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LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

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Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#41

Post by hinkley »

blacksmith_tb wrote: post #12
umvi wrote: post #2

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.

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.

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?

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#42

Post by RonaldRaygun »

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.

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 :)

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#43

Post by adrianpike »

TeMPOraL wrote: post #34
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).

Aha, that does make sense. Thanks for helping me wrap my mind around it. :)

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#44

Post by moron4hire »

cgh wrote: post #6
umvi wrote: post #2

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.

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

Which can be done, because the sail is not just being pushed by the air, it's also redirecting the flow of air. The momentum of the fan pulling the air in does cancel any momentum imparted to the sail as the air pushes it, but there is also momentum gained by the air molecules rebounding back, away from the sail. It's (partially) the same reason you can sail perpendicular to the wind (the shape of the keel is also important), or sail faster than wind. Sailing is not air pushing on a sail, it's air rebounding away from the sail.

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#45

Post by seiferteric »

comboy wrote: post #28
umvi wrote: post #14

Earlier quoted context omitted.

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.

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.

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#46

Post by alpha_squared »

b_tterc_p wrote: post #39

Not really relevant but, if we we had an arbitrarily super fast spacecraft, how would we prevent it from hitting a small rock and exploding?

Ultimately, you can't. Space is really, really, really big and empty, though. Unimaginably so, which would make an incident be some really bad luck. In something like the lightsails, you'd just launch multiple simultaneously.

Space is so big and empty that even for something like the asteroid belt you could pass through any point randomly and on average be thousands of kilometers from the nearest rock. Perhaps even tens of thousands.

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#48

Post by sdenton4 »

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 ksp'er here...) I think the problem is atmospheric drag, which increases when the sail is open. If you're looking to increase apogee, the most efficient place to do so is at the perigee, which also happens to be where the atmospheric drag is the worst.

So setting the mission, you get to choose the orientation of the sun relative to your orbital ellipse. If the sun direction to the sun is perpendicular to the long axis, then you (presumably) want your sail wide at the perigee, where drag will also be greatest, but will also provide the most efficient raising of the apogee.

OTOH, if the sun's direction is parallel to the long axis, you can try to avoid drag at the perigee, and then rotate into position to get more thrust during the journey from perigee to apogee. But this is a less efficient way to raise the orbit, and will deform (circularize?) the orbit. I might have to fire up KSP with 'cubesat' with an ion thruster to simulate this...

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#49

Post by walrus01 »

redorb wrote: post #13
moopling wrote: post #5

Earlier quoted context omitted.

Exactly, it is much like using a fan on a boat to blow a sail.

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

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.

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#50

Post by ceejayoz »

alpha_squared wrote: post #46
b_tterc_p wrote: post #39

Not really relevant but, if we we had an arbitrarily super fast spacecraft, how would we prevent it from hitting a small rock and exploding?

Ultimately, you can't. Space is really, really, really big and empty, though. Unimaginably so, which would make an incident be some really bad luck. In something like the lightsails, you'd just launch multiple simultaneously. Space is so big and empty that even for something like the asteroid belt you could pass through any point randomly and on average be thousands of kilometers from the nearest rock. Perhaps even t…

High hundreds of thousands, if not a million or two, actually.

https://physics.stackexchange.com/questions/26712/what-is-th...

Space is BIG.

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