View Full Version : Shroud Line Length?

18th March 2009, 09:54 PM
Earlier this month I launched a 4" diameter mid-power rocket a couple of times on a G motor. I used a parachute my wife made that's a little too big, I think, for the three-pound rocket. But, since the rocket carried a video camera and didn't go very high, I thought an over-sized 'chute was okay.

But, during both flights, the parachute didn't fully open. Fortunately, even as just a streamer, it brought the rocket down slowly and safely.

At first I assumed the rocket just wasn't heavy enough for the parachute. The rocket weighs a little under three pounds and the 'chute is about 4 1/2' in diameter. But, the other night, we played with the 'chute - running around the house with our dog and cat chasing us.

We discovered that it wouldn't open if we held the end of the shroud lines. But, if we grasped the shroud lines closer to the canopy, the parachute would easily open.

This weekend, I'm going to fly the rocket again with the same parachute, but with the shroud lines shortened.

I found a few references to the minimum length for shroud lines on the 'net, but didn't see anyone talking about the maximum length. Does it make sense that, if the lines are too long, the 'chute won't open?

-- Roger

18th March 2009, 10:05 PM
How long are the shroud lines?

18th March 2009, 10:12 PM
Here is a tid bit I found from Richard Nakka's site (http://www.nakka-rocketry.net/paracon.html).

The inflated diameter (and therefore area) and shape of the canopy are both influenced by the length of the shroud lines (L) in relation to the canopy diameter (D). As the length of the lines are increased, the Cd increases.

and here is the equation to figure the lenght of the shroud lines.

The length of each of these six cords is given by
L = 2.25 * ( D + S ) where D = basic diameter of parachute, S = stitching length. The stitching length, which is the length of cord sewn to the canopy, should be between 5% and 10% of the basic parachute diameter.

19th March 2009, 12:55 AM
Thanks ... I recall seeing and booking that page in the past. It has a lot of useful information on parachutes.

Using his formula, the shroud lines on our 'chute are just about right - they are a little more than a diameter in length from the bottom of the canopy to the attachment point. (His formula is for the length from one point on the canopy to the opposite point, so I divided 2.25 by 2.)

The "optimum" length that we found running around the house is about 2/3 or 3/4 of the diameter. This may be because the 'chute we made is flat ("a parasheet") rather than shaped.

To my eye, the ratio of the length of the shroud lines to the diameter of the 'chute looked longer than on other 'chutes. So, I think it'll work better with shorter lines.

I never really thought about how important the length of the shroud lines might be or that we might have done something else wrong that would cause the 'chute to not deploy right. This is the third or fourth parachute my wife has made, and the others worked well.

-- Roger

19th March 2009, 02:27 AM
I use a 34" chute for my 4" 3pound rocket, but I cheeted some it is a recycled umbrella. I have not had a chance to us my 52" chute yet. Too windy for it.

19th March 2009, 11:12 PM
I had always wondered about the "optimal shroud line length..." Just curious: is your chute a circular or a hemispherical?

21st March 2009, 08:38 AM
I like them to be about the length of the diameter of the chute. The chute actually works better (and I have used chutes made this way) if the lines are ~2x the diameter in length, but I've had more problems with them tangling if I used them at that length.

22nd March 2009, 03:52 AM
I've made a few chutes, from 8" to 53". I've found that for parasheets, the best ratio is shroud line length = 1.5 times the parasheet diameter across the flats. You can use the point to point distance, but I've not found any advantage.
As for tangled lines, I find that is more a function of chute packing then shroud line length.
Other then the Estes plastic chutes, I've never used a parasheet larger then 12". If you are going to sew a chute larger, I would highly recommend using 8 - 12 gore hemispherical chutes with shroud lines 1.5 times the open diameter of the hemi chute.

22nd March 2009, 03:54 PM
One of the guys in our club did a presentation on making competition chutes, and I think he came up with an optimal ratio of around 1.5 as well, give or take 0.1.

I'd like to find out what the optimal ratio is for an X-type chute. I've got a 48" TFR Ultra-X-Type that I'm going to use as the drogue for my L3 rocket, and I had it made with double-length lines (96" instead of the usual 48") so I could test that out. A document I found on recovery systems says X-type chutes can have a line-to-diameter ratio anywhere in the range of 1.0 to 2.0.

23rd March 2009, 07:16 PM
One of the guys in our club did a presentation on making competition chutes, and I think he came up with an optimal ratio of around 1.5 as well, give or take 0.1.

http://www.nar.org/pdf/TCR2.pdf and http://mysite.verizon.net/wjvincent/RandD/shroudline.html

Both point to 1.5 x D being the optimal length for contest chutes.