Launch Controller Design

Motor starters (igniters)

The purpose of a launch controller is to ignite a motor starter, and thus ignite a motor. There are several different kinds of starters, but they are fairly similar in electrical characteristics. When designing a launch controller, you will need to keep these in mind.

Nominal resistance is just the resistance of the starter, including its leads. No-fire current is the maximum current that can be passed through the starter (to check continuity) when ignition is not intended. All-fire current is the minimum current that should be passed through the starter when ignition is intended. Therefore, the continuity circuit must deliver less than the no-fire current to all starter types, and the ignition circuit must deliver more than the all-fire current to all starter types.

There are still lots of blank cells in the table below - if you know where to find data on these, or would like to do some testing on your own and report your results, please contact Danno.

Starter Type	Nominal resistance	No-fire current	All-fire current
AeroTech Copperhead
AeroTech First Fire
AeroTech First Fire Jr.
AeroTech First Fire Mini
Estes Model Rocket Starter	0.5 - 2 ohms		1 amp
Estes Sonic	1.6 ohms	1.25 amps	3.80 amps
Firewire	0.8 - 1.2 ohms	0.3 amps (less than 0.04 amps recommended)	0.6 amps (1 amp recommended)
Firewire Mini	0.8 - 1.2 ohms	0.3 amps (less than 0.04 amps recommended)	0.6 amps (1 amp recommended)
MJG e-match (dip-your-own)	0.8 - 1.3 ohms	0.3 amps	0.7 amps
Quest First Fire Micro
Quickburst Twiggy	1 - 2 ohms
Quickburst Slim Gem	1 - 2 ohms
Quickburst Fat Boy	1 - 2 ohms
Quickburst Super Fat Boy
Wildman Big-Uns
Wildman Little-Uns
Wildman Tiny

Note that proper continuity circuit design is much more critical from a safety standpoint; too little current from the ignition circuit just means the rocket doesn't launch, but too much current from the continuity circuit can cause unintended ignition! In general, test current should be the minimum required to activate whatever lamp or sound device you're using to signal continuity (for instance, this LED specifies 30 mA maximum, and this alarm element only requires 7 mA). You should really never be getting anywhere close to the no-fire limits listed above.

The maximum firing current you can deliver will probably have more to do with the characteristics of the battery being used than the design of the ignition circuit. (TODO: try to compile some data about different types of batteries.)