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 rules for this event, please see the Parachute Duration rules in the Model Rocket 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, as long as one of the models is returned.
GENERAL TIPS
by George Gassaway, NAR 18723 and Trip Barber, NAR 4322
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.
A list of plans and kits is included further down on this page.
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. For years, I have recommended 1/4 mil (.25 mil) plastic dropcloth for parachute material. It is very very thin, thinner than dry cleaner bags. But, it has become a lot harder to find .25 mil dropcloths the last few years. Perhaps 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 PD chutes (a good source for Eggloft Duration chutes, though).
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. You could also use red, I like black for most sky conditions. 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, principally Aerospace Specialty Products, linked below. They sell precut chutes and large sheets. Do not try Mylar “Rescue Blankets”. Those are 1/2 mil thick and are much harder to get to deploy. A good source of really thin aluminized mylar is Homefly; they sell 0.4 micrometer (1/6-mil) mylar in 500 mm wide sheets. 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. I like to use Rayon thread, also known as Embroidery thread. 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. One good material is Gudebrod (brand) “rod wrapping thread” used to repair or make fly fishing poles. The thinnest gauge or weight of this is size “A”, and this is perfect for FAI parachute shroud lines. It is available from fishing tackle vendors such as Fishing Tackle Unlimited and 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 such as that sold by Aerospace Speciality Products.
Attaching Lines. I like to cut up pieces of 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. 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. In some power classes, particularly “A”, the longest delay time currently available for the smallest-diameter available A, the Estes A3-4T, is far too short for maximum altitude but you don’t have any alternative choice.
KITS
–Straight-Up kits (1/2A through D) – Qualified Competition Rockets
–13mm and 18mm kits (1/4A through C) – Aerospace Specialty Products
–Blue Streak kit (B – C) – Apogee Components (can be adapted)
—Competition parachute material – Aerospace Specialty Products
Related Documents:
| Astre A PD-SD (Vincent) | May 28, 2014, 9:26 pm | 4 KB | |
| Darkstar C PD-SD (Talkington) | May 28, 2014, 9:26 pm | 35 KB | |
| Parachute Folding Techniques (Mituriev) | May 28, 2014, 9:26 pm | 253 KB | |
| PD & SD Tutorial (Pinkas) | May 28, 2014, 9:26 pm | 229 KB |