NASA hypothetical engine w/ relativistic ions

[BWV]

Does this violate conservation of momentum or some other law? It claims to take advantage relativistic mass increases to achieve propulsion

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. The engine accelerates ions confined in a loop to moderate relativistic speeds, and then varies their velocity to make slight changes to their mass. The engine then moves ions back and forth along the direction of travel to produce thrust. This in-space engine could be used for long-term satellite station-keeping without refueling. It could also propel spacecraft across interstellar distances, reaching close to the speed of light. The engine has no moving parts other than ions traveling in a vacuum line, trapped inside electric and magnetic fields.

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20190029657.pdf

 

[PeterDonis]

Does this violate conservation of momentum

It does if they are claiming that propulsive force can be obtained. You can't produce an external force on an object by rearranging its internal parts. It looks like that's basically what this is trying to do.

 

[Ibix]

Instant reaction: if he's bouncing charged particles backwards and forwards then he's generating changing EM fields - i.e. radiation. If this works at all, it's a complicated photon rocket. If it doesn't emit radiation then, as Peter says, I can't see how it could possibly work.

 

[berkeman]

What is it with these NASA yahoos? At least he's honest at the end of the presentation...

 

[Ibix]

It's difficult to tell because the slides mention a simulation without stating what was simulated, but I suspect his specific problem is that he thinks he can accelerate his ions in their orbital motion without applying a force in the axial direction. If he's done that (and I'm not sure), it would be wrong.

 

[PeterDonis]

if he's bouncing charged particles backwards and forwards then he's generating changing EM fields - i.e. radiation. If this works at all, it's a complicated photon rocket.

And a very inefficient one, since the EM radiation will not be very coherent or very well collimated.

(I'm not sure the author of the slides even realizes that accelerated charged particles emit EM radiation.)

 

[Ibix]

(I'm not sure the author of the slides even realizes that accelerated charged particles emit EM radiation.)

There's a reference to "fields" in the penultimate slide which might, charitably, cover it. Or it might just be a science-y word - he mentions gravity waves in a context where he means gravitational waves, which isn't a great sign.

 

[Dale]

Does this violate conservation of momentum or some other law? It claims to take advantage relativistic mass increases to achieve propulsion
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20190029657.pdf

From p 6 “increase absolute ion velocity”. On the same page as it introduces relativistic momentum.

I want some of my tax dollars returned.

 

[kent davidge]

(I'm not sure the author of the slides even realizes that accelerated charged particles emit EM radiation.)

He's an engineer at Nasa and you are quesitoning whether he knows that, wow

 

[Vanadium 50]

Next time people get cranky when I say relativistic mass is worse than useless, remember this point.

 

[berkeman]

He's an engineer at Nasa and you are quesitoning whether he knows that, wow

Just for the record, my insult was worse...

 

[kent davidge]

Just for the record, my insult was worse...

oops, on a second thought, I see that the David Burns referred to at the beggining of the article is not the same person who wrote the article?

 

[PeterDonis]

He's an engineer at Nasa

The person given on the first slide is a manager, not an engineer.

I see that the David Burns referred to at the beggining of the article is not the same person who wrote the article?

Since David Burns is a manager, I'm assuming he didn't prepare the slides himself but tasked someone that works for him with doing it. But I don't see any reference to who that was or what credentials or knowledge they had.

 

[Keith_McClary]

I want some of my tax dollars returned.

And that's what NASA engineer David Burns has been doing in his spare time. ...
He's posted it to the NASA Technical Reports Server

Source:
https://www.sciencealert.com/no-this-new-space-engine-isn-t-going-to-break-physics

 

[BWV]

So the consensus here is that the plans for the engine are legit?

 

[PeterDonis]

Source

The slides themselves say "Manager", not "Engineer". Given a choice between the primary source and a second-hand account in an article, I'm going with the primary source.

 

[PeterDonis]

So the consensus here is that the plans for the engine are legit?

Huh? I don't know what would give you that impression.

 

[berkeman]

Huh? I don't know what would give you that impression.

I think he was joking...

 

[jartsa]

So the idea is basically that ions are shot from the rear of the spaceship. At the middle of the spaceship a transverse electric field gives the ions more mass-energy, which supposedly increases the forwards momentum of the ions, without anything gaining any backwards momentum.

Well, I guess that's what the idea is.

 

[Ibix]

So the idea is basically that ions are shot from the rear of the spaceship.

Edit: as noted in the next posts, I misread jartsa's post. The next paragraph is correct but not a correction. The final paragraph is wrong.

No. The ions bounce back and forwards in a tube, being accelerated and decelerated in such a way that they hit the front of the tube harder than the back. But the forward acceleration process doesn't slow the rocket by the same amount the collision speeds it up... somehow. Because relativistic fairy dust, I think. That's why it's nonsense (unless it emits EM radiation, in which case it's accidentally a photon rocket, probably an inefficient one as Peter commented).

You are describing an unconventional rocket that uses relativistic ions as reaction mass. It may or may not be worth investigating as a design, but it is perfectly acceptable physics.

 

[jartsa]

No. The ions bounce back and forwards in a tube, being accelerated and decelerated in such a way that they hit the front of the tube harder than the back.

I was talking about that.

Here's an improved version:

So the idea is basically that ions are shot from the left side of a lab to the rightwards direction. At the middle of the lab a vertical electric field gives the ions more mass-energy, which supposedly increases the leftwards momentum of the ions, without anything gaining any rightwards momentum.

 

[Ibix]

I was talking about that.

I misread - apologies.

Note that the process is supposed to be cyclic, feeding the ions back around to launch again.

 

[jartsa]

Note that the process is supposed to be cyclic, feeding the ions back around to launch again.

Doesn't classical physics say that an observer moving through a room, which has oppositely charged ceiling and floor, observes a magnetic field, which bends the paths of ions that were originally co-moving with the observer, but were then accelerated by the electric field?

I would guess that if the observer considers himself moving, he would see ions being slowed down.

Relativity may have its own idea what magnetic fields really are.

 

[Ibix]

I would guess that if the observer considers himself moving, he would see ions being slowed down.

I don't think so. Or it depends on more detail than you've given.

The EM is a distraction. The device as described is functionally equivalent to a wheel mounted on an axle running the length of the ship. The wheel is free to slide along the (frictionless) axle. You start with the wheel, unspinning, at the back of the ship, and push it forwards. As it slides towards the front of the ship at constant linear velocity, you spin the wheel up to relativistic speeds, so when it strikes the front wall it has increased relativistic mass. It bounces off the front wall and returns, being spun down as it does so. Rinse and repeat.

The argument is that the wheel has more relativistic mass when it bounces off the front wall than when it bounces off the back wall, but the same linear speed. Therefore we get a net push forwards. The flaw is in believing that you can spin up the wheel while it maintains a constant linear speed without needing to apply any component of force parallel to the axle. This is only true if the linear velocity is zero, in which case the wheel never hits either wall. If it's moving, the extra momentum from the bounce off the front is compensated by the momentum the ship lost spinning up the wheel (edit: from that component of force parallel to the axle).

Short version: force and acceleration are not always parallel in relativity. Spinning up the wheel while it's in motion slows the ship, so the wheel bouncing off the front restores it to its original speed. It's a textbook free energy device, with a textbook free energy device inventor's mistake of not quite grasping all of the physics.

Edit 2: I still haven't done any maths, so there may still be effects I haven't accounted for. But the above are definitely effects the slides' author doesn't appear to have accounted for.

 

[jartsa]

The device as described is functionally equivalent to a wheel mounted on an axle running the length of the ship. The wheel is free to slide along the (frictionless) axle. You start with the wheel, unspinning, at the back of the ship, and push it forwards. As it slides towards the front of the ship at constant linear velocity, you spin the wheel up to relativistic speeds, so when it strikes the front wall it has increased relativistic mass. It bounces off the front wall and returns, being spun down as it does so. Rinse and repeat.

The argument is that the wheel has more relativistic mass when it bounces off the front wall than when it bounces off the back wall, but the same linear speed. Therefore we get a net push forwards. The flaw is in believing that you can spin up the wheel while it maintains a constant linear speed

Aha, when we increase wheel's spinning energy, its linear kinetic energy decreases. (Assuming that we do not apply any linear force, which means that the linear momentum stays constant and the linear speed decreases.) That's interesting.

linear kinetic energy = linear momentum * linear speedIf we have pair of spinning wheels, and we start increasing that systems relativistic mass by spinning up that system, by making the wheels to revolve around each other, do the spinning energies of the individual wheels decrease?

 

[DEvens]

When I was a kid I lived on a farm. And a farm nearby had a field that was split by a roadway, with a tunnel for the cows to walk between the field parts as they liked.

The farmer was a bit of a whack-job. He believed that if he could get the cows to walk through the tunnel in just the right way he could increase his cows. Free cows! We would tell him that walking through a tunnel made no difference to the number of cows.

One day, one of his cows wandered into the tunnel to get out of the sun. Then she lay down and gave birth in the tunnel. Upon emerging from the tunnel with a brand new baby calf, the whacky farmer was impossible. See! he said. See! You can make cows by having them walk through a tunnel.

He became quite irate when we disagreed.

Guys, electromagnetism is a conservative force. You conserve energy and momentum. Bouncing things, spinning them, flipping them over, rotating magnets, etc. and tiresome etc. does not change that. Reactionless drives are not possible. Maybe he's got cows giving birth in tunnels, er.., I mean, maybe he's letting photons escape. Or maybe he's just wrong. But he does not have an engine that works without reaction of something.

 

[berkeman]

The farmer was a bit of a whack-job.

Kind of like some NASA engineers...

This thread is closed. When there is a paper that is published in a peer-reviewed journal, we may be able to discuss this more.

What is it with these NASA yahoos? At least he's honest at the end of the presentation...

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