After contributing three or four posts on another forum thread that dealt with parachutes, and seeing responses from a number of members who showed an interest in building a semi-ellipsoid parachute, I decided to post this thread with the hope that it may help those who are looking for detailed information on the process.
Note that this is not necessarily an authoritative 'How To', but rather my experience. What I present here is not just a description of the process, but also the mistakes I made, and my observations of what it's like working with 1.1 oz ripstop nylon. Knowing this can be helpful before you sit down behind a sewing machine and start running stitches.
The parachute I built is 31.5" diameter with a 6" spill hole. The canopy shape is semi-ellipsoidal, with a Cd of 1.50. The sizing of my parachute, stitches, shroud line material and method of attaching it to the gores, is based on a relatively lightweight rocket (20 oz). Heavier rockets will require a larger parachute (of course), and double top stitching. (I'll explain that when I get to that step.)
Note: Because I'm explaining the steps as if the reader has no experience in measuring, folding, pinning and sewing ripstop nylon, this is a very lengthy description of the process. If you do have experience, breeze past the minutiae. Also, this thread assumes you know the basics of using a sewing machine—adjusting thread tension, setting stitch length and width, sewing straight and zigzag stitches.
Materials:
Ripstop Nylon: This should be 1.1 oz in weight, calendered. Whether it is DWR coated (durable water resistant) doesn't matter (mine is). I got my fabric from ripstopbytheroll:
https://ripstopbytheroll.com/collec...ucts/1-1-oz-ripstop-nylon?variant=11187595457
Specify 'calendered', and 'cut and folded' (for the shipping method).
A word about ripstop nylon, which will explain why I'm using the 70/10 needle and Tera 80 thread: This nylon is more like a plastic bread bag than it is like denim or cotton fabric. It has a weave, but that weave has been closed in the calendering process. So the needle doesn't pass between threads as you sew a stitch. It punches a hole. And holes weaken ripstop. Because you can't avoid holes altogether, make them as small as possible.
Needles: Microtex (Singer or Schmetz) 70/10. The 70/10 is a very fine (narrow) needle. The Microtex is super sharp. And because it is a fine, super sharp needle, it will dull faster. When you hear it popping as it pierces the fabric, it's time to change the needle. If you're not sure, change the needle every three hours of sewing.
Oddly, I couldn't find Microtex needles in 70/10 size anywhere in Las Vegas (Hobby Lobby, Joann's, Michaels). So I bought them here:
https://www.wawak.com/Sewing/Needle...tex-home-machine-needles-5pack/?sku=SMNS15110
Thread: Tera 80 (Gutermann). This is the strongest thread that will work with a 70/10 needle. It's polyester, and abrasion-resistant. The only place I could find it in white was here:
https://www.rockywoods.com/Gutermann-TERA-80-Polyester-Thread
Pins: In keeping with the small-holes-are-better-than-big-holes, I used Dritz dressmaker pins. Like the sewing machine needles, these are thinner and sharper than regular pins. If you can't find these at your local fabric store, get them here:
https://www.amazon.com/gp/product/B001HZUFC0/
Shroud line: I'm using braided nylon shade-pull cord. It's strong, and very flexible (unlike Kevlar). I went with 1.5mm diameter.
https://www.amazon.com/gp/product/B07FVN9V4X/
Sewing machine presser feet: Use the standard foot for all straight and zigzag seams. For sewing shroud lines to the canopy, this foot makes it much easier:
https://www.amazon.com/gp/product/B08JM8T7GJ/
For the 1.5mm shroud line, the 4mm presser foot was ideal.
Fabric pen: Used for marking seam allowances. The ink disappears after a day or so. I got mine here:
https://www.joann.com/prym-dritz-disappearing-ink-marking-pen-purple/1027739.html
Before we get rolling, a few observations about sewing ripstop:
First, it will bunch between the presser foot and dogs unless you start your stitch by using the machine's handwheel for at least one stitch (or two). Once the machine takes over, let it pull the fabric (don't pull the fabric from behind the machine). Keeps your hands flat on both sides of the fabric, pulling slight tension sideways from the foot.
Second, I couldn't backstitch at all without wadding the ripstop in the dogs. So, I started the stitch, ran it about an inch, then, with the needle down, lifted the foot and turned the fabric 180 degrees. Lower the foot and sew back to your starting point. Then lift the foot again (needle down), turn 180, then lower the foot and run the entire stitch length. At the end, repeat the reversing process. This is how you lock stitches when you can't use the machine's reverse lever to backstitch.
Locking stitches on the top and bottom hems is not necessary. The ends of these stitches are secured when they are folded into the flat-felled seam when joining gores.
Third, sew slower than you would with a heavier needle. The Microtex 70/10 will bend easily, even break, if you put the pedal to the metal.
Finally, as mentioned above, replace the needle when it starts making an obvious popping noise as it passes through the fabric, or every three hours of sewing.
With that, let's go.
1. Practice stitching.
Before you start running stitches through those perfectly-cut gores, get used to sewing ripstop nylon by practicing stitches on smallish pieces. Jump down to #5 and #6 to see how I folded and sewed the hem, and #7 to see the flat-felled seam. Get comfortable with both.
Note: Before lifting the foot to pull the fabric away from the machine, raise the needle by turning the handwheel until the thread take-up lever (that shiny piece of metal that moves up and down in the slot at the left side of the machine) is at the top. This will avoid fouling the bobbin thread.
The top and bottom stitches should look the same. If there are loops on the bottom stitch, the thread tension is too loose. On the machine's tension dial, turn it to a higher number. Sew another sample. Adjust thread tension as necessary.
With the exception of the zigzag stitch I used for attaching the shroud lines, all stitches are 8 per inch.
2. The pattern.
I used Richard Nakka's 80cm gore pattern for this parachute. It has a .75" (10cm) seam allowance on all sides (the distance between the dashed edge line and the solid inner line).
I printed my pattern on 24 lb copy paper, which is too stiff to pin to the fabric. There are three alternatives that I could think of: use the heavy paper pattern to trace a line around the edge onto the fabric (which is what I did), trace the pattern onto tissue paper and pin it to the fabric, or use a rolling cutter with the fabric on a cutting pad.
This is tissue paper over the pattern. It's cheap, and available at Dollar Stores and craft stores (gift wrapping):
If you use the tissue paper for a pattern, iron it on low heat to flatten the folds. Then trace the pattern with a fine-point Sharpie.
Important! Before cutting gores with the pattern, refer to step #7 in Part II. Watch the video and read the two paragraphs following it.
3. Flatten the folds in the fabric.
Spread your fabric out on the ironing board, and press the folds flat. Use low heat (the setting on my iron is Nylon/Silk). If the iron sizzles when you touch a wet finger to it, it's too hot.
4. Cut the gores.
As mentioned above, I traced the edge of the pattern onto the fabric by pushing a pin at an angle through the pattern and fabric, and into the ironing board padding. Then I traced around it using a disappearing ink pen.
Use sharp scissors to cut the gore. You don't need expensive fabric scissors, but you do need a fairly good quality. I used Fiskars (longer blades are better), which you can find at fabric and crafts stores, Walmart, etc.
Note: The more you handle the gores after they've been cut, the more the edges will fray. Frayed edges complicate the edge folding/ironing process.
5. Folding the top and bottom edges of the gore for hemming.
Before folding and marking, note that the two sides of ripstop are different. The side that is dull will be on the outside of the canopy. The shiny side will be on the inside of the canopy. And because I will be referring to a YouTube video in Part II that demonstrates a flat-felled seam, I'm using the same terminology a seamstress would use:
'Wrong side' refers to the shiny (or inside) of the fabric. 'Right side' refers to the dull (or outside) of the fabric.
On the right side of the fabric, measure .75" from the bottom edge of the gore, and mark it with a straight edge and pen.
I cut a strip of paper .75" wide and used it to space the straight edge. (It's faster.) If your straight edge has a cork strip on the bottom, flip it upside down so there's no gap between straight edge and fabric.
[Continued in Part II]
(https://www.rocketryforum.com/threads/31-5-semi-ellipsoidal-parachute-build-part-ii.164479/)
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