Shroud lines

[Brain]

Being aware of the reigning shock-cord-length wisdom (twice the body length), I wonder if there is some formula for deciding how long to make shroud lines. Does it have to do with the rocket size, or performance characteristics? Or is this digging in too deep?

[gpoehlein]

I don't know if there are any hard and fast rules, but I've always gone by the rule that the shroud line length depends on the chute canopy size. So, a 36" diameter chute needs 36" long shroud lines. If the shroud lines are too long, the chute cannot open fully (or reefs).

Greg

[Brain]

I don't know if there are any hard and fast rules, but I've always gone by the rule that the shroud line length depends on the chute canopy size. So, a 36" diameter chute needs 36" long shroud lines. If the shroud lines are too long, the chute cannot open fully (or reefs).

Greg
Shall I assume that this length is from the 'chute attach point to where it attaches to the nose cone?

[Sunward]

Shall I assume that this length is from the 'chute attach point to where it attaches to the nose cone?
yes, but that is confusing.

The shroud length is typically the size of the parachute. So a 36" parachute (the diameter) would have a 36" shroud line. As the line is typically connected at two points on the parachute, the line itself would be over 72" long - some extra for attaching to the parachute.

[gpoehlein]

Angelo has it exactly right. I thought about adding the part about doubling the shroud line length when attaching at two points, but I was tired and what I wrote didn't make it very clear and I erased that part. Thanks, Angelo for adding the clarification in a way that makes it crystal clear. :D

Greg

[STRMan]

BTW, I NEVER attach my chutes directly to a nose cone. I always have a small piece of shock cord on it, either by attaching the chute a few inches from the nose cone right to the shock cord for a smaller rocket, or for a dual chute model, I'll actually put a small shock cord onto the cone before attaching the chute to it.

If my rocket catches an unexpected gust and then turns downwind giving it an unusually high deployment speed, I don't want to start ripping off the chute's shroud lines because of that.

[blackshire]

BTW, I NEVER attach my chutes directly to a nose cone. I always have a small piece of shock cord on it, either by attaching the chute a few inches from the nose cone right to the shock cord for a smaller rocket, or for a dual chute model, I'll actually put a small shock cord onto the cone before attaching the chute to it.

If my rocket catches an unexpected gust and then turns downwind giving it an unusually high deployment speed, I don't want to start ripping off the chute's shroud lines because of that.But if that single length of shock cord running from the shroud lines' "gathering point" to the nose cone were to break, the rocket would plummet Earthward without a parachute...that's a single-point failure mode. While you might have one or more shroud lines break on a hard deployment with a 'chute whose shroud lines are tied directly to the nose cone, at least the other shroud lines will remain attached and the canopy will provide some drag, even if it acts like a streamer with one or more broken shroud lines.

[blackshire]

Being aware of the reigning shock-cord-length wisdom (twice the body length), I wonder if there is some formula for deciding how long to make shroud lines. Does it have to do with the rocket size, or performance characteristics? Or is this digging in too deep?G. Harry Stine wrote in his "Handbook of Model Rocketry" that little hard data exists on the optimum lengths for shroud lines. After mentioning that the shroud line length should never be less than the greatest dimension of the 'chute canopy (a 12" [between opposing corners] hexagonal or octagonal 'chute should have shroud lines at least 12" long), he goes on to say that he tried using shroud lines 1.5 times the major canopy dimensions and won a few parachute duration contests with them. (As an aside, he also mentioned the odd experiences of having some rockets descend faster under 24" parachutes than the same rockets fitted with 18" parachutes!)

[Brain]

Thanks, gents!
This is all good info to tuck away into my grey matters.
Now, on a related topic (which just now presents itself to me): what about streamers? Are there prevailing thoughts on length, width, attachment set-ups, material, etc.?

I can guess that mylar is probably the best bet for material, but I have to bow to the collected knowledge base for the rest. This is the only rocketry forum I have ever been on, and I can't imagine that I can find a more suitable bunch of know-it-alls! :D

[blackshire]

When he was in college, our current NAR President (Arthur H. "Trip" Barber) experimented with streamers and found that the optimum length/width ratio is 10:1 (a 1" streamer produces maximum drag when it's 10" long, a 2" wide streamer should be 20" long for maximum drag, and so on). Longer streamers actually begin to stream instead of fluttering and flapping, which they must do to produce drag. (I've always thought the German term for them, "flatterbands" [flutterbands], is more accurately descriptive. Having a recovery device called a streamer that flutters is like driving on a parkway.) Trip Barber also found that it is possible to size a streamer to fit the mass of a rocket for maximum duration.

Streamers are usually attached at one end, but they are sometimes attached in the middle (Centuri and Semroc used/use this method). A streamer can be attached to the shock cord or can be free to slide up and down it (passing the shock cord through a reinforced hole in one end of the streamer), or (using a 6" or so length of shroud line) the streamer can be tied to the nose cone.

Regarding the streamer material, G. Harry Stine preferred crepe paper because crepe paper streamers open even in cold weather and don't melt if a little ejection charge gas leaks past the wadding. Competition flyers use streamers made of mylar, tracing tissue, and bond paper.

[Bill]

Regarding the streamer material, G. Harry Stine preferred crepe paper because crepe paper streamers open even in cold weather and don't melt if a little ejection charge gas leaks past the wadding. Competition flyers use streamers made of mylar, tracing tissue, and bond paper.


Serious competitors iron their streamers with a zigzag fold for even more drag.


Bill

[gpoehlein]

Serious competitors iron their streamers with a zigzag fold for even more drag.


Bill

Actually, you don't need to iron them at all - I found that if you crimp mylar between your finger and thumbnail, it will hold a very sharp crease. There is a video on the Apogee website that shows how to fold/crease streamers for maximum effect.

Greg

[blackshire]

"The Model Rocketry Handbook: 21st Century Edition" by Stuart Lodge contains a drawing of a "streamer press" for creasing mylar competition streamers. Also, the BMS (Balsa Machining Service) School Rocket Kit comes with a pre-creased mylar streamer.