LightSail 2 Spacecraft Successfully Demonstrates Flight by Light • Hacker News Board

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#21

Post by zaphoyd » Wed, Jul 31, 2019, 10:20 PM UTC

umvi wrote: post #10

omarhaneef wrote: post #7

Earlier quoted context omitted.

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

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.

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.

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#22

Post by umvi » Wed, Jul 31, 2019, 10:20 PM UTC

omarhaneef wrote: post #15

umvi wrote: post #10

Earlier quoted context omitted.

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.

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 did…

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-Re...

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#23

Post by jimktrains2 » Wed, Jul 31, 2019, 10:21 PM UTC

umvi wrote: post #10

omarhaneef wrote: post #7

Earlier quoted context omitted.

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

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.

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?

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#24

Post by Skunkleton » Wed, Jul 31, 2019, 10:21 PM UTC

umvi wrote: post #8

cgh wrote: post #6

Earlier quoted context omitted.

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

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.

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).

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#25

Post by omarhaneef » Wed, Jul 31, 2019, 10:23 PM UTC

umvi wrote: post #22

omarhaneef wrote: post #15

Earlier quoted context omitted.

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 did…

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-Re...

Okay, I am not more sure you are kidding but to be fair to me, I had no context for your joking ways before.

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#26

Post by umvi » Wed, Jul 31, 2019, 10:23 PM UTC

bmdavi3 wrote: post #18

umvi wrote: post #8

Earlier quoted context omitted.

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.

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.

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.

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#27

Post by adrianpike » Wed, Jul 31, 2019, 10:24 PM UTC

Can someone more familiar with orbital mechanics help me understand why it looks like the perigree is coming down in opposite to the apogee?

I'm but a mere KSP player, but I've always thought the perigee comes down in a fairly linear fashion based on drag, which is mostly a function of altitude - I would assume they'd be using the sail to just "burn prograde" whenever the solar angle is possible, raising both the apogee and the perigee, but the apogee coming up way faster?

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#28

Post by comboy » Wed, Jul 31, 2019, 10:24 PM UTC

umvi wrote: post #14

henryfjordan wrote: post #3

Earlier quoted context omitted.

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

Net effect is not zero if the photon bounces off of the sail

Why use sail then? You can just shoot your photon the opposite way.

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#29

Post by gitgud » Wed, Jul 31, 2019, 10:26 PM UTC

umvi wrote: post #14

henryfjordan wrote: post #3

Earlier quoted context omitted.

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

Net effect is not zero if the photon bounces off of the sail

If it bounces off in the opposite direction then the net effect would be actually be negative.

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#30

Post by umvi » Wed, Jul 31, 2019, 10:26 PM UTC

zaphoyd wrote: post #21

umvi wrote: post #10

Earlier quoted context omitted.

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.

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.

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.