I have been thinking about making a pyro release mechanism (PRM) for some time. It just seems to make sense. Release the drogue through the upper body tube in the conventional manner, then have the drogue attached to the main parachute, only held in place mechanically until dual deployment is desired. The advantage is space savings in the rocket body, resulting in less weight and more room for payloads. The other advantage is a stronger body tube, not having to break the rocket at mid section will allow the two sections to be attached more securely.
I looked at a few designs I found on the web, most used a pin that was displaced by a pyro charge and released a key or pin that in turn would release a quick coupler or similar device. So I made one that did just that, a pyro charge moved a pin, releasing a key that held a quick coupler. It worked, but I really didn't like it. It was very time consuming to make, and required a lot of machining. Then I had a couple of other issues with it, it needed a clean charge or any foreign material may jam the key. The pin also needed a tether of its own to keep it from being lost, it worried me that the pin and tether may damage the body tube upon firing, or tangle in a deployment line. I was concerned that if I used a captured pin design, it could bounce back before the key came out.
I had a lot of rather complex ideas for other designs, but they all required a lot of machine work too. So I sat down to think, let's start from scratch, and try to keep it very simple and very reliable.
Here's what I came up with. It's probably not very original, and I'm sure someone has done this before, but it was at least an original idea for me, not having seen it used before. I decided to use nylon screws to shear, just like we do on nose cones or mid body separations. Now you may be thinking they won't hold, but remember, it only has to hold the drogue chute, not the main chute. And the worst that can happen in the event of an early or late deployment is the screws shear and deploy the main early, in the process absorbing some of the kinetic energy of the high speed deployment.
Here are the main components of the PIRM (Pyro Initiated Release Mechanism).
Top left is the body of the PIRM, bottom left is the bottom closure for the body, to the right is the piston with the eye bolt that gets released.
Here you can see the bottom closure installed. I drilled and tapped four holes through the body into the bulkhead and installed four #6-32x1/2 machine bolts. I drilled out a 5/16" hole through the bulkhead to mount the PIRM to a rocket bulkhead.
A small hole was drilled to allow the initiation charge to be installed in the chamber. The hole was drilled at an angle to make inserting the wires easier.
Two holes were drilled and tapped to #4-40. The nylon screws rest in a groove turned into the piston. Another groove holds an o-ring to seal the chamber.
Some of the benefits of this design.
When the charge goes off, the piston is ejected in the right direction, right out the open nose end of the rocket.
Reloading is fast and easy.
Most any type of initiator can be used, e-match, flash bulb or in my case Xmas bulb soda straw deployment charge.
Fairly small and light. I made this one fairly big, it's 1.75" with 5/16" bolts. But it could be made a lot smaller.
It's easily scalable. You could use more shear screws for a larger rocket, of course a larger deployment charge would be needed too.
It should be very reliable and almost fool proof. So long as the proper charge is installed, it should deploy.
I've done a couple of tests so far, it looks like about .3 to .4 gram deployment charge should be plenty. The nylon screws shear at around 50 pounds plus for each one. That gives me over 100 pounds of holding force, plus the piston o-ring resistance. I suppose I should find a formula to see at what speed a small 20" drogue chute would reach 100 pounds of drag. I can only imagine it would be very fast, and probably result in damage to the chute before the pins sheared. A few more ground tests and I'll have to launch an old, "low and slow" rocket to test the PIRM.
I tried testing the PIRM in an old PVC rocket, and one thing became quickly apparent. The PIRM was way too big for the small 3" diameter rocket. That, and the fact that the plug on the PIRM was large enough I was worried it would damage the parachute when it went off, I tried it anyway in LT-68. I wasn't too optimistic anything would work, as I really had to pack things into the upper body tube to get it all to fit. I used a very small shock cord, 1/8" tubular kevlar to maximize the room in the rocket. Problem was, when the drogue deployed it tangled in the thin kevlar line, and the rocket landed hard before the timer had a chance to deploy the main. So the PIRM never really got tested, but it started me thinking again.
So I went back the original PIRM I had built, looking at its benefits vs. drawbacks. Two things had to be addressed.
First, I needed a clean deployment charge that wouldn't foul the release
mechanism. That meant I would need a new way to initiate the deployment charge using my timers. To that end I ordered
a spool of 40 gauge ni-chrome wire. I was hopeful the ni-chrome wire would fire at low amps, but testing indicated
I still needed at least 1.5 amps for quick firing. So a new timer with a high amp output would be needed, as the
timers I have been using only put out .2 amps. I'll document the high amp timer in the electronics section.
Second, I needed to shorten it up a bit to get it into a reasonable size rocket body. So I removed the pipe thread
fitting on the charge end, and shortened up the body of the PIRM. Then drilled out a small hole for the initiator
wires, and fabricated an end plate to seal the charge end of the PIRM.
Here's my modified, finished PIRM.
Here's a side view.
Here it is unassembled.
I think you get the idea of how it works from the pictures. It's not a thing of beauty that's for sure. I don't have a milling machine. So I cast the body in aluminum using my home foundry, and had to do the cutting with whatever tools I had in the shop. It's about 1.125" square by 1.4" long. The ejection pin is 1" long.
After I received my ni-chrome wire, I made up a bunch of small igniters with a single thin coat of pyrogen. The igniter is inserted into the PIRM from the open charge chamber, then the black powder charge is added and the access plate installed. A few tests indicated about .35 grams of 2F black powder did the trick. To make measuring the small amount of black powder easier, I made a small measure from a straw with one end sealed. One inch of straw equaled about .6 grams, and .6" of straw was about .35 grams.
I tested the PIRM several times using my launch controller, then several more times with my new high amp timer. I would put varying amounts of pull force on the release key by means of a quick coupler and a rope. In all cases, with either light force or heavy force on the key it released quickly and cleanly when the charge went off.
Click Here for a quick video of the PIRM2 firing in a test. The electronics module 3 is suspended by a rope and attached to the release key on the PIRM2. Notice how non violent the release is. The electronics module just falls away at release, the key and rope barely move.
Wyatt Harris did a Cad Drawing of a similar pyro release, thanks for the drawing!
