These are vehicles propelled by solid fuel rocket engines.  They are made of aluminum with solid aluminum wheels on ball bearing hubs.  The front axle is tunable and the wheels have a very low rolling resistance.  Although made of aluminum, because they are 600 to 900 centimeters long, they do have some weight to them.  This helps to keep them grounded.  These were versions built for my nephews and based off of two original designs built for the Fusion Works invitational rocket races