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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
  It only works because the fan pushes against something not directly connected to the car. This is the same reason you can't use a jet engine in space.
  - umvi 7y ago
    So you are saying if I put the car in a vacuum chamber and mounted a CO2 cartridge instead, it wouldn't work?
    - Skunkleton 7y ago
      Of course it would, but what wouldn't work is bouncing that CO2 off of some sail.
      - f5thesystem 7y ago
        I would think a sail, or rudder, could redirect the force from the CO2 canister.
        Skunkleton 7y ago
        It could, but it would be significantly more efficient to just point the CO2 canister in a different direction.
    - Skunkleton 7y ago
      The basic physics at work here is conservation of momentum. Rockets work because they are sending mass in the opposite direction of the main vessel. If you draw a box around the whole system (rocket + spent fuel), you have net zero momentum. There is a whole separate device called an EM Drive, which claims to basically violate conservation of momentum. It is highly controversial, and unlikely to be anything other than a measurement mistake.
  - RonaldRaygun 7y ago
    This is actually a common misconception about how jet engines work. The engine is not pushing against the surrounding medium (the air), it is operating purely due to the conservation of momentum of throwing combustion byproducts at a high velocity in the opposite direction. This is the same principle upon which a rocket engine operates, the difference being that the jet only needs to bring along fuel for the combustion reaction, thereby massively increasing the efficiency of the engine by making use of ambient atmospheric oxygen. In fact, a jet engine could in theory (ignoring things like heat rejection) operate just fine in space if you brought along a massive bag of air and hooked it up to the intake duct. But then you've basically just designed the world's worst rocket engine :)
- zaphoyd 7y ago
  An electric fan on a derby car works the same way a propeller does though right? You don’t need the sail at all. It works by pushing against the surrounding medium. This seems different than a laser in a vacuum.
  - umvi 7y ago
    I'm saying the electric fan is pointed so that it is blowing air in the forward direction. A sail at the front of the car will catch said air and propel the car forward. I'm sure it would work in a vacuum too if you had a CO2 canister mounted to the car blowing forward into the sail.
    - deleted 7y ago
      [deleted]
    - natosaichek 7y ago
      No it will not. If you attach a fan to a derby car and point it backwards, the car will move forwards. If you redirect the fan so it's pointing forward, the car will move backwards. If you put a sail in the way, the car will move backwards less efficiently. If you make a perfect sail that blocks all the air from the fan, it will not move at all.
      - AmericanChopper 7y ago
        > If you make a perfect sail that blocks all the air from the fan, it will not move at all. It should push the fan and the sail apart, which could result in a structural failure.
        tsukikage 7y ago
        Until it does, though, the air from the fan will bounce off the sail and go (mostly) backwards, propelling the craft forwards (for the same reason, albeit much less efficiently, as if you had no sail and just pointed the fan backwards. You've basically made a thrust vectored device, badly).
- jimktrains2 7y ago
  Perhaps I'm being a dunce right now, but doesn't the fan exert a force on the car opposite the direction of flow, and wouldn't that force necessarily be greater than the force of capturing the same flow in a sail? Also, why not just use the fan or laser or led as the means of propulsion directly?
  - umvi 7y ago
    No, because of conservation of momentum. Same reason a jet engine can move a piece of metal to reverse its thrust
    - jimktrains2 7y ago
      Sure, the air hitting the sail would impart a force on the craft, but there is also a force from the fan? I guess I think of reversers as redirecting the jet blast in a controlled manner, I guess I don't think of the sail as redirecting the wind in a controlled manner.
- blacksmith_tb 7y ago
  You'd need to have some very smooth bearings to make that even possible (given that the fan would have to weigh about as much as the car), but the physics seem off, the fan pushes air away from it, the air pushes the fan backwards...
  - umvi 7y ago
    The sail acts as a reverse thruster, transferring the momentum from the air molecules to the craft. Same physics behind jet engine reverse thrusters - it just redirects the air in a different direction.

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
It only works because the fan pushes against something not directly connected to the car. This is the same reason you can't use a jet engine in space.
- umvi 7y ago
  So you are saying if I put the car in a vacuum chamber and mounted a CO2 cartridge instead, it wouldn't work?
  - Skunkleton 7y ago
    Of course it would, but what wouldn't work is bouncing that CO2 off of some sail.
    - f5thesystem 7y ago
      I would think a sail, or rudder, could redirect the force from the CO2 canister.
      - Skunkleton 7y ago
        It could, but it would be significantly more efficient to just point the CO2 canister in a different direction.
  - Skunkleton 7y ago
    The basic physics at work here is conservation of momentum. Rockets work because they are sending mass in the opposite direction of the main vessel. If you draw a box around the whole system (rocket + spent fuel), you have net zero momentum. There is a whole separate device called an EM Drive, which claims to basically violate conservation of momentum. It is highly controversial, and unlikely to be anything other than a measurement mistake.
- RonaldRaygun 7y ago
  This is actually a common misconception about how jet engines work. The engine is not pushing against the surrounding medium (the air), it is operating purely due to the conservation of momentum of throwing combustion byproducts at a high velocity in the opposite direction. This is the same principle upon which a rocket engine operates, the difference being that the jet only needs to bring along fuel for the combustion reaction, thereby massively increasing the efficiency of the engine by making use of ambient atmospheric oxygen. In fact, a jet engine could in theory (ignoring things like heat rejection) operate just fine in space if you brought along a massive bag of air and hooked it up to the intake duct. But then you've basically just designed the world's worst rocket engine :)
zaphoyd 7y ago
An electric fan on a derby car works the same way a propeller does though right? You don’t need the sail at all. It works by pushing against the surrounding medium. This seems different than a laser in a vacuum.
- umvi 7y ago
  I'm saying the electric fan is pointed so that it is blowing air in the forward direction. A sail at the front of the car will catch said air and propel the car forward. I'm sure it would work in a vacuum too if you had a CO2 canister mounted to the car blowing forward into the sail.
  - deleted 7y ago
    [deleted]
  - natosaichek 7y ago
    No it will not. If you attach a fan to a derby car and point it backwards, the car will move forwards. If you redirect the fan so it's pointing forward, the car will move backwards. If you put a sail in the way, the car will move backwards less efficiently. If you make a perfect sail that blocks all the air from the fan, it will not move at all.
    - AmericanChopper 7y ago
      > If you make a perfect sail that blocks all the air from the fan, it will not move at all. It should push the fan and the sail apart, which could result in a structural failure.
      - tsukikage 7y ago
        Until it does, though, the air from the fan will bounce off the sail and go (mostly) backwards, propelling the craft forwards (for the same reason, albeit much less efficiently, as if you had no sail and just pointed the fan backwards. You've basically made a thrust vectored device, badly).
jimktrains2 7y ago
Perhaps I'm being a dunce right now, but doesn't the fan exert a force on the car opposite the direction of flow, and wouldn't that force necessarily be greater than the force of capturing the same flow in a sail? Also, why not just use the fan or laser or led as the means of propulsion directly?
- umvi 7y ago
  No, because of conservation of momentum. Same reason a jet engine can move a piece of metal to reverse its thrust
  - jimktrains2 7y ago
    Sure, the air hitting the sail would impart a force on the craft, but there is also a force from the fan? I guess I think of reversers as redirecting the jet blast in a controlled manner, I guess I don't think of the sail as redirecting the wind in a controlled manner.
blacksmith_tb 7y ago
You'd need to have some very smooth bearings to make that even possible (given that the fan would have to weigh about as much as the car), but the physics seem off, the fan pushes air away from it, the air pushes the fan backwards...
- umvi 7y ago
  The sail acts as a reverse thruster, transferring the momentum from the air molecules to the craft. Same physics behind jet engine reverse thrusters - it just redirects the air in a different direction.