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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

#81

Post by yarg »

mabbo wrote: post #69

My understanding of what's going on here is that the light sail isn't aiming or changing at all. It's just a big sail, in orbit. When it's moving away from the sun, the sail is being pushed "forward", which adds to it's velocity- this increases the apogee on the opposite side of the orbit. When it's moving towards the sun, the sail is being pushed "backwards", lowering it's current velocity and reducing the perigee.…

I don't think you'd even need to retract the sail - just structure it differently.

There's an omni-directional wind turbine (https://newatlas.com/2018-dyson-award-winner-o-wind-turbine/...) which sort of encapsulates what I'm thinking of.

I'm not too sure about how easy it would be to use a non-planar sail to redirect photons in the direction required to constantly build forward momentum - but a photonic "wind" turbine used to power a quantum dot emitter array might be able to do something.

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#82

Post by yarg »

c3534l wrote: post #76
mabbo wrote: post #69

My understanding of what's going on here is that the light sail isn't aiming or changing at all. It's just a big sail, in orbit. When it's moving away from the sun, the sail is being pushed "forward", which adds to it's velocity- this increases the apogee on the opposite side of the orbit. When it's moving towards the sun, the sail is being pushed "backwards", lowering it's current velocity and reducing the perigee.…

Gravity is exponential, which makes lifting fuel off of Earth a losing strategy for long-distance travel. Anything that is fueled by something already in space seems incredibly promising.

Newton's approximation of gravitational force is: G = gMm/d^2

The variable here is d (the distance between the centres of mass) - gravity is inverse quadratic (https://en.wikipedia.org/wiki/Inverse-square_law), not exponential.

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#83

Post by parsimo2010 »

mabbo wrote: post #69

My understanding of what's going on here is that the light sail isn't aiming or changing at all. It's just a big sail, in orbit. When it's moving away from the sun, the sail is being pushed "forward", which adds to it's velocity- this increases the apogee on the opposite side of the orbit. When it's moving towards the sun, the sail is being pushed "backwards", lowering it's current velocity and reducing the perigee.…

I didn’t work on Lightsail, but the article implies that it does aim the sail. It isn’t expanding/contracting, but it does change orientation using the reaction wheels. Basically, they turn the sail broadside to catch the sunlight to raise the orbit, and turn it on edge when they don’t want the extra drag.

This particular satellite won’t ever leave orbit because the sail can’t overcome even the tiny amount of atmospheric drag. If you want to achieve escape velocity with a sail you’ll need a bigger sail and a higher orbit to start out. Right now it’s only feasible for small satellites and we’ve been able to achieve escape velocity using rockets for decades.

Eventually, I do envision a competition to see who can get a payload to Mars with the least amount of fuel. Kind of like the fuel efficiency challenges that college engineering departments participate in today.

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#84

Post by Cogito »

mabbo wrote: post #68
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?

Why would it explode? More likely, the small rock would put a hole into the spaceship. This isn't a disaster. The spaceship residents would notice a gradually decreasing amount of pressure, assess where the leak is, and put a piece of duct tape over the hole. Remember kids, real life isn't like the movies.

The kinetic energies involved can be huge.

In the same way that an object won't explode when hit by a subsonic 9mm round with ~0.5kJ of muzzle energy ('just' causing a leak and decreasing amount of blood pressure) it will when hit by a 0.50 calibre round with 14-20kJ of muzzle energy.

For two objects in low earth orbit, travelling in opposite directions, the speed of impact can be over 14km/s - or 98000kJ per kg. At that speed even a relatively small object is going to cause a massive explosion when it hits you.

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#85

Post by polyfractal »

mabbo wrote: post #69

My understanding of what's going on here is that the light sail isn't aiming or changing at all. It's just a big sail, in orbit. When it's moving away from the sun, the sail is being pushed "forward", which adds to it's velocity- this increases the apogee on the opposite side of the orbit. When it's moving towards the sun, the sail is being pushed "backwards", lowering it's current velocity and reducing the perigee.…

A minor note: the LightSail team says the perigee is lowering due to atmospheric drag not due to the sun slowing it down:

> The perigee, or low point of its orbit, has dropped by a similar amount, which is consistent with pre-flight expectations for the effects of atmospheric drag on the spacecraft.

That's not to say the solar pressure isn't potentially slowing it down too, but it sounds like the main driver is drag in this case.

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#86

Post by jimktrains2 »

umvi wrote: post #37
jimktrains2 wrote: post #23

Earlier quoted context omitted.

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

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.

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#87

Post by natosaichek »

umvi wrote: post #30
zaphoyd wrote: post #21

Earlier quoted context omitted.

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.

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.

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#88

Post by Cogito »

moron4hire wrote: post #44
cgh wrote: post #6

Earlier quoted context omitted.

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 im…

> Sailing is not air pushing on a sail, it's air rebounding away from the sail.

To add just a tiny amount of nuance - sailing is redirecting airflow with a sail.

Surface and flow effects mean that very little air should be bouncing off the sail. If there is you probably have a lot of turbulence, and a 'bubble' will form around the sail around which most of the wind will simply divert, rather than redirect.

Or to put it another way, think of an entire parcel of air rebounding away from the sail, not individual air molecules.

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#89

Post by b_tterc_p »

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…

Is this true about tiny pebbles as well as asteroids? I would think a tiny rock would cause immense damage at high speeds.

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#90

Post by sdenton4 »

jessriedel wrote: post #75
sdenton4 wrote: post #48

Earlier quoted context omitted.

(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…

Unlike Lightsail 1, Lightsail 2 operates at an altitude where the atmosphere is negligible.

Hmm.... But the site says this:

"In the past 4 days, the spacecraft has raised its orbital high point, or apogee, by about 2 kilometers. The perigee, or low point of its orbit, has dropped by a similar amount, which is consistent with pre-flight expectations for the effects of atmospheric drag on the spacecraft."

They also say this on the mission page:

"LightSail 2's attitude control system does not have the precision to maintain a circular orbit and continuously fly the spacecraft higher. Therefore, as one side of LightSail 2’s orbit rises, the other side will dip lower, until atmospheric drag overcomes the forces of solar sailing, ending the primary mission. The spacecraft will remain in orbit roughly a year before entering the atmosphere and burning up."

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