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LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

planetary.org

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Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#31

Post by umvi »

Skunkleton wrote: post #17
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.

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.

So you are saying if I put the car in a vacuum chamber and mounted a CO2 cartridge instead, it wouldn't work?

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#32

Post by bsmith0 »

simplicio wrote: post #19
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.

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.

So just use a directional light source pointing away from the craft...

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#33

Post by blacksmith_tb »

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.

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...

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#34

Post by TeMPOraL »

adrianpike wrote: post #27

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 apo…

My "mere KSP player" guess (without having access to my desktop on which I'd check that, in KSP) is that the angle at which they're reflecting the Sun amounts to burning prograde and radial-in (i.e. forward and down), which when done near perigee should both rise the apogee (the prograde component) and lower the perigee (the radial-in component).

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#35

Post by hinkley »

adrianpike wrote: post #27

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 apo…

Another armchair dweller here, but you can use low thrust to make an orbit more elliptical or more circular. With the earth in the way part of the day, making it more circular is probably not in the cards.

I wonder if there is more solar wind at apogee, too...

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#37

Post by umvi »

jimktrains2 wrote: post #23
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.

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?

No, because of conservation of momentum. Same reason a jet engine can move a piece of metal to reverse its thrust

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#40

Post by umvi »

blacksmith_tb wrote: post #33
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.

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...

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.
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