Tips
In my opinion, the two most important things with rocket cars are their aerodynamic drag, and their weight. Drag force increases as the square of your speed, and so power requirement increases as the cube of the velocity. So, to go twice as fast, you will be looking at 8 times the power (just assuming aerodynamic drag), hence decreasing your drag is a huge factor if you want to go fast. Even more important is weight. You can have the the lowest drag design, but if it weighs as much as a brick, you are not going to be going anywhere. The lighter your car, the faster it can accelerate, and with engines that burn for a little over 1 second, how fast your car can accelerate is very important. As your weight decreases, however, aerodynamics becomes even more important - think about how far you can throw a feather compared to a tennis ball. A feather is very high drag even though it is very light, so it will not go far. So to sum up, in order to get your rocket car to be as fast as possible, you are looking for the lowest drag and lightest weight design you can come up with.
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