Grumpy_Mike:
You need what is known as a logic level n-channel FET, connect the source to ground, the gate through a 100 ohm resistor to the arduino output pin and the drain to your load. Connect the other end of the load to the positive of your power supply and connect the negative of the supply to ground. As simple as that.
Where could i get such a FET? i usually buy my stuff from sparkfun. Could i substitute the FET with another type of transistor? (in the event sparkfun does not carry an FET)
also, how bad is a rocket being top heavy?