Helical Engine- RL Kraken drive or the next EM Drive?

[DAL59]

I'm surprised there isn't a thread for this already, because this is interesting to say the least.

2 weeks ago this paper was published (https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20190029294.pdf) that proposes a helical particle accelerator that would use mass changes to accelerate a craft without propellant¹.  The engine would bounce ions back and forth, but accelerate them to relativistic speeds at one end of the device, increasing their mass and creating an inequality that results in a net movement of the entire device.  This does not seem to violate conservation of energy, as it requires hundreds of megawatts of power.  However, it appears that only one person has worked on this paper, and no experimental evidence exists.  

¹ It technically uses 1.776E-11 kg of ions per year, plus the nuclear fuel needed to power the ship.

Some basic calculations: If this ship had a mass of 2,000 tons, after 1 year it would have a velocity of 17 meters per second.  So, the usefulness of this is very questionable.  It would take 16,000 years to reach even alpha centauri.

[snip]

Edited October 25, 2019 by Starhawk
Enormous text block removed by moderator

[Elthy]

https://arstechnica.com/science/2019/10/corkscrewing-bouncy-ion-drive-would-provide-thrust-in-different-universe/

Its allways the same: https://xkcd.com/2217/

[sevenperforce]

4 minutes ago, Elthy said:

https://arstechnica.com/science/2019/10/corkscrewing-bouncy-ion-drive-would-provide-thrust-in-different-universe/

Its allways the same: https://xkcd.com/2217/

Exactly. From the link:

So, let's just state up front: this drive won't work. The problem is that, even though the author does a very nice simulation, he has left out the fields that do the accelerating. When we accelerate ions using a magnetic or electric field, the ions push back on the field. There is an equal and opposite force exerted on the electrodes and coils that produce the fields, and those just happen to be in the spaceship, too.

In the first step, where we accelerate the mass to a high relativistic speed, we also accelerate the cylinder in the opposite direction. Now, in special relativity, we don't conserve energy and momentum separately. Instead, they are conserved together. If you only consider momentum (and not energy), then you will find net forces everywhere due to inertial mass changes—things get heavier as they approach the speed of light. This is exactly what the author has found. If you consider energy and momentum simultaneously, those forces will suddenly disappear.

This is where the increase in inertial mass comes from in the first place: energy is sucked out of the field and turned into mass. When the particles are slowed, that mass is given up as photons in the field, which slow the cylinder as they are absorbed. What is the net force? Zero, 0N of force.

[Guest]

2 hours ago, Elthy said:

Its allways the same: https://xkcd.com/2217/

For that particular example, I'm reminded of the last panel of this one: https://xkcd.com/804/  

Even if you got the math right, do make sure that you got the right math, too.