Launch Controller questions

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.


Well-Known Member
Apr 13, 2014
Reaction score
I've been thinking about a simple, multipad launch controller.
I've drawn a circuit diagram and I'm after a bit of advice.

1st, will this work?!
2nd, what size battery should I use for the continuity circuit?
3rd and final, anything else i should consider?

A couple of comments about your design...

The led's in series with the ignitors will reduce the voltage and current to the ignitors. Not good,

I think the diode across the positive terminal will not work. Once you closed the safety key all selected pads would launch...without warning. Bad thing.

The safety switch would have to be able to handle the current for all of the ignitors. To be safe, they would have to have at least a 20 amp DC rating. Most RS stuff is 3 amp DC rating.

Here is a web page for a controller that might fit the bill better. You would still need heavy dudy switches, but this design is much more reliable...since it only launches 1 pad at a time, RS switches would be no problem.

Personally, I prefer relay launchers. The High current stuff stays at the launch pad, and the low current stuff is in the controller unit. Here is the circuit I use with my gear.

I have only built the 1-bay version, and it reliably launches 3~4 motor estes clusters, copperheads, and the packed e-matches with RMS reloads. The relay box uses 30A automotive relays and I use a 12 Volt garden tractor battery at the pad. A 12 "AA" pack or a couple of 6V square flashlight batteries power the controller.

Just be sure to hook a diode in reverse polarity across the relay coil to damp back emf.

Good luck on your project!!
Heres a design for my Heavy duty relay launcher with 40amp contacts it will handle any cluster I'd like to put on it. It also is an single pad launcher but it's been working strong for about 15 years:D
here the diagram for my 12/24 volt 17pad master controller that has a lot of extra features you wouldn't need, but the controller circuit works very well. Hope this helps more then the relay circuit;)
Originally posted by womblegs
I've been thinking about a simple, multipad launch controller.
I've drawn a circuit diagram and I'm after a bit of advice.

1st, will this work?!
2nd, what size battery should I use for the continuity circuit?
3rd and final, anything else i should consider?


As drawn its not going to work. Depending on the current limiting provided by the LED's it may fire all selected pads when you push the check button. Not a good thing.

A nice simple relay based launcher would work well. A good place to find components is All Electronics.

ROL's Info Central has some lauch control designs on line. Its not too hard to come up with one. I designed the units used at LDRS 23. Good luck.

ROL is a good spot for information. I have learned a lot from that site.
The circuit initially proposed does indeed have some problems.

I'm not sure what the + and - in the middle of the diagram represent, unless this is intended to indicate an internal battery (bottom) and jacks for an external battery (middle).

12 volts going through 2 diodes might or might not work, depending on the igniters and the diodes. Silicon diodes (the most common) will have a voltage drop of about 0.7 volts per diode. (Germanium diodes drop the voltage by about 0.3 volts each.) Thus, the voltage available to the igniters (assuming no voltage drop in the wiring) would be about 10.6 volts. Most of the igniters I've seen will fire with 10.6 volts applied as long as the current supplied is sufficient. That's where the resistance of the components (wiring, switches, internal resistance of the battery, etc.) come back into play. Lead-acid batteries have low internal resistance. Heavier wire has lower resistance. The diodes would have to be considered, not from a resistance perspective per se, but in terms of the current flowing through multiplied by the 0.7 volts. If the power dissipated is too large, they'll burn, probably cutting the current.

Also note that the 12 volt battery won't provide current to the pad select LEDs unless the launch button is pressed. The other set of + and - terminals are not parallel to the 12 volt battery, as the switches aren't the same for each. The safety switch should be the one directly next to the power source. Etc.

Best to start with (or just use) one of the simpler designs posted on Rocketry On Line (ROL) or elsewhere. ROL is great. Simplicity is a virtue in electronic design. The fewer components, the less that can (and inveitably will) go wrong. A simple user interface makes for fewer human errors too. Murphy was a EE who sidelined as a rocket scientist. :D
Thanks for all you help guys. In answer to a couple of questions, the + / - without a voltage label is for the continuity test, I was unsure as to what size battery to use for it. I am using Estes / Quest ignitors.
The diode by the 12v cell is to stop current flow from the continuity battery to the 12v cell.
A good design generally uses the same battery for the continuity check as for the firing circuit, since that's the circuit you need to verify continuity for. A relay system with a high current circuit at the pad and a low current circuit running from the launch controller to the pad will sometimes use two batteries, and "continuity check" can be construed in several ways. Still, good design will have a continuity check (if one is included) arranged to test as much of the firing circuit as possible, which implies using the same battery(ies).

As the 12-volt source is likely to be the higher voltage in the system, it's the other battery that would need the protection. It wouldn't be a good design to wire them side by side (not quite parallel) like this though, and as stated above, you don't really need or want two batteries here.

Estes and Quest igniters don't really require a 12-volt system unless you're doing clusters, but using a 12 volt a gell-cell or similar will give you a good reliable firing voltage, providing the current needed, with room for growth to Copperheads or clusters.

Before you work with higher current systems though, learn about relays and diode protection against the surge voltage that occurs when the coil is de-energised though. Even though the battery is 12-volts, the voltage across the relay coil can spike very high and run back into the hand unit if the circuit isn't right. I've seen a lot of proposed designs that I'd want a wooden pole and lineman's gloves before I'd use.

Before you do clusters, learn about clip whips and parallel wiring of igniters. Learn about good solid connections for each clip. Learn why these things are important and what can happen when only part of a cluster ignites.

Browse around and look at launch controller designs. Start with Rocketry On Line's site, and do a Google search too. Ignore complex designs with bells and whistles. Your first design probably ought to even be single-pad. Murphy's Law really does apply here. I'm an electrical engineer, and I follow the same advice I'm giving here. Feature-larded systems may be fun to play with at first, but they tend to be maintanance hogs, distracting from the rocketry. Before long, you'll want a basic Estes launcher, and the whiz-bang job will wind up collecting dust in the garage.