S6—Streamer Duration

Event Highlights

• Flown in three standard rounds and, if needed, a flyoff round
• Flown in the 2.5 N-sec (A) power class for both Seniors and Juniors
• Maximum flight duration per standard round is 180 seconds
• Score is the sum of the duration of the three flights
• If tie for 1st, 2nd, or 3rd place after three standard rounds, 1st flyoff round is flown with max of 300 seconds
• Maximum of two models may be entered for the three standard rounds
• One additional model may be entered for the flyoff round
• S6A model requirements: minimum length = 500 mm, minimum diameter = 40mm for at least 50% of length, maximum mass = 100 grams
• Streamer requirements: must have no less than 10:1 length-width ratio, must be made of a single piece of material, must completely unfurl in flight

Major Challenges

• Reliable model (good boosts, no crimps, good ejection that doesn't damage the model)
• Reliable and rapid deployment of streamer at apogee
• Finding thermals/lift

Discussion

To be competitive for a medal at a WSMC, you will need to get an average duration of at least 140 seconds on each of your three flights in the standard rounds, and probably at least one 180-second max. Unless there is huge thermal activity, it is rare to have multiple fliers get three 180-second maxes and have a flyoff round. In order to get a team medal (three person team), it normally takes an average score of at least 400 seconds per team member and no DQ's or poor flights (less than 2 minutes).

Vehicle reliability is very important. If you fly a super-light model that crimps during boost (a frequent experience with models made from Kapton film rather than fiberglass), you will be eliminated from contention. 40mm foam plugs (or ejection pistons) are usually used to prevent scorching or melting of the streamer.

Streamer deployment reliability is very important. If your streamer fails to unfurl promptly and deploy perfectly at apogee, your performance will cause you to be eliminated from contention. Streamers are usually kept tightly furled in storage but then unfurled somewhat when inserted in the rocket body for flight, so that they open fully and instantly on ejection. A streamer that is arranged inside the body in such a manner that its mass is not distributed exactly symmetrically around the roll axis of the model will cause the model to cone during boost and lose altitude.

Recovery of models is very important. Only two models may be used for the three standard rounds. These are flown consecutively, with each being 80 or 90 minutes long. Therefore, at least one of the models from the first two flights must be recovered within less than two hours in order to make the third flight. One additional model may be entered for the flyoff rounds. If that is the only model available, then that model must be recovered in order to fly in the 2nd flyoff round. Fortunately, at a WSMC, the U.S. team members who are not flying that day will be deployed for short, mid, and deep recovery. Exploits and achievements of the U.S. "deep recovery team" are legendary!

Vehicle Construction

• Lightweight fiberglass or vellum (see Construction page)
• Nose cones are typically vacuum-formed plastic (available from Apogee Components) with a foam core shoulder.
• The fins are made out of either lightweight 1/20 or 1/16 balsa wood, or 1/32 balsa that is reinforced with tissue or fiberglass.
• Airframe mass, without motor or streamer, should be less than 8 grams; top-end models may be as little as 5 grams. Light airframes are absolutely critical to success in this event.
• The anchor point for the shock cord line that goes to the nose cone and streamer must be external to the body and attached at a point where the body will hang horizontally from it with a burned-out motor casing. This way the large light fiberglass body adds maximum drag during recovery. To do this drill a small hole at the deployed CG, drill another at the top of one fin root. Kevlar shock line gets threaded through the first hole, runs through the inside of the airframe, out the airframe near the root, and anchored into a flue fillet.
  

Streamer Construction 

S6A streamers are usually made from aluminized mylar. The design goal for the streamer is to have it flapping and coning vigorously during recovery, creating drag from this action. A wide range of creasing and folding techniques are used to achieve this action. Streamer sizes vary from 4 x 40 inches to 5 or 6 inches by 90 inches, depending on the weight of the material used. Heavier materials are stiffer and hold creases better (this is good for drag), but they have to be smaller or their weight becomes a problem that reduces boost altitude and increases the fall rate on recovery despite their drag. Streamer mass should not exceed that which puts the model at optimum boost mass.

The "Roll Fold" (sometimes referred to as the "Scorpion's Tail") is the simplest approach for streamer creasing in common use. It can only be used with stiffer mylars, 1 mil in thickness. This type of mylar is sold in 30-inch-wide rolls as window film for hydroponics greenhouses. Make up streamers in batches at a time by rolling full-width 1-mil mylar around a 10mm body tube then remove the tube, squash the streamer material flat between a long wooden clamp made from 1x 4s and carriage bolts, and bake this inside an oven at low heat (150 degrees) for a few hours. Then slice the roll into individual streamers and add the shroud line. Leave the streamers in their tightly folded condition until they were ready for use, and store together those made from a single batch. Streamers made from the same batch behave very consistently. Straight out of the oven, "Roll Fold" streamers are too tight to be used as-is. As part of the prep routine, open up the streamer by gently pulling on it. Pay close attention to the degree that you open the streamer up. Too little or too much will hurt the streamer action. Pull and release the streamer until it naturally wants to roll up and assume a triangular shape, i.e. folds at 60 degrees, that just fits in the 40mm body.

The "Heat Sink" creasing technique is much more complex and time consuming, but is closer to the state of the art in other countries that are more successful in S6. This technique (described in the Sport Rocketry article posted below) produces a very large number of very tiny folds in the streamer material, and uses thinner (1/2-mil) material than the "Roll Fold". This type of mylar is commonly available, in rescue blankets and from model rocket hobby sources. Streamers made using this technique can be large (6 inches by 90) without being excessively heavy; using a length:width of up to 15 is common in East European models that use techniques and streamers similar to this.

Unlike the NAR Pink Book, FAI rules do not require "single point" attachment of the shroud line to the streamer. Many European modelers use a yoke, with a line tied to each end of one of the narrow ends of the streamer, and then the two lines are joined (offset) to the single shroud line. If a single-point anchor on the streamer is used, place if offset from the center of one of the narrow ends, 1/4 of the width of the streamer from one of the edges and not in the middle. Stiffen the leading edge of that narrow end of the streamer with a piece of fine-diameter music wire.

Propulsion

S6A is not an event where any U.S. motor is even marginally competitive. Maximum possible altitude and minimum possible post-burnout mass are critical to performance. US A motors have far less total impulse than the full 2.5 N-sec that European motors have, their burnout casing mass is significantly heavier, and their 4-second delay time does not permit the model to reach peak altitude before ejection. All U.S. competitors fly in the WSMC using 10.2mm diameter European motors, generally with 2N average thrust and 5 seconds delay. These provide higher boost altitudes and lighter recovery masses, which increase altitude and duration. Some pre-WSMC experience test-flying with these motors is recommended to make sure that the motor and ejection characteristics are understood. Almost all competitors in S6 use piston launchers to increase launch altitude.

Competition Strategy

Finding significant lift is critical; you have no chance to get a max flight if you don't find lift. Methods for finding lift are documented on the Techniques page. At a WSMC, the U.S. team will generally have several Kestrel weather stations plus some thermal streamers on tall poles deployed to help identify wind/temperature patterns that indicate thermal formation. Another strategy is to watch how other models in the air react and to launch accordingly. Frequently, when there is thermal activity, there will be a multiple models in the air at the same time. Timing of the launch here is everything since down air or sink always surrounds a rising up current, and the decision window to piggyback off a competitor's thermal is very short. Experience (or advice from experienced teammates) plays a key role in choosing when or when not to fly.

It is recommended that you not try to use a streamer more than once; the sharp and springy creases that give it its drag characteristics relax in flight and are not as sharp on a second flight by the same streamer. Come to the field with four streamers already folded for flight and stored in body tubes. When you get back one of your models in the first two rounds, remove its streamer and replace it with one that is already prepped.

Each round is fixed in length, with the next round starting exactly when the last one ends. This requires that the modeler be adequately prepared to deal with any situation, be it weather, motor malfunctions or catos, or a misfire.