Launch date and time: March 10, 2006 @ 4:00PM
Launch location: Rock Creek High School near St. George, KS
Launch wind and weather: Light winds and cool temps 5-10MPH at 65F
Last year at about this time, my friend Christian and I drove out to observe the Team America Rocketry Challange qualification flights for the Rock Creek High School. We were both pretty well impressed with the teacher and team, and their test and qualification flights went pretty well (alas, neither Christian nor I documented this event). Since Christian has moved to Oklahoma (and couldnt see his way clear to drive up just for this) I was on my own this time. So on the afternoon of March 10, 2006, I headed West across Eastern Kansas to observe the 2006 teams efforts. It was raining pretty hard in Kansas City so I was a bit apprehensive about launching, but as I cleared the I70 turnpike toll booth the rain stopped and the sun began to peak out from behind the clouds from time to time and the sky began to clear by the time I left Topeka.
This year the challange is to fly a raw large hens egg to an altitude of exactly 800-feet, and at the same time put in a duration of exactly 45-seconds, but not break or even crack the egg. This is reasonably complex in that a score is two-fold since both time and altitude must be accounted for. Points are added to the score if either altitude or time are short or over a score of zero is best. Only one egg is required, not two as in past years, and staging is optional but no additional points are awarded for the extra complexity. This means that the rockets are less complex and can be smaller and lighter, but the math is more complex since one must solve for not just time or altitude, but for both. From what Ive heard from other observers, its resulting in scores that are a bit higher than in previous years. Im personally glad to see a somewhat lower altitude this year. The first year was 1500-feet and the cause of a lot of drifting and wishing for mile-square fields! The Rock Creek teams strategy this year is to hit the target 800-feet and from there a measured descent via a parachute from the motors ejection charge to get the 45-seconds duration. The rocket is a single stager, this year, and uses a cluster of two D11-Ps and a single E9-6 all lit together.
We spent a little time together in the class-room as the team readied their entry. We weighed it, discussed the motors being used, and they showed me their launcher the legs and plate are integral to each other to form a very sturdy platform, well done. The team has a lot of support from the school; the principle came out to watch, and one of the athletic teams turned out to cheer them on. There were also quite a few observers classmates and relatives of the team members were on hand watching and supporting their team.
The first flight was a test flight, and went pretty well. Even a rocket this large begins to look pretty small at 800-feet, but this one went well and flew more or less straight up in the light 10-mph wind. It didnt drift too far, and was recovered intact on the field. The egg survived, with a good time and altitude. Score would have been 23.1.
Second flight was announced as a qualification attempt. The flight profile was almost exactly the same as the last but 3-feet higher despite the extra 10-grams of mass. Score of 26.5.
Third flight was also announced as a qualification attempt. This time it arched over a lot more and only attained an altitude of 749-feet. The resulting duration was notably shorter as well. Score would have been 56, except the egg was cracked.
We discussed the possible reasons for the unexpected performance on the last flight, and Im afraid I was having a somewhat off-day and wasnt too coherent. The possible reasons would be (in order of probability) changes in the wind (and weather-vaning creating an altitude-wasting long arching flight), binding on the rod/lugs (robbing energy at the start), variations in motors (manufacturer guarantees 10%), some change in the airframe gaps between the sections, dirt on the fins or nose, etc. creating drag, and other things.
Even so, a score of 26.5 is pretty good, certainly nothing to sneeze at, and the team should be proud that things went so well.
Another minor concern was with the thrust to weight ratio. The TARC documentation strongly suggests a 5 to 1 ratio which I find to be conservative, even with a short rod. With a six-foot rod, 5 to 1 is very conservative, 4 to 1 is more in the normal range, and Id go lower if it were a calm day. The class was finding their average from the motor designation, not realizing that this is a somewhat misleading value (and that some manufacturers are a bit notorious for this sort of thing.) They also convert the mass to weight in metric rather than converting everything to pounds and ounces! Why have I never thought of that before?!
The team showed me this:
Weight is mass times acceleration due to gravity or (M Kg) x (9.8M/ss).
The mass of the rocket for the qualifying flight was about 750-grams:
0.750Kg x 9.8M/ss = 7.355N
The total average impulse of two D11 and one E9 motors is 18.08N + 9.02 for 27.1N.
So 27.1 / 7.355 is 3.68. A little low, but not dangerously slow by any stretch. And this was borne out by the fact that the rocket flew quite nicely. And thats a whole lot easier to figure as opposed to converting the whole mess to English, whew!
A higher T/W ratio would help overcome the problem with wind and weather-vaning and might make the altitude and times more consistant. The rule of thumb is 4:1 but higher is better. Trip Barber indicates that the documentation suggests 5:1 mostly to get teams thinking about it and that teams wont be held to it at the main event. He also indicates that teams with a too low ratio will probably fail at qualifying (and I have to agree with this logic) and so it hasnt been a problem at the main event before.
So, one team qualified, anyway! Congratulations to the Rock Creek team, and with a score that will hopefully get an invitation to the fly off! Also, our thanks to the AIA and NAR for bringing the Challenge back again this year; and were hoping that they will do it agian next year, too!
|1||2006-02||2 x D11-P, E9-6||734.3g||50g||46.63S
|821-feet||23.1||Test flight; looks good.|
|2||2006-02||2 x D11-P, E9-6||749.3g||65g||47.60S
|824-feet||26.5||Qual flight 1; looks good.|
|3||2006-02||2 x D11-P, E9-6||759.3g||70g||41.13S
|749-feet||54.98||Qual flight 2; Cracked egg DQ|
Submitted by Tim Burger, NAR 78486 L1, photos too.
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Page created on March 17, 2006.
Page last updated on April 10, 2006.