Increasing the motor size to improve performance in model rockets creates safety issues, while reducing weight leads to a loss of payload capacity. Inspired by the Arcas sounding rockets, which could reach an altitude of 100 km using a gas-fed launch tube, Con Hathy studied the tube effect in a small model. The motor’s exhaust was expected to create pressure inside the tube and provide additional thrust. He launched the 3D-printed rocket from both a standard rail and a tube; a sabot that surrounded the rocket and provided a seal with the tube was used. In the first test, the tube launch produced worse results. The simulation showed that, as the rocket moved forward, the volume behind it that needed to be filled with gas grew faster than the exhaust was produced, and the slight vacuum that formed slowed the rocket down. Hathy narrowed the tube and added deployable fins and a smaller sabot reinforced with foam. In the second test, the rocket reached an altitude 72% higher, according to the altimeter. The data showed that a longer tube could improve performance.
Why it matters
The experiment shows that improving performance in small rockets cannot be achieved solely by increasing motor power; the launch setup also determines the rocket’s movement. This result offers an important design criterion for model rocket designers, particularly those trying to balance weight, safety and payload capacity. The fact that the initial failure was explained through simulation indicates that experimental results should be evaluated not only as measurements, but also in conjunction with airflow and pressure changes. However, it remains unclear whether the increase achieved is valid for different rocket sizes, tube dimensions and payload conditions. The contribution that longer tubes could provide has also not yet been tested directly.