8 ms·

Very cool! I know nothing about solar sails, but I wonder if the craft could have LEDs mounted to it so it could bombard different regions of the sail with phot

by umvi 7y ago

Very cool! I know nothing about solar sails, but I wonder if the craft could have LEDs mounted to it so it could bombard different regions of the sail with photons to make it turn appropriately.

henryfjordan 7y ago
If you emit a particle (a photon) and then catch it the net effect will be 0.
- moopling 7y ago
  Exactly, it is much like using a fan on a boat to blow a sail.
  - redorb 7y ago
    mythbusters were able to show a weird effect where the fan on a boat thing worked - I think they chalked it up to some blown wind from the fan drawing in more than from around the fan.. https://youtu.be/uKXMTzMQWjo?t=75 https://youtu.be/uKXMTzMQWjo?t=75
    - walrus01 7y ago
      From a physics point of view it's not like they created an infinite motion machine, the stored energy in the liquid petroleum is being burned, heat from the internal combustion engine radiated into the atmosphere, etc. It's just converting stored energy (some amount of kJ or kWh in a fuel tank, or battery) into an aerodynamically weird highly inefficient thrust method.
- umvi 7y ago
  Net effect is not zero if the photon bounces off of the sail
  - comboy 7y ago
    Why use sail then? You can just shoot your photon the opposite way.
    - seiferteric 7y ago
      That would be a "photon rocket" which is an existing idea, but requires a lot of energy, whereas this uses free energy from the sun.
  - gitgud 7y ago
    If it bounces off in the opposite direction then the net effect would be actually be negative.
    - deleted 7y ago
      [deleted]
- simplicio 7y ago
  But if your emitter emits in every direction (like a lightbulb) and you only catch the ones going in one direction in the sail, you'd propel yourself in that direction. So having a directional lightsource fire into the sail wouldn't work, but having one that emits light in every direction on one side of the sail would.
  - bsmith0 7y ago
    So just use a directional light source pointing away from the craft...
Rychard 7y ago
I would assume the number of photons coming from the sun is many orders of magnitude higher than any LED could ever hope to produce. This is of course overlooking the whole "pull oneself up by one's bootstraps" vibe.
cgh 7y ago
That's like trying to propel your sailboat by holding up an electric fan.
- umvi 7y ago
  I mean, it's only the size of a boxing ring, not a football field. I think LEDs could produce enough light to have a net effect.
  - bmdavi3 7y ago
    I don't know if the strength of an LED is enough to move the craft significantly, but if it is you'd need to point it at anything _but_ the ship itself, otherwise the two forces would cancel out. Just shine it in the opposite direction of where you want to go.
    - umvi 7y ago
      No, the two forces don't cancel out, I can't remember exactly why but elastic collisions don't work like that. That's why you can point a fan into your sailboat's sail and it will still go forward, mythbusters proved it.
      - mkl 7y ago
        Mythbusters demonstrated something different, and something much less efficient than simply pointing the fan backwards like a normal fan boat (no sail required). Here: https://mythresults.com/blow-your-own-sail https://mythresults.com/blow-your-own-sail
  - Skunkleton 7y ago
    No. Think of the photons as producing little pushes. When they are emitted by the LED, they produce a little push in one direction, and then the sail pushes a little bit back in the other direction. If it were possible to create a significant volume of thrust from photons emitted by LEDs, then you would be better off just pointing the LEDs away from where you wanted to go (like a rocket).
omarhaneef 7y ago
That would be like taking fan and blowing it into your own sails. It wouldn't work.
- umvi 7y ago
  It does work though, depending on the material your sail is made of. Try it: attach an electric fan to a pinewood derby car and point it so it is blowing into a tin foil sail also attached to the pine wood derby car. The car will move forward.
  - omarhaneef 7y ago
    I thought you want the LED mounted on the spacecraft itself. That would be the equivalent of the fan being on the sailboat. Yes, if you had another power boat next to the sailboat you could blow the boat, but it would be transferring some energy from the engine of the powerboat. So if you had another spacecraft near the space sail, and that spacecraft was shooting light, it would have some miniscule effect, but I didn't think that is what you meant. Edit: I suspect you're kidding but I can't be sure.
    - umvi 7y ago
      That's what I'm talking about. A handheld fan[1] is on the pinewood derby car, same as the sail. I built one before so I know it works. [1] https://www.amazon.com/Aluan-Handheld-Portable-Adjustable-Rechargeable/dp/B07QXHT67B/ https://www.amazon.com/Aluan-Handheld-Portable-Adjustable-Re...
      - omarhaneef 7y ago
        Okay, I am not more sure you are kidding but to be fair to me, I had no context for your joking ways before.
  - deleted 7y ago
    [deleted]
  - Skunkleton 7y ago
comboy 7y ago
This kind of technology had been demonstrated many years ago and many people around the world had been watching it at the time[1]. 1. https://i.imgur.com/VL0Q9hT.gif https://i.imgur.com/VL0Q9hT.gif
blacksmith_tb 7y ago
Not a sailor (solar or otherwise) but my instinct is you could turn by "furling" parts of the sail (say, if there were vanes that could be turned perpendicularly to the flow of photons). But lots of designs seem to call for propulsion from a laser, so maybe that could just be angled away slightly. At astronomic distances that might be tough, though.
- hinkley 7y ago
  Supposedly, if you are orbiting the sun, you can turn a solar sail in such a way that you can brake against the solar wind, thereby moving in-system rather than out. But the sailing analogy breaks down for turning. A boat is pushing against the water and the wind. That gives it the ability to do things that you can’t in a vacuum, like move faster than the wind. For ‘turning’ you have only thrust vectoring, right?
  - db48x 7y ago
    There are several control methods available to solar sails that can work. They were studied quite extensively by NASA during the 60s. The thrust generated by a solar sail is proportional to the angle between it and the sun, as is the direction of that thrust. If any part of your sail can twist, then you have steering vanes that are quite useful. You can twist the vanes to angles where the vane on one side of the craft is reflecting sunlight while the one on the other side is not in order to get a rotational acceleration. If your sail is carrying a payload, then that payload will likely be in the center of the craft, with the sail spread out around it symmetrically. If you can shift the payload side to side relative to the sail, then that will shift the center of mass of the whole craft. With more of the sail to one side of the center of mass you will again get a rotation. If you want more information, find a copy of the book Space Sailing by Jerome Wright; it's quite good.
walrus01 7y ago
https://en.m.wikipedia.org/wiki/Photon_rocket https://en.m.wikipedia.org/wiki/Photon_rocket It's a real idea, but you would need a truly ridiculous amount of electricity available on a spacecraft. Only possible by a huge nuclear reactor. Doesn't violate any laws of physics.
- mkl 7y ago
  There are key differences with that: either the photons are shot backwards (like a rocket), or the source of the photons is not on the ship itself. ("The limitations posed by the rocket equation can be overcome, as long as the reaction mass is not carried by the spacecraft.")
tfolbrecht 7y ago
I'm curious if you could selectively turn off the reflective effect in different regions of the material and steer it that way.
- andygates 7y ago
  IKAROS used this technique. The bird was spin-stabilised and a section of the sail had LCD shades that could be dark or light. It could darken its, say, venus-side segments as they went round. Gloriously over-engineered.
JshWright 7y ago
Changing orientation is easily done using spinning masses (this is how most satelites maintain their orientation)

by umvi 7y ago

Very cool! I know nothing about solar sails, but I wonder if the craft could have LEDs mounted to it so it could bombard different regions of the sail with photons to make it turn appropriately.

henryfjordan 7y ago

If you emit a particle (a photon) and then catch it the net effect will be 0.

moopling 7y ago
Exactly, it is much like using a fan on a boat to blow a sail.
- redorb 7y ago
  mythbusters were able to show a weird effect where the fan on a boat thing worked - I think they chalked it up to some blown wind from the fan drawing in more than from around the fan.. https://youtu.be/uKXMTzMQWjo?t=75 https://youtu.be/uKXMTzMQWjo?t=75
  - walrus01 7y ago
    From a physics point of view it's not like they created an infinite motion machine, the stored energy in the liquid petroleum is being burned, heat from the internal combustion engine radiated into the atmosphere, etc. It's just converting stored energy (some amount of kJ or kWh in a fuel tank, or battery) into an aerodynamically weird highly inefficient thrust method.
umvi 7y ago
Net effect is not zero if the photon bounces off of the sail
- comboy 7y ago
  Why use sail then? You can just shoot your photon the opposite way.
  - seiferteric 7y ago
    That would be a "photon rocket" which is an existing idea, but requires a lot of energy, whereas this uses free energy from the sun.
- gitgud 7y ago
  If it bounces off in the opposite direction then the net effect would be actually be negative.
  - deleted 7y ago
    [deleted]
simplicio 7y ago
But if your emitter emits in every direction (like a lightbulb) and you only catch the ones going in one direction in the sail, you'd propel yourself in that direction. So having a directional lightsource fire into the sail wouldn't work, but having one that emits light in every direction on one side of the sail would.
- bsmith0 7y ago
  So just use a directional light source pointing away from the craft...

Rychard 7y ago

I would assume the number of photons coming from the sun is many orders of magnitude higher than any LED could ever hope to produce. This is of course overlooking the whole "pull oneself up by one's bootstraps" vibe.

cgh 7y ago

That's like trying to propel your sailboat by holding up an electric fan.

umvi 7y ago
I mean, it's only the size of a boxing ring, not a football field. I think LEDs could produce enough light to have a net effect.
- bmdavi3 7y ago
  I don't know if the strength of an LED is enough to move the craft significantly, but if it is you'd need to point it at anything _but_ the ship itself, otherwise the two forces would cancel out. Just shine it in the opposite direction of where you want to go.
  - umvi 7y ago
    No, the two forces don't cancel out, I can't remember exactly why but elastic collisions don't work like that. That's why you can point a fan into your sailboat's sail and it will still go forward, mythbusters proved it.
    - mkl 7y ago
      Mythbusters demonstrated something different, and something much less efficient than simply pointing the fan backwards like a normal fan boat (no sail required). Here: https://mythresults.com/blow-your-own-sail https://mythresults.com/blow-your-own-sail
- Skunkleton 7y ago
  No. Think of the photons as producing little pushes. When they are emitted by the LED, they produce a little push in one direction, and then the sail pushes a little bit back in the other direction. If it were possible to create a significant volume of thrust from photons emitted by LEDs, then you would be better off just pointing the LEDs away from where you wanted to go (like a rocket).

omarhaneef 7y ago

That would be like taking fan and blowing it into your own sails. It wouldn't work.

umvi 7y ago
It does work though, depending on the material your sail is made of. Try it: attach an electric fan to a pinewood derby car and point it so it is blowing into a tin foil sail also attached to the pine wood derby car. The car will move forward.
- omarhaneef 7y ago
  I thought you want the LED mounted on the spacecraft itself. That would be the equivalent of the fan being on the sailboat. Yes, if you had another power boat next to the sailboat you could blow the boat, but it would be transferring some energy from the engine of the powerboat. So if you had another spacecraft near the space sail, and that spacecraft was shooting light, it would have some miniscule effect, but I didn't think that is what you meant. Edit: I suspect you're kidding but I can't be sure.
  - umvi 7y ago
    That's what I'm talking about. A handheld fan[1] is on the pinewood derby car, same as the sail. I built one before so I know it works. [1] https://www.amazon.com/Aluan-Handheld-Portable-Adjustable-Rechargeable/dp/B07QXHT67B/ https://www.amazon.com/Aluan-Handheld-Portable-Adjustable-Re...
    - omarhaneef 7y ago
      Okay, I am not more sure you are kidding but to be fair to me, I had no context for your joking ways before.
- deleted 7y ago
  [deleted]
- Skunkleton 7y ago

comboy 7y ago

This kind of technology had been demonstrated many years ago and many people around the world had been watching it at the time[1]. 1. https://i.imgur.com/VL0Q9hT.gif https://i.imgur.com/VL0Q9hT.gif

blacksmith_tb 7y ago

Not a sailor (solar or otherwise) but my instinct is you could turn by "furling" parts of the sail (say, if there were vanes that could be turned perpendicularly to the flow of photons). But lots of designs seem to call for propulsion from a laser, so maybe that could just be angled away slightly. At astronomic distances that might be tough, though.

hinkley 7y ago
Supposedly, if you are orbiting the sun, you can turn a solar sail in such a way that you can brake against the solar wind, thereby moving in-system rather than out. But the sailing analogy breaks down for turning. A boat is pushing against the water and the wind. That gives it the ability to do things that you can’t in a vacuum, like move faster than the wind. For ‘turning’ you have only thrust vectoring, right?
- db48x 7y ago
  There are several control methods available to solar sails that can work. They were studied quite extensively by NASA during the 60s. The thrust generated by a solar sail is proportional to the angle between it and the sun, as is the direction of that thrust. If any part of your sail can twist, then you have steering vanes that are quite useful. You can twist the vanes to angles where the vane on one side of the craft is reflecting sunlight while the one on the other side is not in order to get a rotational acceleration. If your sail is carrying a payload, then that payload will likely be in the center of the craft, with the sail spread out around it symmetrically. If you can shift the payload side to side relative to the sail, then that will shift the center of mass of the whole craft. With more of the sail to one side of the center of mass you will again get a rotation. If you want more information, find a copy of the book Space Sailing by Jerome Wright; it's quite good.

walrus01 7y ago

https://en.m.wikipedia.org/wiki/Photon_rocket https://en.m.wikipedia.org/wiki/Photon_rocket It's a real idea, but you would need a truly ridiculous amount of electricity available on a spacecraft. Only possible by a huge nuclear reactor. Doesn't violate any laws of physics.

mkl 7y ago
There are key differences with that: either the photons are shot backwards (like a rocket), or the source of the photons is not on the ship itself. ("The limitations posed by the rocket equation can be overcome, as long as the reaction mass is not carried by the spacecraft.")

tfolbrecht 7y ago

I'm curious if you could selectively turn off the reflective effect in different regions of the material and steer it that way.

andygates 7y ago
IKAROS used this technique. The bird was spin-stabilised and a section of the sail had LCD shades that could be dark or light. It could darken its, say, venus-side segments as they went round. Gloriously over-engineered.

JshWright 7y ago

Changing orientation is easily done using spinning masses (this is how most satelites maintain their orientation)