Custom Rocket Launch Controller

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I built my own launch system to have benefits over the stock controller included in the Estes kit.

Custom model rocket launcher front view.

• A key-based switch so that I can take the key to the launch pad without worrying about the kids accidentally launching the rocket. (Upon further consideration, it is unlikely that the anything the kids have access to could substitute for an Estes peg safety key, since it would need to be conductive, non-tapered, and about the size of the hole.)
• Connects to an external rechargeable lead-acid gel cell (or AGM) to increase the number of launches without consuming alkaline batteries. (Upon further consideration, rechargeable NiMH batteries probably would work fine in the Estes controller without being damaged by a dozen high-drain launches per day.)
• Heavy-duty cable to deliver a larger percentage of the energy to the igniter.
• Power through a relay rather than through the switches to deliver a larger percentage of the energy to the igniter and to increase the selection of usable switches (since current-carrying capacity would no longer be boundary).
• More rugged.
• Power indicator.
• Detects open-circuit and short-circuit.
• Buzzer to prevent silent countdowns.
• Microcontroller safety features such as requiring a countdown, requiring a specific switch order, and time-limit on igniter power to avoid short-circuit damage.
• Cool-looking switches. More fun for the kids.

In fact, this project may have started with nothing more than the desire to create a fancy box with Hollywood movie switches.

The igniter arming control is a covered pilot toggle switch.

Yes, the tip of the switch is a blinking red LED! (You can read more about the pilot switch in a related article.)

Rear and Interior

The rear of the rocket launcher box has four banana jacks and a buzzer.

Rear of custom rocket launch controller with jacks and buzzer.

The igniter is not polarized, so either wire from the cable can be connected to either jack. In the event that I come across a polarized igniter, the orange jack is positive and the brown jack is ground.

The custom controller can operate from between 6 VDC and 20 VDC. That spans the range of 4 AAs, 2 LiPoly, and up to a standard motorcycle or car battery. Because the custom controller contains many active electronics, it won’t operate unless the positive lead of the battery connects to the red jack, and ground to the black jack. However, the circuit is protected such that nothing will happen if the user connects the cables incorrectly.

Rocket launcher circuit board installed in aluminum case.

Although there appears to be plenty of room on the inside, the switches on the top of the case are fairly deep, and their cables and interconnects take up a lot of room.

Because the case material is electrically conductive and the controller can be connected to a potentially lethal power source (automotive battery), I made sure none of the electrical connections made contact with the case. Even the circuit board is attached using nylon screws. The anodized surface is an electrical insulator, so even a stray wire internal wire is unlikely affect the user. Regardless, I wouldn’t use this in the rain or near a body of water.

The nice thing about using an external battery with removable cables is that the launch system can be disabled by pulling a cable if necessary.

Side Views

The sides of the custom launch controller case have instructions and status information.

Side of rocket launcher.

The text is printed on a laser printer onto full sheet stickers (Avery 8665). Unfortunately, the stickers aren’t completely clear after being applied, large areas are prone to bubbles, and the laser toner flakes off with usage.

Other side of rocket launcher.

Launch Sequence

The custom launch controller must be operated in a very specific way, or else it aborts the launch until the key is returned to the safe position.

Launch control sequence.

1. Power on (the switch illuminates)
2. Insert the key in the Lockout switch in the safe position and turn until the key is vertical. You cannot remove the key in this position. So, if you walk away with the key, you know the launch controller must be in the safe position.
3. Test the igniter by flipping up the Test Igniter switch. If the igniter is good, the status light turns green and the Arm switch will light up red.
4. Lift the Arm switch cover and flip the Arm switch to the up position. A five second countdown begins. The launch countdown is accompanied by a buzzer. Thus, you cannot quietly initiate a launch.
5. When the five second countdown is complete, the launch button ring lights up as a rapidly flashing blue. You can continue your countdown if a longer countdown is desired. (You have up to twenty-five seconds to press the launch button.) Pressing the launch button powers the igniter for 200 milliseconds, regardless of whether you continue to hold down the launch button for a longer period.

If you toggle any switch or press any button prematurely (or out of sequence), or if the continuity test fails at any point after being armed, the status light blinks red and the launch is aborted. The same is true if there is a power loss or microcontroller reset. In other words, any unexpected condition puts the launch controller into an error state that requires the lockout switch returned to safe mode.

I was concerned that the launch sequence would be difficult for the kids to learn. But, they memorized it almost instantly. In fact, they narrated as they went: “Power on! Lockout released! Testing continuity! Rocket armed! Countdown begins...”

Let’s take a closer look at the circuitry...