Model Rocket Launcher WiFi ESP8266 Part 6

Introduction 
This blog continues from Part 5 of the ‘Wi-Fi-controlled rocket launcher’, providing a summary of code changes, battery voltage measurements and an alternative idea for the launcher housing.

Launcher Screenshot (Firefox mobile)

Code Change
With 18650 batteries used in the updated launcher design, it was decided to add a basic battery voltage measurement on the served webpage. Voltage measurements are updated only when the user refreshes or presses a button on the mobile device. The WiFi code uses a PUSH/GET method or a request-reply action. The code could be updated to automatically refresh the page or implement a WebSocket solution, which would provide near-instantaneous battery voltage updates.

Battery Voltage
The refresh rate for the ESP ADC battery voltage input was set to 250 ms. To provide a reasonably accurate voltage measurement, the resistor divider and the forward voltage of the reverse polarity diode were included in the scaling factor. It should be noted that the accuracy of the ESP battery voltage will be impacted by differences in the diode forward voltage. During bench measurements, the error was less than 0.1 V for the voltage range 3.5 V to 5 V.

Battery Measurement with Engine Starters
To validate the battery voltage measurements, resistive dummy loads and several Estes starters were used.

Resistor as Load

Starter as Load

Standard Estes Starter

Tests will also be conducted with Aerotech starters in a future post.

The two graphs below show the battery voltage when the starters were initially operated in free air and then fitted to an Estes engine. A single, fully charged 18650 3200 mAh from Naccon was used for the duration of the testing.

The green trace in the plots below represents the actual time that the ESP provides power to the starter.

Battery Voltage with Starters (Free Air)

Battery Voltage with Starters (Installed in engine)

Launcher Case Update
When the original rocket launcher post was first published, model rocketry was still popular in my region, and model rocket engines were not restricted from sale. Due to changes in model rocket supply and to improve design accessibility for the wider community, it was decided in the next circuit board revision to change the launcher housing. An off-the-shelf ABS case will be used. Although this is a return to the original design, it should give more hobbyists access to the design.