults on the lunar surface. 11 years ago I proposed
a loop with winches encircling a planet or a moon.
I call it orbital loop.

PS. I would rather use tethers where they are needed the
most -- low Earth orbit. There is lots of massive junk
there. Hook up a rotating electrodynamic tether to the
space station, and you have a cheap space transportation
system launching a few small payloads a day to low Earth
orbit and beyond.

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

Vincent Cate wrote:

VC Just checking, you are saying that this thread
VC is the first with some novel ideas?

Yes. The first that I have seen. I define novel ideas
as ideas that may solve important global problems, and
require testing to prove that they are wrong.

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

johnhare wrote:

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

As defined above. What I am missing in sci.space.tech is
an in-depth discussion of novel ideas pertaining to
reducing the cost of access to space. Most men hate novel
ideas. Their computer hardware (Mac or PC), programming
language, spoken language, dress, local sports team,
favorite space transportation system, etc. are their
religions. If we were open minded, there would be long
lasting threads leading to a consensus, web sites, and
sharing of related work (library research, programming,
computer graphics, etc.).