Deployment and safe recovery is possibly the most difficult aspect of
flying an experimental rocket. More rockets are lost due to failed recovery than any other reason, likely all other
reasons combined. At least in my case anyway :(
My early rockets used a variety of mechanical or chemical (burning) devices to initiate a charge of black powder to blow the nose cone and parachute out of the forward end of the rocket. Thus deploying the parachute for a safe landing. I finally settled on a home built timer, that resulted in very high rate of recovery success. Later, I added dual timers and dual (redundant) charges that brought the success rate to nearly 100%. Still, these were fairly low power rockets flying to fairly low altitudes and used a single parachute for recovery. As my rockets grew in size and power, dual deployment was needed. The concept of dual deployment is simple; release a small drogue parachute at apogee, then have a large main parachute deploy at a lower altitude for a soft landing. The whole purpose of dual deployment is of course, to keep the rocket from drifting too far from the launch pad.
My first attempt at dual deployment was in the Cosmo Rocket project. Apparently the rocket separated prematurely and there was no recovery at all. The problem with a timer is that everything must go exactly as planned. So with my next rocket I built an all fiberglass body and again used mid separation dual deployment. Only with this rocket I decided to use a Missile Works RRC2x altimeter. The altimeter takes some of the guess work out of deployment by sensing apogee using a barometric sensor. The rockets first flight went half as planned, the rocket separated at mid body and deployed the drogue chute. The altimeter fired the main ejection charge, but the nose cone shear pins did not shear. Luckily the drogue chute was a bit oversized as a precaution, and the rocket landed safely.
The next launch was not so fortunate. While both the apogee and main charges fired, there was no deployment due to an oversight on my part by leaving some holes open in the body tube, venting the ejection charge gases and not allowing separation.
I decided to move on to a different approach to dual deployment, see the PIRM page for details. The PIRM allows for both parachutes to be deployed from the nose cone upper body tube. There are inherent advantages.
In this launch I used the PRIM2 and the fiberglass rocket. Take a look at the launch page for my theory on why the deployment failed.
After giving the problem some more thought, I decided to use a piston behind the nose cone to hold and push out the parachute. Below is a sketch to give you a better idea of what I have in mind.
The drawing is the upper body tube, with all the deployment hardware from just above the electronics module.
The piston is cup shaped, with the cup open at the top next to the nose cone, the parachute is folded and placed inside the cup (piston). The piston is prevented from falling lower into the body tube by a small retainer lip (green) inside the body tube.
If you follow the pink line, (that's the shock cord), it starts at the nose cone where it is tied to an eyebolt in the nose cone. There is about 4 feet of shock cord between the nose cone and the where the shock cord ties to the parachute shroud lines (in yellow). Then about two more feet of slack and the shock cord passes through the center of the piston base, and is epoxied in place to keep the piston from sliding on the shock cord.
As the shock cord passes through and below the piston, there is another 10' of slack in the cord before it attaches to the PIRM2 release key. From the release key there is another couple of feet of slack in the cord before it attaches to the main parachute shroud lines. Then from the main parachute the shock cord has another 10' of slack before attaching to the main eye bolt in the bulkhead.
Here's how I think it will work. When the apogee deployment charge goes off, the nose cone and piston will be ejected basically as one unit. When the piston reaches the end of the shock cord, it will abruptly stop, the nose cone will now move away from the piston in a snapping action, pulling the drogue chute out of the piston and deploying it. Well, that's the plan anyway.
I think for the next series of tests I'll fly the rocket with a little less power. No sense in flying the rocket to 4,000' where I can't even see it. 1,000' should be plenty high enough. I think I'll do about .7 lbs of propellant in an unrestricted grain configuration. That should give me plenty of thrust off the pad to get the rocket into stable flight, keep the altitude down and allow me to use my existing motors and nozzles. Launch Test 79 will be the first test of this new design.
Update 8 May, 2005:
While I've have fairly good success with deployment using the PIRM2 in the above configuration. I have been having
some problems with the main chute, sometimes it comes out when the apogee chute is deployed, and once it remained
in the upper body tube and didn't deploy.
As for coming out too soon, that's was really because the main chute is too loose in the body tube. Either the shock cord friction pulls it out, or the force of the drogue chute deploying jars it loose and it falls out. One thing that seemed to help was to wrap the shock cord between the PIRM2 and main anchor eyebolt around the parachute. That helps hold the chute in place, and also tends to make the parachute deploy a little better when the PIRM2 releases the retaining pin.
But I think a better plan is to use a deployment bag to hold the chute. It holds the chute in place more securely, and also helps to protect the chute from deployment charge heat.
In the above picture you can see the deployment bag. It is fairly heavy nylon, although if I'd had some nomex or other heat resistant material I would have used that. It is retained to the main anchor eye bolt with a short length of 1/8" braided nylon cord.
In this picture you can see how it connects to the PIRM2. When the PIRM2 releases the key, the cord gets pulled out, pulling the chute out with it, leaving the bag attached to the eye bolt.
The only thing I need to worry about now is the main chute not making it all the way out of the body tube, and that could happen when using long shroud lines on the parachute. For example, the PIRM2 fires and releases the key, the parachute is pulled out of the deployment bag as the slack in the shock cord is pulled out of the body tube. But if the shroud lines are too long, the slack in the shock cord may be taken up before the parachute makes it out of the body tube. You'd have a case where the PIRM2 key and shroud lines are out of the rocket, but the parachute itself didn't make it out far enough to deploy.
To prevent the above scenario, the shock cord behind the PIRM2 key must be long enough to make sure the parachute can be completely pulled out of the body tube. Now that I have a bag to prevent early deployment of the main chute, it shouldn't be a problem adding some length to the shock cord.
I really don't think the deployment bag needs any extra holding power. But if it does a flap could be added to the bag with some Velcro to retain the parachute even more. I'll fly it the way it is a few times and see how it goes.
Update 11 June, 2005:
The PIRM 2 and recovery hardware has now had 8 consecutive successful deployments of both chutes. The last 4 have used either the new home built altimeter or the altimeter and timers for back up. I still have the Dual Outboard Rocket to launch witch uses mid body separation. I'll post the results here after the flight.