The diameter of the exhaust nozzle would regulate the expansion and distribution of the combustive gases inside the rocket. Ironically, a simple piece of geometry would make a major difference between whether the rocket achieved liftoff and whether it listed to one side and exploded rapidly.

Using makeshift propellant, the boys experimented with various nozzle sizes but frequently exploded their rockets, leaving them unsuccessful and open to causing potential damage. They realized they needed to understand the math behind the nozzle and so began studying some of the textbooks they had ignored in their classes.

This led them to calculate an accurate diameter and shape for the nozzle—one that would converge before it diverged, allowing for expansion of gases while also funneling the blast into a directed line of exhaust and allowing them to achieve liftoff.

Since they weren't skilled enough to create this nozzle, they had to take the design to the mine machine shop in their town and have someone use the lathe to craft an accurate and stable nozzle. Doing this allowed them to create a sufficient rocket design, and eventually they achieved successful liftoff. The nozzle was successful enough that they were able to reuse it—especially because they had coated it in a ceramic to prevent it from degrading from heat over time.