Dual deployment and recovery systems _ Ematch Ignition Design

I was chatting with @drid and @manthos nthos at OSCW18 about the HPR rocket telemetry boards LSF had done and discussing further development. One thing I suggested was the addition of deployment circuits. For those new to rocketry “deployment” usually refers to the stuff the rocket airframe releases to recover itself, often parachutes… but sometimes payloads etc.

In simpler rockets, fired with a solid rocket motor, the motor contains a timed charge that detonates at a defined time after motor burnout and is often adjusted to coincide with or be just after apogee. This charge blows into the air-frame and pressurises it blowing of the nosecone and therefore pushing out the parachute. This is a great system and pretty foolproof with one notable issue…if your rocket goes very high and there is a slight side wind deploying a parachute at apogee may mean that your rocket lands a looooooooong way away.

So as high power rockets increase in altitude, avionics systems are used to deploy recovery items in a sequence… so often a rocket deploys a small drogue at apogee which slows the rockets descent a little but allows it to still fall quite quickly and then when it returns to a predefined altitude the avionics deploy a second charge to split another part of the air-frame releasing a main chute reducing the airspeed to a safe landing velocity.

The deployment charges are often small amounts of black powder or pyrodex and packed in a small well with an ematch/ignitor. I suggest that future avionics projects might include some driver circuits to have deployment channels available. Deployment channels are sometimes also used for air-starts of second or third stages in solid rocket motor multi stage vehicles and also some commercial altimeter/avionics board have the deployment channels usable to drive servos for mechanical deployment projects. It’s also considered good practice for recovery deployment to have redundancies, sometimes this is as simple as having 2 deployment charges on the avionics but also having the solid rocket motor ejection charge as a backup, through to running an entirely separate avionics stack with its own charges set to deploy 1 second after the main system… if all has gone well these will then just emit a small bit of gas into an open section that has already deployed.

So humbly and nervously (My scant electronics knowledge is very home learnt!) I’m chucking up this schematic of a circuit I’ve used to ignite ematches. Its basically a transistor (I chose the IRF540n as it is used on lots of commercial altimeters). So the 4.7r resistor limits the charge current supplied to the capacitor and the charge time is estimated with t in seconds = 5RC with R in ohms and C in farads. So in the example of my circuit running of a 2C lipo at 7.4v the charge time for the capacitor is under 0.02 seconds. The resistor between gate and source pulls the transistor low making sure the transistor is off until deployed by sending the GPIO input high. This circuit will then push around 1.5 amps through the ematch/ignitor which is way more than most commercially ignitors need to fire. Most deploying altimeters switch the channel live for over half a second with many leaving the circuit open up to 2 seconds (some like the eggtimer quantum allow the fire time to be adjusted).

So perhaps this is useful … if only for people to read a bit about how deployment in rockets happens. Totally up for questions!

ematch_schema.pdf (13.3 KB)

Hey @concretedog. Nice introduction to pyrotechnics. The think that I don’t understand is the usage of the capacitor and the resistor. Why you dont wire the ematch directly with the battery?

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This is based on using a single battery to power the avionics/altimeter and also the ematch ignitor channel. Without using the RC and charging the capacitor you will just brownout the power and it will starve the electronics. The power for the ematch is taken direct from the battery whereas the main board may be regulated off the battery.

OK, that’s makes sense. Thanks.

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Me?.. Make sense?.. Come on! :slight_smile: