Matthew
The least expensive launch system I have seen is found at
http://www.nar.org/NARTS/mahler/ultra.html
The launch controller design is fully in compliance with the NAR regulations which is meant to be interpreted as.
1.) Your launcher must have a removable safety interlock switch that physically disconnects the launcher from the power source.
In this simple launcher referenced above, the removable safety interlock switch is simply a metal "bull dog" clip (normally used to hold a stack of papers together) which is used to make the power connection to the launcher.
You could also use a coaxial power connector similar to the style used with AC adapters for radios and video games as the safety interlcok key, (but don't use stereo or ear plug plugs which can short out the battery on insertion into the jack), or you can use a key switch where the key is removable in tthe off position only.
2.) Your launcher must a separate momentary contact switch for the launch switch. You may not use the safety interlock key switch as both power interlock and the launch button. This would remove the dual redundany built into the safety code that mandates a positive power disconnect in the event that one switch fails in the closed position.
In the article, this is simply a spring metal bar, but a door bell switch works just fine. You could use a second monentary contact key switch but this is expensive.
While not in the safety code it is prudent to have a continuity checker circuit in the launcher. I would add the following.
3.) Your launcher should have a current-limited continuity checker to indicate that your ignitor is properly attached to the clip leads.
This continuity checker can simply be a resistor/led as used in the article (typical current <20 mA), or it can be a piezo buzzer (typical current ~10-20 ma), or it can be a low current flashlight bulb (for Estes ignitors the continuity current should not exceed 0.1 amp, however be careful as some homebrew e-matches might fire at this current). Since leds with current limiting resistors don't burn out and use less current they are preferred. The recommended resistor value is 50 ohms per volt, so for example, if you have a 12 volt system use a ~600 ohm resistor. A current-limited piezo buzzer in series with the led can serve as the current limiter in place of the resistor.
The continuity circuit is powered up when the safety interlock switch is turned on. The ignitor is fired when the launch switch, in parallel with continuity checker, is depressed.
A more detailed description of the minimal launch controller circuit with the math is found on the Estes Educator site.
http://www.esteseducator.com/Pdf_files/1924_launch.pdf
also info on Estes ignitors
http://www.esteseducator.com/Pdf_files/Igniters.pdf
Bob Krech