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A simple single-pad Launch Controller for HPR rockets Print
Written by Tokind   
Thursday, 11 November 2010 20:34

I showed up at a launch once, built an “I” motor, and then discovered that there was no remote controller for an HPR rocket. The experience and the results were very unpleasant. See Last Flight of BURT.

My wife and I decided on the drive home that I was going to build our own controller. Of course, the secret of a controller for launching a high power rocket is that the pad must be 100 to 1500 ft. distant depending on the size and number of motors. The battery and switch which light the igniter cannot run through this length of wire. The current loss is too great. You have to have a relay out at the pad next to your battery.

Assembled Controller

In itself, this would be pretty straightforward. But there are a few requirements, and a few nice features, that you would want to have. The most important requirement is a way to disable the launch power at the pad and also at the console. You do not want to be connecting an igniter if there is any current on hand to set the rocket off in your face. It is nice to have a way to test the continuity of the hookup to the igniter before you walk all of the way back to the launch console. It is also nice to be able to test continuity from the console before you commit to a countdown. The wind may have disconnected something on your walk back.

I also wanted to add a warning feature, a pulsing beeper or siren, to warn that the pad is armed to fire. If someone were to enable the console while you were away, you would hear a warning as soon as you threw the enable switch at the pad. The warning beeper also encourages the user or nearby helper to disable the console immediately after the rocket is launched. This saves a bit on battery life.

I splurged on the connectors. I only needed four wires to do this design, and the wires to the console only needed to carry low current – but I could not resist ordering very durable and color coordinated connectors and plugs. You might easily shave $50. off of the electrical parts order by selecting, say, RJ45 connectors, which are used for Ethernet.

Of course I wanted to make this all very neat, and have around 300 ft. of cable. I am not very likely to ever launch anything bigger than a K motor. With this in mind – this article will explain how I built this system. At the end of the article you will see a link to download a complete PDF plan which includes a schematic, parts list, and illustrations.

All told, I spent around $400, but this also included a nice launch base built on a heavy-duty folding tripod speaker stand, and cable reel. If you think about it, it is not very difficult to spend this kind of money on a good day launching high power rockets.

In addition to the parts listed on the DigiKey order, I purchased Automotive spade plugs and several colors of 16 gauge stranded wire from a nearby Auto Zone. I also happened to have several hundred feet of Ethernet cable on hand. For the igniter clips and battery lines I tore apart an old unused battery charger and used the professional quality molded cord through-ways. Cutting the edges of the pad box to accommodate these proved to be very easy to do, and gave me a great looking result.

Inside the controller

There are two forms of connection used in this work. The crimp-on spade connectors are used for the high current connections: the circuit breaker and the relays, which are automotive type devices which may have to be replaced some time. On the beepers, plugs, and switches I used solder joints. You will also notice a tiny copper wire ring where I attached ground connections to the common ground.

I really liked these little project boxes. Not only are they the right size, but they are molded with card-slot ribs on the inside. This allowed me to cut out pre-perforated fiberglass blanks on which to mount the beepers and relays. The beepers are inserted through the holes in the perf. board and soldered into place with 26 gauge stranded wire. The relays which, as I suggested may need to be replaced some time, are attached to a perf. board with double-sided tape, and then strapped in with solid wire to hold them firmly in place (see the illustrations in the download package.)

In the schematic diagram you will see that the high-current circuit (the one that lights the igniter) is shown with a heavier black line than is the rest of the wiring. You should be sure and use the 18 gauge or heavier wire for these connections. In fact, if you want to light clusters or very high-current igniters, you may choose to buy a somewhat larger pad box and 14 gauge or even 12 gauge wire for the high-current path.

Thomas and his creation

The author with completed launch base, controller, and battery. The inset shows the mounted drill chuck which secures launch rods up to 3/4”. In this picture the launch rail is mounted.

Theory of operation

Referring to the schematic diagram, Relay R1 supplies 12vdc to the pad continuity beeper PZ1, which is connected to S2 (continuity). When you press S2 with the igniter clips shorted, or with an igniter connected, the path to ground will cause current to flow through the beeper which generates an audible tone. If the ignitor is not well connected there is no path to ground and no indication of continuity. The piezoelectric buzzers have the characteristic of limiting current to less than 20ma, which is safe for all current-driven igniters used in our hobby.

Likewise, PZ2 and S3 on the Console unit form the same circuit and perform the same function through pins 2 and 6 on the jacks. Note that continuity may be tested at the pad regardless of the state of S1 (safe).


S1 is the pad-side safety disconnect. Relay R2, the launch relay, will not engage unless S1 is closed (arm position) as there is no voltage available. The pad-side safety is a required feature, and this one works especially well: if  S4 (enable) at the console were on, switching S1 (safe) to the Arm position provides an immediate beeping alert through PZ3 that the pad is armed. The voltage to do this is supplied by pin 87a of R2 (launch relay) when R1 (enable) is energized by S4, and S1 (safe) is closed.

The alert siren is normally activated by S4 only when S1 (safe) has been closed and S4 (enable) is closed after walking from the pad to the console. At this time, pressing S5 (launch) on the console will create a direct path between +12vdc and ground through R1, R2, and the igniter which results in ignition.


Last weekend I had an opportunity to support a rather large group launch. I was pleased and proud that my new controller worked exactly as expected. My buddy Russell asked if I would like to burn in the new setup with a larger rocket launching on a G40. That was fun. It lit right away, flew straight and true, and now my blast deflector is very clearly an experienced one.

You may download the plans with parts list, theory, and illustrations at

Last Updated on Wednesday, 24 November 2010 10:17

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