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'Helical Engine' Concept Could Reach 99% the Speed of Light (nasa.gov)
36 points by SirLJ on Oct 22, 2019 | hide | past | favorite | 9 comments



Okay so here's why this is trivially unsound from the perspective of standard modern physics.

1) Newton II: F= dp/dt. This particular formulation of Newtons Law is valid in relativity.

2) Newton III: My own formulation of this is all force arrows are "double-headed". Again, valid in relativity also.

So any dp/dt on anything inside your engine, requires equal and opposite dp/dt on something else.

The only new thing that they're doing is trying to make the mass relativistic, so p /= mv. I have no need of the hypothesis that p=mv in this refutation. Unless there is something outside that is taking the dp/dt, that this is only ever going to oscillate.

The reason is this. Lets define the "system" as everything inside the engine at t=0, and define the "system" as all particles and fields which would not exist if it weren't for the system at t=0.

The problem is this. The total momentum of the "system" can never change from it's initial momentum.

It looks like the idea in mind is that magic will happen because of the non-linearity in the momentum to velocity relation due to relativity. But if we look at this system as a whole, it's total momentum is fixed at all times at the initial total momentum, which implies an initial total system drift velocity, which only depends on account the total rest mass of the system, even in relativity.

The only way to increase the velocity of the engine over is to decrease the rest mass of the engine over the course of many engine cycles. That is, we need some of the mass-energy of the system to simply escapes the engine, in the form of escaping particles or electromagnetic waves.

Sadly, there is nothing that they've shown here that is an explanation of how we circumvent that part of engine physics.


Extensive discussion https://news.ycombinator.com/item?id=21250577 (271 points, 7 days ago, 134 comments)

Short version of the comments there:

The article mix classical and relativistic formulas, you must be very careful when you do that or you can get weird results.

In Classical Mechanic, momentum is conserved.

In Special Relativity, momentum is also conserved, but the definition of momentum includes the momentum of the electromagnetic radiation.

The device work, but not as the author think. It emits a lot of electromagnetic radiation. It emits more radiation in one direction, and the spaceship is pushed in the oposite direction.

The calculation of the energy is slightly of, I'm not sure why, perhaps because it mix classical and relativistic formulas.

The device is essentially and inefficient Photon Rocket https://en.wikipedia.org/wiki/Photon_rocket . It's equivalent of using a laser or flashlight in the back of your spaceship. It emits light in one direction, and the spaceship is pushed in the oposite direction.

For some reason people that rediscover photon rockets like to call them massless and claim that they break the laws of physic.

The theoretical maximum thrust/energy ratio of photon rockets is very small, that's the reason no one use them. In an inefficient photon rocket it's smaller.


> The engine accelerates ions confined in a loop to moderate relativistic speeds, and then varies their velocity to make slight changes to their mass

Wouldn't that change in momentum also have an equal and opposite reaction on the accelerator?


Description reads like a reaction-less drive [1] so at least quite speculative if not even violating the law of conservation of energy (because kinetic energy goes up with the square of velocity, but a reaction-less drive converts stored on-board energy into kinetic energy linearly).

[1] https://en.wikipedia.org/wiki/Reactionless_drive


Thunderf00t and EEVblog (Dave) entered the chat


What exactly is this? Comments so far are all dunking on it, but it's on nasa.gov, so .. ¯\_(ツ)_/¯


Ever heard of perpetual motion or snake oil? That's what the title alludes to, and here's the summary from the page

A new concept for in-space propulsion is proposed in which propellant is not ejected from the engine, but instead is captured to create a nearly infinite specific impulse.

Yeah, it doesn't look good at all. Maybe some solar freakin roadways: space edition.

EDIT: the end of the slides is telling that this abortion hasn't been validated in the slightest:

Basic concept is unproven •Has not been reviewed by subject matter experts •Math errors may exist!


This is just one person's slide deck. It hasn't undergone peer review and doesn't address conservation of momentum.


It could if momentum isn’t conserved.




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