This is of course overlooking the whole "pull oneself up by one's bootstraps" vibe.
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.
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.
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.
Not really. If a fan was able to blow air in a vacuum it would produce thrust as well. Basically a rocket does that. It's just blowing gasses out the back.
It's Newton's law of motion, "every action has an equal and opposite reaction".
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.
It should push the fan and the sail apart, which could result in a structural failure.
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.
Also, why not just use the fan or laser or led as the means of propulsion directly?
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.
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 didn't think that is what you meant.
Edit: I suspect you're kidding but I can't be sure.
[1] https://www.amazon.com/Aluan-Handheld-Portable-Adjustable-Re...
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 :)
Using atmospheric oxygen for the early phase of flight is a massive weight savings. If/when SABRE works, the Skylon spaceplane could be a practical reusable SSTO. It could carry a very respectable payload mass (the aim is 11-17 metric tonnes) to LEO, and it could do so quite cheaply if the designers' claims pan out.
Fun fact for Kerbal fans: the RAPIER engine in the game is inspired by SABRE.
Either way, an LED is not generating any meaningful thrust, and pointing that thrust at a sail adds no benefit.
A propeller makes thrust like a paddle, by pushing off of something. A jet/rocket engine makes thrust by recoil, using conservation of momentum.
But even if they did not have fans, there is no fundamental difference between using a propeller or fan to speed up the flow of air and heating the air inside a chamber to speed up the flow of air. (The fan is used because at subsonic speeds it can be made more efficient.)
In other words, all 3 means of propulsion work by conservation of momentum.
There is a whole separate device called an EM Drive, which claims to basically violate conservation of momentum. It is highly controversial, and unlikely to be anything other than a measurement mistake.
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.
Which, to bring it back to the origin of this thread of armchair physicists incorrecting each other, means that an LED mounted on a solar sail would not work, because the only force it imparts is collision. Which makes the sail analogy kind of a bad one for the use of solar pressure as a driving force.
Edit: Hmmm, but what if the surface material of the sail were phosphorescent and could gain energy state from the solar pressure, then kick it a photon when dropping energy state? And then what if there were a fly wheel (magnetic axel, in the vacuum of space, extremely low friction) that could act as the keel... Hmmmmmmm
Edit 2: I wonder if a lens that refracted light similarly to how air flows over an airfoil would create a "solar lift" effect? Unfortunately, lenses are heavy.
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).
It's a real idea, but you would need a truly ridiculous amount of electricity available on a spacecraft. Only possible by a huge nuclear reactor. Doesn't violate any laws of physics.
Gloriously over-engineered.
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?
The thrust generated by a solar sail is proportional to the angle between it and the sun, as is the direction of that thrust. If any part of your sail can twist, then you have steering vanes that are quite useful. You can twist the vanes to angles where the vane on one side of the craft is reflecting sunlight while the one on the other side is not in order to get a rotational acceleration.
If your sail is carrying a payload, then that payload will likely be in the center of the craft, with the sail spread out around it symmetrically. If you can shift the payload side to side relative to the sail, then that will shift the center of mass of the whole craft. With more of the sail to one side of the center of mass you will again get a rotation.
If you want more information, find a copy of the book Space Sailing by Jerome Wright; it's quite good.
