by Rincewind » Wed May 24, 2006 4:04 pm
Even if using maneuvering thrusters to keep one object stationary while the other is moved, the net force on the arm is the same. So that is not the limmiting factor.
The limit is in how slow. They are using electric motors which work by lots of little magnetic shoves. Electric motors don't like running a varible speeds, there is only a narrow range of speed for the motor that will work. Below a certain momentum in the drive shaft of the motor, and there will not be enough power to kick round to the next shove point. Also if the motor is running slowly, then it is jerky. Not smooth. To move big things slowly you need smooth.
To get round the speed and jerkyness, the motors are run quite quickly, then geared down to the required speed. To change speed you have to change gear. These are mechanical gears. So they are heavy. So only only take a limited range of gears into space with you. Limited gears = limited speed range = limited momentum range.
They could probably change the load capacity by simply changing the gears without altering the structure of the arm. But since the Canadarm only has to deal with the shuttles own payload, why build it to handle larger.
The Canadarm2 on the space station has enough capacity to move the shuttle about. Indeed could move two shuttles about. This version 2 arm can have more motors and better gearing because it's in space for a long time and is not coming back and forth so it not so expensive to launch. But there is still going to be some limiting factor on how gently the arm can move things. So there will always be a mass limit.
Even if using maneuvering thrusters to keep one object stationary while the other is moved, the net force on the arm is the same. So that is not the limmiting factor.
The limit is in how slow. They are using electric motors which work by lots of little magnetic shoves. Electric motors don't like running a varible speeds, there is only a narrow range of speed for the motor that will work. Below a certain momentum in the drive shaft of the motor, and there will not be enough power to kick round to the next shove point. Also if the motor is running slowly, then it is jerky. Not smooth. To move big things slowly you need smooth.
To get round the speed and jerkyness, the motors are run quite quickly, then geared down to the required speed. To change speed you have to change gear. These are mechanical gears. So they are heavy. So only only take a limited range of gears into space with you. Limited gears = limited speed range = limited momentum range.
They could probably change the load capacity by simply changing the gears without altering the structure of the arm. But since the Canadarm only has to deal with the shuttles own payload, why build it to handle larger.
The Canadarm2 on the space station has enough capacity to move the shuttle about. Indeed could move two shuttles about. This version 2 arm can have more motors and better gearing because it's in space for a long time and is not coming back and forth so it not so expensive to launch. But there is still going to be some limiting factor on how gently the arm can move things. So there will always be a mass limit.