The ARS High Power Relay Launch System

By Mike Bernard (April 98)

Ever wonder what that launch box of our's looks like on the inside, along with those little blue boxes at the pads? Well here is an article by Mike to tell you how it works! The Albuquerque Rocket Society (ARS) launch controller is an easy to build, relatively inexpensive, reliable and safe relay type launch system. It has been used for over two years with little maintenance and expanded once due to our club's growth. Here are some of it's built in design features:

Expandability: It can be built to control one launch pad or as many pads as your club can afford. It is simple to add more pad capabilities as your club's need grow.
Relatively inexpensive: The system is designed to use off the shelf Radio Shack parts. The individual pad boxes can be built for around $35 each.
Safety built in: Several layers of safety interlocks are designed in. These include a locking key switch, pad safety interlock that disconnects both the main power and control power and a flashbulb safe igniter continuity check circuit. The design meets all criteria for launch control systems set out in NAR and Tripoli safety codes.
Versatile: The pad selection switches can be used to fire multiple pads together for drag racing rockets and the pad boxes can be ganged together for high current cluster ignition. The system has been tested on all types of igniters from Estes Solar, to home-brew Thermalite, to high current conductive composition types.
Powerful: The system uses high current relays and pad located ignition batteries to provide up to 20 amps of current. This is more than enough to fire multiple igniters of any type. Control and continuity circuits have their own discreet power sources for reliable operation.

Basic Operational Characteristics

(Note: Refer to the schematic in the newsletter for details on the actual wiring.)

The LCO PANEL is equipped with a set of toggle switches for pad selection, one switch per PAD BOX. The panel also uses a locking key switch wired in series with the pad selection switches and the launch push button. This provides for positive safing of the system by the LCO simply by removing the safing key prior to opening the range head for rocket loading.

A multi-wire cable connects the LCO PANEL to a DISTRIBUTION BOX located some distance from the LCO table. We typically set one distribution box at 95 feet for the rockets using motors up to "H" that connects to four pad boxes via lamp cord wires so that the total distance to the pads is 100 feet from the LCO table. A second distribution box is set at 200 feet to connect up to four more pad boxes for rockets using up to "K" motors. The system has been used for more distant pads up to 400 feet from the LCO table. The system is designed to operate properly up to 1000 feet away. The distribution box and pad boxes are equipped with banana plug type connectors for easy and reliable assembly and disassembly.

The PAD BOXES use a 30 amp rated 12vdc automotive relay to provide power from the ignition battery to the igniter when the launch button is pushed. A DPST switch is wired as an interlock for both the control and power circuits. A 20 amp fuse is wired in series with the igniter to provide over current protection of the pad box wiring. This is particularly important in situations where the igniter clips short together while the launch button is still depressed. A typical ignition battery can provide plenty of power to cause the igniter clips to melt and potentially cause a fire. The fuse prevents this from happening. A piezo electric buzzer is wired in parallel with the igniter through a push button switch and a 9vdc battery to provide CONTINUITY CHECKING. The piezo buzzer limits the current through the igniter to no more than 8ma and has proven to be flash bulb safe. The continuity checker may also be used to test for blown fuses by shorting the igniter clips together. 16 gauge lamp cord is used for both the connections to the ignition battery and igniter clips. Size the length of these wires so that one ignition battery may be shared between two pads and so that the igniter leads are long enough to provide a buffer space between the pad box and the launcher during continuity tests. Always inform others in the area before you are conduct a continuity test and face away from the pad while doing so. We generally use leads 8 feet long and place our pads 10 feet apart.

BATTERIES used for the control power and ignition may be a variety of 12vdc types. Sealed gel cell lead acid are convenient and can be purchased in a large number of sizes. They tend to be somewhat expensive though. The batteries we use are made for lawn tractor use and can be found everywhere. WAL- Mart sells them for about $20. They have proven to be very reliable and powerful. Typical short circuit currents of 500+ amps are possible. You will need one for the LCO panel and one for every two pad boxes.

Part List
bat 1: 9vdc radio battery and clip, Radio Shack #23-583 and #270-324.
bat 2 and 3: 12vdc lawn tractor battery or equivalent.
B1: Piezo buzzer, Radio Shack #273-060 (CAUTION: Do Not use a mechanical type buzzer as they can cause excessive current to be drawn through the igniter).
S1: DPST 20 amp rated toggle switch, Radio Shack #275-710.
S2 and S4: SPST momentary push button, Radio Shack #275-011.
S3: SPST key lock switch 10 amp rated, (a 1/4 inch mono phono jack and shorted plug can be substituted).
S5 through Sn: SPST 5 amp rated toggle switches, Radio Shack # 275-603.
F1: 20 amp automotive fuse and panel mount holder, Radio Shack #270-1074 and #270-367.
R1: 30 amp automotive relay, Radio Shack #275-226.
Plugs and jacks: Standard banana type, Radio Shack #274-725 and # 274-717, 3/4 inch spacing between jacks.
Leads between distribution boxes and pad boxes, igniter and ignition battery leads: 16 gauge stranded lamp or speaker cord, Radio Shack #278-1267 or equivalent.
Pad and distribution boxes: 6 1/4x 3 3/4 x 2 plastic with metal top panel, Radio Shack #270-627.
Cable between LCO panel and distribution boxes: Use a minimum of 24 gauge stranded wire. Use one conductor for each pad box plus common. Only one common is required to each distribution box.. Three conductors will be needed for a two pad setup, Radio Shack #278-871 or equivalent.
Battery clips: Radio Shack # 270-343.
Igniter clips: Radio Shack #270-380 or #270-373 as preferred.
LCO panel Box: Radio Shack #270-216 or equivalent.

Smart shopping and scrounging through used or surplus electronics supply houses can save your club a bundle when constructing this project. Just be sure that the parts carry an equivalent rating to those called for in the parts list and always allow for room in the various boxes for future growth.