Parachute Duration
OVERVIEW
Parachute Duration combines the challenge of building a light rocket that can carry a large enough parachute to descend slowly, yet being able to deploy reliably, to achieve a good duration score. The model has to remain in one piece throughout the flight and cannot be staged.
For the full set of rules see The Sporting Code
SCORING
The contestant may make one or two flights. Up to two models may be used. The score is the sum two flights.
GENERAL TIPS
Design Considerations. There are trade-offs between the model’s performance and the parachute’s size. Increasing the size of the parachute means the chute will need more room inside your rocket. Increasing the size of the rocket to hold a larger chute will make the rocket heavier, so it will not boost as high.
One of the biggest problems in flying PD is getting the chute to deploy. This is not just an issue of packing technique. It concerns how tightly packed the chute is inside of the tube. This is why some fliers will use a longer length tube, and/or a larger diameter tube, so they can have more room to pack a chute in. This also depends on the material used for the parachute, since thinner material packs more easily into a given tube length and diameter than a thicker material.
Proper folding techniques/patterns are essential for ensuring deployment reliability. The Russians have developed and documented a Z-folding method that works well; see the article in the files below. You want to pack the parachute such that as it comes out of the tube the canopy comes out first, with the shroud lines falling downward from underneath the canopy, not packed on top of the canopy in the tube. This will lead to full and rapid deployment.
The simplest and perhaps best models are models that are the same diameter as the smallest-diameter available motor for that power class (13mm for A and below power classes) using a body tube of about 10″ or so greater length than the motor casing. Then use whatever appropriate diameter of chute that you can pack into it that will DEPLOY reliably.
Parachutes. Parachute choice and preparation is important. A simple Estes type chute just is not going to perform very well. The plastic is very thick. One recommendation is 1/4 mil (.25 mil) plastic dropcloth for parachute material. It is very very thin, thinner than dry cleaner bags, but may be difficult to find. Perhaps this is in part because plastic that thin is terrible as a real dropcloth. Still, it is worth trying to find, because it packs so small. You might have to settle for a somewhat thicker dropcloth. Or, even go with plastic taken from cheap ponchos, though those are not nearly as good for 1/2A or 1/4A PD chutes.
If you do use a dropcloth, one drawback is that it is clear, which can be hard to see (though you will want to use talcum powder on the chute, so it will not be totally clear but “frosty” looking). Before you use talcum powder, get a large black magic marker and use it to color part of the plastic to make it more visible. Be sure to dust the chute with talcum powder after using markers on it, as the marker ink remains tacky otherwise.
A lot of fliers use 1/4 mil aluminized Mylar chutes. Those pack smallest of any material, and are very visible when the sun is out to shine on it. You will not find 1/4 mil Mylar locally. You will need to order it from one of the rocketry vendors that sell contest type supplies. Do not try Mylar “Rescue Blankets”. Those are 1/2 mil thick and are much harder to get to deploy. You may need to glue together multiple sheets to form a large sheet. To do this, use the type of contact cement that is used for styrofoam. It does not melt Mylar, and it dries quickly without the remaining exposed bits of cement still remaining tacky.
Shroud Lines. Rayon thread, also known as Embroidery thread works well. It is “smooth” like silk thread, so the lines do not tend to grab each other as thin cotton lines can. This thread is much thinner than the typical thread used for kit chutes, such as Estes chutes. That is a good thing, since the thinner thread packs smaller, and for light models like these the thread is plenty strong enough.
Shroud lines can also be made from very thin braided nylon. The thinnest gauge or weight of this is size “A”, and this is perfect for FAI parachute shroud lines and is available from vendors such as Mud Hole. Use 12 to 16 lines, each 1.5 times the parachute canopy diameter in length. They can be attached to the canopy (before it is dusted with baby powder) with a piece of aluminized mylar tape.
Attaching Lines. One option for line attachment is to cut up band-aids to attach the lines. The band-aid adhesive is VERY sticky, so they will not peel off or left the line slip out easily. Also, the band-aid material is pretty thin and flexible. AVOID using something like Trim Sheet Monokote, as over time the tabs tend to pry off.
Another option is mylar tape. Press the mylar tape and the opposite side of the parachute with your fingers to apply the tape.
Do not use cellophane tape or any other type of tape whose adhesive gets soft in the sun and can then tack your parachute closed. Put a loop in the end of the shroud line before putting it in place with a tape strip.
Shock Cords. Kevlar shock cords of 50 to 100 pound grade are recommended. Use about 24″ length from the main body to the parachute.
Talcum Powder. YES! Thoroughly. Rub it into the parachute material, do not just sprinkle it on.
MOTOR SELECTION
The best motor selection for this event is one with low average thrust, the smallest possible diameter, the highest total impulse that fits within the power class being flown, and a long delay time. If you have a choice between a 13mm motor and an 18mm motor for this event, you should generally use the 13mm motor. Not only is the body diameter less (higher altitude), but the casing weight is less (slower descent rate). Parachute models tend to need medium to long coasting times after motor burnout in order to reach their maximum feasible altitude, so use the longest available delay time for the motor type you choose.
KITS
In the past kits have been available from QCR, ASP Rocketry, and Apogee Components.
Currently Galactic Manufacturing and Apogee Components sell FAI style parachute duration models.
Galactic Manufacturing FAI Competition Model
Apogee Components International Thermal Sailor
NARAM 68 1/4 A Parachute Duration Beginner Tips
1/4A Parachute Duration will be flown at NARAM 68, and presents challenges in using such a low total impulse motor. Most competitors would recommend using a piston launcher in this event. 40 mm diameter FAI rockets, while capable of flying on a 1/4A, may not be the best option for NAR competition.
This event would be a good opportunity to use a 13 mm minimum diameter body tube.
The Arrow plans below show a typical minimum diameter parachute duration model. This model uses elliptical fins and an externally mounted shock cord. The fin shape for this event is likely not too significant. However, externally mounted shock cords typically allow for easier deployment of a relatively large parachute and they also allow the rocket to hang perpendicular during descent which for The Arrow using a 12 inch parachute might decrease descent rate by ~3%.
A reasonable place for a beginner to start is to use a 10 or 12 inch diameter dry cleaner bag parachute in a 13 mm rocket. As a significant challenge with this event is getting the parachute to fully deploy, practice flights are a good idea. Pistons are typically not recommended for the first few flights for a beginner, however for 1/4A, even for a beginner, a fixed head piston launcher might be reasonable to improve performance after some practice flights with successful deployments.
More advanced modelers will likely use FAI style rockets with a 24 mm or 30 mm diameter body tube. The current Junior and Senior division records in 1/4A PD, were both set by piston launched larger diameter rockets which allowed for larger parachutes. Plans for the current Senior record holder are in the links section.
Links
S3-Parachute-Folding-Techniques.pdf
1/4 PD Senior Record Holder
Sagita The Arrow American Spacemodeling June 1990.pdf