When dealing with rocketry, certain environmental factors and physical restraints must be kept in mind. Launching an effective craft is not simply a matter of packing as many rockets as possible onto the end of a ship and hoping for the best - this will often lead to failure upon launch or lack of stability that can cause you to fail the challenge. Below are some laws of physics to keep in mind when constructing rockets.
Thrust is the energy expelled behind a craft that allows it to be propelled in a direction. The thing to keep in mind regarding thrust is that the location of whatever generates the thrust is as important as the thrust itself. When packed in tight clusters, Rockets tend to wobble or cause drifting in the vehicle proper due to unequal thrust. To maximize the power of your vehicle, try and situate the rockets in equal distances from the core of the ship.
Vector is the energy expelled by the craft to change direction of thrust. In Crashtastic, with the exception of the Steering Wheels which enable you to change direction of craft, there is no vector. Make sure your thrusters are set equidistant (i.e., if you're using four rockets, put them on struts in a + shape radiating from the core to ensure stability) from the core in equal numbers. This will prevent wobble and loss of vehicle control.
The location and distribution of weight across your craft will largely determine its success. Top heavy craft tend to immediately bob or topple over, whereas bottom heavy craft are typically better at maintaining vector. Misplaced weight can cause the ship to turn due to increased weight, and will cause rotation or loss of control.