A couple of q's on dual chutes...

[Stones]

Is there some sort of formula for how much distance there should be between 2 chutes when deployed?
Here's what I'm thinking. 40" ripstops with 50" shroud lines (16 per chute). I will have 10' of shockcord from the airframe to the NC. I'd like to attach 2 smaller shockcords from the NC to the swivel on the shroud lines, to each chute. My best guess says about 5' each on these 2 smaller shockcords. I'm looking to give as much room as possible for the chutes to deploy with the least chance of tangling.
Any input is, as always, greatly appreciated.

 

[edwardw]

Here is what a guy in our club flys when he uses dual chutes. He used this setup last weekend with two 8' chutes on a 42 lb rocket that was his L3 project.

He took one long line and put two sewn ends with quicklinks to attach the chutes. To secure it in the middle to the rocket he used a Larks foot knot. He then used a lenght between the chutes that was 1/4 the length of the one long line. He put two sewn ends on that and put those all together with his quick links to the chutes. He then again used a Larks foot knot to keep the nose cone where it should be.

HTH,
Edward

 

[rstaff3]

It so happens I saw an article by Bob Stroud the other day Like you mention, he recommends running a shock cord from each chute to a common connect point along the main shock cord. This is required to ensure the chutes have space to open.

He says you should ideally have three chutes vs. 2. The third chute greatly enhances stability. Also, he also said you actually lose 5-10% of performance over the equivalent single chute since the chutes will not be centered over the load.

 

[edwardw]

I just have a question for you on shroud line length. You said they are 40" chutes, and you have 16 50" shroud lines. That is 1.25 the chute size. Do you always use this? Is there an advantage I'm overlooking? When I make my chutes I go 2x on everything. It's just a personal preference of mine, and I thought I read somewhere that it helps a little.

Edward

 

[AndyC]

As a general principle, I'd make the shock cord to each chute as long as possible. Very short, or no cord, will place the chutes very close together. This is bad for 2 reasons :1. more risk of tangling,
2. a 'bubble' of high pressure air builds around the outside of each chute. This will tend to force the chutes apart, meaning that they fly at a large angle to the direction of descent, and so are less efficient.

I've witnessed a skydiver (using a round canopy) have a malfunction, deploy his reserve, then have the main re-inflate. Because the chutes fly at an angle to one another, the descent rate is almost identical to a single chute - but more stable.

 

[Stones]

Originally posted by edwardw
I just have a question for you on shroud line length. You said they are 40" chutes, and you have 16 50" shroud lines. That is 1.25 the chute size. Do you always use this? Is there an advantage I'm overlooking? When I make my chutes I go 2x on everything. It's just a personal preference of mine, and I thought I read somewhere that it helps a little.

Edward

Edward...
I'm not sure if you are talking about a single shroud line connected at 2 points on the chute or not. My shroud lines are actually 100" long and are connected to the chute at 16 points, for a total of (8) 100" lines.
I went with Rocketman chute dimensions which show a R4C (aprox. 43") with 50" lines.
Seems like plenty of line. pic below.

 

[Stones]

Originally posted by rstaff3
...
Also, he also said you actually lose 5-10% of performance over the equivalent single chute since the chutes will not be centered over the load.

Dick...
Just so I'm understanding this correctly, is that "5-10%" performance loss based on whether the 2 chutes are centered over the load or meaning that I'd be better off with just the one chute. I'm guessing that even with the chutes slightly "off center", they would still bring down the rocket at a slower rate vs. the one chute.

 

[edwardw]

I think that he means that each chute is only 90-95% effective because they are canted and spilling some of their air. So overall you are getting two chutes at 90-95% rather than 1 at 100%. So it's a better bet

With the shroud lines, I thought that they were 50" long to te connection point...looks like a good setup

Edward

 

[DynaSoar]

Originally posted by Stones
Is there some sort of formula for how much distance there should be between 2 chutes when deployed?
Here's what I'm thinking. 40" ripstops with 50" shroud lines (16 per chute). I will have 10' of shockcord from the airframe to the NC. I'd like to attach 2 smaller shockcords from the NC to the swivel on the shroud lines, to each chute. My best guess says about 5' each on these 2 smaller shockcords. I'm looking to give as much room as possible for the chutes to deploy with the least chance of tangling.
Any input is, as always, greatly appreciated.

Just an idea:

Use snap swivels on the chutes/cords that will slide along the main shock cord, not simply by their weight, but requiring only a few pounds pull to make them slide along.

Snap one to the NC. Snap the other about 2/3 of the way down the main shock cord. With the NC ejected and the main shock cord pulled taught by the first chute (doesn't have to be open at this point), the lower chute will still be completely below the first (50" shrouds plus 20" radius, if still collapsed). Thus, they'll be opening entirely separate from each other.

When the lower chute catches air AND the shock cord stays taut because the upper chute is full, the swivel will slide along the main cord until they're both at the nose. If the upper chute is full, the tension on the shrouds should keep it from tangling. If it's not, there's not yet enough tension on the cord for the lower chute to slide up.