Move in Space without mass exchange

[Irvin Atkins]

TL;DR Summary

I came up with a system that allows you to move in space without exchanging mass. Where did I make a mistake?

I did a thought experiment and I can't figure out what the mistake is.

There is a system of 2 electric motors weighing 1 kg each with batteries in the Earth's orbit.
The motors are rigidly connected by a 1-meter-long bar.

If one motor starts rotating in one direction on a signal, the entire system should start rotating in the opposite direction around(???) the motor's rotation axis.
After rotating the system 180 degrees, we start rotating the motor in the opposite direction to stop the system from rotating.
As a result, we have a system in the same state but shifted 1 meter to the side.

Repeat the same with the second motor.

If this works, then you can make the motors' rotation axes perpendicular and move in any direction without exchanging masses.

 

[Frabjous]

You are assuming that the active motor is stationary.

 

[Baluncore]

If this works, then you can make the motors' rotation axes perpendicular and move in any direction without exchanging masses.

That would be a reactionless drive.
https://en.wikipedia.org/wiki/Reactionless_drive

Flywheels cannot move the centre of mass of the machine.
Reaction wheels are used on satellites to maintain, or to change the orientation, not the position of the centre of mass.

 

[Nugatory]

Whatever force the active motor exerts on the rest of the machine, by Newton's third law the rest of the machine must exert an equal and opposite force on the active motor. Thus as @Frabjous says above the active motor is not stationary - the boom and passive motor moves one direction, the active motor moves in the other, and as @Baluncore says the center of mass of the system stays put.

It's also worth taking a moment to think about what happens when a motor is turned off. Does it keep spinning on impossible unrealistic frictionless bearings or does it slow down and stop? Of course it's the latter, meaning that now the rest of the machine is exerting a force on the motor to slow it down, by the third law there's an equal and opposite force on the rest of the machine, and these will end up reversing the initial kick from starting the motor up in the first place.

 

[Nugatory]

The forum rules do not allow discussion of reactionless drives so this thread is closed.