In designing and building the rockets, both the materials used and the design of the nozzle were significant factors. As with other components, the nozzle's materials had to withstand much greater heat (caused by the burning fuel) than the boys had originally anticipated. They switched to a higher-grade steel, which they had to get via mail order.
The nozzle design was a significant feature that influenced speed and required both calculations and trial-and-error to attain. The crucial factor was the diameter of the hole in the bottom. If it was too small, the rocket would blow up; if it was too big, the rocket would not lift off at all. Figuring out the exact diameter required the boys to improve their math skills.