Parachute reefing isn't anything new, nor is it new to amateur rocketry. But from what I've seen most attempts
at reefing have used a slider reefing mechanism to allow for a more gentle opening of a parachute. What I'm looking
at doing is to reef a main at apogee so it functions as a drogue chute until the usual main deployment altitude,
then the chute is unreefed and the rocket descends at a low rate of speed for a soft landing.
In my internet searching I found a US Air Force document detailing reefing of different parachutes. Here's a link to the document: http://www.dtic.mil/cgi-bin/GetTRDoc?Location=U2&doc=GetTRDoc.pdf&AD=ADA036497
From what I gathered reading that document, a flat sheet circular chute is a good candidate for even very tight reefing. With that in mind I decided to proceed with my tests.
Reefing the chute should be easy, a line is simply extended around the parachute skirt to restrict the open diameter of the chute at the base. The problem is releasing this constricting line at the proper moment. Of course we have altimeters that can fire a charge at main deployment altitude, but what you need is the line to be cut cleanly or released, and it can't have any pins or even knots in the line, those obstructions would/could prevent the line from passing through the rings that hold the line on the skirt of the chute.
That still leaves options, one easy option would be to simply cut the line with a heat source, a nichrome cutter would work, but the power required might be excessive. I did try a Pyrodex pellet just to see how it would work, it worked fine, but there are a lot of flames that make me nervous. A late firing of the main could start a ground fire, even more likely is the parachute or shroud lines getting burned. Troubling too is the melted blob of nylon line that may get stuck in the reefing rings.
Another option is just a line cutting device, I have used such devices on nylon tubing on hybrids, but a clean reliable cut may take some serious engineering when cutting nylon line.
In the end I decided on an ultra simple device that should also be ultra reliable. It is a small aluminum box with a lid that is retained by nylon screws, the reefing line passes into the box, over a pin and out the box. The entry and exit points of the line into and out of the box have half diameter holes drilled, so when the box is assembled the nylon line is pinched in the half hole. A small pyro charge inside the box blows the cover off and releases the reefing line.
I've done a couple of tests and it seems to work fine. I haven't tested it to over 20 pounds of holding force, other than me just trying to pull the line out by hand. I can't imagine there will be huge forces acting on the reefing line, it's not like a shroud line, in fact the opposite may be true, it may be that it needs to open under a very light load. Which this design will do.
Here's a video of a couple of early tests. The first test is a Pyrodex pellet cutting the line, the next two tests are of the release mechanism I made: http://www.thefintels.com/aer/reefreleasetests.wmv
Out of curiosity I pulled out my 10' diameter cute and played with it in the wind today. First I just let it go in the wind to see how much pull it had, then I reefed it to a diameter of 3'. The pull force was much less, but I can see I'll need more restraining points than just the 8 shroud line attachment points. The chute was smaller, but not nearly reduced to 3'. Doubling or even tripling the attachment points would likely do the trick, or even a continuos line holder, perhaps some tubular nylon sewn to the skirt.
Here's a poor quality picture of the device. (I really need a better still camera)
More to come....