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Some thoughts on solar sails



 
 
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  #1  
Old March 22nd 04, 04:15 PM
nafod40
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Default Some thoughts on solar sails

Most stuff I've seen on solar sails really reminds me of solar
parachutes, where your drag happens to exceed gravity, so you go with
the solar flow, so to speak.

Sail boats work by exploiting the boundary between two different moving
masses, the air above and the sea below. running with the wind at the
stern is actually a pretty poor way to go. running with the wind abeam
results is fast travel.

not too many analogies to the sea/air interface is space. But in flying
model gliders, there is another type of soaring that exploits this type
of interface, between two masses of air. It's called dynamic soaring.
Basically you dip into one mass, then another, in an alternate fashion,
to exploit their differences.

http://www.wfu.edu/albatross/atwork/dynamic_soaring.htm

Using the effect, guys flying radio control gliders are able to get
their gliders, flying in 10-20 knots of wind, up to 200 MPH by flying on
the lee side of slopes, and bouncing between the moving air above and
the still air below. Lots of energy to extract at the boundary.

I was wondering if the downstream eddies and shadow behind a planet
might offer an opportunity to exploit such effects. Or if their magnetic
fields might offer the sort of sea/air interface that sailing craft use.

Clearly comets are subject to differing forces on their tails, as
exhibited by the fact that comets will have two different dust tails.
Opportunity for exploiting?

Just some rambling thoughts. Anywhere that you could find forces pushing
in different directions could possibly be usable.

  #2  
Old March 23rd 04, 06:15 PM
Gordon D. Pusch
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Default Some thoughts on solar sails

nafod40 writes:

Most stuff I've seen on solar sails really reminds me of solar
parachutes, where your drag happens to exceed gravity, so you go with
the solar flow, so to speak.


VERY Bad Analogy. First, there is no "drag" to speak of in space.
Second, solar sails change their orbital momentum by reflecting sunlight
in some chosen direction; since sunlight carries momentum, and since
momentum must be conserved, the sail "recoils" when photons bounce off it,
with a change in momentum that is equal and opposite to the change in
photon momentum. As a result, solar sails do =NOT= just "go with the flow;"
by tipping the sail so that the reflected photons have a momentum component
parallel to the sail's orbital velocity around the Sun (or whatever body it
happens to be orbiting), a solar sail's orbit will slowly spiral outward,
while by tipping it so that the reflected photons have a momentum component
antiparallel to the sail's orbital velocity around the Sun (or whatever),
the sail's orbit will slowly spiral inward.


Sail boats work by exploiting the boundary between two different moving
masses, the air above and the sea below. running with the wind at the
stern is actually a pretty poor way to go. running with the wind abeam
results is fast travel.


Again a Bad Analogy, since in space there is not even _ONE_ "fluid medium"
worth speaking of, let alone two !!!


not too many analogies to the sea/air interface is space.


Which is why such analogies are not very helpful.


But in flying
model gliders, there is another type of soaring that exploits this type
of interface, between two masses of air. It's called dynamic soaring.
Basically you dip into one mass, then another, in an alternate fashion,
to exploit their differences.

http://www.wfu.edu/albatross/atwork/dynamic_soaring.htm

Using the effect, guys flying radio control gliders are able to get
their gliders, flying in 10-20 knots of wind, up to 200 MPH by flying on
the lee side of slopes, and bouncing between the moving air above and
the still air below. Lots of energy to extract at the boundary.


Still a very bad analogy, since in space there is _NOTHING_ analogous
to even _ONE_ moving mass of air.


I was wondering if the downstream eddies and shadow behind a planet
might offer an opportunity to exploit such effects.


False analogy. There are _NO_ "eddies" in the flow of sunlight "downstream"
of a planet to be "take advantage of." Sunlight is _NOT_ a "fluid."


Or if their magnetic fields might offer the sort of sea/air interface
that sailing craft use.


You persist in making false analogies. Magnetic fields do =NOT= have sharp
"interfaces;" you cannot "tack" against them. A spacecraft equipped with
a long linear or circular current-carrying conductor (which are called an
"electrodynamic tether" and a "magsail," respectively) can interact with the
planetary or interplanetary magnetic field to produce electrodynamic forces
that can alter its trajectory, but there is nothing "fluid like" about these
electromagnetic forces, and reasoning using "fluid like" analogies will
lead you down the proverbial garden trail to false conclusions.

The only case where interplanetary "sail" propulsion has an even
_approximately_ accurate "fluid like" analogy are the so-called
"magsail" or "M2P2" concepts, where a magnetic field is used to
interact with the supersonic plasma of the "solar wind" --- and
even then, the analogy to "sailing" is quite bad, because sailboats
are not used in supersonic winds, there is nothing analogous to water
to "tack" against, and sailboats obey nothing analogous to the orbital
mechanics that results from the interplay between gravity and inertia.
Even sailplaning is a bad analogy, since aircraft are governed by an
quasistatic balance between lift, drag, and gravity, and while magsails
and M2P2 have qualities that we misleadingly _call_ "lift" and "drag,"
magsails and M2P2 do =NOT= operate under a quasistatic balance between
"lift," "drag," and gravity.

And before you ask: While it certainly is possible to combine a "light sail"
and a "magsail," that =STILL= does not give you the ability to "tack"
or "soar" --- it simply gives you more knobs to tweak your newtonian
orbital trajectory with.


Clearly comets are subject to differing forces on their tails, as
exhibited by the fact that comets will have two different dust tails.


Clearly you do not understand comets, let alone how comet tails are formed.
Comets have a "dust tail" and an "ion tail." Comet tails appear to point
"away" from the Sun because the effect of solar "light pressure" on the
comet's nucleus is negligible, so that to a first approximation it follows
a newtonian orbit (modulo the "rocket" effect exerted by geysers of dusty gas
being ejected from its surface). The "dust tail" _appears_ to "point away"
from the sun because dust particles are small enough and "fluffy" enough
(i.e., they have a large surface to volume ratio) so that the force exerted
on them by "light pressure" is a significant fraction of the gravitational
force exerted on them by the Sun; they therefore follow a slightly different
orbit than the comet's nucleus does, so that their relative trajectories
diverge. The "ion tail" is formed of gas that has become ionized and has
been "picked up" by magnetohydrodynamic effects in the supersonic flow
of the solar wind plasma.

In summary, comet nuclei are dominated by newtonian orbital mechanics,
slightly perturbed by newtonian reaction forces. Dust particles are
dominated by newtonian mechanics, slightly perturbed by "light pressure."
Gas ions are totally dominated by magnetohydrodynamic forces.


Opportunity for exploiting?


No.


Just some rambling thoughts. Anywhere that you could find forces pushing
in different directions could possibly be usable.


You persist in making false analogies between "solar sails" and sailboats.
These false analogies are false analogies.


-- Gordon D. Pusch

perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;'
  #3  
Old March 24th 04, 02:38 AM
Christopher M. Jones
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Default Some thoughts on solar sails

nafod40 wrote in message ...
Most stuff I've seen on solar sails really reminds me of solar
parachutes, where your drag happens to exceed gravity, so you go with
the solar flow, so to speak.

Sail boats work by exploiting the boundary between two different moving
masses, the air above and the sea below. running with the wind at the
stern is actually a pretty poor way to go. running with the wind abeam
results is fast travel.

not too many analogies to the sea/air interface is space.

[mega-snip]

No, not many. The lack of friction really messes with
our common sense.

With a solar sail you've got basically two ways of working
it in getting from here to there. You can tilt it so that
the thrust is close to parallel with the direction of
travel in orbit (this is kinda tricky because moving the
mirror exactly edge on to the Sun will reduce thrust to
near zero, so you have to choose a middle ground which
produces sizable thrust in the right direction, this is
roughly about 60 deg. from edge-on). The thrust from the
Solar Sail will, mostly, speed up your orbit, which will
push the orbit farther out (like a continuous series of
aphelion boost burns). You can do this in reverse, point
the thrust in the opposite direction and slow down the
orbit, forcing it to fall inward (like a continuous
series of perihelion lowering burns). Between these two,
and coasting, you can navigate just about anywhere.

However, because continuous application of thrust along
the direction of the orbit necessarily pushes the sail
farther and farther from the Sun, causing the photon
flux to diminish, there are fairly inherent maximum
limits to Solar Sail speed. About the best way to
achieve maximum speed with a Solar Sail, if you're
heading out to the deep, outer Solar System, for
example, is to use the oberth effect, and try to put
most of your thrust close to the Sun (first you fall in,
then you boost out).
  #4  
Old March 24th 04, 05:54 AM
Richard Lamb
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Default Some thoughts on solar sails



I'd speculate that "tacking" a light sail
(and yes, that's the term that will probably be used)
will be quite different from tacking a sailboat.

"The next tack is scheduled for mission elapsed time of
18 years, 4 months, 3 days, 6 hours, 44 minutes, 42.2 seconds.

If completed sucessfully, we will our rondeveau with our next
destination on schedule at MET 54 years,... "


duh!
  #5  
Old March 26th 04, 03:43 PM
nafod40
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Default Some thoughts on solar sails

Christopher M. Jones wrote:
However, because continuous application of thrust along
the direction of the orbit necessarily pushes the sail
farther and farther from the Sun, causing the photon
flux to diminish, there are fairly inherent maximum
limits to Solar Sail speed. About the best way to
achieve maximum speed with a Solar Sail, if you're
heading out to the deep, outer Solar System, for
example, is to use the oberth effect, and try to put
most of your thrust close to the Sun (first you fall in,
then you boost out).


That is the rub, isn't it?

I posted another note (was it moderated out?) where I pondered whether
you couldn't hang out close to the sun, use the sails to spin up a
platform, thus storing up energy where the flux is high, then use this
stored energy to make a launch platform. The thought was park an
asteroid or other large mass in close, hang sails on it so it'd be a
light driven anemometer to spin it up, then fling probes off of it like
a space version of a trebuchet.

The asteroid would not change it's orbit, so it could stay in close to
the sun, building up energy at a maximum rate.

Or it could be a vehicle, with a stationary inner section and a rotating
outer section that is spun up by insolation, and then uses the stored
rotational energy to toss off mass to get the action/reaction of a rocket.


  #6  
Old March 27th 04, 05:45 AM
Makhno
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Default Some thoughts on solar sails

The only case where interplanetary "sail" propulsion has an even
_approximately_ accurate "fluid like" analogy are the so-called
"magsail" or "M2P2" concepts, where a magnetic field is used to
interact with the supersonic plasma of the "solar wind" --- and
even then, the analogy to "sailing" is quite bad, because sailboats
are not used in supersonic winds,


What is a 'supersonic plasma' ?


  #7  
Old March 28th 04, 08:24 PM
Paul F. Dietz
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Default Some thoughts on solar sails

Makhno wrote:

What is a 'supersonic plasma' ?


A plasma that is travelling faster (with respect to the vehicle)
than the speed of sound in the plasma(*),

Paul

(*) details of what 'speed of sound' means in a plasma omitted
  #8  
Old April 4th 04, 03:58 PM
Martha H Adams
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Default Some thoughts on solar sails

I'm working on a story that mentions a freighter, the Millie Gee, that
cycles regularly between Luna and Deimos.

The Millie Gee uses solar power from large arrays to power a
vasimr engine whose ejection mass is lunar water. She never touches
down, is reached by tugs/landers from both destinations. (Her cargo
for her first trip out included a tug for the Deimos end of her
schedule.) She follows an elliptical orbit between her two ports; but
has options for other travel if the need arises.

I think my solar powered Millie Gee would serve better for local
travel inside the asteroid belt, than solar sail propulsion, because
(assuming appropriate design) she could accelerate in any direction at
any time. Which a solar sail cannot do.

Why "tug" in story space, rather than "shuttle"? Because in story
environment, "shuttle" has bad connotations.

Cheers -- Martha Adams

 




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