Just Like 3 - light sails
Frank
windward. Can light sails do the same things in space?
No.
Consider the physics of a sailboat or old-fashioned sailing ship. It
exists in two fluids: air and water. It doesn't function unless
these
have a relative velocity. If they do, then the force of the wind on
the
sails can be combined with the force of the sea on the keel to push
the
boat in a third direction just as the pressure of your finger and
your
thumb can send a sliver of soap off in a direction much different
from
the direction in which either digit is pressing.
The light streams radially from the Sun. The only other force
permeating
the solar system is the force of solar gravity in the opposite
direction, but still radial. The resultant is a radial force. (But
see
later on.)
One problem with considering light sails is that the attraction
works on
mass while the repulsion works on area. The first requirement is a
sail
which has sufficient area per mass so that it actually is driven
outward. So far, we haven't material for such a sail, so estimating
the
other problems runs into the indeterminacy of the material. This is
the
first step, but hardly the last one. The mass of the entire system
--
sail, payload, and connecting lines -- must be less than the lifting
power of the area of the sail.
SAIL
Whatever the nature of the sail, it can have only a small tensile
strength and essentially no compression strength along its large
dimensions. So, if the line running the payload to the sail meets
the
sail at two acute angles with sail in between, the sail will
collapse
until they do not. That leaves the sail at least as curved as a
section
of a sphere with the payload in the center of the sphere. This
section
is less than a hemisphere, if only so that the light pressure will
be
pulling against the lines instead of pushing against them.
Actually, the curvature must be greater. Picture the sail between
two
connectors each pulling perpendicular to the sail. Compared to the
acceleration of the system as a whole, the connectors have a force
sunward and the space between has a force away from the sun. That
would
cause the space between to bulge out and pull the connectors
together.
If the curvature were greater (if, in other words, the payload were
sunward from the center of the sphere implied by the spherical
section
of the sail), then the outward one of the two connectors would be
pulling away from the inward one. This pull could be calculated to
overcome the force bunching up the sail.
What happens when the sail is turned so that the net force on it is
other than radial. Since the force on the payload is radial, there
is a
torque on the entire system. Depending on the torque, it turns the
system so that the net force on the sail is radial or it turns it so
that the sail is edge-on to the light stream; then, the sail
collapses.
Some proponents of light sails want stiffeners, serving a purpose
like
that of masts, in the sail to make them rigid bodies. So long as the
material of the sail has not been described, there is no data to
prove
that this is impossible. I would, however, like to meet somebody
willing
to bet that it can be done with a payload of more than 1 kg.
LINES
Get the design which is of maximum efficiency for some mass of
payload
and lines combined. Then design for a mass 4 times as great. In that
case, you must have sails four times as large. That means lines
joined
to the sail at four times as many points and lines twice as long.
That's
eight times as much mass of lines. Without a design, we can't tell
where
that point comes, but there is some point at which increasing the
area
of the sail does not increase the possible mass of the payload.
And then, if you want to trim sail, you have to wind up the lines.
What
is the length of the line? What is the spool on which you wind it?
WHAT IS POSSIBLE
I've been negative up to now. A Clarke light-sail race is beyond
possibility. OTOH, another effort might make sense.
You have a ship in circular orbit around the sun. It deploys a light
sail that reduces the net force on the ship towards the sun by half.
It
is no longer in a circular orbit. Since the only forces on the ship
are
radial, it's angular momentum remains the same. OTOH, it's net
centripetal force is half as great. It goes into an elliptical orbit
whose aphelion can be calculated by V^2/R = .5*v^2/r, v*r=V*R.
Thus R = r*(2^1/3) = 1.26 r. And, if the ship releases the sail at
that
point, it goes into another elliptical orbit with perihelion
significantly less than r.
IF you can collapse the sail and take it with you, then you can
repeat
the process every leg from r to aphelion. This will give you -- over
a
great length of time -- as eccentric an ellipse as you could wish.
This isn't much when compared to the dreams, but it does give a
method
of using a light sail when the net force on the system still points
sunward, and it does provide one method of using light sails to move
something sunward -- albeit after some delay.
Raven
news:4cfbeab4-5ae8-4c4a...@l33g2000vbi.googlegroups.com...
> Sail boats can do all sorts of cool things, including beating to
> windward. Can light sails do the same things in space?
> No.
Correct. You cannot tack with a lightsail any more than you can with a
balloon. And if your sun-sailing ship has a net force inward because gravity
is stronger than the opposing light pressure it cannot reach escape velocity
even by turning to accept the full force of the sunlight.
> WHAT IS POSSIBLE
> I've been negative up to now. A Clarke light-sail race is beyond
> possibility. OTOH, another effort might make sense.
Consider tilting the lightsail at 45 degrees angle WRT the radius vector.
You get a gentle push outward; mathematically it is comparable to a
hypothetical slight lessening of gravity. You also get a gentle push to one
side, and a thing you cannot have with the sail pointed directly towards the
Sun: a change in angular momentum. Somewhat paradoxically, by tilting it at
45 degrees so that the sunlight is reflected backwards WRT the ship's orbit,
you now can reach escape velocity. You can also tilt the sail in the
opposite direction, and spiral inward. Or tilt it to reflect sunlight out
of your orbital plane, changing that. You can reach many places in the
solar system and beyond with a solar sail - if you have a workable one, and
lots of patience. Especially the former commodity is currently in short
supply, of course.
And forget the solar wind, except to fine-tune calculations. Even if you
can capture it, you must also be able to reflect it at 45 degrees, give or
take, and at any rate sunlight carries an order of magnitude or two greater
momentum density.
Jon Lennart Beck.
Greg Goss
> And forget the solar wind, except to fine-tune calculations. Even if you
>can capture it, you must also be able to reflect it at 45 degrees, give or
>take, and at any rate sunlight carries an order of magnitude or two greater
>momentum density.
>
>Jon Lennart Beck.
"forget the solar wind"?
You must be able to reflect it before it is useful. Exactly. But a
"sticky" solar wind will it mangle your calculations by pushing your
sail when you're trying to calculate the vector from bounced light?
--
apart from one noisy guy up in Canada, no-one wants
a three-cylinder tissue box on bicycle tires.
Brian Davis
> Sail boats can do all sorts of cool things, including beating to
> windward. Can light sails do the same things in space?
I was thinking of a potential response... and then decided it might be
better to ask you to read up on the problem a little bit first. The
first part of Forward's "Flight of the Dragonfly" would be a really
good start.
--
Brian Davis
Luke Campbell
> And forget the solar wind, except to fine-tune calculations. Even if you
> can capture it, you must also be able to reflect it at 45 degrees, give or
> take, and at any rate sunlight carries an order of magnitude or two greater
> momentum density.
It is claimed that a plasma magnet can deflect the solar wind over a
very wide area for its mass, giving a higher specific thrust than
solar sail material. http://www.ess.washington.edu/Space/PlasmaMag/
Similarly, it is claimed that mini-magnetospheric plasma propulsion
can produce a large area "sail" with relatively low mass and compact
equipment. And of course a magnetic field can deflect the solar wind
at an angle, although the dynamics are considerably more complex than
the simple specular reflection of solar sail material.
Luke
Raven
news:7kc7e2F...@mid.individual.net...
>> And forget the solar wind, except to fine-tune calculations. Even if
>> you can capture it, you must also be able to reflect it at 45 degrees,
>> give or take, and at any rate sunlight carries an order of magnitude
>> or two greater momentum density.
> "forget the solar wind"?
> You must be able to reflect it before it is useful. Exactly. But a
> "sticky" solar wind will it mangle your calculations by pushing your
> sail when you're trying to calculate the vector from bounced light?
Yes, hence "except to fine-tune calculations". But I have before read
sci-fi books in which a starship used a magnetized metal mesh as a solar
sail, capturing the solar wind.
Jon Lennart Beck.
Frank
<jon.lennart.beck.its.my.n...@mail.its.in.danmark> wrote:
> "Frank" <fpfrankpal...@gmail.com> skrev i meddelelsennews:4cfbeab4-5ae8-4c4a...@l33g2000vbi.googlegroups.com...
2) I started to snip, but decided that might not be fair. Um, how do
you turn the sail at a 45 degree angle and keep it turned?
This imposes a torque on the system. If you have something like a
mast, you have to have a rigid body of km length and minimal mmass.
What's that made of?
If you simply pull the lines, why does the sail move instead of the
payload, which is lighter and not subject to light pressure.?
1) Hypothetically, a system with VERY SLIGHT inward acceleration
starting from orbit would have -- due to the orbit -- an escape
velocity for that inward acceleration.
Of course, if you got that close to balance between light pressure and
solar gravity, you could construct a lighter system furher from the
sun -- the acceleration goes inverse square for both light pressure
and gravity, but the force decreases as the radius increases, allowing
a less strong and therefore less massive, system.
And, in any of these cases, "escape velocity" ain't something which
would get you to the next stellar system in the next millenium.
Frank
Is this the one which has a ship and a seperable mirror driven by
sunlight concentrated by a magic lens?
I've read it.
It's cheap to reply to an appeal to physics by sayng the person hasn't
read enough.
Which of my startements has your disgreement?
Brian Davis
> Is this the one which has a ship and a seperable mirror driven by
> sunlight concentrated by a magic lens?
Yep... but that's not the part I'm talking about. The very first
chapter talks about solar sailing within the solar system, including
how to change your orbit.
> I've read it.
Then I respectfully submit you either didn't understand it, or didn't
remember it. If you'd like a more fact-based application, go read
through some JBIS articles (I think Forward had a couple on solar
sailing).
> It's cheap to reply to an appeal to physics by sayng the person
> hasn't read enough.
Not when you're a teacher of physics, and the person asking the
questions hasn't shown that they've even done the equivalent of
reading the book.
> Which of my startements has your disgreement?
Well, for a start, understanding very very basic forces. Put a
reflector at an angle to the light, and there are no torques upon it.
Once I saw you mysteriously jump to "no radial accelerations", I
realized trying to discuss physics with you was rather like trying to
teach a squirrel to play cards... kind of pointless unless you can get
them to evolve thumbs first.
Solar sailer in circular orbit, with the sail held "out" by light
pressure. Reel in shroud lines on leading edge of sail, just slightly,
"tipping" the sail (light pressure will actually do the tipping for
you here, with different-length shroud lines). Now the force on the
sail is not radial to the Sun, while the force on the payload still
is; yep, that's a torque, so the *system* as a whole starts rotating.
When the angle between the radial vector and the sail-payload vector
is what you want it to be, you readjust the shroud lines to provide a
torque to cancel the net rotation. Now the solar sailer is in a non-
radial orientation with the mirror at an angle, and a portion of the
light "thrust" is tangential, speeding you up or slowing you down in
an otherwise Newtonian orbit. If you have any other torques, you just
tilt the sail, again, to cancel them.
That's... well, that's basic physics. The mass of the payload don't
enter into it, although admittedly you can't do this with a point-
source payload. But you certainly don't need a rigid structure for the
entire sail, or the payload-to-sail connection.
If you *don't* understand this... do some research. JBIS is a good
source for this, but really this is basic physics.
--
Brian Davis
Brian Davis
> And, in any of these cases, "escape velocity" ain't something which
> would get you to the next stellar system in the next millenium.
I'm not sure I've seen anybody imply that - not even in fiction.
Forward certainly didn't in "Rocheworld". Aldrin avoided it in the
more recent "Encounter with Tiber". Heck, even Pohl didn't use it way
back in "THe Mote in God's Eye".
What *have* you read, on solar sailing?
--
Brian Davis
James Nicoll
Brian Davis <brd...@iusb.edu> wrote:
>Heck, even Pohl didn't use it way back in "THe Mote in God's Eye".
ITYM Niven and Pournelle.
--
http://www.livejournal.com/users/james_nicoll
http://www.cafepress.com/jdnicoll (For all your "The problem with
defending the English language [...]" T-shirt, cup and tote-bag needs)
Brian Davis
> Brian Davis moronically wrote:
>
>> ...Pohl didn't use it way back in "The Mote in God's Eye".
>
> ITYM Niven and Pournelle.
Ugh. Thanks, I knew my memory of that story was going, I just didn't
catch that even my memory of the *authors* is fading as well. Grrrr...
--
Brian "could I have catsup with my crow?" Davis
Raven
> Um, how do you turn the sail at a 45 degree angle and keep it
> turned? This imposes a torque on the system.
Does it? As long as the entire sail is illuminated the linear pressure
is unvarying - in the reasonable limit that the sail is so small compared to
the distance to the Sun that the lightflux is constant across it.
Jon Lennart Beck.
Frank
> On Oct 23, 11:27 am, Frank <fpfrankpal...@gmail.com> wrote:
>
> > Is this the one which has a ship and a seperable mirror driven by
> > sunlight concentrated by a magic lens?
>
> Yep... but that's not the part I'm talking about. The very first
> chapter talks about solar sailing within the solar system, including
> how to change your orbit.
>
> > I've read it.
>
> Then I respectfully submit you either didn't understand it, or didn't
> remember it. If you'd like a more fact-based application, go read
> through some JBIS articles (I think Forward had a couple on solar
> sailing).
>
> > It's cheap to reply to an appeal to physics by sayng the person
> > hasn't read enough.
>
> Not when you're a teacher of physics, and the person asking the
> questions hasn't shown that they've even done the equivalent of
> reading the book.
>
> > Which of my startements has your disgreement?
>
> Well, for a start, understanding very very basic forces. Put a
> reflector at an angle to the light, and there are no torques upon it.
Upon the sysrtem there will be.
> Once I saw you mysteriously jump to "no radial accelerations", I
> realized trying to discuss physics with you was rather like trying to
> teach a squirrel to play cards... kind of pointless unless you can get
> them to evolve thumbs first.
>
> Solar sailer in circular orbit, with the sail held "out" by light
> pressure. Reel in shroud lines on leading edge of sail, just slightly,
> "tipping" the sail (light pressure will actually do the tipping for
> you here, with different-length shroud lines). Now the force on the
> sail is not radial to the Sun, while the force on the payload still
> is; yep, that's a torque, so the *system* as a whole starts rotating.
> When the angle between the radial vector and the sail-payload vector
> is what you want it to be, you readjust the shroud lines to provide a
> torque to cancel the net rotation. Now the solar sailer is in a non-
> radial orientation with the mirror at an angle, and a portion of the
> light "thrust" is tangential, speeding you up or slowing you down in
> an otherwise Newtonian orbit. If you have any other torques, you just
> tilt the sail, again, to cancel them.
>
But once the light pressure on the reflector -- you do remember that
the reflector must be curved, don't you? -- provides non-radial force,
it also provides torque on the system. Just as you noted when you
wanted it.
Frank
I was presenting a method, together with the limitations of the
method.
That is considered honest presentation among people who desire honest
presentation.
Frank
<jon.lennart.beck.its.my.n...@mail.its.in.danmark> wrote:
> "Frank" <fpfrankpal...@gmail.com> skrev i meddelelsennews:4f048ca1-25ef-453d...@j9g2000vbp.googlegroups.com...
>
> > Um, how do you turn the sail at a 45 degree angle and keep it
> > turned? This imposes a torque on the system.
>
> Does it? As long as the entire sail is illuminated the linear pressure
> is unvarying - in the reasonable limit that the sail is so small compared to
> the distance to the Sun that the lightflux is constant across it.
>
> Jon Lennart Beck.
It ain't the distance from the sun that's the problem.
You have a lightsail hauling a payload. (In my originial post, I
pointed out why the payload had to be at a greater distance from the
lightsail than the center of the lightsail is from teh edges.)
Then, the force on the lightsail is not radial (towards or area from
the sun) whil the force on teh payload is, Thus, you have a torque on
the SYSTEM -- a net force n one end of the system which isn't parallel
with the net force on teh other end of the system.
Brian Davis
> Upon IT there may be no torques.
> Upon the sysrtem there will be...
>
> But once the light pressure on the reflector -- you do remember that
> the reflector must be curved, don't you? -- provides non-radial force,
> it also provides torque on the system. Just as you noted when you
> wanted it.
Let's see, I've got a sail at (for instance) a 45° angle to the Sun-
payload line... the effect of light reflecting off that sail will be a
force perpendicular to the surface of the sail. As long as that force
vector passes through the center of mass... no torque. As long as the
gravitational force vector passes through the center of mass... no
torque.
At least for a rigid structure, like the payload. For the sail, we
can't get away that easily.
So... there's a force (due to reflected light) pushing the sail away
from the payload, and the tensions from the shroud lines pulling
towards the payload. Huh, no torque on the sail either, from either of
those forces. Ah, but what about gravity! Oh, wait... the center of
mass of the sail material would be at the center of the sail too.
Oops, no torque.
The only way you get a torque to turn the ship is if the perpendicular
from the sail doesn't point through the center of mass of the system
(generally, that center of mass will be located along the line
connecting the center of mass of the sail to the center of mass of the
payload, although where it is along that line depends on the sail/
payload mass ratio). You can get such torques by reeling shroud lines
in and out, as I mentioned. You can also eliminate such torques by
reeling shroud liones in and out, as I mentioned.
If you *do* have a net torque on the system for some reason, the
solution is just as simple - cant the sail until you have an equal and
opposite torque. Not really a difficult physics concept.
I'll re-quote another portion of my previous post here:
>> If you *don't* understand this... do some research. JBIS
>> is a good source for this, but really this is basic physics.
There was a reason I asked (in a different post) what you had read.
It's not because what you read makes your viewpoint on a subject
correct or not (it doesn't). But it *does* means a large group of
people don't have to re-teach things that are generally really well
understood... and that understanding can be gotten from books. There's
a *reason* these things turn up again & again in both science and
science fiction, and it's not because everybody prior to you missed
the "obvious flaws". Grab a copy of Mallove & Matloff's "Starflight
Handbook", and at the very least read chapter 6 and the references.
They're *old* references (heck, my copy is 1989), but the problems you
seem to be stumbling over were very old (and solved) long before then.
Come to think of it, I've thought about ordering Matloff's "Deep Space
Probes" - has anyone else any opinions on it?
--
Brian Davis
Mike Ash
<c16df49e-2ea6-4891...@l31g2000vbp.googlegroups.com>,
Frank <fpfran...@gmail.com> wrote:
You have a torque on the system IFF the differential force between the
two components does not point through the center of mass. In other
words, you tilt the light sail, this begins to rotate the whole system,
but the torque disappears when the payload is once again lined up with
the middle of the light sail.
--
Mike Ash
Radio Free Earth
Broadcasting from our climate-controlled studios deep inside the Moon
Frank
> On Oct 24, 3:46 pm, Frank <fpfrankpal...@gmail.com> wrote:
>
> > Upon IT there may be no torques.
> > Upon the sysrtem there will be...
>
> > But once the light pressure on the reflector -- you do remember that
> > the reflector must be curved, don't you? -- provides non-radial force,
> > it also provides torque on the system. Just as you noted when you
> > wanted it.
>
> Let's see, I've got a sail at (for instance) a 45° angle to the Sun-
> payload line... the effect of light reflecting off that sail will be a
> force perpendicular to the surface of the sail. As long as that force
> vector passes through the center of mass... no torque. As long as the
> gravitational force vector passes through the center of mass... no
> torque.
>
Now show me as ystem with a lightsail and a pyloard where both forces
pass throught the center of gravity of the SYSTEM.
> At least for a rigid structure, like the payload. For the sail, we
> can't get away that easily.
>
The question ain't the torque on the payload and the torque on the
sail. (Although a curved sail whose average was not normal to the
sun's rays would expereince torque.) The problem is the torque on the
system.
You have any system which consists of two distant objects connected --
I'm ignoring he forces (external to the system) on the lines, although
feel free to consider tham -- whith forces in different directions on
each end will experience torque.
> So... there's a force (due to reflected light) pushing the sail away
> from the payload, and the tensions from the shroud lines pulling
> towards the payload. Huh, no torque on the sail either, from either of
> those forces. Ah, but what about gravity! Oh, wait... the center of
> mass of the sail material would be at the center of the sail too.
> Oops, no torque.
I have a crowbar. I put the tip under a stone. I pull on the handle.
The force on the tip is the force of teh stone downward -- no torque.
the force on the handle is my pulling east -- no torque. Is tehre
torque on teh crowgbar? I say yes.
> The only way you get a torque to turn the ship is if the perpendicular
> from the sail doesn't point through the center of mass of the system
> (generally, that center of mass will be located along the line
> connecting the center of mass of the sail to the center of mass of the
> payload, although where it is along that line depends on the sail/
> payload mass ratio). You can get such torques by reeling shroud lines
> in and out, as I mentioned. You can also eliminate such torques by
> reeling shroud liones in and out, as I mentioned.
>
Um, you now consider teh system. (Rather late, but you're entitled to
your choice.) The light is refelcted so that teh force from teh sail
runs through the center of mass of the system. Now, that makes the
pull outward through the payload. The payload also experiences a
gravitational pull towards the sun.
Whyc does that not pull the payload off the line of the outward pull?
> If you *do* have a net torque on the system for some reason, the
> solution is just as simple - cant the sail until you have an equal and
> opposite torque. Not really a difficult physics concept.
Right. and since, despite your "no torque upon either part, so no
torque on the system" analysis, you do -- always when the seil is not
on a line passing throught the center of gravity of the payload and
the center of the sun -- have a torque on the system. This means that
the tension of the lines from the outward edge is greater than the
tension from the inward edge. So, you deal wit that by reeling in line
from the inward edge. (Actually, real lines will lengthen due to
tension, so -- even if you do not respond -- the same problem will
occur.)
This tips the sail a greater amount -- which increases the torque --
which increases what you have to do to gight the torque -- until
you've tilted the sail to the point that its curvature reduces the
pull from the inward edge significantly enough for it to satrt
folding.
Frank
> In article
> <c16df49e-2ea6-4891-8e7f-8693568c4...@l31g2000vbp.googlegroups.com>,
>
> - Show quoted text -
Um,
I see your general point. If the differential force doesn't point
through the center of mass, there is a torqu -- not, not.
But i don't see your specific. IF the lightsail's pull points through
the center of mass, and the solar gravity on the payload does not, why
is the differential force pointing through the center of mass?
Mike Ash
<5d43ede1-45e6-49b5...@d34g2000vbm.googlegroups.com>,
Frank <fpfran...@gmail.com> wrote:
> I see your general point. If the differential force doesn't point
> through the center of mass, there is a torqu -- not, not.
> But i don't see your specific. IF the lightsail's pull points through
> the center of mass, and the solar gravity on the payload does not, why
> is the differential force pointing through the center of mass?
Both the sail and the payload are equally affected by solar gravity. It
is in free fall. Gravity can be completely ignored except for orbital
dynamics, unless the distance between your payload and your sail is
EXTREMELY long, long enough for tidal effects to become important.
Brian Davis
> I have a crowbar. I put the tip under a stone. I pull on the handle.
> The force on the tip is the force of teh stone downward -- no torque.
> the force on the handle is my pulling east -- no torque. Is tehre
> torque on teh crowgbar? I say yes.
Sigh. I give up. Faced with your superior understanding of basic
physics, I see you have proved that my analysis (and, incidentally,
most others that have looked at the problem) is tragically flawed, and
solar sails can't work. Thank goodness we resolved that.
Or, alternatively, I've got better things to do with my time.
--
Brian Davis
Frank
>
> Frank <fpfrankpal...@gmail.com> wrote:
> > I see your general point. If the differential force doesn't point
> > through the center of mass, there is a torqu -- not, not.
> > But i don't see your specific. IF the lightsail's pull points through
> > the center of mass, and the solar gravity on the payload does not, why
> > is the differential force pointing through the center of mass?
>
> Both the sail and the payload are equally affected by solar gravity. It
> is in free fall. Gravity can be completely ignored except for orbital
> Mike Ash
> Radio Free Earth
> Broadcasting from our climate-controlled studios deep inside the Moon
Damn!
I hadn't thought of that.
I think you're right.