Auto-Reefing Parachutes

Discussion about deployment systems including altimeters, timers, air speed flaps, servo systems, and chemical reactions.
SaskAlex
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Auto-Reefing Parachutes

Post by SaskAlex »

One of my goals this year is to develop a reliable high speed parachute deployment system. I want to deploy long after apogee, when the rocket is close to the ground. Your rocket won't drift as far, and I think it just looks cool.

One element of this project is the parachute itself. I've thought about reefing rings (and I should thank air command for the video they made on that topic), but it seems to me they are not the greatest solution. The faster the rocket is falling, the greater the forces in the shroud lines, and the faster the ring will move down the lines. In essence, it will have less effect when you need it more. What I want is a parachute that somehow responds to air speed- it should have a lower coefficient of drag (Cd) at higher speeds. The other day I thought of something that might just work- instead of cutting out a circle for your parachute, cut a short section of a cylinder. Fold over the top inch or so of the cylinder, and put a rubber band inside. This closes up the top a bit, so it folds into a bit of a dome with a fair sized spill hole. At higher speeds, the rubber band will stretch, increasing spill hole size and decreasing the Cd. If the rubber band is large enough, the parachute will be able to "blow-out" right back to a cylinder, having much less drag.

Here's my first attempt at such a chute. I cut a flat piece of wood so that it just fits in the tube. I stretched a rubber band over the bag, and put small pieces of two-sided tape below the band. In the picture, the red plastic hasn't yet been pulled off of the tape. I then folded over the end of the bag, and removed the plastic from the tape pieces. And that's all there is to it.
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And here it is completed. The flattened cylinder had a width-to-length ratio of about 2:1. This would it allow to fold into a rough hemisphere with no spill hole, but it turned out to to be excessively long. I've done another one with a width-to-length ratio of about pi:1, and that turned out to be a better shape. I don't have any pictures of that one yet.
DSCF2679.JPG
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I haven't done any testing yet, and this very well might not be worth the effort. On the other hand, for a system which consistently deploys at high speed, such a parachute design could be useful.

Alex
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Re: Auto-Reefing Parachutes

Post by air.command »

I think that's pretty clever Alex. :) It will be interesting to see what results you get with the testing. Even if one doesn't want to achieve high speed deploys, inevitably you sometimes have to deal with them. I know during high speed deploys there are very strong forces involved, I wonder if you might end up snapping the rubber band from over extension. If that happens maybe a larger diameter string loop on the inside with the rubber band to prevent it stretching too far. It will also be interesting to see how much the spill hole will stay open while the rocket is falling under normal terminal speed.

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SaskAlex
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Re: Auto-Reefing Parachutes

Post by SaskAlex »

George- I chose the rubber bands so that they can expand to the full circumference of the cylinder without breaking, so the cylinder itself plays the role of the string you describe. The cylinder material would have to tear or stretch significantly for the bands to snap. Well, there is still the possibility of uneven stretching of the band as the chute suddenly opens, so I guess that's not fool proof, but I certainly don't think that's the most likely problem. Spill hole opening at terminal speed could certainly be an issue. So far I can't see any way of easily changing out the rubber bands, so it will be difficult to experiment with that. It likely would take a fair bit of work to get all the details mastered.

Alex
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Re: Auto-Reefing Parachutes

Post by bugwubber »

That's a beautiful parachute. Think I read somewhere that the NASA rovers took a 14g hit when their parachutes deployed with no ring.
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arjan
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Re: Auto-Reefing Parachutes

Post by arjan »

Hi Alex,
Maybe you won't believe it, but I had exactly the same idee a few weeks ago when I was walking downwind to get my rocket back, it drifted ~150m away from me.
I think this should work very well.
Looking forward to your first test results.
Arjan

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Asupremeflight
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Re: Auto-Reefing Parachutes

Post by Asupremeflight »

Interesting concept. If I didn't know any better I would think you were using old cassette tape for your parachute lines. If not what is that cord?
SaskAlex
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Re: Auto-Reefing Parachutes

Post by SaskAlex »

arjan wrote:Hi Alex,
Maybe you won't believe it, but I had exactly the same idee a few weeks ago when I was walking downwind to get my rocket back, it drifted ~150m away from me.
I think this should work very well.
Looking forward to your first test results.
Hi arjan- do you mean you thought of the same parachute design, or just deploying at low altitude? In any case, I guess great minds just think alike. I don't know if I'll get these chutes to the point where they are worthwhile, but I'm confident I'll get my late deployment system working well enough with or without them.
Asupremeflight wrote:If I didn't know any better I would think you were using old cassette tape for your parachute lines. If not what is that cord?
Haha, it really does look like tape in the picture. No, it's not cassette tape, but something from the same era- braided nylon fishing line (20lb). I just ran out of it the other day, which is a shame. It holds knots so much better than modern fibers.

Alex
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Asupremeflight
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Re: Auto-Reefing Parachutes

Post by Asupremeflight »

yeah I thought maybe you knew something I didn't know. because if I remember correctly tape from cassette tapes break rather easily. do they still make that kind of fishing line? I don't know why but I always have a hard time tying knots. I use the bow line knot. which is a great knot but it's hard for me to tie I do not know why I think maybe because I am left handed.
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arjan
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Re: Auto-Reefing Parachutes

Post by arjan »

SaskAlex wrote:
arjan wrote:Hi Alex,
Maybe you won't believe it, but I had exactly the same idee a few weeks ago when I was walking downwind to get my rocket back, it drifted ~150m away from me.
I think this should work very well.
Looking forward to your first test results.
Hi arjan- do you mean you thought of the same parachute design, or just deploying at low altitude? In any case, I guess great minds just think alike. I don't know if I'll get these chutes to the point where they are worthwhile, but I'm confident I'll get my late deployment system working well enough with or without them.

Alex
I meant I had exactly the same design in my mind, pretty scary when I saw your pictures!
Arjan

n-bwaterrockets.blogspot.com
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U.S. Water Rockets1
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Re: Auto-Reefing Parachutes

Post by U.S. Water Rockets1 »

arjan wrote:
SaskAlex wrote:
arjan wrote:Hi Alex,
Maybe you won't believe it, but I had exactly the same idee a few weeks ago when I was walking downwind to get my rocket back, it drifted ~150m away from me.
I think this should work very well.
Looking forward to your first test results.
Hi arjan- do you mean you thought of the same parachute design, or just deploying at low altitude? In any case, I guess great minds just think alike. I don't know if I'll get these chutes to the point where they are worthwhile, but I'm confident I'll get my late deployment system working well enough with or without them.

Alex
I meant I had exactly the same design in my mind, pretty scary when I saw your pictures!
That means that it could be a hood idea if several people came to the same answer!
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