Diametric Drives...limits and how they work

[HvP]

As the others here have made clear, this proposal is just another "free energy device."

In other words, a perpetual motion machine.

There is a reason they have never worked.

They disobey the laws of physics.

Edited May 7, 2021 by HvP

[SpaceFace545]

37 minutes ago, HvP said:

As the others here have made clear, this proposal is just another "free energy device."

In other words, a perpetual motion machine.

There is a reason they have never worked.

They disobey the laws of physics.

Even an EM drive isn’t a perpetual motion machine. 

[wumpus]

4 hours ago, mikegarrison said:

This is actually a really popular idea in science fiction. One of the FTL tropes that gets used a lot is the one where a ship generates a black hole at its nose and then "falls into it". Of course, this is the same thing as the magnet car up above, except that it uses gravity instead of magnetism.

The difference is that once you pass the Schwarzschild radius in a black hole, there  are no known laws of physics to break.  I'd have to agree with you that whatever laws are governed by such areas are unlikely to allow the magnetic car to work.

2 hours ago, SpaceFace545 said:

Even an EM drive isn’t a perpetual motion machine. 

It violates the law of conservation of momentum, instead of the law of conservation of energy.  Slightly more plausible (especially knowing that mass isn't necessarily conserved), but still extremely unlikely.

I like to point out when the EM drive comes up that an LED is perfectly capable of taking electricity in and producing a momentum out, much like the goal of the EM.  It just doesn't produce nearly as much momentum as they want.

[HvP]

2 hours ago, wumpus said:

an LED is perfectly capable of taking electricity in and producing a momentum out

As photons do have momentum despite having no mass this works in an open system. It's essentially the reverse of a solar sail where solar photons impinge on the surface of the sail and transfer their momentum to it. You could theoretically power an LED sail to eject photons behind you instead. It's just that whatever you use to generate the electricity will either run out eventually or be too heavy to use effectively. And using solar panels to gather the light to make electricity to power the LEDs is less efficient than just reflecting the sunlight in the first place.

But it's assumed that when attempted in a closed cavity, such as the EM drive proposes, the net forces from ejection to reflection to absorption would simply cancel out leaving no overall thrust.

Edited May 7, 2021 by HvP

[sh1pman]

Your spaceship will perpetually accelerate, turn into a black hole due to its kinetic energy, eventually growing up to the size of the entire universe and consuming all matter.

That’s  why perpetual motion spaceships that break Newton’s Third Law are banned by the Galactic Code. Don’t even try building such a thing or they’ll send space marines after you.

[magnemoe]

11 hours ago, SpaceFace545 said:

more like a slow space airplane but yeah

Buzard ramjet more like, its works just like this collection hydrogen for reaction mass. 
Gigantic, very hard to build, much harder if you try to use the hydrogen for fusion while it passing trough. 
But breaks no physical laws as it it, one problem is that you will be braking against the hydrogen trying to redirect it.

Using the hydrogen to brake is much easier and probably works pretty well if you move very fast. No reaction mass is needed. 

[SpaceFace545]

9 hours ago, wumpus said:

I like to point out when the EM drive comes up that an LED is perfectly capable of taking electricity in and producing a momentum out, much like the goal of the EM.  It just doesn't produce nearly as much momentum as they want.

Pretty much just a photon rocket, sans the giant laser.