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Basically I thought I could tell you how it is going so far and use any feedback you give. I realize that we are now down to the wire since the deadline for qulaification is a mere three weeks away. As of now, we have made several practice flights and are pretty close on altitude and getting closer on time. The main problem we are having now is repeatability. It just seems almost impossible to constantly get very close to 850 ft and 45 sec with the percent error of the rocket engine and varying wind conditions.
Our rocket is about 30 in tall, 2.5 inches in diamter, and uses a F42-8 engine. The top 8 inches or so is the payload section that contains the egg (so far no cracked eggs even with and 1100 foot free fall) and altimeter.
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This is why this is such a challenge.
What are you getting regarding just the altitude consistency? Duration consistency is to some extent directly linked to altitude consistency (higher altitude = longer time, lower altitude = shorter time), so that is why I ask about altitude first.
Altitude can vary for various reasons. First, of course, the engine could vary either in its newton-second total, or by the time delay. But composite engines tend to be pretty consistent. And whatever minor inconsistency there may be, that is part of the game everyone else is having to deal with.
Areas to look at for altitude consistency be the launcher, the wind, and any launch angle. If the launch rod is flimsy, or the rocket otherwise does not take off consistently time after time, then that can throw it off right from the start. So, a larger diameter (stiffer) rod, and perhaps longer, could help it if that is an issue. If you are already using a 1/4 inch rod 4 feet long, then that ought to be sufficient.
Wind could also affect it. Has the rocket always flown straight up, or has it sometimes curved into the wind? Any flights that are not vertical would fly lower, and the more it curved into the wind, the lower it would end up flying.
Although, with an F42, that thrust level probably is enough to prevent it from being affected a lot by wind, unless you flew on a really windy day. If you were using a rocket that had lower thrust, then I would suspect wind to have had a lot of effect on altitude consistency.
Now, let's assume that you got the altitude consistency worked out, and focus on duration consistency.
Duration can be affected widely by the air conditions when you fly. A rocket could make ten flights to 850 feet and could easily have a wide range of durations.
The biggest variation in the flying conditions would be updrafts and downdrafts. Usually in the form of thermals, which are small spots of rising air, that sometimes are visible as "dust devils". Also, when air rises in a thermal, it displaces surrounding air, making it move downwards, so outside of a thermal there is downward moving air. So, for duration consistency, you want to avoid flying when there is a thermal nearby since the model could either hit rising air or falling air. Now, how to really spot a thermal, that is a big issue since it is more of an art than a science, contest flying model rocketeers (and model airplane contest fliers) have one put in a lot of time learning how to detect thermals, and even the best miss detecting them by 10-20% of the time.
About the best I could suggest to help detect thermals is to use a $10 Wal-Mart bubble machine (Gazillion Bubbles has a great machine), or a several students, to blow bubbles and see if the bubbles rise (thermal) , fall quickly (probably down air), or drift a bit and fall (probably sort of neutral air). In addition, make use of a 15 to 20 foot pole (like a telescoping fishing pole) and tape a 15 to 20 foot long piece of videotape from a worn out T-160 VHS tape, the T-160 is thinner than the T-120 tapes. The long tape streamer is very sensitive, so it shows what the wind is doing. When there is a thermal nearby it can actually start pointing upwards a bit. Although the more wind there is, the less useful bubbles and streamer poles are to be of use.
If you wanted to read more about thermals, see this article that Ken Mizoi and I wrote long ago ,which is on the Apogee website:
http://www.apogeerockets.com/education/detecting_thermals.aspThe GOOD thing is that you are not looking to find a thermal, you are trying to assure there is no thermal around, but still it could be tricky to learn enough to help you avoid a thermal. Although, later in this message I'm going to give you a pretty sure-fire way to avoid thermals and downdrafts.
Another tip I can give in this area is that a thermal sucks air into it near the ground, and that affects the relative wind you feel. On a mostly calm day, it will make the wind direction change, so if you notice a shift to a certain direction that the wind generally has not blown, that probably is a thermal sucking in air near you. On a windier day, you might not notice a direction shift, but you can notice a wind velocity shift. If say the average general wind was 6 miles per hour, and the thermal sucked in air at 4 miles, per hour, you might notice the wind getting slower, from 6 MPH to 2 MPH ( 6MPH - 4 MPH), as the thermal might be upwind of you, sucking in air at 4 mph. Then when the thermal passed by, downwind of you, it would get a lot stronger, to about 10 mph (6 mph + 4 MPH). So, if you were flying on such a day and noticed these cycles, you could wait for the thermal to have passed far away, the wind to have returned to normal, and then fly. Take note that otherwise, for regular sport rocket flying we usually thing it is a GOOD thing for the wind to drop and then fly, but that may well mean a thermal is near which you do not want due to how it would affect the consistency.
Here is the sure-fire way. The best thing of all would be if you could just go out and fly early, shortly after dawn. The winds are pretty calm, and thermals usually do not start till at least after 9 AM. The worst time to fly would be mid-day. If I were competing in this myself, I'd do everything shortly after dawn when the air would be the most consistent. Note quite as good, an alternative would be to fly very late in the day, in the hour before sundown.
Here is a link to my favorite weather website to use a day or two before a BRB launch, or for days when I am considering going out to fly at a little local site:
http://tinyurl.com/2klecNote the graphs for the wind velocity, how it usually drops at night and picks up in the middle of the day. How nice it would be to fly about 7-8 AM tomorrow.
Have you worked out the descent rates of your previous flights? What are they? Take the altitude in feet and divide by the time in the air, to find out how many feet per second the model descends. If the descent rate from flight to flight is not very close, and assuming you are using the same chute and it deploys 100%, then you would pretty much be looking at how the air conditions are affecting the model (so, this type of analysis helps you separate out how the varying altitude is affecting the duration). Compare your average descent rate to what the ideal descent rate should be (850 feet divided by 45 seconds = ideal descent rate in feet per second).
Now, let's imagine you have a rocket that always goes 850 feet, and you are flying in air that is consistent. So then it ought to come down to dialing in the parachute performance to achieve that ideal descent rate. I would assume the parachute is packed consistently, and always deploys 100% correctly (otherwise, you have to fix that).
If the time comes in under 45 seconds, you'd need a parachute that would be a little bit bigger. If the time was say 10% under 45 seconds, then you'd want to make a new chute about 10% larger in area (not in diameter, but in area).
If you had a flight time that was consistently a little over 45 seconds, then you could either go to a chute a little bit smaller, or else "reef" the shroud lines a little bit. Reefing the lines is simply shortening them a bit so the chute does not deploy to the same effective diameter as it does with regular length shroud lines. It would not take much reefing. But reefing might be a bit tricky to dial in without doing number of flights, so again it might be better to make a new chute, this time making diameter a little bit smaller.
Also when making a new chute, it would need to use the same material and shroud line length as the original. You would not want to use a fabric chute for one flight, and plastic for another flight, as the chute performance likely would not be the same for the same diameter, so it would be tricky to try to extrapolate (or plain guess) what correction factor to use between a fabric chute or cloth chute. If you are using a commercially made fabric chute, then you'd not have time to get a new one made, so if you were a bit over 45 seconds then you might be better off reefing the lines a bit.
Lastly, do not wait till the last day or two before the deadline to make your qualifying flight. The weather might not be good then, a rainout would be very bad after all your work. And rain is not the only possible issue, a very windy day also would hurt your chances.
- George Gassaway
