S2P – Precision Payload
- This is the FAI equivalent of the US Team America Rocketry Challenge (TARC) event.
- Rockets may be any size, shape, or weight and may use any motor as long as all are within the limits of the FAI definition of “model rocket” (1500 grams, 160 N-sec total power, no motor above 80 N-sec).
- Goal is to achieve precisely 300 meters altitude and 60 seconds duration while carrying a 60 gram “fragile payload” that is the size and weight of a single raw size-large egg and that uses a parachute to recover the entire rocket as one connected whole.
- Flown in three rounds, with only one rocket and only one “fragile payload”; if the fragile payload (egg) is broken at any time in any flight, score for the event is “DQ”.
- Assuming that the egg survives, event score is the sum of scores from all three rounds. Score in a round is the distance by which the altitude goal is missed plus three times the time by which the duration goal is missed.
- Repeatability of flight performance as weather varies during the event and in the face of variations in rocket motor performance (total impulse, delay time).
- Durability of the rocket against landing or other forms of damage and ability to protect its fragile payload over the course of three flights.
This video on “Flight Testing in TARC” covers all the considerations in how to get your S2P model to reliably achieve its specific altitude and duration performance target: https://www.youtube.com/watch?v=lge0QT0uPKw
All of the “lessons learned” from the many years of US experience with student TARC teams apply to the approach to designing and building for this event:
- AP composite rocket motors are preferred due to their superior repeatability of total impulse performance compared to black powder. However, some competitors have achieved good results using clusters of Estes D12 motors. Use motors from the same production batch in practice flights and throughout the event (note: FAI rules do not permit flying metal-casing motors, so reloadable motors are not be permitted when S2P is flown internationally).
- Use motor(s) with 30 grams or less of propellant per motor. Motors with more than 30 grams of propellant cannot be shipped internationally and will not be available at a WSMC.
- Use motor delay lengths that ensure the rocket flies to or through its natural apogee before ejection. Early ejection completely destroys repeatability of altitude performance. (note: FAI rules do not prohibit use of pyrotechnic charges the way that TARC rules do, so using an apogee-sensing altimeter to deploy the parachute precisely at apogee rather than relying on motor delay/ejection is an option for S2P).
- Use higher-thrust motors for flight and a rail rather than a rod for launching, to control boost trajectory in presence of any wind by minimizing weathercocking and rod-whip.
- Build the rocket so that it is about 10% under-weight relative to what flight simulation software such as RockSim says is the weight needed to achieve a 300-meter altitude with the motor that is planned (simulations often over-estimate altitude), and include a provision to add ballast weight following test flights in order to bring altitude performance on target.
- Optimize airframe size and weight for hitting the altitude goal first, then adjust the size and configuration of the parachute to hit the duration goal from this altitude.
This is not a “finesse” event where airframe weight matters. If your rocket gets heavy, add more power. Build strong; the rocket has to not just last three flights (and 20 ft/sec landings), it has to remain identical in drag and shape for all those flights plus an appropriate number of test flights. You cannot afford to have broken fins, zippered bodies, etc. that change the airframe’s drag coefficient between flights. Use plywood or basswood fins, through-the-wall mounting or fiberglassed to the body with strong root fillets so that nothing cracks on landing. Use anti-zippering techniques for your shock cord anchor so ejection dynamics do no damage. Put a long shock cord on the rocket so the heavy payload section and the heavy booster do not smack into each other at ejection. Use a large enough body diameter to permit the egg to be completely surrounded with cushioning foam so that no landing orientation puts it at risk.
Use a cloth (ripstop nylon, silk, etc.) parachute for this event. Plastic parachutes, even with “over the top” shroud lines, do not have the durability or strength to handle the repeated loads of these relatively heavy rockets. Once you get the diameter roughly right to get a 60-second duration from 300 meters, trim performance by minor “reefing” of the shroud line length (first choice since it is reversible) and/or increasing the spill hole size of the parachute.
Use of an electronic timer to control release of many of the shroud lines at a particular time (say 58 seconds) to precisely control duration is permitted. Actually doing it, without breaking the payload due to high landing speed; or tangling the shroud lines at ejection and defeating the controlled release, is not easy. Remember that the FAI definition of a parachute is that three or more shroud lines are used and that this event’s rules require use of a parachute for recovery, so be sure not to release so many shroud lines that fewer than three remain attached, or you will be DQ’d.
To succeed at this event, you have to practice-fly with the same (or an identical) rocket many times beforehand and take data with every flight. What launch angle do you need to use in order to get a perfectly vertical flight as a function of wind speed? How does ambient temperature (or launch site altitude) affect flight performance? How many grams of ballast weight does it take to change altitude performance by one meter, and how does this weight affect duration performance? Answers to these questions based on data and experience will accurately inform key choices made during the actual competition.
The key variable under the competitor’s control is flight weight. Typically altitude varies by several feet (1-3 depending on rocket overall weight/size) for each gram of weight. You must keep track of and control the liftoff weight of your rocket to 1 gram of precision. Remember that the “fragile payload” (egg) that you are issued can be any weight within the range of 57 to 63 grams, so you need to have a rocket that is optimized for the 63-gram egg, and ballast weights to add if your egg is less than 63 grams.
There are a few commercial kits that can be adapted to fly this event. Basically, any kit that has a diameter larger than an egg and that can survive multiple medium-powered flights can be used. Smaller models, such as the Quest Aerospace “High Q”, have an advantage that the target altitude can be reached with an “E” motor. However, there is less room for egg padding. Larger kits, such as the Semroc “Brighton”, have lots of room for an egg and altimeter. However, you’ll require an “F” motor to achieve 300 meters.
A few kit suggestions are listed below. Many more suitable kits are available from vendors who sell medium-power kits. [Note: at this time, you should probably NOT use an all-fiberglass kit. There is no FAI competition experience to date to determine if this type of material/construction is accepted under FAI rules.]
- Quest High Q
- Estes Pro Series II Ascender
- LOC Precision Hi-Tech
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