B Da Vinci's flying machine meets solar sail?

[Stefen]

so the question has been previously answered as to what would happen if a fan was in the vacuum of space. but the responses I seen all ask what the fan would push on or up against. what if the fan where to push against light? solar sails are able to have a force push against them in space?

 

[Stefen]

https://www.physicsforums.com/threads/fan-in-vacuum.440620/#post-2947649 this is the forum I read about the effects of a fan in space, saying that there is nothing in space for a fan to push against. But what about light?

 

[Stefen]

Could u not make a solar sail kinda like Davinci's flying machine ? Or does that just sound cool ... And then if the base must rotate then why not make it corkscrew shaped and also solar sail? ... One word a day? Today's word is procrastinanigans... The act of procrastinating your shenanigans

 

[A.T.]

what if the fan where to push against light? solar sails are able to have a force push against them in space?

So the question here is: Can a spinning fan like solar sail produce more thrust, than a non spinning one, with the same surface area?

 

[phinds]

So the question here is: Can a spinning fan like solar sail produce more thrust, than a non spinning one, with the same surface area?

Well, it seems to me that's hardly the only question. The big question I see is, what powers the fan? The OP clearly envisions the fan pushing against light, not passively being pushed BY light.

 

[Stefen]

So the question here is: Can a spinning fan like solar sail produce more thrust, than a non spinning one, with the same surface area?

That's something I have been wondering... Which I think if we could harness an extra thrust some how from the light.. I mean I doubt a fan will but maybe more a screw?

 

[Stefen]

That's something I have been wondering... Which I think if we could harness an extra thrust some how from the light.. I mean I doubt a fan will but maybe more a screw?

With no resistance in space I guess it wouldn't really help

Well, it seems to me that's hardly the only question. The big question I see is, what powers the fan? The OP clearly envisions the fan pushing against light, not passively being pushed BY light.

The fan is powered by light of course... ?? Solar energy... Or does light work different in space?

 

[phinds]

With no resistance in space I guess it wouldn't really help
The fan is powered by light of course... ?? Solar energy... Or does light work different in space?

Sure, that works, but my point is that I don't think you can use the energy in the light twice, so all of the energy that goes into turning the fan does not go into moving the vehicle forward, so you get less efficiency than a plain old solar sail.

 

[A.T.]

Well, it seems to me that's hardly the only question. The big question I see is, what powers the fan? The OP clearly envisions the fan pushing against light, not passively being pushed BY light.

Yes, but the first question is, if rotation can really increase the force on a propeller shaped solar sail. And unlike with a propeller in a tailwind of air, the answer is not that obvious to me for light.

 

[Vanadium 50]

harness an extra thrust some how from the light

What extra thrust? Consider a simple mirror. Light with momentum p comes in and light with -p goes out. How do you get any more momentum? That's all there is.

 

[phinds]

Yes, but the first question is, if rotation can really increase the force on a propeller shaped solar sail. And unlike with a propeller in a tailwind of air, the answer is not that obvious to me for light.

My thought was that the amount of light energy arriving at the vessel is independent of whether or not the struck surface is rotating so the only difference would be the awesomely trivial amount of (possible) extra energy give to the impact by the speed of the sail. Thus you have essentially a tie, BUT you have to power a rotating sail and that uses up far more power than anything that could be gained from the fact that the sail is rotating.

 

[A.T.]

Light with momentum p comes in and light with -p goes out. How do you get any more momentum?

Only by interacting with more light. So the question is whether rotation can achieve this.

 

[Vanadium 50]

But an efficient solar sail reflects all the light in its path. How do you reflect more than "all"?

 

[Stefen]

Only by interacting with more light. So the question is whether rotation can achieve this.

I would love to find out... Thank u guys I was curious.. . I'd love to see a live test... That would be cool

 

[A.T.]

But an efficient solar sail reflects all the light in its path.

We talking about a fan-shaped solar sail, which doesn't reflect all the light that passes through the rotor disc. Something like the heligyro in the middle.

The question is whether it's rotation can increase the amount of reflected light. It seems to me not. But a good and simple argument would be nice.

 

[Vanadium 50]

We talking about a fan-shaped solar sail, which doesn't reflect all the light that passes through the rotor disc

But it does reflect all the light that impinges the sail.

 

[mrspeedybob]

I think it may be useful to think of the light as a kind of photon-gas tail wind. If the blades of the fan are flat, then I agree, rotating would not change anything. However if they are angled like fan blades, then thy could sweep a greater volume of the photon gas then flat, stationary blades would. The effect would be very slight though, unless the speed of the blades was a significant fraction of C.

 

[Vanadium 50]

However if they are angled like fan blades, then thy could sweep a greater volume of the photon gas then flat, stationary blades would. The effect would be very slight though, unless the speed of the blades was a significant fraction of

I'm not sure this isn't zero. But in any event, by tilting the blades you reduce the forward thrust by cos(theta).

 

[mrspeedybob]

That would be true for a stationary angled blade, but a moving angled blade would blue shift the reflected photons. I'm thinking you could gain a little more Δp from the blue shift then you'd loose to the change in angle.

I'm pretty sure most of us could do the math to actually figure this out. I'm also pretty sure I'm feeling too lazy for anything deeper then a qualitative analysis right now. :-)

 

[Stefen]

That would be true for a stationary angled blade, but a moving angled blade would blue shift the reflected photons. I'm thinking you could gain a little more Δp from the blue shift then you'd loose to the change in angle.

I'm pretty sure most of us could do the math to actually figure this out. I'm also pretty sure I'm feeling too lazy for anything deeper then a qualitative analysis right now. :-)

It would be awesome if we could master light travel through a method like this... All though it would take lifetimes to get any real speed

 

[sophiecentaur]

then thy could sweep a greater volume of the photon gas

I can't see why. Do you have a diagram to show what you mean? (What is a photon gas?) But the swept area of spinning rotor would be no more than that of a mirror of the same diameter. And how would you get any more momentum out than 2p? (Vanadium's remark earlier) A point worth making is that the mechanics of such big and spindly craft is very different from earthly flying machines and there will be different trade-offs of mass and strength. There would be higher forces in a rotating structure and that could militate against the idea (even if it did have legs - which I doubt).
This link is about a solar sailing.

 

[mrspeedybob]

Instead of a rotating blade, simplify the mental picture to a 1x1 meter square sail. The sail is operating in a region of space with a photon density of 1 photon / m³. The photons are all moving in the same direction, perpendicular to the sail, so the sail is hit by 3x10⁸ photons / second. Each photon has momentum p and reflects away with momentum -p so the momentum transfer to the sail is 6x10⁸p / second.

Now replace the photons in this mental picture with stationary particles, and imagine the sail is moving through the particles at 3x10⁸ m/s. As each particle hits the sail, it bounces away at 6x10⁸ m/s which is equivalent to a momentum of 2p. The momentum transfer to the sail in this scenario is also 6x10⁸p. I realize this second mental picture is non-physical as it neglects relativistic effects, but it's easy to see, in a Newtonian sense, that these two scenarios are equivalent. Only the frame of reference has changed.

Now let's modify the second scenario by angling the sail, and moving it sideways at the same time we move it forward. At the same time we're moving the sail forward by 3x10⁸ meters, we'll also move it sideways by 10⁸ meters. The angle of the sail will be set so that the particles still impact it at a right angle, but now that it's moving at a diagonal, it's going to be moving at 3.16x10⁸ m/s, so the particles will be bouncing off at 6.32x10⁸ with momentum 2.11p. Also, the sail will now be sweeping 3.16x10⁸ m³ and so impacting the same number of particles, so the total momtum transfer to the sail will be 6.66x10⁸p. Of course this momentum transfer is now angled instead of straight back so we'd have to split the vector into perpendicular components and find that the rearward component is 6.32x10⁸

Now change back to the original reference frame and the scenario looks like a sail with a 3:1 angle moving sideways at 1/3 c while being hit with photons with momentum p and the forward momentum transfer to the sail is 6.32x10⁸p

Now back up to the original helio-gyro. 1x1 meter sail I've been describing could be the last meter of a very long blade. Long enough that the tangential velocity of the blade tip can be treated as straight sideways movement.

 

[Stefen]

U just blew my mind!