Magnetic force-driven propulsion system for an interplanetary/interstellar spacecraft

Question

I have come up with a conceptual idea of a magnetic force-driven space propulsion system. What I believe is very unique about this propulsion system concept is that its working principle should not violate the Law of Conservation of Momentum nor the Law of Conservation of Energy, rather it should work in conjunction with these two laws of physics.

In order to help convey this conceptual propulsion idea, I have created a drawing of a conceptual mechanical device that would use magnetic force to generate propulsion.

In reference to this drawing, I first want to point out a few things. This is showing a side view perspective of a non-metallic board with two non-metallic rods fastened to it and rotating around these rods are two non-metallic levers with each lever having either a permanent magnet or an electromagnet fastened to the end of it. Each lever has a non-metallic ball bearing built into it, such as an all-glass ball bearing. The board, rods, and levers would be made out of a non-metallic material such as PVC, carbon fiber, or wood.

The two magnets are labeled with the letter M and their direction of travel are indicated by the arrows on the drawing. The gray filled square located to the left of P3 is a rectangular block of non-metallic material that is fastened to the board.

The working principle is that one permanent magnet will be positioned at position P1 and the other at position P2 and then they will be released (or turned on if they are electromagnets). Due to magnetic attraction they will begin moving and will accelerate towards position P3. When they reach position P3, the two magnets will impact the rectangular block and should come to a dead stop. The levers’ end positions will be at a 90 degree angle to their starting positions.

Say that this device is located out in interstellar space, far away from any stars and planets, and is at first motionless. Then the two magnets are released from positions P1 and P2. As they travel towards P3, the device\spacecraft should begin moving in the opposite direction and then when the magnets come to a complete stop at P3 due to impacting with the block, the device should immediately experience a decelerating force which will bring the device's movement to a complete stop. The end result should be that the device has moved a certain distance from its original position to a new position in space. I have indicated the direction of this movement with the position A to position B arrow on the drawing.

If there will be movement, then continuous propulsion may be possible via mechanical means. To make the device (and thus the spacecraft) move continuously in only one direction, this could be accomplished with 180 degree rotations of the board. Without the 180 degree rotations of the board, mechanically moving the two electromagnets back to positions P1 & P2 would propel the board/spacecraft in the opposite direction.

For example, after the two levers have come to a stop at position P3, the electromagnets would be turned off, the board would then rotate 180 degrees, the levers would then be mechanically moved back to positions P1 and P2, the board would then be turned 180 degrees, and then the electromagnets would be turned on causing the two levers to move towards position P3. This would be a continuous process. I think the only way to provide enough electricity for this system to work would be via an on board nuclear reactor.

I have shown this conceptual idea to a Physics professor at a college in my local area and I asked him for his professional opinion on whether the motion of the electromagnets would generate any propulsion. This was his answer:

“The answer is that the movement would occur exactly as you state. The system starts out with zero momentum. Then as the magnets swing left, the base has to move right so that the total momentum remains zero. When the two magnets hit the backstop at P3 they stop moving, and the base stops moving, and again the total momentum is zero. To put it another, equivalent, way, the center of the base has moved right, but the center of mass of base plus magnets has not moved at all.

Energy conservation is only slightly more intricate. The system starts with a lot of magnetic energy. As the magnets swing the magnetic energy gets smaller but the kinetic energy of the magnets plus base increases by the same amount. When the magnets hit the backstop at P3 then the magnetic energy is at a minimum, the kinetic energy is zero (nothing is moving any longer), and there is thermal energy due to the collision between the magnets and the backstop.”

Answer 1

This is another example of a perpetual motion machine. Although you may believe it will produce net motion without violation of conservation of momentum:

1. Any net motion produced would be a violation of conservation of momentum, so it cannot occur
2. You've not fully accounted for the net center of mass motion throughout your machine's cycle, or how momentum is transferred between components of the system at different points in the machine cycle, never created or destroyed.

We can disregard the specific use of magnetic force because it's not relevant; any method of moving masses around will confront the same underlying issue.

Let's say we had a mass at one end of a slide. We move the mass by whatever means we choose (cable, for example). As we move it to the opposite end of the slide, conservation of momentum says our vehicle must move in the opposite direction to maintain the center of mass. Now we rotate the slide 180 degrees so that the mass goes back to where it started, relative to our vehicle. Well, conservation of momentum says our vehicle must move back to where it started. It doesn't matter whether the reaction mass is translated or rotated, momentum must be conserved.

EDIT

A few other points...

Energy and momentum are always conserved.

Kinetic energy and momentum typically accompany each other, but conversion of kinetic energy to some other form e.g. thermal does not require or imply removal of momentum.

Kinetic energy scales with square of velocity, momentum scales linearly with velocity.

Consider two objects undergoing an inelastic collision (they do not rebound). Each object will undergo a change in its velocity vector, and a change in its kinetic energy. The sum total kinetic energy will decrease, with the difference becoming thermal energy (energy is conserved). The momentum vector of the combined objects post-collision is the same as the sum of the individual momentum vectors pre-collision (momentum is conserved).