Electronics Module 3

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This electronics package, designated EM-3 is the electronics to replace the altimeter lost in Launch Test 61. While not giving up on altimeters, this unit uses dual deployment via three separate timers. One timer to deploy the drogue chute at apogee, another backup timer for the drogue, and a high amp timer to deploy the main chute by means of the PIRM2. In the future I will likely replace one of the timers with an altimeter, and use the remaining two timers in back up mode.

There is a main power switch to "power on" the module once the rocket is on the pad. The timers will be started by magnetic reed switches. One magnetic switch will be wired through a 10 amp relay, the relay will power the primary drogue timer and the PIRM2 main parachute deployment. The second magnetic switch will go directly from the main power switch to the backup drogue timer.

The upper and lower bulkheads are 1/2" oak plywood. The vertical mounting board is 1/4" birch plywood. The mounting board is retained by an aluminum angle bracket at each end. For support two 1/4" threaded rods run the length of the board. The module will drop in from the top of the upper body tube, and is retained by a 5/16" steel bolt from a bulkhead in the upper body tube.

Notice the aluminum tube extending from the top of the module. That is an extension for the drogue deployment charge. Since both the main and drogue chutes will be packed into the same body tube, I needed to keep the deployment charge above the main parachute. The extension tube is epoxied in place, and is also prevented from pushing into the electronics module by the edge of the tube resting on the upper aluminum angle bracket.

Here's a look at the parts of the module labeled.

Here is the top end of the module. You can see the main harness attachment eyebolt, the PIRM and the deployment extension tube.

Here is the back side of the module. One more battery will be added before flying this module. I want some redundancy, and by paralleling two batteries I'll also double the amps available. There is also room on the back side for more electronics, probably a transmitter. You can see the back side of the main power switch below the battery. The switch is rated at 12 volts and 20 amps. Just below the switch you can see the nut that retains the module in the rocket. It's counter sunk slightly, and epoxied in place.

Update: After Launch Test 80 I discovered there was a little problem in the way I had the timers wired. I had all the timers running from one power supply, and as soon as the first timer timed out, it would draw so much current it would drop the battery voltage to the point where the other timers would sense the voltage drop. When they sensed the voltage drop it was the same as if they had timed out, and all the timers would then fire at the same time. Which of course meant the main chute timer to the PIRM2 would fire at apogee rather than at a later time like it should.

The fix was easy, I simply used a single throw double pole switch, and wired the apogee timers on one battery circuit, and the main timer on another battery.

I should also mention the relay isn't used. I decided to take it out before I even flew this module the first time. The magnetic reed switches can handle two amps each, and the timers never draw that much current. So the relay wasn't needed in the first place.