I have had this idea in my head for years and I want to know once and for all if it will work or not. I understand that a flywheel gains in mass when it spins. If this is true, is my thinking below correct? Any feedback would be appreciated! Inertial Mass Propulsion: I am going to try to describe a working model just to explain the concept that I have in mind: You have two flywheels on a common axis that can spin in opposite directions. Each flywheel has a motor/generator attached to it that can spin-up or stop the flywheels. Each of these motor/generators is connected together so they do not produce any torque. The flywheels can be spun-up or stopped at the exact same rate so the net effect is that no torque is produced (you could hold it in your hand during spin-up and not feel anything other than the gyroscopic effect when they were spinning). Hope you are with me so far. You are floating in space (wearing your space suit of course) and you have this counter-rotating flywheel device with you. You attach a long length of strong extension cord to it. You have a battery pack with you that is capable of spinning up both of the flywheels to their maximum speed. Here goes: You hold the device in your hands, activate the motors to spin-up the flywheels to their maximum speed, and you throw it away from you as hard as you can. Now, before the slack it taken up in the extension cord, you activate the generators to stop the flywheels and recharge the battery pack. When the assembly reaches the end of the extension cord, you pull it back to you. You push away from the device while the flywheels are spinning and pull it towards you when they are stopped. Push away from the device while it is “heavy” and pull it back while it is “light”. So here is my question: would you start moving in the direction opposite the way you were throwing the device? Is the net effect “thrust”? Can you create mass, push away from it, and then convert the mass back to energy and the net effect be thrust? Forget electrical losses and efficiencies etc, I am not concerned with that. I just want to know if it really works the way I perceive it. This means you could make thrust in any direction without giving up any mass (just giving up energy instead). Can this be?