Eject, …, eject!

As mentioned last week, I’ve decided to go with a pressurized CO2 ejection system instead of the traditional black powder explosive charges. So, what is it all about? Let’s see why you need an ejection system in the first place. The goal is to eject one or two parachutes at set altitude(s), for a controlled and gentle return to the ground of the rocket and its payload. In theory. Let’s take the case of a single (parachute) deployment scheme. At apogee, when the forces are minimal, the parachute is ejected. Under minimal stress, it can deploy and eventually allowing the smooth landing of the rocket. With a dual system, instead of one parachute deployed at apogee, two parachutes are deployed. At apogee, a small one (drogue), allows a controlled and rapid descent to avoid the drift of the rocket due to altitude winds. Then, at a set altitude, let’s say 1000ft, the main parachute is deployed. The flight computer is used to detect these altitudes using a barometric altimeter and to eject the chutes. It is during the ejection process that the black powder gets into the picture. Indeed, the goal is to pressurize the inside of the rocket so it disassembles and frees the parachute.

The pressurization is achieved using an electric match – ignited by the flight computer and its battery – that explodes a small charge. The rapid – explosive – expansion of the gases does the magic! With a pressurized CO2 ejection system, the explosive charge is minimal, so it can push a piston to pierce a canister of pressurized CO2. It is the rapid decompression of the CO2 that pressurizes the inside of the rocket. There are two main reasons why you would go with such a system (vs. the simplicity of the black powder charges). First, you avoid the corrosive action of the sulfur that deposits on everything inside the rocket during the explosion. Second, the higher the rocket goes, the harder it is to ignite/burn the black powder because of the oxygen rarefaction (~40k+ft). And I throw one more good reason: it is cool (literally :-), so you will not burn your chute). If you look at the pictures, the aluminum casing (red) contains the electric match holder (white) and the plunger (metallic) that pierces the CO2 canister (with the warning label). The casing has holes to release the pyrogen gas, and on its base (close to the CO2 canister and to the mounting flange (gold)), few bigger holes to release the CO2. The flange is mounted onto the bulkhead of the electronic bay. The CO2 canister is sitting inside the bay, and the casing outside the bay, toward the parachute. Now you probably see why I had to plan to move the telemetry platform into the nosecone to free up space for the canisters in the bay. Finally, the size of the CO2 canister is picked based on the diameter and the length of the rocket. Enjoy the WE!