I am trying to build a model of free-floating spacerobot which has a 6dof arm.The gravity is zero. When the motor turns on and the arm moves, the base's position will change because of the reacting force.  I have seen the examples of the "rollercoster",it uses two"Prismatic" and one "Revolute" to simulate the track of rollercoaster ,and the example  "quadrocopter". So, I want know if the "free motion" component is very useful or efficient for my model.  I have tried several times,when the base is fixed to the"free motion" which is next to the  "fixed frame", it will not work unless the arm is fixed to a "fixed frame" too. So i use a "XYZ translational" and three "revolute"s instead. It worked ,but  the simulation  was slow. And when the arm has morn than four joints, there will be a "java heap space" message.whether or not it is because the performance of my computer is low. Or I shouldn't  build it like that?  

I also want to ask, when maplesim is applied to the realm of spacerobot , what's the advantage,and which things are fittest for maplesim? (Like Michel Lambert does, a researcher at the AMTC,his example on the website  is  studying the effect of dynamic parameters such as elasticity on robotized contact tasks).Can you introduce some?

Thank you very much!