Can you "tack" a solar sail?
Niven/Pournelle had a cool idea in The Mote in God's Eye of charging your craft when traversing a magnetic field to change direction (not specific to sails, but another passive technique).
LightSail 2 Spacecraft Successfully Demonstrates Flight by Light
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Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light
#122Can you "tack" a solar sail? Niven/Pournelle had a cool idea in The Mote in God's Eye of charging your craft when traversing a magnetic field to change direction (not specific to sails, but another passive technique).
Using Earth's magnetic field for attitude control is already a thing [1].
Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light
#123Can you "tack" a solar sail? Niven/Pournelle had a cool idea in The Mote in God's Eye of charging your craft when traversing a magnetic field to change direction (not specific to sails, but another passive technique).
Using Earth's magnetic field for attitude control is already a thing [1]. [1] https://en.wikipedia.org/wiki/Magnetorquer
In fact, that’s exactly what LightSail is using to control its attitude: http://www.planetary.org/blogs/jason-davis/2016/20160418-lig...
Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light
#124Not 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.
Actually that's exactly sounds like a movie series Expanse. Space combat scene. They even depressurize the ship in anticipation of getting holes in it.
Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light
#125My 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.…
Solar sails are intended for interplanetary travel in heliocentric orbit. Providing a continuous radial low thrust, resulting in an expanding spiral trajectory. In this application, the wind is always favorable.
This is a proof of concept test vehicle in low earth orbit, it isn't going anywhere.
Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light
#126Earlier quoted context omitted.
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.
> 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.
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).
Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light
#127We need more of this, and more spending and results in space research. Not only because we are destroying our own planet, but also because we as a species have been stagnating for quite some time.
Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light
#128Can 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…
I'd guess the sail orientation is not changing relative to the sun, so at perigee this provides an acceleration (because it's mostly in the velocity direction), which raises apogee. At apogee it provides deceleration (because it's in the anti-velocity direction), which lowers perigee. There are also probably other effects depending on the sail orientation that muck with the inclination and other elements. It's a litt…
Since the relative position to the sun changes pretty slowly, I imagine they would need to wait half a year to raise the current periapsis. I doubt this is a sail that allows to beat to "wind"-ward.
Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light
#129Earlier quoted context omitted.
> 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.
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).
A sail is used to create drag. If the air from the fan hit the sail and dispersed perfectly perpendicular to it, then it wouldn’t move the craft at all.
The more realistic outcome would be that the sail did not disperse the air with perfect efficiency, and the craft would just move backwards.
What you’re describing would require redirecting the flow of air backwards, like a curved exhaust. Which is really just a less efficient version of turning the fan around.
Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light
#130Earlier quoted context omitted.
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 combusti…
This is just a complicated way of saying the same thing. Of course a jet engine is not just a really fast propeller. And if you hooked a "bag of air" to a propeller, that would also work in space. Either way, an LED is not generating any meaningful thrust, and pointing that thrust at a sail adds no benefit.
No, it's not.
A propeller makes thrust like a paddle, by pushing off of something. A jet/rocket engine makes thrust by recoil, using conservation of momentum.