CO2 drag cars are a popular project for school shop classes, engineering students and hobbyists alike. With the attraction they hold for competitive "hands-on engineers," it should come as no surprise that there is some truly intense science behind dragster design. What constitutes the "best" design can depend on the class, rules and even altitude and humidity.
Perhaps the biggest misconception behind CO2 dragster design is in the name itself. CO2 "drag" cars are far more similar in design and principle to jet-powered land speed racers than they are a fuel car. Top Fuel dragsters look the way they do because their primary design criteria is to channel more than 8,000 horsepower through a little bit of rubber. As a CO2 car is essentially a jet-powered vehicle, it requires no power transfer to the ground, so the main priorities are the car's friction, mass and aerodynamic drag.
There are two basic designs: "rail" cars (wheels exposed and connected by a set of rails) and "shell" cars (wheels enclosed inside of the hollow body). Dragster-like rail cars often offer the lowest mass, but can be fragile. Although it may seem counter-intuitive for such a small body, rail cars do not have the best aerodynamics when compared to a larger, enclosed shell car. Remember, total frontal area (the primary aerodynamic consideration) is dictated by the height of the CO2 cartridge and launch system. So, all else being equal, the more blunt-edged, exposed-wheel rail cars will produce more drag and frontal pressure area than slipperier shell-cars. Aerodynamic advantage always goes to the shell car.
To reduce the size of the drag pocket behind the car, "boat-tail" (taper) the rear of the body with the lowest possible angle from the top of the body to the back of the CO2 cartridge.
The fastest CO2 cars of five years ago are no match for some of the newest generation of cars arising from MIT and professional designers. Common practice says that a slick surface should produce the least amount of drag, but one need only look at a golf ball to see the truth. Golfers long-ago discovered that putting tiny diviots into the ball's surface made it go further with the same amount of force, and the same thing holds true for cars.
The small pits in a golf ball's surface helps the air flow over its curved surface (the boundry layer) go from laminar (straight) to turbulent. A turbulent boundary layer sticks to the surface of the vehicle better, curving around behind it and reducing the size of the "drag pocket." This can increase the vehicle's aero-efficiency by as much as 50 percent, which yields huge increases in top speed and a reduction in times. Golf-balling your car's surface will almost certainly give you an edge over a car that is otherwise identical. Of course, this assumes you're using the thinnest possible wheels, which are always better than wider ones.