Solar Sail will not work?

[Kayra]

At least according to this.

http://arxiv.org/html/physics/0306050

 

Any truth to it?

[InfiniteNow]

Well, when they launched it, the rocket booster failed, and that's part of why it did not work. As for the technology behind solar sailing, we have not yet found out since it never made outside the earth's atmosphere for testing.

 

http://www.planetary.org/programs/projects/solar_sailing/

[Kayra]

InfiniteNow, that article I pointed to is saying that the physics of the sail will not work, at least not without violating the second law of thermodynamics.

[InfiniteNow]

From your article:

 

However a detailed examination of this proposal shows it to be in direct conflict with Carnot's rule, and no such pressure can be expected.

 

 

From the Wiki:

http://en.wikipedia.org/wiki/Radiation_pressure

Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the energy flux density divided by the speed of light. If the radiation is totally reflected, the radiation pressure is doubled.

 

The article you are referencing shows that perpetual motion machines cannot work because heat is lost in the exchange, but is trying to tie this to a statement that there is no such thing as radiation pressure, which is false.

 

Radiation pressure exists, and the sail is built to reflect it... pushing it forward.

[Kayra]

Hmm

I thought he was trying to say that if you could reflect 100% of the radiation, you get work with no losses.

 

If you consider the radiation to be heat, then you have a perfect heat engine.

 

i think I need to re-read it several times to comprehend what he is trying to say :lol:

[InfiniteNow]

Granted, I did just skim the article, but of course solar sailing is not 100% energy efficient. I don't think anyone is claiming that it is (at least not anyone credible if they are). The basic premises are what I have issue with.

 

Of course it's not 100%... so that blows up idea of going against Carnot and 2nd Law of ThermoD right way.

 

 

Let's say you have a toy sailboat in your bath tub, and you throw little rocks at it. Will it move? Of course. Will 100% of the energy of the moving rock transmit to the the boat? Of course not. The solar sail concept is pretty much the same thing, except much bigger and the rocks are radiation pressure.

[Kayra]

Granted, I did just skim the article, but of course solar sailing is not 100% energy efficient. I don't think anyone is claiming that it is (at least not anyone credible if they are). The basic premises are what I have issue with.

 

Of course it's not 100%... so that blows up idea of going against Carnot and 2nd Law of ThermoD right way.

 

The reflecting surface on a solar sail is usually something like 99.9% effective, isn't it?

 

Let's say you have a toy sailboat in your bath tub, and you throw little rocks at it. Will it move? Of course. Will 100% of the energy of the moving rock transmit to the the boat? Of course not. The solar sail concept is pretty much the same thing, except much bigger and the rocks are radiation pressure.

 

If you throw a rock at your sailboat, 100% of the energy (- the friction of the rock through the air) will transfer to the boat. But that is mechanical energy, and not heat energy. Newtons law applies.

 

You may be right about the radiant energy striking the sail not being considered heat energy. at least that would make more sense in regards to it's efficiency.

[InfiniteNow]

http://science.howstuffworks.com/solarsail6.htm

 

most solar sails are made of thin, metal-coated, durable plastics such as Mylar or Kapton. For example, the solar sail of Cosmos-1 is made of aluminum-coated Mylar, has a thickness of 0.0002 inches or 5 microns (ordinary Saran Wrap is about 0.001 inches or 25 microns thick) and an area of 6,415 square feet (600 square meters).

 

 

Pressure from Sunlight

 

Using the following equations and values, you can calculate the force of sunlight on and acceleration of the spacecraft:

 

Force (F) = 2(P x A)/c

Acceleration (a) = F/M

 

At 1 astronomical unit (AU), the power of sunlight is about 131 watts/foot2 (1,400 watts/meter2). Our spacecraft weighs 2.2-lb (1-kg) and has a sail area of 0.38 mi2 (1 km2 or 1-million m2), so:

 

P (power) = 1,400 watts/m2

A (area) = 1-million m2

c (speed of light) = 3x108 m/s

M (mass) = 1 kg

 

This works out to a force (F) of about 2 lb or 9 newtons (N). This force leads to an acceleration (a) of about 29 ft/s 2 (9 m/s2), slightly less than the acceleration due to Earth's gravity. In comparison, a space-shuttle main engine can produce 367,000 lb (1.67-million N) of force during liftoff and 462,000 lb (2.1-million N) of thrust in a vacuum.

[Kayra]

Yep, I see where the fella went wrong I think.

 

Radiation is not heat. You can not apply Carnot's theory to radiative energy, only to heat.

 

The same radiative energy can induce heat in whatever absorbs it, and he assumes from this that they are one and the same.

 

Does that sound about right?

[InfiniteNow]

I think heat and radiation are pretty similar, however, the distinction should be heat versus radiation pressure. Pressure being the key word, and it is the result of the radiations effect on the surface (in whatever form that radiation comes).

[Pyrotex]

Yep, I see where the fella went wrong I think.

Radiation is not heat. You can not apply Carnot's theory to radiative energy, only to heat....

Yup, that sounds right. If you consider just the photons in the solar wind, then heat doesn't even come in to play. You're dealing with the momentum of the photons bouncing off the mylar like... little rocks. If you consider the particles in the solar wind--protons, Helium nuclei, Carbon ions, whatever, then you're dealing with absorbed momentum when those particles "stick", like... little spitwads.

 

Metalized coating on telescope mirrors are not as efficient as you might think. Aluminized mirrors are about 96% efficient when brand new. I have heard of more expensive coatings that go as high as 99%.

 

Oh, another item. The photon and particle portions of the solar wind have another difference. You can "tack" against the photon portion in somewhat the same way a sailboat tacks against the wind. But when the particles hit and stick, all momentum is transferred in the same direction as the particle was going. You can't "tack" with particles because they don't bounce.

[Kayra]

Thanks, that makes perfect sense

[GAHD]

Heat *is* radiation, just at a lower wavelength. For a solar sail to work it must either

A) Convert incoming energies into kinetic energy

;) Capture kinetic energy from charged particles.

Can you figure out which one is more likely for the oversized tinfoil bedsheet?

[Kayra]

Heat *is* radiation, just at a lower wavelength.

 

I thought heat was the measure of the average velocity of molecules (or?).

You can only apply the Carnot cycle to something with moving molecules.

Radiation has no molecules. When radiant energy strikes a molecule, it imparts velolcity (or heat) onto it. So while they are equivalent, they are not the same.

 

 

For a solar sail to work it must either

A) Convert incoming energies into kinetic energy

;) Capture kinetic energy from charged particles.

Can you figure out which one is more likely for the oversized tinfoil bedsheet?

 

Lets take that sail-boat/rock example above. If you throw rocks at the sail (instead of the boat), and the sail bounced the rocks back at exactly the same speed and direction they were thrown, then the boat gets double the energy from the rock and moves along the trajectory that the rock was thrown from. If the sails are angled, then the rock is bounced off at the same speed, but in a different direction then was thrown, and the boats trajectory is the average of the two trajectories. Does that sound right?

 

No conversion takes place, simply redirection.

[GAHD]

Yes yes, you are halfright on the hair-splitting of heat (t's valence layer and energy states of of electrons) but...

 

Lets take that sail-boat/rock example above. If you throw rocks at the sail (instead of the boat)' date=' and the sail bounced the rocks back at exactly the same speed and direction they were thrown, then the boat gets double the energy from the rock and moves along the trajectory that the rock was thrown from. If the sails are angled, then the rock is bounced off at the same speed, but in a different direction then was thrown, and the boats trajectory is the average of the two trajectories. Does that sound right?

 

No conversion takes place, simply redirection.[/quote']

 

:surprise:

 

it should read "he sail bounced the rocks back at nearly the same speed and direction they were thrown" part of the kinetic energy from that rock gets absorbed by the ship.With a flexible surface like a sail 'most' of the energy gets absorbed (unless the rock is going fast enough to tear-through the sails) and the rock will most likely just fall to the deck(which absorbes "most" of the kinetic energy imparted by gravity and redistributes it to the water the ship is floating on). Google conservation of momentum, for somebody using fancy words like Carnot cycle you seem to be missing out on one of the basics of thermodynamics.

[Kayra]

:surprise:

 

it should read "he sail bounced the rocks back at nearly the same speed and direction they were thrown" part of the kinetic energy from that rock gets absorbed by the ship.With a flexible surface like a sail 'most' of the energy gets absorbed (unless the rock is going fast enough to tear-through the sails) and the rock will most likely just fall to the deck(which absorbes "most" of the kinetic energy imparted by gravity and redistributes it to the water the ship is floating on). Google conservation of momentum, for somebody using fancy words like Carnot cycle you seem to be missing out on one of the basics of thermodynamics.

 

Interesting. I thought we were using the rocks as an analogy for light (albeit not a perfect one). One would hardly expect those photons to fall to the deck after striking the sail. They move at the speed they like to move at © in a vacuuming, hence when they bounce off of the sail, they impart twice the energy that they have as motion to the sail.

 

The Carnot cycle and laws of thermodynamics refer to heat and heat engines. Radiative energy can be converted to heat (it even happens a bit when they strike the sail), but in radiative form the Carnot cycle should not apply (I think). Even infrared energy is not heat. It is radiative energy at a frequency that is readily more absorbed as heat.

[Kayra]

Sorry if it appeared I was trying to split hairs on the heat thing. It was not intended. The "Heat thing" was essential to the argument.

 

From Wikipedia

 

http://en.wikipedia.org/wiki/Heat

 

"In physics, heat is defined as energy in transit.[1] Generally, heat is a form of energy associated with the motion of atoms, molecules and other particles which comprise matter. "

 

I think what you were getting at with the "valence layer and energy states of of electrons" was that if an atom is struck by a photon, and that photon does not have enough energy to cause the electron to move to a higher energy state, then the energy is transformed into heat.

 

Good point on missing out on the thermodynamics though. It was my lack of knowledge and my ongoing struggles in understanding in this area that brought me to this forum in the first place.

 

(My first thread http://hypography.com/forums/physics-mathematics/5113-why-carnot-cycle-most-efficient-heat.html)