Launch Controller Design

Motor starters (igniters)

The purpose of a launch controller is to ignite a motor starter, and thus ignite a motor. There are several different kinds of starters, but they are fairly similar in electrical characteristics. When designing a launch controller, you will need to keep these in mind.

Nominal resistance is just the resistance of the starter, including its leads. No-fire current is the maximum current that can be passed through the starter (to check continuity) when ignition is not intended. All-fire current is the minimum current that should be passed through the starter when ignition is intended. Therefore, the continuity circuit must deliver less than the no-fire current to all starter types, and the ignition circuit must deliver more than the all-fire current to all starter types.

There are still lots of blank cells in the table below - if you know where to find data on these, or would like to do some testing on your own and report your results, please contact Danno.

Starter Type	Nominal resistance	No-fire current	All-fire current
AeroTech Copperhead
AeroTech First Fire
AeroTech First Fire Jr.
AeroTech First Fire Mini
Estes Model Rocket Starter	0.5 - 2 ohms		1 amp
Estes Sonic	1.6 ohms	1.25 amps	3.80 amps
Firewire	0.8 - 1.2 ohms	0.3 amps (less than 0.04 amps recommended)	0.6 amps (1 amp recommended)
Firewire Mini	0.8 - 1.2 ohms	0.3 amps (less than 0.04 amps recommended)	0.6 amps (1 amp recommended)
MJG e-match (dip-your-own)	0.8 - 1.3 ohms	0.3 amps	0.7 amps
Quest First Fire Micro
Quickburst Twiggy	1 - 2 ohms
Quickburst Slim Gem	1 - 2 ohms
Quickburst Fat Boy	1 - 2 ohms
Quickburst Super Fat Boy
Wildman Big-Uns
Wildman Little-Uns
Wildman Tiny

Note that proper continuity circuit design is much more critical from a safety standpoint; too little current from the ignition circuit just means the rocket doesn't launch, but too much current from the continuity circuit can cause unintended ignition! In general, test current should be the minimum required to activate whatever lamp or sound device you're using to signal continuity (for instance, this LED specifies 30 mA maximum, and this alarm element only requires 7 mA). You should really never be getting anywhere close to the no-fire limits listed above.

The maximum firing current you can deliver will probably have more to do with the characteristics of the battery being used than the design of the ignition circuit. Generally speaking, batteries with larger cells can deliver more current. This makes standard nine-volt batteries the least desirable choice, as they are actually six tiny 1.5 volt cells connected in series; a pack of six AA batteries, although larger and heavier, will be able to push more current to your igniter. At club launches, where many rockets will be launched over the course of a day, it is common to see a launch controller hooked up to a large 12V battery such as a car battery or lawn tractor battery. (TODO: try to compile some data about different types of batteries.)

Basic Controller Design

Fig. 1. Simple launch controller circuit.

At its simplest, a safety-code compliant launch controller will consist of the components shown above. A battery is connected first to a safety interlock switch, sometimes called a safety key. This does not have to be a literal key (although it can be), but it must be something that can be physically removed from the launch controller to render the circuit safe with zero chance of an unintended ignition. The handheld controller which comes with Estes starter kits uses a small pin that is inserted in a hole to arm the system and removed to make it safe. In Days of Yore, this safety key would be tethered to the plastic cap intended to keep you from poking your eye out on the launch rod, so you couldn't forget to remove that cap before launch; Estes seems to no longer do this, though.

With the safety switch closed, the continuity circuit is active. Current from the battery flows through the closed safety switch but not through the launch button (since we're not pushing the button yet), causing it to instead flow through the LED and resistor R1. The resistance of R1 is chosen based on the battery voltage to restrict the continuity current to an appropriate value for the LED and to prevent the current from exceeding the no-fire limit of any igniter which may be connected (see above). Finally, a second resistor may be used to restrict the launch current, if necessary to avoid damaging the battery. When the launch button is pressed, the resistance of the circuit becomes the sum of R2, the resistance of the igniter, the resistance of the wiring and the internal resistance of the battery, which may only add up to a few ohms, so a 2 or 3 ohm resistor here can be a significant control on the current.

Ohm's Law

The foundation of circuit design is Ohm's Law, which states that the voltage across a resistor (measured in volts) is equal to the resistance (measured in ohms) multiplied by the current (measured in amperes, or amps). This is frequently written as V = I x R, but for this purpose we will rearrange it as R = V / I. Given a certain voltage provided to the circuit and a certain current that is desired, what resistance must be used?

This equation must be solved twice - once for the launch circuit and once for the continuity circuit. The launch circuit only has one variable, so that should be done first:

R2 + R_wire + R_igniter = V_battery / I

Looking at the table above, we'll estimate 1 ohm for the igniter and aim for a launch current of 2 amps. (Data sheets for common alkaline batteries seem to be reluctant to publish a maximum current value, but random Googling is telling me that two or three amps is not unreasonable for regular AA cells. A TODO for this page is finding better battery data.) In this example, the measured resistance of the wire (30 feet of audio speaker wire) is approximately 1 ohm and the battery used is a pack of six AA cells in series, nominally 9 volts but with a measured voltage of 9.5 volts when brand new. The equation for the launch circuit is:

R2 + R_wire + R_igniter = V_battery / I

R2 + 1 + 1 = 9.5 / 2

R2 = 2.8 ohms

As the designer, you can decide whether to round that value up or down, but it's good practice to plug the actual value back into the equation to make sure the current is still reasonable. Rounding down to a 2 ohm resistor will increase the current, so let's try that first:

2 + 1 + 1 = 9.5 / I

2.4 amps = I

That seems reasonable, especially since the voltage may droop a bit as the batteries run down, so we'll stick with R2 = 2 ohms.

A slight complication for the continuity circuit calculation is the fact that the LED in the continuity circuit, like all diodes, does not behave like a resistor. Instead, it is accounted for as a constant voltage drop regardless of the amount of current delivered to it (within reason). The voltage and current values will be found on the data sheet for the LED. For instance, the data sheet for the LED used in the example circuit gives the "forward voltage" as 2.1 V typical, 2.6 V maximum, and the ideal drive current as 30 mA maximum. The equation for this continuity circuit is:

R1 + R_wire + R_igniter + R2 = V_battery / I

Now, the desired current is 0.03 A (30 milliamps) and we already know the value of R2:

R1 + 1 + 1 + 2 = 9.5 / 0.03

R1 = 310 ohms

As you can see, the 4 ohm resistance of the launch circuit is not significant compared to the resistor needed for the continuity circuit. This can be rounded off to 300 ohms, which is a commonly available resistor size.

Congratulations! You just designed a launch controller.

Advanced Controller Design

TODO