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Solar sailing DOESN"T break laws of physics'



 
 
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  #12  
Old July 4th 03, 10:54 AM
Dale Trynor
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Default Solar sailing DOESN"T break laws of physics'



Uncle Al wrote:

"Geoffrey A. Landis" wrote:

Despite a recent article in New Scientist, a solar sail does not break
the laws of physics.

[snip]

Actually, it does as proposed. The sail will come into thermal
equilibrium with the radiation field and emit photons from its other
side, counter-thrusting.


[snip]

Dale Trynor wrote:
I was going to point out that you needed to think about this a bit more.
That's unnecessary now but I am surprised you missed it. I am also very
surprised that an expert like Gold would make such a mistake and then get it
published. Makes me feel better about my own mistakes however I am just an
amateur.
I do remember reading that the solar force is something like 5 pounds per
square mile if that's of much use. It could really add up to a lot of
stresses if one were to have a 1000 mile wide mirror without using some
means to deal with that. Thickness would of course have to increase by a lot
if it was unsupported and no tricks used.

I did some posts recently on the idea that you could cover an entire planet
to 15 psi with a very thin film if each inch of area had a 15 pound weight
attached. In principle ones thickness would never need to be more than that
required for the 1 square inch of area no mater how much area was covered.



If you push optimistic numbers,
http://solarsails.jpl.nasa.gov/intro...struction.html
you get a bowl of moldy farina.

Oh yeah... even chemically tough Kapton in orbit gets chewed -
especially its reflectance,
http://setas-www.larc.nasa.gov/esem/..._append_b.html

--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
"Quis custodiet ipsos custodes?" The Net!


  #13  
Old July 4th 03, 01:03 PM
Gregory L. Hansen
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Default Solar sailing DOESN"T break laws of physics'

In article ,
Allen Thomson wrote:
(Gregory L. Hansen) wrote

Dang, I don't know what to say. Gold is a putz,


Gold is, ah, interesting... Check out his theory on the origin of
petroleum.

and I thought New Scientist was better than that.


Life teaches us bitter lessons.

I didn't realize a solar sail was a heat engine.


It *is* a heat engine, properly analyzed. Others in this thread
have done that; Gold did not. In a sense, what a solar sail does
is thermodynamically not that much diffrent than what a piston in
your car's engine does. Although, thermodynamics aside, Newtonian
mechanics does just fine for understanding how a solar sail works.


Yeah, I was just thinking that a few hours after I'd sent out the message.
You could imagine the solar sail craft as a piston, the Sun at so many
milions of degrees, and the "sink" at about 3 kelvin. Not a bad Carnot
efficiency, actually.

--
"Is that plutonium on your gums?"
"Shut up and kiss me!"
-- Marge and Homer Simpson

  #14  
Old July 4th 03, 02:19 PM
Mitchell
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Default Solar sailing DOESN"T break laws of physics'

But there is energy lost from the photons. Do the math. That's where
the Doppler shift matters.


That's just not true. I'm a senior year physics undergrad at Cornell,
and I was in a group of 6 people who got to sit down and talk with
Gold about his "perfect mirror and violations of conservation of
energy/momentum" paper before it was published. Among the arguments we
attempted to refute is theory was the idea of Doppler shift. He
correctly countered that a Doppler shift does not affect the total
energy, only the power.

Another post mentioned that if a Doppler shift could occur without
interaction with the sail, there would be nowhere for the lost energy
to go. Well, a moving galaxy puts out Doppler shifted light, and since
each star radiates basically isotropically, there's no acceleration
from that. So where's that energy going when we view the galaxy? The
answer is that it's still all there, it's just coming in at a
different rate. Doppler shift only changes power, not total energy.

Mitchell
  #15  
Old July 4th 03, 04:14 PM
Henry Spencer
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Default Solar sailing DOESN"T break laws of physics'

In article ,
Mitchell wrote:
He correctly countered that a Doppler shift does not affect the total
energy, only the power.


You'll need to explain this in more detail, because it's not convincing
the way you've told it. Light is reflected from a perfectly-reflecting
moving solar sail; say it's moving away from the Sun for purposes of
argument. The light comes back red-shifted, having lost energy. Same
number of photons, less energy per photon. Where did the energy go?

Another post mentioned that if a Doppler shift could occur without
interaction with the sail, there would be nowhere for the lost energy
to go. Well, a moving galaxy puts out Doppler shifted light...


Yes, but it's an emitter, not a reflector. As measured by an observer
motionless with respect to us, that light *started out* Doppler shifted;
at no point did its energy *change*. It's the change in energy as a
result of perfect reflection that requires explanation.

...Doppler shift only changes power, not total energy.


However, changing the power carried by a specific beam of photons -- it
goes out from the Sun carrying X watts, it comes back after reflection
carrying X-Y watts -- still requires explanation. The missing power has
to go somewhere. Where?
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |
  #16  
Old July 4th 03, 05:37 PM
Dr John Stockton
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Default Solar sailing DOESN"T break laws of physics'

JRS: In article , seen in
news:sci.space.policy, Henry Spencer posted at
Thu, 3 Jul 2003 21:48:17 :-

The JPL Halley design was
2um of Kapton, topped by 100nm of aluminum, with 12.5nm of chromium on the
back. 2-3um Kapton is commercially available.


Such thicknesses have the property that solar radiation pressure
approximately balances solar gravity, at any distance from the Sun.

With exact balance (not forgetting payload), a sailing vessel would have
a constant-velocity path, and would reach Root2 AU in 1/2pi years, if
started with Earth's orbital velocity from an altitude where Earth's
gravity becomes unimportant.

--
© John Stockton, Surrey, UK. Turnpike v4.00 MIME. ©
Web URL:http://www.merlyn.demon.co.uk/ - FAQqish topics, acronyms & links;
some Astro stuff via astro.htm, gravity0.htm; quotes.htm; pascal.htm; &c, &c.
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  #17  
Old July 4th 03, 09:01 PM
Christopher M. Jones
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Default Solar sailing DOESN"T break laws of physics'

"Henry Spencer" wrote:
In article ,
Dr John Stockton wrote:
Such thicknesses have the property that solar radiation pressure
approximately balances solar gravity, at any distance from the Sun.


Not quite, alas. For that you need about 0.75g/m^2 -- about 1/4 of the
mass JPL was looking at, and somewhat better than the best they expected
for near-term advanced materials. And then, as John points out, it has to
go somewhat lighter yet because there is payload and other overhead to be
accommodated.

Near-term advanced materials with low-overhead designs do offer potential
for at least achieving thrust of a significant fraction of the Sun's
gravity, which is significant because it makes interplanetary trajectories
much more efficient than with lower thrusts.


Just to add a little clarification for the folks that aren't
quite up on the details of solar sails, any thickness is
sufficient for operation at some level, the thicknesses
Henry and John are talking about are only those which would
make for excellent solar sails. The trick is that you
aim your thrust ahead or behind the direction of travel
along the orbit, that way you don't have to fight the Sun's
gravity, you work with it. With any solar sail at all it's
possible to change to any orbit or to leave the Solar System
entirely, it's just a matter of how long it would take, with
a very poor solar sail it may take longer than the age of
the Solar System, with the sorts of sails they can build
these days (see above) it would take longer than using
modern chemical rockets, but not by all that much.

  #18  
Old July 4th 03, 11:18 PM
David M. Palmer
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Default Solar sailing DOESN"T break laws of physics'

In article , Steve Harris
wrote:

Come on. You're like the guy who says: when I heat up an
object, its weight doesn't change (to first order).
Therefore the equivalence of mass and energy is violated.
Duh.

There is no energy cost to move the stationary sail *to
first order.* Carnot's law is broken to exactly the degree
that you simplify the problem with approximation. But don't
confuse your approximation with violation of physical law.


There is no energy cost to move the stationary sail to first order.
There is no change in photon energy to first order

There is an energy cost to move the stationary sail to second order.
There is an equal change in photon energy to second order.

There is no breaking of Carnot's law to any order.

--
David M. Palmer (formerly @clark.net, @ematic.com)
  #19  
Old July 4th 03, 11:20 PM
Dr John Stockton
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Default Solar sailing DOESN"T break laws of physics'

JRS: In article , seen in
news:sci.space.policy, Dale Trynor posted at Fri, 4
Jul 2003 09:54:45 :-

I do remember reading that the solar force is something like 5 pounds per
square mile if that's of much use.


I make it, for total reflection, about 9.6E-7 kgF/m^2, or nearly one
kilogram per square kilometre; I leave conversion to archaic units as an
exercise for the reader.

--
© John Stockton, Surrey, UK. Turnpike v4.00 MIME. ©
Web URL:http://www.merlyn.demon.co.uk/ - FAQqish topics, acronyms & links;
some Astro stuff via astro.htm, gravity0.htm; quotes.htm; pascal.htm; &c, &c.
No Encoding. Quotes before replies. Snip well. Write clearly. Don't Mail News.
  #20  
Old July 5th 03, 12:33 AM
Gregory L. Hansen
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Default Solar sailing DOESN"T break laws of physics'

In article ,
David M. Palmer wrote:
In article , Steve Harris
wrote:

Come on. You're like the guy who says: when I heat up an
object, its weight doesn't change (to first order).
Therefore the equivalence of mass and energy is violated.
Duh.

There is no energy cost to move the stationary sail *to
first order.* Carnot's law is broken to exactly the degree
that you simplify the problem with approximation. But don't
confuse your approximation with violation of physical law.


There is no energy cost to move the stationary sail to first order.
There is no change in photon energy to first order

There is an energy cost to move the stationary sail to second order.
There is an equal change in photon energy to second order.

There is no breaking of Carnot's law to any order.


And yet, there is an energy cost to move a stationary sail, and there is a
change in photon energy. And no matter what thermodynamics arguments are
brought up, light pressure has been used to orient satellites, there's
nothing controversial about it. Looks like someone is going to have to
figure out where their analysis went wrong.

--
"Is that plutonium on your gums?"
"Shut up and kiss me!"
-- Marge and Homer Simpson

 




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