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.
> It works by pushing against the surrounding medium.
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".
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.
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.
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.
