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Comment by omarhaneef | Hacker News Reader

That would be like taking fan and blowing it into your own sails. It wouldn't work.

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.

I thought you want the LED mounted on the spacecraft itself.

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.

That's what I'm talking about. A handheld fan[1] is on the pinewood derby car, same as the sail. I built one before so I know it works.

[1] https://www.amazon.com/Aluan-Handheld-Portable-Adjustable-Re...

Okay, I am not more sure you are kidding but to be fair to me, I had no context for your joking ways before.

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.

So you are saying if I put the car in a vacuum chamber and mounted a CO2 cartridge instead, it wouldn't work?

Of course it would, but what wouldn't work is bouncing that CO2 off of some sail.

I would think a sail, or rudder, could redirect the force from the CO2 canister.

It could, but it would be significantly more efficient to just point the CO2 canister in a different direction.

The basic physics at work here is conservation of momentum. Rockets work because they are sending mass in the opposite direction of the main vessel. If you draw a box around the whole system (rocket + spent fuel), you have net zero 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.

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 :)

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.

Almost all modern jet engines have (ducted) fans. In planes that never go supersonic, e.g., all current commercial airliners, the fan provides most, but not all, the propulsion. And the only difference between a fan an a propeller is how many blades it has. (The presence of a duct around the fan does not change anything basic.)

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.

Well put. For anyone interested in the similarities of jet engines and rocket engines, read up on the SABRE engine that the UK company Reaction Engines is working on for the proposed Skylon spaceplane. SABRE is a hybrid that's designed to work like a jet engine straight from its horizontal take-off up to about Mach 5.5+ and 25Km in altitude. Higher/faster than that, it switches from using atmospheric oxygen to oxygen stored in its fuel tanks and works like a rocket engine. (Of course the technical details are far more complex than that, but that's the TL;DR.)

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.

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.

> 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.

> 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".

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.

You'd need to have some very smooth bearings to make that even possible (given that the fan would have to weigh about as much as the car), but the physics seem off, the fan pushes air away from it, the air pushes the fan backwards...

The sail acts as a reverse thruster, transferring the momentum from the air molecules to the craft. Same physics behind jet engine reverse thrusters - it just redirects the air in a different direction.

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