shroud line length?

[Starfire73]

I am correct in guessing that shroud lines for a parachute, should be roughly the same length as the parachute size?

i.e. an 18" parachute should have sihroud lines about 18" long.

I am making my own chutes (or at least trying to), and the chutes I have that I've checked seemed to have shroud lines the same length as the parachute size. Is there a formula for calculating the shroud line length? Or should I just go with what I'm assuming and quit over thinking it?

 

[quickburst]

You are correct. The shroud lines are the same length as the canopy is wide.

18" chute ... 18" shroud lines, 24" chute ... 24" shroud lines ... and so on.

 

[powderburner]

For sport chutes, yes, length = diam works great.

If you are asking in regards to competition applications, I think there are some old NAR research projects where people found a 'sweet spot' at something like length = 1.5 x diam.

If you are really trying to improve chute function (especially for 'parasheets' that are cut from a flat pattern) I think you can greatly improve performance (and safety) by increasing from six shroud lines to eight or ten. The extra lines do a better job of pulling the chute into a circular shape, for minimal weight and cost.

 

[cjl]

I know someone who swears by the rule of length = 2x chute diameter.




However, for sport applications, 1x is fine, just don't go much below it.

 

[edwardw]

That would be me that swears by the 2x rule. It works for me and gives me nice inflation.

Edward

 

[shreadvector]

1.5 D works great for any parachute that has 8 sides or more for competition.

For sport flying it also works for 6 sided chutes, but if you attempt to use a 6 sided chute in ponpetition it might be a little happier with 2D.

Originally posted by powderburner
For sport chutes, yes, length = diam works great.

If you are asking in regards to competition applications, I think there are some old NAR research projects where people found a 'sweet spot' at something like length = 1.5 x diam.

If you are really trying to improve chute function (especially for 'parasheets' that are cut from a flat pattern) I think you can greatly improve performance (and safety) by increasing from six shroud lines to eight or ten. The extra lines do a better job of pulling the chute into a circular shape, for minimal weight and cost.

 

[Weldo]

Old thread bring back!

Is it possible to have shroud lines that are too long? I made a mylar chute recently, haven't flown it yet, but it is 30" round while flat. I read that 1.5x the diameter is a good number so I cut 4 pieces of line about 96" long. I arrived at that length since one thread will do two shroud lines, plus a bit extra for knotting. The chute has 8 shroud lines total.

After it was all said and done it just seems too long. Was it supposed to be 1.5x the "inflated" diameter?

 

[ebruce1361]

Old thread indeed!

I don't know if there's such thing as truly "too long" for shroud lines providing you have sufficient space to pack the whole thing when it comes to hobby rocketry. As for the 1.5x diameter to line length ratio, I'm pretty sure that referrers to flat diameter. I almost never see a parachute size listed as an inflated diameter.

I did manage to dig up some information circa 2011 or 2012 on computer simulation experiments regarding shroud line length, but it's in the context of manned spacecraft design:

"...as shown in Figure 3, we made changes in the lengths of the cables known as suspension lines, which connect the canopy to the riser that connect to the spacecraft, and we performed computations. These changes had a significant impact on the dynamics of the parachutes. There were various results, including increased descent speed and gliding speed. We are repeating simulations such as these and refining the parachute designs to meet the objectives. The design choices will be confirmed with drop tests in the end, but numerical simulations help reduce the number of tests, and as a result, we can keep the cost down and shorten the development period as well."

You can take a look at the full article HERE. As for our applications, it might be worth it to run a few experiments. Launch a single rocket multiple times with the exact same configuration except swap the parachute each time with one of identical design except for shroud line length. Combine ground observations of drift and descent stability with onboard altimeter data for descent rate and perhaps even GPS positioning, and you might be able to dial in on an ideal canopy diameter to shroud line length ratio.

 

[Weldo]

Cool, thanks!

I can imagine that as the shroud line length increases, the inflation diameter of the parachute will increase as well. Some recommend "reefing" your chute to increase rate of descent. In essence what reefing does is shorten the shroud lines in order to limit the inflated diameter upon descent. The smaller inflated diameter will increase descent speed due to lack of air resistance or drag.

Conversely, increasing shroud line length must increase inflated diameter. As a mental exercise in extremes, I'm imagining a 36" circular mylar chute with 8 shroud lines at a ludicrously short 6". The chute, assuming it even could inflate would look like an upside down test tube, very small diameter.

Now imagine the same 36" round chute but with shroud lines of an infinite length. Assuming there is no wind and the weight pulling downward on the parachute is minimal, the inflated diameter in this case would likely be at or near the full 36".

I also think that as shroud line length and thus inflated diameter is increased, the ease with which the parachute catches air for the initial inflation will also increase.

 

[tfish]

For the flat chutes I make I use the Flat diameter times .89. They seem to work fine. I too, loop 2 (side by side) lines together.



Tony

 

[ebruce1361]

The diameter would increase, but up to a point. Somewhere, there will be an equilibrium between the decrease in the parachute canopy height from skirt to apex, and the increase in diameter. As you said, with no wind resistance and nearly zero speed, the canopy would fully spread out into a flat disk, but that can't happen in reality. As a mass falls under a parachute, the canopy will constantly change geometry as air density increases lower to the ground to a wider diameter, to say nothing about crosswinds.

I would venture to say that a parachute will be at its fullest and widest inflation in ideal conditions when closest to the ground. With that logic, to achieve the lowest touchdown velocity, you would want the parachute as close to the ground as possible and as wide as possible when the mass makes contact, so shorter lines would be ideal since that places the parachute closer to the mass. But as you said, too short makes for a condition of a parachute that cannot achieve full inflation.

Conversely, supposing touchdown velocity isn't as crucial, I am curious as to what would happen with absurdly long lines. The shroud lines themselves don't have much surface area, but they do have some. As such, I think longer lines would be more prone to be affected by crosswinds, and would make for undesirable lateral movement. On top of that, longer lines means additional mass pulling the skirt of the parachute down, increasing descent speed and reducing the inflated diameter.

One thing is for sure, I totally want to make a parachute with ridiculously long shroud lines just for the fun of it and see what happens. Something like a 24" parachute with 8' lines. I'll bet that will be an absolute pain to pack!

 

[BABAR]

An interesting use of long shroud lines is for camera rockets.

I used a BT-80 tube with three shock cord attachments equally spaced, each with two shroud lines with a six sided chute. Packed carefully so would deploy without twisting. Small spill hole. The three long lines spread out around the rocket diameter allowed the rocket to descend without the usual spinning associated with a single shock cord, so got a smooth descent video.

 

[Joekeyo]

Would longer lines increase the probability of entanglement (no, not the quantum kind)?

 

[BABAR]

Would longer lines increase the probability of entanglement (no, not the quantum kind)?

Absolutely yes. I am thinking from a parachute OPENING perspective, the optimum length would be infinite. Obviously infinite isn’t possible, but 10, 50, or even 100 foot lines are POSSIBLE but nobody is gonna do that on a LPR ROCKET. Among the limiting factors are weight of the line, packing room in the rocket for the line, cost of the line, and, as you mentioned, the longer the line the more likely it will get tangled. The same constraints apply to the shock cord. Some experienced rocketeers have given suggestions above. I don’t have empirical knowledge about the difference between 1.5 and 2 diameters, I would hazard a SWAG that going beyond 2 diameters likely buys you much more trouble than any potential incremental efficiency gain is worth,

I will put in a plug for 8 shroud lines over 6 for four reasons.

One is redundancy, shroud lines do break or pull off or through the plastic parasheet type chutes. Losing 3 lines on a 6 line chute is gonna be worse than on 8.

Two is you spread out the opening stress a little more with 8 rather than 6. An additional “insurance” for this is to continue the line over the top center of the canopy and down the other side (making the opposite side shroud line), but that’s a bit messy for me

Three is efficiency, 8 lines gives a slightly more round canopy than 6, although the difference is incremental and I certainly wouldn’t replace a stock 6 shroud chute for that reason.

For all these, you can argue that 10 or 12 or more is better, but you gotta stop somewhere.

Four is if you make your own, I find it is really easy to cut out a perfect octagon than a hexagon. If you ever made paper snowflakes in kindergarten you may remember this.

Start with a square piece of plastic.

Fold in 1/2

Fold in 1/2

Fold in 1/2

Draw or imagine a line bisecting the angle made by the point

Make a single cut along the “near” edge to the point, perpendicular to the line above. (Try it with a piece of paper first)

Open it up, Voila! Perfect octagon.

 

[ep29030]

As I recall, Knacke's Parachute Manual says that you get about 10% increase n parachute efficiency if you make the shroud lines 150% of canopy width, rather than the standard 100%. As I recall, the manual also recommended at bridle (shock cord) length be at least 2X tube diameter,with 3-5X recommended for clear air when chute inflates. If you are using an elastic shock cord, greater length = less load on any portion of the elastic. For that reason, I've read 3-5X rocket length as recommendation. Myself, I use 4X rocket length for my elastic shock cords. If packed properly, chute entanglement should not be an issue. If you just stuff it all in there, anything can (and will) happen.

 

[Weldo]

In the past I've carefully packed the shroud lines but I've never given much thought to the shock cord. I'm guilty of just shoving it in the body tube. Any recommended methods of packing elastic shock cord?

 

[rharshberger]

I generally run on the principal of shroud line length is 1.5xdiameter of canopy in part because of the slightly increased efficiency and part as it works for my method of folding chutes better (I flake the chute, then one z-fold of lines on top of flaked chute, fold tip of chute to base, then z-fold lines over that and either roll or repeat folding process depending on chute bundle size desired). I have done no testing to verify that 1.5 length shrouds are actually more efficient but they work well for me.