Thread: Linear motor vs. angular motor in space

I was just wondering why we haven't used linear motors as a form of space propulsion.

To me it seems as though an angular motor works perfectly fine in space; i.e., we can have a motor that creates angular momentum. Nevertheless, we can't convert that angular momentum into linear momentum in space without displacing some mass. However, why can't we use a linear motor, which directly creates linear momentum, as a form of space propulsion?

If the previous post didn't make much sense, I'll clarify it with an example.

Maglev trains use linear motors to propel themselves forward. Let's examine two scenarios: a wheel based space propulsion system vs a maglev based propulsion system.

Assume the following scenarios happen in outer space.

Let's say you had a treadmill that was connected to the bottom of a car-like vehicle. Furthermore, assume that the treadmill spins at the same velocity as the car, so that when the car goes faster, the treadmill also spins faster (the car and treadmill are moving in opposite directions to prevent the car from driving off the treadmill). Then, after a certain period of time, once the car and treadmill are moving at a certain velocity, the treadmill is lowered away from the car so that there is no contact between the treadmill and the car. In this instance, I wouldn't expect the car to gain any velocity, as the car's velocity comes from the conversion of angular momentum to linear momentum (which only happens due to friction). Instead, I would expect the car's wheels to continue to spin while the car remains stationary.

Now, let's consider a similar situation with a maglev train. Imagine we had a special treadmill that had maglev rails on it instead of a rubber mating. There is an angular motor that is spinning the treadmill, while there is simultaneously an alternating current travelling along the maglev rails, effectively creating a linear motor. The treadmills angular motors are built in such a manner that they keep up with the velocity of the maglev (again, the maglev and treadmill are moving in opposite directions so that the maglev doesn't drive off the treadmill). After some period of time, we lower the treadmill. What happens now? I hypothesize that the maglev would propel itself forward, because unlike the car which had angular momentum and required friction to create linear momentum, the maglev actually has linear momentum and should be able to propel itself forward.

Now, we could just do the same thing but rather than having a treadmill, just attach the rail to the train car and pass an alternating current through it (a linear motor). Depending on how much current is in the rail, the train car will accelerate or decelerate, thereby allowing for non-mass-ejecting space propulsion.

Aren't you forgetting about conservation of momentum? How does the vehicle propel itself forward without imparting any momentum in the opposite direction.

I suggest you attend a physics class.

Harold, how would you explain conservation of momentum in an angular electric motor? Initially the angular momentum of a stationary axle is 0, but after some energy is applied to the motor, the axle begins to spin and there is an increase in angular momentum. There is a definite change in the angular momentum, but this is because energy is added to the system, thereby acting within the bounds of conservation of momentum.

Why can't we apply this same theory to a linear motor and linear momentum? I.e., if a linear motor adds energy to a system, is it not possible that the system will experience an increase in linear momentum (while still abiding by the law of conservation of momentum)?

Originally Posted by railnet

Harold, how would you explain conservation of momentum in an angular electric motor? Initially the angular momentum of a stationary axle is 0, but after some energy is applied to the motor, the axle begins to spin and there is an increase in angular momentum.

Wrong. the angular momentum imparted to the motor and its load is equal and opposite to the angular momentum which is imparted to the base to which the motor is mounted, and ultimately the earth.

There is a definite change in the angular momentum, but this is because energy is added to the system, thereby acting within the bounds of conservation of momentum.

Why can't we apply this same theory to a linear motor and linear momentum? I.e., if a linear motor adds energy to a system, is it not possible that the system will experience an increase in linear momentum (while still abiding by the law of conservation of momentum)?

We can apply the same to a linear motor, or anything else that moves in a straight line. The momentum is imparted to the earth, or whatever moving platform to which the linear motor is mounted. If it is mounted on nothing (as in space) it cannot accelerate.

You are absolutely correct.

Let's change some of the assumptions.

Assume the following events occur on the Moon (where the atmospheric density is 10^-13 torr -- practically a vacuum). You have a treadmill with maglev tracks rotating at a certain velocity while a maglev train runs in the opposite direction with an equal magnitude velocity. The treadmill is anchored to the Moon; however, the maglev train car is not. After the maglev/treadmill system has been accelerated to a sufficiently high velocity, a magnetic barrier is placed between the treadmill and the maglev, effectively removing the hold the treadmill had on the maglev car. What would happen?

Harold, you seem to have a better grasp of physics than I do; however, if I may hypothesize, I believe that the maglev train car would fly off the track with its attained velocity.

Originally Posted by railnet

You are absolutely correct.

Let's change some of the assumptions.

Assume the following events occur on the Moon (where the atmospheric density is 10^-13 torr -- practically a vacuum). You have a treadmill with maglev tracks rotating at a certain velocity while a maglev train runs in the opposite direction with an equal magnitude velocity. The treadmill is anchored to the Moon; however, the maglev train car is not. After the maglev/treadmill system has been accelerated to a sufficiently high velocity, a magnetic barrier is placed between the treadmill and the maglev, effectively removing the hold the treadmill had on the maglev car. What would happen?

Harold, you seem to have a better grasp of physics than I do; however, if I may hypothesize, I believe that the maglev train car would fly off the track with its attained velocity.

If you mean that the train will go into orbit at high enough velocity, you are right.