Below presented a concept for a electromagnetic thruster.

Basic Concept

Suppose you have two identical current carrying solenoids, Solenoid A and Solenoid B that are firmly mounted to the inside of a container.

The solenoids are aligned along the x axis, have current flowing in the same direction and have a separation distance d, as shown in Figure 1.

Figure 1

Let the force on Solenoid A due to the magnetic field of Solenoid B be denoted as $F_a$, the force on Solenoid B as $F_b$. It follows that $F_a – F_b = 0$ and the container holding the solenoids is initially at rest.

At a certain time $t_0$, Solenoid’s B current flow is disrupted. The magnetic field generated by Solenoid B will start to decay, decreasing the attraction force $F_b$ to a lower value of $F_b’(t) < F_b$. Due to the finite propagation speed of electromagnetic waves, the information bubble will reach Solenoid A only at $t_1 = d / c$.

In the time interval $T = t_1 – t_0$, Solenoid A is “unaware” of the change and will continue to experience force equal to Fa in the positive x direction. The net force acting on the container during interval $T$ must be $F = F_a – F_b’(t) > 0$ i.e. force in the positive x direction as shown in Figure 2. that should propel to container to the right.

Fighre 2

After the information bubble will propagate to reach Solenoid A, the force will stop.

Scheme for a Continuous Motion

It is possible to adjust the mechanism above to create continuous force production by periodically changing the current in the solenoids with frequency $f=c/d$ and a phase difference of half a cycle.

instead of disrupting the current in Solenoid B, we change the current's direction and in doing so, change the polarity of the solenoid.

This will further increase the magnitude of $F = F_a – F_b’(t)$ compared to the basic scheme above, because now $F_b'(t) < 0$ and Solenoid B is pushed by the field of Solenoid A in the positive x direction.

In order for solenoid A to continue to produce force to the right, when the information bubble will reach it after period $T$, we switch it's polarity.

this will produce two effects:

1. Solenoid A will be pulled to the right

2. We create an information bubble that will propagate towards Solenoid B

after another period of $T$ we switch the polarity of Solenoid B, which will push solenoid B to the right.

we can continue this alteration of polarities to produce force in a "push-pull" configuration.

The Question

Is it physically possible to produce thrust using this setup?

it is clear that there are many specific issues that where neglected such as transient effects, difficulty of crating high frequency alternating currents in solenoids and many others.

but can it fly? :)


closed as unclear what you're asking by sammy gerbil, Jon Custer, ZeroTheHero, Yashas, JamalS Jul 1 '17 at 9:44

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  • $\begingroup$ I think the most likely explanation is that your scheme explicitly radiates EM waves, which carry momentum away. There may be some things you didn't take into account, but generically any radiation you generate will not be perfectly symmetrical, so you'll get thrust. $\endgroup$ – Peter Kravchuk Jun 30 '17 at 22:19
  • $\begingroup$ The magnitude of that thrust is a different question. $\endgroup$ – Peter Kravchuk Jun 30 '17 at 22:20
  • 4
    $\begingroup$ @sammygerbil, knowing a general result is one thing, understanding why it works is something completely different. $\endgroup$ – Peter Kravchuk Jun 30 '17 at 22:23

I suspect that if you sit down and do the math, you'll find that there is in fact no net force in this particular situation, but that's not particularly relevant - if that's the case, you can modify the problem (e.g. by some strategically-placed electric charges) such that the net force will be nonzero.

The reason this is not a problem is that the scheme you describe is not closed, i.e. it ends with the currents in a different configuration than they were at the beginning. If you want to make your device into a true reactionless drive, then you need to bring back the currents to where they were, so that you can repeat the process and get some more momentum out of it. However, in this configuration (as in all others in that class), reversing the changes in the current will induce forces of equal magnitude and opposite direction, so the net force over the cycle is zero.

  • $\begingroup$ The argument in the question may be completely fine. I think the important point is not closed cycles, but that this process radiates EM waves. Thus the momentum is carried away. $\endgroup$ – Peter Kravchuk Jun 30 '17 at 22:15
  • $\begingroup$ @PeterKravchuk Probably that, too, but I don't see the point in examining the radiation in detail for a non-periodic process. $\endgroup$ – Emilio Pisanty Jun 30 '17 at 22:25
  • $\begingroup$ If the system is closed, it is not important whether the process is periodic, because of momentum conservation. Radiation is the only thing that makes the system open. Periodicity would be important for displacement, not momentum gain. $\endgroup$ – Peter Kravchuk Jun 30 '17 at 22:28

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