We've established below that I am lazy. Here's more proof... a table of precalculated measurements based on the figures shown below, rounded to the nearest quarter inch (close enough for my purposes).
|8||2 1⁄4"||3 1⁄2"|
|16||4 3⁄4"||6 1⁄2"|
|18||5 1⁄4"||7 1⁄2"|
|20||5 3⁄4"||8 1⁄2"|
One of the things that has always given me trouble is doing the math for an octagonal parachute, mostly because I've been doing it from the top down every time. Stupid of me. Observe the diagram:
Regardless of the size of the octagon, the ratio of the size of the side b to the total width of the octagon X will be the same. Rather than belabor the math one more time, I'll just tell you: b is approximately 41.42% of X. To cut a square into an octagon that size, you need to mark in from the corners by a, which is obviously (100% - 41.42%) / 2, or 29.29% of X.
Honestly, I think it's safe enough to go with a = 30%, b = 40%.
Why did I post this here? So next time I'll know where to look for the answer. I am planning to make a few of the parachutes described below; my Baffler and Big Daddy 4x18 are both noted for destroying lesser parachutes. The math was just on my mind, and so now the answers are on the net.
I sell and service computers for a living, and for probably a year now my distributor has been sending me computers packaged in Tyvek bags. The bags protect the shiny parts of the computer from scratching; when we deliver a computer to a customer, we don't take all the packaging. Since I'm a pack rat, the bags have been piling up.
Tyvek is lightweight and tear resistant. I've thought for a while that I should be able to make parachutes out of these bags, but the material is too bulky to make smaller chutes out of. So here, I've created a parachute for my Ultimatum rocket. It's a BT-70 sized rocket, with a large chute compartment, and I already know it needs a big chute.
I used braided kite line bought at the dollar store for the shroud lines, held in place with epoxy. I put little strips of Tyvek over the ends of the lines, possibly adding some strength, or maybe just for looks.
If it works well, Justin may want one for his 4-H project rocket... BTH-70, 5'6" long, fitted for a cluster of 3 E engines. Egad.