There are a couple varieties of rockets that use Nitrous Oxide as an oxidizer to burn some variation of a plastic fuel grain. I won't talk much about how they work here, but go over a launch system I've built for them. If you don't know how the nitrous type rockets work, you may want to do a google search for "Hypertek" or "RATT Motors".
The basic design started with a hand truck modified slightly so that there was room for a wooden box between the upright bars. Due to the bar that you hold on to being in the way, it ended up with an upper box and lower box. The upper box contains the relays, and the fill/dump switches. The lower box contains the battery and the solenoids.
You can see a little of the 'electronics' here. It is set up for two pads, so there are switches for right and left side. The fill and dump switches are at the pad so the person filling will monitor the nitrous as well. On the front, the top white square is a wall switch (hard to tell below). It disables the LCO ability to press the fire button. The outlet on the bottom is the LCO control line - it basically puts 12 volts out the ground line and uses the other two lines as fire button. I've tested it with 200' of cord and it doesn't have too much voltage drop for driving the relays.
From the side below, you can see the Oxygen and Nitrous out, as well as the 12volt out for the igniter.
Not quite completed below. Plan is to build a box for holding the tanks so that there is no risk of them falling over. May make the 'box' out of something water tight so that the nitrous can be iced down. Got some one foot hoses made for the short connect.
More of a close up of the switch panel. Key switch for turning on the system. Then switches for fill / dump / oxygen / fire. The oxygen is mostly for testing. The fire switch is only enabled with a key switch. It isn't intended for actual launch as you're only 10' from the rocket. There's also a switch for either nitrous or normal APCP mode (solid propellant). The APCP mode disables the solenoids and the fire button will just put 12 volts to an igniter.
The insides of the relay box:
The insides of the solenoid box.
