Model Rocket Igniter Controller

Model rocketry is a fun and educational hobby that starts with a ready-to-fly model rocket kit. As your experience grows, you can upgrade your system with parts that you design or build yourself. Some people like to assemble more complex rockets, others like to paint and decorate, and some people add interesting payload such as a camera.

Because I come from a software and electronics background, I chose to focus on improving my rocket ignition system. The ignition system is the piece that supplies power to an electric match, which in turn starts the engines burning to propel the rocket.

Higher performance rocket ignition system in a box

Higher-performance rocket ignition system in a box

I wrote an extensive article about the starter kit rocket launcher controller and my first couple of replacement controllers. (Read the article if you haven’t already, as this article assumes that background knowledge.) Since then, many people have written to me asking for instructions on how to build their own fancy rocket launch controller. So, I designed this new ignition system to be something you can make yourself.

In this article, I’ll show you how it works, step through the schematics, and direct you on how you can make your own.

Comparison

Starter kit rocket launcher controllers are simple and inexpensive. The battery connects to the igniter (electric match) through a pair of switches. Both switches must be pressed for power to reach the igniter. The switches and cable must be heavy-duty enough for the amount of energy being supplied.

Standard rocket ignition system

Standard rocket ignition system

The Model Rocket Safety Code requires that everyone is at least 15 feet away from the launch pad at liftoff. The resistance of the long wire (15 feet away from the battery and 15 feet back) means that a significant amount of power is burned off in the wire, instead of the igniter.

A solution is to place the battery and an ignition circuit board near the launchpad, with the control switches located remotely. Now, only the local wire needs to be heavy duty, as the ignition energy won’t travel all the way to the person with the switches. A keyed switch is added to the ignition circuit board to prevent remote operation when someone is working near the launchpad.

Improved rocket ignition system

Improved rocket ignition system

Example: When someone is working near the rocket, they turn the keyswitch to the ’safe' (disconnected) position. The signals from the remote switches are therefore unable to control the battery. Before walking away, the keyswitch is turned to the 'unlocked' (connected) position.

Then, the user presses and holds down the 'arm' pushbutton. The ignition circuit detects this, emits a warning tone, and begins the minimum countdown. After the countdown is complete, the user can press the 'launch' pushbutton to tell the ignition circuit to connect the battery to the igniter.

The ignition current doesn’t flow through the 'arm' or 'launch' pushbuttons -- only a signal. The 'arm' and 'launch' pushbuttons are wired separately rather than in series, so that the ignition system can detect their presses individually.

Features

The rocket ignition system presented in this article has the following benefits over the inexpensive launch controllers in starter kits:

  1. Delivers more power to the igniter (short cable to battery), which means:
    • More launches per battery charge
    • Wider choice of igniter brands
  2. Supports switches and pushbuttons in a wide variety of colors and shapes (not limited to only heavy-duty switches)
  3. Can be wireless
  4. Can be connected to a master control for multiple launch pads
  5. Safety features (with the addition of a microcontroller)
    • Enforced switch sequence
    • Minimum countdown
    • Audible buzzer
    • Igniter short-circuit protection (1 second power limit)

What Does it Look Like?

Because you can use your choice of switches, LEDs, buzzers, and case, the appearance is really up to you. I used a large plastic box (Romanoff Half Stowaway Red). The box provides enough room to store a small motorcycle battery inside. Weight isn’t a concern because the ignition system is placed on the ground nearby the rocket, rather than being carried by the operator.

Battery stored inside

Battery stored inside

Here’s the ignition circuit board:

Rocket ignition system circuit board

Rocket ignition system circuit board

Ⓐ High-current connector for battery input and igniter output

Ⓑ Voltage regulator with overcurrent limiter (circuit breaker) and reversed-battery blockage (diode) to protect the electronics

Ⓒ White rectangular connectors for the remote pushbuttons (arm, launch) or a wireless add-on board

Ⓓ Relay to provide maximum battery power to the igniter

Ⓔ Optional status LEDs

Ⓕ Optional microcontroller to enforce safety rules and drive buzzer

Ⓖ Connector for optional buzzer that can be driven by 5V or raw battery voltage

My board is attached upside down underneath the container lid for ease of wiring. This means my LEDs needed to be soldered to the opposite side of the board to poke out the top of the lid.

LEDs soldered to other side of board

LEDs soldered to other side of board

Safety

While the ignition system is an important piece for the safe launching of model rockets, it is the people and process that ultimately determine the outcome. You must learn and always follow the model rocket safety code.

If after reading this article you don’t understand the circuit, or if any step in this article intimidates you, then don’t attempt to build this igniter system. Instead, purchase a commercially-available launch system. That will save you time and pain.

Also, before using it on a real igniter, be sure to thoroughly test any ignition system (whether you make it or not) using a lightbulb. Try various combinations of button sequences and cable flexes to make sure you can’t set it off accidentally.

Plan

First, we’re going to go through schematics of each aspect of the ignition system. Then, the PCB layout is presented along with a home-etched example. Finally, we’ll discuss controller options and software.

Let’s get started with the igniter power supply.