15th December 2011, 01:09 AM
What Size(s) of parachutes should I make?
I have a fairly large sheet of ripstop nylon (5'x4'). I would like to construct parachutes for two rockets out of it. One is 4" diameter by 4' tall. It weighs about 2 pounds without recovery, motor, and a couple things I am still working on like the altimeter sled. It has 0.2 inch thick fins that do not protrude below the body. The other one is a dual configuration 3" rocket. It varies from about 4.5' long and 2.5 pounds to about 7' and 4 pounds. This is of course without motor or recovery. The rocket has fins made from the same plywood (0.2), but they extend about 2" below the body tube. I think I may need a larger parachute for it. I fly at CMASS and MMMSClub. Both fields have fairly soft ground. I would not like to make more than two parachutes, and I would like everything to come down as quickly as safely possible. I have used many calculators and they all return different results. Do you have experience with similar rockets? What size/shape parachutes did you use with them?
Total Impulse for 2016: 684 Ns - A 6.875% J
A:0, B:0, C:0, D:0, E:0, F:0, G:0, H:0, I:0 J:1
2015: 1049 Ns - G:1, H:1, J:1 | 2014: 1428 Ns - G:1, H:6 | 2013: 953 Ns - H:5 | 2012: 1293 Ns - D:2, E:1, F:2, G:4, H:4 | 2011: 945 Ns - A:4, B:5, C:13, D:4, E:7, F:5
15th December 2011, 01:21 AM
15th December 2011, 01:46 AM
15th December 2011, 02:29 AM
It apears you have less than 2 sq yard of fabric. One way to decide on chute diameter is to sim the rockets in Rocksim or OpenRocket and shoot for a decent rate of about 15 ft per second. You might be able to get away with 20-25 fps on a soft field. An accurate flying weight is important. You could also use this calculator. http://www.onlinetesting.net/cgi-bin/descent3.3.cgi
Originally Posted by eggplant
My guess is that you will make a 36" - 42" chute for the lighter of the two rockets, and then go buy more fabric for the heavier rocket...
That's just a guess.
NAR 91621 L2
TRA 15982 L2
17th December 2011, 07:20 AM
If you want two chutes from that material, you will probably have to make 8 gore chutes. This will allow you to cut the gores out and get the most efficient use of the materials.
Also by making the chutes from gores, you can make elliptical chutes. These are much more efficient then chutes made from flat sheets and use a lot less material.
A 24" elliptical chute should be fine for the lighter rocket. 36" or 40" elliptical would work for the larger one.
Another options for the larger one is to get an old medium sized umbrella and us the fabric from that. A golf umbrella will get you about a 53" chute, the smaller standard umbrellas make about a 40" chute. Ask friends, family, co-workers, etc to save any broken umbrellas for you. You'll be surprised how many chutes you'll end up with.
BTW, the 150 lb Dacron line works great for shroud lines. I got mine on special at the Kite Studio. I paid $15 for 3000 feet several years ago. It's now $28, but that is still a great deal. You can also get rip-stop nylon there.
17th December 2011, 05:00 PM
For a 15 ft/s descent rate, and flat para-sheet, I would use around 36-40" for the smaller rocket and 54-60" for the larger rocket. For a dome shaped chute, I would use around 30" for the smaller one, and no less than 42" for the larger.